HR: 1340h
AN: T13A-1335    [Abstracts]
TI: Geologic structure of Middle Mountain within the San Andreas Fault zone near Parkfield, California
AU: * Thayer, M R
EM: maurits@asu.edu
AF: Arizona State University, P.O. Box 871404, Tempe, AZ 85287 United States
AU: Arrowsmith, R
EM: ramon.arrowsmith@asu.edu
AF: Arizona State University, P.O. Box 871404, Tempe, AZ 85287 United States
AU: Young, J
EM: jeri.young@asu.edu
AF: Arizona State University, P.O. Box 871404, Tempe, AZ 85287 United States
AU: Fayon, A
EM: fayon001@umn.edu
AF: University of Minnesotan Duluth, Heller Hall 229 1114 Kirby Drive , Duluth, MN 55812 United States
AU: Rymer, M
EM: mrymer@usgs.gov
AF: United States Geological Survey, 345 Middlefield Road , Menlo Park, CA 94025 United States
AB: Knowledge of the geometry and history of motion of rock bodies within fault zones such as the San Andreas fault (SAF) is essential input into mechanical models of earthquake rupture dynamics and fault evolution. The Parkfield segment of the SAF is the focus of significant geophysical characterization and borehole studies. In order to enhance the geologic information about the SAF structure in this area, we undertook an intensive high-resolution geologic mapping effort (1:6000 scale) of the Middle Mountain area (about 40 km^2). The geologic structure differs dramatically across the San Andreas fault zone. The northeast side contains numerous sub-parallel faults that likely accommodated significant strike slip motion. These high-angle faults bound granite, marble, and sedimentary rock slivers. The density and complexity of these faults increases toward the center of the fault zone. The Gold Hill reverse fault on the northeast side of the SAF is a low-angle southwest-dipping fault that locally displaces the older Tertiary Monterey Formation over the younger Tertiary Etchegoin Formation. Folds with axes trending parallel to the strike of the Gold Hill reverse fault are present within the hanging wall. The Plio-Pleistocene Paso Robles Formation dominates the southwest side of the SAF and is a formidable cover. Fault-bounded granitoid slivers are also present within the southwest terrain. One fault striking nearly normal to the SAF cuts rock units near the mid-section of Middle Mountain. To the northwest of this fault, older Tertiary formations are present. The folds within the hanging wall of the Gold Hill reverse fault and the reverse fault itself indicate SAF-normal shortening near the SAF zone. The Gold Hill fault most likely cuts the numerous high-angle sub-parallel faults at depth. With the northeastward-verging nature of this fault, the cross-section on the northeast side is a roughly hewn half-flower structure. The sedimentary basin into which the Paso Robles Formation was deposited has been inverted due to subsequent deformation adjacent to the SAF and now comprises the eroding apex of Middle Mountain. Cross-sections of the southwest side of the fault vary remarkably and increase in complexity to the northwest. In the southeastern half, a pronounced syncline within the Paso Robles Formation trends sub-parallel to the SAF. In the northwestern portion, the steeply dipping Buzzard Canyon fault repeats some Tertiary units. Apatite fission track dating of the granitoid blocks will provide us with the exhumation history of the basement blocks and thus an indication of their motion and vertical offset within the fault zone. Intensive field mapping is the essential tool that allows scientists to link varying thicknesses and other intrinsic characteristics of a fault zone to the mechanical properties of the actual rock bodies. This high-resolution study significantly constrains the geology and geometry of the crustal blocks of this area and allows us to better understand the tectonic history of the Middle Mountain uplift feature. This in turn provides us with a greater knowledge of how mechanical properties of these blocks influence the observed geophysical and geodetic properties within the area.
DE: 8180 Tomography
DE: 7200 SEISMOLOGY
DE: 7205 Continental crust (1242)
DE: 1219 Local gravity anomalies and crustal structure
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting