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Concepts Developed in the Franciscan Complex, California Geological Society of America Special Paper 338 1999 Subduction and the rock record: Concepts developed in the Franciscan Complex, California John Wakabayashi 1329 Sheridan Lane, Hayward, California 94544; e-mail: [email protected]. INTRODUCTION and pointed out the far-traveled nature of some Franciscan Com- plex rocks. Hsü (1968, 1971) formalized the concept and princi- Ernst’s (1970) paper was chosen as the classic paper for the ples of melange (Bailey et al., 1964, had recognized the Franciscan Complex because it related high-pressure, low-tem- shear-zone character of what were later called melanges). Dick- perature (high P-T) metamorphism to subduction. Perhaps most inson (1970) placed the Franciscan in the context of an arc-trench significantly, the paper explained the association of low geother- system, with the Franciscan, Great Valley Group, and Sierra mal gradients and the metamorphism. The paper also pointed out Nevada as the subduction complex, forearc basin, magmatic arc the difficult tectonic problem of the exhumation of the high P-T (and main terrigenous sediment source), respectively. Bailey et rocks, a problem still vigorously debated today, and proposed a al. (1964) set the stage for these papers by compiling and evalu- tectonic model explaining the exhumation of the deeply buried ating an enormous amount of data and presenting ideas that fore- rocks. In addition, the paper explained the tectonic contact of the cast the plate tectonic interpretation of the Franciscan; this is still Great Valley forearc over the Franciscan subduction complex in a useful reference on the Franciscan. the context of plate tectonics theory (Hamilton, 1969, gave a sim- Many major conclusions of these landmark papers have not ilar explanation; see following). Ernst’s paper was one of the key been significantly challenged since their publication. Subsequent advances in the plate tectonics revolution. Variations of Figure 3 research has continued to provide insight into fundamental of Ernst (1970) have become the textbook model of subduction- processes in subduction zones. Some developments in Francis- zone metamorphism. Geologists now regard high P-T (including can geology since 1970, as well as major controversies, are dis- blueschist facies) metamorphism as the strongest evidence of an cussed in the following. The general geology of the Franciscan exhumed subduction complex. Evaluation of thermal gradients Complex is shown in Figure 1. associated with subduction and their connection to metamorphic assemblages, introduced by Ernst (1970), has become an impor- DEVELOPMENTS IN FRANCISCAN GEOLOGY tant concept in understanding the evolution of orogenic belts SINCE 1970 (e.g., Ernst, 1975, 1988). An example of this type of analysis is the premise that Franciscan subduction was continuous, from its Reexamination of the tectonic boundary between the Fran- inception in the late Mesozoic to conversion to a transform plate ciscan Complex and the Coast Range ophiolite and/or Great boundary in the late Cenozoic, because Franciscan high P-T Valley Group rocks lack thermal overprints (such as late greenschist facies assemblages) which should have resulted from any cessation of Various studies led to the formulation of a general model subduction (Cloos and Dumitru, 1987; Ernst, 1988). for the arc-trench system, in which a forearc basin (the Great Ernst’s (1970) paper was one of several key papers that Valley Group) and its basement (the Coast Range ophiolite) related the Franciscan Complex to subduction processes and rode passively and undeformed on the upper plate of the arc- established the Franciscan as the type subduction complex. trench system, while the Franciscan Complex was complexly Hamilton (1969) equated the Franciscan to a subduction com- deformed structurally beneath as the subduction complex plex, related subduction to arc volcanism in the Sierra Nevada, (e.g., Dickinson and Seely, 1979). Recognition of east-vergent Wakabayashi, J., 1999, Subduction and the rock record: Concepts developed in the Franciscan Complex, California, in Moores, E. M., Sloan, D., and Stout, D. L., eds., Classic Cordilleran Concepts: A View from California: Boulder, Colorado, Geological Society of America Special Paper 338. 