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Tectonic inversion of an asymmetric graben: insights from a
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combined field and gravity survey in the Sorbas basin
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Damien Do Couto1,2,3, Charles Gumiaux4,5,6, Romain Augier4,5,6, Noëmie Lebret4,5,6, Nicolas
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Folcher7, Gwenaël Jouannic8, Laurent Jolivet4,5,6, Jean-Pierre Suc2,3, Christian Gorini2,3
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1
6
(
[email protected])
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2
8
3
9
[email protected])
Total S.A., 2 Place De La Coupole, 92078 Paris La Défense Cedex, France
UPMC Université Paris 6, UMR 7193, ISTEP, 75005, Paris, France CNRS,
UMR
7193,
ISTEP,
F-75005,
Paris,
France
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4
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orleans.fr,
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[email protected])
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5
CNRS/INSU, ISTO, UMR 7327, 45071 Orléans, France
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6
BRGM, ISTO, UMR 7327, BP 36009, 45060 Orléans, France
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7
16
Nouvelle-Calédonie (
[email protected])
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8
18
Haie, 54510 Tomblaine, France (
[email protected])
(
[email protected],
Université d’Orléans, ISTO, UMR 7327, 45071, Orléans, France (
[email protected]@univ-orleans.fr,
[email protected],
PPME (EA n° 3325), Université de la Nouvelle-Calédonie, BP R4 98851 Nouméa Cedex,
CETE de l’Est Laboratoire Régional des Ponts et Chaussées de Nancy, 71 Rue de la Grande
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Corresponding author: Damien Do Couto, Institut des Sciences de la Terre de Paris, CNRS-
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UMR 7193-Université Pierre et Marie Curie, 4 Place Jussieu-75005 Paris, France.
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(
[email protected])
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Key points
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The Sorbas Basin shows a strong asymmetric structure
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The basin has been initiated during an extensional regime
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It then experienced a severe tectonic inversion from ca. 8 Ma characterized by a ca. N-S
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shortening
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Abstract
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[1] The formation of sedimentary basins in the Alboran domain is associated with the
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exhumation of metamorphic core complexes over a ca. 15 Ma period through a transition from
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regional late-orogenic extension to compression. An integrated study coupling field analysis and
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gravity data acquisition was performed in the Sorbas basin in the southeastern Betic Cordillera.
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Detailed field observations revealed for the first time that extensional tectonics occurred before
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shortening in this basin. Two extensional events were recorded with NW-SE to N-S and NE-SW
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kinematics respectively; the first of which being likely responsible for the basin initiation.
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Tectonic inversion of the basin then occurred around 8 Ma in an overall ca. N-S shortening
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context. 2D-gravity sections reveal that the basin acted as an active depocenter as the basin floor
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locally exceeding 2km-depth is characterized by a marked asymmetric architecture. Based on
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this integrated study, we explore a new evolutionary scenario which can be used as a basis for
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interpretations of the Neogene tectonic history of the southeastern Betics.
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1. Introduction
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[2] During the Neogene, the geodynamics of the Mediterranean region was marked by the
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development of backarc basins, from the Alboran Sea in the west all the way to the Aegean Sea
47
in the east, initiated after a major change in the subduction regime, during the Oligocene
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[Rehault et al., 1984; Faccenna et al., 1997; Jolivet and Faccenna, 2000]. The Pannonian Basin,
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Aegean Sea, Liguro-Provençal Basin, Tyrrhenian Sea and Alboran Sea formed above retreating
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slabs. The present-day complex geometry of the subduction zones and backarc basins results
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from the initial geometry of the African margin [Frizon de Lamotte et al., 2011] and progressive
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slab tearing and detachments [Wortel and Spakman, 2000] associated with a complex 3D mantle
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convection pattern [Faccenna and Becker, 2010]. At the western end of the Mediterranean
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region, the Alboran domain has recorded this regional episode of backarc extension and slab
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retreat [Faccenna et al., 2004; Spakman and wortel, 2004] until ~7-8 Ma before the resumption
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of a compressional regime due to Africa-Eurasia convergence [Jolivet et al., 2008]. Offshore and
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onshore sedimentary basins have recorded this evolution and the respective contributions of early
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extension and late compression in the final architecture of these basins is still debated, especially
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in the Huércal-Overa and Sorbas basins [Weijermars et al., 1985; Ott d’Estevou and Montenat,
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1990; Augier et al., 2005a; Meijninger and Vissers, 2006; Pedrera et al., 2010]. This study aims
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at deciphering the respective contribution of extension and subsequent compression in the
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shaping of the Sorbas basin.
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[3] The Sorbas basin, located in the internal zones of the Betic Cordillera (southern Spain), is a
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key peripheral basin where upper-Serravallian to Pliocene sediments have recorded interactions
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of both tectonic and eustatic changes of the southwestern Mediterranean region. Several
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scenarios of the Neogene evolution of the whole Alboran region were based upon the
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stratigraphy and structure of this basin. However, different tectonic settings, from strike-slip to
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pure extension, have been proposed for the initiation of the basin in the Serravallian-lower
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Tortonian. Since the upper Tortonian, the Sorbas basin has been subjected to NW-SE shortening
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that has shaped most of its current geometry [Weijermars et al., 1985]. The debate on the
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tectonic origin of the Sorbas basin is mainly based on contrasting interpretations of the structural
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relationships between sedimentary basins and metamorphic domes of the adjacent “sierras” (i.e.
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Sierra de los Filabres and the Sierra Alhamilla) [Martínez-Martínez and Azañón, 1997; Martínez-
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Martínez et al., 2002, 2004; Behr and Platt, 2012]. While the Huércal-Overa basin shows rather
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well preserved shape and distribution of depocenters linked to the pre-8 Ma extension
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[Meijninger and Vissers, 2006; Pedrera et al., 2010], late shortening had a major influence on the
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present-day geometry of the Sorbas basin [Weijermars et al., 1985; Ott d’Estevou and Montenat,
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1990]. The effect of the early extension has not already been discussed and a better description
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of the deep structure of the basin is needed to access its early tectonic history. In order to better
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constrain the geometry of the basin during its evolution, a gravity survey has been carried out
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through the basin together with new field structural analyses. From these new structural and
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geometrical constraints, a new tectonic evolutionary scenario is proposed for the Sorbas Basin.
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2. Geological setting
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2.1. The southeastern Betic Cordillera
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[4] The Betic Cordillera forms the northern branch of the Betic-Rif orogenic belt that results
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from the combination of a continental collision between the Alboran domain and the Iberian-
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Moroccan passive margins, during the Miocene [Crespo-Blanc and Frizon de Lamotte, 2006],
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and a strong westward movement of the Alboran domain [Lonergan and White, 1997; Faccenna
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et al., 2004]. The former south Iberian Mesozoic-Cenozoic passive margin currently forms the
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External Zones of the Betics and corresponds to a fold-and-thrust belt [Platt et al., 2003]. To the
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South, the finite structure of the Internal Zones results from the stacking of metamorphic units
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and their subsequent exhumation in the core of metamorphic domes [Martínez-Martínez et al.,
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2002; Augier et al., 2005a, 2005b]. From bottom to top, the alpine metamorphic basement is
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composed of the Nevado-Filabride (NF) and the Alpujarride (Alp) complexes, overlain by a third
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one, the Malaguide (Mal) complex [Torres-Roldán, 1979]. These three complexes are presently
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separated by crustal-scale extensional shear zones that played a major role during the
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exhumation of the deeper complexes [García-Dueñas et al., 1992; Lonergan and Platt, 1995;
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Crespo-Blanc, 1995; Platt et al., 2005; Augier et al., 2005a]. Exhumation of the Alpujarride
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complex occurred during the Early Miocene (22 to 18 Ma) in a N-S to NNE-SSW extensional
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setting [Monié et al., 1994; Crespo-Blanc et al., 1994; Crespo-Blanc, 1995; Platt et al., 2005].
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Timing of peak metamorphic conditions and the inception of exhumation of the NF complex is
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still debated [López-Sánchez Vizcaino et al., 2001; de Jong, 2003; Augier et al., 2005b; Platt et
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al., 2006]. Conversely, timing of the final exhumation stages in the greenschists facies conditions
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spans over the Middle and the Late Miocene (from 15 to 9 Ma) [de Jong, 1991; Monié and
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Chopin, 1991; Johnson et al. 1997; Augier et al., 2005b; Platt et al., 2005; Vázquez et al., 2011]
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through an E-W regional-scale extension [Jabaloy et al., 1992]. Final exhumation stages in the
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brittle regime at ca. 13-12 Ma was accompanied by the development of extensional sedimentary
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basins such as the Huércal-Overa basin [Vissers et al., 1995; Augier, 2004; Meininger and
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Vissers, 2006; Augier et al., 2013].
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[5] The Sorbas basin belongs to a mosaic of E-W oriented Late Neogene intramontane basins
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with the Huércal-Overa basin and the Almanzora Corridor, to the North, the Alpujarran Corridor
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to the West and the Vera basin to the East [Ott d’Estevou and Montenat, 1990; Sanz de Galdeano
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and Vera, 1992; Iribarren et al., 2009; Pedrera et al., 2010]. It is bounded to the North by the
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Sierra de los Filabres and to the South by the Sierra Alhamilla (Figure 1), two E-W elongated
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metamorphic domes [Platt and Vissers, 1989; Vissers et al., 1995; Martínez-Martínez and
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Azañón, 1997] formed by the exhumation of the NF complex and amplified by later folding
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[Weijermars et al., 1985].
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2.2. Tectonic and geodynamic framework
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[6] The extensional regime which affected the Betic Cordillera and, in a more regional extent,
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the entire Alboran domain started from the Late Oligocene – Early Miocene (see Vergés and
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Fernàndez, 2012 for a review of tectonic evolution) and is widely documented both offshore
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[Watts et al., 1993; Comas et al., 1999] or onshore [García-Dueñas et al., 1992; Galindo-
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Zaldívar et al., 2003], especially in the Huércal-Overa basin [Briend et al., 1990; Augier et al.,
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2005a; Meijninger and Vissers, 2006; Pedrera et al., 2009, 2010, 2012; Augier et al., 2013].
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This latter displays an overall half-graben geometry accompanied by meso- to small-scale syn-
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sedimentary normal faults. Its formation and its development have been associated with the
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exhumation of the NF metamorphic complex in the Sierra de Los Filabres metamorphic dome
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[Augier et al., 2005a; Meijninger and Vissers, 2006; Augier et al., 2013] (see Figure 1).
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According to Augier et al. [2013], the half-graben geometry results from the development of
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north dipping normal faults that are probably rooted in the detachment separating the NF from
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the Alp complexes, called the Filabres Shear Zone. A similar tectonic contact (called the
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Alhamilla detachment [García-Dueñas et al., 1992; Martínez-Martínez and Azañón, 1997]) also
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crops out in the Sierra Alhamilla. Last motions on that detachment in ductile conditions occurred
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at ca. 14 -13 Ma [Martínez-Martínez and Azañón, 1997; Augier et al., 2005b, Platt et al., 2005]
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while exhumation tectonics under the brittle regime lasted until ca. 8 Ma.
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[7] Unlike the Huércal-Overa basin, the Sorbas basin presents evidences for compression
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tectonics (Figure 2). The northern boundary of the basin is characterized by an overall gentle
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onlap of the sediments over the basement of the Sierra de Los Filabres (Fig. 2b). Conversely, in
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the southern part of the basin, Tortonian sediments are folded, locally overturned or even
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overthrusted by basement rocks (Figure 3) [Weijermars et al., 1985; Ott d’Estevou and
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Montenat, 1990; Giaconia et al., 2012a,2013]. The onset of compression in the region has been
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ascribed to the upper Tortonian into the basin at around 8 Ma [Weijermars et al., 1985; Comas et
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al., 1999; Augier, 2004; Jolivet et al., 2006; Meijninger and Vissers, 2006; Augier et al., 2013].
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This compressional regime resulted in the formation of the current E-W oriented open-fold
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syncline geometry of the Messinian to Quaternary series [Weijermars et al., 1985; Ott d’Estevou
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and Montenat, 1990] and the (Figure 2B).
