ROCKIES OF NORTHERN MEXICO 



445 



section is about 7350 feet thick. The lower part is Late Triassic, and the 

 upper part is Early Jurassic. 



EL PASO-RIO GRANDE THRUST BELT 



When the Late Cretaceous seas of the Mexican geosyncline finally 

 withdrew from the El Paso-Rio Grande area, the sedimentary veneer on 

 the Precambrian crystallines was of appreciable, although variable, 

 thickness. In the nortiiern Quitman Mountains, it was 10,000 feet thick. 

 The ancestral Diablo Range along the north side of the Rio Grande was 

 an area of thinning of Pennsylvanian and Permian strata ( see the paleo- 

 tectonic maps of Plates 7, and 8 ) , and the Coahuila platform to the south 

 and west was an area of marked thinning of the Lower Cretaceous 

 (see paleotectonic map, Plate 15). 



The Laramide compressional forces then gripped the El Paso-Rio 

 Grande area and subjected it to intense deformation. Examine the map 

 of Fig. 28.1. The rocks were highly folded and thrust-faulted (Huffington, 

 1943 ) . Three thrusts are prominent, namely, the Devil Ridge, Red Hills, 

 and Quitman. See cross section Y of Fig. 25.16. They occur in the Quit- 

 man and Malone Mountains and in Devil Ridge and, altogether, make 

 a zone about 75 miles long from the Hueco basin on the north to the 

 Chinati Mountains on the south. Folds and thrusts are known in a 

 broad belt west of the Quitman Mountains in Chihuahua, Mexico. The 

 areas of bedrock are few in northern Chihuahua, and these are little 

 known geologically. Consequently, the western limit of the Laramide 

 Sonoran Rockies there is indefinite. They probably merged with the 

 Sierra Madre Occidental Rockies to form a great broad belt of defor- 

 mation. Much of this area is now in the Basin and Range structural prov- 

 ince because of the superposition of later block faults on the Laramide 

 folds and thrusts. 



The El Paso-Rio Grande thrust belt probably extends far enough south- 

 eastward to merge with the Sierra Madre Oriental system of Laramide 

 ranges in Coahuila. Its eastern boundary is sharply defined between the 

 Malone Mountains and the Sierra Blanca. Folding and thrusting are 

 prominent in the Malone Mountains, whereas the strata in the Sierra 



Blanca are but slightly folded and cut by a few small normal faults. 

 Igneous activity was conspicuous in the thrust belt, principally in the 

 Quitman Mountains. The same igneous province spread over an extensive 

 part of the domes and basins of the foothill province, viz., the Davis 

 Mountains volcanic field, the Chisos Mountains, and the Serranias de 

 Burro uplift. See the Tectonic Map of the United States. In the Quitman 

 Mountains, the folding and thrusting are followed by extensive erosion 

 and then eruption of a volcanic series of rhyolites, trachytes, and andes- 

 ites. The volcanic rocks sagged and were intruded by a ring dike of 

 diorite. Then an intrusion of quartz diorite followed, and afterward the 

 quartz monzonite Quitman pluton (Huffington, 1943). This activity, if 

 related to the Davis Mountains volcanics, occurred in Eocene and Oligo- 

 cene time. 



PLATEAU CENTRAL AND SIERRA MADRE ORIENTAL 



The Jurassic and Cretaceous geosyncline of Mexico and the Coahuila 

 peninsula have already been described. Refer particularly to the paleo- 

 tectonic maps of this book. During the Laramide orogeny, the thick sedi- 

 ments of the geosyncline were caught in compressive forces and severely 

 folded, but the thinly veneered peninsula was only slightly deformed. In 

 places, later block faults are superimposed on the Laramide folds; but 

 for the most part, the present-day mountains of the Sierra Madre Oriental 

 and the high surface of the Plateau Central are the result of a long 

 chronicle of erosion and alluviation. The two provinces from the eastern 

 half of the highlands of northern Mexico. They are closely related tectoni- 

 cally but are somewhat different in surface features. The Plateau Cen- 

 tral is an area of relatively low relief, but high altitude, and consists of 

 wide bolson plains from which rise mountains composed largely of the 

 folded sedimentary rocks. The Sierra Madre Oriental is a region of 

 high relief along the east side of the Plateau Central and is composed 

 of parallel mountain ranges also of folded sedimentary rocks. 



Figure 28.4 is a typical example of the folds in the geosynclinal parts 

 of the province. They are tight to the point of being isoclinal, overturned, 

 and even fan-shaped in places, and closely packed; but in spite of the 



