northern California (Fig. 1). Our findings also suggest that a long-lived partitioning of regional transpressional strains has occurred adjacent to the. San Andreas ...
How long do structures take to form in transpression zones? A cautionary tale from California Enrico Tavarnelli Centro di Geodinamica, Università della Basilicata, Via Anzio, 85100 Potenza, Italy Robert E. Holdsworth Reactivation Research Group, Department of Geological Sciences, University of Durham, South Road, Durham DH1 3LE, UK ABSTRACT It is generally assumed that individual sets of coplanar and colinear deformation structures form together during events that are of relatively short duration (1–5 m.y.). The record of deformation in a sequence of Late Cretaceous to Holocene sedimentary rocks from the northern Salinian block of California spans at least 30 m.y. and illustrates that this assumption is sometimes incorrect. At different localities, geometrically and kinematically identical contractional structures either predate or postdate local unconformities of varying age within the succession, so that it is possible to define at least four chronologically distinct, but otherwise indistinguishable, deformation episodes. In the absence of the unconformities, the punctuated nature of the deformation would not be apparent, therefore suggesting that subparallel structures may form during successive, distinct deformations spread out over long time periods. In the northern Salinian block, the inferred contractional strain field is approximately normal to the adjacent San Andreas fault and appears to have been consistently oriented in this direction during deformation events recorded over the past 30–45 m.y. The strain pattern is most easily explained by efficient partitioning of transpressional strains into fault-normal shortening and right-lateral faulting during episodic regional deformations. We propose that reactivation of preexisting structural anisotropies controls the observed partitioning of deformation in many transpression zones. INTRODUCTION It is generally agreed that orogenic cycles lasting tens of millions of years may involve one or more regional deformation events that produce structures distinct in terms of their geometry,
kinematics, and style. Compilations of data from well-constrained, relatively young fold belts and imbricate thrust terrains suggest that the durations of deformation events typically range from 1 to 30 m.y.; there are marked maxima between 1
Figure 1. Sketch map of northern Salinian block study area. Inset shows separation of Salinian block from Sierra Nevada magmatic arc along San Andreas. General distribution of main deformation stages (a– d) recognized during this study (Fig. 2) is shown together with locations of cross sections A-A′, B-B′, and C-C′ (Fig. 3).
Geology; December 1999; v. 27; no. 12; p. 1063–1066; 4 figures; 1 table.
and 5 m.y. (Pfiffner and Ramsay, 1982). Precise stratigraphic controls on the timing of events are generally lacking, however, and, together with rheologic and strain rate considerations, permit individual events to have durations of about 100 k.y. (Pfiffner and Ramsay, 1982; Rutter et al., 1985). As a result, many geologists tend to assume that individual deformation structures observed in the field, such as folds, faults, and fabrics, formed during single events of relatively short duration (S¸engör, 1991). However, several studies (Saleeby and Sharp, 1980; Tobisch and Fiske, 1982) have cast significant doubts on the universal validity of this assumption by showing that essentially similar, subparallel structures can form during different deformation events that are widely separated in time. In this paper we present compelling evidence to support the latter model using structural and stratigraphic data collected from part of the northern Salinian block of central northern California (Fig. 1). Our findings also suggest that a long-lived partitioning of regional transpressional strains has occurred adjacent to the San Andreas fault during oblique convergence of the Pacific and North American plates over at least the past 30 m.y. GEOLOGIC SETTING OF THE NORTHERN SALINIAN BLOCK In California, the North American and Pacific plates are juxtaposed along a subvertical belt of anastomosing northwest-southeast–striking faults, the San Andreas fault system, which defines a major ocean-continent transform boundary active since the late Oligocene (Wilson, 1965; Atwater, 1989). Right-lateral slip along the San Andreas and associated faults was accompanied by northeast-southwest–directed shortening, accommodated by regionally important northwest-southeast–trending folds and thrusts (Compton, 1966; Aydin and Page, 1984). Prior to the onset of the transform regime at the end of the Oligocene, the history of California was dominated by the Andean-type convergence and subduction of the Pacific plate beneath the North American continent (Dickinson, 1981). The Pacific and North American plates are locally separated by a continental fragment, the Salinian block, which was detached in early Miocene time from the southern termination of the subduction-related Sierra Nevada batholith and was translated at least 500 km northwestward along the San Andreas fault to its present position in western California (Fig. 1 inset; Graham, 1978; 1063
Ross, 1983; Whidden et al., 1998). The Salinian block is further subdivided into four smaller terrains—the southeastern, central, western, and northern blocks—by right-lateral strike-slip faults of the San Andreas transform system (Ross, 1983). The structures described in this paper are all within the northern Salinian block. STRATIGRAPHY AND STRUCTURE The northern Salinian block consists of granitoid basement of Cretaceous age overlain by Late Cretaceous to Holocene strata that are locally divided by unconformities into a series of timeslice units, reflecting a complex history of deposition, deformation, and erosion (Fig. 2). These rocks are moderately to weakly affected by folds, reverse faults, thrusts, and related contractional fabrics that accommodate regional northeastsouthwest–directed shortening (Tavarnelli, 1998). At different locations, individual structures either predate or postdate unconformities of different ages (Fig. 3), so it is possible to define and broadly correlate four chronologically distinct deformation stages across the region, which we refer to here as stages a–d, ranging in age from late Eocene to Holocene (Fig. 2).
