HR: 0800h
AN: T21A-0458    [Abstracts]
TI: Fault Zone Structure of Middle Mountain, Central California
AU: Thayer, M
EM: maurits@asu.edu
AF: Department of Geology, Arizona State University, Tempe, AZ 85287 United States
AU: * Arrowsmith, J R
EM: ramon.arrowsmith@asu.edu
AF: Department of Geology, Arizona State University, Tempe, AZ 85287 United States
AB: The Middle Mountain uplift feature in Central California provides an exceptional laboratory in which to study the characteristics of an active fault zone. This complex geological terrain is not distinguished by a single master fault, but rather a 1-3 kilometer wide zone with numerous sub-parallel faults and folds. In order to further understand this deformation, detailed 1:6000 scale geologic field mapping was undertaken during the 2004 and 2005 field seasons in order to establish the shallow fault zone structure. From this information, we can address questions such as the fault geometries, distribution of deformation, fault zone development and migration, and fault strength. Offset along the San Andreas fault zone has juxtaposed contrasting rock units. The northeastern side is Miocene to Pliocene marine sedimentary units and Plio-Pleistocene terrestrial sedimentary units. Dominant structures include several high angle faults striking sub-parallel to the main SAF trace that bound marble, granites, and Tertiary sedimentary units. Fault density increases near the active SAF trace. The Gold Hill fault is a reverse fault of varying southwest dip that surfaces along the eastern margin of Middle Mountain and places the Miocene Monterey shale over Pliocene Etchigoin sandstone. Alternating synclines and anticlines with axes trending parallel to the strike of the Gold Hill fault are present within the hanging and foot walls. An 800 meter thick package of the Plio-Pleistocene Paso Robles Formation dominates the southwestern terrain and is deformed by en echelon folds and secondary faults. Some of the faults strike nearly normal to the SAF, offset Tertiary and Quaternary units, and tend to be northwest-side up. Several SAF-parallel striking faults slice Tertiary and Quaternary sedimentary units and granitic bodies. A fault on the southwestern side of Cholame Creek juxtaposes Tertiary rhyolitic rocks (Pinnacle-Neenach equivalent) against Salinian granite. SAF sub-parallel fault density increases three-fold within the overlying sedimentary units, thus confirming that faults bifurcate near the surface. Folding of the mid-Tertiary units on the northeast side of fault probably results from SAF-normal compression and movement along the Gold Hill reverse fault. The en echelon folding of the Paso Robles formation on the southwest side of the SAF is consistent with simple shear parallel to the main SAF. The transverse faults on the SW-side of the SAF are most likely high-angle normal faults either developed by SAF-parallel simple shear and later rotated to their current position, or possibly created by slip along a bend in the SAF at depth. The fault that offsets rhyolite against granite is a remnant SAF trace, and indicates ~3 km northeastward migration of the SAF trace. However, southwestern units are found on the northeastern side of the active trace, thus indicating the ability of the active trace to jump 10s to 100s of meters back to the southwest.
DE: 8010 Fractures and faults
DE: 8108 Continental tectonics: compressional
DE: 8111 Continental tectonics: strike-slip and transform
SC: Tectonophysics [T]
MN: Fall Meeting 2005