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[8] Based on field investigations, several hypotheses were proposed to explain the initiation
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phase of both the Sorbas [Montenat et al., 1987; Weijermars et al., 1985; Ott d'Estevou et al.,
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1990, Poisson et al., 1999] and Tabernas basins [Kleverlaan, 1989]. These mainly differ on the
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interpretation of the regional-scale tectonic regime occurring at that time:
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- compressional to transpressional models with an overall N-S oriented maximum shortening
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have been put forward [Montenat et al., 1987; Weijermars et al., 1985; Ott d’Estevou et al.,
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1990; Sanz de Galdeano and Vera, 1992; Poisson et al., 1999]. They all consider the
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Africa/Europe convergence as the principal driving force controlling crust deformation [Sanz de
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Galdeano, 1985; Ott d’Estevou and Montenat, 1990]. In these models, the Sorbas basin is
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interpreted either i) as a purely flexural basin developed along a major EW trending/northward
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verging thrust [Weijermars et al., 1985] or ii) a transpressional basin linked to a wide NE-SW
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strike-slip shear-zone [Sanz de Galdeano, 1985; Weijermars, 1987; Montenat et al., 1987; Ott
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d'Estevou et al., 1990; Sanz de Galdeano and Vera, 1992; Stapel et al., 1996].
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- a strike-slip faults network is active since the Middle/Upper Miocene [Sanz de Galdeano, 1985;
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Booth-Rea et al., 2004a; Sanz de Galdeano et al., 2010; Rutter et al., 2012] and corresponds to
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the onshore extension of the Trans-Alboran shear zone (Figure 1) [de Larouzière et al., 1988]. In
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that context, the deposition of Tortonian sediments would have taken place in a pull-apart trough
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[Kleverlaan, 1989; Stapel et al., 1996; Haughton, 2001; Martínez-Martínez et al., 2006; Rutter et
167
al., 2012].
168
- extensional models in which the formation of asymmetric intramontane basins are linked to the
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late exhumation stages within the metamorphic domes of the NF metamorphic complex during
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the collapse of the Internal zones [García-Dueñas et al., 1992; Vissers et al., 1995; Martínez-
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Martínez and Azañón, 1997; Meijninger and Vissers, 2006; Augier et al., 2013]. Basins would be
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initially extensional during the upper Serravallian-lower Tortonian [Meijninger and Vissers,
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2006; Augier et al., 2013] before they experience inversion tectonics from upper Tortonian to
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Quaternary.
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[9] Recent gravity surveys show an irregular and/or asymmetrical distribution of the residual
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gravity anomalies in the Almanzora Corridor (Figure 1) [Pedrera et al., 2009], in the Sorbas
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basin [Li et al., 2012] and in the Huércal-Overa basin [Pedrera et al., 2010]. The Huércal-Overa
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area has been particularly well documented: modeled gravity profiles evidence that the deepest
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parts of the sediment/basement interface, reaching 1000 to 1500 m depth are confined to the
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southern part of the basin [Pedrera et al., 2009, 2010] and more particularly along normal faults
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[Augier et al., 2013]. Besides, tectono-stratigraphic and palaeostress analyses from brittle fault
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show that two main depocenters were first controlled by SW-NE trending normal faults probably
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rooting in the Filabres Shear Zone [Augier et al., 2013]. This brittle extensional event occurred
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during the latest Serravallian to lower Tortonian (~12-11 Ma) and is linked to the exhumation of
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the NF metamorphic complex [Augier et al., 2013]. Sedimentation started during the late
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denudation stages of the NF complex as evidenced by zircon and apatite fission tracks ages and
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U-Th/He analyses on apatites (respectively at 11.9 ± 0.9 Ma, 8.9 ± 2.9 Ma and 8.7± 0.7 Ma)
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[Johnson et al., 1997; Vázquez et al., 2011] in both the Sierra de los Filabres and Alhamilla
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[Augier et al., 2005b; Platt et al., 2005].
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3. The Sorbas basin
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3.1. Basement units bounding the basin
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[10] While the Malaguide complex is poorly represented within the study area, the Alpujarride
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and the Nevado-Filabride complexes are both well exposed within the metamorphic domes
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forming the sierras [Vissers et al., 1995; Martínez-Martínez and Azañón, 1997]. The Alpujarride
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complex is mostly represented by Permian purple to reddish phyllites overlain by Triassic
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metacarbonates [García Monzón et al., 1973b, 1974; Azañón and Crespo-Blanc, 2000]. The
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Nevado-Filabride complex is made of three main units [García-Dueñas et al., 1988; de Jong,
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1991] called, from bottom to top, the Ragua Unit, the Calar-Alto Unit and the Bédar-Macael Unit
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with respective average structural thicknesses of about 4,000, 4,500 and 600 m [Martínez-
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Martínez et al., 2002]. The Ragua Unit is made up of Palaeozoic dark micaschists with
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intercalated quartzites and rare dark marbles [Martínez-Martínez, 1985] while the Calar-Alto and
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the Bédar-Macael units mainly contain Palaeozoic black micaschists, Permo-Triassic quartzites,
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metapelites and Triassic metacarbonates. Dense eclogite and blueschist bodies or their
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retrogressed equivalent amphibolite-facies metamorphic rocks are described as metric to
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kilometric scattered lenses into both Calar-Alto and Bédar-Macael units [García Monzón and
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Kampschuur, 1972; García Monzón et al., 1973a; Martínez-Martínez and Azañón, 1997].
208 209
3.2. Stratigraphy
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[11] The oldest sedimentary record of the Sorbas basin surroundings is represented by scarce
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Burdigalian-Langhian marly limestones badly preserved north to the Sierra Cabrera and
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Serravallian shallowing-upward turbidites exposed along the Sierra Alhamilla and Sierra Cabrera
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(Figure 1) [Ott d’Estevou and Montenat, 1990; Sanz de Galdeano and Vera, 1992]. The rest of
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the series is quite similar to nearby basins [Sanz de Galdeano and Vera, 1992] and starts with
215
two different units [Ott d’Estevou and Montenant, 1990]: (i) a ~100 m thick upper Serravallian-
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lower Tortonian continental coarse-grained deposits evolving basinward to yellowish marls and
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turbidites (Figure 2C), which outcrop in the western (i.e. Tabernas area) and eastern (i.e.
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Gafarillos area) parts of the basin (Figure 3A); (ii) a rather thick (up to 2000 m according to [Ott
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d’Estevou and Montenat, 1990]) upper Tortonian marine unit, composed of grayish to yellowish
220
sandstones and turbidites (Figure 2C). It is worth mentioning that deposition of the upper
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Serravallian-lower Tortonian sediments appear coeval with the latest exhumation of the NF
222
complex in the surrounding sierras [Ott d’Estevou and Montenat, 1990; Johnson et al., 1997;
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Vázquez et al., 2011]. Latest Miocene sediments unconformably overly Tortonian sediments and
224
are composed of six units including from bottom to top (Figure 2C):
225
- a shallow marine fossiliferous calcarenite
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Tortonian/Messinian boundary [Ott d’Estevou and Montenat, 1990; Sierro et al., 1993;
227
Krijgsman et al., 2001]; its thickness ranges from 10 to 80-90 m depending on its
228
paleogeographical context [Ott d’Estevou and Montenat, 1990; Puga-Bernabéu et al., 2007]
229
- an early Messinian marly unit known as the Abad marls composed of ~120 m of marls
230
alternating with diatomites. The upper part passes laterally to plurimetric fringing reefal
231
carbonates (Cantera member; Figure 2) outcropping both in the northern and in the southern part
232
of the basin (e.g. Cariatiz and Morrón de la Cantona localities; Figure 2A), bounding the basin
233
during the early Messinian [Martín et al., 1999]
(Azagador member) dated from the
234
- a 120 m thick evaporitic unit (Yesares member) consisting in gypsum alternating with clayey to
235
marly laminites [Dronkert, 1976] deposited during the Messinian Salinity Crisis [Clauzon et al.,
236
1996; CIESM, 2008] whose onset is calibrated at 5.971 Ma [Manzi et al., 2013]
237
- a latest Messinian-early Pliocene lagoonal calcarenite (Sorbas member) [Roep et al., 1998]
238
passing to a Gilbert-type fan delta (dated from 5.46 to 5.00 Ma) [Clauzon et al., in revision]; the
239
thickness of this marine unit reaches 75 m in the center of the basin at Sorbas locality
240
- finally, a mostly continental sedimentary unit made of reddish conglomerates called the
241
Zorreras member and reaching 60 m in the basin [Montenat et al., 1980; Martín-Suárez et al.,
242
2000].
243 244
4. Aims and approach
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[12] Most of the studies on the Sorbas basin, having poor structural claims, focused on the
246
Messinian history [Rouchy and Saint-Martin, 1992; Clauzon et al., 1996; Roep et al., 1998;
247
Riding et al., 1998, 2000; Martín et al., 1999; Krijgsman et al., 2001; Braga et al, 2006;
248
Bourillot et al., 2009; Roveri et al., 2009]. Besides, while the overall structure of the post-
249
Tortonian deposits is documented as the consequence of a ~N-S compressional context (see
250
above), the tectonic control during the onset of subsidence and early development of the basin
251
remains debated and in fact mostly unexplored. This study aims at constraining the kinematics of
252
the Sorbas basin while the basin underwent its first subsidence phase and sedimentary deposition
253
during the upper Serravallian-lower Tortonian. This paper thus focuses on the structure of the
254
early deposits/basement contact which outcrops along the southern margin of the basin but also
255
on the 3D geometry of this contact, to depth. It also deals with the kinematics and stress
256
conditions that prevailed during the upper Serravallian-lower Tortonian period. Geological and
257
geophysical data have been used and integrated in this work including structural geology,
258
mapping, palaeostress computations and gravimetric survey and modelling.
259
[13] At first, a synthesis on the structure of the southern margin of the basin (i.e. the northern
260
edge of the metamorphic Sierras) is made with a particular focus on the contact between
261
Serravallian-Tortonian sedimentary units and the basement. It is based on a compilation of
262
existing data and own field observations. Then, structural and kinematical constraints are given
263
from structural field observations made in the older deposits of the basin, in specific areas. As
264
shown before, earliest deposits of the upper Serravallian–lower Tortonian are poorly exposed in
265
the study area because of the extension of post-Tortonian sedimentary series (Figure 2C, 3A) and
266
the ongoing deformation. However, systematic and accurate structural measurements have been
267
realized along the southern basin margin where outcropping conditions allow observing various
268
kinematic constraints. To first describe the initial development of the Sorbas basin and to insure
269
an accurate determination of the deformation kinematics, palaeostress orientation patterns were
270
evaluated by the computer-aided inversion method of fault-slip data [Angelier, 1984, 1990,
271
1994]. Thanks to the recognition of numerous stratigraphic time-markers including major
272
unconformities [Ott d’Estevou and Montenat, 1990; Sanz de Galdeano and Vera, 1992], the
273
succession of stress regimes is reasonably well constrained in time. As detailed in the followings
274
(see Structural Analysis), the study focused on (i) the structure of the basement/sedimentary
275
rocks interface, (ii) structural analyses and inversion of fault-slip data to obtain palaeostress
276
tensors carried out in the Serravallian-Tortonian series, and (iii) their internal structures of .
277
[14] Gravity data modeling has already proven its efficiency for imaging deep geological
278
structures at crustal-scale, especially for “basement” rock types imaging in plutonic and
279
metamorphic geological environments [Rousset et al., 1993; Lefort and Agarwal, 1999; Martelet,
280
1999; Martelet et al., 2004; Talbot et al., 2004; Joly et al., 2009; Turrillot et al., 2011]. While
281
gravity modeling is more rarely applied to sedimentary geological environments [Szafián and
282
Horváth, 2006; Pedrera et al., 2009, 2010; Li et al., 2012; Salcher et al., 2012], this method is
283
used here, in particular, to constrain the geometry of the interface between the rather dense
284
basement rocks of the metamorphic complexes and the lighter sediments of the basin. Deep
285
structures of the basin are thus constrained along a series of 2D gravity profiles overlapping the
286
basin and the surrounding basement units. Gravity models and field observations, including
287
bedding orientations measurement, were finally used to interpolate the 3D basin floor geometry.
