Paper No. 4
Presentation Time: 8:45 AM


MAHONEY, J.B., University of Wisconsin, Eau Clare, Eau Claire, 54701, HAGGART, James W., Geol Survey Canada, 625 Robson St, Vancouver, BC V6B 5J3, Canada, KIMBROUGH, David L., Geological Sciences, San Diego State University, San Diego, CA 92182 and GROVE, Marty, Department of Geological Sciences, Stanford University, Stanford, CA 94305,

Rapid orogenic exhumation in the southern Canadian Cordillera is recorded by the tectonic inversion of the Cretaceous Methow basin, coupled with rapid basinal subsidence and initiation of the peripheral foreland Nanaimo basin.

The composite Methow/Methow-Tyaughton basin records a complex basin evolution that initiated with Hauterivian(?) to Albian sedimentation in a forearc basin receiving easterly-derived volcanoplutonic detritus. Facies variations, subsidence rates, and the association between sedimentation and contractional deformation all suggest that the forearc basin was superseded in middle Albian time by structurally controlled subsidence in a complex hybrid foreland basin setting. Sedimentation in the Methow/Methow-Tyaughton basin ended by at least Coniacian time, coincident with the culmination of deformation in the Coast Belt thrust system along the western margin of the basin.

The Nanaimo basin is a peripheral foreland basin formed along the western edge of the Insular Superterrane in southwestern British Columbia. Subsidence initiated in Turonian time in response to contractional crustal thickening in the southern Coast Belt and Cascade Range to the east and southeast. Subsidence increased dramatically during the late Santonian to Maastrichtian, and episodic sedimentation produced a thick succession (>4 km) of complexly intertonguing shelf and submarine fan deposits. The basin contains a distinctly bimodal detrital zircon signature, with a major population of Cretaceous (68 to 100 Ma) zircon representing first cycle plutonic detritus eroded during active contractional tectonism within the Coast Plutonic Complex. A smaller population of Jurassic and Cretaceous zircon was derived from uplifted plutons and Methow basin strata to the east. A small yet distinct population of Meso- and Paleoproterozoic (1300-1800 Ma) zircon is consistently present, and is strikingly similar to that of the Mesoproterozoic Belt Supergroup from the western edge of Laurentia. These zircon data suggest that the arc system was locally breached by one or more large rivers by early Campanian time, allowing extraregional sediment from the distal back arc region to prograded northwestward into the basin.