Structural geology of the Bishop Cap Hills in south-central New Mexico

Joshua John Rohret, University of Texas at El Paso


The Bishop Cap Hills (BCH) of south-central New Mexico consist of deformed Ordovician and Pennsylvanian sedimentary rocks. Structures in the BCH record Laramide shortening and Rio Grande Rift extension. This deformation history has been analyzed using the geometry and field relations of faults, veins, and folds. Numerous well-preserved normal faults and fewer poorly-preserved reverse faults have slickensides, tool marks, and fibrous veins on their surfaces which are inferred to parallel directions of maximum shear strain rate. Orientations of these lineations on fault planes are used in a quantitative kinematic analysis that yields the orientations of the principal incremental strain axis. In addition, Laramide folding of Paleozoic strata, and veins created from rifting and local caldera activity, have distinct geometries which place constraints on the deformation history. ^ Absolute ages of rock units, cross-cutting relations of faults and veins, and retro-deformation of stereonet data define a sequential history as follows: (1) NE directed Laramide shortening recorded by folds and reverse faults; (2) Early veins consistent with NNE oriented extension; (3) NE directed extension indicated by normal faults that cut the BCH into tilted blocks; and (4) NNW extension dated at 5.43 ± 0.34 Ma. This data shows that Rio Grande Rift extension in the BCH has been heterogeneous and involved components of NE and nearly N-S extension. These nearly orthogonal extension directions are consistent with finite radial horizontal extension. This pattern of extension can be explained by vertical shortening or magmatic inflation of the crust if assumed to represent a single progressive deformation. ^

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Recommended Citation

Rohret, Joshua John, "Structural geology of the Bishop Cap Hills in south-central New Mexico" (2006). ETD Collection for University of Texas, El Paso. AAI1435305.