Deep structure of the Rio Grande Rift from seismic reflection profiling

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Abstract
Seismic reflection surveys carried out by the Consortium for Continental Reflection Profiling (COCORP) in the Rio Grande Rift near Socorro, New Mexico, have successfully mapped large‐scale structural variations down to the base of the crust. A total of 155 km of 24‐fold common reflection point coverage was obtained parallel and transverse to major rift structure, including a continuous east‐west profile which spans the central rift from the Sierra Lucero on the west (at the edge of the Colorado Plateau) to the Manzano Mountains on the east. Stacked seismic reflection sections derived from these surveys indicate coherent reflected energy from depths as great as 35 km, with substantial evidence of structural complexity extending throughout the crust. Among the major features apparent on these sections are (1) large‐scale relief on the floor of the Albuquerque Basin, including a major intragraben horst standing over 3 km above the surrounding deep basin floors, and an extensive, shallow structural bench beneath the southeast portion of the basin, (2) well‐developed, antithetic Tertiary normal faulting on the east side of the rift, (3) marked lateral and vertical variation in intrabasement reflection character, including relatively long correlatable events, distinct bands of short, discontinuous reflections, and seismically transparent zones characterized by few, if any, reflections, (4) a complex band of reflections from depths appropriate for the Moho, and (5) unusually strong, coherent reflections from a depth of approximately 20 km, corresponding closely with the top of an active magma body previously postulated to exist beneath the rift in this area on the basis of microearthquake studies. The magma body reflections are among the most prominent deep events yet recorded by COCORP; their geometry and relationship to surrounding structural elements provide important constraints on the depth of brittle faulting and new insight on magma migration and accumulation. Extensional faulting appears to be primarily near vertical near the surface with no clear evidence of large scale listric faulting at depth. The eastern rift boundary at Abo Pass is marked by a high‐angle planar zone devoid of coherent reflections, possibly representing intense structural disruption extending through the crust. In contrast, the seismic section across the western rift boundary is defined by a gently dipping, relatively simple reflection of uncertain origin, with little evidence of a throughgoing fault in the lower crust. The seismic characteristics of many of the intrabasement features suggest particular igneous and/or metamorphic origins and support models which emphasize lateral and vertical heterogeneity at depth.