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.

The Solitario displays geologic features that span virtually the entire regional history of Trans-Pecos Texas since Cambrian time. The visible structure (cover) is the eroded remnant of the roof of a radially symmetric late Eocene (38 Ma) laccolith. Erosion of the laccolith roof has exposed a remarkably complete stratigraphic section. The rock record begins with Upper Cambrian Dagger Flat Sandstone. Deposition of Upper Cambrian sand and shale in a shallow sea gave way during Ordovician to deposition of black shales interbedded with some sand and black chert, reflecting more restricted circulation. About 1 km of sediments, from the craton to the north and northwest, accumulated in the Ouachita Trough during Late Cambrian and Ordovician time. The area was elevated and slightly tilted, but not significantly deformed, by the Llanorian Orogeny during Silurian time. Silurian rocks are missing, and the Lower Devonian-Mississippian Caballos Novaculite rests unconformably on the Upper Ordovician Maravillas Formation. More than 1.4 km of flysch, from a source to the southeast, forms the Mississippian-Pennsylvanian Tesnus Formation. No Paleozoic rocks younger than Early Pennsylvanian (Morrowan Series) have been found. The measured thickness of Paleozoic rocks in the Solitario is approximately 2.6 km and represents a time span of 240 m.y., with a single break of ~30 m.y. during Silurian, one of the longest depositional records known. The Paleozoic rocks presently found in the Solitario are allochthonous and were intensely deformed during the Ouachita Orogeny. The orogeny affected the Solitario area from Middle Pennsylvanian (Desmoinesian) until Early Permian (middle Wolfcampian). Transport of the allochthon during the Ouachita Orogeny was at least tens of kilometers from the southeast. Deformation was primarily by folding, with the development of nappes, S-folds, boudinage structures, and local and regional thrust faults evident in the exposed Paleozoic rocks. After the Ouachita Orogeny, the Solitario area remained positive from Early Permian (middle Wolfcampian) on the structural block known as the Tascotal Uplift that formed the southern margin of the Permian sea. Throughout early Mesozoic, the area remained elevated on the West Texas-Coahuila Platform, and was extensively eroded as part of the Wichita paleoplain. In Early Cretaceous (late Aptian), the area was covered by a shallow sea, and 1.2 km of carbonates were deposited. These rocks are now magnificently exposed in cross section in the shutups that cut the rim of the Solitario dome. The Cretaceous rocks are correlative with carbonate units found to the east and south in the Gulf Coast area. At the end of the Cretaceous (Gulfian), the area was elevated once again as the Laramide Orogeny migrated eastward. Regionally, the Solitario lies on a large structural block that is defined by gravity data as a remnant of the Tascotal Uplift. The block appears to have responded to Laramide compression by uplift and rigid-body rotation without undergoing extensive internal deformation. Deformation associated with the Laramide Orogeny had no discernible effect on the later emplacement of the Solitario laccolith. Within the mapped area, Laramide compression is, at most, presently evident only as sparse stylolites in the Cretaceous rim rocks. Mid-Eocene basal conglomerate of the Devils Graveyard Formation, shed from Laramide folds to the west, is found in Fresno Canyon, and is the only Tertiary rock that predates the formation of the Solitario dome. The oldest reliably dated igneous rock in the Solitario is a 37.5 ± 0.8 Ma rhyolite sill. The sill intruded the base of the Cretaceous section immediately prior to the formation of the Solitario dome. The dome was formed by intrusion of ~100 km 3 of silicic magma that formed the present granite laccolith shortly after emplacement of the rim sill. The structural relief of the dome is 1.6 km, and the roof underwent 400 m of radial extension from the center. A crestal graben formed during doming, and the graben block collapsed less than 1 m.y. after formation of the dome, foundering and rotating down to the south after the roof was deeply eroded. The foundering of the crestal graben block was probably contemporaneous with the emplacement of a granite intrusion on the eastern side of the collapsed block and formation of a small caldera south of the crestal graben block. The series of intrusive and extrusive volcanic rocks found within the dome includes 14 mappable rock types, with a wide range of compositions. The Solitario igneous suite was emplaced over a total time span of 11 m.y.; silicic igneous activity was probably limited to the first 3 m.y. of this time. Younger, more mafic rocks have vents within the Solitario dome, and are thus included within the suite, but appear to be genetically and temporally related to the Bofecillos volcanic center, immediately west of the dome. The oldest units of the central basin-filling Needle Peak Tuff were deposited in late Eocene within 1 m.y. after the dome was formed. The roof of the dome was therefore eroded to virtually its present level by the end of the Eocene. The emplacement of the Needle Peak Tuff is associated, at least in part, with the collapse of a small caldera in the south part of the central basin. Volcaniclastic rocks accumulated in surrounding areas during the Oligocene and early Miocene, particularly those erupted from the Bofecillos volcanic center to the west. Early Oligocene Chisos Formation pinches out against the western flank of the dome. These volcanic units eventually lapped high onto the eroded rim of the dome, but did not spill over into the central basin. From early Miocene until the Quaternary, the area was an elevated plain, with the streams at or near their base level. There is no evidence in the map area for significant erosion or deposition from early Miocene until the Pleistocene, when the Rio Grande began actively downcutting its bed to the south. The base level of all local streams was lowered as a result. The map area is presently being rapidly eroded, and the late Eocene topography has been partially resurrected.

