Constraining Upper Mantle Flow Using Seismic Anisotropy & Geodynamic Modeling

1. Constraining Upper Mantle Flow Using Seismic Anisotropy & Geodynamic Modeling Donna Blackman Scripps Institution of Oceanography John Collins, Don Forsyth, Jim Gaherty, Cecilio Rebollar

2. Overview • Upper mantle flow associated with plate motion and melt-enhanced upwelling • Influences of ridge segmentation & asymmetric opening • Development of mantle seismic anisotropy • Modeling flow-induced anisotropy • Ideas for further work in Gulf of California

3. Aspects of Flow • Plate-driven mantle flow • Upwelling and decompression melting of peridotite • Buoyancy-enhanced upwelling if melt is retained within crystal matrix • Spreading rate (and/or regional viscosity) control on nature of flow and, therefore, the rate/structure of new crust produced

4. Motion of plates induces flow in upper mantle Decompression melting Along-strike variation in melt production and/or migration >> stronger flow velocity gradients

5. Ridge/Rift Segmentation • Evidence of 3-D upwelling and/or melt supply? • Variation in lithospheric structure • Cooling with age & changes in plate thickness at ridge offsets • Central vs end-of-segment crustal structure? • Evolution in time • Changes in rift/ridge-transform plate boundary geometry • Response to change in plate motion? • Reflection of changes in melt supply? (entrained heterogeneities) >>> degree of coupling between lithosphere and asthenosphere…

6. Model of Flow in Vicinity of Spreading Axis Offset 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 weak Visco-elastic, power law rheology with separate upper crust, lower crust, & mantle properties Melt production follows McKenzie & Bickle 1988 Axial zone ~2 orders of magnitude weaker than surrounding lithosphere

7. Plate-Parallel Flow in Vicinity of Spreading Axis Offset 3 Myr of Spreading, 100 km offset Non-transform offset top top kilometers Map View top transform offset (free slip) 55 km mm/yr Vertical Profile Depth (km) km km 12 km W of axis 45 km W of axis

8. Vertical & Axis-Parallel Flow in Vicinity of Spreading Axis Offset 3 Myr of Spreading Map View, 20 km depth Transform offset kilometers kilometers kilometers Melt buoyancy enhanced upwelling Small component of along-strike flow near segment end

9. Variation in Melt Fraction: Effect of Plate Boundary Geometry & Pre-existing Crustal Structure Map View Vertical Profiles along spreading axis

10. Aspects of Seismic Anisotropy • Mantle minerals have inherently anisotropic elastic structure • Flow-induced alignment of crystal orientations • Poly-crystalline aggregates undergo deformation • Possible contribution of retained/migrating melt • Distribution of melt >> anisotropic signature • Influence on bulk seismic velocity • Effective elastic constants & modeling surface seismic signature >> direct link between flow & seismic anisotropy models

11. Development of mineral texture along flowlines Strain-induced alignment of mineral grains Distribution of grain orientations Peridotite is olivine (black poles) + pyroxene (green poles), ~70:30

12. ol1341 en1341 Compute Effective Elastic Constants Voigt average over all the individual grain contibutions Oriented single-crystal EC projected onto global frame to get x,y,z contribution ol1322 en1322

13. ol1341 en1341 Compute Effective Elastic Constants Voigt average over all the individual grain contibutions Oriented single-crystal EC projected onto global frame to get x,y,z contribution Fast direction Magnitude anisotropy ol1322 en1322

14. Buoyancy Enhanced Upwelling Passive Flow Model symbols show different models of melt 'inclusion' geometry predicted body-wave anomaly due solely to presence of melt relative travel time delay (secs) (Blackman & Kendall, 1997)

15. Passive vs. Buoyancy-Enhanced Upwelling slow, symmetric spreading Flowlines & finite strain P-wave anisotropy (degree & fast-axis direction)

16. Case Studies • East Pacific Rise • MELT Experiment 17°S • Suite of models & comparison with OBS data • Western US • CSEDI Project with Thorsten Becker & Vera Schulte-Pelkum • Mantle Wedge flow behind subducting plate • With/without backarc shearing • Proposal for Gulf of California • Collaboration with Frank Vernon & Graham Kent, Harold Magistrale, & Gary Pavlis

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

18. Reference EPR 17°S Flow Model Migrates 32 mm/yr Constant asthenospheric viscosity Finite strain P-Wave Anisotropy Maximu S Splitting S Splitting at Vertical Incidence Mnimum S Splitting

19. Flow across 600 km depth is not allowed Flowlines & finite strain P-wave anisotropy S-wave Splitting at near-vertical incidence Incidence + 20° Incidence - 20°

20. Temperature Anomaly associated with Pacific Superswell influences flow(Toomey et al., EPSL 2002) Predictions for seismic anisotropy & heterogeneity match MELT data better than the other models tested

21. Subduction Zone Anisotropy Direction of fast-seismic propagation has been determined to parallel the trench in several cases Broadscale mantle flow? Backarc shearing (Hall et al., 2000)? Effect of water on fast-axis orientation during texturing (Jung & Karato, 2001)?

