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Nappes, Tectonics of Oblique Plate Convergence, and Metamorphic Evolution Related to 140 Million Years of Continuous Subduction

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Nappes, Tectonics of Oblique Plate Convergence, and Metamorphic Evolution Related to 140 Million Years of Continuous Subduction Nappes, Tectonics of Oblique Plate Convergence, and Metamorphic Evolution Related to 140 Million Years of Continuous Subduction, Franciscan Complex, California Author(s): John Wakabayashi Source: The Journal of Geology, Vol. 100, No. 1 (Jan., 1992), pp. 19-40 Published by: The University of Chicago Press Stable URL: http://www.jstor.org/stable/30082316 Accessed: 10/04/2010 17:15 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. 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The University of Chicago Press is collaborating with JSTOR to digitize, preserve and extend access to The Journal of Geology. http://www.jstor.org Nappes, Tectonics of Oblique Plate Convergence, and Metamorphic Evolution Related to 140 Million Years of Continuous Subduction, Franciscan Complex, California' John Wakabayashi Earth Sciences Associates Inc., 701 Welch Rd., Palo Alto, CA 94304 ABSTRACT This paper presents a new synthesis of Franciscan Complex tectonics, with the emphasis on the pre-San Andreas fault history of these rocks. Field relations suggest that the Franciscan is characterized by nappe structures that formed during sequential accretion at the trench. The presence of these structures along with other field relations, including the lack of evidence for large offset of conglomerate suites, indicates that strike-slip fault systems of large displacement (>500 km) did not cut the FranciscanComplex during subduction. Regional geology and comparisons to modern arc-trench systems suggest that strike-slip faulting associated with oblique subduction took place inboard (east) of the Franciscan in the vicinity of the magmatic arc. The Franciscan varies along strike, because individual accreted elements (packets of trench sediment, seamounts, etc.) did not extend the full length of the trench. Different depths of underplating, distribution of post-metamorphic faulting, and level of erosion produced the present-day surface distribution of high P/T metamorphism. FranciscanComplex tectonic history can be summarized as follows: (1) East-dippingFranciscan subduction initiated beneath, and shortly after formation of, the Coast Range ophiolite. (2) High-temperatureprecursors to Franciscanhigh-grade tectonic blocks formed as a dynamothermal aureole during subduction initiation beneath the hot hanging wall and were underplatedto the upper plate. (3) Subduction continued, the high-grade metamorphic rocks were overprinted with assemblages of increasing P/T ratio as the hanging wall cooled, and the aureole was dismembered into blocks. (4) As subduction progressed,more material was underplated and metamorphosed as coherent blueschist. Peak metamorphic temperatures of successively subducted units de- creasedwith time as hanging wall heat continued to dissipate. (5) Trench sediments with parts of seamounts, oceanic rises, and other small masses on the downgoing plate were underplatedor offscrapedduring approximately 140 m.y. of continuous subduction, forming stacks of nappes. (6) The Mendocino Triple Junction migrated northward, and the subduction zone was replaced by a transform plate boundary associated with the San Andreas fault. Deformation and faulting related to the Neogene transform tectonic regime obscured many subduction-related structures. Introduction The Franciscan Complex of the California Coast sent fragments of seamounts, other oceanic rises, Ranges is an assemblage of variably deformed and and the pelagic cover and upper part of the sub- metamorphosed rock units that formed as a sub- ducted oceanic crust (e.g., Hamilton 1969; Mac- duction complex associated with east-dipping sub- Pherson 1983; Tarduno et al. 1985), possibly with duction at the western North American plate mar- a component of olistostrome blocks from the upper gin from the late Jurassic through the Tertiary, a plate (MacPherson et al. 1990). period of over 140 m.y. (e.g., Hamilton 1969; Page The Franciscan is well known for high P/T 1981). Franciscan lithologies are predominantly de- metamorphism (e.g., Ernst et al. 1970; Ernst 1970, trital sedimentary rocks with subordinate basaltic 1971). The Coast Range ophiolite