Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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Chronology of tectonic and landscape evolution of the southern Sierra Nevada Foothills-eastern San Joaquin Basin transition, CA

Abstract

An extensive range front bedrock pediment landscape crops out along the southwest Sierra Nevada Foothills-eastern San Joaquin Basin transition, CA and extends an unknown distance into the basin as the basement nonconformity. Apatite (U-Th)/He thermochronometric data from a 150 km long roughly horizontal transect sampling plutonic rocks along this bedrock pediment landscape constrain the timing of cooling through apatite closure (∼65 degrees C) of the swath of the composite Sierra Nevada Batholith currently exposed at the surface to be Late Cretaceous (75 +/- 7 Ma). At the northern and southern termini of this study area the modern bedrock landscape maps into lateral continuity (+/- 10's of meters) with the basal Cenozoic nonconformity where mid Eocene sedimentary rocks and volcanic deposits (40.1 +/- 0.3 Ma) overlie plutonic basement. Assuming no major transverse faults cut the study area, the modern bedrock landscape in the study area exposes a swath of the composite Sierra Nevada Batholith which is isostructural with the batholithic rocks directly subjacent to the mid-Eocene nonconformity at its northern and southern termini. Thus the rocks exposed along the study area were at earth's surface (+/- 10's of meters) by mid-Eocene time.

This analysis constrains the following chronology of post magmatic tectonics and landscape evolution: Phase 1) deep initial batholith exhumation by ca. 75 Ma; Phase 2) Late Cretaceous to mid-Eocene complete exhumation to Earth's surface; Phase 3) burial by mid Cenozoic strata; and Phase 4) Plio-Pleistocene final exhumation of the pre mid-Eocene bedrock landscape as the modern foothills-basin transition. We pursue the details of each phase of this chronology by utilizing multiple applications of the (U-Th)/He chronometer as well as field, sub-surface and GIS based mapping techniques. We find that 1) Late Cretaceous deep exhumation was rapid, probably tectonic, and roughly contemporaneous with profound tectonic exhumation of the southernmost Sierra Nevada/Mojave region (Chapman et al, 2012), 2) pre mid-Eocene landscape evolution is preserved as remnant elements of an early Paleogene tropical weathering regime and bedrock pediment geomorphology, and 3) Cenozoic epeirogeny and faulting are recorded by 3D fluctuations in the relict pre-Eocene bedrock landscape due to the ongoing mantle lithospheric foundering event (Saleeby and Foster, 2004).