ORIGIN OF DEEP CRUSTAL REFLECTIONS - IMPLICATIONS OF COINCIDENT SEISMIC REFRACTION AND REFLECTION DATA IN NEVADA

We compare seismic refraction and reflection results along the PASSCAL/COCORP 40-degrees-N transect in the northern Basin and Range of Nevada in order to determine the origin of the prominent reflections from the deep crystalline crust. Reflection data along the transect show a thick zone of discontinuous, subhorizontal reflections, beginning at 4-6 s two-way traveltime (10-20 km depth) and ending at 9-11 s (27-35 km). Two independently derived velocity models, based on refraction data, are largely similar and agree on many important aspects of the reflectivity-velocity relation. Both models show that the top of the reflective zone lies 3-8 km above a prominent mid-crustal velocity discontinuity, which is interpreted to separate bulk silicic from bulk dioritic-gabbroic crust; in most places, the silicic mid-crust is more strongly reflective than the mafic lower crust. This pattern is expected in areas where ductile shearing is the mechanism responsible for the reflectivity. One of the velocity models, however, suggests that, in places, the strongest reflectivity spans both the middle (6.1 -6.3 km/s) and lower (6.6 km/s) crust; this pattern suggests that the combined influence of ductile strain fabrics and mafic intrusions gives rise to crustal reflections. Both model show that the lowermost crust and crust/mantle transition are highly reflective, also suggesting the presence of mafic and/or ultramafic intrusions. Thus the observed reflection patterns suggest that ductile shearing and the intrusion of mantle-derived magma-both of which are likely to have accompanied the extreme Cenozoic extension-are important factors in generating deep crustal reflections.