Abstract

COCORP seismic-reflection profiles across Nevada at about 40°N image a prominent, essentially continuous band of reflectors at a two-way traveltime of 9 to 11 s. The approximate correspondence of this reflection time with estimates of the two-way traveltime to the Moho in this area provided by seismic-refraction data suggests that the prominent reflections are from the Moho. The relief on these reflectors (the “reflection Moho”) beneath Nevada, across a latitudinal transect of <450 km, is only 2.2 s (∼7 km) after correction for basin effects, or only ∼50% of the Moho topography previously inferred from refraction data. Observed variations in travel-time to the reflection Moho are gradual, with no evidence for major offsets. The reflection Moho in this part of the Basin and Range province is interpreted as the base of a complex group of reflections. A striking aspect of the reflection data is the resolution of this group of reflections, in many places, into 2 distinct reflections as much as 1.2 s (∼ 4 km) apart, which elsewhere merge into a single reflection. The reflection Moho appears to be continuous beneath terranes that experienced very different tectonic histories in the Paleozoic and Mesozoic and apparently truncates crustal reflectors in some areas. The reflection Moho in its present configuration thus appears to be a young feature in the Basin and Range province, and it may be related to Cenozoic magmatism and extension. In some areas of rifting and volcanism outside the Basin and Range province, the observed reflection Moho closely resembles that seen in the Basin and Range province. In other areas with other tectonic settings, including the Sierra Nevada and the Colorado Plateau, the reflection Moho has variable seismic character and may represent different kinds of geologic boundaries from that of the Basin and Range province.

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