20 Sierra Nevada Microplate-Basement and Basins I

Friday, 28 May 2010
8:30 AM - 12:00 PM
Platinum 2

MID-CRETACEOUS REGI0NAL EXHUMATION OF THE SIERRA NEVADA-GREAT VALLEY BATHOLITH AND A POSSIBLE TECTONIC DRIVING MECHANISM

Paper number: 20-3
Program: Theme Sessions
Time: 9:05 AM – 9:25 AM

Authors

Abstract

Petrographic and select geochronological data on ~300 basement cores from the Great Valley (GV) indicate that at least the eastern half of the GV is underlain by Early Cretaceous batholithic plutons with pendants of Foothills metamorphic belt-type rocks. The buried plutons yield U/Pb zircon ages with a range of 140-115 Ma, and were emplaced at ~3-4 kb. Forearc basin strata nonconformably above this westernmost batholithic zone only reach back to Coniacian-Campanian (88-80 Ma). Igneous barometric data for the Sierra Nevada batholith (SNB) typically show 1-2 kb more exhumation along the western Early versus eastern Late Cretaceous zones, with western zone exhumation comparable to that of the GV basement. New He zircon data from the SNB show rapid exhumation of Early Cretaceous plutons at 92 to 88 Ma, followed by slow exhumation (~0.06 mm/yr) continuing to a similar rate through much of Cenozoic time as recorded by He apatite. The GV batholith and western to axial SNB shared a common mid-Cretaceous exhumation event. The southern ~100 km of the SNB has a steep longitudinal exhumation gradient whereby ~11 kb level plutons are exposed in the extreme south. Exhumation occurred very rapidly at 95-80 Ma (Ar hornblende through He zircon), involving first rapid erosion followed by large magnitude extensional faulting. Much of the southernmost SNB upper crust is dispersed as detachment sheets along the Garlock and San Andreas faults. An updated plate kinematic model for the Pacific-Farallon ridge and the Hess-Shatsky Rise and conjugate rise system, complimented by inverse tomography of subducted Farallon plate seismic velocity anomalies of the mid-mantle beneath the eastern U.S. track the Shatsky conjugate with impact of its main massif along southernmost SNB-Mojave region at 95-90 Ma. Shallow flat subduction of the massif correlates with the disruption and uplift of this arc segment, followed by large magnitude extension as the back edge of the massif subducted. Inspection of bathymetric data around the Shatsky Rise reveals broad shoulders, particularly off its northern flank. We posit that the subduction of the northern conjugate shoulder induced enough of a slab flattening component to drive the regional GV-SNB exhumation event.