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Discontinuous kinematics of the Pacific - North America plate boundary

Discontinuous kinematics of the Pacific - North America plate boundary. Outline of Talk. Relation of surface and subsurface velocity fields Western US velocity field Where Earthscope can help. Western North America. How to characterize the deformation: Possibilities.

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Discontinuous kinematics of the Pacific - North America plate boundary

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  1. Discontinuous kinematics of the Pacific - North America plate boundary IRIS June 2004

  2. Outline of Talk • Relation of surface and subsurface velocity fields • Western US velocity field • Where Earthscope can help IRIS June 2004

  3. Western North America IRIS June 2004

  4. How to characterize the deformation: Possibilities • Plate-like Faults penetrate lithosphere as high strain areasShear in mantle lithosphere ‘localized’ • Floating blocks in continuumWeak faults extend only through thin brittle upper layerDistributed shear in mantle/lower crust • Plates with wide boundariesCombination of above IRIS June 2004

  5. Thatcher, W., International Geology Review, 45, p. 191, 2002. IRIS June 2004

  6. Thatcher, W., J. Geophysical Res., March 1995. IRIS June 2004

  7. Marlborough region NZ Bourne et al., 1998 IRIS June 2004

  8. Deviatoric stress from gravitational potential energy variations Implied lithospheric viscosity from stress and strain rate estimates Flesch, L., et al. Science 287, 2000. IRIS June 2004

  9. Thatcher, W., International Geology Review, 45, p. 191, 2002. IRIS June 2004

  10. Pollitz, F., Geophys. J. Int. 153, 2003. IRIS June 2004

  11. Let’s examine the plate possibility for western US Analysis • Use geodetic, geologic, seismologic data to estimate simultaneously crustal block rotation poles, coupling on block-bounding faults, internal strain rates, and GPS reference frame • Each GPS velocity solution rotated into reference frame by least-squares fit • No velocity data excluded due to proximity to faults • 3D coupling distribution on faults parameterized by nodes along fault contours • Minimize reduced cn2 by simulated annealing & downhill simplex IRIS June 2004

  12. First a stop in NZ: M. K. Savage, K. M. Fischer, and C. E. Hall, Strain modeling, seismic anisotropy and coupling at strike-slip boundaries: Applications in New Zealand and the San Andreas Fault, Geol. Soc. London Special Publications, 227, 9-40, in press. Surface velocity field Wallace, Laura, et al., in prep. IRIS June 2004

  13. Rotational and Elastic parts of velocity field Wallace, Laura, et al., in prep. IRIS June 2004

  14. In North Island rotation accommodates 2/3 and faulting 1/3 of transverse motion (gray lines) -- rotation is our friend IRIS June 2004

  15. Region is divided into ‘blocks’, contiguous areas that are thought to rotate. The relative long-term slip vectors on the faults are determined from rotation poles. Each block rotates about a pole. Back-slip is applied at each fault to get surface velocities due to locking. Velocities due to fault locking are added to rotations to get full velocity field. The rotating blocks are separated by dipping faults. IRIS June 2004

  16. The strain rate tensor near a locked fault represents a spatial transition from the velocity of one block to the velocity of the other. In other words, a locked fault allows one block to communicate information about its motion into an adjacent block. Program described at www.rpi.edu/~mccafr/defnode/defnode.html IRIS June 2004

  17. Data • Slip vectors • Harvard CMT, NUVEL-1, C. DeMets, Jackson & Molnar (1990) • Transform azimuths • C. DeMets • Slip rates • NUVEL-1, C. DeMets, several compilations • Fault outline data • Jennings GPS velocities • PNW1, our PNW solution • SCEC CDM3, Southern California • WUSC version 2, Western US (Bennett et al.) • Northern California (Freymueller et al., 1999) • BARD (Murray and Segall, 2001) • Sierra Nevada (Dixon et al., 2000) • ECSZ (McCluskey et al. 2001, Gan et al. 2001) • Basin and Range (Thatcher et al. 1999) • Baja (Dixon et al. 2002) • Pacific –North America (Beavan et al. 2002) IRIS June 2004

