Abstract

Initial development of the Nacimiento uplift occurred during the Laramide orogeny by what may have been shear folding of the brittle Precambrian basement rocks and stretching of the incompetent overlying sedimentary strata along a monoclinal fold on the west side of the uplift. Antithetic and synthetic normal faults formed in the strata along the monocline as it developed. The normal faults and the fold were cut by the Nacimiento fault which is nearly vertical at depth but flattens upward and has westward movement of the hanging wall block over the San Juan basin. Where the Nacimiento fault is preserved at high structural and stratigraphic levels it is gently dipping and has normal faults in the hanging wall block. These longitudinal normal faults formed as the uplift was arched and stretched.

A model based on primary vertical uplift integrates the different stress fields that caused the complex sequence of monoclinal folding, normal faulting, and reverse faulting with concomitant tensional features in the hanging wall block.

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