HR: 16:15h
AN: T24C-02 INVITED     [Abstracts]
TI: Low-Angle Normal Faults in the Gulf of California Extensional Province: Constraints on Mechanics
AU: * Axen, G J
EM: gaxen@ees.nmt.edu
AF: Dept. Earth and Environmental Science, New Mexico Tech, Socorro, NM 87801 United States
AB: The mechanics and tectonic role(s) of low-angle normal faults (LANFs or detachments) remain controversial. Four LANFs evolved in the Gulf of California rift as integral parts of the late Cenozoic Pac-N.Am. dextral-extensional plate boundary. They cut older intrusive and metamorphic rocks and late Cenozoic syntectonic marine and nonmarine upper plate strata. These LANFs display only brittle tectonites, with slip generally <20 km. Only the southern Cañada David fault remains active; its northern part was deactivated by the Laguna Salada fault. Unlike "typical" LANFs, isostatic footwall rebound is minimal or absent, especially for east-dipping LANFs that root toward axial basins underlain by new mafic crust, the density of which may have impeded rebound. Significant LANFs formed only north of Puertecitos, B.C., where the rift axis and western flank trend about 20 degrees more northerly than elsewhere, and where the angle between the rift axis and the relative plate-motion vector is largest. Thus, LANF formation may be favored above some threshold (e.g., of extension rate, minimum work criterion, etc.) reflected by this angle. The west Salton detachment cuts gently across older, steeper foliation or isotropic granitoids, so its orientation was not controlled by anisotropy. It is exposed within kilometers of its breakaway, where it intersected an older, low relief erosion surface, demonstrating that the fault dipped gently to within ~1 km or less of Earth's surface. Thus, if models are correct in which LANF dip is controlled by rotated stress-fields, then stress rotation occurred even at very shallow depths. Some such models invoke basal shear traction to rotate the stress field; such boundary conditions might fit the NW Gulf if it opened largely in response to shear between oceanic microplates and the overlying continent. These models predict uniform LANF dip directions, but three LANFs dip east and one (Cañada David) dips west, so it seems unlikely that this, or any regionally consistent boundary condition, controlled LANF dip direction. Other models invoke lateral gradients in basal vertical normal stress to rotate the stress field. These may operate on spatial scales compatible with observed along-strike dip reversals.
DE: 8105 Continental margins: divergent (1212, 8124)
DE: 8106 Continental margins: transform
DE: 8109 Continental tectonics: extensional (0905)
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8163 Rheology and friction of fault zones (8034)
SC: Tectonophysics [T]
MN: Fall Meeting 2005