TWO LITHOSPHERIC PROFILES ACROSS SOUTHERN CALIFORNIA DERIVED FROM GRAVITY AND SEISMIC DATA

Abstract

We report results from density modeling of the crust and uppermost mantle along a transect across southern California. A linear gravity inversion technique was used to calculate crustal and mantle densities along a two-dimensional profile. We used borehole measurements, seismic velocities, and petrologic information to constrain the model where possible. We further assumed that the lithosphere is close to isostatic equilibrium in the deep ocean and east of the Mojave Desert. We used the linear equation r = a + bVp (where r is density, Vp is seismic P-wave velocity) to approximate the mantle density-velocity ratio. A value of b=0-0.2 corresponds to a purely thermal effect on density in the mantle, whereas a coefficient of b > 0.3 implies that petrological or metamorphic changes play an important role in determining density. These factors are likely responsible for creating a high-velocity and high-density mantle body beneath the San Andreas fault. Analysis of the isostatic balance of the models and the average density of the consolidated crust did not allow us to clearly define a preferred density model. We found that for a larger coefficient b in the mantle density-velocity ratio, the less dense was the crust in the Mojave block.