• Upper mantle flow associated with Upper mantle flow associated with plate motion and melt-enhanced plate motion and melt-enhanced upwellingupwelling

• Influences of ridge segmentation & Influences of ridge segmentation &

asymmetric opening asymmetric opening

• Development of mantle seismic Development of mantle seismic

anisotropyanisotropy

• Modeling flow-induced anisotropyModeling flow-induced anisotropy

• Ideas for further work in Gulf of Ideas for further work in Gulf of

CaliforniaCalifornia

• Plate-driven mantle flowPlate-driven mantle flow

• Upwelling and decompression melting of Upwelling and decompression melting of

peridotiteperidotite

• Buoyancy-enhanced upwelling if melt is Buoyancy-enhanced upwelling if melt is

retained within crystal matrixretained within crystal matrix

• Spreading rate (and/or regional viscosity) Spreading rate (and/or regional viscosity)

control on nature of flow and, therefore, control on nature of flow and, therefore,

the rate/structure of new crust producedthe rate/structure of new crust produced

Motion of Motion of plates plates induces induces flow in flow in upper upper mantlemantle

Along-strike variation in melt Along-strike variation in melt production and/or migrationproduction and/or migration

>> stronger flow velocity >> stronger flow velocity gradientsgradients

DecompressioDecompression meltingn melting

• Evidence of 3-D upwelling and/or melt supply?Evidence of 3-D upwelling and/or melt supply?• Variation in lithospheric structureVariation in lithospheric structure

• Cooling with age & changes in plate thickness at Cooling with age & changes in plate thickness at ridge offsetsridge offsets

• Central vs end-of-segment crustal structure?Central vs end-of-segment crustal structure?• Evolution in timeEvolution in time

• Changes in rift/ridge-transform plate boundary Changes in rift/ridge-transform plate boundary geometrygeometry

• Response to change in plate motion?Response to change in plate motion?• Reflection of changes in melt supply? (entrained Reflection of changes in melt supply? (entrained heterogeneities)heterogeneities)

>>> degree of coupling between lithosphere >>> degree of coupling between lithosphere and asthenosphere…and asthenosphere…

Van Wijk & Blackman, Tectonophysics, in press (basic results similar to earlier work by us/others)

Top velocity (10 mm/yr, half rate), initial plate boundary position prescribed

T-dependent viscosity, coupled deformation (finite element, Tekton revised for 3-D) & temperature (finite difference) calculation

Axial zone is weakVisco-elastic, Visco-elastic,

power law power law

rheology with rheology with

separate upper separate upper

crust, lower crust, lower

crust, & mantle crust, & mantle

propertiesproperties

Melt production Melt production

follows McKenzie follows McKenzie

& Bickle 1988& Bickle 1988

Axial zone ~2 Axial zone ~2

orders of orders of

magnitude weaker magnitude weaker

than surrounding than surrounding

lithospherelithosphere

Map Map ViewView

toptoptoptop

55 55 kmkm

Non-transform offsetNon-transform offset

transform offset(free slip)

toptop

kilometers

Depth (km)

km km

mm/yr

VerticVerticalalProfilProfilee

12 km W of axis12 km W of axis 45 km W of axis45 km W of axis

Map View, Map View, 20 km depth20 km depth Transform offsetTransform offset

kilometers

kilometerskilometers

Melt buoyancy enhanced upwellingMelt buoyancy enhanced upwelling

Small component of along-strike flow near Small component of along-strike flow near segment endsegment end

Map Map ViewView

Vertical Vertical Profiles Profiles along along spreadinspreading axisg axis

• Mantle minerals have inherently anisotropic Mantle minerals have inherently anisotropic elastic structureelastic structure

• Flow-induced alignment of crystal Flow-induced alignment of crystal orientationsorientations• Poly-crystalline aggregates undergo deformationPoly-crystalline aggregates undergo deformation

• Possible contribution of retained/migrating Possible contribution of retained/migrating meltmelt• Distribution of melt >> anisotropic signatureDistribution of melt >> anisotropic signature• Influence on bulk seismic velocityInfluence on bulk seismic velocity

• Effective elastic constants & modeling Effective elastic constants & modeling surface seismic signaturesurface seismic signature

Development of mineral texture along Development of mineral texture along flowlinesflowlines

Strain-induced alignment Strain-induced alignment of mineral grainsof mineral grains

Distribution of grain Distribution of grain orientationsorientations

Peridotite is olivine Peridotite is olivine (black poles) + pyroxene (black poles) + pyroxene (green poles), ~70:30(green poles), ~70:30

