Early Basin and Range Development in Trans-pecos Texas and Ad3acent Chihuahua: Magmatism and Orientation, Timing, and Style of Extension

Magmatism during early Basin and Range extension in Trans-Pecos Texas consisted of widespread but volumetrically minor alkali basalts (24-17 Ma), an alkalic mafic to intermediate composition stratovolcano (28-27 Ma), and a dominantly rhyolitic, possibly bimodal, caldera complex in adjacent Chihuahua (3128 Ma). This magmatism contrasted sharply with the much more voluminous and compositionally more diverse Eocene-Oligocene magmatism. The age and orientation of dikes eraplaced during this magmatic episode have been used to determine the orientation of least principal stress, o 3 . An ENE orientation of o3, similar to early extension throughout the Basin and Range province, can be well documented by 28 Ma and possibly as early as 31 Ma. Faulting, consisting almost exclusively of movement along high-angle normal faults, was underway by 23 Ma. Some initial extension was taken up along preexisting WNW trending structures, oblique to ENE oriented o 3. The least principal stress probably changed to WNW as it did elsewhere in the Basin and Range province, but the timing of this change is not well constrained in Texas. ENE and NE striking silver-copper-lead veins in Precambrian and Cretaceous red beds may have formed during the later Basin and Range extension. The present Basin and Range topography of this region is the result of movement along high-angle and dominantly NNW trending faults established early under o 3 oriented ENE and reactivated after o 3 changed to WNW.