123 124 J. Wakabayashi 125˚ W 124˚ 123˚ 122˚ 121˚ LEGEND MODOC Franciscan Complex fsch blueschist grade, jadeite rare or lacking in sandstones, not schist PLATEAU blueschist grade, mostly sandstone and shales, blueschist grade, 41˚N KLAMATH 41˚ jadeite common, not schist MTNS schist, blueschist or higher grade: csch grain size exceeds 0.5 mm; fsch grain size commonly less than 0.3 mm Mendocino GO Triple Fp prehnite pumpellyite grade, not schist Fp/b prehnite pumpellyite or blueschist grade, not schist Jct. Fp C O A S T Fz zeolite grade 40˚ A 40˚ fsch Other rock units and faults Fp? GO Great Valley Group and/or Coast Range ophiolite M Fig. 2A Ksb Salinian block KJN Nacimiento block Ca Calaveras fault G Greenville fault fsch SIERRA GV Green Valley fault H Hayward fault SA Fz M Maacama fault SA San Andreas fault GO 39˚ Fp 39˚ SGH San Gregorio-Hosgri fault P a c i f i c E H-RC Healdsburg-Rodgers Creek fault Fp CENTRAL VALLEY A transects for which Franciscan nappe columns are shown in csch Figure 4 GO GO D H-RC SA Fp GO approximate western limit of subduction complex rocks GV Fz? csch 0 100 km 38˚ Ksb H 38˚ San G O c e a n GO NEVADA Francisco 120˚ 119˚ Ca Fp F GO 124˚ Fb B R A N G E S S A Fig. 2B 37˚ GO 37˚ G ▲ SGH C csch Ksb? csch csch Fp/b Ksb 123˚ GO 36˚ H 36˚ Fp/b N KJN 122˚ 121˚ 120˚ 119˚ Figure 1. Distribution of Franciscan and other basement rocks of central and northern California, show- ing Franciscan Complex rocks of different metamorphic grades. Map derived from Jennings (1977) with modifications from Wakabayashi unpublished field data. structures, comprising part of a tectonic wedge system that similar relationships in other high P-T belts. Wakabayashi and faulted the Great Valley Group and Coast Range ophiolite, and Unruh (1995) proposed that normal slip took place during one locally placed Franciscan Complex rocks over Great Valley period of time along the ophiolite–Franciscan Complex contact Group rocks, greatly complicated this model (Wentworth et al., and that tectonic wedging (and periodic thrust faulting along the 1984) (Fig. 2). contact) were operative during other periods (Fig. 2). Godfrey et The tectonic contact of the comparatively unmetamorphosed al. (1997) presented evidence for a buried ophiolitic suture Great Valley Group and Coast Range ophiolite with the struc- beneath the Great Valley Group that may be a consequence of an turally underlying Franciscan Complex was initially called the east-vergent collision event similar to that originally proposed by Coast Range thrust (Bailey et al., 1970). This nomenclature Moores (1970; see Moores and others, this volume) (Fig. 2A). reflected the recognition of the tectonic contact as a subduction boundary (Hamilton, 1969; Ernst, 1970). However, Suppe (1973) Metamorphism noted that the contrast of metamorphic assemblages (low-P rocks on high-P rocks) across this contact was consistent with normal Franciscan metamorphic rocks occur as tectonic blocks in rather than reverse slip. Neogene normal fault movement melange and as intact units or thrust sheets (e.g., Bailey et al., between the Great Valley Group and Franciscan was proposed by 1964). Intact (termed coherent) Franciscan units range from Ernst (1970). Platt (1986) indicated that the metamorphic con- blueschist-greenschist transition assemblages to zeolite facies trast across the tectonic contact implied normal slip, and noted (e.g., Blake et al., 1988) (Figs. 1 and 2), and grain sizes of meta- Subduction and the rock record: Concepts developed in the Franciscan Complex, California 125 fsch Central 1989; Wakabayashi, 1990; Krogh et al., 1994). Petrologic studies km Fb O Valley show that the P-T conditions of these blocks evolved progres- 0 G Fz Fp G sively from high to low geothermal gradients, probably in less 10 O Fp (inferred) SN than 5 m.y. (Wakabayashi, 1990) (Fig. 3). This type of thermal 20 fsch Fp/b evolution has been suggested to be the result of metamorphism at 30 Oceanic CrustFb/j Mantle the inception of Franciscan subduction (Wakabayashi, 1990) 40 A 40 80 120 160 (Fig. 3). Alternatively, high-grade blocks may have been meta- 50 morphosed prior to Franciscan subduction (e.g., Coleman and Fbj csch Central km Lanphere, 1971; Moore, 1984). Fb GO csch GO Fb GO Valley 0 Coherent rocks are part of an inverted metamorphic gradient Fz 10 Fp Fp (inferred) csch from zeolite to blueschist-greenschist grade (e.g., Blake et al., 1988; Fbj SN/O/M 20 Fb/j inverted metamorphic gradient first recognized by Blake, 1967). Oceanic Crust Fp/b 30 Thrust faults apparently separate the rocks of different metamorphic 40 B 40 80 120 grade, but the metamorphic gradient may record changing thermal 50 conditions within the subduction zone, and the structurally higher Figure 2. Schematic cross sections along lines shown in Figure 1. San parts of the metamorphic gradient reflect the thermal influence and Andreas fault system dextral slip is restored according to Wakabayashi proximity of the hanging wall of the subduction zone (e.g., Ernst, and Hengesh (1995), and subsurface relations are restored to hypotheti- 1971; Platt, 1975; Suppe and Foland, 1978; Cloos, 1985; Peacock, cal geometry immediately prior to conversion to transform margin. 1988) (Fig. 3). The P-T paths of representative high-grade blocks Cross-section A is modified from Wakabayashi and Unruh (1995) and Godfrey et al. (1997). Abbreviations as in Figure 1, except: G—Great and coherent metamorphic rocks are shown in Figure 3. Valley Group; O—Coast Range ophiolite; SN/O/M—undifferentiated Sierran basement, ophiolitic rocks, and mantle beneath ophiolitic rocks; Melanges Fb/j, Fp/b designations reflect higher grade of metamorphism for deeper parts of equivalent unit.
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