Plates,Plates, slabs,slabs, andand keelskeels DecipheringDeciphering Earth'sEarth's convectiveconvective historyhistory fromfrom seismology,seismology, mineralmineral physicsphysics andand geodynamicsgeodynamics ThorstenThorsten WW BeckerBecker UniversityUniversity ofof SouthernSouthern CaliforniaCalifornia LosLos Angeles,Angeles, CaliforniaCalifornia DGG C.-F.-Gauß Lecture European Geosciences Union General Assembly Vienna, April 16, 2008 2/51 PreviousPrevious GaußGauß LecturesLectures HeinerHeiner IgelIgel (2007)(2007) Rupture,Rupture, WavesWaves andand ImagesImages UlrichUlrich ChristensenChristensen (2006)(2006) GeodynamoGeodynamo ModelingModeling 3/51 PlatePlate tectonicstectonics –– Top,Top, coldcold boundaryboundary layerlayer temperature hot MantleMantle convectionconvection cold CoreCore heatheat flowflow –– Bottom,Bottom, hothot boundaryboundary layerlayer 4/51 Problem:Problem: MantleMantle ¹¹ thermalthermal convectionconvection RheologicalRheological controlscontrols onon plateplate tectonics?tectonics? – WhyWhy doesdoes (only)(only) EarthEarth havehave plateplate tectonics?tectonics? – HowHow areare platesplates coupledcoupled toto mantlemantle flow?flow? ChemicalChemical controlscontrols onon boundaryboundary layers?layers? – RoleRole andand evolutionevolution ofof thethe continents?continents? – RoleRole ofof volatilevolatile variationsvariations andand fractionation?fractionation? 5/51 ContentContent ofof thisthis talktalk AppliedApplied GeodynamicsGeodynamics approachapproach – CombineCombine disciplinarydisciplinary constraintsconstraints andand modelingmodeling toto understandunderstand mantlemantle systemsystem interactionsinteractions – EstablishEstablish aa geodynamicgeodynamic referencereference modelmodel SeismicSeismic anisotropyanisotropy ofof thethe upperupper mantlemantle – TectonicTectonic strainstrain gaugegauge – ConstrainingConstraining laterallateral viscosityviscosity variationsvariations – ExploringExploring thethe rolerole ofof continentalcontinental keelskeels – VolatileVolatile variationsvariations inin thethe asthenosphereasthenosphere 6/51 SurfaceSurface constraintsconstraints –– plateplate tectonicstectonics Crustal velocities in HS-3 Hot spot reference frame e.g. DeMets et al. (1991); Gripp & Gordon (2002);Sella et al. (2002); Kreemer et al. (2003) 7/51 GlobalGlobal circulationcirculation modelsmodels Tractions from plate motions acting on asthenosphere MantleMantle viscousviscous StokesStokes flow,flow, nono inertiainertia – InstantaneousInstantaneous solutionsolution forfor boundaryboundary conditionsconditions andand internalinternal loadsloads – Semi-analytical,Semi-analytical, ifif viscosityviscosity isis NewtonianNewtonian andand onlyonly depthdepth dependentdependent ForceForce modelmodel – PlatePlate motionsmotions prescribedprescribed – EvaluateEvaluate mantlemantle tractionstractions andand plateplate drivingdriving forcesforces Hager & O'Connell (1981) 8/51 DeepDeep structuralstructural constraints:constraints: SeismologicalSeismological mappingmapping 9/51 SeismicSeismic tomographytomography CAT-scanCAT-scan likelike techniquetechnique EarthquakesEarthquakes :: sourcessources SeismometersSeismometers :: receiversreceivers MeasureMeasure traveltravel timestimes toto invertinvert forfor 3-D3-D velocityvelocity structurestructure 10/51 MantleMantle shearshear velocityvelocity anomaliesanomalies MapMap atat 12001200 kmkm depthdepth e.g. Grand and van der Hilst (1997); Becker and Boschi (2001) 11/51 MantleMantle temperaturetemperature anomaliesanomalies MapMap atat 12001200 kmkm depthdepth UseUse mineralmineral physicsphysics toto convertconvert velocityvelocity intointo temperaturetemperature (density)(density) anomaliesanomalies 12/51 MantleMantle flowflow drivendriven byby tomographytomography IsoviscousIsoviscous geoidgeoid predictionprediction ObservedObserved geoidgeoid anomaliesanomalies FlowFlow modelmodel drivendriven byby densitydensity ViscosityViscosity increaseincrease geoidgeoid predictionprediction anomalies,anomalies, surfacesurface freefree slipslip ObservedObserved geoidgeoid anomaliesanomalies requirerequire lowerlower mantlemantle viscosityviscosity increaseincrease Overall,Overall, geopotentialgeopotential (stress(stress related)related) constraintsconstraints non-uniquenon-unique e.g. Richards & Hager (1984); Hager (1984); Forte & Mitrovica (2000); Moucha et al. (2006) 13/51 VelocitiesVelocities asas aa constraintconstraint forfor laterallateral viscosityviscosity variationsvariations FlowFlow modelmodel withwith onlyonly radialradial viscosityviscosity variationsvariations PoloidalPoloidal componentcomponent NoNo toroidaltoroidal flowflow withoutwithout laterallateral viscosityviscosity variationsvariations Strain-ratesStrain-rates