New Mexico Geological Society Annual Spring Meeting — Abstracts

Regional piercing points in Early Proterozoic rocks, defined by the intersections of subhorizontal isobaric surfaces with steeply dipping stratigraphic markers and structures, allow restoration of 50-150 km of right lateral strike-slip along a network of N-S striking Laramide (?) faults that formed the precursor of the Rio Grande Rift in northern New Mexico. The restoration of slip provides new insight into the structure of Proterozoic rocks and the regional correlation of Proterozoic provinces.

Realignment of Proterozoic rocks in the Tusas, Picuris, Truchas and Rio Mora uplifts shows an originally E-W to NW-trending ductile thrust belt that may extend westward to the Needle Mts. of southern Colorado. This thrust belt probably formed near the southern margin of the Yavapai province and records continental collision of a younger Proterozoic province (1.65 Ga) in central New Mexico (Mazatzal province). Proterozoic rocks in New Mexico can be placed into the Yavapai (1.74-1.72 Ga) or Mazatzal (1.65 Ga) province based upon age and isotopic characteristics. A broad boundarybetween provinces is formed where the volcanogenic basement (1.74-1.72 Ga) of northern NM and southern CO (Yavapai province) and overlying sedimentary cover (Hondo Group, Uncompahgre Formation) were deformed together during the 1.65 Ga Mazatzal Orogeny.

The brittle character ofthe Picuris-Pecos, Borrego, Nacimiento, and other faults suggests post-Precambrian movement. Although some Pennsylvanian displacement is likely, the right lateral strike-slip displacement is probably Laramide age. This interpretation is supported by the association of faults with en echelon folds of Jurassic-Cretaceous rocks in the Nacimiento Mountains and the regional balance of Laramide foreland shortening in the Wyoming province and extension in Arizona. Right lateral displacement was related to decoupling and northward movement of the Colorado Plateau relative to the midcontinent. This movement was driven by body forces associated with gravitational collapse of the overthickened Arizona segment of the Cordilleran orogen. These crustal-scale faults localized later Tertiary extension resulting in the opening ofthe Rio Grande rift.