HR: 0800h
AN: T11B-0582    [Abstracts]
TI: Crustal Structure And Magmatism, Coast Mountains Orogen, Latitude 52-53 degrees North, British Columbia, Canada
AU: * Rusmore, M E
EM: rusmore@oxy.edu
AF: Occidental College, Dept. of Geology, Los Angeles, CA 90041, United States
AU: Gehrels, G
EM: ggehrels@email.arizona.edu
AF: University of Arizona, Department of Geosciences, Tucson, AZ 85721, United States
AU: Woodsworth, G J
EM: tricouni@telus.net
AF: Geological Survey of Canada, 101-605 Robson St, Vancouver, BC V6B 5J3, Canada
AB: New geologic data and U-Pb ages reveal complex history of arc accretion, crustal thickening and migration of magmatic fronts during deformation. Plutonic ages define distinct western and eastern Jurassic – mid Cretaceous arcs that share a common history after ~90 Ma. Juxtaposition of these arcs occurred during mid- Cretaceous crustal shortening in a dominantly SW-vergent crustal-scale thrust belt. Significant crustal thickening buried 151 Ma granitic clasts to pressures > 6 kb, and mid-Cretaceous plutons were emplaced at this depth along the axis of the orogen. Thrusting continued after establishment of the 90 Ma arc; a regional SW-verging thrust emplaced high-grade metamorphic rocks of the Yukon-Tanana terrane and deep-seated plutons over low- grade rocks of the Alexander and Wrangellia terranes. The shear zone is coincident with the western boundary of 82-89 Ma plutons and a regionally extensive, late-kinematic, sill-like pluton. Dextral shear zones preserved on the flanks of the orogen suggest a component of Late Cretaceous transpression. By 75 Ma, metamorphism, deformation, and magmatism had migrated central portions of the orogen and there is no evidence of ductile deformation and syn-kinematic metarmorphism younger than ~70 – 65 along the western flank of the orogen. The Coast shear zone localized 62-58 Ma synkinematic plutons during NE-side up displacement, creating a sharp western magmatic front. Sparse cooling ages suggest plutons and metamorphic rocks adjacent to the CSZ cooled through 500-600 deg between 54-58 Ma during exhumation along the shear zone. Voluminous granitic plutons were emplaced from ~55-50 Ma, but significant crustal extension that affected the eastern side of the orogen farther north is not evident along this transect. This history supports previous models of crustal subcretion and the generation of arc magmas in thickened crust. Definition of two pre-90 Ma arcs negates models calling for simple Andean-style orogen prior to mid-Cretaceous times. Late Cretaceous crustal shortening involved dextral transpression. The pattern of Eocene magmatism and apparent lack of significant coeval crustal extension suggest that widespread Eocene magmatism was related to gradual eastward migration of the magmatic arc, rather than extension-related processes.
DE: 1115 Radioisotope geochronology
DE: 8012 High strain deformation zones
DE: 8035 Pluton emplacement
DE: 8104 Continental margins: convergent
SC: Tectonophysics [T]
MN: 2007 Fall Meeting