Seismic reflectivity of the Whipple Mountain shear zone in southern California

Abstract

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. 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. -from Authors