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Abstract: Impact of Late Cenozoic Extension on Laramide Overthrust Belt and Diablo Platform Margins, Northwestern Trans-Pecos Texas

E. W. Collins, J. A. Raney

Late Cenozoic basins and normal faults were superimposed upon preexisting Tertiary, Mesozoic, and older structures of northwestern Trans-Pecos Texas. We analyzed the structural and stratigraphic framework of the region using borehole, seismic, outcrop, and aerial-photograph interpretations; regional cross sections were prepared to record Mesozoic and Cenozoic structural attributes. Laramide thrusting displaced Cretaceous rocks northeastward and produced northwest-trending thrust faults and related normal faults, folds, and monoclines. Cenozoic extension that began 24 to 30 m.y. caused normal faulting that produced northwest- and north-northwest-trending, 3- to 30-mile-wide fault-bounded basins and adjacent mountain ranges. Tectonism continues to the present. The pre-late Cenozoic stru tural grain has at least partly controlled geometries of the late Cenozoic basins and associated faults.

The deepest Cenozoic basin of this area, the 105-mi-long Hueco Bolson, developed along the leading edge of the Laramide thrust belt and the southwest margin of the Diablo Plateau, and has as much as 9800 ft of Cenozoic basin fill. The most active Quaternary faults vertically displace middle Pleistocene deposits by about 33 to 105 ft. The 56-mi-long Red Light Bolson, containing more than 2000 ft of Cenozoic deposits, also formed near the leading edge of the older thrust belt. A northwest-striking Quaternary fault zone bounds this basin on its east margin. A fault in southeast Red Light Bolson vertically displaces middle Pleistocene deposits by about 30 ft.

Two series of Cenozoic basins also have formed northeast of the overthrust belt. The 50-mi-long Eagle Flat-Green River basin system consists of three basins that contain 900 to more than 2000 ft of basin fill. Late Tertiary-Quaternary tectonism has not been as active there as in the other large basins. The 124-mi-long Salt Basin graben system comprises five basins and marks the east edge of Quaternary faulting in this region. Cenozoic fill is more than 2000 ft thick, and the most active Quaternary faults vertically displace middle Pleistocene deposits by at least 13 to 36 ft.

AAPG Search and Discovery Article #90980©1994 AAPG Southwest Section Meeting, Ruidoso, New Mexico, April 24-26, 1994