HR: 0830h
AN: S11C-0298    [PDF]
TI: The Orocopia-Pelona Schist Terrane, southern California and southwest Arizona: A Calibrated Target for Crustal Seismic Anisotropy Studies
AU: Haxel, G
EM: ghaxel@usgs.gov
AF: USGS, 2255 N. Gemini Drive, USGS, Flagstaff, AZ 86001 United States
AU: * Christensen, N
EM: chris@geology.wisc.edu
AF: Univ. Wisconsin, Dept. Geology, U. Wisconsin, Madison, WI 53706 United States
AU: Okaya, D
EM: okaya@usc.edu
AF: Univ. So. California, Dept. Earth Sciences, USC, Los Angeles, CA 90089 United States
AU: Jacobsen, C
EM: cejac@iastate.edu
AF: Iowa State Univ., Dept. Geol. Sci., ISU, Ames, IA 50011 United States
AB: The Orocopia-Pelona Schist belt represents an enigmatic element within the Late Cretaceous to early Tertiary tectonic framework of the southern Cordillera. This oceanic supracrustal terrane is widely believed to underlie much of southern California and the southwest corner of Arizona. Alternative interpretations of geologic and geophysical data suggest that the schist may have a considerably more limited subsurface distribution, restricted to the vicinity of the curvilinear belt defined by the pre-San Andreas distribution of outcrops of the Orocopia and Pelona Schists. Regional depth and thickness, breadth, and large-scale internal configuration of the schist differ according to the current evolutionary models: (1) low-angle, eastward subduction, (2) trapping of marine rocks along a transpressional continental margin, or (3) tectonic burial of Great Valley-like forearc strata beneath overriding Mesozoic batholithic arc crust. The three-dimensional distribution of the schist could help to constrain these models, but is poorly known. The Orocopia-Pelona Schist is thus a promising candidate for mapping via crustal seismic anisotropy. The Orocopia-Pelona Schist terrane is exposed in some 15 tectonic windows, stretching from the central Transverse Ranges, California, to the Kofa region, southwest Arizona. Along the eastern two-thirds of its extent, exposures of the schist are tightly aligned along the Chocolate Mountains anticlinorium. The schist terrane comprises predominantly homogeneous quartzofeldspathic metagraywacke, with subordinate to minor interlayered metabasalt, metachert, siliceous marble, and ultramafic rock. The schist exhibits strong foliation and pronounced lineation, with regionally uniform orientation. Metamorphic mineral assemblages indicate tectonic burial to depths approx. 25 - 35 km. We have performed petrophysical laboratory measurements on samples of Orocopia Schist from the Chocolate and Trigo Mountains, determining seismic velocities and densities at pressures up to 10 kb. Owing to the pronounced fabric anisotropy of the schist, compressional and shear wave velocity measurements were made not only parallel to the three principal axes but also in the directions of the three axial-plane diagonals (at 45 degrees). The schist exhibits large-magnitude seismic anisotropy ranging between hexagonal to strongly orthorhombic symmetry. VP ranges between 5.27-6.79 km/s (14-26% anisotropy) at 1 kb and 5.85-7.14 km/s (12-19% anisotropy) at 6 kb (22 km). VS anisotropy is as large as 28% at 1 kb (3.05-4.03 km/s) and 24% at 6 kb (3.29-4.14 km/s). (In contrast, a sample of Jurassic granodiorite, which forms much of the continental crust tectonically overlying the Orocopia Schist, is isotropic with VP = 6.38 km/s and VS = 3.69 km/s, both at 6 kb). We present data for the fabric of the schist, the anisotropic velocity ellipsoid (that is, Christoffel-based quantification of propagation and splitting behavior in non-axial propagation directions), and the regional distribution and internal character of the schist terrane from the perspective of seismic wave propagation. We also discuss types of seismic experiments needed for anisotropic subsurface imaging of the Orocopia-Pelona Schist.
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
DE: 7203 Body wave propagation
DE: 7260 Theory and modeling
DE: 8025 Mesoscopic fabrics
DE: 8110 Continental tectonics--general (0905)
SC: Seismology [S]
MN: 2003 Fall Meeting