FULL-CRUSTAL (P-T-t-D-Z) TECTONICS: COMBINING METAMORPHIC P-T PATHS, IN SITU PETROCHRONOLOGY (t), AND MOHOMETRY (Z) TO REVEAL THE EVOLUTION OF ANCIENT MOUNTAIN BELTS

Metamorphic rocks exposed at Earth’s surface preserve evidence of their journey through the crust during the tectonic evolution of orogenic belts. Despite the wealth of information they hold, metamorphic rocks are still somewhat limited in one major respect: they provide information mainly about the processes at and above the crustal level of their metamorphism, i.e, prograde, peak, and retrograde temperatures and pressures. Without constraints on total and evolving crustal thickness, metamorphic data cannot resolve events and processes below the level of the metamorphism, e.g., in the lower crust. Here, we provide three examples to demonstrate the insight gained by combining geochemical crustal thickness constraints from plutons (mohometry), with petrologic analysis (P-T-history) and monazite/xenotime petrochronology (time). (1) In the New England Appalachians, collapse of the thick Acadian altiplano crust (ca. 340Ma) involved a significant component of deep crustal ductile flow, perhaps analogous to the deep crust of the modern Himalayas. (2) In the Mesoproterozoic Picuris orogeny of SW Laurentia, middle crustal thickening was accompanied by an additional ≥10km of thickening in the lower crust, probably related to magmatic underplating. (3) In the Athabasca Granulite Terrane of NW Canada, granulite facies metamorphism occurred near the Precambrian Moho as the orogenic crust thickened above it. Integrating these methods of analyses provides insight into full-crustal tectonics (evolution and processes) and strengthens comparisons between ancient orogens and the crust under modern mountain belts. The implications are particularly significant for illuminating processes in the deep crust, mechanisms of exhumation, and the transition from orogenic crust to stable post-orogenic continental crust.