Seismic reflection and tomographic velocity model constraints on the evolution of the Tofino forearc basin, British Columbia
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| Authors: | Hayward, N; Calvert, AJ |
| Year: | 2007 |
| Journal: | Geophys. J. Int. 168: 634-U2 Article Link (DOI) |
| Title: | Seismic reflection and tomographic velocity model constraints on the evolution of the Tofino forearc basin, British Columbia |
| Abstract: | The Tofino Basin is a sedimentary forearc basin that overlies the continental shelf of the Cascadia margin to the southwest of Vancouver Island. The basin, which contains up to similar to 4 km of marine clastic sedimentary rocks, formed following accretion in the Early Eocene of the Crescent and Pacific Rim Terranes, and subsequent accretionary wedge basement. Subduction of the Juan de Fuca plate has since been the primary tectonic driving force in the development of the basin's structure. Investigations using coincident seismic reflection profiles, tomographic velocity models and recently reassessed biostratigraphic well data show that basement composition has largely controlled deformation of the overlying Tofino Basin sediments. Anticlinal folds overlying the accretionary wedge exhibit low P-wave velocities at the apex of the fold, which may be related to fracturing of older, more lithified sediments accompanied by fluid expulsion from the accretionary wedge. In contrast the velocity variation across folds over the Crescent Terrane mimics the fold geometry, and does not appear anomalous. A sub-basin (containing up to similar to 3 km of Oligocene to Holocene sediment) has developed in the central part of the Tofino Basin at the boundary between the Crescent and Pacific Rim Terranes. Seismic interpretation suggests that deposition has increased more rapidly in the Late Miocene to Holocene. Subsidence within the sub-basin is likely to have been controlled by sediment loading, flexure and regional tectonic forces, localized by pre-existing zones of weakness such as the Tofino Fault. The development of the sub-basin may also have been influenced by the displacement landward of part of the lower forearc crust during subduction erosion. Diapiric structures along the axis of the sub-basin suggest that fluid expulsion into the Tofino Basin from the deeper accreted terranes is localized by the terrane-bounding fault. Further seaward, fluid expulsion from the accretionary wedge may be more pervasive. The seismic data demonstrate that subsidence of the sub-basin beneath the inner continental shelf has been occurring relatively late in the history of the Tofino Basin, and that the Cascadia accretionary complex cannot be viewed as growing gradually seaward with no inboard deformation. |
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