288
These results allow improving our knowledge on the tectonic evolution during the development
289
of the Sorbas basin and precising the structural control on deposits geometry.
290 291
5. Structural analysis
292
5.1. Evidence of compressive tectonics
293
[15] The Sierra Alhamilla, Sierra Polopos and Sierra Cabrera represent three E-W elongated en
294
echelon antiforms (Figure 3). The Alhamilla and Polopos ranges form two north-verging
295
asymmetric folds with a north steep-dipping forelimb and a gentle south-dipping backlimb [Platt
296
et al., 1983]. They are together separated from the Sierra Cabrera by the dextral North Gafarillos
297
strike-slip fault (NGF, Figure 3). The northern front of the Sierra Alhamilla, Polopos and
298
Cabrera antiformal structures are composed of three major faults called, from East to West, the
299
North Cabrera reverse fault (NCRF), the North Gafarillos fault (NGF) and the North Alhamilla
300
reverse fault (NARF) [Giaconia et al., 2012a,b,2013] (Figure 3).
301
[16] The North Cabrera reverse fault affects the entire sedimentary filling and shows inverted or
302
strongly dipping Serravallian and Tortonian sediments with a N to NW transport striking [Booth-
303
Rea et al., 2004b] (cross-section I-I’, Figure 3). According to Booth-Rea et al. [2004b], the
304
activity of the NCRF initiated before the upper Tortonian as upper Tortonian conglomerates
305
unconformably overlies Langhian-Serravallian deposits. The fault activity is related to the
306
initiation of major sinistral Palomares and Carboneras strike-slip faults [Booth-Rea et al., 2004b;
307
Gràcia et al., 2006; Giaconia et al., 2012a]. The North Gafarillos and South Gafarillos faults
308
(NGF and SGF, Figure 3) represent two transpressive dextral faults accommodating the
309
deformation and the relative displacement of the Sierra Alhamilla with the Sierra Cabrera
310
[Giaconia et al., 2012a]. The E-W segment of the NGF, bounding the Sorbas basin to the south,
311
affects the Tortonian series (cross-section II-II’ in Figure 3) and most of the deformation
312
occurred during the Tortonian as Messinian reefal carbonates seal the contact toward the West
313
(Cantera Member, Figure 2B, 3) [Ott d’Estevou and Montenant, 1990; Stapel et al., 1996; Jonk
314
and Biermann, 2002]. The E-W North Alhamilla reverse fault zone, which is linked to the South
315
Gafarillos dextral strike-slip fault, can be followed over ~20 km along the southern border of the
316
Sorbas-Tabernas basin. This thrust fault zone cuts the northern limb of the asymmetric Alhamilla
317
antiform where Serravallian-Tortonian deposits display overturned bedding just below (Figure 2;
318
cross-section III-III’ in Figure 3). Its activity lasted from the upper Tortonian to the Late
319
Pleistocene with an overall top-to-the-NNW movement and controls the front of the Sierra
320
Alhamilla range by a drastic topographic gradient [Jonk and Bierman, 2002; Giaconia et al.,
321
2012a]. Several studies pointed out the syn-compression filling of upper Tortonian series
322
[Weijermars et al., 1985; Ott d’Estevou and Montenat, 1990; Haughton, 2001], reporting a
323
southward progressive gradient of deformation. Toward the west of the Sierra Alhamilla, the
324
NARF disappears and the nature of the contact separating Serravallian-Tortonian deposits from
325
the Alpujarrides metamorphic series appears unconformable (cross-section IV-IV’ in Figure 3).
326
There, the El Alfaro hill which is made of plurimetric conglomeratic and sandy interval belongs
327
to the upper Tortonian turbiditic series [Hodgson and Haughton, 2004]. According to Haughton
328
[2000] and Hodgson and Haughton [2004], the timing of deformation in the Alfaro area is also
329
estimated as Tortonian and Messinian in age from the sedimentary record (thick slump deposits
330
and internal unconformities).
331
[17] The overthrust of the Sierra Alhamilla basement range over the southern border of the basin
332
marks the strong asymmetry of both the basin and the basement range [Ott d’Estevou et al.,
333
1990; Martínez-Martínez and Azañón, 1997]. The NARF disappears toward the East of
334
Lucainena de las Torres (Figure 3), Figure 4 displays a landscape interpretation accompanied
335
with bedding measurements made across the basement/sediments contact (see Figure 2 for
336
location). The bedding attitude of the Tortonian series shows a progressive steepening over more
337
than 2 km and is even overturned in the vicinity of the Sierra Alhamilla (Figure 4); fold
338
asymmetry being illustrated by the more pronounced clustering of bedding poles in stereonet
339
projection (Figure 4). Structurally below, to the south, meta-sediments of the basement units
340
show parallel overturned bedding orientation to the basal layers of the basin marking an
341
overturned forelimb of the Sierra Alhamilla. There, upper Tortonian sediments are separated
342
from the basement by an unconformity or a stratigraphic break as the upper Serravallian-lower
343
Tortonian continental series are missing. East of the NARF, the geometry of the
344
basement/sediment interface argues in favour of a north-verging asymmetric fold in the Sierra
345
Alhamilla with an overturned steep-dipping forelimb, highlighting the recumbent syncline of
346
Tortonian deposits. Such folded structure could be interpreted as a fault-propagation fold.
347
[18] Progressive unconformities (e.g. growth-strata) in upper Tortonian sediments (8-7.24 Ma),
348
mark the growth of the Sierra Alhamilla basement range and determine a good age-control on
349
that deformation episode. This compressional episode mostly shaped the current geologic and
350
topographic features of the southeastern parts of the Internal zones of the Betic Cordillera and
351
pre-dates the Messinian to present evolution of the basin. Indeed, the overlying Messinian
352
carbonates seal most of the folding linked with the growth of the Sierra Alhamilla range (Figure
353
2, 3) [Weijermars et al., 1985; Ott d’Estevou and Montenat, 1990] but are in turn involved in the
354
present-day large-scale open fold of the Sorbas basin. Internal deformation of the upper-
355
Tortonian series is characterised by small-scale wrench faulting with only rare reverse faults as
356
shown along the road-cut near Lucainena de las Torres (Figure 4). Analysis of fault-slip data
357
yielded a strike-slip palaeostress tensor solution with a N-S maximum compression direction [1]
358
and a W-E minimum compression direction [3] in that case, consistent with the large-scale
359
folding of the northern front of the Sierra Alhamilla (Figure 2B and 4).
360 361
5.2. Evidence of extensional deformation
362
[19] Upper Serravallian-lower Tortonian deposits, which are particularly abundant in the
363
Huércal-Overa basin, are in contrast very limited within the Tabernas-Sorbas basin. They are
364
encountered along the northern limb of Sierra Alhamilla (Figure 3). These deposits are of prime
365
importance with respect to the deep structure of the basin and consequently with the buried
366
history of the Sorbas basin. Hereafter, two groups of outcrops are described as representative of
367
that discussion, the Tabernas and the Gafarillos areas (see location in Figure 2).
368
[20] In the Gafarillos area, upper Serravallian-lower Tortonian formations crop out well along
369
roads that cross the hills, particularly the ca. N020E striking road linking Gafarillos to Gacia-
370
Bajo. There, the sedimentation, characterized by conglomerates and coarse-grained sandstones
371
presents general subhorizontal bedding and is affected by a pervasive network of small-scale
372
normal faults (Figure 5A) locally showing synsedimentary character (Figure 5B). The entire
373
normal faults population, generally developed as 50-70° dipping roughly planar surfaces,
374
presents two distinctive orientations: a dominant ca. N140E and a subordinate ca. N030E
375
conjugate sets of normal faults. Associated stress tensors correspond to subhorizontal extension
376
presenting respectively N060E and N140E minimum compression direction [3] and a common
377
subvertical maximum compression direction [1] (Figure 5A). Analysis of younger rocks
378
belonging to the basal upper Tortonian in the same area (inset C in Figure 5), revealed that only
379
the ca. N140E set of normal faults was present, suggesting that the former N30E system was
380
restricted to the upper Serravallian-lower Tortonian formations.
381
[21] The Tabernas area provides an example of geometrical relationships between faulting and
382
folding (see location on Figure 2). There, upper Serravallian to upper Tortonian formations
383
experienced a large-scale folding episode (i.e. the so-called Tabernas fold) [Ott d’Estevou and
384
Montenat, 1990] particularly well exposed along the incised Rambla de la Sierra. A landscape
385
picture is given on Figure 6. In details, the whole area presents a pervasive network of small-
386
scale faults systems including mostly normal and wrench faults whose relative chronology and
387
relationships with folding remained so far unexplored. Close-up views of two parts of the
388
northern limb of the fold are given as insets in Figure 6; inset B documents faulting in the upper
389
Serravallian-lower Tortonian formations and inset C, the upper Tortonian. Moreover, Figure 6D
390
provides a decomposition of the total heterogeneous fault population into homogeneous,
391
cogenetic fault populations. Normal faulting appears clearly related to pre-folding extension as
392
suggested by the coexistence of both subvertical and subhorizontal normal and reverse faults (in
393
the present attitude of bedding) passively rotated with respect to bedding (Figure 6B and 6C). In
394
addition, it is to be noted that some of them reveal a syn-sedimentary character. Computed
395
principal stress regimes, in agreement with an Andersonian model after back-tilting [Anderson,
396
1942] yielded a succession of two subhorizontal extensional regimes with N50E and N140E
397
minimum compression direction [3] for an almost vertical common maximum compression
398
direction [1]. Wrench faults that often reactivate former normal fault planes present N20-80E
399
sinistral faults accompanied by N120-170E dextral faults. Analysis of fault-slip data yielded a
400
strike-slip palaeostress tensor solution characterised by a N-S maximum compression direction
401
[1] and a W-E minimum compression direction [3]. This result is consistent with the large-
402
scale W-E folding that affect passively the Sierras [Weijermars et al., 1985; Ott d’Estevou and
403
Montenat, 1990] and the basins (Figure 2 and 3) with a ca. 10-20° dispersion as exemplified by
404
the WSW-ENE Tabernas anticline (Figure 6D).
405
[22] The structural development and the sedimentation of the Tabernas-Sorbas basin thus appear
406
controlled by normal faulting from upper Serravallian-lower Tortonian onward and at least
407
during a part of the upper Tortonian. These traces of extension whose origin appear coeval and
408
may be related with the regional-scale exhumation of the Nevado-Filabride metamorphic
409
complex [Martínez-Martínez and Azañón, 1997; Augier et al., 2005a, 2013] are now mostly
410
hidden by thick younger deposits in the Sorbas basin area.
411 412
6. 2D gravity survey
413
6.1. Acquisition and treatment
414
[23] A gravity survey was conducted throughout the Sorbas basin and surrounding basement
415
areas. Ground measurements were performed using a SCINTREX CG5-M micro-gravimeter in
416
195 gravity stations emplaced along 6 ~north-south trending sections, orthogonal to regional-
417
scale structures, and one additional longitudinal (i.e. ~east-west) section for crosschecking
418
(Figure 7). Measurement stations were spaced ca. 1 km apart along the two longer sections
419
aiming at imaging regional-scale structures (see N-S trending SCS1 “Sorbas Cross-Section 1”
420
and SCS2 E-W trending section; Figure 7). Along other profiles, stations were spaced ca. 500 m
421
apart within the Sorbas basin, and ca. 250 m apart along the segments in the vicinity of the
422
basement/sediment contact. To better control the raw data quality, measurements were repeated
423
three times at each gravity station. Geographic coordinates and altitude were measured using a
424
MAGELLAN Promark3 post-D-GPS system. For each station, location was recorded and
425
averaged during more than one minute and values were adjusted through a post-treatment
426
correction process by using a secondary antenna that was fixed for the entire survey period. Such
427
acquisition method ensures ca. 10 cm and 20 cm of precision for horizontal and vertical
428
coordinate values, respectively. To ensure consistency between the different gravity profiles
429
measured and to control the temporal instrumental drift, a unique gravity reference station was
430
used during the overall field survey; it was systematically measured twice a day, before and after
431
daily surveys (Figure 7B, Los Martinez locality). To calculate the Bouguer anomaly, standard
432
free air, plateau and terrain corrections were successively performed; a 2.67 g.cm-3 Bouguer
433
reduction density was chosen as an average density value for crustal rocks. Inner terrain
434
corrections (for 50 m of distance away from station) were routinely performed using Hammer
435
charts [1939]. Following Martelet et al. [2002], far field terrain corrections (up to 167 km away
436
from the station) were computed numerically using the SRTM digital elevation model. Finally,
437
the Bouguer anomaly values were calibrated (with a constant shift) to the available regional-
438
scale gravity data [Torne et al., 2000] in order to refer to a common and absolute gravity
439
reference with other existing data sets (see Figure 8A).
