Role of faulting and gas hydrate in deep-sea landslides off Vancouver Island
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Role of faulting and gas hydrate in deep-sea landslides off Vancouver Island. or. Recipe for slumping: Lift, cut, shake, but maybe freeze first. George Spence. Collaborators include: Carol Lopez Ross Haacke Tark HamiltonMichael Riedel + many others.

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Role of faulting and gas hydrate in deep-sea landslides off Vancouver Island

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Role of faulting and gas hydrate in deep-sea landslides off Vancouver Island

or

Recipe for slumping:

Lift, cut, shake, but maybe freeze first

George Spence

Collaborators include:

Carol Lopez Ross Haacke

Tark HamiltonMichael Riedel

+ many others


Storegga Slide : mother of all landslides

  • mass failure area equiv to Iceland

  • headwall ~250 km long

  • runout ~800 km

  • Multiple events (3?)

  • oldest, biggest 250 ka

  • most recent 8.2 ka


1929 Grand Banks earthquake (M 7.2), slump and tsunami

  • tsunami : 28 deaths; observed in Portugal

  • undersea cable breaks out to 500 km (turbidity currents)

  • failure area 20,000 km2, sed vol 100-150 km3 (thickness ~5 m)

(Fine et al. 2005)


1998 Papua New Guinea earthquake (M 7.1) and tsunami

  • tsunami : 2200 deaths

  • tsunami source : motion on low-angle fault plus slump


  • slump sediment volume only 1-4 km3 (max thickness 600 m)

slump amphitheatre

(Synolakis et al. 2002)

Papua New Guinea slide


Cascadia margin, Vancouver Island

Swath bathymetry, U Washington 2004


U1326

U1326 : IODP drilling, 2005


U1326


U1326


Cascadia margin setting

BSR (Bottom Simulating Reflector)

Base of gas hydrate

Deformation

front

Basin

Sediments

Accretionary Prism

Sediments

Oceanic Crust


Methane Hydrate Structure

Carbon + hydrogen (centre) trapped in ice lattice


Multichannel seismic 10-50 Hz

BSR

BSR


U1326 :

array of ocean bottom seismometers


OBS high-vel (85-110 mbsf)

BSR at 240-260 mbsf

U1326 downhole log high-vel: hydrate

OBS velocities


High vel: hydrate?

BSR

  • BSR depth well-constrained at 250-260 mbsf

  • high-vel shallow hydrate layer extends laterally for 4-6 km

U1326 :

Final Velocity Model – Line 2

Depth (km)


slump

seismic reflection lines


slump

Scarps: up to 75 m high

BSR

NW

SE

NW

SE

2.4 s

3.0 s

Line 13

Line 21


Margin-perpendicular faults

: extensional, with motion parallel to least-compressive stress direction


extension cracks

What produced these margin-perpendicular faults?


Expansion cracks on ridge are due to longitudinal flexure, i.e. tension on outside edge

tension

compression

Better analogy : bend a baguette


Lateral extent of slump controlled by margin-normal faults


Reconstruct original ridge by interpolating across slump: C to A


Volume of slumped material : 0.6 km2

Vertical extent of slump coincident with base of hydrate


Slump Mechanisms

  • Gas hydrate dissociation

  • High pore fluid pressures

  • Contrasting seds & physical properties, e.g. glacial vs. de-glacial vs. interglacial

  • Earthquakes


1.

  • Hydrate may increase sediment strength by cementing grains (but increase depends on how hydrate is distributed, and how much hydrate is present)

  • Is there coincidence between glide plane and base of hydrate?


2. High pore fluid pressures

High fluid flux (e.g. high sed rates; compaction at convergent margin) produces high pore pressures

High pore pressure reduces sed strength (i.e. reduces grain-to-grain contact)

Frontal ridge is region of greatest deformation and greatest fluid flux


Mounds and slumps, offshore Nicaragua

slope seds

decollement

Overpressure at decollement

(Talukder et al. 2008)


key core

3. Contrasting sed properties

Coring program Aug 2008 : Haacke, Riedel, Pohlmann, Hamilton, Enkin, Rose, and others


Key core at intersection of headwall and glide plane

Bottom of core contains older seds, much stiffer and stronger than overlying seds found found everywhere else, which are likely weakde-glacial deposits (~14 kyr)

Top of stiff sediments may provide the glide plane.


4. Earthquakes

  • acceleration-induced sliding

  • earthquakes may produce excess pore pressures

  • Coring cruise Aug 2008 :

  • series of 10-17 turbidites found overlying the slumped deposits, which is comparable to the number of earthquakes since last glacial period, i.e. consistent with slumps occuring at de-glacial time


Ridge on slope off Van Is

Original data

bubble pulse

BSR


bowtie

Predictive deconvolution


Migration


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