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

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

Global Ocean Floor Age AGE (Ma) Müller et al., JGR,1997

Global spreading rates

Spreading asymmetries Müller et al., Nature,1998

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

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 Do Tectonic Plates Care about Mantle Motion?

Subduction Geometry Illustration modified after: Scholz and Campos, JGR, 1995 V UP = Upper Plate ‘Absolute’ Velocity V S = Subduction Plate Velocity F SA = Sea Anchor Force F SU = Trench Suction Force F SP = Slab Pull Force F R = Resistance Force  F n = Normal Force at Interface = F SA sin  + F SU cos 

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

Philippine Plate

Slab Dip (Izu-Bonin-Mariana) Mean Dip (Degrees) F sa N/m (x10 12 ) Scholz and Campos, JGR, 1995

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

Harabaglia and Doglioni, GRL, 1998 Topography and Gravity profiles

Stacked Profiles Topography Gravity East directedWest directed Harabaglia and Doglioni, GRL,

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

Density at 813 km Gurnis et al., GRL, 2000 Earthquake “Density” ( km) Earthquakes > 100 km and > M5  rr at 542 km

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

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

Plate BoundariesGravity Anomalies Ocean Floor Age Topography

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

Active and Recent Volcanos (various sources)

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?

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?