notnot veryvery plate-likeplate-like ObservedObserved plateplate velocitiesvelocities inin hothot spotspot referencereference frameframe PoloidalPoloidal componentcomponent ToroidalToroidal componentcomponent Sources and sinks Strike slip motion, spin O'Connell et al. (1991); Ricard et al. (1991); Forte & Peltier (1993); Thoraval & Richards (1997) 14/51 TheThe rolerole ofof thethe plateplate boundariesboundaries Observed plate motions VelocityVelocity modelmodel – PrescribePrescribe weakweak plateplate boundariesboundaries – ComputeCompute plateplate dragdrag couplingcoupling andand drivingdriving torquestorques – SolveSolve forfor EulerEuler Modeled plate motions vectorsvectors forfor rigidrigid platesplates CorrelationsCorrelations good,good, butbut oceanicoceanic platesplates movemove asas fastfast asas continentalcontinental onesones Ricard & Vigny (1989) Ricard & Vigny (1989); Lithgow-Bertelloni & Richards (1998); Becker & O'Connell (2001); Conrad & Lithgow-Bertelloni (2002) 15/51 AdditionalAdditional constraints:constraints: SeismicSeismic anisotropyanisotropy RadialRadial :: LoveLove (SH(SHorizontal)) vs.vs. RayleighRayleigh (SV(SVertical)) surfacesurface waveswaves PREM radial average Radial average from S362WMANI model depth depth [km] (V /V )2 SH SV core mantle e.g. Dziewonski & Anderson (1981); Debayle et al. (2005); Panning & Romanowicz (2006); Kustowski et al. (2008) 16/51 AdditionalAdditional constraints:constraints: SeismicSeismic anisotropyanisotropy RadialRadial :: LoveLove (SH)(SH) Azimuthal vs.vs. RayleighRayleigh (SV)(SV) anisotropy surfacesurface waveswaves RMS from DKP2005 AzimuthalAzimuthal :: RayleighRayleigh SV1SV1 fasterfaster thanthan SV2SV2 PREM radial average Radial average from S362WMANI model depth depth [km] (V /V )2 SH SV core mantle e.g. Dziewonski & Anderson (1981); Debayle et al. (2005); Panning & Romanowicz (2006); Kustowski et al. (2008) 17/51 AzimuthalAzimuthal anisotropyanisotropy patternspatterns SVSV (Rayleigh)(Rayleigh) anomaliesanomalies atat ~75~75 kmkm depthdepth e.g. Tanimoto & Anderson (1984); Montagner & Nataf (1986); Ekström (2001); Debayle et al. (2005) 18/51 LatticeLattice preferredpreferred orientationorientation texturestextures Lower hemisphere proJection of the alignment concentration of crystallographic axes fast MantleMantle mineralsminerals (olivine)(olivine) intrinsicallyintrinsically (single(single crystal)crystal) anisotropicanisotropic MultiMulti graingrain assembliesassemblies formform anisotropicanisotropic texturestextures underunder dislocationdislocation creepcreep SeismicSeismic anisotropyanisotropy isis aa recordrecord ofof tectonictectonic strainstrain andand flowflow Zhang & Karato (1995) 19/51 AnisotropyAnisotropy andand mantlemantle convectionconvection Montagner (1998) e.g McKenzie (1981); Tanimoto & Anderson (1984); Ribe (1989) 20/51 NewNew developmentsdevelopments I:I: OlivineOlivine alphabetalphabet dry wet stress 0 Water content 300 ppm wt Karato et al. (2007) 21/51 NewNew developmentsdevelopments II:II: EfficientEfficient mineralmineral physicsphysics theoriestheories ofof texturingtexturing Zhang & Karato Kaminski & Ribe (1995) lab (2001) theory IncludeInclude simple recrystallizationrecrystallization shear effectseffects WeWe useuse KaminskiKaminski etet al.al. ((2004)2004) DREXDREX implementation,implementation, uniaxial tunedtuned toto lablab resultsresults compression LinkLink flow,flow, strain,strain, texture,texture, andand seismicseismic anisotropyanisotropy quantitativelyquantitatively Kaminski & Ribe (2001) 22/51 NewNew developmentsdevelopments III:III: SphericalSpherical modelsmodels withwith laterallateral viscosityviscosity variationsvariations WithWith largelarge scalescale computing,computing, wewe cancan nownow solvesolve thermo-thermo- chemicalchemical convectionconvection problemproblem inin spheresphere WillWill focusfocus onon instantaneousinstantaneous solutionssolutions withwith temperature,temperature, stress,stress, andand depth-depth- dependentdependent visco-visco- plasticplastic rheologiesrheologies e.g. Tackley (1994); Bunge et al. (1996);Zhong et al. (2000); Tackley (2000a,b) 23/51 QuantitativeQuantitative modelsmodels ofof upperupper mantlemantle flowflow andand anisotropyanisotropy ComputeCompute globalglobal flowflow withwith dislocation/diffusiondislocation/diffusion creepcreep ComputeCompute globalglobal anisotropyanisotropy usingusing mineralmineral physicsphysics forfor upperupper mantlemantle DeriveDerive seismicseismic propertiesproperties andand comparecompare withwith observationsobservations 24/51 CirculationCirculation modelmodel UseUse geologicgeologic informationinformation (stiff(stiff cratonscratons andand seafloorseafloor ages)ages) forfor lithospherelithosphere