Alternate origins of the Coast Range Ophiolite (California); introduction and implications

Abstract

Correctly interpreting the tectonic evolution of the California continental margin requires understanding the origin of the Jurassic Coast Range Ophiolite, which represents a fragment of mafic-to-ultramafic crust of oceanic character lying depositionally beneath the western flank of the Great Valley forearc basin in fault contact with the Franciscan subduction complex of the California Coast Ranges. Three contrasting hypotheses for genesis of the ophiolite as seafloor are each based on internally consistent logic within the framework of plate tectonics, but are mutually exclusive and lead to strikingly different interpretations of regional tectonic relations, even though each assumes that the Sierra Nevada batholith to the east represents the eroded roots of a magmatic arc linked to subduction along the Mesozoic continental margin. To encourage the further work or analysis needed to develop a definitive interpretation, summary arguments for each hypothesis of Coast Range Ophiolite genesis in mid- to late Jurassic time are presented in parallel: (1) backarc spreading behind an east-facing intraoceanic island arc that then collided and amalgamated with the Sierran continental- margin arc; (2) paleoequatorial midocean spreading to form oceanic lithosphere that was then drawn northward toward a subduction zone in front of the Sierran continental-margin arc; and (3) forearc spreading within the forearc region of the Sierran continental-margin arc in response to transtensional deformation during slab rollback.

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