THERMOCHRONOLOGY LINKS GROWTH OF THE ROCKY MOUNTAIN FRONT RANGE TO INVERSION OF A NEOPROTEROZOIC RIFT SYSTEM

Localization of Phanerozoic deformation in the Southern Rocky Mountains (USA) has been hypothesized to reflect reactivation of a Proterozoic rift system formed during the breakup of the supercontinent Rodinia. The inference that orogenic deformation in continental interiors may be controlled by ancient structures is geologically compelling; however, clear evidence supporting such a linkage is commonly sparse. We document the Neoproterozoic-Recent tectonic history of Paleoproterozoic crystalline rocks in Colorado’s Front Range through thermal history modeling of zircon (U-Th)/He (He) thermochronologic ages that range from 50-607 Ma and vary as a function of crystal U-Th composition (i.e., radiation damage). Contacts between the crystalline rocks and Neoproterozoic and Paleozoic sedimentary rocks provide unique constraints that place our samples near Earth’s surface (<2 km depth) at ca. 700, 500, and 300 Ma. Thermal histories that reproduce the zircon He ages require heating to >250˚C (324±20˚C) and then cooling to <60˚C during late Neoproterozoic time. We argue that these results document the origin of the Front Range as a zone of rifting that experienced km-scale burial and exhumation coeval with Neoproterozoic fragmentation of western Laurentia and concurrent global glaciations.