The Hueco bolson of Trans-Pecos Texas formed in response to Cenozoic extensional tectonism and lies within the southern Rio Grande rift near the poorly defined boundary between the rift and the southern Basin and Range province. The bolson is composed of a northwest subbasin that contains north-striking normal faults and a southeast subbasin that contains northwest-striking normal faults. Cenozoic basin fill is thin (less than 150 to 200 m) on the east and northeast bolson margins and is thick (as much as 2,850 m) in the central bolson and on the west and southwest bolson margins where major normal faults bound a graben that is 15 to 25 km wide. Major faults bounding the graben on the west and southwest have been more active and exhibit greater offset than do boundary faults on the east. This disparity in displacement between the graben margins has resulted in an asymmetric graben. Isopach maps of lower and upper basin fill sequences, differentiated from seismic data, indicate that much of the southeast Hueco bolson subsided more than the northwest Hueco bolson during deposition of the lower basin fill. Thicker upper basin fill within the northwest basin indicates that the basin subsided more in the northwest than in the southeast during deposition of the upper basin fill. The two major faults that bound the Hueco graben on the west and southwest, the East Franklin Mountains fault and Amargosa fault, respectively, have had the most recent (late Pleistocene-Holocene) surface ruptures. Scarp-slope angles of these faults are commonly steeper than 20°, and middle Pleistocene surficial deposits that contain indurated calcic soils having stage IV to V morphology are offset between 24 and 32 m. Maximum throw on these faults during single surface-rupture events has been between 1.6 and 3 m. Major faults bounding the southeast Hueco graben on the northeast (Campo Grande, Caballo, and an unnamed fault) had their most recent surface ruptures during the late Pleistocene. Scarp-slope angles of these faults are rarely as much as 15° and more commonly between 4 and 7°. Middle Pleistocene surficial deposits that contain indurated calcic soils having a stage IV to V morphology are offset between 1.6 and 24 m. Maximum throw on these faults during single surface-rupture events has been between 0.6 and 2 m.

In the Big Bend segment of the Rio Grande rift (Trans-Pecos Texas), the formation of deep grabens began in early Miocene time (about 23 Ma) and was accompanied by basaltic volcanism of small volume. Graben faults that cut Quaternary deposits attest that extensional deformation continues. Volcanism accompanied only the early phase of rifting, between 23.3 and 21 Ma. Alkaline basalt flows, dikes, and sills predominate, and the felsic suites of the northern Rio Grande rift are not present. The only voluminous rift volcanism was in Black Gap graben in easternmost Big Bend and in the Bofecillos Mountains on the west. The young eruptive centers of the northern rift are absent, but heat flow is nonetheless elevated in both rift basins and transverse structural zones in the Big Bend. Although geophysical data are far sparser here, there is no persuasive evidence for a massive mafic intrusive complex beneath the southern rift. Rupture of the entire lithosphere is suggested by the relatively primitive compositions of the basalts, several of which bear lower crustal or mantle xenoliths. Rift basins include Presidio (northern and southern segments), Redford, Santana, and Black Gap grabens, as well as the Sunken Block (centered on Big Bend National Park), which encompasses Castolon and Tornillo grabens and their continuations into Mexico. Basal sedimentary deposits in earlier formed grabens are dated at 23 to 22 Ma and rest unconformably across older rocks. Younger grabens of more northerly orientation began to develop at about 10 Ma. Bounding faults strike northwest to north-northwest, and basins of various sizes terminate or change size and/or geometry at west-trending discontinuities. The Shatter, Presidio, and Terlingua transverse zones are long-lived fault complexes that link and/or segment Big Bend rift grabens. None of these westerly structural corridors has yet functioned as a transform. All predate late Cenozoic extensional deformation, all extend well beyond their intersections with rift-basin-bounding faults, and none shows significant lateral offset, although right-lateral movement is documented. These west-trending zones subdivide the region into smaller structural units with steeper bounding faults than is typical of the northern rift. Deep early Miocene grabens, synextensional volcanism, elevated heat flow, Quaternary normal faulting, and evidence for full lithospheric rupture indicate that the Rio Grande rift extends at least as far south as the continuations of the Castolon and Tornillo grabens into Chihuahua and Coahuila, Mexico. The southern terminus of the rift, however, remains undefined.