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Deformation and Metamorphism of the Franciscan Subduction Zone Complex Northwest of Pacheco Pass, California DARREL S. COWAN* Shell Oil Company, P.O. Box 527, Houston, Texas 77001 ABSTRACT juxtapose rock units that bear no apparent tion and metamorphism, along a major stratigraphie, deformational, or metamor- fault system of regional extent that is an The Franciscan Complex northwest of phic relation to one another. The structural important crustal expression of lithospheric Pacheco Pass, California, includes three units were separately deformed ' and consumption at depth. fault-bounded units, each characterized by metamorphosed under a variety of condi- Miyashiro (1961) suggested that the a different deformational style and suite of tions prior to their tectonic juxtaposition Franciscan and grossly parallel Sierra metamorphic mineral assemblages. Struc- during late Mesozoic continental margin Nevada batholith and related high- turally highest is jadeitic pyroxene-bearing subduction. Field and pétrographie evi- temperature, low-pressure metamorphic metagraywacke semischist. The areally ex- dence permit, but do not prove, the rocks are part of a series of circum-Pacific tensive Garzas tectonic mélange separates hypothesis that both the semischist and paired metamorphic belts. As such, they are the semischist from the structurally lowest exotic, high-grade mélange inclusions once in essence late Mesozoic analogs of rock as- pumpellyite-bearing Orestimba meta- were more deeply buried and have been semblages forming in active subduction graywacke. The Garzas mélange is emplaced upward into their present zones and associated magmatic arcs, re- representative of Franciscan mélanges in anomalously shallow structural positions. spectively (Takeuchi and Uyeda, 1965). general. These mappable bodies have an in- Key words: structural geology, Franciscan, When viewed in the context of plate tec- ternal fabric dominated by penetrative, blueschist metamorphism, tectonic tonic theory and paired metamorphic belts, mesoscopic shear fractures and contain tec- mélange, subduction, California. the Franciscan is hardly a unique assem- tonic inclusions of all sizes immersed in a blage, nor is it a particularly anomalous pervasively sheared, generally fine-grained INTRODUCTION one. It offers an excellent opportunity to matrix. The shear fractures record brittle The Franciscan Complex, a heterogene- learn about the metamorphic and deforma- deformation of consolidated rock bodies. ous assemblage of metagraywacke, chert, tional history of subduction zone assem- Many mélanges contain exotic inclusions, mafic volcanic rocks, and minor isolated blages and to further our understanding of clearly not derived from adjacent units, and blocks and slabs of high-grade blueschist- how plate interactions are expressed in the metamorphic mineral assemblages and tex- and amphibolite-facies metamorphic rocks, geologic record. tures indicate that they were metamor- is widely exposed in the California Coast The purpose of this paper is to sum- phosed before being tectonically mixed Ranges and parts of southwestern Oregon marize structural and petrologic data from with more voluminous, generally lower and Baja California. The complex is cur- a representative Franciscan terrane in the grade, metagraywacke inclusions. rently interpreted as having accumulated in Diablo Range northwest of Pacheco Pass, The structural units are grossly sheetlike a late Mesozoic-early Tertiary subduction California, and to emphasize certain as- in external form and are separated from zone at the western margin of the North pects of the tectonic framework of the study one another by gently to steeply dipping American plate (for example, Hamilton, area that may characterize the Franciscan, major faults. Unlike low-angle thrusts in 1969; Bailey and Blake, 1969; Ernst, 1970; and perhaps some other subduction zone imbricated Cordilleran terranes, the faults and many others). Franciscan "trench" complexes, in general. A more detailed pre- do not systematically repeat or offset a materials were carried against and beneath sentation of lithologic and petrographic normal stratigraphie sequence but rather the coeval Great Valley sequence, a thick data is in Cowan (1972), and the reader is succession of clastic rocks of Tithonian to referred to Bailey and others (1964) for a Maestrichtian age that contrasts strikingly * Present address: Department of Geological Sciences, comprehensive summary of Franciscan University of Washington, Seattle, Washington 93195. with the Franciscan in its lesser deforma- geology. Geological Society of America Bulletin, v. 85, p. 1623-1634, 5 figs., October 1974 1623 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/10/1623/3418147/i0016-7606-85-10-1623.pdf