A deterministic and stochastic velocity model for the Salton Trough/Basin and Range transition zone and constraints on magmatism during rifting

Abstract

As a high resolution addition to the 1992 Pacific to Arizona Crustal Experiment (PACE), a 45‐km‐long deep crustal seismic reflection profile was acquired across the Chocolate Mountains in southeastern California to illuminate crustal structure in the transition between the Salton Trough and the Basin and Range province. The complex seismic data are analyzed for both large‐scale (deterministic) and fine‐scale (stochastic) crustal features. A low‐fold near‐offset common‐midpoint (CMP) stacked section shows the northeastward lateral extent of a high‐velocity lower crustal body which is centered beneath the Salton Trough. Off‐end shots record a high‐amplitude diffraction from the point where the high velocity lower crust pinches out at the Moho. Above the high‐velocity lower crust, moderate‐amplitude reflections occur at midcrustal levels. These reflections display the coherency and frequency characteristics of reflections backscattered from a heterogeneous velocity field, which we model as horizontal intrusions with a von Kármán (fractal) distribution. The effects of upper crustal scattering are included by combining the mapped surface geology and laboratory measurements of exposed rocks within the Chocolate Mountains to reproduce the upper crustal velocity heterogeneity in our crustal velocity model. Viscoelastic finite difference simulations indicate that the volume of mafic material within the reflective zone necessary to produce the observed backscatter is about 5%. The presence of wavelength‐scale heterogeneity within the near‐surface, upper, and middle crust also produces a 0.5‐s‐thick zone of discontinuous reflections from a crust‐mantle interface which is actually a first‐order discontinuity.