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Metamorphic and Deformational Processes in the Franciscan Complex, California: Some Insights from the Catalina Schist Terrane

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Metamorphic and Deformational Processes in the Franciscan Complex, California: Some Insights from the Catalina Schist Terrane Metamorphic and deformational processes in the Franciscan Complex, California: Some insights from the Catalina Schist terrane J. P. PLATT Department of Geology and Mineralogy, University of Adelaide, Adelaide, South Australia 5001 ABSTRACT INTRODUCTION Maxwell, 1974; Blake and Jones, 1974). The paleogeography suggested by Dickin- On Santa Catalina Island, blueschist is The Franciscan Complex (Berkland and son (1971) is adopted here, however. The structurally overlain by glaucophanic others, 1972) of western California consists purpose of this paper is to describe the greenschist, which is overlain in turn by a largely of graywacke, chert, and basaltic Franciscan schist on Santa Catalina Island, unit of amphibolite and ultramafic rock. volcanic rocks deposited in Late Jurassic to present some tectonic hypotheses, and These three units are juxtaposed along sub- Eocene time on simatic basement (Bailey show how they might help explain some horizontal postmetamorphic thrusts; tec- and others, 1964). The present disruptive problems posed by the Franciscan Com- tonic blocks of amphibolite are distributed deformational style and high-pressure, plex. along the thrust between the greenschist low-temperature metamorphism are the re- and the blueschist. Physical conditions of sults of eastward lithospheric underthrust- CATALINA SCHIST TERRANE metamorphism are estimated to be approx- ing beneath the western margin of the imately 300°C and 9 kb for blueschist, North American plate during late Mesozoic General Description 450°C and 8 kb for greenschist, and 600°C and early Cenozoic time (Hamilton, 1969; and 10 kb for amphibolite. I suggest that Bailey and Blake, 1969; Ernst, 1970). The In California south of the Transverse metamorphism occurred in a newly started Franciscan Complex lies beneath a major Ranges, the Franciscan Complex crops out subduction zone, where an inverted thermal tectonic discontinuity, the Coast Range as two small areas of schist, one in the Palos gradient developed below the hot thrust (Bailey and others, 1970). Above this Verdes Hills, and the other on Santa hanging-wall peridotite. Postmetamorphic thrust lie Paleozoic and Mesozoic granitic Catalina Island (Fig. 1). The distribution of eastward underthrusting along surfaces of and metamorphic rocks in the Klamath clasts of similar schist in the Miocene San varying dip can explain the present struc- Mountains and the Great Valley sequence Onofre Breccia indicates that the schist ter- tural relationships. with its ophiolitic basement in the Coast rane may underlie much of the offshore Tectonic blocks of glaucophane-epidote Ranges (Fig. 1). The Great Valley sequence area of southern California (Woodford, schist, amphibolite, and eclogite elsewhere consists of Late Jurassic and Cretaceous 1925; Vedder and others, 1974). The fol- in the Franciscan Complex may be dis- graywacke turbidites; it lies unconformably lowing description of the metamorphic rupted remnants of similar metamorphic on older continental rocks in the east and rocks on Santa Catalina Island is sum- zones. The inverted thermal gradient will directly on ophiolite, interpreted as an marized from Piatt (1975). only exist in the early stages of subduction, oceanic crustal remnant (Bailey and others, The Catalina Schist consists of three tec- which explains why the blocks are the old- 1970), in the west. tonic units, distinguishable by their est rocks in the Franciscan Complex. Prior to major right slip on the San An- metamorphic mineral assemblages. The The gross decrease in age and metamor- dreas fault (Hill and Dibblee, 1953; units are separated by regionally subhori- phic grade westward across the Franciscan Crowell, 1962), which doubled up the zontal folded thrusts (Figs. 2, 3). Mineral results from successive underthrusting and north-northwest—trending Mesozoic belts assemblages in the three units are sum- accretion of progressively younger slices of (Fig. 1), California probably was an active marized in Figure 4. supercrustal material, concurrent with up- continental margin resembling the west The Catalina Blueschist Unit is struc- lift and erosion. Pressure-temperature (P-T) coast of South American (Hamilton, 1969). turally lowest and the most extensive in conditions of metamorphism in each east- According to this model, the site of the outcrop. It consists of metagraywacke, dipping tectonic slice will increase down- Franciscan Complex corresponds to the metachert, mafic metavolcanic rocks, and dip. At any given time, older, more easterly oceanic trench and inner trench wall, the ultramafic rock, uniformly metamorphosed slices