Late Quaternary characteristics of the northern Embudo fault, Taos County, New Mexico
K. I. Kelson, P. W. Bauer, J. R. Unruh, and J. D. J. Bott
The seismic potential of transfer fault zones in active rifts is poorly understood, with no known historical examples of surface-rupturing earthquakes along structures that trend obliquely across rifts. The Embudo fault, which is a transfer zone that strikes N60??E across the north-trending Rio Grande rift, accommodates differential movement between the east-tilted San Luis Basin on the north and the west-tilted Española Basin on the south. Detailed geologic mapping along the northeastern18 km of the Embudo fault provides evidence of late Pleistocene and possibly Holocene displacement along many discontinuous fault strands that are distributed over a width of several kilometers. The northeastern part of the Embudo fault has had left-oblique slip during the late Quaternary, as suggested by substantial along-strike variations in scarp heights, subhorizontal slickensides on fault planes, and reverse faulting within a right step-over. Fault-scarp heights along the northeastern part of the Embudo fault appear to correlate more with local strikes of fault strands than with age. The fault strands with northeasterly strikes exhibit prominent scarps, and strands with more westerly strikes have low scarps and subtle geomorphic expression. In comparison, the north-striking, rift-margin Sangre de Cristo fault, directly north of the Embudo fault, has prominent, continuous geomorphic expression and well-de???ned scarps suggesting late Pleistocene and possibly Holocene activity. These relations are consistent with roughly east-west extension across the rift, and left-lateral slip on the northeast-striking northeastern section of the Embudo fault, rather than reverse slip as previously postulated based on limited roadcut exposures. It is unclear whether the Embudo fault is an independent or dependent seismic source, although evidence of late Pleistocene and possibly Holocene activity on the northeastern section of the Embudo fault suggests that it should be considered as an independent seismogenic source.
- Kelson, K. I.; Bauer, P. W.; Unruh, J. R.; Bott, J. D. J., 2004, Late Quaternary characteristics of the northern Embudo fault, Taos County, New Mexico, in: Geology of the Taos Region, Brister, Brian S.; Bauer, Paul W.; Read, AdamS.; Lueth, Virgil W., New Mexico Geological Society, Guidebook, 55th Field Conference, pp. 147-157.