HR: 08:00h
AN: T31G-01 INVITED     [Abstracts]
TI: Transpressional Mechanics of West-Central California
AU: * Unruh, J R
EM: unruh@lettis.com
AF: William Lettis & Associates, Inc., 1777 Botelho Dr., Suite 262, Walnut Creek, CA 94596 United States
AU: Sawyer, T L
EM: tom@piedmontgeosciences.com
AF: Piedmont Geosciences, 10235 Blackhawk Drive, Reno, NV 89506 United States
AB: Analyses of seismicity, seismic reflection and geodetic data, and map-scale patterns of active faulting in west-central California do not support the hypothesis that plate-boundary stress is regionally refracted across a weak San Andreas fault, resulting in "fault-normal compression" and partitioning of plate motion onto parallel strike-slip and thrust faults. Specifically, this model is not consistent with the following observations: (1) Dextral faults of the eastern San Andreas system (i.e., the Greenville, Mt. Lewis, Northern Calaveras, Concord and Green Valley faults) typically strike 10-20ø more northerly than the San Andreas and Hayward faults, but despite this apparent "releasing" geometry do not accommodate components of extension; (2) Major late Cenozoic folds commonly are oriented about 45ø to the strike-slip faults, rather than parallel to the faults as assumed by some partitioning models; (3) Anticlines typically exhibit right-stepping, en echelon patterns, typical of dextral wrench tectonics; and (4) Loci of crustal shortening primarily are located in restraining bends or restraining step-overs between major strike-slip faults. These relations are consistently expressed along a SE to NW traverse that follows progressive transfer of dextral slip among segments of the Tesla-Ortigalita, Greenville, Concord, Green Valley and Maacama strike-slip faults for a distance of about 200 km. Rather than discrete partitioning of strike-slip and thrust deformation predicted by the "fault-normal compression" model, these observations document a 10ø-20ø CW rotation of macroscopic dextral shear from west to east across the Pacific-Sierran plate boundary (Unruh and Lettis, 1998). Viable mechanical models for transpressional tectonics in west-central California must account for this observed rotation of the principal strain rates (and presumably principal stresses). Because velocity gradients can be measured across the plate boundary with high precision, a logical modeling approach would be to test whether a constant velocity boundary condition and variable viscous rheology could produce the observed variations in strain rate and strain geometry.
DE: 7221 Paleoseismology
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
DE: 8107 Continental neotectonics
DE: 1200 GEODESY AND GRAVITY
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting