Late Cretaceous to Early Tertiary Deep Crustal Ductile Flow in the Coast Mountains Batholithic Complex of British Columbia

Andronicos, C L (1), Chardon, D H (2), Davidson, C (3), Gehrels, G E (4), *Hollister, L S (1), Morozov, I B (5), Smithson, S B (5)

(1) Department of Geosciences, Princeton University, Princeton, NJ 08544 United States

(2) CEREGE-CNRS, Universite d'Aix-Marseille Aix en Provence, 13545 France

(3) Dept. of Geology, Beloit College Beloit, WI 53511 United States

(4) University of Arizona, Tucson, AZ 85721 United States

(5) Dept. of Geology and Geophysics, University of Wyoming, Laramie, WY 82071 United States

We interpret the crustal structure of a 180 km long transect across the Coast Mountains of British Columbia, using surface geology, controlled source seismology, and geochronology. The seismic profile clearly images features within the crust and down tothe Moho. The exposed rocks, primarily high grade metamorphic or plutonic, plunge into the seismic profile, and important structures identified on the surface can be recognized in the seismic profile. The resulting 3-d image and the age constraints lead to interpretations of how the deep crust responded to a prolonged period of large magnitude ductile transpressive flow, and how it responded when the period of transpression ceased. The combination of age and surface structural and metamorphic data show that the core of the orogen was at or above upper amphibolite facies metamorphic conditions continuously from 85 Ma to 50 Ma. The crust responded ductily during transpression by partitioning strain along wide orogen parallel dextral strike slip shear zones and orogen normal, top to the southwest, reverse shear zones between 85 and 58 Ma, and by lateral extension between 53 and 50 Ma.During the entire 85 - 50 Ma history, the core where the ductile flow occurred was bordered to the southwest by a relatively cooler, rigid wall. This wall forms the southwest boundary of the Coast shear zone that was imaged seismically as a 5-10 km wide vertical zone extending from the surface to the Moho. A relatively high velocity keel near the base of the crust within the core is interpreted to be due to magmatic underplating and intraplating of basalt sills that intruded during the ductile deformation. The basalts were the primary source of heat for maintaining high temperatures in the lower crust of the core of the orogen. The crust is abnormally thin for an orogenic belt, averaging 31 km thick; this is attributed to be mainly due to Tertiary extensional deformation.

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