Abstract

Reprocessed seismic reflection data from near the Picacho Mountains metamorphic core complex in southeastern Arizona show that the crust is seismically reflective from 1.5 s two-way travel time (TWTT) to the refletion Moho at 10 s TWTT. These high amplitude, subhorizontal, and laterally discontinuous reflections may be a common seismic signature of highly extended areas This study presents the results of synthetic seismogram modeling of these reflections, based on drill-hole and outcrop data from the Picacho Mountains and adjacent core complexes.

Modeling studies suggest that the prominent reflectivity beneath the Picacho Mountains results from a thinly layered crust where the layers are of alternating high and low velocity and their lateral extent is on the order of 500 m. Moreover, the lack of distinctive diffractions in the seismic data is interpreted to mean that lateral changes in velocity orlayer thickness are smooth rather than abrupt on the scale of a seismic wavelength. Soure for the reflectivity include fracture zones, compositional layering of micaceous gneisses and granitoid rocks, and zones of velocity anisotropy (particularly in micaceous rocks). The association of the reflectivity in metamorphic core complexes with Cenozoic deformation suggests that the reflections are a by-product of extension.

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