STRUCTURES OF BASIN RANGES 



CHARLES R. KEYES 



New Mexico School of Mines, Socorro, N. M. 



Several memoirs recently published have awakened a new inter- 

 est in the geotectonics of the Great Basin region. The main struc- 

 tural features about which discussion centers appears to be whether 

 the Basin ranges are the result of normal faulting and form "block" 

 mountains; or whether the "block" aspect is only apparent, in reality 

 the "block" originally being a sharp asymmetric fold, in which subse- 

 quent erosion wears off the steeper limb faster than the other. 



Tn the elucidation of the arguments by specific example, it is 

 unfortunate that some of the illustrations selected have not been 

 chosen with greater discernment. It is now well understood that 

 some of the instances noted furnish the most conclusive proofs of 

 directly the contrary of the purpose for which they were cited. With- 

 out entering into detail in regard to many of these cited examples from 

 other parts of the Basin region, it seems pertinent at this time to call 

 attention briefly to certain features displayed in the New Mexican 

 part of the field. These may help to explain similar phenomena in 

 other districts. 



The geologic sequence in central New Mexico is especially note- 

 worthy on account of the almost complete absence of the Lower 

 Paleozoic rocks and the enormous development of the Cenozoic 

 strata. The important member of the sequence above the Protero- 

 zoic metamorphics is the Upper Carboniferous limestone which 

 attains a normal thickness of 2,000 feet. 



In all of the mountain ranges of central New Mexico, appearing 

 as tilted blocks, the crests of which rise 3,000-5,000 feet above the 

 bases, the great Carboniferous limestone forms the backslope, usually 

 reaching to the summit. On the opposite or steep face of the ridges 

 the Proterozoic elastics and schists, which usually stand on edge, 

 are exposed for a vertical distance of 2,000 feet, or more. Above 

 the Carboniferous limestones at the foot of the mountains are Red 

 Beds and then Cretaceous sandstones. 



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