300 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Fig. 19.4. Beltian trough (vertically ruled) and the belts of the Laramide orogeny (white). 



Colorado and New Mexico Rockies 



The Ancestral Rockies of Pennsylvanian and Permian age were gradually 

 buried by Triassic, Jurassic, and Cretaceous sediments, and it was upon 

 this crustal make-up that the Laramide belt of deformation was super- 

 posed in Colorado and New Mexico. The back of the ancestral Colorado 

 Range was broken, and two modern ranges were created, both with 

 subparallel elements such that the older range seemed to have exerted 

 some control over the younger. The western half of the ancestral range 

 with a thin sedimentary veneer on the Precambrian crystallines developed 

 a number of thrust sheets. A transverse prophyry belt carries numerous 

 stocks and much ore. 



The Laramide belt of deformation in New Mexico was a narrow one 

 through the central part of the state, and aside from the common north- 

 south orientation of both the Ancestral and Laramide Rockies, their 

 relation seems to be a matter of chance, viz., the younger ranges rose in 

 part where the older ones stood and in part in the sites of the older basins. 



The Laramide structures are large asymmetrical anticlines like those in 

 Wyoming, with gravity slide thrusting on the steep flanks. Graben or 

 rift faulting broke through the Laramide uplifts in a north-south zone in 

 Late Cenozoic time. Much Tertiary volcanism occurred in Colorado and 

 New Mexico. 



Rockies of Northeastern Mexico 



The Laramide system of northeastern Mexico includes the El Paso-Rio 

 Grande thrust belt, the Sierra Madre Oriental, the Sabinas foothill belt, 

 and the Parras synclinorium. The Pennsylvanian Marathon and Coal- 

 huila systems, the Permian ranges, platforms, basins, and shelves of the 

 Marathon foreland, and the late Mesozoic Mexican geosyncline are the 

 foundational elements upon which the Laramide structures were super- 

 posed. The strata of the Mexican geosyncline were generally closely 

 folded lengthwise of the basin, the thin veneer on the old Coalhuila system 

 — the Coalhuila peninsula — was domed broadly and locally flexed, and 

 the basin beds along the east side of the peninsula were folded and thrust 

 eastward. The Parras trough at the south end of the peninsula was in- 

 tensely compressed from south to north, and tight east-west folds and 



