TECTONIC EVOLUTION OF THE BEAR MOUNTAINS FAULT ZONE , FOOTHILLS TERRANE , CENTRAL SIERRA NEVADA

The Late Jurassic-Early Cretaceous Bear Mountains fault zone (BMFZ) is the westernmost strand of the Foothills fault system in the Western Metamorphic belt of the Sierra Nevada. The tectonic significance of this 300-km-long fault zone has been downplayed in the past, but we contend that it is a major discontinuity within the Foothills terrane. The BMFZ is an 5-km-wide shear zone consisting of slate-metagraywacke-matrix melange that experienced intense polyphase deformation and metamorphism. Small-scale structures indicate reverse slip along most of the extent of this steeply east dipping shear zone. A host of lithologically diverse tectonic blocks are enclosed in the BMFZ. Blocks of sandstone, metavolcanic rocks, and plutonic rocks of intermediate composition have counterparts in the adjacent eastern and western zones of the Foothills terrane, whereas other block types are exotic. Exotic blocks include ultramafites that locally contain pods of gabbro, garnet amphibolite, greenschist, sedimentary breccia, and volcaniclastic rocks. The breccia is composed primarily of clasts of amphibolite and minor garnet-bearing impure quartzite (metachert?); it probably accumulated at the base of fault scarps. The exotic blocks comprise a broadly ophiolitic assemblage similar to that inferred to form basement to the Foothills terrane, and they record a deformation prior to incorporation in the slate-metagraywacke matrix. Amphibolite blocks and quartzite preserved as clasts within the breccia probably were deformed during overthrusting of oceanic lithosphere in a setting similar to that beneath the Tuolumne ophiolite east of the BMFZ. The presence of exotic blocks implies large-scale mixing and large displacements in the BMFZ, as do differences in stratigraphy across the fault zone. Metamorphic grade does not vary significantly across the BMFZ, however, suggesting that dip slip did not exceed several tens of kilometers if the fault zone originally dipped gently eastward as we propose. There is no direct evidence for major strike slip along the BMFZ as has been proposed by others; any such slip must have occurred before the reverse-slip related structures of the zone. We thus interpret the BMFZ as an intra-arc reverse fault of moderate to large displacement, with possible earlier strike slip, that incorporates ophiolitic basement showing an older, complicated history. Copyfight 1991 by the American Geophysical Union. Paper Number 91TC00862. 0278-7407/91/91TC-00862510.00 INTRODUCTION The concept of terrane analysis has focused attention on the importance of understanding as completely as possible the extent of individual terranes and the nature of terrane-bounding faults. For example, since Clark [1960] originally defined the Foothills fault system as a series of Late Jurassic, steep faults in the Western Metamorphic belt, Sierra Nevada, California (Figure 1), much of the confusion about the evolution of this region has centered around the various interpretations of the age, kinematics, and regional significance of these faults [e.g., Burchfiel and Davis, 1972, 1975; Schweickert and Cowan, 1975; Saleeby, 1981; Edelman et al., 1989; Schweickert et al., 1988; Sharp, 1988]. The age and kinematics of most of the structures included by Clark in the Foothills fault system are still not well known. This uncertainty has therefore allowed a variety of conflicting tectonic interpretations for the Jurassic evolution of the Western Metamorphic belt, including arc-arc collision [Schweikert and Cowan, 1975], autochthonous ophiolite and arc construction and subsequent dismemberment [Burchfiel and Davis, 1972, 1975; Saleeby, 1981, 1982], and eastward ophiolite obduction followed by westward backthrusting [e.g., Day et al., 1985]. In the central Sierra Nevada the westernmost exposed strand of the Foothills fault system is the Bear Mountains fault zone [BMFZ] [Clark, 1960; Paterson et al., 1987; Tobisch et al., 1989]. The tectonic significance of this 300-km-long fault zone has until recently been downplayed, because of the general impression that the fault zone occurred entirely within one terrane [Foothills terrane of Schweickert, 1981; Schweickert et al., 1984]. In contrast, Paterson et al. [1987] and Tobisch et al. [1989] noted significant differences in the central Foothills terrane east and west of this fault zone and suggested that the BMFZ is an important tectonic boundary [see also, Newton, 1986; Ave' Lallemant and Oldow, 1988; Edelman and Sharp, 1989]. They also pointed out that the fault zone was a locus of complex polydeformation and metamorphism and that it was active, at least episodically, for a minimum of 30 m.y. Previous studies [Paterson et al., 1987; Tobisch et al., 1989; Vernon et al., 1989] have concentrated on Late Jurassic to Early Cretaceous deformation and metamorphism near the southern end of the BMFZ. Here we consider: (1) the protoliths of materials within the fault zone; (2) the metamorphism and deformation preserved in these materials; (3) the overall structural and metamorphic history of the fault zone; and (4) the implications of this history for tectonic scenarios proposed for the region. The major conclusions are: (1) The BMFZ has a long and complex structural and metamorphic history. (2) The fault zone forms a major discontinuity in the Western Metamorphic belt. We see two likely interpretations for this discontinuity. Our favored interpretation is that the BMFZ is a long-lived intra-arc fault with moderate to large amounts of reverse slip and possible earlier strike slip. Alternatively, the close association of the BMFZ with tectonic melange containing ophiolitic blocks indicates that it is a candidate for a suture between two Mesozoic arcs in the Western Metamorphic belt. (3) In either 996 Miller and Paterson: Bear Mountains Fault Zone, California