Late Cretaceous to early Neogene tectonic development of the southern Sierra Nevada region, California

Abstract

The Sierra Nevada batholith is an ~600-km-long, NNW-trending continental arc generally exposed from epizonal to mesozonal levels and showing a distinct strike-perpendicular zonation in structural, lithologic, petrologic, geochronologic, and isotopic patterns. South of 35.5° N, in the southern Sierra Nevada–northern Mojave Desert region, the depth of exposure increases markedly and a tectonostratigraphy consisting of three distinct, fault-bounded assemblages is observed. From high to low structural levels, these units are (1) fragments of shallow-level eastern Sierra Nevada batholith affinity rocks, (2) deeper-level western to axial zone rocks, and (3) subduction accretion assemblages (e.g., the Rand schist). This multi-tiered core complex is the product of shallow subduction that occurred over ~500 km of the plate margin in Late Cretaceous time. Slab shallowing was accompanied by intense contractile deformation within the crust and along the subduction megathrust; crustal thickening, uplift, and denudation of the residual arc to midcrustal levels; removal of the forearc and frontal arc by subduction erosion; and replacement of sub-batholithic mantle with underplated subduction assemblages. As the slab reverted to a "normal" trajectory, previously thickened crust no longer compensated at depth by a shallowly dipping slab became gravitationally unstable and underwent a profound phase of extensional collapse. Two subparallel shear zones, one separating assemblages 1 and 2 (the southern Sierra detachment) and the other juxtaposing units 2 and 3 (the Rand fault), comprise an integrated Late Cretaceous detachment system that accommodated extensional collapse. These Late Cretaceous events preconditioned the southern California crust for imprints of subsequent tectonic regimes. For example, subduction of the Pacific-Farallon slab window in early Neogene time created an extensional stress regime in the overriding plate, facilitating high-angle normal faulting across the previously extended region and volcanism associated with upwelling astheno-spheric material. The invasion of hot and buoyant asthenosphere destabilized dense sub-batholithic root material still affixed beneath the central Sierra Nevada batholith, leading to Pliocene–Quaternary delamination of the high-density rocks. Replacement of dense sub-batholithic root materials with asthenosphere has led to ~1 km of uplift across the southern Sierra Nevada and into the eastern San Joaquin Basin. The purpose of this trip is to highlight structural and petrologic records of multiple phases of tectonism in the southern Sierra Nevada–Mojave Desert region, illustrating the profound and lasting effect that shallow subduction may have on a continental margin.

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