Geophysical study of the crust and upper mantle in the Qinghai-Tibet Plateau, China:

Broadband seismic observations and tomography imaging of the

lithosphere in southern and central Tibetan

USA,2008-11-11

He Rizheng, Gao Rui, Zheng Hongwei (CAGS, Cnina)

Zhao Dapeng (Northeastern University, Japan )

Since Paleozoic, Multiple collisions in diferent geological eras have sequentially produced near E-W tendings structures, such as sutures, thrust and folds, thrust and normal faults.

Since ～ 17Ma, strong E-W extending took place in the Tibet, as some sequence of collision of India plate with Eurasian plate( Armijo et al,1989; Yin An et al ， 2000).

（ Kapp and Guynn ， 2004 ）

Typical Models for Tibet uplifting

http://www.geo.arizona.edu/~ozacar/models~1.html

(Argand, 1924)

(Dewey and Burke, 1973)

(England and Houseman, 1989) (Tapponnier et al., 1982)

(Zhao and Morgan, 1987)

(Hamburger et al., 1992)

The Key question was deep structure under the central Tibet needed to be known.

To understand those key questions, hundreds of broadband stations deployed in Tibet in past two decades.

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To understand above-mentioned questions, we collect the datasets of several projects, such as INDEPTH-I,II,III, HIMNT, ISC etc, which had been carried out in the central Tibet.

All of stations used sum up to 305 stations.

3-D Tomography method used was from Zhao Dapeng (1992,1994, 2001) .

305 Stations used in this study

P-wave images at various depths:From 0 km to 100 km

1. Low P-velocity under Tethyan Himalaya is shallower than Moho depth.

2. Velocity structure of Lhasa Terrane was more complicated.

3. Low-velocity anomaly boday located beneath YGR, which is the most scale rift in the Central Tibet.

4. As a whole, the strike of velocity anomaly body is similar to suface tectonic tr

end.

1. The trending of P-velocity anomalous bodies had varied from E-W in the crust to nearly S-N.

2. low P-velocity anomalous body beneath Tethyan Himalaya disappered.

P-wave images at various depths:From 150 km to 400 km

(Tilmann et al., 2003)

Comparing with the existing results

（ He et al.,2006 ）

Comparision between the profile along Tibetan Highway from our group and the result of Owens and Zandt （ 1997 ） .The above one is from Owens an

d Zandt （ 1997),the below one is from this study.

（ Zheng et al.,2006)

Tomographic images of E-W profiles along 25°N show that Indian Lithosphere had a bend.

（ Zheng et al.,2006 ）

Tomographic images of the Indian Lithosphere can be continuously traced from N24°to N34° under E88° profile.

（ Zheng et al.,2007 ）

The frontier of subducted Indian Lithospheric Mantle

White solid-lines show the location of the frontier of subducted ILM projected to the surface. Black dash-lines denote the location needing to be confirmed. The dash-line contour shows a low Pn and high Sn attenuation zone (McNamara, 1994 ).

（ He et al.,2006 ）

Our Conclusions1. The frontier of ILM presently located beneat

h the central Qiangtang Terrane, about N34°.2. ILM deformed, which was similar to that of t

he SCOOP , with about NE strike. The deformation of ILM resulted of Extending in the region.

3. The partial melting materials beneath Tethyan Himalaya maybe resulted of lecuogranites in the High Himalaya and Tethyan Himalaya.

Cenozoic Potassic magmatism in the central Tibetan happened during Tibetan uplifting (Hacker et al., 2000; Ding et al., 2003William et al,2004; Chung et al ， 2004; Guo et al., 2006).

The zone of uneffect Sn-Low Pn

Is similar to the potassic vocanic zone

Vocanism in Northern Tibet is hard to understand

Seismicity Map (Mb>3.0) from USGS

From 01,Jan to 08, Nov

Ando

HotanJSS

DangxiongZongba

Nyma

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So, we make a plan that we will deploy hundreds of BB stations with ~10km space along the S-N profile along E88.5 ° from JSS to KLS, and E-W profile along N34° or N35°from E83° to E95°to cross the Lavas zone in Northern Tibet to capture some images of deep structure under the zone.

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(Yin & Harrison, 2000 ) (Tilmann et al., 2003)

(He et al.,2006)

Cartoon of ILM under central Tibet (E85° to E90°)