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Paul W. Bauer and Steven Ralser

Abstract--The Picuris-Pecos fault is arguably the largest-displacement, exposed, strike-slip fault in the state, and yet its kinematic history is poorly constrained. Although it clearly displays 40 km of dextral strike-slip separation, the separation is based on Proterozoic-age piercing lines, and therefore only yields a net slip for the last 1.4 Ga. Although the original Picuris-Pecos fault mappers concluded that major strike-slip faulting occurred in Proterozoic time and lesser dip-slip movement occurred in Laramide time, recent investigators have speculated that most or all of the 40 km of strike-slip separation is actually Laramide in age. In contrast, we conclude that Proterozoic slip on the Picuris-Pecos fault is a reasonable interpretation based on plastic deflection and attenuation of strata along the fault, geometries and kinematics of Paleozoic and Laramide fault structures, the relative fracturing of Proterozoic versus Paleozoic strata, and the presence of other high-angle, Proterozoic-age faults. New field data from faults parallel to the Picuris-Pecos fault indicate that Laramide deformation produced concurrent, transpressional(?), strike-slip and dip-slip movements, and we speculate that the Picuris-Pecos fault and adjacent sub-parallel structures represent a positive flower geometry of Laramide age. Although new fission-track data by Kelley (1995) imply Neogene vertical movement along the southernmost Picuris-Pecos fault, cross-cutting field relationships along the northern fault exposure demonstrate that little or no post-26 Ma vertical faulting occurred. Perhaps this along-strike variation resulted from different amounts of Neogene extension in the southern and northern parts of the Española Basin.

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