LATE NEOGENE TECTONIC HISTORY OF THE PRESIDIO BOLSON AND THE INTEGRATION OF THE RIO GRANDE FROM TRANS-PECOS TEXAS TO THE GULF OF MEXICO

The Red Hills intrusive complex (~64 Ma) is a unique Laramide magmatic system for Trans-Pecos Texas but is contemporaneous with other Laramide systems (75–54 Ma) that host major porphyry copper-molybdenum deposits in Arizona, southwestern New Mexico, and northern Mexico. The Red Hills pluton crops out near the southern margin of the 33-Ma Chinati Mountains caldera and on the eastern flank of the alluvium-filled Presidio Bolson through which the Rio Grande flows.

Fluid inclusion studies of the mineralized quartz stockwork in the Red Hills pluton constrain pressures from 20 to 30 MPa, corresponding to depths of formation of 2 to 3 km, thus confirming major unroofing of the magmatic complex. Ar/Ar dates for supergene alunite veins overprinting the mineralized stockwork range from 5.8 to 3.4 Ma. The older ages may reflect increased regional moisture related to the late Miocene opening of the Gulf of California. The younger ages are for alunite veins from the deepest part of the Red Hills weathering profile, in the transition zone to unoxidized igneous rocks. Rapid uplift and erosion at Red Hills also may account for the preservation of only a thin oxidized cap. The apparent lack of ages younger than 3.4 Ma is interpreted to reflect rapid erosion of the weathered profile of the Red Hills intrusive complex associated with Pliocene extension. The extension, flank erosion, and associated sedimentation created and filled the Presidio Bolson with as much as 1,500 m of sediments at the southern terminus of the ancestral Rio Grande drainage. Subsequent breaching of the natural barriers along the southern terminus of the Presidio Bolson resulted in the integration of the through-going Rio Grande system in the Pleistocene. Thus, the combined Rio Grande-Rio Bravo system began to produce a small fluvial-deltaic depocenter and renewed progradation of the continental margin in the western Gulf of Mexico in the Pleistocene.