In the Southern (URSEIS) and Middle (ESRU and Alapaev) Urals the East European .... into the linear thrust belt of the Middle Urals, where it is cored in the east ...
The Crustal Architecture of the Southern and Middle Urals From the URSEIS, ESRU, and Alapaev Reflection Seismic Surveys 1
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D. Brown , C. Juhlin , A. Tryggvason , D. Steer , P. Ayarza , M. Beckholmen , A. Rybalka and M. Bliznetsov 5
The Urals Seismic Experiment and Integrated Studies (URSEIS), Europrobe's Seismic Reflection Profiling in the Urals (ESRU), and reprocessed Russian reflection/refraction seismic surveys have shown the known Uralides to be bivergent, with a crustal root along the central volcanic axis of the orogen. In the Southern (URSEIS) and Middle (ESRU and Alapaev) Urals the East European Craton crust thickens eastward from —40 km to —48 km, and is imaged by sub-horizontal to east-dipping reflectivity that can be related to its Paleozoic and older evolution. The suture zone between the East European Craton and the accreted terranes, the Main Uralian fault, is poorly imaged in the URSEIS section, but in the ESRU and Alapaev sections it is imaged as an abrupt change from a zone of east-dipping reflectivity that extends from the surface into the middle crust. East of the Main Uralian fault, the Magnitogorsk (Southern Urals) and the Tagil (Middle Urals) volcanic arcs display moderate to weak upper crustal reflectivity, and diffuse middle to lower crust reflectivity. The Moho beneath both arc complexes is poorly imaged in the reflection data, but based on refraction data is interpreted to be at 50 to 55 km depth. East of the arc complexes, the Uralide structural architecture is dominated by a wide zone of anastomosing strikeslip faulting into which numerous syntectonic Late Carboniferous and Permian granitoids intruded. This area is imaged in the seismic sections as clouds of diffuse reflectivity interspersed with, or cut by sharp, predominantly west-dipping reflections. In the Southern and Middle Urals, west-dipping
institute Jaume Almera, Barcelona, Spain Department of Earth Sciences, Uppsala University, Uppsala, Sweden University of Akron, Akron, US Department of Geology, University of Salamanca, Salamanca, Spain Bazhenov Geophysical Expedition, Scheelite, Russia 2
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Mountain Building in the Uralides: Pangea to the Present Geophysical Monograph 132 Copyright 2002 by the American Geophysical Union 10.1029/132GM03 33
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CRUSTAL ARCHITECTURE OF THE URALIDES reflectivity of t h e T r a n s - U r a l i a n l o w e r c r u s t w h e r e it a p p e a r s t o i n t e r n a l faults of t h e strike-slip anomalies, allowing them to be
zone extends from the middle crust into the merge with the M o h o . The boundaries and fault s y s t e m a r e well m a r k e d b y m a g n e t i c c o r r e l a t e d b e t w e e n t h e seismic s e c t i o n s .
1. I N T R O D U C T I O N The Uralide orogen developed during the Late Paleozoic as the continental margin of the former East European Craton was subducted eastward (current coordinates) beneath a chain of intra-oceanic island arcs (Magnitogorsk and Tagil) during the Late Devonian and Early Carboniferous [Puchkov, 1997; Brown and Spadea, 1999]. This was followed by closure of the paleo-Uralian ocean basin to the east and accretion of volcanic arcs and continental crust along the eastern margin of the grow ing orogen from the Late Carboniferous through to the Late Permian-Early Triassic [e.g., Zonenshain et al., 1984, 1990; Puchkov, 1997]. In the south, the Uralides began to form during the Middle to Late Devonian as the continental margin of the former East European Craton subducted eastward beneath the Magnitogorsk island arc. As a consequence, an accretionary complex was developed and emplaced over the subducting slab, and by the Early Carboniferous the arc was sutured to the continental margin along the Main Uralian fault zone [e.g., Puchkov, 1997; Brown et al., 1998; Brown and Spadea, 1999]. A deformation hiatus until the Late Carboniferous followed along the eastern margin of the East European Craton. Farther north exposure is poor and tectonic interpretations are heavily reliant on geophysical and borehole data. It is generally accepted, however, that the Tagil volcanic arc collided with the eastward subducting East European Craton in the Early Carboniferous [e.g., Puchkov, 1997]. This interpretation is based on the presence of Early Carboniferous (Visean) flysh-type deposits with an eastern provenance overlying bathyal deposits in the upper reaches of the Pechora River [Puchkov, this book]. F r o m the Late Carboniferous through to the Late Permian-Early Triassic, the paleo-Uralian ocean basin to the east closed, as island arcs and continental fragments were accreted to the eastern flank of the developing Uralides. It is not clear to what extent terrane amalgam ation, deformation and metamorphism occurred within these fragments prior to accretion. Closure of the paleoUralian ocean basin was accompanied by westward thrusting of the East European Craton Precambrian basement and its late Paleozoic platform cover to form a foreland thrust and fold belt and a largely Permian-age foreland basin [Kamaletdinov, 1974; Brown et al., 1997].
