Assessment of submarine slope failures off Vancouver Island, British Columbia

Abstract

Multibeam bathymetric data acquired off Vancouver Island across the accretionary prism of the Cascadia subduction zone reveal a prominent segmentation of the deformation front with dominant azimuths of the ridges at ~120° and ~150° and abundant submarine landslides. Both these ridge-orientations are oblique to the direction of subduction (~45°). Ridges at a strike of ~120° show dominantly rectangular-shaped failure head-scarps and intact blocks of sediments within the failed sediment mass, whereas ridges with an azimuth of ~150° show curved head-scarps and incoherent debris in the failure mass. We propose that this systematic change in failure-style is related to the underlying thrust fault system producing steeper and taller ridges for azimuths around 150°, but less steep and tall ridges at 120°. Thus, debris-flow style failure is simply a result of higher kinetic forcing of the down-sliding sediment mass: more mixing and destruction of the coherent blocks for taller and steeper ridges, and blocks of intact sediment for gentle slopes and less elevated ridges. A segmentation of the deformation front and ridge alignment into two dominant azimuths could be a result of: a) complex interaction and competing forces from overall slab-pull (45°), b) re-activated faults orientated almost N-S (~175°) on the oceanic plate and overlying sediment cover (reflected in the magnetic stripes and abyssal plain strike-slip faulting), and c) relative orientation of the back-stop off Vancouver Island and accreted terranes (at ~127° following the coastline between Nootka Island and Port Renfrew). Extensional faulting is observed only at ridges with debris-flow style failure, which also are the ridges with larger height and steeper slopes. These extensional faults may be the result of over-steepening of the ridges and collapse of the sediment pile that can no longer withstand its own weight due to limited internal shear strength.

Fracture Alignments in Marine Sediments Off Vancouver Island fromPsSplitting Analysis

“…12b). Indeed, a stress field analysis based on the borehole breakouts in the accretionary prism shows nearly margin-normal horizontal compression (N25°-35°E; Riedel et al, 2016). The latter, margin-parallel fast axes, could be inferred in this study in a limited area (Fig.…”

Horizontal compressive stress regime on the northern Cascadia margin inferred from borehole breakouts

“…Note also the apparent fragmentation of the deformation front and entire prism indicated by several representative line‐segments (black dotted lines; for further details see Riedel et al . []).…”

“… Rose diagram of strike orientations of digitized lineaments of accretionary ridges across the Northern Cascadia margin using high‐resolution seafloor multibeam data (modified from Riedel et al . []). Lengths of individual segments (binned at 5°) represent the number of occurrences of the individual strike angle.…”

“…Ridge and thrust fault segments a few kilometers in length consistently appear from west of the deformation front to east of the shelf‐edge. Structural lineaments of individual ridge segments are oriented ∼110° ± 10° and ∼150° ± 10° [ Riedel et al ., ], subperpendicular to the direction of relative plate motion (Figure ). Most obvious at Site U1326 (Figure ), the fact that our observed S Hmax is not in the plate convergence direction may be due to the orientation and rotation of these ridge segments.…”

“…The inset shows orientations of maximum compressive stress inferred from crustal earthquakes for the inner fore-arc region along the west coast of Vancouver Island (dark blue rosette symbol) from Balfour et al [2011] and farther inland (magenta rosette symbol) from Wang et al [1995]. Note also the apparent fragmentation of the deformation front and entire prism indicated by several representative line-segments (black dotted lines; for further details see Riedel et al [2016]).…”