THE SIERRA NEVADA EARTHSCOPE PROJECT: MOTIVIATIONS, STATUS, AND EARLY RESULTS

Our understanding of the geodynamics of the Sierra Nevada has been in flux since we finally moved away from the idea that it was a range standing high on a Mesozoic crustal root. The discovery that the root does not exist, combined with recognition that the material under the Moho has changed since 10 Ma in at least some areas of the Sierra, has led us to be interested in how mafic material is removed from beneath the batholith and what the consequences of this removal are. The Sierra Nevada EarthScope Project (SNEP) seeks to determine the extent of removal of garnet pyroxenites from under the range and the seismological and structural signatures of the removal. The core of the experiment is the first deployment of broadband seismometers of EarthScope's FlexArray facility. Some 50 seismometers were deployed in summer 2005 from Kings Canyon National Park north to near Lake Tahoe and from the western foothills east into Nevada. These seismometers will remain in the field until late summer 2006 and will then be reconfigured in a second phase lasting to fall 2007. From this we hope to better understand the role of this foundering event in the history of the Sierra Nevada and to provide better constraints on the dynamics of lithospheric foundering. For instance, some of us have proposed that the foundering occurred under a limited area and thus cannot be the cause for Sierra-wide events, while others of us have suggested that the foundering is Sierra-wide and responsible for a number of tectonic events in the past 5 My. On a broader front, if we find that foundering material used to underlie a region of vastly different shape in map view than the presumed downgoing body under the southern Great Valley, then delamination is not a viable mechanism, and if it is of different strike-length, then existing 2-D simulations of downwelling cannot capture the full range of deformation of these systems. Some initial travel time variations will be presented to illustrate the kinds of data being acquired.