2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 2
Presentation Time: 1:50 PM


JOHNSTON, Stephen T., School of Earth & Ocean Sciences, Univ of Victoria, PO Box 3055 STN CSC, Victoria, BC V8P 3P6 and BOREL, Gilles, Museum of Geology, UNIL-BFSH2, Lausanne, CH-1015, Switzerland, stj@uvic.ca

Are accretionary orogens constructed of exotic lithosphere, or do they consist of re-accreted para-autochthonous blocks? To address this question, we use the Cache Creek terrane (CCT) of the Canadian Cordillera to constrain paleogeographic models of Cordilleran evolution. Key observations are: (1) CCT carbonates overlie OIB basalts indicating deposition on seamounts; (2) Reefal carbonates span the Late Carboniferous into the Middle Triassic, requiring CCT remain tropical until 230 ma; (3) CCT fauna are Tethyan - Verbeekinid (Yabeina) fusilinids in Permian carbonates contrast with American Swagerinid fuslinids, and the only non-Asian occurrence of the Late Permian conodont Wardlawella is in CCT; (4) Carnian (230 Ma) accretion of the CCT to the Stikine arc is recorded by onlap of arc-detritus, and by deformation and blueschist metamorphism; (5) Carnian reefs with Tethyan affinities characterize Stikinia, requiring a tropical latitude until 200 Ma; (6) Attempted subduction of a ribbon continent (Yukon-Tanana / Cassiar) beneath Stikinia at 185 Ma resulted in collision, thick-skinned accretion and exhumation of UHP rocks; (7) Continued subduction beneath the composite arc-ribbon continent (Saybia) led to docking with North America at 150 Ma, ending coastal phosphorite deposition and inundating the continent with westerly derived flysh. We assume a starting point for the CCT in easternmost Tethys at 280 Ma, and keep CCT and Stikinia tropical until 200 Ma. If, as is commonly assumed, accretion to Stikinia occurred adjacent to North America, then unrealistic rates of translation (40 cm/a for 50 Ma) are required. Using reasonable translation rates (11 cm/a), accretion to Stikinia occurs in western Panthalassa, consistent with the mixed Tethyan fauna of Stikinia. Subsequent collision with the ribbon continent occurs in central Panthalassa, >6000 km west of North America. Docking with North American at 150 Ma is assumed to occur 2000 km west of the continental margin. Final accretion at 55 Ma was preceded by significant margin-parallel displacements and coeval crustal shortening of the North America margin. The palegeographic constraints provided by CCT indicate that much of the Canadian Cordilleran accretionary orogen is exotic. The accreting crustal block, Saybia, grew through repeated collisional events within Panthalassa prior to docking with North America.