HR: 1330h
AN: T22B-0512    [PDF]
TI: Examination of Exhumed Faults in the Western San Bernardino Mountains, Southern California: Implications for Fault Mechanics, Earthquake Rupture, and Slip Evolution
AU: * Jacobs, J R
EM: jrjacobs@cc.usu.edu
AF: Utah State University, 4505 Old Main Hill Geology Department, Logan, UT 84322
AU: Evans, J P
EM: jpevans@cc.usu.edu
AF: Utah State University, 4505 Old Main Hill Geology Department, Logan, UT 84322
AB: Detailed mapping of small-displacement, predominantly high-angle reverse faults of the late Miocene Cedar Springs Fault System was performed in the Silverwood Lake area in the western San Bernardino Mountains, California in order to compare structural and lithologic variations between faults with varying degrees of slip. The faults have been exhumed from 1-5 km depth due to the late Miocene to early Pleistocene uplift of the western portion of the range, and range in slip from several cm to 3.5 km. The host rock is the Mesozoic crystalline basement complex of granodiorite, diorite, and quartz-monzonite, with Precambrian to Paleozoic metasedimentary roof pendants. All these lithologies are cut by abundant dikes which serve as excellent offset markers. The Miocene Crowder Formation (17-9.5 Ma), consisting of arkosic sandstone and conglomerate, is also a useful marker bed. We use a rock mechanics and structural petrologic approach to study fault-related rocks. Fault zone structure and composition data were determined by using the detailed transect method to representatively sample the host rock, damage zone, and fault core. Fault kinematics were determined from exposure of an offset marker and slip indicators. Thin sections were prepared from transect samples to study the variation in microstructure across fault zones. Geochemical analysis (XRD, XRF, and ICP-MS) of samples was performed to help determine the pressure and temperature conditions of brittle deformation (indicating formation depth of the studied faults) and the nature and extent of fluid-rock interactions. The majority of the studied faults have a well-defined fault core, typically consisting of mm to tens of cm of clay gouge and/or (ultra)cataclasite, surrounded by a much thicker damage zone, on the order of m to tens of m, exhibiting a significant increase in subsidiary faults and fractures as compared to the host rock. Faults through the more competent dikes are manifested as a zone of dense fractures and faults instead of the defined fault core characteristic of the softer granitic rocks. An asymmetric slip preference to the footwall was observed in faults placing granitic rocks on top of the Crowder Formation. Damage elements, such as subsidiary faults, fractures, and veins, were inventoried in order to define the extent of the damage zone. These elements often exhibit a bimodal orientation, one sub-parallel to and the other at a high angle to the main fault. Quantitative data indicate that damage zones of larger-displacement faults contain more subsidiary faults than smaller-displacement damage zones, which are characterized more by fractures. Calculated slip from the high-angle reverse faults spans five orders of magnitude, from the cm scale to the hundreds of m scale. Fault core thickness initially increases with slip amount, but core thickening slows substantially between 10-100 m of slip. This suggests that the fault core develops early and large amounts of displacement can be accommodated on relatively narrow, discrete slip surfaces.
DE: 8010 Fractures and faults
SC: Tectonophysics [T]
MN: 2003 Fall Meeting