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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