Abstract Gas and oil fields producing from Devonian reservoirs in the Permian basin of West Texas and New Mexico form a remarkably uniform pattern—fields producing only gas on the south and west, fields with high gas/oil ratios and gas-condensate fields near the center of the region, and fields which produce only oil on the north. The methane content of gases in these fields has a similar regional variation from more than 90 per cent methane on the south and west to less than 30 per cent on the northeast. Analysis of variation of other gas components and crude oil suggests that the methane variation is independent of both other gases and crude oil composition. Relation of methane variation to variation in gas volume suggests that variation in gas distribution is primarily a function of methane distribution. Limited data suggest that carbon dioxide may be anticipated in increasing volumes along the southern margin of the region. Uniformity of the observed pattern indicates that only gas will be found in Silurian-Devonian reservoirs in the Delaware and Val Verde basins. The gas-distribution pattern bears no consistent relation to Silurian-Devonian lithofacies or structural relief. There is simply an increase in the amount of methane toward the Marathon thrust belt. It is proposed that the gas pattern may have been effected by distribution of methane in solution in formation water migrating away from the Marathon thrust belt under the influence of supernormal pressures resulting from overthrusting. Carbon dioxide may have been distributed similarly at a later time and in smaller volumes than methane. Differential distribution of both gases may have been a function of volume of methane or carbon dioxide in solution in a given area and of the volume of gas-bearing water reaching the area.

The evolution of crystalline continental crust probably has been dominated by arc magmatism. Olivine, pyroxene, and garnet are largely crystallized from rising arc melts in the subcontinental mantle. Residual hot, dry gabbroic magmas cross the density filter of the Mohorovičić discontinuity and spread out in the basal crust, heating preexisting crustal rocks and producing widespread secondary melting. The basal crust is dominated by two-pyroxene gabbro, increasingly fractionated and contaminated upward, variably metamorphosed in granulite facies. Migmatites and restites, of granulite or uppermost amphibolite facies, dominate the higher part of the lower crust, and from these rise mixed and secondary melts, of intermediate to felsic compositions, that evolve with complex combinations of crystallization, fractionation, and assimilation. The rising melts heat the middle crust, causing hybridization, migmatization, and secondary melting. The melts absorb much water derived mostly by breakdown of wall-rock micas, in continental crust, and of hornblende in island-arc crust. A hydrated melt cannot rise past the level at which load pressure equals fluid pressure, so hydrous melts expel their volatiles and crystallize in the middle crust, often as two-mica granitic rocks in continental crust. Only melts not hydrated by middle-crust reactions remain hot and dry enough to rise to the upper crust to crytallize as shallow batholiths and to erupt as ash flows. Rift-related continental magmatism has also been important, although criteria for identifying its products are much less definitive than generally assumed. Volcanic-rift assemblages are in many cases basaltic or bimodal basaltic and rhyolitic, but they can include voluminous rocks of intermediate compositions. Arc assemblages also can be strongly bimodal, on modest scales of space and time. Metaluminous rhyolites are abundant in both rifts and arcs, but peralkaline ones occur primarily in rifts. Ancient rift complexes include upper-crustal layered gabbro-granite-rhyolite complexes. Ancient and modern rift systems display distinctively layered basal crust on reflection profiles, likely recording widespread injection of gabbroic magmas from the mantle. Foreland thrust belts, wherein preexisting stratal wedges are imbricated craton-ward, have formed in both collisional and noncollisional settings. Paleozoic imbrication in Appalachian and Ouachita-Marathon regions was a byproduct of arc-continent and continent-continent collisions; basement overthrusting also affected the southern Appalachians. Cretaceous thrust-belt imbrication in much of the Cordillera occurred in an Andean setting, and gravitational spreading due to magmatic thickening of the crust in the magmatic-arc belt may have been responsible. The latest Cretaceous and early Paleogene Laramide shortening of the Rocky Mountain sector of the craton represents a clockwise rotation of the Colorado Plateau region of about 4° relative to the continental interior about a New Mexico Euler pole. This, and the synchronous tectonic erosion from beneath of continental crust farther southwest, may have been byproducts of drag on subducting Pacific lithosphere that did not sink out of the way of the advancing continent. Early Basin and Range extension occurred in back-arc-spreading mode, behind a trench and subducting margin; late extension has been in oblique mode as the continent has adjusted its shape to that of the evolving San Andreas boundary. The dominant mode of extension has been normal faulting wherein footwalls rise and deform and hingelines migrate between inactivated, undulating sectors of faults and still-active dipping sectors. Basin and Range extension followed widespread magmatic heating of the crust; it has doubled the width of a broad region and has continued for 30 m.y. Extension starting with cold crust may be geologically more common and may result in extension of much lesser extent and shorter duration before a continent is sundered and oceanic spreading begins. The Midcontinent Rift system is of this type, although extension there stopped before reaching the stage of normal oceanic spreading. The Atlantic continental shelf may be largely prograded over thick Mesozoic basaltic crust, rather than having been built atop severely thinned continental crust.

Abstract Persimmon Gap is unique in that the effects of three major Persimmon Gap. A wind gap that was part of the historic orogenies are exposed. A Paleozoic inlier features the southern- Comanche Trail crosses the southern Santiago Mountains at the most exposed thrust faults (Upper Ordovician over Lower Pennnorthern entrance to Big Bend National Park. The Persimmon sylvanian) of the Ouachita system in Texas. These rocks and the Gap site is 42 mi (68 km) south of Marathon, Texas, along U.S. Cretaceus cover (predominantly limestone) were involved in 385 (Fig. 1). The main outcrop is a prominent peak 0.5 mi (0.8 oblique overthrusting during Laramide left-slip transgression, lokm) east of the highway that can be reached easily by walking up tally creating a positive flower structure. Right-slip transtensional an arroyo into the hills (Fig. 2). Beware of cactus! Permission to faulting of the present cycle of Basin and Range deformation is walk off road should first be obtained at Persimmon Gap Ranger evident via downdropped fault blocks. The geologic history of the station 0.2 mi (0.3 km) south of the outcrop. The park has a region is dominated by repeated movements of basement-rooted nominal entrance fee. northwest-trending faults.