Seismic reflection and tomographic velocity model constraints on the evolution of the Tofino forearc basin, British Columbia

Abstract

S U M M A R YThe 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 ∼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 ∼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.

Systematic characterization of morphotectonic variability along the Cascadia convergent margin: Implications for shallow megathrust behavior and tsunami hazards

“…Static backstop geology and geometry have been characterized at numerous locations along the margin using a combination of crustalscale seismic, potentialfield, and geologic mapping data (e.g., Hyndman et al, 1990;Snavely and Wells, 1996;Parsons et al, 1999;Fleming and Tréhu, 1999;McCrory, 2000;Hayward and Calvert, 2007;Tréhu et al, 2012;McCrory and Wilson, 2013). The dynamic backstop along the Cascadia margin is interpreted to be the seaward boundary of the PlioPleistocene wedge, and its general location along the entire margin has been inferred previ ously by Snavely (1987) and has been mapped in more detail based on seismicreflection imagery and deepsea drilling information offshore portions of Vancouver Island, Washington, and Oregon (e.g., Silver, 1972;Mann and Snavely, 1984;Hyndman et al, 1990;Adam et al, 2004).…”

Seismic Velocity Structure Beneath the Tofino Forearc Basin Using Full Waveform Inversion

“…The amount of displacement along the faults and style of deformation varies along the margin, which indicates localized variations of pore fluid pressure. Two-dimensional traveltime tomography of MCS firstarrival times suggest that sediment deposition increased more rapidly in the later half of the Tofino basin history [10,12]. According to this study, the development of the basin is related to the reactivation of the terrane-bounding Tofino fault.…”

“…P-wave velocities derived from tomography show that anticlinal folds in the accretionary wedge sediments indicate low velocities at the apex of the fold. In contrast, folded sediments over the Crescent Terrane exhibit a velocity pattern that more closely mimics the shape of the folds [12]. The lower velocities at the apex of the fold are interpreted as the result of fracturing of older (Late Pliocene), more lithified sediments that contain fluids derived from deeper sediments and the accretionary wedge as opposed to overlying younger (Pleistocene to Holocene) sediments where the low-velocity is due to their lower burial depth.…”

“…A series of multi-channel seismic (MCS) lines were collected across Tofino basin in 1985 and 1989 by the Geological Survey of Canada (Figure 1). Interpretation of seismic reflection profiles, traveltime inversion velocity models and biostratigraphic well data suggest that character of the basement has largely controlled deformation of the overlying Tofino basin sediments [5,[11][12][13][14][15]. On migrated seismic sections, some thrust faults appear to penetrate close to the top of the subducting oceanic crust [6,16].…”

Uncertainty Based Multi-Step Seismic Analysis for the Near Surface Imaging

“…Our motivation for this work is to develop a simple but effective method that will use shallow seismic data more completely (Figure 1). The application of similar procedures to those used for deep experiments [22,23] but in a simplified form, would allow us to overcome multiple processing problems and obtain a final result with an estimated uncertainty. The use of additional analyses which are effective in near-surface studies (e.g.…”