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Seismic reflectivity of the Whipple Mountain shear zone in southern California

1989; American Geophysical Union; Volume: 94; Issue: B3 Linguagem: Inglês

10.1029/jb094ib03p02989

2156-2202

Chi‐Yuen Wang, D. A. Okaya, Charles Ruppert, G. A. Davis, Tie‐Shuan Guo, Zengqiu Zhong, Hans‐Rudolf Wenk,

Seismic Waves and Analysis

The Whipple Mountains shear zone of southeastern California comprises part of the Whipple Mountains metamorphic core complex of the North American Cordillera. The 3.9‐km‐thick shear zone displays excellent exposures of ductilely deformed mid‐Tertiary mylonitic gneisses, as well as kinematically related, but younger cataclasites which underlie a major low‐angle normal fault (the Whipple detachment fault). Prominent crustal reflections beginning at 3–4 s were recorded on CALCRUST seismic profiles southwest of the Whipple Mountains. In order to interpret these seismic reflections and to understand why detachment faults are imaged on some profiles but not on others, we collected oriented rock specimens from all major structural units in the Whipple Mountains and determined P wave velocities for these samples parallel to three principal fabric directions under laboratory‐induced confining pressures up to 500 MPa. Using a geologic section through the mylonitic gneisses from a previous investigation, we then constructed a detailed acoustic impedance section for the Whipple Mountain shear zone based upon laboratory results. This section was in turn used to compose two dimensional synthetic reflection seismograms for comparison with the CALCRUST records. Significant impedance contrasts are enhanced by the opposing fabric orientations between the interlayered but structurally isolated relict domains of nonmylonitized rocks and the surrounding mylonitic gneisses. The finite thickness and lateral extent of these rock units cause constructive interference in seismic modeling to create the “seismic fabric” of strong, laterally discontinuous but subparallel reflections as seen in the CALCRUST profiles southwest of the Whipple Mountains. Chloritic breccias beneath the Whipple detachment fault (WDF) have much lower seismic impedance than that of rocks occurring both structurally above and below. However, the significant thinning of the chloritic breccia zone to the southwest of the Whipple Mountains may have caused the poor image of the WDF on the CALCRUST profiles. The absence of a detachment reflection is thus not necessarily indicative of the absence of a subsurface fault zone.