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USArray and Active Tectonic Deformation in the Western United States

USArray and Active Tectonic Deformation in the Western United States. SW US GPS Block Model. Western North America S-Splits. Wayne Thatcher U. S. Geological Survey, Menlo Park, CA.

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USArray and Active Tectonic Deformation in the Western United States

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  1. USArray and Active Tectonic Deformation in the Western United States SW US GPS Block Model Western North America S-Splits Wayne Thatcher U. S. Geological Survey, Menlo Park, CA Thanks to : Bill Hammond, George Zandt, Gene Humphreys, Don Forsyth, Martha Savage & Rob McCaffrey for Sharing Results

  2. Applied Forces & Rheology Determine Continental Deformation Forces + Rheology = Deformation . (F) () eij(x, y) • plate flexure • post-glacial rebound • post-seismic deformation • GPS & InSAR • tectonic geology • seismic anisotropy (z) • seismic constraints • dynamic modeling

  3. Model Grid & Applied Forces Humphreys & Coblenz [2006]

  4. Model Constraints from Focal Mechanisms Humphreys & Coblenz [2006]

  5. Modeled Stress Field in Western US Humphreys & Coblenz [2006]

  6. Forces + Rheology = Deformation . (F) () eij(x, y) • (x, y) ~ easy (GPS) • (z) difficult (S-splits) The hardest part!

  7. Western US Topography, Holocene Faults & Plate Tectonic Setting Sierra Nevada Colorado Plateau GPS Transect Faults from M. Machette & R. Dart, USGS

  8. GPS Velocities Across Western US Basin & Range Province Sierra Nevada Colorado Plateau WEST EAST Hammond & Thatcher (2004)

  9. Geodetically Even Topographically Impressive Toiyabe Range is Unimportant

  10. Southwest U. S. Block Model & GPS Sites McCaffrey [2005 JGR]

  11. GPS maps surface deformation but provides no strong constraints on depth extent of blocks How can we learn more about this?

  12. ....If Upper Crust is Coupled to Mantle

  13. Western North America S-Splits ABSOLUTE PLATE MOTION M. Savage (2003)

  14. California & Nevada S-Split Orientations (Polet & Kanamori 2002 GJI)

  15. Southwest U. S. Block Model McCaffrey [2005 JGR]

  16. Observed Mantle Deformation Fabric Orientation& Modeled Absolute Velocity of Surface Blocks Observed S-Split Orientations (Polet & Kanamori 2002 GJI)) Modeled Absolute Surface Velocity (Hot-Spot Reference Frame) Acceptable Agreement in Central & Northern California (Motion of Lithosphere Blocks Causing Ductile Flow of Asthenosphere?) Notable Disagreement in Southern California (Local Perturbation by Transverse Ranges Tectonics?)

  17. Southern California Surface Wave Tomography Yin & Forsyth [2006]

  18. Surface Wave Tomography Shows High Seismic Velocity Blobs under Transverse Ranges Possibly Related to Anomalous S-Split Orientations? EAST WEST Yin & Forsyth [2006]

  19. Sierra Nevada Earthscope Project (SNEP) • Lithospheric X-section Images: • Based on CCP stacking Of ~1300 RFs from 66 FA/PASSCAL &19 TA stations • Results relevant to active deformation processes: • Basin & Range crustal character extends far into the Sierran Range • Disappearance of Moho beneath western foothills may be related to Lithospheric “Drip Tectonics” • High levels of crustal seismicity and young volcanism in eastern Sierra may be linked to delamination processes Zandt, Gilbert, Owens, Jones, & SNEP Field Team, [2005 IRIS mtg.]

  20. EarthScope Field Area

  21. Alternative Kinematic Descriptions of Continental Deformation • Each argued, to exclusion of the other, since early days of plate tectonics • Based on seismicity, active faulting, eq focal mechanisms… • Elements of each have observational support Plate-Like Continuum Sea • Space geodesy (GPS, InSAR) can quantify and distinguish between extreme end-member models

  22. GPS Data, Blocks, Rotation Axes & Small Circles Thatcher (2006)

  23. Aegean GPS-Based Block Model Nyst & Thatcher (2004 JGR)

  24. ENGLAND, MOLNAR HOLT, HAINES, SILVER… EQUIPE TAPPONNIER

  25. Take Home Points • GPS uniquely quantifies present-day Tibetan deformation • Deformation simply&usefully described by relative block motions (i.e microplates follow rules of plate tectonics) • Internal deformation of microplates small but measurable • GPS slip rates on major Tibet strike-slip faults low, 5-12 mm/yr • GPS slip rates << geologic estimates of ~12-30 mm/yr Cause of discrepancy is unknown •S-Splits suggest microplate drag of lithospheric mantle

  26. Some Unanswered Questions • Are larger microplates fragmented into smaller blocks? • Are ‘GPS microplates’ geologically long-lived or transitory? • To what depth in lithosphere do block structures extend? • Why do GPS and geologic slip rates disagree in Tibet?

  27. Comparison of Zhang et al (2004) GPS Data (blue) and Data Used in this Study (red)

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