HR: 10:50h
AN: T42B-03    [Abstracts]
TI: Seismotectonics of an Evolving Intracontinental Plate Boundary in Eastern California
AU: * Unruh, J R
EM: unruh@lettis.com
AF: William Lettis & Associates, Inc., 1777 Botelho Drive, Suite 262, Walnut Creek, CA 94596 United States
AU: Hauksson, E
EM: hauksson@gps.caltech.edu
AF: California Institute of Technology, Seismological Laboratory, Mail Code 252-21, Pasadena, CA 91125 United States
AB: Analysis of seismicity in the northern Mojave block, southern Walker Lane belt and southern Sierra Nevada provides a detailed snapshot of the kinematics of active deformation within a young and probably evolving intra-continental plate boundary in eastern California. Earthquakes were relocated using joint hypocentral inversion, double-difference and cross-correlation techniques. Groups of focal mechanisms were inverted for the components of a reduced deformation rate tensor. The inversion results are synthesized in maps of the seismogenic deformation field. In general, seismogenic deformation east of the Sierra Nevada is characterized by horizontal plane strain and primarily accommodates northwest translation of the Sierra Nevada-Central Valley (Sierran) microplate relative to stable North America. Crustal thinning in the Walker Lane belt is subordinate to NW-directed dextral shear. There is no obvious variation in strain geometry in the transition from the eastern California shear zone to the Walker Lane belt across the Garlock fault, which suggests that this structure is being deformed by distributed NW dextral shear. In contrast, seismogenic deformation in the southeastern Sierra Nevada near lat 36øN is characterized by horizontal extension and locally by oblate flattening; i.e., extension in two perpendicular horizontal directions. The extensional deformation occurs directly east of the "Isabella anomaly", a high-velocity anomaly in the upper mantle interpreted to be lower Sierran lithosphere that detached in late Neogene and descended into the asthenosphere. There is a progressive east-to-west transition from strike-slip faulting in the Walker Lane belt to horizontal extension and vertical crustal thinning within the southern Sierra Nevada, indicating that NW dextral shear extends into the eastern Sierran microplate. The seismotectonics of this region may reflect an early stage in the process whereby the Walker Lane expands westward in discrete steps, progressively narrowing the rigid Sierran microplate and possibly diverting a greater percentage of total Pacific-North American plate motion east of the San Andreas system (Jones et al., 2004). This process may be intimately connected with late Cenozoic detachment and descent of lower Sierran lithosphere.
DE: 8107 Continental neotectonics
DE: 8110 Continental tectonics--general (0905)
DE: 8123 Dynamics, seismotectonics
DE: 8158 Plate motions--present and recent (3040)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting