2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 3
Presentation Time: 8:45 AM


ZANDT, George and GILBERT, Hersh J., Department of Geosciences, Univ of Arizona, Gould-Simpson Building 1040 E. 4th Street, Tucson, AZ 85721, zandt@geo.arizona.edu

A major goal in Cordilleran tectonics is to understand the processes of the construction and destruction of giant continental arc batholiths. In southern California and Baja Mexico, large portions of three composite batholiths were constructed during a Late Cretaceous magmatic flareup: the Sierra Nevada, Mojave/Salinian, and the Peninsular Ranges batholiths. Despite their similar origins, these three batholiths have undergone distinct tectonic histories during Cenozoic time. Recent interdisciplinary studies have suggested that the different Cenozoic histories of these batholiths may be controlled, in part, by the segmentation of the flat Laramide slab and the different fates of their ultramafic batholithic roots. The Sierran batholith has remained a relatively coherent and strong block within the widely extended western United States. The Mojave/Salinian batholith was disaggregated into myriad pieces and sliced in two by the Cenozoic San Andreas transform system. And, the Peninsular Ranges batholith has remained relatively intact but has been rifted from the North American plate and accreted to the Pacific plate.

Utilizing seismic techniques that isolate P-to-s converted phases, we investigated the composition, fabric, and structure of each batholith and relate the seismic attributes to specific processes of batholith root removal and subsequent extensional tectonics. Results from both the southern Sierra Nevada and northern Peninsular Ranges show a large-amplitude Moho converted phase consistent with a sharp boundary, which may have resulted from root removal. Similarities in crustal structures between these two regions may reflect the comparable tectonic histories of each batholith and their relations to adjacent extensional provinces. Differently, portions of the Mojave terrane are characterized by extremely complex seismic signals that may be diagnostic of anisotropy resulting from an underlying subducted and accreted schist terrane. While it appears that similar crustal structures suggest similar tectonic development of both the Peninsular Range and Sierran batholiths, the different appearance of the Mojave province suggest that it has undergone a significantly different tectonic history.