Compute Effective Compute Effective Elastic ConstantsElastic Constants

Voigt average over all the individual grain contibutions

Oriented single-crystal EC projected onto global frame to get x,y,z contribution

ol1341

en1341

ol1322

en1322

Compute Effective Compute Effective Elastic ConstantsElastic Constants

Voigt average over all the individual grain contibutions

Oriented single-crystal EC projected onto global frame to get x,y,z contribution

ol1341

en1341

ol1322

en1322

Magnitude anisotropyMagnitude anisotropy

Fast directionFast direction

Passive Flow Model Buoyancy Enhanced Upwelling

relative travel time delay (secs)

predicted body-waveanomaly due solelyto presence of melt

symbols showdifferent modelsof melt 'inclusion'geometry

(Blackman &Kendall, 1997)

Passive vs. Buoyancy-Enhanced UpwellingPassive vs. Buoyancy-Enhanced Upwellingslow, slow, symmetrisymmetric c spreadinspreadingg

FlowlineFlowlines & s & finite finite strainstrain

P-wave P-wave anisotroanisotropy py (degree (degree & fast-& fast-axis axis directiodirection)n)

• East Pacific RiseEast Pacific Rise• MELT Experiment 17°SMELT Experiment 17°S• Suite of models & comparison with OBS dataSuite of models & comparison with OBS data

• Western USWestern US• CSEDI Project with Thorsten Becker & Vera Schulte-CSEDI Project with Thorsten Becker & Vera Schulte-

PelkumPelkum

• Mantle Wedge flow behind subducting plateMantle Wedge flow behind subducting plate• With/without backarc shearingWith/without backarc shearing

• Proposal for Gulf of CaliforniaProposal for Gulf of California• Collaboration with Frank Vernon & Graham Kent, Harold Collaboration with Frank Vernon & Graham Kent, Harold

Magistrale, & Gary PavlisMagistrale, & Gary Pavlis

Shear Wave Splitting Shear Wave Splitting determined along the MELT determined along the MELT OBS arrayOBS array (Wolfe et al., Science 1998)(Wolfe et al., Science 1998)

Reference EPR Reference EPR 17°S Flow 17°S Flow ModelModel

Migrates 32 mm/yrMigrates 32 mm/yr

Constant Constant asthenospheric asthenospheric viscosityviscosity

Fin

ite s

trai

nP

-Wav

e A

niso

trop

yM

axim

u S

S

plitt

ing

Mni

mum

S

Spl

ittin

g

S Splitting at Vertical Incidence

Flow Flow across across 600 km 600 km depth depth is not is not alloweallowedd

Flowlines & finite strain

P-wave anisotropy

S-wave Splitting at near-vertical incidence

Incidence + 20° Incidence - 20°

Temperature Temperature Anomaly Anomaly associated with associated with Pacific Pacific Superswell Superswell influences flowinfluences flow (Toomey et al., EPSL (Toomey et al., EPSL 2002)2002)

Predictions for Predictions for seismic anisotropy & seismic anisotropy & heterogeneity match heterogeneity match MELT data better than MELT data better than the other models the other models testedtested

Subduction Zone Subduction Zone AnisotropyAnisotropy

Direction of fast-Direction of fast-seismic propagation seismic propagation has been determined has been determined to parallel the to parallel the trench in several trench in several casescases

Broadscale mantle Broadscale mantle flow?flow?

Backarc shearing Backarc shearing (Hall et al., 2000)?(Hall et al., 2000)?

Effect of water on Effect of water on fast-axis fast-axis orientation during orientation during texturing (Jung & texturing (Jung & Karato, 2001)?Karato, 2001)?

2-D Corner Flow2-D Corner Flow Add Along-trench Add Along-trench FlowFlow

P-wave anisotropy

max S- wave Splitting

Vertical S-wave Splitting

P-wave anisotropy

max S- wave Splitting

Vertical S-wave Splitting

Mantle flow and Mantle flow and predicted predicted anisotropy in the anisotropy in the Lau BasinLau Basin

Conder et al., GRL Conder et al., GRL 20022002

Backarc spreading flow Backarc spreading flow and melting in and melting in addition to addition to subduction-induced subduction-induced mantle wedge flowmantle wedge flow

Western US Western US Becker et al, EPSL submitted 2005Becker et al, EPSL submitted 2005

