The metamorphic core complex belt in southeastem Califomia and westem Arizona is a NW-SE trending zone of unusually large Tertiary extension and uplift. Midcmstal rocks exposed in this belt raise questions about the crustal thickness, crustal structure, and the tectonic evolution of the region. Three seismic refraction/wide-angle reflection profiles, acquired and analyzed as a part of the U.S. Geological Survey's Pacific to Arizona Crustal ExpeAment, were collected to address these issues. The results presented here, which focus on the Whipple and Buckskin-Rawhide mountains, yield a consistent three-dimensional image of this part of the metamorphic core complex belt. The seismic refraction/wide-angle reflection data are of excellent quality and are characterized by six principal phases that can be observed on all three profiles. These phases include refractions from the near-surface and crystalline basement, reflections from boundaries in the middle and lower cmst, and reflections and refractions from the upper mantle. The final model consists of a thin veneer (<2 km) of upper plate and fractured lower plate rocks (1.5-5.5 km s -1) overlying a fairly homogeneous basement (-6.0 km s -l) and a localized high-velocity (6.4 km s -1) body situated beneath the westem Whipple Mountains. A prominent midcrustal reflection is identified beneath the Whipple and Buckskin-Rawhide mountains between 10 and 20 km depth. This reflector has an arch-like shape and is centered beneath, or just west of, the metamorphic core complex belt. This event is undefiain by a weaker, approximately subhorizontal reflection at 24 km depth. Together, these two discontinuities define a lensshaped midcrustal layer with a velocity of 6.35-6.5 km s -1. The apex of this midcmstal layer corresponds roughly to a region of major tectonic denudation and uplift (-10 km) defined by surface geologic mapping and petrologic barometry studies. The layer thins to the northeast and is absent in the Transition Zone. The 6.35-6.5 km s -1 velocities are ccmpatible with a diorite composition or a mixture of mafic and silicic rocks.This midcmstal layer is underlain by a higher-velocity lower cmstal layer that is modeled as only 3-6 km thick beneath the metamorphic core complex belt and regions to the southwest. To the northeast, however, this layer thickens to 8-10 km as the midcrustal layer pinches out above it. The velocity of the lower crest is constrained by traveltime modeling and is 6.6 + 0.15 kms -1 beneath the westem Transition Zone and the metamorphic core complex belt; higher velocities may be present farther to the southwest where the layer is thin. The velocity of the lower crust is too low to accommodate significant amount of mafic underplating at the base of the crust. Instead, we interpret the velocities to indicate that the lower crust is passively thinned beneath these regions without significant addition of mafic mantle-derived intrusions. The crest-mantle boundary does not dome up beneath the core complexes but remains approximately subhorizontal at...

Abstract.Stochastic models for the crystalline crust produce synthetic seismograms that compare well with recorded data for a variety of crustal ages and tectonic environments. In this paper, we explore the parameter space describing such stochastic models as a basis for formulating the inverse problem; that is, we wish to estimate the parameters which define a stochastic model from the recorded backscattered wave field. We base the estimation on approximate relations between the primary reflectivity series, which is the ideal wave field response of a medium, and various seismic gathers. A two-dimensional lateral correlation method is used to investigate the sensitivity of synthetic wave fields to horizontal characteristic length. A derived empirical relationship relates the scale length to the half width of the correlation coefficient. A horizontal wavenumber-time domain spectral analysis successfully estimates the horizontal characteristic length (a•) and the fractal dimension (D) of the stochastic medium from which the wave field was backscattered.

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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.