Fault Border of the Sangre de Cristo Mountains North of Santa Fe, New Mexico

Abstract

The eastern prong of the southern Rocky Mountain province in New Mexico, called the Sangre de Cristo Mountains, is usually described as a great anticlinal structure. Its eastern border and general internal structure conform to this definition but its western border is a fault-line scarp facing the down-faulted structural basin of the Rio Grande depression. This depression is underlain by partly consolidated fan deposits-the Santa Fe formation of Miocene or early Pliocene age. A water-laid tuff, the Picuris formation, with interbedded basalt in places underlies the Santa Fe unconformably. These two formations are separated from the older rocks of the Sangre de Cristo both by erosional and by fault contacts. The fault pattern is irregular and consists largely of en echelon faults trending northwest, but so overlapping as to produce a north-south trend of the scarp from Santa Fe to Chimayo. North of this point the Picuris re-entrant forms an extension of the depression eastward, and the Picuris prong, bounded by a complex fault pattern, forms an extension of the mountain area westward. The Rio Grande depression has been eroded in several successive stages. At the earliest stage the area of outcrop of the Santa Fe was reduced to a plain and the fault scarp was reduced to maturity by erosion. Later revival of streams has accentuated the topography and given the mountain front an apparent height about twice as great as it had in the early stages of erosion.