FIELD MAPPING & REGIONAL REMOTE SENSING ANALYSIS OF STRUCTURES IN THE BUCK HILL VOLCANIC GROUP, TRANS-PECOS TEXAS (BREWSTER COUNTY)

Two distinct fault sets have been identified near the southern extension of the Rio Grande Rift in the area of the 02 Ranch, approximately 270 sections, in central Brewster County, Texas. Although this wild and scenic area was viewed by millions on the PBS series Texas Ranch House, the regional structure has not been explored in detail until recently. The first set of faults strike northwest, offsetting flows and ignimbrites of the Buck Hill Volcanic Group (Eocene to Oligocene in age). Extensional movement of these faults has created mesoscale graben-like structures. Displacement of outcrops in this area suggests the faults have most recently undergone dip-slip as well as right lateral strike-slip motion. Both the relative and absolute depth of these surface manifestations is currently unknown. The second set of faults strike due west and exhibit predominant strike-slip motion. These west striking faults also offset the units of the Buck Hill Volcanic Group, as well as offsetting underlying Cretaceous carbonates in the southern areas of the ranch. Based on the offset of surface units, the most recent slip on these faults appears to be right lateral. Among the faults in this second set is the Chalk Draw Fault, a strike-slip fault in Trans-Pecos, Texas which extends at least 100 km in length. Field mapping is being supplemented by Principle Components Analysis (PCA) of 15m resolution merged multispectral Landsat imagery. This well-established technique has proven useful for quantifying compositional and vegetation differences associated with surface contacts and lineaments. It has also proven useful for establishing regional variances in mapped formations, some of which have been used in the past to infer specific structures. 1m resolution aerial photography is used to supplement regional scale mapping of formation contacts identified in the multispectral data and lineaments. Crosscutting relationships suggest that these structures are related to ongoing extensional events in the western United States. However, orientations of east-west trending strike slip faults suggest that reactivation of older structures may be indicated. These events would be originating from pre-existing structures being overprinted by the more recent extensional tectonics of the Rio Grande Rift.