THE CORDILLERAN UNCOVERED: APPLIED LOW-TEMPERATURE THERMOCHRONOLOGY OF EASTERN WASHINGTON AND THE BLUE MOUNTAINS PROVINCE

Bode D Trappett

Collapsed, or collapsing, orogens incorporate a complex mixture of tectonic interactions observed around the world in the mountain belts of the Appalachians, Himalayas, and Central Asian orogenic belt. The North American Cordillera is a prime present-day example of a collaps-ing orogen with large portions remaining understudied. In the north, the Canadian Cordillera shows episodic extension from gravitational collapse of overthickened crust, while further south Basin-and-Range extension involves a more convoluted combination of overthickened crust and influences of Farallon slab rollback. The emplacement of the Columbia River Flood Basalts, a large igneous province, has covered major structures and outcrops of Eastern Washington and Eastern Oregon, hampering the understanding of the full scope their collapse. Utilizing low-temperature thermochronology, this study works to establish constraints on cooling and potential extensional mechanisms. Application of apatite fission-track and apatite (U-Th-Sm)/He analysis reveals distinct periods and regions of cooling across the North American Cordillera. Thermal his-tory modeling shows that Eastern Washington and the Blue Mountains Province of Oregon have undergone completely separate cooling histories following regional transitions to extension. Com-parison to exterior extensional settings show that Eastern Washington has experienced a similar cooling history to that of the southern Canadian Rockies, with Paleocene-Eocene onset of cooling in an episodic fashion. Conversely, the Blue Mountains Province had completed its major phases of cooling in the Cretaceous, long before extension dominated the region, and largely remained unaffected by Cenozoic Cordilleran collapse. Minor localized events are observed in the Blue Mountains, but they do not appear to correlate with any broad scale events. This gap in tectonic history across the Cordilleran front, in the form of the Blue Mountains Province, shows a complex interaction of tectonic processes that have ultimately removed tensional forces from the Blue Mountains and concentrated them elsewhere. This study discusses these tectonic processes and correlates them to the influence of Laurentian basement, regions of over-thickened crust, slab roll-back, slab tears, and hotspot/plume interactions with the lithosphere.

THE CORDILLERAN UNCOVERED: APPLIED LOW-TEMPERATURE THERMOCHRONOLOGY OF EASTERN WASHINGTON AND THE BLUE MOUNTAINS PROVINCE

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