Anatomy of a metamorphic core complex: Seismic refraction/wide‐angle reflection profiling in southeastern California and western Arizona

Abstract

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...

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“…These observations align with those of the Pacifi c to Arizona Crustal Experiment (PACE) profi le (McCarthy et al, 1990), which was designed to determine lithospheric structure along a transect spanning the southern Basin and Range, near the Gulf of Mexico, to the northeast into the Colorado Plateau. Thin crust that increases from ~28 km in the southern Basin and Range (McCarthy et al, 1991) to 40-45 km beneath the Colorado Plateau was similarly observed along the PACE profi le (McCarthy and Parsons, 1994). Additional sampling of the southern Basin and Range provided by USArray presented here indicates that similarly thin crust to that imaged along the PACE profi le extends eastward throughout the southern Basin and Range toward the Rio Grande rift (Fig.…”

Crustal versus mantle core complexes

“…Basin and Range: Allmendinger et al, 1987;Aegean: Tirel et al, 2004b) and the local absence of Moho offset in relation with core complex detachments (e.g. Allmendinger et al, 1987;McCarthy et al, 1991) suggested that the lower crust was weak and able to flow laterally to compensate thickness variations due to variations of upper crust stretching (Gans, 1987;Buck, 1988;Wernicke, 1990;Block and Royden, 1990;Kruse et al, 1991;McKenzie et al, 2000).…”

Seismic reflection evidence for detachment polarity beneath a major accommodation zone, west-central Arizona

“…Using a two-way traveltime to the Moho of 9.5 s (Fig. 7) and an average crustal velocity of 6.25 km/s, as determined by refraction seismic data below the Buckskin-Harcuvar core complex (profile 1 of McCarthy et al, 1991), the depth to the Moho below line PW-21 is about 30 km. This depth is in good agreement with the refraction Moho determined by Sinno et al (1981) in this area.…”