CULEBRA GRABEN: A MAJOR INTRARIFT STRUCTURE IN THE SAN LUIS BASIN, RIO GRANDE RIFT

The Culebra graben in southern Colorado is one of three north-trending structural depressions along the eastern margin of the San Luis basin, which is the largest basin in the northern part of the late Cenozoic Rio Grande rift. Recent geologic mapping, subsurface data from an oil test well, a new high-resolution aeromagnetic survey, and existing regional gravity data are combined with previous studies summarized by Wallace (2004) to characterize the structural framework of the Culebra graben. This new structural framework improves our understanding of earthquake hazards in this part of the rift.

The Culebra graben coincides with a topographically anomalous region in southern Colorado named the Culebra Reentrant by Upson (1939). To the north and south of the reentrant, the western front of the Sangre de Cristo Mountains is sharp and abrupt, is controlled by the Sangre de Cristo fault, and has the typical geomorphology of a tectonically active range front. Within the reentrant, the range crest is recessed as much as 30 km eastward, and foothills underlain by syn-rift sedimentary and volcanic rocks of the Santa Fe Group occupy a broad area between the range crest and valley floor. Proterozoic rocks exposed at the range crest are downdropped into the Culebra graben by multiple late Cenozoic faults, including the active Sangre de Cristo fault.

Much of the post-Miocene extension across the Rio Grande rift at this latitude is restricted to the narrow part of the Culebra graben, which is bounded on the east by the Sangre de Cristo fault and on the west by the San Luis fault, a newly named structure on the eastern margin of San Pedro Mesa and the Basaltic Hills. Miocene deformation occurred along faults as far east as the range crest and beyond. Proterozoic rocks within the graben are downdropped about 3.6 km relative to equivalent rocks at the range crest. If rifting initiated 25 m.y. ago, then the average long-term uplift rate of the east side of the graben is about 145 m/m.y. Pliocene Servilleta basalt flows are offset at least 0.5 km across the Sangre de Cristo fault, which yields a vertical slip rate of 123-136 m/m.y., depending on whether the top or bottom of the flow sequence is used in the calculation.