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Walter R. Roest & R. Dietmar Müller

The University of Sydney. Geological Survey of Canada. How Important is the Motion of Subducting Slabs Relative to the Underlying Mantle: A Proposed Study. Walter R. Roest & R. Dietmar Müller. Illustration: W. Jacquelyne Kious & Robert I. Tilling, This Dynamic Earth (on-line), USGS.

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Walter R. Roest & R. Dietmar Müller

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  1. The University of Sydney Geological Survey of Canada How Important is the Motion of Subducting Slabs Relative to the Underlying Mantle:A Proposed Study Walter R. Roest & R. Dietmar Müller

  2. Illustration: W. Jacquelyne Kious & Robert I. Tilling, This Dynamic Earth (on-line), USGS Do Tectonic Plates Care about Mantle Motion?

  3. Do Tectonic Plates Care about Mantle Motion? • How about Mid-Ocean Ridges? • Stein et al., EPSL,1977 predicted asymmetry between leading and trailing plates • Analysis of spreading symmetry, Müller et al., Nature,1998

  4. Global Ocean Floor Age Müller et al., JGR,1997 0 10 20 180 200 AGE (Ma)

  5. Global spreading rates

  6. Müller et al., Nature,1998 Spreading asymmetries

  7. Conclusion, Müller et al. • No systematic asymmetry in seafloor spreading rates, with respect to absolute plate motion • Ridges move independent of mantle • However, we do observe repeated spreading axis jumps towards hotspot

  8. Do Tectonic Plates Care about Mantle Motion? • How about Subduction Zones ? • Scholz and Campos, JGR 1995 • On the mechanism of seismic decoupling and back arc spreading • Harabaglia and Doglioni, GRL 1998 • Topography and gravity across subduction zones • Gurnis, Ritsema and van Heijst, JGR 2000 • Tonga slab deformation: The influence of lower mantle upwelling

  9. Subduction Geometry VUP = Upper Plate ‘Absolute’ Velocity VS= Subduction Plate Velocity FSA= Sea Anchor Force FSU= Trench Suction Force FSP= Slab Pull Force FR= Resistance Force DFn = Normal Force at Interface = FSA sin Y + FSU cos Y Illustration modified after: Scholz and Campos, JGR, 1995

  10. Effect of Sea Anchor Force TIME • Western Pacific • Steep subduction, slab roll-back • Upper plate extension • Back-arc spreading • Acadia Subduction Zone • Locked zone • Deformation in upper plate • Mega-thrust earthquakes

  11. Philippine Plate

  12. Slab Dip (Izu-Bonin-Mariana) 14 12 10 8 Fsa N/m (x10 12) 6 4 2 0 45 55 65 75 85 95 Mean Dip (Degrees) Scholz and Campos, JGR, 1995

  13. Conclusion, Scholz and Campos • Sea anchor force resists lateral motion of slab relative to mantle • Sea anchor force model predicts state of seismic decoupling for 80% of the world’s subduction zones • Simplistic model, other forces may play a role in seismic decoupling in some cases

  14. Topography and Gravity profiles Harabaglia and Doglioni, GRL, 1998

  15. Stacked Profiles West directed East directed 0 0 Topography 2000 2000 4000 Gravity Harabaglia and Doglioni, GRL, 1998

  16. Conclusion, Harabaglia and Doglioni • Significant differences between east and west directed subduction • Back arc spreading associated with west directed subduction (however, see Maria Sdrolias talk about periodicity) • Explain observations by overall westward motion of lithosphere with respect to mantle

  17. Earthquake “Density” (400 -700 km) Earthquakes > 100 km and > M5 Density at 813 km srr at 542 km Gurnis et al., GRL, 2000

  18. Conclusion, Gurnis et al. • Tonga slab more deformed than other slabs • Result of relatively young subduction • Underlain by seismically slow structures • Upward flow from mantle plume • In other, long-lived, subduction system, the mantle tends to pull down, here pushes up

  19. Proposed Study • Analyse global data sets: • Absolute and relative plate motions • Gravity, Topography • Mantle Tomography • Ocean Floor Age, Plate boundaries • Earthquakes • Volcanos • ….. And more…..

  20. Plate Boundaries Gravity Anomalies Ocean Floor Age Topography

  21. Earthquakes (Engdahl et al., Bull.Seism.Soc.Am., 1998) 0-70 km 70-300 km 300-700 km

  22. Active and Recent Volcanos(various sources)

  23. Conclusion - 1 • Large scale mantle flow seems to have an effect on subduction characteristics: • What has the mantle been doing to subducting lithosphere? • Do we understand the mechanics? • What drives slab roll-back?

  24. Conclusion - 2 • There are many data sets, but relatively few quantitative analyses / compilations: • What parameters do geodynamic modellers want to see quantified? • Are there other data sets (e.g. geochemistry, heatflow….)? • How do we go from here?

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