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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.

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