structurally over- volcanic rocks and chert, and minor limestone. lies the Franciscan and is depositionally overlain The detrital rocks may represent offscraped and by well-bedded sandstones and shales of the Great underplated trench-fill sediments (e.g., Dickinson Valley sequence coeval with the Franciscan (e.g., 1970). The pelagic and volcanic rocks may repre- Dickinson 1970). The Coast Range ophiolite and Great Valley sequence lack high P/T metamor- 1 Manuscript received May 1, 1991; accepted August 18, phism. From west to east, the three sub-parallel 1991. geologic provinces of the Franciscan, Great Valley [Journalof Geology, 1992, volume 100, p. 19-40] © 1992 by The University of Chicago.All rights reserved.0022/1376/92/10001-003$1.00 19 20 JOHN WAKABAYASHI PickettPk. terrane Yolla Bolly terraneor similar CentralBelt CoastalBelt GVSGreat Valley Sequence Coast Range ophiolite MF Maacamafault CF Calaveras fault HF Haywardfault H-RCF Healdsburg-RodgersCreek fault MFZ MendocinoFracture Zone Figure 1. Generalized map of Franciscan Complex exposures in California. Adapted from Jennings (1977), Blake et al. (1984), and Jayko et al. (1986) and Wakabayashiunpub. data. sequence, and Sierra Nevada batholith (see Figure strike-slip faulting took place within the Francis- 1) may represent respectively the subduction com- can (e.g., McLaughlin et al. 1988, in favor; Seiders plex, forearc basin deposits, and arc of an ancient 1991 against). (2) The structural relationship of arc-trench system (e.g., Dickinson 1970; Ingersoll many Franciscan tectonostratigraphic terranes to 1978). Few dispute this general model, but major one another is poorly understood. (3) Opinions dif- questions and controversy regarding Franciscan fer on whether coarse-grainedblueschist, eclogite, history remain, for example: (1) Disagreement ex- and amphibolite tectonic blocks (called "high- ists over whether or not large-scale (>500 km) grade blocks" or "knockers" in Franciscan litera- Journalof Geology TECTONICS OF OBLIQUE PLATE CONVERGENCE 21 ture) are vestiges of a pre-Franciscanmetamorphic Range; and (4) the Nacimiento block (Sur terrane event (e.g., Coleman and Lanphere 1971; Blake of McWilliams and Howell 1982). et al. 1988) or instead formed during the initial 1) Northern Coast Ranges. In this region, the stages of Franciscan subduction (e.g., Platt 1975; Franciscan has traditionally been subdivided into Suppe and Foland 1978; Cloos 1985; Wakabayashi three NW-SE trending lithologic belts: the Eastern, 1990). Central, and Coastal belts (Berklandet al. 1972). In this paper the above problems and other as- Easternmost and structurally highest, the blue- pects of Franciscan geology will be evaluated in- schist grade Eastern Belt consists of thrust sheets cluding: (1) the occurrence and significance of of metaclastic and metavolcanic rocks with fine- low-angle faults that formed during subduction grained metamorphic minerals generally <0.2 mm and accretion, particularly those separating Fran- in size (Suppe 1973; Worrall 1981; Blake et al. ciscan tectonostratigraphic terranes; (2) obliquity 1988). The belt has been divided into the structur- of plate convergence and its consequences in Fran- ally highest and highest metamorphic grade Pick- ciscan tectonics; (3) metamorphism and its rela- ett Peak terrane, schistose rocks commonly with tionship to tectonic evolution; and (4) aspects of complete textural reconstitution, and the structur- Franciscan accretion that may not fit a "classical" ally lower Yolla Bolly terrane, unfoliated to foli- (e.g., Karigand Sharman 1975) model of accretion, ated rocks with variable preservation of protolith such as tectonic wedging (e.g., Wentworth et al. textures (Blake et al. 1982). The Yolla Bolly terrane 1984) and distribution of high-gradeblocks (a prob- also includes a subunit of shale matrix melange lem that bears on the mechanisms of Franciscan (Worrall 1981; Jayko and Blake 1989). Yolla Bolly melange formation). terrane metaclastic rocks have yielded Tithonian to Cenomanian fossils (Blake et al. 1988). Eastern Belt and similar rocks are referredto as "coherent blueschists" in this paper. Among coherent (non- The Franciscan Complex: Regional Overview melange) Franciscan rock units, only the coherent The present outcrop pattern and structure of Fran- blueschists exhibit penetrative fabrics, with the ciscan rocks has been heavily influenced by Neo- exception of local weakly-developed
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