During late stages of the collision, extensive wrench or transpressive faulting appears to have dominated along the central axis of the orogen, fragmenting the Tagil arc and juxtaposing metamorphic terranes within the East and Trans-Uralian zones [Ayarza et al., 2000a; Friberg et al., 2000; Hetzel and Glodny, 2002]. This stage of orogenic development was accompanied by widespread crustal and mantle melting and the intrusion of numer ous plutons [Bea et al., this book, Gerdes, 2002] (Plate 1). The Middle Urals was also affected by Mesozoic to Tertiary deformation that resulted in folding and thrust ing of Lower Triassic deposits in the Tagil zone [Puchkov, 1997] and the development of the West Siberian Basin. The easternmost zones (East and Trans-Uralian zones) are widely covered by Mesozoic and Cenozoic sedi ments. The Uralides are preserved, relatively intact, within Eurasia and provide a rare opportunity to investigate the crustal architecture of an entire Paleozoic orogen. With this in mind, nearly 1000 k m of new deep reflec tion seismic data have been acquired in the Southern (URSEIS) and Middle (ESRU) Urals since 1993 under the auspices of E U R O P R O B E [e.g., Berzin et al., 1996; Ecthler et al., 1996; Knapp et al., 1996; Steer et al., 1998; Juhlin et al., 1996, 1998], and c. 500 k m of shallow Russian data (Alapaev, R114 and R115) have been reprocessed [e.g., Steer et a l , 1995; Brown et al., 1998] (Plate 1). In addition, two refraction/wide-angle reflec tion profiles ( U W A R S and URSEIS) have been acquired [Thouvenot et a l , 1995; Carbonell et al., 1996, 2000]. These experiments have shown the Uralide crust to be highly reflective, to have what appears to be a bivergent structural architecture, and to have clear reflection characteristics for each tectonic unit [Echtler et al., 1996; Knapp et al., 1996; Steer et al., 1998; Juhlin et al., 1998; Friberg, 2000]. The M o h o appears to have distinct characteristics from west to east across the orogen that can be related to the different tectonic units and events [Steer et al., 1998; Juhlin et al., 1998]. Despite similarities, the U R S E I S , E S R U and Alapaev sections display crustal reflectivity patterns that suggest the Uralide crust is different in the Southern Urals than in the Middle Urals [e.g., Steer et al., 1995, 1998; Juhlin et a l , 1998; Ayarza et al., 2000], indicating that different late- or post-tectonic processes may have been active in the two areas. The aim of this paper is to integrate
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the sesimic data with the known surface geology, placing special emphasis on the structures that bound the major tectonic units, and to develop a unified crustal-scale model for the architecture of the Uralides from the U R S E I S section in the south and the E S R U section in the north.