Peraluminous rhyolites that are chemically somewhat similar to topaz rhyolites and anorogenic granites occur in an orogenic setting near Sierra Blanca in the Tertiary Trans-Pecos magmatic province. The Sierra Blanca rhyolites are even more enriched in most incompatible trace elements than are topaz rhyolites. Some of the extreme enrichments may in part be the result of chemical modification by crystallization from an F-rich vapor phase. The rhyolites were intruded as laccoliths at 36 Ma, during the main phase of Trans-Pecos igneous activity, which is characterized by ash-flow eruptions from numerous calderas and widespread mafic, intermediate, and silicic intrusions. A dominant east-northeast orientation of dikes and veins throughout the region indicates mild compression that was residual from Laramide deformation. This compressive tectonic setting, coupled with concurrent volcanism in Mexico and the east-northeast change in magma chemistry from calc-alkalic in western Mexico through alkali-calcic to alkalic in Texas, suggests that the rhyolites were emplaced in a continental arc. Extension did not begin in Trans-Pecos Texas until after 32 Ma; 31- to 17-Ma dikes are dominantly oriented north-northwest, perpendicular to the direction of extension during early Basin and Range deformation. Thus, the tectonic setting of the Sierra Blanca rhyolites contrasts with that of typical topaz rhyolites, most of which were emplaced during periods of crustal extension. The Sierra Blanca rhyolites are chemical and mineralogic oddities for the region, where most rhyolites are peralkaline or metaluminous. The rhyolites are depleted in the same elements as topaz rhyolites (Mg, Ca, Ti, Sr, Ba) but are more highly evolved than topaz rhyolites. Extreme trace-element enrichments (Li, F, Zn, Rb, Y, Zr, Nb, Sn, Ta, Pb, HREE, Th, U) are accommodated in Li-rich white mica, Zn-rich biotite, Rb-rich feldspars, and numerous trace minerals, including cassiterite, changbaiite, columbite, thorite, xenotime, yttrium- and REE–rich fluorides, and zircon. The rhyolites are large-tonnage, low-grade resources of several rare metals. Also enriched in Be (as much as 180 ppm), the rhyolites are the sources of Be and F in beryllium deposits in fluoritized limestones along the contacts with the laccoliths. Interaction with the limestones probably locally elevated the Ca, Mg, and Sr contents of the rhyolites. Vapor-phase crystallization has modified the original magmatic chemistry of the rocks. Evidence of vapor-phase crystallization includes the presence of minerals typical of pegmatites: cryolite (from 0 to 3 volume percent), alkali feldspars with nearly end-member compositions, polylithionite-zinnwaldite mica, and rutilated quartz, plus fluid inclusions defining quartz overgrowths on magmatic grains. Extreme HREE enrichments (Yb to 72 ppm; chondrite-normalized REE patterns with positive slopes) may also be the result of vapor-phase crystallization.

The Taylor Creek Rhyolite, a group of Tertiary high-silica-rhyolite lava domes and flows in southwestern New Mexico, contains cassiterite-bearing veins whose tin was derived from the host rhyolite as it degassed, cooled, and devitrified immediately after emplacement. Theoretical considerations and studies of fumarolic deposits at many volcanoes worldwide indicate that tin is highly mobile in a vapor phase, probably as halogen complexes, thus favoring the occurrence of such auto-mineralization in a cooling-lava environment. Mass-balance calculations for the New Mexico situation indicate that much of the tin evolved during devitrification of the rhyolite cannot be accounted for in the mineralized deposits. Some of this “missing” tin almost certainly was dispersed into alluvium during erosion of mineralized parts of the lavas, and some may have been transferred to the atmosphere around fumaroles rooted in the cooling lavas. In addition, tin may have been lost to the atmosphere from Taylor Creek Rhyolite magma that was erupted in fountains. The recent recognition of fountain-fed fallback in the New Mexico rhyolite field suggests that this third means of moving tin out of erupting magma may indeed have contributed to the overall tin history in the Taylor Creek Rhyolite magma system. Fountain-fed flows of silicic lavas are not well known, whereas mafic counterparts are known to be common as a result of observations of many eruptions of basaltic magma. Characteristic properties of silicic magmas that collectively tend to result in relatively high viscosity inhibit the occurrence of eruption columns whose fallback is hot enough to thoroughly weld and perhaps totally rehomogenize into a melt that subsequently feeds lava flows. However, high volumetric rates of eruption, high magmatic temperature (relative to solidus temperature), and any other conditions that help to reduce viscosity, in concert with factors that result in relatively brief periods of trajectory for lava clots, favor the formation of fountain-fed silicic lava flows. Fluorine-rich magma of about 830°C produced such silicic lavas in the Taylor Creek Rhyolite, and rocks with a similar mode of emplacement in other volcanic areas are herein hypothesized to be far more abundant than presently recognized. Possible examples elsewhere include large-volume sheets of silicic lavas in southwestern Idaho (eruption temperature of 950° to 1,100°C) and in Trans-Pecos Texas, where lava-flow and welded pyroclastic textures intermixed within individual eruptive units have led to confusion and difficulty in interpreting the mechanism of emplacement. Documentation of fountain-fed silicic lavas is rare; eruptions of silicic magmas are infrequent relative to the average human lifespan, and very few have occurred during historic time. Moreover, evidence of a lava-fountain origin may be only weakly preserved in the rocks so formed; the evidence also may be entirely lacking, as is commonly the case for the closely observed mafic examples.