Sierra Nevada Aquatic Research Laboratory

01 Jul, 05 AM - 06 Jul, 03 AM

This work is part of a Mildred E. Mathias Grant-funded proposal to look at the evolution of the Hilton Creek fault adjacent to SNARL. An abstract of the project follows below: The Hilton Creek fault (HCF) is a prominent range-front normal fault bounding Long Valley caldera to the mountains of the southern Sierra Block. The 760 ka eruption of the Bishop Tuff is hypothesized to have occurred along the HCF, though little is known about the fault?s long term history of slip and its relationship to the magmatic evolution of Long Valley caldera. Here we use geomorphologic and thermochronologic methods to better constrain motion on the HCF over pre-caldera and post-caldera timescales. A suite of well-preserved shorelines line the east rim of Long Valley caldera and are visibly downwarped toward the Hilton Creek fault. Preliminary analysis of shoreline elevations measured from a 5 m NASA TOPSAR DEM reveals that tilting occurred after emplacement of the youngest shoreline (~200 ka). This ~500 kyr gap in normal faulting in Long Valley Caldera during a period of prolonged regional extension suggests a dynamic relationship between regional tectonics, magma chamber evolution, and crustal rheology in Long Valley caldera that is at present poorly understood. To examine the pre-caldera slip history, we employ apatite (U-Th)/He and fission track thermochronology. Establishing constraints on motion of the Hilton Creek fault over million-year timescales will: 1) provide a baseline for determining the initial boundary conditions of Long Valley before the eruption of the Bishop Tuff and formation of its caldera; 2) provide a long-term, integrated slip rate that can be compared to other primary faults in the Eastern California Shear Zone; 3) allow comparison with slip rates measured over much shorter timescales and 4) paired with geomorphic observations of a temporal gap in intra-caldera extensional tilting, help toward understanding the influence of magma intrusion on the rheology of the fault and the associated tectonic evolution of Long Valley caldera.

Visit #29009 @Sierra Nevada Aquatic Research Laboratory

Approved

Under Project # 21625 | Research

Late Pleistocene Surface Deformation at Long Valley Caldera

faculty - University of California, Santa Cruz

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Group of 3 Graduate Student Jul 1 - 6, 2012 (6 days)

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