by guest on 30 September 2021 1624 D. S. COWAN FRANCISCAN COMPLEX Northwest of the Garzas Creek Narrows, both s surfaces locally have been folded on NORTHWEST OF PACK E CO PASS the Tesla-Ortigalita fault is sharp and ;i small scale. Interbedded graywacke and nearly vertical (Fig. 2). Southeast of the phyllite display groups of kinklike folds The core of the northern part of the Di- narrows, an unconsolidated, locally pow- with sharp hinges and amplitudes of a few ablo Range (Fig. 1) consists of the Francis- dery, bluish-gray matrix of thoroughly centimeters; whereas individual graywacke can Complex, flanked by Upper Jurassic to comminuted rock debris containing iso- beds commonly contain isolated shear Upper Cretaceous sedimentary and vol- lated blocks of graywacke, semischist, bands, transverse to the foliation, that canic rocks of the Great Valley sequence greenstone, thinly b:dded red and green define the axial planes of small asymmetric that are lithologically similar to Great Val- chert, and glaucophane-lawsonite schist angular folds with amplitudes that range ley strata elsewhere in the Coast Ranges occupies the fault zone and expands to a from a few millimeters to several centime- (Dickinson and Rich, 1972). Vickery width of 400 m at Richard Creek. The con- ters. Apparently, no secondary axial plane (1924), Huey (1948), Leith (1949), Briggs tact of the shear zone debris with fractured foliation was developed in either case. The (1953), Schilling (1962), Maddock (1964), Great Valley strata is approximately verti- rather limited, nonrepresentative exposures and Raymond (1973a) mapped parts of the cal or dips steeply southwest. The sand- of this unit prevented a systematic analysis northern Diablo Range and together estab- stone, conglomerate, and shale of the Great of the folds, but their existence does not de- lished that the entire boundary of the Fran- Valley sequence were not examined in de- tract from the remarkably consistent at- ciscan mass is a series of steeply dipping tail during this study, but the age of the titude of bedding and foliation, which faults now collectively termed the "Tesla- strata immediately adjacent to the Francis- grossly parallel the lower tectonic contact Ortigalita fault" (Page, 1966; Raymond, can is probably Turonian. Both Maddock cf the unit and seem to preclude the pres- 1973b). (1964) and Schilling (1962) found fossils of ence of any large folds within it. The area mapped during this study is ap- this age in stratigraphically equivalent Microscopically, the foliation that proximately 20 km northwest of Pacheco rocks to the northwest and southeast. characterizes the entire unit is defined by Pass (Fig. 1). The northern part of the area the planar preferred orientation of the slightly overlaps the southern edge of the METAGRAYWACKE SEMISCHIST boundaries of mineral grains and grain Mount Boardman quad mapped by Mad- The structurally highest unit within the aggregates (Fig. 3A). It is clear that the dock (1964). Raymond (1973a, 1973b) Franciscan locally is a semischistose meta- rocks are tectonites with planar fabrics ac- mapped the Mount Oso area to the north, graywacke that ranges in thickness from ap- quired during recrystallization and flow and Cotton (1972) differentiated Francis- proximately 200 m to 600 m. This unit oc- under stress. There is very little evidence for can bedrock units in the western half of the curs immediately adjacent to the sharp simple cataclasis or mechanical granulation range north of California F.oute 152. The trace of the Tesla-Ortigalita fault, or ac- unaccompanied by concurrent recrystalliza- Franciscan Complex at Pacheco Pass has companying shear zone debris, and is sepa- tion. Metamorphic white mica, chlorite, been exhaustively studied by McKee rated from structurally underlying tectonic and lawsonite have grown parallel and (1962a, 1962b), Ernst and Seki (1967), and mélange by a fault that dips moderately to subparallel to the foliation. Recrystallized Ernst and others (1970). Ernst (1971b) steeply eastward. No fossils were found in volcanic and metamorphic lithic fragments later made a petrographic reconnaissance the unit, but whole-rcck K-Ar dates (Table and chert grains in metagraywacke and of Franciscan metagraywacke from the core 1) indicate it is at least: as old as Albian age. conglomerate are markedly flattened and of the northern Diablo Range and deter- The dates also show the unit was attenuated in this plane. Deformed clastic mined the approximate areal distribution of metamorphosed before the deposition of quartz grains, the most abundant detrital metaclastic rocks containing jadeitic pyrox- presently adjacent Great Valley strata. ene. Investigations to date indicate the Bedding in the semischist is