The nature of deep crustal structures in the Mojave Desert, California

Abstract

The character of multi-offset reflections from the deep crust in the Mojave Desert are examined to reveal the physical nature of the reflecting structures. We focus on distinguishing classical abrupt discontinuities, such as traditional models of the Conrad and Moho boundaries, from more unusual structures. Finite-difference modeling and simple interference relations show that pre-critical reflections exhibiting an increase in peak frequency with offset arise from thinly-layered horizontal structures, while reflections from step discontinuities show no change in frequency with offset. In the deep crust thin layers may result from sill intrusion or fault motion. The sense of changes in Poisson's ratio and the relative strength of density changes determine whether reflection amplitudes will increase or decrease with offset. A simple linear regression on pre-critical reflection amplitudes against offset is adequate to separate reflections arising from increases in Poisson's ratio from those arising from decreases in Poisson's ratio and/or density changes. The latter condition may be the result of strong anisotropy or the presence of pore fluid. Comparisons of the properties of major deep reflectors across the Mojave Desert suggest that the effects of tectonic motion and fluid injection have penetrated all levels of the crust.