  18. Block model GPS Fault slip rate EQ slip vector IRIS June 2004

  19. North America reference frame is estimated by minimizing 248 GPS velocities (Nrms=1.1, Wrms=1.0mm/yr). Pacific angular velocity from 5 spreading rates, 73 eq slip vectors (Nrms = 1.2), and 56 GPS velocities (Nrms = 1.2, Wrms=1.0mm/yr). Juan de Fuca Euler vector from 28 PAC-JdF spreading rates, 1 transform azimuth (Blanco FZ; res = 1º). IRIS June 2004

  20. Block boundaries placed along major fault systems. Slip vectors Observed Calculated IRIS June 2004

  21. Block motion Total NW componentNE component IRIS June 2004

  22. Rotational component N componentE component IRIS June 2004

  23. GPS residuals with 70% confidence ellipses IRIS June 2004

  24. Predicted fault slip rates IRIS June 2004

  25. Block rotations Lamb, S., Earth Planet. Sci. Lett. 84, p. 75, 1987. IRIS June 2004

  26. Luyendyk, B. GSA Bull., 1991. Jackson & Molnar, J. Geophys. Res., 1990. IRIS June 2004

  27. Vertical axis rotations in North America frame (Negative is clockwise) IRIS June 2004

  28. GPS residuals in Transverse Ranges don’t indicate rapid rotations (presently) IRIS June 2004

  29. Strain rates within blocks needed in 5% to 15% of area Inverted strain rates – sigma < 10 ns/yr 10 < sigma < 20 ns/yr Strain rates – residual inverted 10 ns/yr = 1 mm/yr over 100 km distance IRIS June 2004

  30. Velocity field for Pacific Northwest derived from campaign and continuous sites. Reference frame is North America and ellipses are 70% confidence. In collaboration with Tony Qamar, Bob King, Herb Dragert, Charles Williams IRIS June 2004

  31. East profiles of East component East profiles of North component 47.2N 46N 47.2N 47.2N 44.5N 42.5N mm/a 46N 46N mm/a W E Distance from deformation front, km 44.5N 44.5N 42.5N 42.5N Distance from deformation front, km IRIS June 2004

  32. IRIS June 2004

  33. Allowing Oregon and Washington to behave as 5 independent, rotating blocks shows which regions take up the slip. • The poles of 4 of the Oregon blocks fall close the the ‘whole Oregon’ pole. • All rotations are clockwise. • Could be ~ 1 mm/a extension along arc • No indications of strike-slip along arc Block rotations relative to North America. Block rotations relative to NE Oregon. W Washington (0.69) SE Oregon (0.20 o/Ma) NE Washington (0.19) NE Oregon (0.72) Cape Blanco (0.72) IRIS June 2004

  34. Rotation rates GPS – derived P’mag – Ray Wells 12 Ma Pomona 15 Ma Ginko Rotating Oregon block IRIS June 2004

  35. Western US ‘discontinuous’ velocity field North America frame Hotspot frame IRIS June 2004

  36. Splitting observations on map of shear wave velocities at 150 km depth. (Silver, P., and W. Holt, Science 295, 2002) Summary of shear-wave splitting measurements in California and Nevada. From M. K. Savage, K. M. Fischer, and C. E. Hall, Geol. Soc. London Special Publications, 227, 9-40, in press. IRIS June 2004

  37. Observations Mantle = hotspot Surface velocity Mantle velocity (hot spot frame) Mantle moves east Silver, P., and W. Holt, Science 295, 2002 IRIS June 2004

  38. Steady-state, discontinuous, hot-spot frame velocity field Straining block inversion Rigid block inversion IRIS June 2004

  39. Thoughts: • ‘Block’ representation appears to work for about 85 to 95% of western US at the mm/yr (2%) level • PBO will provide improved surface velocity distributions • USArray can provide length-scales of strain variations in mantle • USArray can provide length-scales of mantle wavespeed variations (viscosity) in mantle IRIS June 2004

  40. THE END IRIS June 2004

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