Southern Trans-Pecos Texas contains parts of two of the major overthrust belts of North America, the older Ouachita and the younger Cordilleran. In addition, this area has been deformed by two other major tectonic episodes. As early as 1,000 m.y. ago, the Van Horn mobile belt was formed by the closure of an inner arc basin during the formation of a proto-Pangea. This mobile belt provided the base upon which the Diablo platform formed. Recent petrologic evidence suggests that the Van horn mobile belt continues southward into Mexico and underlies the Coahuila platform. During Late Pennsylvanian to Early Permian time, this platform, a continental promontory, impeded the northward movement of the overriding Ouachita orogenic thrust sheets, bending them southwestward at the in ersection of this thrust complex.

During the late Mesozoic, the Diablo platform acted as a stable buttress, against which sediments of the Chihuahua trough were deformed and thrust. These folds and thrusts comprise the Chihuahua tectonic belt, which forms part of the Cordilleran thrust belt of North America. East of this platform, faulted monoclines may represent the southern limit of the fault-bounded, basement-cored uplifts of the front ranges.

Finally, western Trans-Pecos Texas was overprinted by extensional basin and range faulting during the Cenozoic, with concomitant igneous intrusive and extrusive activity. The igneous intrusions occur in a belt trending roughly north-northwest, following the trend of the basin and range faulting. These intrusions are scattered through most of Trans-Pecos except for an area to the south where the four tectonic belts intersect. Here, extensive crustal fracturing and extension have resulted in the emplacement of a greater density of igneous material into the overlying crust.

End_of_Article - Last_Page 1437------------