2. G E O L O G I C A L F R A M E W O R K A generally accepted division of the Uralides involves a number of longitudinal zones that are largely based on the ages and paleogeography of the dominant rocks within them [e.g., Ivanov et al., 1975; Khain, 1985; Puchkov, 1991]. In this paper, however, we use a modified subdivision that includes the recognition of tectonic units. This subdivision consists of a foreland thrust and fold belt composed of East European Craton rocks, the accreted Magnitogorsk and Tagil island arcs, the East Uralian zone, and the Trans-Uralian zone. Each of these tectonic units and their bounding faults is described below. Additionally, the area of the Uralides discussed in this paper has been traditionally divided geographically into the Southern and Middle Urals at about 56° N . 2.1. The Foreland Thrust and Fold Belt South of approximately 53° N , the foreland thrust and fold belt consists of a narrow (—20 km) zone of upright to west-verging folds and thrusts that record only minor amounts of shortening. Between 53° N and 56° N it forms a —120 km wide west-verging, basement involved thrust stack that records —20 k m of Uralide shortening [Brown et al., 1997; Perez-Estaun et al., 1997]. M u c h of this part of the foreland thrust and fold belt is composed of the gently south-plunging, Precambriancored Bashkirian Anticlinorium (Plate 1) that was de formed and metamorphosed during the Late Proterozoic [Shatsky, 1963; Puchkov, 1993], and which was only mildly deformed and metamorphosed during the Paleo zoic [Glasmacher et al., 1999; Geise et al., 1999]. In the Southern Urals, an accretionary complex, related to the collision of the Magnitogorsk island arc with the East European Craton, overlies the foreland thrust and fold belt (Plate 1) [Brown et al., 1998; Brown and Spadea, 1998; Alvarez-Marron et al., 2000]. N o r t h of 56° N , the foreland thrust and fold belt is a narrow, N-S trending, west-verging basement-involved thrust stack measuring —50 to 75 km in width (Plate 1). It extends northward into the linear thrust belt of the Middle Urals, where it is cored in the east by the Kvarkush Anticlinorium. Rocks in the Kvarkush Anticlinorium were deformed and meta morphosed during the Precambrian, and only mildly
reworked during the Paleozoic. The amount of short ening in the foreland thrust and fold belt has not been calculated for this part of the orogen. 2.2. The Main Uralian Fault Zone The Main Uralian fault zone (the arc-continent suture) in the Southern Urals is an u p to 10 km wide melange containing material that was tectonically eroded from the volcanic arc, including a number of mantle fragments [e.g., Savelieva et al., 1997]. In the northern most part of the Southern Urals (—55° N), it was intruded by an undeformed phase of the Syrostan batholith (Plate 1), dated at 327 ± 2 [Montero et al., 2000]. Intrusion of the batholith marked the end of tectonic activity along the Main Uralian fault in this area. In the Middle Urals, the Main Uralian fault is poorly exposed. Juhlin et al. [1998] defined it as a 10 km wide zone containing an underlying thrust stack that con tains strongly deformed and metamorphosed sandstones, quartzites, and quartz-mica schists of apparent East European Craton affinity. Here we define it to be the contact between the East European Craton and the Tagil arc, and the metamorphic thrust stack to be in its footwall (see also Knapp et al. [1998]). Ayarza et al. [2000a] suggest that there are significant differences in the evolu tion of the Main Uralian fault between the Southern and Middle Urals, and in the latter area it may be exten sively reworked.
2.3. The Magnitogorsk and Tagil Volcanic Arcs In the Southern Urals, the Silurian to Late Devonian Magnitogorsk volcanic arc is composed of a complete island arc volcanic sequence that begins with Emsian boninite-bearing arc-tholeiites in the forearc region, followed by Emsian to Givetian arc-tholeiite to calcalkaline volcanism typical of a mature arc [Seravkin et al., 1992; Brown and Spadea, 1998; Spadea et al., this book]. These volcanic units form the basement on which up to 5000 m of westward-thickening, Frasnian- to Famennianage forearc basin volcanoclastic sediments were deposited [Maslov et al., 1993; Brown et al., 2001]. Lower Carbon iferous shallow water carbonates unconformably overlie the arc edifice. Locally, Lower Carboniferous granitoids intrude the arc. Deformation in the Magnitogorsk volca nic arc is low, with only minor, open folding and minor thrusting [Brown et al., 2001]. The metamorphic grade barely exceeds seafloor metamorphism. In the Middle Urals, the Middle Silurian Tagil arc has also been interpreted to be an intra-oceanic island arc [Yazeva and Bochkarev, 1996; Bosch et al., 1997] with predominantly Silurian andesitic magmatism in the west
BROWN ET AL. and Lower Devonian trachytes and volcanoclastics in the east. These volcanic and volcanoclastic rocks are over lain by 2000 meters of Lower and Middle Devonian limestone that in the east is intercalated with calc-alkaline volcanics [Antsigin et al., 1994; Yazeva and Bochkarev, 1994]. The Tagil arc forms an open synformal structure [e.g., Bashta et al., 1990; Ayarza et al., 2000b] that has been metamorphosed to lower greenschist facies.
