PRELIMINARY ANALYSIS OF THE TASCOTAL MESA FAULT AND ASSOCIATED FEATURES NEAR THE PRESIDIO-BREWSTER COUNTY LINE, DALQUEST DESERT RESEARCH STATION, TRANS-PECOS TEXAS

The Texas Trans Pecos is rife with structural features. The youngest of these are fault systems that generally trend northwest-southeast. These are largely consistent with Basin-and-Range tensional features. There are a few lengthy east-west fault systems; their relationship to regional tectonics is more contested. The Tascotal Mesa Fault (TMF) is perhaps the most prominent of these. It trends east-west at roughly 29° 33’ N latitude, transecting most of the western Big Bend region. Our study area between W103° 46.5’ and W103° 49’ is in the central TMF. Here, the TMF intersects several other faults. It also cuts through and offsets the exposed Eocene-Oligocene lithologies that include the stratigraphic section (from oldest to youngest) of the Devils Graveyard Formation, the Mitchell Mesa Rhyolite, the Tascotal Formation, and the Rawls Formation. The fault also cuts the Terneros Creek Rhyolite, intrusive rocks that date at an interval between the deposition of the Devils Graveyard Fm. and the Mitchell Mesa Rhyolite. Additional kinematic indicators are preserved in the lithologies along and adjacent to the fault.

As noted by previous workers, the near-vertical fault records 300-400 m of right-lateral movement. This is best seen by the offset in the Tascotal-Rawls contact in the westernmost part of the study area. The Terneros Creek rhyolite is also offset by the fault, but it is difficult to assess given the irregular margins of the shallow intrusion. And in the central portion of the study area, the TMF offsets a near-vertical fault striking 328°. The TMF also records significant vertical movement, perhaps as much as 200 m.

Broad deformation trends give way to extensive deformation on the downthrown, southern side of the fault trace. Within the 100 meters south of the trace, the Tascotal Formation is upturned to near vertical, and basalt layers in the Rawls Formation are extensively brecciated by calcite filled fractures. Beyond this deformation zone, a broad south plunging anticline stretches to the south. To the immediate north of the TMF, the dips indicate a broader anticline and syncline with a northeast trending axes. In the badlands canyons on the east side of the study area, the TMF runs through the Devils Graveyard Formation, and exhibits 40 m of within-section offset and lesser deformation adjacent to the trace.