will have been uplifted further, hence Great Valley sequence to the arc-trench under blueschist-facies conditions (Fig. 4). metamorphic grade in the exposed edges gap, and the Sierra Nevada—Salinia— A large proportion of the volcanic rocks will increase eastward and structurally up- Peninsular Range batholith belt to the con- were deposited as well-bedded basaltic sand ward. tinental volcanic-plutonic arc (Dickinson, and conglomerate, which have been recrys- If erosion is faster than accretion for a 1971). tallized to distinctive glaucophane- time, younger slices will be metamorphosed The environment of deposition and the lawsonite schist and phyllite. Massive at lower pressures than were the older sources of Franciscan sediment continue to omphacite-lawsonite "greenstone" was de- higher ones. Simple reverse faulting can be debated (for example, Bailey and Blake, rived from diabase, flow breccia, and pillow then produce the observed interleaving of 1969, p. 229; Scholl and Marlow, 1972, lava. Much of the metagraywacke lacks rocks of different metamorphic grade. Key 1974; Matthews and Wachs, 1973), and glaucophane, but the presence of lawsonite words: metamorphic petrology, tectonics, several alternative models for Franciscan and jadeitic pyroxene indicate that it was Franciscan, subduction, blueschist, eclogite. tectonics have been advanced (Hsu, 1971; metamorphosed under the same conditions Geological Society of America Bulletin, v. 86, p. 1337-1347, 9 figs., October 1975, Doc. no. 51002. 1337 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/10/1337/3418137/i0016-7606-86-10-1337.pdf by guest on 23 September 2021 1338 J. P. PLAIT as the metavolcanic rocks. Synmetamorphic deformation produced isoclinal folds and a schistosity, and there were several subse- quent folding events, but primary textures are sufficiently well preserved locally to allow determination of original rock types. The Catalina Greenschist Unit struc- turally overlies the Blueschist Unit in the central part of Santa Catalina Island (Figs. 2, 3). It is derived from a rock as- semblage similar to that of the Blueschist Unit, although the proportion of mafic vol- canic rocks is higher and the metasediments are predominantly pelitic. Intense deforma- tion and thorough recrystallization have largely destroyed primary textures and have produced a pervasive schistosity. The schist and phyllite of the Greenschist Unit are readily distinguished from those of the Blueschist Unit by the conspicuous por- phyroblasts of clinozoisite and epidote in mafic schist and of albite, almandine gar- net, and, locally, biotite in the metasedi- ments. The critical blueschist-facies miner- als, lawsonite, aragonite, and jadeitic pyroxene are absent, but glaucophane and crossite are locally abundant in mafic schist (Fig. 4). Crossite and biotite occur locally in equilibrium association in metachert (Piatt, 1975). Piemontite was reported from Greenschist Unit metachert by Bailey (1940). An episode of retrograde metamorphism, accompanied by a second deformation, caused chloritization of biotite and garnet, renewed growth of albite and other lower greenschist-facies minerals, and local de- velopment of pumpellyite. The structure has been complicated further by two post- metamorphic periods of folding. Greenschist-Blueschist Thrust. The con- tact between the Blueschist and Greenschist Units is regionally subhorizontal (Fig. 3), with the Greenschist Unit on top. Experi- mentally determined phase relations (Fig. 5) suggest that the two units were meta- morphosed under distinctly different condi- tions, and neither unit has been affected by the conditions attending the metamorphism of the other. The contact is therefore a postmetamorphic fault, which I name the Greenschist-Blueschist thrust. Based on the Figure 1. Generalized geologic map of California, showing the distribution of the principal late outcrop pattern of the Greenschist and Mesozoic tectonic elements. Modified from U.S. Geological Survey (1966) and Bailey and others Blueschist Units, displacement must exceed (1970, Fig. 5). CRT: Coast Range thrust. NF: Sur-Nacimiento fault zone. 9 km. The thrust is a zone locally up to 200 m The Catalina Amphibolite Unit overlies originally have been a body of differen- thick, filled with tectonic blocks of both the Blueschist and Greenschist Units in tiated gabbro. The semipelitic schist con- amphibolite-facies rocks and serpentinite in the center of the island (Figs. 2, 3). It con- tains quartz -I- plagioclase + garnet + bio- a matrix of talc-chlorite-actinolite schist. sists largely of green hornblende—zoisite tite -I- muscovite + kyanite + zoisite; no Many of these blocks are massive schist, with minor brown hornblende- primary textures remain, but the aluminous garnet—brown hornblende rocks, identical garnet schist, semipelitic schist, and garnet composition indicated by the mineralogy to garnet amphibolite
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