Crustal thickening and tectonic denudation within the Thor-Odin culmination, Monashee complex, southern Canadian Cordillera

Upper amphibolite facies rocks within the Canadian Cordillera are exposed within the hinterland of the Columbian orogen as domal culminations (e.g. Thor-Odin and Frenchman Cap culminations of the Monashee complex). These culminations contain parautochthonous Paleoproterozoic basement (core gneisses) complexly folded with unconformably overlying Paleoproterozoic to early Cambrian platformal metasediments (cover gneisses). This crustal pile underwent Jurassic to Eocene east-directed crustal thickening coeval with the obduction and accretion of allochthonous terranes to the ancestral margin of the North American craton. The overthickened pile experienced an episode of major crustal extension, during the Eocene, that initiated the development of the Cordilleran metamorphic core complexes (e.g. the Shuswap metamorphic core complex). The Monashee complex, a high-grade culmination within the Shuswap core complex, offers a view into deeper levels of tectonism within the orogen.; Most cover gneiss metapelites and some core gneisses within the Thor-Odin culmination contain a second generation sillimanite metamorphic assemblage. Outcrops in the northwest part of the culmination contain kyanite laths, whereas the central culmination contains similar laths pseudomorphed by sillimanite and fibrolite. The western margin of the Monashee complex displays evidence of an inverted thermal overprint as sillimanite replaces kyanite at higher structural levels.; Compiled K/Ar and 40Ar/39Ar ages (58–52 Ma) indicate a cooling event related to rapid tectonic unroofing of the Shuswap metamorphic core complex during the early Tertiary. However, new hornblende 40Ar/39Ar step-heating analyses of amphibolites from the Thor-Odin culmination of the southern Monashee complex indicate older cooling ages (∼70 Ma) at deeper structural levels that escaped the influence of the early Tertiary thermal event. They indicate a slow cooling event related to an earlier thermal event involving crustal thickening and transposition of fold nappes. The younger cooling ages (58–52 Ma) recorded at higher structural levels reflect a later cooling event following renewed heating that produced a thermal inversion (a reversed metamorphic mineral zonation). This renewed thermal activity is thought to have occurred by heating at high structural levels by radiometric decay. (Abstract shortened by UMI.)...