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

1. 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

2. 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

3. 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)

4. 1998 Papua New Guinea earthquake (M 7.1) and tsunami • tsunami : 2200 deaths • tsunami source : motion on low-angle fault plus slump

5. slump sediment volume only 1-4 km3 (max thickness 600 m) slump amphitheatre (Synolakis et al. 2002) Papua New Guinea slide

6. Cascadia margin, Vancouver Island Swath bathymetry, U Washington 2004

7. U1326 U1326 : IODP drilling, 2005

8. U1326

10. U1326

11. Cascadia margin setting BSR (Bottom Simulating Reflector) Base of gas hydrate Deformation front Basin Sediments Accretionary Prism Sediments Oceanic Crust

12. Methane Hydrate Structure Carbon + hydrogen (centre) trapped in ice lattice

13. Multichannel seismic 10-50 Hz BSR BSR

14. U1326 : array of ocean bottom seismometers

15. OBS high-vel (85-110 mbsf) BSR at 240-260 mbsf U1326 downhole log high-vel: hydrate OBS velocities

16. 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)

17. slump seismic reflection lines

18. slump Scarps: up to 75 m high BSR NW SE NW SE 2.4 s 3.0 s Line 13 Line 21

19. Margin-perpendicular faults : extensional, with motion parallel to least-compressive stress direction

20. extension cracks What produced these margin-perpendicular faults?

21. Expansion cracks on ridge are due to longitudinal flexure, i.e. tension on outside edge tension compression Better analogy : bend a baguette

22. Lateral extent of slump controlled by margin-normal faults

23. Reconstruct original ridge by interpolating across slump: C to A

24. Volume of slumped material : 0.6 km2 Vertical extent of slump coincident with base of hydrate

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

26. 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?

27. 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

28. Mounds and slumps, offshore Nicaragua slope seds decollement Overpressure at decollement (Talukder et al. 2008)

29. key core 3. Contrasting sed properties Coring program Aug 2008 : Haacke, Riedel, Pohlmann, Hamilton, Enkin, Rose, and others

30. 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.

31. 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

32. Ridge on slope off Van Is Original data bubble pulse BSR

33. bowtie Predictive deconvolution

34. Migration