1. 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 1.Institute of Earth Sciences, Academia Sinica, Taiwan 2.Institute of Geophysics, National Central University, Taiwan 3.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 Manila Trench South China Sea CWB M L  3.5 Kuo, 2003 IES DMC 80 mm/yr WEP

4. Tectonic Evolution of the Northern Taiwan Mountain Belt 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 NWSE

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

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

7. Upper Mantle Anisotropy in the vicinity of Northern Taiwan (SKS) WFSB ANPB 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 Okinawa Trough YNG IGK

8. Upper Mantle Anisotropy beneath Northern Taiwan (SKS) One Backbone Network BA TSBroadband 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) 50 km 100 km 150 km Tatun Volcanic Area Central Range Taipei Hsinchu

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

10. Mantle Wedge Anisotropy beneath Northern Taiwan (tele. S) Waveforms generated from deep earthquakes in the Tonga-Kermedec region  = 74  Incidence angle= 20  T 0 = 12-15 s 210 km 410 km SKS tele. S S Trench direction

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 Pacific Plate Philippine Sea Plate Eurasian Plate Taiwan Kamchatka ? ? 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 76 mm/yr 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 Kim et al., 2005 N S Offshore Mantle wedge *Note the color scale is not the conventional one 100 A low velocity layer attached on the slab surface A’ CWB dataset

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

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

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 Kneller et al., 2005 Nakajima & Hasegawa, 2004 Jung and Karato, 2001 Katayama et al., 2004

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

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. - Thank you - Syueshan, Taiwan

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

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 Wang et al., 2006 Local/Regional/Teleseismic events joint inversion Prescribed slab

29. Wang et al., 2004

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

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