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The metamorphic core complex belt in southeastern California and western Arizona is a NW-SE trending zone of unusually large Tertiary extension and uplift. Midcrustal 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 Experiment, 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 crust, 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 (approximately 6.0 km s-1) and a localized high-velocity (6.4 km s-1) body situated beneath the western 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 underlain by a weaker, approximately subhorizontal reflection at 24 km depth. Together, these two discontinuities define a lens-shaped midcrustal layer with a velocity of 6.35-6.5 km s-1. The apex of this midcrustal layer corresponds roughly to a region of major tectonic denudation and uplift (approximately 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 compatible with a diorite composition or a mixture of mafic and silicic rocks. This midcrustal layer is underlain by a higher-velocity lower crustal layer that is modeled as only 3-6 km thick beneath the metamorphic core complex belt and regions to the southwest. To the northest, however, this layer thickens to 8-10 km as the midcrustal layer pinches out above it. The velocity of the lower crust is constrained by traveltime modeling and is 6.6 +/- 0.15 km s-1 beneath the western 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 crust-mantle boundary does not dome up beneath the core complexes but remains approximately subhorizontal at a depth of 26-28 km or, in the case of the Whipple Mountains, actually deepens; a 3-km crustal root is modeled.

This lack of upward doming of the Moho, together with the vertical alignment of the metamorphic core complex belt over what are believed to be extension-related structures in the middle and lower crust, suggest that there is no lateral offset of upper crustal deformation from deeper zones of extension, as one would expect if extension occurred along crust-penetrating shear zones (Wemicke, 1981; Wemicke et al., 1985). Instead, domed and inflated middle crust and thinned lower crust directly underlie the region of greatest thinning of the upper crust.