PROTEROZOIC TECTONIC EVOLUTION OF THE NEW MEXICO- SOUTHERN COLORADO REGION: A SYNTHESIS

Proterozoic crystalline basement of the New Mexico region records two protracted orogenic episodes. 1) A 1.80-1.65 Ga episode involved assembly of continental lithosphere, when arc crust was amalgamated to a southward-expanding Laurentia and crust was progressively thickened and stabilized. This led to a period of lithospheric stability with no tectonism from 1.6-1.5 Ga. 2) A 1.45-1.35 Ga episode involved pervasive intracratonic tectonism, when the lithosphere was destabilized by heating, magmatism, and transpressive tectonism (and inferred development of an orogenic plateau) behind an inferred active margin to the south. Metamorphic data indicate that the rocks that are now at the surface resided at depths of 10-20 km at 1.4 Ga (some were perhaps at similar depths at ca. 1.65 Ga). Thus, New Mexico rocks record polymetamorphism (Paleo- and Mesoproterozoic), but the metamorphic signature was fundamentally established during the Mesoproterozoic event. From 1.35 to1.0 Ga, intracratonic tectonic events took place behind a long-lived active plate margin along southern Laurentia. The inboard effects of this (Grenvillian) tectonism involved deposition of Mesoproterozoic successions (1.25- 1.1 Ga), emplacement of granite and gabbro bodies (~1.1 Ga), and ~1.1 Ga Ar-Ar K-spar cooling ages from basement rocks. At 0.8- 0.55 Ga, faulting and thermal events related to the breakup of the supercontinent of Rodinia resulted in the establishment of the Rocky Mountain trend, as recorded by ~800 Ma Ar-Ar cooling ages and an east-down N-S fault system between the granite- rhyolite provinces to the east and deeper 1.4 Ga rocks of the Rocky Mountains. By Cambrian time, Paleoproterozoic and Mesoproterozoic middle crustal rocks were at the Earth?s surface, and they were then re-buried by 2-3 kilometers of Phanerozoic sediments. Basement rocks were differentially re-exhumed (or further buried) during 0.3 Ga Ancestral Rockies orogeny, 70-50 Ma Laramide orogeny, and 25-0 Ma Miocene extensional event. Present lithospheric structure is the cumulative product of all these events; it is characterized in the upper crust by a polygonal network of ~10-km-scale blocks and in the lower crust and mantle by regions of different fertility and strength many of which were inherited from Paleoproterozoic assembly structures and/or 1.4 Ga tectonism.