MIDDLE CRUSTAL CROSS-SECTIONS AND GEOCHRONOLOGIC INSIGHTS ON POLYPHASE TECTONISM IN THE PROTEROZOIC OF NEW MEXICO: 1.5 TO 1.3 GA FORMATION AND DEMISE OF AN OROGENIC PLATEAU

Metasedimentary and igneous basement rocks in northern New Mexico record episodic pulses of tectonism during cratonic growth from 1.8 to 1.38 Ga. Continued challenges involve parsing the deformational features attributable to the Yavapai (1.71-1.68 Ga), Mazatzal (1.66-1.60 Ga), and Picuris (1.5-1.38 Ga) orogenies in this region and understanding how older structures may have been overprinted and reactivated to explain the observed strain. A series of regional cross-sections of the Vadito, Hondo, and Trampas groups of northern New Mexico, combined with new geochronologic and thermochronologic constraints, suggest the following. An early bedding-parallel fabric is common in pre-Trampas Group rocks and interpreted to be related to early isoclinal folds and thrusts. New U-Pb monazite core dates of about 1.5 Ga in aluminous gneisses from the Vadito-Hondo group contact may record earliest Picuris tectonism. Onset of heating/deformation may have been close in age to deposition of the Trampas Group at ~1488 Ma. Growth of garnet at 1.45- 1.40 Ga and monazite rims ~1.45 Ga may record metamorphism close in age to deposition of the nearby syntectonic <1.45 Ga Marqueñas Formation. We interpret the juxtaposition of high grade rocks with similar-aged sediments to be the product of progressive thrusting and development of syntectonic basins. A modern analog is the Tien Shan - Tarim basin region of the Tibetan Plateau. New ⁴⁰Ar/³⁹Ar dates on hornblende and muscovite constrain cooling through ~500 °C and ~350-400 °C respectively and provide insight into cooling rates following the Picuris Orogeny. Hondo and Trampas group rocks cooled through 500 °C 1420-1381 Ma and through 350 °C from 1378-1359 Ma. We compile these new data with ⁴⁰Ar/³⁹Ar cooling data from muscovite from the Petaca pegmatite district (mean age of 1375 ± 10 Ma), and similar ages in basement exposures from neighboring mountain ranges. Slow cooling from >550 to <350 °C from ~1420 to ~1360 Ma suggest cooling rates of ~3 °C/Ma. We propose that an orogenic plateau was built in northern New Mexico during the Picuris Orogeny. The slow erosional removal of this plateau would provide a mechanism to explain the protracted cooling. For the Rio Mora area, K feldspar ⁴⁰Ar/³⁹Ar data suggest there was long term net exhumation from 1400 to 300 Ma with a significant pulse at ~1000 Ma.