440
[24] Two dimensional (2D) direct gravity modeling was performed using the Geosoft-GM-SYS
441
software. Gravity Bouguer anomalies depend on both the geometry and the density contrasts
442
between geological units. In order to avoid multi-solutions models from the gravity analysis,
443
densities of the geological units were measured at the laboratory using the hydrostatic weighing
444
method. Densities used for each lithology considered in the model are listed in Table 1 and
445
detailed below. These were also systematically compared to published density range values
446
usually considered for classical lithologies [Robinson and Çoruh, 1988; Telford et al., 1990,
447
Jacoby and Smilde, 2009]. While density values were fixed, the modeling process then consisted
448
to adjust geometries of the geological units along the sections. The Moho depth variation
449
generally induces large wavelength variations in gravity anomaly profiles and, for this study, the
450
geometry of this particular interface is fixed from the available reference dataset of Fullea et al.
451
[2006] or Ziegler and Dèzes [2006]. The geometry of each geological unit was drawn in
452
accordance to the geological maps [García Monzón et al., 1973b, 1974] (Figure 2).
453 454
6.2. Rock densities
455
[25] Rock densities were directly measured from several rock samples for each geological unit.
456
Indeed, the determination of average standard densities for a geological formation highly
457
depends on the definition of the lithological assemblage [Jacoby and Smilde, 2009]. Gravity
458
modeling based on measured densities ensures a much better control. Both the Alpujarride and
459
the Nevado-Filabride complexes metamorphic basement were sampled (Figure 2). The purple
460
micaschists of the Alpujarride complex were sampled near Lucainena de Las Torres at the
461
southern edge of the basin. The corresponding measured density (2.69 g.cm-3; see Table 1) is
462
consistent with the general bulk densities usually considered for micaschists and phyllites (i.e.
463
from 2.64 to 2.74 g.cm-3) [Jacoby and Smilde, 2009]. Due to the occurrence of dolomitic layers
464
at all scales, the density of the Triassic metacarbonate rocks from the upper part of the
465
Alpujarride complex was fixed from the literature [e.g. Jacoby and Smilde, 2009] with a bulk
466
density of 2.7 ±0.2 g.cm-3. An average density of 2.69 g.cm-3 was thus attributed to the overall
467
Alpujarride complex for modeling. Micaschists samples of all three units composing the
468
Nevado-Filabride complex (i.e. the Ragua Unit, the Calar-Alto Unit and the Bédar-Macael Unit)
469
yielded densities of 2.65, 2.67, and 2.69 g.cm-3 respectively (Table 1). These densities are in
470
good agreement with both average densities published in the literature [Jacoby and Smilde,
471
2009] and the fact that the metamorphic grade of these complexes increases in this same order.
472
Indeed, despite a relatively common lithological succession, one main distinction between those
473
units is ascribed to the presence of metabasite bodies in both Bédar-Macael and Calar-Alto units
474
(which include eclogite and amphibolite rocks) that were not directly measured [García Monzón
475
et al., 1973a; Martínez-Martínez and Azañón, 1997]. Average densities of such basic rocks are
476
respectively about 3.3 and 2.9 g.cm-3 [Jacoby and Smilde, 2009]. Hence, the adjacent Bedar-
477
Macael and Calar-Alto units have been considered as a single unit for gravity modeling,
478
hereafter called the Upper NF unit, with a slightly increased average density of 2.7 g.cm-3 (Table
479
1). The lowermost metamorphic unit has been defined as the Lower NF unit (corresponding to
480
the above mentioned Ragua unit) with a measured average density value of 2.65 g.cm-3. On the
481
regional-scale cross-section SCS1 (Figure 7A), the use of dense bodies was not necessary to
482
adjust the modeled residual Bouguer anomaly with the measured one. At that scale, far-field
483
effects of these scarce and dense bodies can be neglected. On the contrary, adjustment of the
484
modeled residual Bouguer anomaly on small-scale cross-sections required the use of such
485
bodies, in accordance with their exposures [García Monzón et al., 1973b, 1974].
486
[26] Sampling of the sedimentary cover has been performed throughout the entire basin (Figure
487
2). Apart conglomeratic layers (from the upper Serravallian to lower Tortonian), whose density
488
cannot be obtained by laboratory measurement, densities of all the sedimentary units have been
489
measured (see Table 1). Two groups have been distinguished for modeling: one including
490
Serravallian and Tortonian deposits and another one from lowermost Messinian to uppermost
491
Pliocene layers. The average density obtained for Tortonian deep marine deposits (2.54 g.cm-3;
492
Table 1) has directly been used in the 2D modeling process. Densities measured for the
493
lowermost calcarenite, messinian marls-reefs-evaporites, and the overlying latest Messinian-
494
early Pliocene deltaic deposits are summarized in Table 1. The density measured for the
495
Messinian marls, yielding a density of 1.6 presents much lower values than the density usually
496
considered for such rock type (2.4 g.cm-3; Jacoby and Smilde, 2009). Consequently, and in order
497
not to unbalance the gravity modeling, an average density of 2.4 g.cm-3 has been considered for
498
his unit. Finally, a mean 2.37 g.cm-3 density has been calculated for the entire lowermost
499
Messinian-uppermost Pliocene group. Note that the density of the topmost Pliocene
500
conglomeratic layers has not been measured because of their sample-scale lithological
501
heterogeneity and their non-cohesive structure. It has been considered equal to the Pliocene
502
marine deposits, which yielded averaged density of 2.36 g.cm-3.
503 504
6.3. Gravity profiles modeling
505
[27] A 55 km-long profile, crossing the Sierra Alhamilla to the South and the Sierra de Los
506
Filabres to the North, was acquired to image the effect of the overall crustal structure on the
507
gravity signal (Figure 8). Indeed, wavelength of the gravity anomaly primarily depends on the
508
depth (for given density contrasts) of the “source” geological bodies and structures; rather long
509
profiles are thus required to image the deepest crustal geometries. Here, the long N-S Bouguer
510
anomaly profile, hereafter named the “regional-scale” profile, shows a large northward decrease
511
from +15.15 mGal to -42.10 mGal (Figure 8A); this long wavelength anomaly corresponds to the
512
regional Gravity anomaly and, at first order, this can be related to a north-dipping Moho (Figure
513
8B). This feature is in good agreement with the Moho depth model/compilation published by
514
Ziegler and Dèzes [2006] or Diaz and Gallart [2009] showing a progressive northward
515
deepening, from 22 km to 32 km, beneath the Sierra de Los Filabres.
516
[28] The residual Bouguer anomaly values corresponding to this regional profile (Figure 8) were
517
calculated by substracting the measured Bouguer anomaly and the regional Bouguer anomaly as
518
given by Torne et al. [2000] or Fullea et al. [2010]. The obtained profile then appeared very
519
similar to the one recently published by Li et al. [2012] – who surveyed approximately the same
520
regional-scale section – with similar amplitudes and shapes of the two residual Bouguer anomaly
521
curves (compare Figure 8A and [Li et al., 2012]:figure 3). As the two surveys and studies have
522
been strictly conducted independently, such features highlight a good reproducibility of the
523
measurements and treatments and the quality of the gravity data used for this study. Here, all the
524
2D gravity models were computed from the “total” Bouguer anomaly values (instead than from
525
the residual Bouguer anomaly) by taking into account the geometry of the moho, as mentioned
526
above [Torne et al., 2000; Fullea et al., 2006; Ziegler and Dèzes, 2006]. It is noteworthy that,
527
below the lowermost NF metamorphic unit, standard densities have been used for the crust with
528
2.7 and 2.8 g.cm-3 for respectively the “mean” and lower crust levels.
529
[29] On the other hand, short-wavelengths of the measured gravity profiles highly depend on the
530
shallow crustal structure. In this study, this part of the modeling was constrained by the geometry
531
of the outcropping post-Tortonian deposits within the Sorbas basin (see Figure 2) and of the
532
metamorphic unit, into the domes, as deduced from field structural measurements. The mid-
533
wavelength anomaly (Figure 8A) constituted the only remnant part of the measured signal to be
534
fitted. If considering the density contrast in between units (see Table 1), this latter depends on the
535
depth and shape of the base of the basin but also on the geometry of internal boundaries within
536
the underlying metamorphic complexes (Figure 8B). The residual mid-wavelength anomaly was
537
then modeled by adjusting the geometry of the contact between metamorphic basement and
538
sediments (Figure 8B), as is the case to the northern boundary of the basin (Figure 8C). As a
539
whole, the modeled Bouguer anomaly can be separated in three components along this regional-
540
scale profile (see Figure 8C), displaying the combined effects of (i) the deep crustal structures
541
(red curve) [ii] the upper crust structure into the outcropping metamorphic units (blue curve) and
542
(iii) the shape and thickness of sedimentary basins (green curve in Figure 8C). Note that, even
543
though a large part of the “total” Bouguer anomaly range value is directly linked to the Moho
544
depth (see red curve in Figure 8C), a significant part of its variations is also due to the structure
545
of the upper crust of both the metamorphic and sedimentary units, with similar proportions (see
546
similar amplitudes of the blue and green curves in Figure 8C). Finally, on the regional-scale
547
SCS1 profile the mid-wavelength Bouguer anomaly decreases from 5 to 25 km, which
548
corresponds to an asymmetric shape of the basin floor as shown in the model (Figure 8D).