22. 2-D Corner Flow Add Along-trench Flow P-wave anisotropy P-wave anisotropy max S- wave Splitting max S- wave Splitting Vertical S-wave Splitting Vertical S-wave Splitting

23. Mantle flow and predicted anisotropy in the Lau Basin Conder et al., GRL 2002 Backarc spreading flow and melting in addition to subduction-induced mantle wedge flow

24. Finite Strain Ellipses Color-coded for depth Western US Becker et al, EPSL submitted 2005 Surface velocity condition (white arrows), global seismic tomography proxy for density, radially varying viscosity, Kaminsky & Ribe method for LPO Predicted Fast S Polarization Direction (black bar) Profile view of finite strain

25. Western US initial results Comparison of predicted (black bars) & observed (white bars) SKS splitting Gray wedge indicates variation in prediction due to method of synthetic calculation (single layer approximates 375 km deep region vs. multiple, variable layers) Overall fit is reasonable for central/southern area. NW and Basin & Range are not matched well. Local structure & effects on flow (not included in low resolution global model) preclude match in complex areas (S Great Valley)

26. Mantle Structure in the Gulf of California Relation to complete crustal transect? Anomalously hot? Depth extent of upwelling? Southern (oceanic spreading) vs. Northern (rifting) Gulf structure? Buoyancy vs plate-driven flow? Gaherty, Collins, Rebollar surface wave study designed to address several aspects Continue to pursue additional work (SIO, SDSU, CICESE, Indiana)

27. Seismometer Deployments • NARS • ~5 yr, 18 broadband • Collins et al. OBS • ~15 mo, 18 broadband • Proposed Array • 60 PASSCAL • 22 OBS • All broadband • 15-18 months

28. Seismometer Deployments • NARS • ~5 yr, 18 broadband • Collins et al. OBS • ~15 mo, 18 broadband • Previously Proposed Array • 60 PASSCAL • 22 OBS • All broadband • 15-18 months

29. Scientific Objectives • Architecture of the crust across the rift system • Teleseismic and local event tomography • Mapping of Moho across onshore & offshore parts of system • Local EQ source parameters • Strength and deformation of the lower crust • Imaging of forward scattered P-S conversions • Upper mantle thermal structure and flow • seismic velocity structure & attenuation (scale of ~10 km) • Shape of seismic discontinuities (upward or downward deflection?) • Seismic anisotropy (splitting, polarization direction of fast S wave) • Linked models of mantle flow, lithospheric deformation, development of textural (+/- melt) anisotropy, and seismic anisotropy • Converted phase imaging for detecting possible foundered slab • Coordinate with findings of other Gulf of CA studies

30. (Poppeliers & Pavlis, 2002)

31. Scientific Objectives • Architecture of the crust across the rift system • Teleseismic and local event tomography • Mapping of Moho across onshore & offshore parts of system • Local EQ source parameters • Strength and deformation of the lower crust • Imaging of forward scattered P-S conversions • Upper mantle thermal structure and flow • seismic velocity structure & attenuation (scale of ~10 km) • Shape of seismic discontinuities (upward or downward deflection?) • Seismic anisotropy (splitting, polarization direction of fast S wave) • Linked models of mantle flow, lithospheric deformation, development of textural (+/- melt) anisotropy, and seismic anisotropy • Converted phase imaging for detecting possible foundered slab • Coordinate with findings of other Gulf of CA studies

32. Jones et al., 1994

33. Summary • Mantle structure & flow are likely to vary both laterally and with depth on scale of several km • Along-strike changes in GoC rifting • Influence of transform offsets • Large scale flow at depth • Combination of research approaches needed to fully address problems • Separate effects of T, melt, textural anisotropy • Series of inverse studies and selected forward modeling tests to assess possible contributions of various structure and implied geodynamic consequences • Opportunity to assess both crust & mantle structure to infer processes of rifting as a full system • Combine body wave, surface wave and active source seismics • Current data will provide resolution of mantle on several 10 ‘s km scale • Denser onshore/OBS array would improve resolution to several km • Avoid ‘bias’ due to possible superpostion of segmentation and longer wavelength rifting signatures • Recognize cross-axis structure which could be key to understanding rifting processes

34. Key Constraints Needed • Surface velocity • Plate boundary geometry, temporal evolution • Cooling rates of crustal rocks • Guide vertical flow predictions • Information on melting • Degree/depth of melting • Composition • Seismic/EM measurements • All phases (P, S, Surface Waves) • As many backazimuths & angles of incidence as possible Caution about possible ‘bias’ due to limited areas (core complexes) emphasized in many studies …

35. Savage & Sheehan, 2000

37. Texture predicted depends on assumptions but fundamental result is often similar between methods Wenk & Tomé, JGR 1999, model recrystallization via strain-controlled nucleation & growth of new grains Sub-vertical shear imparts strong texture in upwelling zone; diffusion occurs in corner; subhorizontal shear generates plate-spreading signature Pole Figures illustrate development of texture

38. P-wave anisotropy along flowline: different models

39. Comparison of Texture & Finite Strain Anisotropy Prediction P-wave fast axis orientation for slow-spreading, passive model