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Upper Mantle Seismic Anisotropy around the Plate Edge beneath northern Taiwan

Upper Mantle Seismic Anisotropy around the Plate Edge beneath northern Taiwan. Wen-Tzong Liang 1 Yih-Zhen Hsu 2 Bor-Shouh Huang 1 Char-Shine Liu 3. Institute of Earth Sciences, Academia Sinica, Taiwan Institute of Geophysics, National Central University, Taiwan

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Upper Mantle Seismic Anisotropy around the Plate Edge beneath northern Taiwan

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  1. Upper Mantle Seismic Anisotropy around the Plate Edge beneath northern Taiwan Wen-Tzong Liang1 Yih-Zhen Hsu2 Bor-Shouh Huang1 Char-Shine Liu3 • Institute of Earth Sciences, Academia Sinica, Taiwan • Institute of Geophysics, National Central University, Taiwan • Institute of Oceanography, National Taiwan University, Taiwan

  2. Outline • Taiwan Tectonics • Upper Mantle Anisotropy in the Taiwan Region • New Measurements • Implications • Conclusions

  3. Taiwan Tectonic Setting CWB ML 3.5 WEP 80 mm/yr South China Sea Manila Trench Kuo, 2003 IES DMC

  4. Tectonic Evolution of the Northern Taiwan Mountain Belt NW SE • Southwest-propagating arc-continent collision • Westward extended Ryukyu subduction and caused flipping of subduction in the northern part of the collision orogen • Lithospheric stretching induced by the trench suction • Collision orogen collapsed as a result of crustal thinning Teng, 1996

  5. Upper Mantle Anisotropy in the Taiwan Region (I) • Splitting in regional and teleseismic shear waves (ScS, S) ignoring source side anisotropy • Mountain-parallel anisotropy • Taiwan Orogen Rau et al., 2000

  6. Upper Mantle Anisotropy in the Taiwan Region (II) T32A-01 10:20 MCS 302Seismic anisotropy beneath an active collision orogen of Taiwan from dense array observationsHuang et al. GRL (2006)

  7. Upper Mantle Anisotropy in the vicinity of Northern Taiwan (SKS) • SKS phases radiated from the 2006/02/22 Mozambique Earthquake (Mw=7.0) occurred ~97 away from N. Taiwan • Seismic Stations of BATS/ MT/ YM/ HC/ F-Net Networks • Mountain-parallel (on land) vs. Trench-parallel fast direction (in agreement with Long et al. 2005, 2006) • t = 0.2 ~ 1.5 s WFSB ANPB Okinawa Trough YNG IGK

  8. 50 km Upper Mantle Anisotropy beneath Northern Taiwan (SKS) One Backbone Network • Broadband Array in Taiwan for Seismology (BATS) 3 Portable BB Netowroks • Metropolitan Taipei BB Network (MT) • Yangmingshan National Park Network (YM) • Hsinchu (HC) BB Network Contours of Wadati-Benioff zone are adopted from Chou et al. (2006) Tatun Volcanic Area Taipei 150 km 100 km Hsinchu Central Range

  9. 74 < d < 154 km ANPB 60 < d < 192 km WFSB Crustal anisotropy Mantle Wedge Anisotropy beneath Northern Taiwan (local S) Trench-parallel anisotropy in the mantle wedge derived from local deep events

  10. Mantle Wedge Anisotropy beneath Northern Taiwan (tele. S) • Waveforms generated from deep earthquakes in the Tonga-Kermedec region •  = 74 • Incidence angle= 20 • T0 = 12-15 s Trench direction 210 km S tele. S SKS 410 km

  11. Possible Origins of Upper Mantle Anisotropy in the Northern Taiwan • Edge mantle flow? • Melt preferred orientation • Collision induced mantle deformation beyond the mantle wedge • B-type olivine fabric vs. corner flow

