Initial paleoseismic and yydrogeologic assessment of the Southern Sangre de Cristo fault at the Taos Pueblo site, Taos County, New Mexico
K. I. Kelson, P. W. Bauer, S. D. Connell, D. W. Love, G. C. Rawling, and M. Mansell

Abstract:

The most-recent surface-rupturing earthquake on the southern Sangre de Cristo fault at the Taos Pueblo site occurred about 10 to 30 ka, based on stratigraphic and soils relations in trench exposures. Detailed topographic surveys at the site show that the primary fault scarp, which is formed on late Pleistocene alluvial deposits, has about 3 m of net vertical  tectonic displacement. A bevel in the scarp suggests that two surface ruptures occurred since deposition of the alluvial fan, each with about 1.5 m of vertical displacement. These displacements imply the occurrence of earthquakes having magnitudes in the range of M6.8 to M7.0. Stratigraphic relations exposed in a 30-m-long trench across the primary fault scarp show the presence of middle Pleistocene eolian and alluvial deposits faulted against late Pleistocene alluvial and colluvial deposits on the eastern, downthrown side of the fault. Stratigraphic evidence of surface-fault rupture includes scarp-derived colluvium and fissure-fill deposits that probably were deposited immediately following the surface rupture. The main fault strand consists of a primary west-dipping fault plane and secondary faulting in a 6-m-wide zone in the hanging wall. The fault zone contains multiple anastomosing strands in pervasively sheared alluvium, and probably has lower permeability than surficial deposits east of the fault, but higher permeability compared to deposits west of the fault. Calcium-carbonate cementation along fault planes in fine- and medium-grained deposits may be indicative of preferential groundwater flow downdip along the fault. Thus, major strands of the southern Sangre de Cristo fault probably provide preferential pathways for the downward flow of vadose-zone groundwater in unconsolidated near-surface deposits.


Citation:

  1. Kelson, K. I.; Bauer, P. W.; Connell, S. D.; Love, D. W.; Rawling, G. C.; Mansell, M., 2004, Initial paleoseismic and yydrogeologic assessment of the Southern Sangre de Cristo fault at the Taos Pueblo site, 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. 289-299.

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