LOREN A. RAYMOND Department of Geography-Geology, Appalachian State University, Boone, North Carolina 28607 Tesla-Ortigalita Fault, Coast Range Thrust Fault, and Franciscan Metamorphism, Northeastern Diablo Range, California ABSTRACT increase in metamorphic grade toward the thrust, whereas structural burial would result Fault contacts in the northeastern Diablo in an increase in metamorphic grade with struc- Range, California, between the partially tural depth. Distribution of mineral para- melanged late Mesozoic Franciscan Complex geneses in the northeastern Diablo Range, and the broadly coeval, less deformed sedi- revealed by analyses of more than 300 thin mentary rocks of the Great Valley sequence sections of metaclastic rocks, shows no spatial have been called, by definition, the Tesla- relation between highest grade rocks and either Ortigalita fault. "Coast Range thrust" is the exposed segments of the Coast Range thrust name applied by Bailey and others (1970a) to fault or the margins of ultramafic masses under- the fault of regional extent that originally going present-day serpentinization. Thus the separated subducted oceanic crust and sedi- available evidence fails to support the meta- mentary rock of the Franciscan Complex from stable recrystallization, metasomatic, and tec- structurally overlying ophiolite plus shelf-slope tonic overpressure concepts. Only the hypothe- facies sedimentary rock of the Great Valley sis of structural burial is not negated by the sequence. The two faults are not equivalent. observed relations. High-angle Neogene segments of the Tesla- Ortigalita fault truncate older fault surfaces of INTRODUCTION the Coast Range thrust-fault system. During the past decade, field work through- Franciscan metamorphic rocks contain low- out the Coast Ranges of western California has to high-pressure, low-temperature mineral provided evidence of a fault of regional extent parageneses characterized by the phases pum- (Fig. 1). The existence of this fault, recently pellyite, prehnite, aragonite, lawsonite, glau- named the Coast Range thrust by Bailey and cophane, and jadeitic pyroxene. Metamorphism others (1970a), was earlier proposed as a solu- has been variously ascribed to metastable re- tion to the enigmatic juxtaposition of two crystallization, metasomatism by fluids gener- great coeval masses of rock, the Franciscan ated in and around serpentinite, structural Complex1 and the Great Valley sequence burial produced by subduction of an oceanic (Bailey and others, 1964; Irwin, 1964). How- lithospheric plate, and burial plus tectonic ever, the recently developed concept of plate overpressures generated beneath the Coast tectonics (for example, see Isacks and others, Range thrust fault. Suppe (1970) and Ernst 1968; Morgan, 1968) has provided California (1971a) argued against metastable recrystalli- geologists with new insight into the significance zation and metasomatism on the basis of of this fault and the juxtaposed masses of rock. available field and laboratory evidence. Contrasting lithologic, structural, and meta- If metamorphism is related to serpentiniza- morphic characteristics of the Franciscan Com- tion, a metamorphic aureole should surround plex and Great Valley sequence are now ex- ultramafic bodies undergoing present-day ser- 1 Berkland and others (1972) have shown that the pentinization. Metamorphism resulting from Franciscan is adequately described by the term "com- tectonic overpressures generated beneath the plex" defined in the American Code of Stratigraphic Coast Range thrust fault will be revealed by an Nomenclature (1961). Geological Society of America Bulletin, v. 84, p. 3547-3562, 7 figs., November 1973 3547 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/11/3547/3428425/i0016-7606-84-11-3547.pdf by guest on 23 September 2021 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/11/3547/3428425/i0016-7606-84-11-3547.pdf by guest on 23 September 2021 FAULTS AND METAMORPHISM, DIABLO RANGE, CALIFORNIA 3549 plained commonly in terms of a plate-tectonics North America plate at the time of under- model (Ernst, 1970, 1971a; Moores, 1970; thrusting by the Franciscan-bearing oceanic Page, 1970; Hsu, 1971). In this model, the Late plate. Because they were on the upper plate, Jurassic to Eocene2 Franciscan Complex (con- Great Valley sequence rocks were only slightly sisting of a moderately deformed to chaotic to moderately deformed. Mineral parageneses assemblage of graywacke sandstone, dark silt- of the zeolite and prehnite-pumpellyite facies stone and shale, minor conglomerate, chert, occurring at the base of the Great Valley se- minor limestone, mafic volcanic rocks, and quence have been attributed by Dickinson their metamorphic