Surface velocity condition (white arrows), Surface velocity condition (white arrows), global seismic tomography proxy for global seismic tomography proxy for density, radially varying viscosity, density, radially varying viscosity, Kaminsky & Ribe method for LPOKaminsky & Ribe method for LPO

Predicted Fast S Polarization Direction (black bar)Predicted Fast S Polarization Direction (black bar)

Finite Strain EllipsesFinite Strain EllipsesColor-coded for depthColor-coded for depth

Profile view of finite strain

Profile view of finite strain

Western USWestern US

initial initial resultsresults

Comparison of predicted (black Comparison of predicted (black bars) & observed (white bars) bars) & observed (white bars) SKS splittingSKS splitting

Gray wedge indicates variation in Gray wedge indicates variation in prediction due to method of synthetic prediction due to method of synthetic calculation (single layer approximates calculation (single layer approximates 375 km deep region vs. multiple, variable 375 km deep region vs. multiple, variable layers)layers)

Overall fit is Overall fit is reasonable for reasonable for central/southern central/southern area. NW and Basin & area. NW and Basin & Range are not Range are not matched well.matched well.

Local structure & Local structure & effects on flow (not effects on flow (not included in low included in low resolution global resolution global model) preclude model) preclude match in complex match in complex areas (S Great areas (S Great Valley)Valley)

Relation to Relation to complete crustal complete crustal transect?transect?

Anomalously hot?Anomalously hot?

Depth extent of Depth extent of upwelling?upwelling?

Southern (oceanic Southern (oceanic spreading) vs. spreading) vs. Northern Northern (rifting) Gulf (rifting) Gulf structure?structure?

Buoyancy vs Buoyancy vs plate-driven plate-driven flow?flow?

Gaherty, Collins, Rebollar surface wave study designed to address several aspects

Continue to pursue additional work (SIO, SDSU, CICESE, Indiana)

Seismometer Seismometer DeploymentsDeployments

NARSNARS~5 yr, 18 ~5 yr, 18 broadbandbroadband

Collins et al. Collins et al. OBS OBS

~15 mo, 18 ~15 mo, 18 broadbandbroadband

Proposed ArrayProposed Array

60 PASSCAL60 PASSCAL

22 OBS22 OBS

All broadbandAll broadband

15-18 months15-18 months

Seismometer Seismometer DeploymentsDeployments

NARSNARS~5 yr, 18 ~5 yr, 18 broadbandbroadband

Collins et al. Collins et al. OBS OBS

~15 mo, 18 ~15 mo, 18 broadbandbroadband

Previously Proposed Previously Proposed ArrayArray

60 PASSCAL60 PASSCAL

22 OBS22 OBS

All broadbandAll broadband

15-18 months15-18 months

1.1. Architecture of the crust across the rift systemArchitecture of the crust across the rift system

• Teleseismic and local event tomographyTeleseismic and local event tomography

• Mapping of Moho across onshore & offshore parts of systemMapping of Moho across onshore & offshore parts of system

• Local EQ source parametersLocal EQ source parameters

2.2. Strength and deformation of the lower crustStrength and deformation of the lower crust

• Imaging of forward scattered P-S conversionsImaging of forward scattered P-S conversions

3.3. Upper mantle thermal structure and flowUpper mantle thermal structure and flow

• seismic velocity structure & attenuation (scale of ~10 km)seismic velocity structure & attenuation (scale of ~10 km)

• Shape of seismic discontinuities (upward or downward Shape of seismic discontinuities (upward or downward deflection?)deflection?)

• Seismic anisotropy (splitting, polarization direction of Seismic anisotropy (splitting, polarization direction of fast S wave)fast S wave)

• Linked models of mantle flow, lithospheric deformation, Linked models of mantle flow, lithospheric deformation, development of textural (+/- melt) anisotropy, and seismic development of textural (+/- melt) anisotropy, and seismic anisotropyanisotropy

• Converted phase imaging for detecting possible foundered Converted phase imaging for detecting possible foundered slabslab

Coordinate with findings of other Gulf of CA studiesCoordinate with findings of other Gulf of CA studies

(Poppeliers & Pavlis, 2002)

1.1. Architecture of the crust across the rift systemArchitecture of the crust across the rift system

• Teleseismic and local event tomographyTeleseismic and local event tomography

• Mapping of Moho across onshore & offshore parts of systemMapping of Moho across onshore & offshore parts of system

• Local EQ source parametersLocal EQ source parameters

2.2. Strength and deformation of the lower crustStrength and deformation of the lower crust

• Imaging of forward scattered P-S conversionsImaging of forward scattered P-S conversions

3.3. Upper mantle thermal structure and flowUpper mantle thermal structure and flow

• seismic velocity structure & attenuation (scale of ~10 seismic velocity structure & attenuation (scale of ~10 km)km)

• Shape of seismic discontinuities (upward or downward Shape of seismic discontinuities (upward or downward deflection?)deflection?)