549
[30] Profiles SCS3, SCS4, SCS5, SCS6 and SCS7 (Figure 7), located in the center of the Sorbas
550
basin, are described from West to East (Figure 9). In comparison to the above described ~55 km
551
long regional-scale profile, these latter shorter profiles stand for more local-scale variations in
552
the structure of the basin. For sake of clarity, modeled geometries of the units are only displayed
553
down to 4 km on the sections but, as for the processing of SCS1 profile, the whole crust is taken
554
into account for gravity anomaly computation, down to the mantle, in order to include depth
555
variations of the Moho. Profile SCS6 extends to the North from Uleila del Campo to the west of
556
Sorbas city (Figure 7). The short wavelengths of the Bouguer anomaly are well fitted by taking
557
into account the extension of the northern boundary of the Tortonian trough beneath the
558
Messinian and Pliocene sediments (see Figure 2). On the other hand, the gravity high displayed
559
in the Bouguer profile (between 2 and 4 km) and forming a mid-wavelength anomaly cannot be
560
simply reproduced by the presence of standard metamorphic rock units below the post Tortonian
561
deposits. Such wavelength can neither be explained by variations in the Moho depth or in the
562
lower crust. Gravity modeling shows here that the occurrence of rather dense material (of 2.9
563
g.cm-3) in the upper crust is mandatory, at that local-scale, to properly model the Bouguer gravity
564
anomaly. Slices of such dense rocks are displayed in black on Figure 9. Note that such small
565
bodies are only necessary to adjust short wavelength of the anomaly in “local” gravity profiles
566
and are not mandatory for modeling the regional scale profile as described above. SCS4 profile,
567
which extends 3 km south of Sorbas, is the southern extension of the SCS6 profile. The first
568
order trend of the modeled Bouguer anomaly was adjusted with the geometry of the
569
metamorphic basement units at depth. Shortest wavelengths have been then modeled with the
570
depth of the interfaces of shallow geological units exposed at the surface. SCS3 profile is almost
571
parallel to SCS6 and extends from Sorbas to the north of El Pilar (Figure 7). The same modeling
572
protocol as described above was used here to adjust the computed Bouguer anomaly curve to the
573
measured anomaly values. Here again, this section displays the extension of the northern
574
boundary of the Tortonian trough below the younger sedimentary series. The N-S oriented SCS7
575
profile extends from Los Feos (northern part of the Níjar basin) to Los Perales (Figure 7). This
576
profile crosses a metamorphic basement high connecting the Sierra Alhamilla in the west to the
577
Sierra Cabrera in the east and where metamorphic rocks have been sampled to constrain density
578
values (Figure 2). The northward decrease of the Bouguer anomaly values, from +21 to +8 mGal
579
(Figure 9), was ascribed to the anticline shape of the metamorphic units as known laterally in
580
both Sierra Alhamilla and Sierra Cabrera. At 2.8 km distance along the profile, a trend break is
581
visible in the anomaly curve and this feature most probably coincides with the
582
basement/sediments contact outcropping there (Figure 9). This argues for a rather steep
583
downward extension of the contact. Besides, the sedimentary cover is here entirely composed of
584
Tortonian sediments and this structure may correspond to the termination of the Tortonian
585
extensional trough to the SE. The westernmost gravity section extends from La Herreria to north
586
of El Chive (SCS5 profile; Figure 7). Three main segments can be distinguished along the
587
Bouguer anomaly profile:- the first one, between kilometer 0 and 2, is characterized by a
588
northwestward strong decrease; the best fit of the model was obtained by an important deepening
589
of the sedimentary infill of the basin ;
590
- the second segment, between 2 and 7 km, is marked by a rather constant low anomaly which
591
corresponds, after adjusting the model, to the thickest sediment filling along the cross-section,
592
- the third segment, beyond 7 km, highlights a rather weak increase of the anomaly before a new
593
stabilization around +3 mGal. This “step” in the trend of the Bouguer anomaly profile may
594
correspond to the extent of the Upper NF units from the Sierra de Los Filabres southward below
595
the post Tortonian series (Figure 2, 7). Except for the extreme northern boundary of the SCS6
596
profile, the measured and modeled Bouguer anomalies show a good consistency and the resulting
597
misfit error is negligible for all profiles.
598 599
7. 3D geometry and depth constraints
600
[31] 2D gravity sections were integrated into the 3D GeoModeller software [Calcagno et al.,
601
2008; Guillen et al., 2008] in order to interpolate the 3D geometry of the basin-floor. The
602
computation principles of this software are based on a joint inversion of both the places, within
603
the 3D volume considered, where geological contacts have been observed or deduced, and
604
measured orientations of corresponding geological surfaces. Here, modeled 2D gravity sections
605
were thus used to constrain both the depth of the basin-floor and its geometry. The modeling box
606
(dashed border rectangle in Figure 7B) extends over 20 x 15 km from 37°10’12.33” N -
607
2°15’03.00” W (top-left corner) to 37°01’48.44” N – 2°02’12.75” W (bottom-right corner) and
608
from 5 km below sea-level to 3 km in altitude.
609
[32] The geometry of the basin floor is illustrated in Figure 10 by both an iso-depth curves map
610
(Figure 10A) and an oblique 3D view of the model in which the sediment infill have been
611
removed (Figure 10B). Iso-depth curves of the basin floor, as interpolated during the 3D
612
modeling process, show a strong marked asymmetry of the basin geometry (Figure 10A)
613
characterized by a steep North-dipping boundary in the southern part of the basin, passing to a
614
gently South-dipping contact to the North. This first-order geometry of the model is well
615
constrained by both (i) the gravity 2D modeling along the profiles and (ii) the very steep dip of
616
the first Tortonian layers even characterized by locally overturned bedding close to the basement
617
contact. The deepest part of the basin, a 2000-2800 m trough, is located in the South, facing the
618
North Alhamilla front zone separating the basin from the metamorphic basement of the Sierra
619
Alhamilla.
620
[33] While the asymmetrical shape of the basin evidenced by our computed 3D model is
621
compatible with the 2D image proposed by Li et al. [2012], discrepancies arise when considering
622
the modeled depth of the basement/sediment basement below the basin. Indeed, previous work
623
estimates a ~1500 m maximum depth of the Sorbas basin in its southern part whereas our 3D
624
computations result in a 2000-2800 m depth localized along the southern part of the basin. When
625
considering the SCS1 regional scale gravity section of this study, comparison of the residual
626
Bouguer anomaly values with the one obtained by Li et al. [2012] clearly shows that such
627
discrepancy could not be attributed to measurement deviations. Thus, differences in depth
628
computations must be assigned to the choice of the density values used for gravity modeling. On
629
the one hand, previous gravity studies conducted in the Sorbas basin and surrounding areas
630
where considering geological units in, basically, two groups: (i) sedimentary rocks with a
631
constant density fixed at 2.35 g.cm-3 and (ii) basement rocks with a constant density fixed at 2.67
632
g.cm-3 [Pedrera et al., 2009, 2010; Li et al., 2012]. We have tested this same approach on the N-
633
S regional cross-section to model the basal contact separating the sediments from the
634
metamorphic basement (dashed line on Figure 8D). The overall asymmetric shape of the basin
635
fill can be reproduced, however the main difference between these approaches is the resulting
636
depth of the basin-floor estimated between 1.6 to 2.8 km (Figure 8D). However, the Bouguer
637
anomaly primarily depends on density contrasts [Martelet, 1999; Martelet et al., 2004; Jacoby
638
and Smilde, 2009; Joly et al., 2009]. Accurate determination of the density is thus essential to
639
model a more reliable geometry to depth. The Sorbas basin, as well as its basement, is composed
640
of a large spectrum of rocks and average densities assigned to each geological layer sufficiently
641
contrasts with both adjacent units to be considered as a representative density interval (Table 1).
642
Our study strives to consider more specific density values for geological units or groups as
643
recognized in the area as well as measuring densities from field samples rather than using
644
standard average density values. When comparing densities of rock units measured in this study
645
to standard ones, we suggest that the distinction of as many geological units as possible to
646
separate “geophysical” units is a more accurate approach to estimate both the geometries and
647
depths of the geological boundaries.
648 649
8. DISCUSSION
650
[34] Contrasted tectonic settings have previously been proposed to explain the initiation and the
651
development of sedimentation within the Sorbas basin: (i) N-S compression to transpression
652
[Montenat et al., 1987; Weijermars et al., 1985; Ott d’Estevou et al., 1990; Sanz de Galdeano
653
and Vera, 1992; Poisson et al., 1999], including strike-slip tectonics [Kleverlaan, 1989;
654
Haughton, 2001] or (ii) extension [García-Dueñas et al., 1992; Vissers et al., 1995; Martínez-
655
Martínez and Azañón, 1997; Augier et al., 2005a; Augier et al., 2013]. Based on field structural
656
investigations and the gravity survey, the 3D geometrical model shows a marked asymmetric
657
shape with the deepest parts localized to the South, along the contact with the Sierra Alhamilla.
658
The southern basin/basement rocks contact is steeply dipping, so as the bedding within the
659
Tortonian basal deposits. The southern deepest parts of the basin correspond to the Tortonian
660
trough, as previously defined. The strong asymmetry of the basin might result from the
661
compressive event, when the Sierra Alhamilla “thrusted” over the southern edge of the Sorbas
662
basin. Indeed, the North Alhamilla Reverse Fault has been described as a prominent structure
663
and its development could have been accompanied by a significant subsidence in its footwall.
664
However, structural analysis highlighted that while north-verging thrusting can be evidenced
665
along this margin, some segments of the north Alhamilla front display a preserved structure of
666
unconformable stratigraphic contact of the Tortonian series on the basement rock units. Such
667
interface was subsequently tilted or even overturned in front of the Sierra Alhamilla but no trace
668
of thrusting can be evidenced then. In these areas (as presented in Figure 4) the deformation
669
marks the growth of a fault-propagated fold at the tip of a blind thrust and such quick change in
670
tectonic style along the north Alhamilla front argues for a rather limited dip slip movements
671
along the NARF. On the other hand, while previous works estimate ~1500 m of maximum depth
672
for the Sorbas basin, our new gravimetric models and 3D computations result in a 2000-2800 m
673
maximum depth localized along the southern part of the basin. This result reinforced the
674
asymmetric character of this basin. It also highlights a much larger cumulated subsidence in this
675
area compared to the Huércal-Overa basin for instance.
676
[35] Moreover, several syn-sedimentary normal faults have been locally described from the
677
upper Serravallian to the upper Tortonian series, in close surroundings to the Sorbas basin
678
(Figure 5, 6). Two extensional paleostresses, whose minimum compression direction [3] are
679
N140E and N50-60E oriented, controlled the deposition of upper Serravallian to upper Tortonian
680
sediments (Figure 5, 6). The analysis of upper Tortonian series, in particular in the Gafarillos
681
area, suggests that the N140E oriented system was mainly restricted to the upper Serravallian-
682
Early Tortonian. In the Tabernas area, both normal faults sets appear clearly as pre-folding
683
extensional faulting as suggested by tilted conjugate sets of normal faults (Figure 6B and 6C).
684
The asymmetry of the basin floor most probably results from the successive extensional and
685
compressive deformations evidenced in the Sorbas basin. Indeed, the limited dip-slip movements
686
along the northern “thrust” front of the Sierra Alhamilla could hardly explain the ~2500 m of
687
subsidence in the south Sorbas trough. In addition, the amount of shortening due to the folding of
688
the Alhamilla range remains hardly assessable because of the dextral movements of the Polopos
689
and North Alhamilla Fault Zone [Jonk and Biermann, 2002; Giaconia et al., 2012a] and the
690
involvement of the strike-slip Cantona Fault Zone in the deformation [Haughton, 2001].
691
Therefore, we assume that a significant imprint of the early extensional event contributed on the
692
finite global architecture and structure of the Sorbas basin.
693 694
8.1. The initial extension history of the Sorbas basin and surroundings
695
[36] The onset of sedimentation roughly occurred at the same time in all intramontane basins of
696
SE Betics, during upper Serravallian to lower Tortonian [Cloething et al., 1992; Sanz de
697
Galdeano and Vera, 1992; Iribarren et al., 2009]. The Huércal-Overa and Sorbas basins
698
underwent a common subsidence history during their early development stage (Figure 11): this
699
first phase is characterized, in both basins, by the deposition of a thick coarse-grained breccia
700
containing Nevado-Filabride clasts [Sanz de Galdeano and Vera, 1992]. Both basins display a
701
synchronous onset and a similar evolution of sedimentation [Sanz de Galdeano and Vera, 1992]
702
while their basement were exhumed by normal faulting. Evidences of extensional tectonics
703
during the lower Tortonian have also been extensively described into the Huércal-Overa basin, to
704
the north (Figure 12A) [Briend et al., 1990; Augier et al., 2005b; Meijninger and Vissers, 2006;
705
Pedrera et al., 2010; Augier et al., 2013]. In that area, the latest exhumation stages of the
706
metamorphic Nevado-Filabride complex (green arrows in Figure 12A) [Johnson et al., 1997;
707
Augier et al., 2005b] coincides with the kinematics of normal faults which controlled the
708
sedimentation into the basin [Augier et al., 2013]. Both basins may have experienced common
709
extensional tectonics during the initiation of sedimentation (Figure 12A). The effect of extension
710
is obviously hardly detectable on the southern edge of the Sorbas basin. However, similar
711
extensional regime boarding metamorphic core-complexes in other surrounding areas led to the
712
formation of half-graben basin such as the Granada basin [Ruano et al., 2004], Fortuna-
713
Guadalentin basins [Amores et al., 2001, 2002], and in the Alboran Sea [Mauffret et al., 1992;
714
Watts et al., 1993; Comas et al., 1999]. We assume that the Sorbas basin which initiated under
715
similar tectonic conditions (Figure 12) may have formed an asymmetric half-graben tilted toward
716
the south. The fact that extensional paleostresses calculated in this study, and especially the
717
N140E oriented system, is consistent with the latest ductile deformation highlighted in the Sierra
718
Alhamilla (green arrows in Figure 12A) reinforces the comparison with the Huércal-Overa basin
719
and the interpretation of half-graben initiation. In the Sorbas basin, gravity modeling evidences
720
that the rather dense rocks of the Alpujarride complex with respect to the sedimentary cover,
721
cannot be prolonged northward, below the basin (Figure 9, 13E), without inducing a large misfit
722
on the measured Bouguer anomaly. Hence, the southern fault wich probably controlled the
723
sediments deposition during the initiation of the subsidence may cross-cut and post-date the
724
Alhamilla detachment zone itself (Figure 13E). This is fairly consistent with the tectonic history
725
of the Huércal-Overa basin and the ca. 12-11 Ma ages for the initiation of sedimentation during
726
latest exhumation stages of the sierras under brittle conditions [Platt et al., 2005].
