402 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



The Sangre de Cristo Range in New Mexico is less complicated than in 

 Colorado. The eastern front resembles that of the Front Range of Colo- 

 rado (Smith and Ray, 1941), and the flat-lying sedimentary formations 

 of the plains are abruptly upturned along the mountain front, and the 

 Dakota sandstone makes prominent ridges. See section E of Fig. 25.15. 

 A normal fault follows the contact between the sedimentary strata and 

 the Precambrian core of the range for more than 7 miles. 



Still farther south, the structure becomes a broad arch out of which 

 Glorieta Mesa is now eroded. See sections F and G of Fig. 25.15. The 

 east flank is fairly sharply flexed at the north end, but toward the south 

 through Cuervo Butte the arch is broad and regular. The low Pedernal 

 Range, once one of the Ancestral Rockies, is covered in the Glorieta Mesa 

 area; the overlapping of the Pennsylvanian strata on its north end is pic- 

 tured in the lowest cross section. The faults along the west side of the 

 Glorieta Mesa are younger than the arch and flexures, and are classed 

 as Basin and Range. They probably resulted in the valley fill of the 

 Estancia basin. 



The extent of the original Sangre de Cristo uplift in Laramide times is 

 difficult to decipher because of the graben faulting and the extensive 

 volcanism. The rendition of it in Fig. 25.12 is very approximate. 



Still another Laramide uplift, the Sierra, appears to have formed in 

 southwestern New Mexico. Throughout its extent chiefly Precambrian 

 and Cambrian rocks are exposed, but through it the major Rio Grande 

 depression now exists. On the west it probably became part of the ex- 

 tensive Laramide uplift southwest of the Mogollon Rim, but the region 

 is so extensively covered with Tertiary volcanics that the relations cannot 

 be well established. 



The uplifts of the Colorado Plateau and the central Laramide belt of 

 Colorado and New Mexico as depicted in Fig 25.12 all seem to be re- 

 lated tectonically, and their origin by vertical uplift is emphasized. Most 

 geologists who have mapped in the Laramide belt of Colorado and New 

 Mexico have considered the thrust faults to indicate compressional orog- 

 eny, and especially intense compression in Colorado. The Williams Range, 

 Gore Range, and Never Summer Range thrusts in Colorado are probably 

 the most impressive, but these have in no respect the stratigraphic throw 

 of the thrust sheets of the central Rockies. 



Vertical uplift of the magnitude of 2 or 3 miles is indicated by the 

 Front Range, and where the vertical movement has been abrupt along 

 one flank or the other, and steep fronts of imposing elevation have formed, 

 the mechanical elements for major gravity slide blocks are set up. The 

 uplifts of the Colorado Plateau where the Precambrian rocks are not ex- 

 tensively exposed represent a less amount of vertical movement, and it is 

 noted that thrusts have not formed on their margins. It seems logical to 

 the writer, therefore, to regard the thrusts of the Laramide Rockies of 

 Colorado and New Mexico as gravity slide phenomena. 



In Chapter 33 on the igneous provinces of the western part of the con- 

 tinent the theory is advanced that the uplift of the Colorado Plateau and 

 other adjacent areas in the Rocky Mountains in Laramide time and after- 

 ward was due to expansion of a column of the mantle underneath, and 

 that this expansion was at least partly due to its partially melting. Also 

 considerable magma made its way up to the crystalline complex, and 

 there spread out in megasills to elevate the crust above as great blisters. 

 These are the uplifts shown in Fig. 25.12. The concept is illustrated in 

 Fig. 36.4. Lagging somewhat after the intrusion of the megasills came 

 the near-surface and surficial igneous activity, so widespread throughout 

 the Plateau and marginal areas. 



Rio Grande Rift Belt 



Kelley (1952) has reviewed the Rio Grande Rift Belt very well, and 

 summarizes his conclusions as follows: 



In about middle Tertiary time volcanic activity that extruded rhyolitic to 

 andesitic rocks developed on an enormous scale. These eruptions, together 

 with their great outwash of alluvial material, accumulated to thicknesses of 

 several thousand feet. The volcanic suites occur mostly in the western half 

 of the Rocky Mountain belt and in the adjacent Colorado Plateau; but locally, 

 as in the Raton, Cerrillos-South Mountain, and Sierra Blanca areas, the erup- 

 tions developed along the Great Plains border. Nevertheless, the uplifts border- 

 ing the east side of the depression are notably lacking in this suite of rocks. 

 Littie or no sharp folding or overthrusting accompanied the volcanic episode. 

 High-angle faulting, however, appears to have accompanied and followed 

 the great igneous activity. In several places there appears to have been two 

 or three distinct volcanic stages separated by intervals of tectonic disturbance 

 and erosion. Although local basins of accumulation appear to have developed 

 during this epoch of Tertiary deposition and deformation, the areas of accumu- 



