Temporal and tectonic relations of early Mesozoic arc magmatism, southern Sierra Nevada, California

Abstract

Early Mesozoic arc magmatism of the southern Sierra Nevada region records the onset of plate convergence–driven magmatism resulting from subduction initiation near the end of Permian time along a prior transform margin. We provisionally adopt the term California-Coahuila transform for this complex boundary transform system, which bounded the southwest margin of the Cordilleran passive margin, its offshore marginal basin, and fringing island arc. In Pennsylvanian–Early Permian time, this transform cut into the arc-marginal basin and adjacent shelf system, calved off a series of strike-slip ribbons, and transported them differentially southward through ∼500–1000-km-scale sinistral displacements. These strike-slip ribbons constitute the principal Neoproterozoic–Paleozoic metamorphic framework terranes for the superposed Mesozoic batholithic belt in the Sierra Nevada and Mojave plateau regions. The southern Sierra Nevada batholith intruded along the transform truncation zone where marginal basin ribbons were juxtaposed against the truncated shelf. Strike-slip ribbons, or blocks, liberated from the truncated shelf occur today as the Caborca block in northwest Mexico, and possibly parts of the Chortis block, farther south. The oldest arc plutons in the Sierra region were emplaced between 256 and 248 Ma, which matches well with ca. 255 Ma high-pressure metamorphism recorded in the western Sierra Foothills ophiolite belt, interpreted to approximate the time of subduction initiation. The initial phases of arc plutonism were accompanied by regional transpressive fold-and-thrust deformation, kinematically marking the transition from transform to oblique convergent plate motion. Early arc volcanism is sparsely recorded owing to fold-and-thrust–driven exhumation having accompanied the early phases of arc activity. By Late Triassic time, the volcanic record became quite prolific, owing to regional subsidence of the arc into marine conditions, and the ponding of volcanics in a regional arc graben system. The arc graben system is but one mark of regional suprasubduction-zone extension that affected the early SW Cordilleran convergent margin from Late Triassic to early Middle Jurassic time. We interpret this extension to have been a dynamic consequence of the subduction of exceptionally aged Panthalassa abyssal lithosphere, which is well represented in the Foothills ophiolite belt and other ophiolitic remnants of the SW Cordillera. Middle and Late Jurassic time was characterized by important tangential displacements along the SW Cordilleran convergent margin. In Middle Jurassic time, dextral impingement of the Insular superterrane intra-oceanic arc drove a migrating welt of transpressional deformation through the SW Cordillera while the superterrane was en route to its Pacific Northwest accretionary site. Dextral transtensional spreading in the wake of the obliquely colliding and translating arc opened the Coast Range and Josephine ophiolite basins. In Late Jurassic time, a northwestward acceleration in the absolute motion of the North American plate resulted in an ∼15 m.y. period of profound sinistral shear along the Cordilleran convergent margin. This shear is recorded in the southern Cordillera by the Mojave-Sonora megashear system. Late Jurassic intrusive units of the southern Sierra region record sinistral shear during their magmatic emplacement, but we have not observed evidence for major Late Jurassic sinistral displacements having run through the Sierran framework. Possible displacements related to the megashear in the California to Washington regions are likely to have: (1) followed preexisting transforms in the Coast Range ophiolite basin and (2) been accommodated by oblique closure of the Josephine ophiolite basin, and the northern reaches of the Coast Range ophiolite basin, proximal to the southern Insular superterrane, which in Late ­Jurassic–earliest Cretaceous time was obliquely accreting to the inner Cordillera terranes of the Pacific Northwest.