ARCHITECTURE OF THE GEORGIA BASIN SOUTHWESTERN BRITISH-COLUMBIA

Georgia Basin is a Cretaceous to Cenozoic forearc basin that overlaps the Wrangellian part of Vancouver Island and the Coast Belt at the lea ding edge of the North American Plate. The basin overlies the presentl y subducting oceanic Juan de Fuca Plate. The basin postdates the mid- to Late Cretaceous amalgamation of the Wrangellia Terrane into the Nor th American Cordillera at the latitude of Vancouver Island. Major epis odes of sedimentation in Georgia Basin, based on the remnant sedimenta ry record, are linked to periods of rapid convergence between the Fara llon/Kula and North American plates; reduced rates of plate convergenc e resulted in uplift and erosion in the basin. The primary phase of ba sin subsidence in the Late Cretaceous accommodated 3 to 5 km of domina ntly marine siliciclastic sediments of the Nanaimo Group. In late Maas trichtian to early Paleocene time, diminished rates of plate convergen ce resulted in local uplift, erosion, and recession of marine waters f rom the basin. Plate convergence increased greatly in the Late Paleoce ne to Late Eocene, resulting in a second phase of rapid subsidence and the accumulation of 3 to 6 km of siliciclastic, mainly nonmarine sedi ment of the Huntingdon and Chuckanut formations. Basin depocentres mig rated to southeastern Georgia Basin. The contemporaneous acme of magma tic activity in the Coast Belt and in Georgia Basin during the Eocene reflects rapid subduction of hot, juvenile oceanic crust beneath the N orth American Plate at this time. In the mid-Eocene, Georgia Basin was contracted on a series of basement-involved thrusts coincident with o utboard accretion of the Pacific Rim and Crescent terranes. Since Eoce ne time, increasingly oblique and diminished rates of plate convergenc e has resulted in widespread basin uplift and erosion of western Georg ia Basin. Only a small depocentre in southeastern Georgia Basin is bei ng maintained by pull-apart along a Neogene strike-slip fault. Georgia Basin remains in the are-trench gap at present and part of it remains topographically low, but subsidence of the central part of basin (if any) under the Strait of Georgia may be more related to downwarping ca used by uplift of western Vancouver Island above the outboard Late Cen ozoic accretionary wedge.