• Seismic anisotropy (splitting, polarization direction of Seismic anisotropy (splitting, polarization direction of fast S wave)fast S wave)

• Linked models of mantle flow, lithospheric deformation, Linked models of mantle flow, lithospheric deformation, development of textural (+/- melt) anisotropy, and development of textural (+/- melt) anisotropy, and seismic anisotropyseismic anisotropy

• Converted phase imaging for detecting possible foundered Converted phase imaging for detecting possible foundered slabslab

Coordinate with findings of other Gulf of CA studiesCoordinate with findings of other Gulf of CA studies

Jones et al., 1994

• Mantle structure & flow are likely to vary both laterally and with Mantle structure & flow are likely to vary both laterally and with depth on scale of several kmdepth on scale of several km• Along-strike changes in GoC riftingAlong-strike changes in GoC rifting• Influence of transform offsetsInfluence of transform offsets• Large scale flow at depthLarge scale flow at depth

• Combination of research approaches needed to fully address problemsCombination of research approaches needed to fully address problems• Separate effects of T, melt, textural anisotropySeparate effects of T, melt, textural anisotropy• Series of inverse studies and selected forward modeling tests to assess Series of inverse studies and selected forward modeling tests to assess

possible contributions of various structure and implied geodynamic possible contributions of various structure and implied geodynamic consequencesconsequences

• Opportunity to assess both crust & mantle structure to infer Opportunity to assess both crust & mantle structure to infer processes of rifting as a full systemprocesses of rifting as a full system• Combine body wave, surface wave and active source seismicsCombine body wave, surface wave and active source seismics• Current data will provide resolution of mantle on several 10 ‘s km scaleCurrent data will provide resolution of mantle on several 10 ‘s km scale• Denser onshore/OBS array would improve resolution to several kmDenser onshore/OBS array would improve resolution to several km

• Avoid ‘bias’ due to possible superpostion of segmentation and longer wavelength Avoid ‘bias’ due to possible superpostion of segmentation and longer wavelength rifting signaturesrifting signatures

• Recognize cross-axis structure which could be key to understanding rifting Recognize cross-axis structure which could be key to understanding rifting processesprocesses

Key Constraints NeededKey Constraints Needed

• Surface velocitySurface velocity• Plate boundary geometry, temporal evolutionPlate boundary geometry, temporal evolution

• Cooling rates of crustal rocksCooling rates of crustal rocks• Guide vertical flow predictionsGuide vertical flow predictions

• Information on meltingInformation on melting• Degree/depth of meltingDegree/depth of melting• CompositionComposition

• Seismic/EM measurementsSeismic/EM measurements• All phases (P, S, Surface Waves)All phases (P, S, Surface Waves)• As many backazimuths & angles of incidence as As many backazimuths & angles of incidence as possiblepossible

Caution Caution about about possible possible ‘bias’ due ‘bias’ due to limited to limited areas (core areas (core complexes) complexes) emphasized emphasized in many in many studies …studies …

Savage & Sheehan, 2000

Pole Figures illustrate development of Pole Figures illustrate development of texturetexture

Texture predicted depends on Texture predicted depends on assumptions but fundamental result is assumptions but fundamental result is often similar between methodsoften similar between methods

Wenk & Tomé, JGR 1999, model Wenk & Tomé, JGR 1999, model recrystallization via strain-recrystallization via strain-controlled nucleation & growth of new controlled nucleation & growth of new grainsgrains

Sub-vertical shear imparts strong Sub-vertical shear imparts strong texture in upwelling zone; diffusion texture in upwelling zone; diffusion occurs in corner; subhorizontal shear occurs in corner; subhorizontal shear generates plate-spreading signaturegenerates plate-spreading signature

P-wave anisotropy along flowlineP-wave anisotropy along flowline: : different modelsdifferent models

Comparison of Comparison of Texture & Finite Texture & Finite Strain Anisotropy Strain Anisotropy PredictionPredictionP-wave fast axis P-wave fast axis orientation for slow-orientation for slow-spreading, passive modelspreading, passive model