727 728
8.2. Evolutive scenario of the Sorbas basin
729
[37] A N-S evolutive cross-section of the Sorbas basin is given on Figure 13. Situations are
730
restored for key main steps from the upper Serravallian-lower Tortonian stage to the present
731
view by deconvoluting the tectonic history of the Sorbas basin. As discussed before, we argue
732
here for an onset of sedimentation occurring in an extensional regime in the Sorbas basin. The
733
strong asymmetry of the basin is compatible with a half-graben geometry controlled by a
734
supposed north dipping fault zone, located along the southern margin during Serravallian and
735
part of the Tortonian (Figure 5 and 6). Geometry and major bounding faults of this initial basin
736
can obviously not be retrieved accurately as most of this basin is now hidden by younger
737
deposits; a significant part of the early basin even being eroded in particular during Messinian
738
evolution. Extensional kinematics, as studied within the preserved and outcropping parts of the
739
basins, is compatible with a NW-SE (~N140E) and NE-SW (~N50-60E) directed subhorizontal
740
extension accompanied by a subvertical maximum compression direction (Figure 6). This event
741
may have favoured an initial subsidence pulse and the deposition of coarse-grained sediments
742
grading to marine deposits. Such pulse thus seems recorded over most the SE Betic basins [Sanz
743
de Galdeano and Vera, 1992; Meninger and Vissers, 2006; Augier et al., 2013]. Inception of the
744
main sedimentation occurred shortly after the crossing of the ductile-brittle transition at ca. 14-
745
13Ma during the final exhumation and the denudation of the Nevado-Filabride-cored domes
746
[Johnson et al., 1997; Augier et al., 2005b; Vasquez et al., 2011]. In the Huércal-Overa basin,
747
where the corresponding oldest deposits are exposed and even very well preserved, this time
748
interval corresponds to sedimentation of about 1 km thick series along the normal-faulted
749
southern boundary [Pedrera et al., 2009, 2010; Augier et al., 2013]. As the two basins show a
750
similar tectonic subsidence evolution during upper Serravallian-lower Tortonian, one can assume
751
that, in the Sorbas basin, the subsidence pulse led reasonably to the deposition of the same
752
amount sediments.
753
[38] Later, during the upper Tortonian, the Sorbas basin experienced a drastic inversion tectonics
754
stage (Figure 4 and 6). This formation shows a progressive steepening over more than 2 km and
755
is even overturned in the vicinity of the Sierra Alhamilla in the footwall of the North Alhamilla
756
reverse fault zone (Figure 4). Besides, the development of upper Tortonian growth strata
757
highlights that deformation occurred during sedimentation (Figure 4). Second order folding (e.g.
758
the Tabernas fold) and small-scale faults are all consistent with a N-S to NW-SE shortening
759
(Figure 4 and 6). Progressive steepening on the southern margin of the basin (i.e. the mobile
760
border) and the significant uplift of the Sierra Alhamilla [Braga et al., 2003] probably resulted in
761
a continuous tectonic subsidence within the basin and the persistence of marine deposits
762
[Weijermars et al., 1985; Kleverlaan, 1989]. Flexural type subsidence then superimposed on the
763
former extensional depocenter and migrated northward (Figure 13D). This marked inversion of
764
the basin appears mechanically in agreement with the development of dextral strike-slip faults
765
recognised at the western and eastern edges of the basin [Giaconia et al., 2013].
766
[38] Many studies pointed out an important strike-slip component of the northern and southern
767
Alhamilla-Cabrera faulted boundaries during the Tortonian [Martínez-Martínez et al., 2006;
768
Rutter et al., 2012] leading to drainage captures [Mather, 2000 ; Giaconia et al., 2012b, 2013].
769
In addition, several authors suggested that strike-slip tectonics parallel to the E-W depocenter
770
have been important in the evolution of the Sorbas basin in particular during the Tortonian
771
turbiditic filling [Ott d´Estevou and Montenat, 1990; Haughton, 2001]. This assumption is
772
mainly based on the occurrence of contained turbidites and a reversal location of its ponding
773
depocenter in the late Tortonian [Haughton, 2001]. However, the initial sinistral strike-slip
774
movement advanced by the authors cannot be observed in the basin due to its intense late
775
deformation. The associated primary east-flowing sedimentation could result from the first-step
776
extension highlighted in this study. Moreover, the tilted-block geometry inferred by Haughton
777
[2001] seems unrealistic with respect to the 2D gravity inversion.
778
[39] Compared to the Huércal-Overa basin, inversion tectonics appears much stronger in the
779
Sorbas basin [Weijermars et al., 1985; Augier et al., 2013]. Onset of regional inversion has been
780
proposed to approximately occur before the Tortonian/Messinian boundary (e.g. ~7.7 Ma)
781
[Weijermars et al., 1985]. Besides, inversion tectonics affecting the northern flank of the Sierra
782
de Los Filabres occurred roughly at the same time, around 8.2 Ma [Augier, 2004]. This period
783
coincides in a broad sense with a period of geodynamic reorganisations of the southeastern
784
Betics (Figure 12B) as described throughout the Alboran region, around 8 Ma [Comas et al.,
785
1999; Jolivet et al., 2006].
786
[40] At the Tortonian-Messinian transition, the basin experienced an overall uplift and erosion as
787
attested by the deposition of the Azagador shallow marine calcarenite over the upper Tortonian
788
(Figure 4 and 13C). The large open-fold syncline of the lowermost Messinian calcarenite shows
789
the relatively limited shortening deformation affecting the basin after its deposition (Figure 13B).
790
From that period, and despite a global sea-level fall of 35 m [Haq et al., 1987], deep marine
791
marls were deposited into the Sorbas basin between 7.3 and 6.5 Ma (Figure 13B). The
792
subsequent deposition of Messinian clays and diatomites, known as the Upper Abad Member,
793
and of the coeval fringing reef, in a restricted marine environment, is once more coeval with a
794
global sea-level fall estimated at 60 to 70 m (Figure 10) [Haq et al., 1987; Miller et al., 2005].
795
These features highly suggest that the global eustatic variations have locally weaker effects on
796
sedimentation than the tectonic control during the Sorbas basin tectonic inversion.
797 798
9. Conclusions
799
[41] While most of the previous studies on Sorbas basin focused on the Messinian salinity crisis
800
event or on compressive tectonics acting from middle- to upper-Tortonian times, this work aims
801
at constraining the early tectonic context during the initiation phase of the basin. Two principal
802
targets were identified as key points to address this problem: the geometry/structure of the early
803
deposits with the basement units and the internal deformation/stress fields taking place during
804
the onset and first stages of sedimentation into the basin. At first, structural analysis and
805
compilation point out that the well-known northward thrusting movement observed along the
806
northern edge of the Sierra Alhamilla may be limited; deformation being characterized by
807
pervasive shearing and folding in some places. Besides, structural analysis of the earlier deposits
808
along the southern margin of the basin, including computation of palaeostress tensors from fault
809
kinematics analysis, highlights the prime effect of an initial extensional event during the onset of
810
basin development. This phase of deformation observed in sedimentary series is coeval with the
811
late exhumation stages of the Nevado-Filabride complex, further south, into the Sierra Alhamilla,
812
which is characterized by a top-to-the NW normal sense of shear [Augier et al., 2005a]. This
813
extensional context must probably controlled the inception of sedimentation in the continental
814
basin.
815
[42] On the other hand, gravity modeling revealed the deep and asymmetric structure of the
816
Sorbas basin. This asymmetry is highlighted both by a gently south-dipping basal unconformity
817
in the north and a steep south-dipping southern contact. Gravimetric survey and modelling
818
results in the determination of a maximum ~2500 m of sediments accumulated and preserved
819
along the southern margin of the basin. The Sorbas basin thus displays as a particularly thick
820
basin in comparison to the other neighbouring continental basins of the area and its well-marked
821
asymmetry could hardly be explained by the limited thrust movements as observed along the
822
northern Sierra Alhamilla. All these results thus argue for a two stages evolution of the Sorbas
823
basin development with (i) extension driving at first the initiation and first stages of subsidence
824
and (ii) inversion of the basin and flexural control of the sedimentation during a subsequent
825
compression.
826
[43] Our integrated study allows estimating the evolution through time of the basin behaviour.
827
We propose that during the early extensional stage, a normal to normal-strike-slip fault system
828
initiated the subsidence along the northern edge of the metamorphic range, south of the basin,
829
during the last stages of exhumation of the metamorphic units. Later on, the asymmetry of the
830
basin was reinforced during the basin inversion by the N-S compressional event occuring from
831
ca. 8 Ma. Since the deposition of the Azagador Member, the Sorbas basin evolved as a flexural
832
basin, resulting from the ongoing overall N-S shortening (acting since ca.~8 Ma). This tectonic
833
setting is marked by the strengthening of the subsidence and leads to the deposition of deep
834
marine sediments (Abad marls). Then, Messinian and post-Messinian deposits are only slightly
835
folded (Figure 13).
836 837
[43] Acknowledgments
838
We warmly thank Georges Clauzon without whom this study would not have been achieved. He
839
provided us a new vision of the evolution of the Sorbas basin based on thorough
840
geomorphological and biostratigraphic researches. He could not see the achievement of this
841
study but his memory will be forever associated to the Sorbas basin.
842
Field investigations have been supported by the CIFRE PhD grant N° 584/2010
843
(TOTAL/UPMC). This paper is a contribution to the Projects AMEDITER (“Actions Marges”
844
CNRS/INSU Programme), “Bassins néogènes et manteau en Méditerranée” (TerMEx
845
CNRS/INSU Programme). We are grateful to Michel Diament (INSU/IPGP) for providing us a
846
SCINTREX CG5-M micro-gravimeter. Geophysical interpretations were drawn using Geosoft
847
software and the 3D modeling process was performed using the 3D Geomodeller (Intrepid
848
Geophysics) developed by the BRGM (Orléans, France). We are grateful to Guillaume Martelet
849
(BRGM) for his help and assistance on the gravity modeling process and also to Gabriel
850
Courrioux (BRGM-GEO) for providing us a 3D Geomodeller licensing and for his technical
851
support. We thank two anonymous referees for their constructive contribution to improving the
852
paper.
853 854
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855
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Spain) by low temperature thermochronology, Terra Nova, 23, 257–263.
1209 1210 1211 1212 1213 1214
Vergés, J., and Fernàndez, M. (2012), Tethys-Atlantic Interaction along the Iberia-Africa Plate Boundary: The Betic-Rif Orogenic System, Tectonophysics, 579, 144–172. Vissers, R.L.M., Platt, J., and van der Wal, D. (1995), Late orogenic extension of the Betic Cordillera and the Alboran Domain: A lithospheric view, Tectonics, 14, 786–803. Watts, A.B., Platt, J. and Buhl, P. (1993), Tectonic evolution of the Alboran Sea basin, Bas. Res., 5, 153–177.
