ABSTRACT
Proterozoic basement rocks in northern New Mexico consist of mid-crustal (10-15 km) metamorphic and metaigneous lithologies that were exhumed to their present day elevations in the Rocky Mountains during punctuated Meso- and Neoproterozoic, Ancestral Rocky Mountain, Laramide, and Rio Grande tectonism. The Precambrian-Paleozoic unconformity in the region indicates that the crystalline basement was deeply eroded by the Mississippian if not earlier, however, the general lack of older sedimentary rocks in the southwestern U.S. precludes identification of Precambrian orogenic events from the sedimentary record. Instead, geochronologic and thermochronologic techniques, particularly the ⁴⁰Ar/ ³⁹Ar method, are used to recognize low temperature cooling ages and thermal histories of mineral geochronometers in order to reconstruct a regional tectonic history. Overall, ⁴⁰Ar/ ³⁹Ar dates for hornblende and mica from the Sangre de Cristo Range support protracted cooling (<400°C) following ca. 1.4 Ga regional metamorphism. K-feldspar yield modeled thermal histories that support progressive, yet punctuated, cooling, faulting, and denudation of the crystalline basement during the Grenville orogeny (1100 to 1000 Ma) and the subsequent break up of Rodinia (850 to 750 Ma). In the Pecos area, metasomatic K-feldspar is abundant and has yielded ⁴⁰Ar/ ³⁹Ar dates ranging from ~420 Ma to >1.0 Ga. These varied ages indicate multiple fluid flow events that can be linked, in several cases, to episodic basement deformation.