2.4. The East Magnitogorsk-Serov-Mauk Fault System The Magnitogorsk arc is structurally juxtaposed against the East Uralian zone along a melange zone that has been named the East Magnitogorsk fault [Ayarza et al., 2000a]. It has tentatively been correlated with the Serov-Mauk fault, which sutures the Tagil arc to the East Uralian zone in the Middle Urals. The Serov-Mauk fault is a strike-slip fault zone that can be traced as a prominent magnetic anomaly throughout the Middle Urals (Plate 1). Along the entire length ( > 700 km) of the East Magnitogorsk-Serov-Mauk fault system there is a significant j u m p in metamorphic grade from the volcanic arcs in the west to the East Uralian zone in the east.
2.5. The East Uralian Zone The East Uralian zone is composed of deformed and metamorphosed Precambrian and Paleozoic conti nental-type crust and island arc fragments [e.g., Puchkov, 1997, 2000; Friberg, 2000]. It was intruded by numerous granitoid batholiths and subordinate diorite and gabbro intrusions during the Carboniferous and the Early Permian [Sobolev et al., 1964; Fershtater et al., 1997; Bea et al., 1997, this book]. The regional metamorphic grade ranges from greenschist to granulite facies. In the northern part of the Southern Urals and in the Middle Urals a number of metamorphic complexes have been identified. These include the Salda, Murzinka-Adui, Sisert, Krasnogvardeiskii, Petrokamensk, and Alapaevsk complexes [e.g., Antsigin et al., 1994], which are juxta posed along tectonic contacts that have been extensively reworked by a roughly north-south striking strike-slip fault system. M a n y of the granitoid batholiths intruded syntectonically into this fault system. The eastern contact of the East Uralian zone is only known in the Southern Urals, where it is a melange containing local relics of harzburgite. In the area crossed by the U R S E I S section, the melange is intruded by a late, undeformed phase of the Dzhabyk granite that has been dated at 291 + 4 M a [Montero et al., 2000]. The late orogenic, dextral strike-slip Troitsk fault lies within the melange.
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2.6. The Trans-Uralian Zone The Trans-Uralian zone is not well known due to its poor exposure; it only outcrops in the Southern Urals. The best known units are Devonian and Carbonifer ous calc-alkaline volcano-plutonic complexes which are composed predominately of volcanoclastics and lava flows that are intruded by co-magmatic gabbro-diorite and diorite plutons [e.g., Puchkov, 1997, 2000]. Ophiolite units and high pressure rocks have also been reported [Puchkov, 2000]. The volcano-plutonic complexes are overlain by terrigenous red-beds and evaporites. Defor mation has not been well studied, although it appears that the Devonian and Lower Carboniferous units are affected by open to tight folds. 3. T H E S E I S M I C P R O F I L E S The acquistion and processing parameters for the U R S E I S section can be found in Tryggvason et al. [2001], for the E S R U section in Juhlin et al. [1998] and Friberg et al. [2000], and for the Alapaev section in Steer et al. [1995]. All sections have been presented in earlier papers, except for the E S R U 9 9 segment (the easternmost —70 k m shown in Plate 3) which is unpublished u p to now. I m p o r t a n t to note for this paper is that the U R S E I S data were acquired with two source types, vibroseis [Echtler et al., 1996] and explosives [Steer et al., 1998]. The vibroseis sections presented in this study image the upper crust much better than the explosive ones (due to the higher fold), but the signal did not penetrate to the M o h o along the entire U R S E I S section with the vibroseis source. The line drawings shown here (Plates 1 to 3) were generated from coherency filtered stacks with the events picked automatically and then depth migrated. The migration velocities used are based u p o n refraction/wide-angle data presented in Carbonell et al. [1996] for the Southern Urals and Juhlin et al. [1996] for the Middle Urals. 3.1. The Foreland Thrust and Fold Belt In the U R S E I S section (Plate 2), from kilometer 0 to about kilometer 50, the foreland thrust and fold belt is characterized by a —20 k m thick zone of continuous, subhorizontal reflectivity that is related to the Riphean to Permian sediments. A thin band of weak, openly concave upward reflectivity at —20 k m depth is interpreted to m a r k the top of the Archean crystalline basement. Below this, the crust is transparent and the M o h o is not imaged. F r o m near kilometer 50 to the Main Uralian fault at about kilometer 146, the foreland thrust and fold belt is characterized by patchy, east and west dipping reflectivity in the upper and middle crust. F r o m kilometer 117 to the
CRUSTAL ARCHITECTURE OF THE URALIDES
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