Neotectonic History and Structural Style of the Campo Grande Fault, Hueco Basin, Trans-Pecos Texas

The Hueco Basin of Trans-Pecos Texas and Chihuahua, Mexico, formed in response to Basin and Range extensional tectonism that began about 24 Ma ago and continues to the present. The basin is curvilinear, with the northwestern part of the basin trending north to south and the southeastern part trending northwest to southeast. The Campo Grande fault is located in the southeastern part of the basin. Here the basin is asymmetrical, the thickest sediments having been deposited along the fault-bounded basin axis near the southwestern flank. Approximately 45 km long and striking northwestward, the Campo Grande fault is 4 to 12 km from the northeastern basin edge; it divides the downthrown, central part of the basin (>2,000 m of fill) from the shallower (175 m of fill) northeastern flank. Another major northwest-striking fault dips northeastward and bounds the southwestern basin margin in Mexico. The Campo Grande fault is composed of en echelon fault strands that are 1.5 to 10 km long and have strikes of N25" to 7S0W. Dips are between 60" and 90" southwest. Displacements decrease near terminations of strands. Grooves on fault planes indicate mostly dip-slip movement. Fault scarps have been modified by erosion of the footwall and deposition on the hanging wall. Erosion-resistant caliche (Stages IV to V) at the surface aids in preserving scarp heights of between 1.5 and 11.5 m and scarp slopes of 4" to 17". Analysis of faulted upper Tertiary and Quaternary units indicates that successively younger units have less displacement. Maximum vertical offset measures 10 m across fault strands that cut the middle Pleistocene Madden Gravel (0.6 to 0.4 Ma old), which caps the Camp Rice Formation. Repeated arroyo incision and fluvial aggradation since the middle Pleistocene have developed Pleistocene terraces that are locally correlative and are mapped as parts of the regionally outcropping Ramey and Balluco Gravels.Holocene terraces also occur. Maximum throws across fault strands that cut Ramey terraces (0.4 to 0.1 Ma old?) are 2.5 to 3 m, but some Ramey deposits overlie fault strands and are not faulted. Offset of Balluco (0.1 to 0.025 Ma old?) and Holocene terraces has not been observed at fault strands that cut Ramey terraces. The average recurrence interval is 0.1 Ma (maximum), and the last faulting episode was late Pleistocene. On the downdropped block of one fault strand, faulted calcic horizons (0.5 to 1.0 m thick; Stage 111) with vertical separations of 1 to 2 m indicate at least five episodes of movement, deposition, and surface stabilization during the last 0.6 to 0.4 Ma. Maximum vertical offset during the last faulting event was about 1 to 1.5 m.