Early Mesozoic tectonic evolution of the western Sierra Nevada, California RICHARD A. SCHWEICKERT Department of Geological Sciences and Lamont-Doherty Geological Observatory of Columbia University, New York, New York 10027 DARREL S. COWAN Department of Geological Sciences, University of Washington, Seattle, Washington 98195 ABSTRACT  Prebatholithic rocks of Mesozoic age in the Sierra Nevada can be interpreted as remnants of ancient volcanic arcs, sub- duction complexes, and sequences of oceanic lithosphere. Two partly coeval subparallel volcanic arcs, one in the western foothills and the other in the northern and eastern Sierra Nevada, are juxtaposed. The western arc was an east-facing island-arc complex that evolved through a series of steps including formation of a remnant arc and interarc basin. The eastern arc was a west-facing marginal arc that was con- structed on the edge of North America. Both arc-subduction complexes consumed intervening oceanic lithosphere and col- lided during the Late Jurassic Nevadan orogeny. Generation of magmas in both arcs apparently ceased at about this time, and renewed subduction was initiated west of the island arc in latest Jurassic time, giv- ing birth to the Franciscan-Sierran arc- trench complex. Fault zones and mélanges in the western Sierra Nevada reflect the complex suturing at the collision boundary. Pre-Tithonian ophiolite at the base of the Great Valley sequence in the Coast Ranges originated in a back-arc or marginal basin setting with respect to the coeval Sierran foothills arc. Key words: California, Mesozoic geology, tectonics, Sierra Nevada, igneous rocks. INTRODUCTION  Prebatholithic rocks exposed along the western flank of the Sierra Nevada (Fig. 1) can best be characterized as a series of elon- gate northwest-trending belts separated from one another by major, steeply dipping fault zones. Pioneering mapping by Turner (1893a, 1893b, 1894a, 1894b, 1894c, 1895, 1897, 1898), Turner and Ransome (1897a, 1897b), Ransome (1899), Lindgren (1894, 1900), Lindgren and Turner (1894, 1895), and later Clark (1964) established that each belt contains rocks of different ages, lithologies, and deformational styles. However, a satisfactory hypothesis to ac- count for their origin and juxtaposition has remained elusive. In accord with plate-tectonics theory, we believe that certain distinctive rock assem- blages in the western Sierra Nevada can be interpreted as remnants of ancient volcanic arcs, subduction-zone complexes, and se- quences of oceanic crust and upper mantle. Our reconnaissance field work, together with published stratigraphic and structural data, suggests to us that the assemblages re- cord the Late Jurassic collision of an oceanic island arc with a west-facing arc- trench system constructed on the North American continental margin in early Mesozoic time. The purposes of this paper are to outline our model for the tectonic evolution of the western Sierra Nevada, discuss its implications, and more impor- tantly, summarize the critical field data upon which it is based. However, we want to stress that our model, anticipated in some respects by Hamilton (1969) and Moores (1972), is simply a working hypothesis that must be rigorously tested by additional field work. REGIONAL SETTING  Most rocks older than the Late Jurassic-Late Cretaceous composite Sierra Nevada batholith are exposed in a belt 50 to 80 km wide and nearly 400 km long (Fig. 1). Rocks of the "foothills" or "western metamorphic" belt are overlain uncon- formably by sedimentary rocks of the Great Valley on the west and are intruded by granitic rocks of the Sierra Nevada batholith on the east. Similar rocks are known from drill-hole and geophysical data to underlie a major part of the Great Valley, but the nature and location of their buried boundary with the Franciscan Complex, widely exposed in the California Coast Ranges west of the valley, are controversial. Scattered granitic, gabbroic, and ultramafic bodies occur within the terrane but are rela- tively small and areally insignificant when compared to much more voluminous metasedimentary and metavolcanic rocks. Rocks and structures correlative with those in the foothills belt crop out in the Klamath Mountains to the northwest. Highly metamorphosed rocks of Middle Jurassic and earlier ages form pendants in the east- ern and southern parts of the Sierra Nevada batholith. Prebatholithic rocks can be conveniently subdivided into three major lithologic and stratigraphic subprovinces, here informally designated as follows: (1) the predomi- nantly Jurassic "western belt"; (2) the "central belt," probably largely Paleozoic in age; and (3) the "eastern belt," mostly of Mesozoic age but in part containing Paleozoic rocks. Broad fault zones contain- ing lenses of sheared serpentinite and schis- tose metavolcanic rocks separate the west- ern and central belts, and an angular un- conformity occurs between the central and eastern belts (Clark and others, 1962). Before presenting our tectonic model, we will summarize pertinent petrologic, stratigraphic, and structural data from each belt and discuss the nature of the major fault zones that affect them. WESTERN BELT  The western belt is separated from the central belt to the east by a system of major faults whose southern segment is called the Melones fault zone (Clark, I960) and whose northern segments are marked by major ultramafic masses (Fig. 1). The west- ern belt actually consists of several discrete fault-bounded structural units. To simplify our discussion, we will divide the belt south of the American River into three tectoni- cally significant units that we have infor- mally named A, B, and C (Fig. 1). North of the American River, the Smartville block separates A and C. From the American to the Tuolumne River, A and C are separated by unit B, and south of the Tuolumne River, they are separated by the Bear Mountains fault zone. Both B and C con- tain appreciable thicknesses of submarine basaltic and andesitic pyroclastic rocks, lava, and interbedded volcaniclastic rocks (Clark, 1964; Morgan, 1973). B, locally sandwiched between the Bear Mountains Geological Society of America Bulletin, v. 86, p. 1329-1336, 3 figs., October 1975, Doc. no. 51001. 1329  Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/10/1329/3418117/i0016-7606-86-10-1329.pdf by guest on 15 September 2021