Abstract

The Consortium for Continental Reflection Profiling (COCORP) transect across the southwest margin of the Colorado Plateau in Arizona reveals prominent subhorizontal zones of reflections, here termed the Bagdad reflection sequence (BRS), which dominate the upper and middle crust in the transition zone (TZ) between the Colorado Plateau and the Basin and Range province. A Cenozoic origin for the BRS is considered most likely; it may represent fragmented or sheared zones related to detachment faulting or a series of intrusions, possibly emplaced along preexisting zones of weakness. Moho reflections observed beneath the TZ and the Basin and Range province contrast with the nonreflective Moho beneath the Colorado Plateau, suggesting that extension and its associated igneous processes probably played a key role in the development of reflectors at the Moho. The Moho and a prominent midcrustal reflector are offset by about 3 km in a normal sense in at least one place; this is the best example of offset Moho yet found by COCORP and contrasts sharply with COCORP observations of a smooth Moho elsewhere. This offset is probably a late tectonic expression of crustal extension and thinning across the TZ.

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