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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 Hamilton Michael Riedel + many others.

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slide1

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 Hamilton Michael Riedel

+ many others

slide2

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
slide3

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)

slide4

1998 Papua New Guinea earthquake (M 7.1) and tsunami

  • tsunami : 2200 deaths
  • tsunami source : motion on low-angle fault plus slump
slide5

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

slump amphitheatre

(Synolakis et al. 2002)

Papua New Guinea slide

slide6

Cascadia margin, Vancouver Island

Swath bathymetry, U Washington 2004

slide7

U1326

U1326 : IODP drilling, 2005

slide11

Cascadia margin setting

BSR (Bottom Simulating Reflector)

Base of gas hydrate

Deformation

front

Basin

Sediments

Accretionary Prism

Sediments

Oceanic Crust

slide12

Methane Hydrate Structure

Carbon + hydrogen (centre) trapped in ice lattice

slide14

U1326 :

array of ocean bottom seismometers

slide15

OBS high-vel (85-110 mbsf)

BSR at 240-260 mbsf

U1326 downhole log high-vel: hydrate

OBS velocities

slide16

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)

slide17

slump

seismic reflection lines

slide18

slump

Scarps: up to 75 m high

BSR

NW

SE

NW

SE

2.4 s

3.0 s

Line 13

Line 21

slide19

Margin-perpendicular faults

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

slide20

extension cracks

What produced these margin-perpendicular faults?

slide21

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

tension

compression

Better analogy : bend a baguette

slide24

Volume of slumped material : 0.6 km2

Vertical extent of slump coincident with base of hydrate

slide25

Slump Mechanisms

  • Gas hydrate dissociation
  • High pore fluid pressures
  • Contrasting seds & physical properties, e.g. glacial vs. de-glacial vs. interglacial
  • Earthquakes
slide26

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?
slide27

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

slide28

Mounds and slumps, offshore Nicaragua

slope seds

decollement

Overpressure at decollement

(Talukder et al. 2008)

slide29

key core

3. Contrasting sed properties

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

slide30

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.

slide31

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
slide32

Ridge on slope off Van Is

Original data

bubble pulse

BSR

slide33

bowtie

Predictive deconvolution

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