LOW-TEMPERATURE CHRONOLOGY RECORD OF TECTONIC AND EROSIONAL EXHUMATION OF GNEISS DOMES IN THE NORTHERN NORTH AMERICAN CORDILLERA

It has been long recognized that exogenic and endogenic processes control mass and heat transfer within the crust, and production/reorganization of topography at the surface, yet their relative contribution to total denudation history is not easy to differentiate. To determine the timing, rates, and style of exhumation, we focus on the Shuswap metamorphic core complex, where a series of migmatite-cored gneiss domes formed during collapse of the thickened crust in the Cenozoic, at both its narrow, high-relief northern end (Thor-Odin Dome) and its wider, lower-relief southern termination (Okanogan Dome). We use multiple low-temperature thermochronologic techniques (apatite fission track and apatite and zircon U-Th/He) combined with existing structural, geo- and thermochronologic data in order to document complete T-t paths of mid-crustal rocks. In both domes a very high cooling rate (>60 °C/My) is observed through zircon He closure temperature (~180 °C). Apatite fission track and He ages vary between 51-32 Ma in the Okanogan Dome, suggesting exhumation to shallow crustal levels occurred subsequent to Eocene extension. In contrast, the Thor-Odin Dome reveals a more complex history. Here, apatite fission track ages range between 44-14 Ma and show a strong correlation with elevation. Thermal modeling of samples from higher altitudes (~2100 to 1300 m) verify only rapid Eocene cooling, whereas the lower-elevation samples reveal an additional Plio-Quaternary cooling event. Apatite U-Th/He chronometry of these samples yields late Miocene (6-12 Ma) ages. Overall our results show that both migmatite-cored gneiss domes experienced a rapid cooling phase during initial ascent of dome rocks facilitated by detachment tectonics following Eocene extension. A subsequent protracted cooling history suggests a very slow erosional exhumation in the Okanogan Dome, whereas a recent rapid exhumation pulse at ~3 Ma resulted in >2km of erosional exhumation of the Thor-Odin Dome.