1215
Weijermars, R., Roep, T.B., van den Eeckhout, B., Postma, G., and Kleverlaan, K. (1985), Uplift
1216
history of a Betic fold nappe inferred from Neogene-Quaternary sedimentation and tectonics
1217
(in the Sierra Alhamilla and Almeria, Sorbas and Tabernas Basins of the Betic Cordilleras,
1218
SE Spain), Geol. Mijnbouw, 64, 397–411.
1219 1220 1221 1222
Wortel, M.J.R., and W. Spakman (2000), Subduction and slab detachment in the MediterraneanCarpathian region, Science, 290, 1910-1917. Ziegler, A., and Dèzes, P. (2006), Crustal evolution of Western and Central Europe, Mem. Geol. Soc. London, 32, 43–56.
1223 1224
1225
FIGURE CAPTIONS
1226 1227
Figure 1: Simplified structural map of Southeastern Spain. Located are the main metamorphic
1228
domes of the Sierra de los Filabres/Nevada and the Sierra Alhamilla/Cabrera and the main
1229
sedimentary basins modified after [Augier et al., 2005b]. Also indicated are the main tectonic
1230
contacts such as the Filabres and Alhamilla shear zones and the prominent sinistral fault zones
1231
pertaining to the Trans-Alboran transcurrent zone (CF : Carboneras Fault, PF : Palomares Fault
1232
et AMF : Alhama de Murcia FaultF) and smaller-scale structures.
1233 1234
Figure 2: Structure and stratigraphy of the Sorbas Basin.
1235
(A) Geological map of the Sorbas Basin, modified from [Ott d’Estevou and Montenat, 1990;
1236
García Monzón et al., 1974]. Projection UTM Zone 30 (WGS84 ellipsoid). (B) Synthetic N-S
1237
cross-section X-Y representing the main architecture of the Sorbas basin (location in Figure 2A).
1238
(C) Synthethic stratigraphic log of the Sorbas basin. Modified after [Montenat and Ott
1239
d’Estevou, 1977; Ott d’Estevou and Montenat, 1990; Krijgsman et al., 1999, 2001; Manzi et al.,
1240
2013; Clauzon et al., revised].
1241 1242
Figure 3: Simplified geological map showing the four main faults accommodating the
1243
deformation along the Sierra Alhamilla, Sierra Polopos and Sierra Cabrera: from East to West,
1244
the North Cabrera reverse fault (NCRF), the North Gafarillos fault (NGF), the South Gafarillos
1245
Fault (SGF) and the North Alhamilla reverse fault (NARF). Four cross-sections, orthogonal to
1246
the sediments/basement contact, present the geometry and architecture of the sedimentary units
1247
with respect to the metamorphic ridges (location of cross-sections in the geological map). See
1248
text for more details.
1249 1250
Figure 4: Evidences of upper Tortonian compressive tectonic.
1251
Panoramic view of the southern folded boundary of the Sorbas Basin East of Lucainena de las
1252
Torres (see location in Figure 2). Here, the metamorphic basement describes a north-verging
1253
asymmetric fold with an overturned steep-dipping forelimb. Tortonian series, separated from the
1254
basement by an unconformity, display a rather continuous growth-strata structure forming a
1255
recumbent syncline. Fault-slip data inversion indicates a N-S maximum compression direction
1256
[1].
1257 1258
Figure 5: Field evidences of extensional deformation affecting upper Serravallian to upper
1259
Tortonian sediments, (location on Figure 2). (A) Uninterpreted and interpreted panorama of
1260
lower Tortonian reddish conglomerates affected by two sets of small-scale normal faults: a
1261
dominant ca. N140E and a subordinate ca. N30E conjugate sets of normal faults. Inset (B) show
1262
a close-up view of syn-sedimentary normal faults affecting sandy to microconglomerate intervals
1263
and interrupted by a coarse-grained conglomerate. (C) Upper Tortonian marine sediments
1264
affected by a N140E oriented set of small-scale normal faults.
1265 1266
Figure 6: Geometrical relationships existing between the upper Serravallian-Tortonian
1267
extensional deformation and the upper Tortonian inversion.
1268
(A) Uninterpreted and interpreted panoramic view of the northern flank of the so-called Taberns
1269
fold [Ott d’Estevou and Montenat, 1990] showing (B) the presence of tilted normal faults
1270
affecting the lower Tortonian deposits and also (C) the upper Tortonian marine sediments. (D)
1271
The decomposition of the total heterogeneous fault population into homogeneous, cogenetic fault
1272
populations shows the presence of two subhorizontal extensional regimes with N50E and N140E
1273
minimum compression direction (3) that pre-date inversion tectonics visible at all scales form
1274
microfaults to large-scale folding.
1275 1276
Figure 7: (A) Simplified geological map showing gravimetric cross-sections realized across the
1277
basin. Red circle represents the reference station. (B) Location of the gravimetric stations and
1278
gravity anomaly plotted over the geological map of the Sorbas Basin. All measurement stations
1279
are represented by small circles colored in function of their relative Bouguer anomaly (see the
1280
color bar in the upper right corner). The dashed border rectangle represents the extension of the
1281
modeling box used to model in 3D the sediments/basement interface at depth.
1282 1283
Figure 8: 2D gravity profile treatment (A) Comparison between the measured total Bouguer
1284
anomaly (yellow diamond shape) and the regional gravity deviation exposed in Torne et al.
1285
[2000] of the N-S regional cross-section (SCS1). White diamond-shape points represent the
1286
calculated residual gravity anomaly profile (see text for details). (B) Superposition of the 2D
1287
gravity model anomaly (green curve), calculated from the density summarized in Table 1, with
1288
the measured total Bouguer anomaly (Black dots). The red curves correspond to the difference
1289
between the modeled gravity anomaly and the measured one. (C) Deconvolution of the measured
1290
gravity signal (black dashed line) showing the combined effects of the deep structures of the
1291
crust and more particularly the Moho geometry (red curve); the upper crust structure of the
1292
outcropping metamorphic units (blue curve) and the shape and thickness of the sedimentary
1293
basins (green curve). (D) Shallow field based cross-section and interpretation of the modeled 2D
1294
gravity anomaly across the Sorbas Basin. Note the strong asymmetry of the sedimentary
1295
depocenter.
1296 1297
Figure 9: Total gravity anomaly, shallow field-based cross-section and corresponding 2D model
1298
constructed for the 5 cross-sections called SCS3 to SCS7. The gravity modeling of these cross-
1299
sections is equal to that detailed in the text for the basin-scale SCS1 cross-section. Vertical
1300
exaggeration = 2, see Figure 7 for location.
1301 1302
Figure 10: Basin-floor 3D geometry. (A) Iso-depth curves of the base of the Tortonian deposits
1303
are plotted over the geological map of the modeled area. The depth is indicated in meters below
1304
sea-floor. (B) 3D oblique view of the base of the Sorbas Basin. Vertical exaggeration = 3.
1305 1306
Figure 11: Tectonic subsidence and uplift curves for the Sorbas and the Huércal-Overa basins
1307
during the Neogene (thick black and red curves, respectively). The subsidence curve of the
1308
Sorbas Basin is drawn from the average estimated paleobathymetries compiled in Table 2.
1309
Absolute eustatic curves are from Haq et al. [1987] and Miller et al. [2005]. Exhumation and
1310
cooling path of the Nevado-Filabrides complex is estimated from ZFT, AFT and U-Th/He
1311
methodologies [Johnson et al., 1997; Augier et al., 2005b; Platt et al., 2005; Vázquez et al.,
1312
2011].Grey rectangles noted A to E refer to steps of the evolutive cross-section of the Sorbas
1313
basin in Figure 13.
1314 1315
Figure 12: Simplified tectonic map showing average upper Serravallian-lower Tortonian
1316
extension directions and average upper Tortonian compressive directions of the southeastern part
1317
of the Internal Zones of the Betic Cordillera. Stretching direction and associated sense of shear
1318
for the Nevado-Filabride complex are synthetized from [Galindo-Zaldívar et al.,1989; Jabaloy et
1319
al.,1992; Augier et al., 2005a,b]. Extension and compression directions shown in the Huércal-
1320
Overa basin are derived from Meijninger [2006] and Augier et al. [2013].
1321 1322
Figure 13: Interpretative N-S cross-sections of the Sorbas basin representing its evolution from
1323
Present (A) to the deposition of Messinian evaporites (B), the Late Tortonian syntectonic
1324
sedimentation of turbidites (C) and the period of tectonic inversion (D) post-dating the extensive
1325
deformation during the upper Serravallian-lower Tortonian (E).
1326 1327
Table 1: Sampling and density measurements.
1328 1329 1330 1331 1332
Table 2: Palaeobathymetry and age of the sedimentary formations of the Sorbas Basin.
IBERIA
5°E
sin ir Ba
Lorca Lorca
Vélez Rubio
aF ur ci
Al ha m a
SIERRA DE LAS ESTANCIAS
Alger
Alboran Sea
de M
Malaga
TASZ
35°N Gharb Basin
2°00'W
2°30'W
e Bal
uiv
dalq
Gua
s
land
Is aric
TELL
Oran
RI F
37°30'
200 km
MOROCCO
Huercal Overa
Baza
s Fau lt
Almanzora corridor
Flysch nappes InternalGuadix Zones
mare
Foreland Basins External Zones
Vera
GRANADA
Palo
38°N
0°
au lt
5°W
N
Fig. 2A
SIERRA DE LOS FILABRES
Alfaix Sorbas
SIERRA NEVADA
SIERRA CABRERA Tabernas
Trevenque idor
an corr
Alpujarr
SIERRA
37°00'N
LA
ALHAMIL
Nijar
SIERRA DE GADOR
r Ca
Murtas
LAS ALPUJARRAS
Polopos
bo
ne
r
as
Fa
ul
t
20 km
ALMERIA
Lujar
Motril Adra
Salobreña
Cabo de Gata
Campo de Dalias
ALBORAN SEA 3°30'
Major depocenter Sedimentary basins Volcanics
3°00'
Alpine metamorphic complexes Alpujarride Filabres & Alhamilla shear zones
Nevado-Filabride
2°30'
Main structures
2°00' W
Secondary structures
Strike-slip faults
Normal fault
Thrust Antiforms
Strike-slip fault Undifferenciated fault
La Mela
X
* *
*
* *
*
* * * * * * * * *
*
*
* * * * * ** * Cariatiz * * * * * * Los Martínez Martinez 4 * ** * * * * ** * * * Moras *
* * * *
Gochar Góchar
* * ** * * * * * * * *
Cortijo del Hoyo
Sorbas 2
2
*
3 5
Los Yesos *
*
Fig. 6
*
*
*
Lucainena de las Torres Turrillas
B
* *
Gacia Bajo
* * *
*
*
CFZ
Morrón de la *Cantona
*
8
Fig. 4
Cerrón de Hueli
Y
* * * * * * * *
* ** * * * *
Penas Peñas Negras 7
CFZ
13°
25°
Cariatiz reef
Moras (projected) 27°
3°
Rio Aguas
16°
25°
85°
45° *
*
*
* *
*
400 200 0
0
1000
?
2000 m
Pliocene-Pleistocene conglomerates (Zorreras)
* *
Messinian reef (Cantera)
Turbidites (upper Tortonian)
Main structures
Messinian diatomites & marls (Abad)
Messinian gypsum (Yesares)
Tortonian-Messinian rich fossiliferous calcarenites (Azagador)
Upper Serravallian-lower Tortonian Breccias and conglomerates
CFZ: Cantona Fault Zone
Alpujarride nappe
Syncline axis
Névado-Filabride nappe
Anticline axis
Undifferenciated fault
?
Other Cities
Thrust
Pliocene marine sediments (Sorbas)
7.24
Serr. Tort. Basement
10
Morrón de la Cantona
800
5.97
Gafarillos
10
* *
5.60 5,60
Fig. 5
X
600
* *
*
Tabernas
1000
* 6
*
* ** *
4 100 000
* * *
*
5.46 5,46
1
*
*
*
Plio.
Tortonian
Casa Blanca
Q.
