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 Franciscan Complex 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. Franciscan Complex tectonic history can be summarized as follows: (1) East-dipping Francistan subduction initiated beneath, and shortly after formation of the Coast Range ophiolite. (2) High-temperature precursors to Franciscan high-grade tectonic blocks formed as a dynamothermal aureole during subduction initiation beneath the hot hanging wall and were underplated to 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 decreased with 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 underplated or offscraped during 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.
Published Version
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