Island-arc evolution and fracture-zone tectonics in the Mesozoic Sierra Nevada, California, and implications for transform offset of the Sierran/Klamath convergent margins

Abstract

In the foothills of the northern Sierra Nevada of California, an early Mesozoic island-arc terrane, composed of Jurassic plutonic and volcanic rocks of the Smartville, Slate Creek, and Lake Combie complexes, tectonically overlies a late Paleozoic to early Mesozoic melange with a continental margin affinity. Fragments of this N/S-trending arc terrane are bounded to the east by E-directed thrust faults that are in turn cut by E-dipping folds and reverse faults, and to the north by the E/W-trending Big Bend fault zone. Triassic to lower Jurassic amphibolite-grade mafic-ultramafic rocks within this fault zone form discontinuous and dismembered exposures of an ophiolite complex that demonstrates an origin within an oceanic fracture zone. This fracture zone is interpreted as part of a tectonic boundary at the northern edge of the early Mesozoic island-arc terrane. Eastward thrusting and accretion of the arc complex onto the continental margin may have occurred around Middle Jurassic time, as suggested by a 165 ± 2 Ma stitching pluton. The presence of Upper Jurassic dikes and plutons that intrude the Smartville complex indicates a post-accretion intrusive event and in situ rifting within the arc. This Late Jurassic intra-arc rifting and magmatism in the northwestern Sierra Nevada was coeval with development of the Josephine back-arc basin and its ophiolite in the western Klamath Mountains, suggesting a period of regional extension and associated magmatism in the western United States. The regional extension was followed by a contractional episode, known as the Late Jurassic Nevadan orogeny, during which the Upper Jurassic and older rocks in both regions were deformed. The nature and chronologyof early Mesozoic tectonic and magmatic events in the northern Sierra Nevada are similar to those documented in the Klamath Mountains, indicating a common tectonic history for both regions. We suggest that a trench/trench/transform-fault plate boundary may have separated the early Mesozoic arc-trench systems of the northern Sierran and western Klamath provinces.