C
4 105 000
Sierra de los Filabres
N
El Chive
Messinian
9
Uleila Del Campo
Projection: UTM 30/WGS84 (coordinates in meters)
A
580 000 4 115 000
570 000
4 110 000
560 000
10
Sampling sites for density measurement Location of the presented outcrops
Y
Sierra Alhamilla
2°20’ W Pliocene
Alpujarrides detachment Nevado-Filabrides
Messinian gypsum Messinian
2° W
II
Thrust
Sorbas
Tabernas
Lucainena de las Torres
NARF
III’
Sierra Alhamilla
IV’
Pliocene continental sediments
Tortonian
Pliocene marine sediments
Upper Serravallian Lower Tortonian
Messinian gypsum
Basal messinian calcarenite
II
N
Sierra Polopos II’
SGF
37° N
Sierra Cabrera
Anticlinal Thrust
Nevado-Filabrides
Rio Aguas
20°
40°
I’
60°
NCRF
1000 m
Cariatiz
27°
Gafarillos
9°
Rio Aguas
25°
15°
30°
21°
500 m
28°
NGF
I
Alpujarrides detachment
Messinian reef and marls
Mojàcar
Sierra Cabrera
500 m
IV
I’
NCRF
Strike-slip
El Alfaro
37°10’ N
III
Anticlinal
Tortonian Serravallian Tortonian
10°
I
II’
Sierra Polopos
NGF
18°
SGF
1000 m Sierra Los Filabres
Moralla
16°
15-20°
26°
20° 47°
53°
Sierra Alhamilla
CFZ 80°
III’
NARF
500 m
III
1000 m IV
IV’
500 m
El Alfaro 37°
1000 m
S
N
Sierra Alhamilla
S
N
Basement
Overturned Tortonian
Basement/Cover contact
Calar Alto
80° 75°
80°
Main stress axes
σ1 σ2 σ3 Strain axes Stretching N:27 faults 09 t-gashes
N:23 planes Fold axis: N086 06NE
Shortening
Messinian
86°
Fault nature Normal fault Sinistral fault Dextral fault Reverse fault pole of t-gashes pole of bedding Fold axis (inversion of bedding data)
90°
Cerro Los Llanos 60°
25°
20°
68°
S
N
B
S
A
N
0
3m
N:38 faults
N:06 faults
S
B
N:27 faults
C
N
0
1m
SE
NW
SE
NW
Tilted normal fault sets Tabernas fold NW-dipping fold flank B C 42° 24°
59°
A
Rambla de la Sierra Present geometry (Classified by fault nature)
Post-tilting faults
Present geometry Original geometry (restored) (Homogenous sets of fauts)
Folding
SE
NW
Early Tortonian
N:27 faults
N:23 planes Fold axis: N245 04SW
B S
Total population Present geometry
0
5m
N Quaternary terrace
N:09 faults
Pre-tilting faults
D
Late Tortonian N:34 faults
C
0
2m
±
2°20’W
2°W
±
Huercal-Overa
Gravity anomaly (mGal) +25
El Pilar
Uleila Del Campo
El Chive
SCS 1
S6
Filabres Sierra de Los
SC
Casa Blanca
La Mela
Vera
Los Martinez
SCS 3
37°20’N
SC
S
Gochar
5
Sorbas S
SCS 4
2
Sie
Campico
SCS
1
Tabernas Tabernas 37°N
La Herreria
SCS 7
SCS
ra
abre C a rr
lhamilla Sierra A
-17
Penas Negras
Lucainena de las Torres Turrillas 0
A
5
10
Km
B
0
5
10
Km
Sedimentary basins
Alpujarride nappe
Messinian and Pliocene
Alpujarride nappe
Miocene volcanic rocks
Nevado-Filabrides nappe
Tortonian
Nevado-Filabrides nappe
Measurement station: (1) reference station (2) basin scale cross-section (3) 500m spaced station
SCS
Name of the cross-section
Gravimetric station
10 0 -10 -20 Total Bouguer anomaly and its linear regression
-30 -40
B
+20
Gravity (mGals)
Gravity (mGals)
A
0
0
Regional gravity variation Torne et al. (2000) Residual Bouguer anomaly
10
20
30
40
50
-20
Measured
-40
Calculated
Error = 0.831
Gravity (mGals)
C 0 -20
Deep crust
-40
D
Metamorphic units
Sedimentary units
S
N
Depth (km)
0 5 10
0
V.E.=1
10
20
Post-Tortonian deposits
Alpujarrides
Serravallian? to Tortonian
Upper NF units Lower NF units
Distance (km)
40
Geometry of the basement/sedimentary cover contact: Sediments: 2,35 g.cm
50
N
-5 Mesuread
Calculated
2 4
0
2
4
6
+10 0 20°
Depth (km)
0
2
0
2
4
Section length (km) NW
SCS6
Error = 0.205
-10 -15
2
0
2
4
8
6
N
+20
Error = 0.138
+10 25° 45°
Depth (km)
Depth (km)
0
SCS7
S
17°
8°
12°
4
6
10
Section length (km)
RBa (mgals)
RBa (mgals)
SE
12°
7°
6°
0
4
8
NW Error = 0.235
16°
7°
8°
Depth (km)
Error = 0.953
SCS5
SE
Section length (km)
20°
2
2
4
Section length (km)
Post-Tortonian deposits
Alpujarrides
Upper NF units
Serravallian? to Tortonian
Dense units
Lower NF units
3°
25°
2
4
N
Error = 0.338
-10
0
0
SCS4
0
0
4
8
55° 35°
S RBa (mgals)
-10
RBa (mgals)
RBa (mgals)
SCS3
S
0
2 Section length (km)
A
* *
*
*
*
*
* * * * * * *
* 0* m *
*
±
B
*
0m
-2000
m 1000 *
0m
200 *
*
0m
±
*
20
*
* * ** * * * * * * * *
0
km
* *
*
-10 0
*
*
*
0 1
*
*
* * *
0m
*
2
1000 m
*
*
3
**
* * * *B* * * * * * * * *
0
*
1
*
2 Km
4 5 km
15 km
Gypsum
Huércal-Overa Basin
E
Serr. 12,0
11,5
11,0
D
10,5
10,0
9,5
Tortonian 9,0
8,5
Global sea-level curves Haq et al., 1987
Sorbas Basin 200
-200
Messinian Salinity Crisis
-200
600 m
C
8,0
Messinian 7,5
7,0
6,5
6,0
-600
B
Pliocene 5,5
5,0
4,5
4,0
3,5
Huércal-Overa basin (Augier, 2004)
A
Quaternary 3,0
2,5
Subsidence curves Miller et al., 2005
Paleobathymetry (meters)
?
ZFT: 250-290 °C
Regional uplift Gilbert delta Sorbas limestone - Zorreras
200
Regional Compression
Basin filling
Reef
AFT: 60-110 °C
Azagador Member
U-Th/He: 60-50°C
Tortonian turbidites
Tortonian turbidites
Subsidence
Sea-level (meters)
Local subsidence
Uplift
Abad marls and diatomites
Regional Extention
Sorbas basin (this study)
2,0
1,5
1,0
0,5
0 Ma
Upper Serravallian 2°30'W Lower Tortonian N (~12 - 8 Ma) de Sierra
A Mu lham rci a a F de au lt
2°00'W
N
s
tancia
las Es
2°00'W
2°30'W Late Tortonian (from 8 Ma)
de Sierra
37°30'
s
tancia
las Es
37°30'
H-O
SIERRA DE LOS FILABRES
SIERRA DE LOS FILABRES
S
bo
ne
Sierra Cabrera
37°00'N
Sierra Alhamilla r Ca
S
Sierra Cabrera
?
Palo
Palo
mare
mare
s Fau lt
s Fau lt
H-O
r
as
F
l au
Sierra Alhamilla
t
r Ca
bo
ne
r
as
Fa
ul
t
ALMERIA
ALMERIA Volcanics 20 km
2°30'
20 km
Alpujarride
Cabo de Gata
Shear zone
2°00' W
Nevado-Filabride
Amphibolite lineation in the Nevado-Filabrides [Galindo Zaldivar et al.,1989; Jabaloy et al.,1992; Augier et al., 2005a] Greenschist lineation and flow lines in the Nevado-Filabrides [Augier et al., 2005a]
Cabo de Gata
2°30'
Extensional shear zones
2°00' W
Mean extension direction
Thrust Normal fault Strike-slip fault
Mean compression direction
37°00'N
A
1000
Basin-scale cross-section at present
800 600
Uplift of the basin since the Pliocene and the deposition of marine deltaic sediments
400 200 0
Local overthrusting of the Sierra Alhamilla over the sedimentary basin
0
0
B
400 200
2
1000
4 km
2000 m
Messinian: deposition of the gypsum unit Deposition of an alternance of gypsum/marls layers relative to the first sea-level drop of the Messinian Salinity Crisis
0
C
0
1000
2000 m
0
1000
2000 m
Depocenter axis
Upper Tortonian: syn-kinematic sedimentation of late Tortonian marine sediments 200
CFZ
0
Tilting of preceding normal faults sets
0
1000
North Alhamilla blind thrust
2000 m
Vertical exaggeration: 2
D
200
Lower Tortonian: syn-kinematic sedimentation of late Tortonian marine sediments Tectonic inversion of the area: once the metamorphic complexes forming the Sierras are finally exhumed, the Africa/Europe convergence implies a N-S compression in the basin
0
0
E
1000
2000 m
Depocenter axis
Lower Tortonian: syn-kinematic sedimentation of late Tortonian marine sediments
Depocenter axis Vertical exaggeration: 2
200 0
Normal faulting 0
1000
2000 m
Depocenter axis Vertical exaggeration: 2
Syn-tectonic sedimentation following the denudation of the Nevado-Filabride core complex in the Sierra Alhamilla antiform.
Sedimentary units Latest Messinian early Pliocene Gilbert-delta
Lithologies
Messinian evaporites Messinian reef and marls Lowermost Messinian Tortonian turbiditic Alpujarride nappe Bedar-Macael Unit Calar-Alto Unit Ragua Unit
Gyps um
N° Sample
Fluviatile conglomerates Coarse-grained conglomerates Marls and clays Reef Wi ti s h di a tomi te a nd l a mi ni te Ri ch fos s i l i ferous ca l ca reni te Sa nds tone Purpl e s chi s ts
NevadoFilabride complex
Lubri n gra ni te, ma rbl e Gra phi ti c a nd bl a ck s chi s ts Bl a ck s chi s ts
1 2 3 4 5 6 7 8 9 10
Density Measured Reference 2.36 2.4 2.36 2.32 2.31 2.31 1.82 2.4 1.57 2.46 2.46 2.54 2.54 2.69 2.69 2.67 2.9 2.67 2.67 2.65 2.65
Literature Limestone: 2.49 ± 0.12 Marl: 2.4 ± 0.1; clay marl: 2.06 ± 0.3 Gypsum: 2.35 -/+ 1.5,2.5 Marl: 2.4 ± 0.1 Conglomeratic limestone: 2.48 Schist: 2.64 ± 0.26 Gneiss, schist: 2.65 -/+ 0.25,0.35 Phyllite: 2.74 ± 0.6 Amphibolite: 2.96 ± 0.8 ; Eclogite: 3.37 ± 0.17
Forma on Zorreras coquina Sorbas calcarenite Gypsum Messinian reef
Age Palaeobathymetry (m) ≈ 5.2 Ma 20 m 5.67 - 5,60 Ma 20 - 40 m 5.96 - 5.67 Ma 0 - 40 m 6.04 - 5.89 Ma 0 - 80 m
Abad marls
7.24 - 5.96 Ma
200 - 600 m
Tort/Mess calcarenites Upper Tortonian
≈ 7.24 Ma > 7,24 Ma
20 - 40 m 350 - 600 m
References Montenat et O d'Estevou, 1977 Montenat et al., 1980 Dronkert, 1976; Goubert et al., 2001 Braga et al. , 2003; Sanchez-Almazo et al. , 2007 Dronkert, 1976; Troelstra et al., 1980; O d'Estevou et Montenat, 1990; Poisson et al., 1999; Krijgsman et al. ,2001 O d'Estevou et Montenat, 1990. Montenat et Seilacher, 1978; O d'Estevou et al., 1981
