RESOLVING THE CHARACTER OF THE CRYPTIC CRUST OF THE WESTERN SIERRA NEVADA, CALIFORNIA

Although seismology has been a key discipline in understanding the modern framework of the Sierra Nevada, application of seismology alone revealed a dilemma. The 1993 active source experiment traversing the Sierra (with Randy Keller as a participant) showed that the crust was thicker under the western foothills than under the higher eastern part of the range, a sort of anti-Airy situation confirmed by receiver function studies. The confusion is further compounded by teleseismic body waves, which arrive earlier in the foothills and later in the High Sierra, more in keeping with a thicker crust to the east than to the west. The identity of this material requires more information. Here we review constraints from the surface geology and gravity to try and understand the geology of this material and place it within the context of the evolution of the Sierra Nevada. Using seismological models derived in part from surface waves and teleseismic body waves, we have shown that the crust of the western Sierra probably has a temperature-corrected density approaching 3.0 g/cc; thus the apparent conflict between the thick crust, early teleseismic arrivals and low elevation of the western foothills is largely resolved by this crust being unusually thick, dense, and high-wavespeed. There are still puzzles in the area, though. The original observations of a thick Sierran crust, made by Perry Byerly in the 1930s, have yet to be fully explained. These were observations of delayed arrivals of regional (Pn) waves from earthquakes west of the Sierra observed on the eastern side of the range. Waveform simulations of these waves with the seismic structure used in the density and gravity analysis fails to reproduce the observed delays. Intriguingly, published magnetotelluric work and unpublished seismic attenuation results suggest that part of the lower crust under the western Sierran foothills is warm and not cold. The geometry of these results resembles a westward pointing wedge of warm material; this might be a product of ongoing deformation or, perhaps, some poorly understood characteristic dating to the Mesozoic. Resolving this structure and interpreting its tectonic implications will continue to require the use of multiple kinds of geophysical and geologic information.