equivalents) is considered and others (1969) to burial metamorphism. to represent ocean-floor and trench rocks, the The Coast Range thrust fault, which is the upper part of a lithospheric slab subducted structural boundary separating the Franciscan (underthrust) and metamorphosed beneath the Complex from the Great Valley sequence, pre- overriding North America plate. Mélanges, sumably formed the plate boundary during characteristic of the Franciscan Complex (Hsu, subduction. Genesis of Franciscan metamor- 1968,1969; Blake, 1970), were probably formed phic rocks has been ascribed to: (1) structural by submarine landsliding in the trench and burial of lower plate rocks below the fault by tectonic mixing during subduction. Region- plane and overlying Great Valley sequence ally metamorphosed terranes within the Fran- (Ernst, 1970, 1971a); (2) tectonic overpres- ciscan Complex contain mineral parageneses of sures developed below the fault plane (Blake the zeolite, prehnite-pumpellyite, and blue- and others, 1967, 1969); (3) metasomatism schist facies indicative of low- to high-pressure (Taliaferro, 1943; Gresens, 1969); and (4) and low-temperature conditions of metamor- metastable recrystallization (Hlabse and Klep- phism (McKee, 1962; Ernst, 1965, 1971a; pa, 1968; see also Ernst, 1971a, for discussion). Blake and others, 1967, 1969; Taylor and Cole- Metasomatism and metastable recrystallization man, 1968; Bailey and Blake, 1969; Ernst and are incompatible with field and laboratory evi- others, 1970; Liou, 1971). These parageneses dence as discussed by Suppe (1970), Ernst are believed to develop as a result of processes (1971a), and this paper. If structural burial operative at consuming plate margins (Ernst, were the controlling factor, metamorphic grade 1970, 1971b; Coleman, 1971; Miyashiro, should increase with depth. If tectonic over- 1971). pressure exercised the primary control, meta- morphic grade should increase upward, toward The juxtaposed Late Jurassic to Paleocene3 the Coast Range thrust fault. On the basis of Great Valley sequence consists of an orderly, preliminary investigations in the Diablo Range slightly to moderately deformed pile of gray- and elsewhere in the Coast Ranges, Blake and wacke to arkosic sandstone, siltstone, shale, others (1967, 1969), Blake and Cotton (1969), and conglomerate that conformably to uncon- Bailey and Blake (1969), and Bailey and others formably overlies various remnants of a Late (1970b) concluded that an "upside-down" Jurassic ophiolite sequence (Moiseyev, 1966, zonation, indicative of tectonic overpressure 1970; Bezore, 1969, 1971; Bailey and others, control, exists. In contrast, recent reconnais- 1970b). In a plate-tectonic model, the sedi- sance in the Diablo Range by Ernst (1971a, mentary rocks represent shelf and slope de- 1971c) suggests that no such relation exists. posits laid down on continental crust, which is The strength of both arguments rests, in part, now exposed in the Sierra Nevada to the east, on the assumption that the distribution of ex- and oceanic crust (the ophiolite) exposed in posed segments of the Coast Range thrust the Coast Ranges. Both of these basement- fault is known. Without this knowledge, a sediment sequences were probably part of the spatial relation between faulting and metamor- phism can be neither demonstrated nor dis- 2 J. C. Kramer and M. O'Day report Paleocene and proved. Yet, as reflected in the generalized Eocene ages for part of the Coastal Belt Franciscan small-scale maps presented by both Blake and (1972, commun.). his colleagues and by Ernst, most of the present s The upper age limit of Paleocene given here for the tectonic contact between the Franciscan Com- Great Valley sequence is required because the Moreno plex and Great Valley sequence is assumed to Formation, the uppermost unit of the Great Valley se- be equivalent to the Coast Range thrust fault. quence exposed in the Diablo Range, ranges in age from It is the purpose of this paper to document the middle Maestrichtian (Late Cretaceous) to Paleocene nature of that tectonic contact in part of the (Payne, 1951). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/11/3547/3428425/i0016-7606-84-11-3547.pdf by guest on 23 September 2021 3550 L. A. RAYMOND northeastern Diablo Range and to examine in Tesla fault by Vickery in 1924. Later, Briggs detail the distribution of Franciscan mevamor- (1953) mapped a thrust 100 km to the south, phic rocks in the area with respect to exposed naming it the Ortigalita thrust. Mapping of ultramafic bodies and segments of the Coast the entire eastern flank of the Diablo Range Range thrust fault. has shown that these and connecting faults form a continuous boundary between Franciscan COAST RANGE THRUST FAULT