Quartz monzonite and porphyritic quartz latite. General description. ____________________ Petrography ___________________________ Porphyritic quartz monzonite ________________ General description _____________________ Petrography-_-_-___-___-__________ Dikes associated with the monzonitic rocks_ __ Petrology of the monzonitic rocks.____________ Age of the monzonitic rocks__________________ Tertiary volcanic and sedimentary rocks.__-_____-Older volclanics-____________________________ Extrusive and sedimentary rocks..________ General description.________________ Lava flows_________________________ Petrology._____________________ Pyroclastic rocks _______________ Epiclastic rocks_______________ Origin and environment___-______._-Intrusive rocks _________________________ General description _________________

No abstract

No abstract

No abstract

No abstract

Age and correlation. _____________ _______ Environment of deposition.._____________ 32 6. Modal, chemical, and spectrographic analyses and norms of dacifcic to andesitic rocks of the middle and upper members of the Bathtub Formation _______________________________________________________ 41 7. Chemical and spectrographic analyses and norms of members of the Salero Formation___________________ 72 8. Modal analyses of rhyodacite from the welded-tuff member of the Salero Formation____________________ 72

The Cordilleran erogenic belt forms a continuous zone of compressive deformation of Late Jurassic to Eocene age resulting from collision of the converging North American plate with northeast Pacific plates. The segment of the belt between El Paso, Texas, and Las Vegas, Nevada, differs from adjacent segments in that subordinate tectonic zones are truncated along an unusual kind of frontal zone. This difference is believed to result from a combination of oblique collision of the deformation zones with the semicratonic Colorado Plateau block, the effects of pre-orogenic basement faults along the southwest margin of that block, southward tilting and deep erosion along this frontal segment, and overprinting of extensional tectonism. The style of deformation across this segment of the erogenic belt varies systematically. In the fold-and-thrust zone (northern Chihuahua, Mexico), folds are large, open to moderately tight, and only slightly inclined, but disharmonic, over (that is, truncated against) genetically related thrust faults and small backthrusts in Permian and Mesozoic rocks. In an eastern intermediate zone (southwestern New Mexico and southeasternmost Arizona), folds are smaller, tighter, and more markedly inclined northeastward toward related shingled thrust faults and larger backthrusts. Likely, detachment occurred along the top of the crystalline basement of Precambrian age. In a western intermediate zone (much of southeastern and central southern Arizona), many folds are also small and are strongly inclined northeastward toward related thrust faults, but some erosional remnants of larger folds involve major thrust plates. Locally, crystalline basement rocks occur in these major plates. In the hinterland zone, many folds are recumbent, and thrust faults are numerous and commonly are parallel to bedding in ductily deformed thin plates of Phanerozoic rock and to foliation in massive plates of basement rock. Areas of mild deformation of a foreland zone lie northeast of, and beneath the fold-and-thrust zone. They are viewed as being external to the main part of the erogenic belt because their faults probably are not continuous with those of the erogenic belt proper in the level of relatively brittle deformation. This zone extends across the Colorado Plateau and east to the Rocky Mountain front. The style and age of deformation in this zone and that of the fold-and-thrust zone are most typically the Laramide phase of the orogeny.

Rocks across the Cordilleran orogenic belt between El Paso, Texas, and Las Vegas, Nevada, show the systematic changes in deformational style, age, and cover thickness found along other transects of the belt. Along the northern part of the El Paso-Las Vegas region, continuity of the tectonic zones-foreland, fold and thrust, intermediate, and hinterland-is disrupted by strike-slip, oblique-slip, and other kinds of structures, as well as by some areas in which erosion has removed overriding plates.Typical fold and thrust deformation occurs in Mesozoic rocks of the Sierra Juárez, southwest of El Paso, where folds are abundant, large, and disharmonic to large thrust faults and small backthrusts. Paleocene-Eocene fold and thrust zone and foreland zone deposits occur in small continental basins. Late-orogenic Eocene igneous rocks are sparse. Deformation at exposed levels occurred beneath 2 to 3 km of cover and was of relatively short duration.In the foreland zone, folds are few, widely dispersed, and associated with reverse faults whose downdip continuity with the fold and thrust zone faults seems unlikely. This Paleocene-Eocene deformation involves the continental basin deposits. Similar mild deformation probably occurred in a subfold and thrust terrane, now exposed through erosion of fold and thrust zone rocks, and possibly is exemplified in the Florida Mountains of central southern New Mexico.In southwestern New Mexico and southeastern Arizona the intermediate tectonic zone (or zones) contains more intensely deformed Mesozoic to Precambrian rocks, voluminous and varied orogenic igneous rocks, and older foreland basin deposits partly of marine origin. Folds are smaller, tighter, and subparallel to shingled thrusts and large backthrusts. Deformation occurred beneath 6 to 8 km of cover during early Late Cretaceous-Paleocene time. This zone(s) contains remnants of earlier and shallower fold and thrust zone and foreland zone structures that were overprinted by the more intense, deeper seated, longer lasting deformation characteristic of this zone.