Upper Mantle Seismic Anisotropy around the Plate Edge beneath

northern TaiwanWen-Tzong Liang1 Yih-Zhen Hsu2 B

or-Shouh Huang1 Char-Shine Liu3

1. Institute of Earth Sciences, Academia Sinica, Taiwan2. Institute of Geophysics, National Central University, Taiwan3. Institute of Oceanography, National Taiwan University, Taiwan

Outline

• Taiwan Tectonics

• Upper Mantle Anisotropy in the Taiwan Region

• New Measurements

• Implications

• Conclusions

Taiwan Tectonic Setting

Manila T

rench

South China

Sea

CWB ML 3.5

Kuo, 2003 IES DMC

80 80 mm/yrmm/yr

WEP

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

NW SE

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

Upper Mantle Anisotropy in the Taiwan Region (II)

T32A-01 T32A-01 10:20 MCS 302Seismic anisotropy beneath an active collision orogen of Taiwan from dense array observationsHuang et al. GRL (2006)

Upper Mantle Anisotropy in the vicinity of Northern Taiwan (SKS)

WFSBANPB

• 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

YNGIGK

Upper Mantle Anisotropy beneath Northern Taiwan (SKS)

One Backbone Network

• BBroadband AArray in TTaiwan for SSeismology (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 Central RangeRange

Taipei

Hsinchu

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

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

210 km

410 km

SKStele. S

S

Trench direction

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

Seismic Anisotropy around the Slab Edges in the Western Pacific

Pacific Plate

Philippine Sea Plate

Eurasian Plate

Taiwan

Kamchatka

?

?• Similar anisotropy pattern obs

erved in the Kamchatka region (Peyton, et al., 2001; Levin et al., 2004)– Trench-parallel in the mant

le 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

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

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

100

• A low velocity layer attached on the slab surface

A’

CWB dataset

Subduction beneath North Taiwan

N S

N S

SN

Chou et al., 2006b

Vp %

Vp/Vs %

Vs %

checkerboard test

A’

AA’A

CWB+JMA datasets

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

Slab Collision and Folding

Chou et al., 2006a

NS

N

SLateral compression along slab at 50~100 km depth range

Slab folding

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

B-type Olivine Fabric

Kneller et al., 2005

Nakajima & Hasegawa, 2004

Jung and Karato, 2001 Katayama et al., 2004

N

3D Geodynamic Structure in the Taiwan Region

Eurosian Plate

Philippine Sea Plate

NSLin et al., 2004

Kim et al., 2005 Chou et al., 2006 Wang et al., 2006

?

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!

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

- Thank you -

Syueshan, Taiwan

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

Lin et al., 2004

Melting Features along the Western Ryukyu Slab Edge

N

Measurement Examples

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

Subduction beneath South Taiwan

Wang et al., 2006

Local/Regional/Teleseismic events joint inversion

Prescribed slab

Wang et al., 2004

Seismic anisotropy and mantle creep in young orogens

Meissner et al., 2002

Fast LPO LPO of olivine in the direction of mantle mantle creepcreep that is responsible for the Lithospheric CollisionLithospheric Collision

Effect of Water and Stress on LPOLPO of Olivine

Jung and Karato, 2001 Katayama et al., 2004 J

ung et al., 2006