New crustal-scale information on the interaction between normal faulting and magmatic activity is provided by recent COCORP deep seismic reflection profiles in Death Valley, California, and by reprocessing of the COCORP data from the Socorro area of the Rio Grande rift, New Mexico. The most striking feature on the seismic sections from these areas is a prominent, subhorizontal series of reflectors at mid-crustal depth. Previous studies have suggested that thin tabular magma bodies lie within the mid-crustal reflective zones. In addition, because they are traced without apparent offset beneath faults both mapped at the surface and interpreted from the COCORP data, these mid-crustal horizons are here inferred to be detachments or zones of tectonic decoupling.
Upper-crustal Cenozoic faults do not appear to penetrate deeper than 15 km in Death Valley and 13 km in the Rio Grande rift. In Death Valley, these faults are relatively planar, with moderate dips (20° to 35°), and appear to bound large basement rocks. One such zone of normal faults can be traced from the magma body inferred at 15 km depth beneath central Death Valley to the surface location of a 690,000-yr-old basaltic cinder cone. Listric and low- to moderate-angle normal faults are evident on the reprocessed New Mexico data and constitute the structural component of upper-crustal extension. In particular, a listric master fault traceable to depths as great as 13 km is inferred to underlie the Albuquerque basin. Unlike the Death Valley data, no faults are observed to merge with the Socorro magma body per se. Rather, subhorizontal to moderately west-dipping packages of reflections are imaged between the base of the faulted upper crust (13 km depth) and the mid-crustal magma body (about 20 km depth).
The middle crust marks a major rheological boundary between the faulted upper crust and a ductile lower crust extending by penetrative flow and intrusion. Events seen in the middle and lower crust are generally subhorizontal, and prominent layering is observed. A band of reflections attributed to the crust-mantle boundary is evident on most seismic sections. The upper mantle appears seismically transparent. On some of the profiles, the events attributed to the base of the crust are the deepest in a series of strong and continuous reflections, at least one of which is a layer of magma. This association supports the suggestion that magmatic intrusions are a probable cause for the high reflectivity observed in the deep crust of many extensional terranes.