The open to isoclinal folds produced during stage a are the most abundant structures in the northern Salinian block, where they affect Cretaceous to Eocene rocks (Figs. 1, 3, and 4; Table 1). These structures are postdated by an unconformity overlain by deposits of Oligocene and Miocene age (Figs. 2 and 3C). Open to tight stage b folds and thrusts affect middle-late Miocene and older rocks (Figs. 1, 3, and 4; Table 1). Stage b structures are postdated by the unconformably overlying upper Pliocene Purisima Formation. Open to closed stage c folds and thrusts are observed at Point Reyes, Point Año Nuevo, and in the Montara Mountains (Figs. 1 and 4; Table 1), and are postdated by unconformably overlying Pleistocene deposits (Fig. 3B). Open stage d folds occur widely in limited numbers (Figs. 1 and 4; Table 1), and are generally postdated by unconformably overlying Holocene deposits, although some thrust faults are locally reactivated and truncate the Holocene basal unconformity (e.g., the southern cliffs of Montara Beach). It is important to emphasize that the marker unconformities that locally produce sharp truncations of preexisting structures overstep both limbs of earlier fold pairs of different sizes (e.g.,
Figure 2. Simplified composite stratigraphic columns and inferred timing of deformation stages from selected locations within northern Salinian block (see Fig. 1 for location). Note that time scale is nonlinear. Timing of regional events proposed by Page et al. (1998) is also shown. Unconformities labeled 1–4 correspond to those shown in Figure 3. 1064
see Fig. 3). The resulting geometries are inconsistent with deposition synchronous with folding, and suggest instead a series of deformation episodes that are separated by periods of erosion. The duration of each deformation stage can be inferred from the age of the strata separated by the unconformities in the various localities studied. Local differences may exist in the exact timing, duration, and, possibly, number of events that are below the resolution of the existing stratigraphic controls, especially during stage a (Fig. 2). Each deformation stage is associated with northwest-southeast–trending fold structures developed on all scales (Figs. 3 and 4; Table 1), apart from stage d, which is only observed on a mesoscopic scale. Associated pressure-solution cleavages are generally axial planar to mesoscopic folds, although those related to stage a and, to a lesser extent, stage b structures display marked fanning patterns around fold hinge zones (Fig. 4). Conjugate thrust faults are associated with each set of folds, and all preserve abundant slickensides and slickenfiber lineations that indicate a mean northeast-southwest transport azimuth (Fig. 4; Table 1). The four deformation stages recognized by us in the northern Salinian block correlate well with the timing of events proposed in recently published models for the regional tectonic evolution of western California. For example, Page et al. (1998) ascribed the formation of the central Coast Ranges of California (including the Salinian block) to structures developed during four main deformation phases (from oldest to youngest): (1) late Oligocene disturbances (29–25 Ma); (2) late Miocene tectonism (11–7 Ma); (3) major Pliocene tectonism (ca. 3.5– 1 Ma); and (4) rise of the present Coast Ranges (0.4 Ma onward). The clear correspondence between these major tectonic phases (1–4) and the main deformation stages (a–d) in the northern Salinian block (Fig. 2) indicates that the latter can be considered to be of regional significance. Paleomagnetic data suggest that folds and thrusts in the region, although crosscut by right-lateral faults, were not significantly reoriented (Duane Gibson, 1983). DISCUSSION AND CONCLUSIONS The relationships between the structures and the unconformities are consistent with at least four short-lived (