  12. Seismic Anisotropy around the Slab Edges in the Western Pacific • Similar anisotropy pattern observed in the Kamchatka region (Peyton, et al., 2001; Levin et al., 2004) • Trench-parallel in the mantle wedge • Trench-normal beyond the slab edge • Suggesting mantle flow at the slab edge driven by the slab retreat • Slab edge subjected to an oblique subduction in the Taiwan region ? Kamchatka 76 mm/yr Eurasian Plate Pacific Plate ? Taiwan Philippine Sea Plate 80 mm/yr

  13. Possible Origins of Upper Mantle Anisotropy in the Northern Taiwan • Edge mantle flow • Melt preferred orientation? • Collision induced mantle deformation beyond the mantle wedge • B-type olivine fabric vs. corner flow

  14. 3D Vp and Vs Structural Models • A low velocity layer attached on the slab surface N S 100 A’ Offshore Mantle wedge 100 *Note the color scale is not the conventional one CWB dataset Kim et al., 2005

  15. Subduction beneath North Taiwan Vp % Vs % A A’ A N S N S A’ N S Vp/Vs % checkerboard test CWB+JMA datasets Chou et al., 2006b

  16. Possible Origins of Upper Mantle Anisotropy in the Northern Taiwan • Edge mantle flow • Melt preferred orientation • Collision induced mantle deformation beyond the mantle wedge • B-type olivine fabric

  17. Slab Collision and Folding S N Slab folding N Lateral compression along slab at 50~100 km depth range S Chou et al., 2006a

  18. Possible Origins of Upper Mantle Anisotropy in the Northern Taiwan • Edge mantle flow • Melt preferred orientation • Collision induced mantle deformation beyond the mantle wedge • B-type olivine fabric

  19. B-type Olivine Fabric Nakajima & Hasegawa, 2004 Jung and Karato, 2001 Katayama et al., 2004 Kneller et al., 2005

  20. N ? 3D Geodynamic Structure in the Taiwan Region Eurosian Plate Philippine Sea Plate N Lin et al., 2004 Kim et al., 2005 Chou et al., 2006 Wang et al., 2006 S

  21. Conclusions • Mountain-parallel anisotropy beyond the mantle wedge implies mantle deformation due to the collision tectonics • Trench-parallel anisotropy exists in the mantle wedge beneath Northern Taiwan • Significant spatial variation of anisotropy beneath ANPB indicates different origins of anisotropy – weak coupling between mantle wedge and continental lithosphere? • Preserved upper mantle deformation /B-type olivine fabric/ Edge mantle flow/ melt preferred orientation • Deployment of Broadband OBS is necessary to study the subduction process in the Taiwan region – ongoing!

  22. IES Broadband OBS Experiment The 1st deployment of BB OBS experiment in the eastern offshore region Kuo and Chi, Sep. 2006 http://obs.earth.sinica.edu.tw

  23. Syueshan, Taiwan - Thank you -

  24. Flow Fields for the Obliquely Subducted Slab 3D perspective plots of the particle paths of different scenarios : (a) rotating Euler vectors onto the slab surface (b) Flow field minimizing the integrated in-plane deformation rate of the subducted slab (c) Flow field minimizing the overall in-plane deformation rate in the entire modeling region Chiao, et al., 2001

  25. Melting Features along the Western Ryukyu Slab Edge N Lin et al., 2004

  26. Measurement Examples

  27. Crustal Deformation in the Central and Southern Ryukyu Arc • Trench-parallel extensional strain • Southward migration of Ryukyu Arc • Bending/retreat of Philippine Sea slab Interaction between the Taiwan-Luzon Arc Collision and the bending of Ryukyu Arc. Nakamura, 2004

  28. Subduction beneath South Taiwan Local/Regional/Teleseismic events joint inversion Prescribed slab Wang et al., 2006

  29. Wang et al., 2004

  30. Seismic anisotropy and mantle creep in young orogens Fast LPOof olivine in the direction of mantle creep that is responsible for the Lithospheric Collision Meissner et al., 2002

  31. Effect of Water and Stress on LPO of Olivine Jung and Karato, 2001 Katayama et al., 2004 Jung et al., 2006

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