Assessment of slope failures off Vancouver Island revealed in EM300 multibeam bathymetry data

Riedel, Michael, Naegeli, K. and Cote, M. (2016) Assessment of slope failures off Vancouver Island revealed in EM300 multibeam bathymetry data Geological Survey of Canada Open File, 8008 . UNSPECIFIED. DOI 10.4095/297904.

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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.

Document Type: Report (Research Report)
Keywords: Vancouver Island; marine geology; surficial geology/geomorphology; structural geology; continental slope; slope deposits; slope failures; slope stability; slope stability analyses; bathymetry; submarine features; submarine transport; landslides; landslide deposits; faults; faulting; Cascadia subduction zone
Research affiliation: OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-GDY Marine Geodynamics
DOI etc.: 10.4095/297904
ISSN: 1701-4387
Date Deposited: 23 Sep 2016 12:21
Last Modified: 23 Sep 2016 12:21

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