Architecture of Colliding Tectonic Plates in Tibet by Passive Source

SeismologyJunmeng Zhao

Institute of Tibetan Plateau Research, Chinese Academy of Sciences

Acknowledgments:Research: Hongbing Liu, Shunping Pei, Lin Ding, Xing Gao, Qiang Xu, Wei Wang (Institute of Tibetan Plateau Research, CAS) Xiaohui Yuan, Prakash Kumar, Rainer Kind (Potsdam, Germany) Zhongjie Zhang and Jiwen Teng (Institute of Geology and Geophysics, CAS)Research Funding: Talent Project, CAS; Innovation Project CAS; NNSF

The crust and mantle structure and the activities control the formation and evolution of the entire geodynamic regime.

The uplifting of the Tibetan Plateau is one of the most important tectonic events in geoscience since Meso-Cenozoic. It affects global climate change, especially the northern semi-sphere.

The Third Pole

Some models for Tibetan Plateau uplifting

QuestionWhy one plateau, more

models?

Kind et al., 2002

Kumar et al., 2006

ANTILOPE (Array Network of Tibetan International Lithospheric Observation and Probe Experiments)

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ANTILOPE progress till 2010

Sections will be finished in 2009ITPCAS Array

ANTILOPE-I

ANTILOPE-II

Bad traffic condition Instruments were carried to assigned position by horses

Tutoring at night for a Tibetan student (middle) in a tent at 5300m highDonation to a Tibetan school

SourceSource(Earthquake)(Earthquake)

Seismic Wave Seismic Wave PropagationPropagation

Receiver ArrayReceiver Array(seismometers)(seismometers)

苏门答腊 2005-10-11 Ms6.0

Architecture of Colliding Tectonic Plates in Tibet

A map showing teleseismic events used for calculation of P and S receiver functions

478 teleseismic earthquakes of highsignal∕noise ratio have been recorded and used for receiver functionanalysis.

7421 P receiver functionsfrom 354 events at epicentral distances of 30–95°

3476 S receiver functions [including core phase (SKS) receiver functions]from 249 events at epicentral distances of 60–115°

Distribution of events with incidence angles estimated by minimizing the SV amplitudes on the P-component of seismograms. We have kept a threshold incidence angle of 52° and beyond that the events were rejected due to possible contamination by high noise level. For nearly 94% of total events the incidence angles determined are less than 30°, a reasonable range of S wave incidence angles.

Architecture of Colliding Tectonic Plates in Tibet

Seismic P Wave receiver functions

Location and Data of ANTILOPE-I 、 II

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Map of seismic stations (black triangles) of the two recent profiles. Black lines denote major sutures and faults.

Crosses denote surface projection ofthe P-to-S conversion points (piercing points) at different depths. Yellow circles are piercing points of the S-to-P conversions at the LAB depth.

(Zhao et al.,2010 PNAS)

S wave receiver functions of ANTILOPE-I

(Zhao et al., 2010)

S wave receiver functions, sorted by piercing points at the 150 km conversion depth.

Positive amplitudes are coded red, indicating an increasing velocity jump with depth, and vice versa. The Moho and LAB phases are marked by dashed lines.

羚羊 -I 羚羊 -IIANTILOPE-I

LAB

Location of ANTILOPE-I

(Zhao et al., 2010)

羚羊 -I 羚羊 -IIANTILOPE-II

LAB ?

S receiver functions, sorted by piercing points at the 150 km conversion depth.

S wave receiver functions of ANTILOPE-II

Positive amplitudes are coded red, indicating an increasing velocity jump with depth, and vice versa. The Moho and LAB phases are marked by dashed lines.

Positive amplitudes are plotted in red, negative in blue. The dashed lines follow the observed phases of the 410 and 660 km discontinuities. The blue and green lines indicate the positions of Indian and Asian plates determined by S wave receiver functions, respectively.

ANTILOPE-I (Zhao et al., 2010)

ANTILOPE-I

Locations of the profiles

Behavior of upper mantle transitional zone when disturbed

Upper mantle transitional zone

P wave receiver functions of ANTILOPE-I

ANTILOPE-II

The blue and red lines indicate the positions of Indian and Asian plates determined by S wave receiver functions, respectively.

P wave receiver Functions of ANTILOPE-II

（ Zhao et al., 2010 ）

Locations of the profiles

ANTILOPE-II

Behavior of upper mantle transitional zone when disturbed

INDEPTH

Location of the profile

INDEPTH

Upper mantle transitional zone

P wave receiver functions of INDEPTH

The blue, red and green lines indicate the positions of Indian, Tibetan and Asian plates determined by S wave

receiver functions, respectively.Behavior of upper mantle

transitional zone when disturbed

An exception would be eastern Tibet at the border of the Sichuan Basin (LMS line ), where lithospheric delamination, possibly acompanied by a vertically asthenospheric flow, may change the equilibrium status of the mantle transition zone. Therefore eastern Tibet seems the only region in our data, where plate tectonics is reaching the mantle transition zone.

Comparison of the transitional zones along four lines in Tibet

Figure4. Rose diagram of each station showing the measurements calculated by the cross-correlation method.The concentric circles are scales. The diameter of the big one is 2 second and the small one 1 second. The number under each circle is number of the seismic station with 3 in the southern end and 66 as the northern end. The straight lines in rose diagrams are measurements of seismic anisotropy calculated by the cross-correlation method. The length stands for the delay time and direction of the straight line stands for the fast polarization direction.

Seismic anisotropy along ANTILOPE-II

Seismic anisotropy along ANTILOPE-II

A) the elevation.B) The delay times. C) The green segment orientation shows the direction of station’s fast wave polarization, and upward is north. D) Cartoon illustrating the collision of the Indian and Tibetan plates by summarizing the results of seismic anisotropy from shear wave splitting analysis and the result of S receiver functions along the same profile.

(Zhao et al., 2010)D

Seismic anisotropySKS anisotropy data at Tibet along the two new profiles (red) and from a collection of other papers ( blue). In south-ern and western Tibet, no or very weak anisotropy is observed. In contrast, the region in northern central and eastern Tibet, which acts like a mobile buffer zone between the collidinglithospheres, is characterized by strong anisotropy.

(Zhao et al.,2010 PNAS)

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ATK Section

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Lift: Simple cartoons showing the relationship of the Indian, Tibetan and Asian mantle lithospheres

Upper: Map of the Tibetan plateau and seismic profiles Transparency colors denote the locations of the Indian, Tibetan and Asian plates. The India-Tibet boundary indicates the northern edge of the Indian plate; The Tibet-Asia boundary is partly defined by the locations of Moho offsets. Blue and Green arrows denote the dominant subduction of the Indian and Asian plates, respectively.

Architecture of Colliding Tectonic Plates in Tibet

The boundary between the Indian andAsian tectonic plates below Tibet

The west and central lines are our recent profiles. East line is a compilation of two French experiments, the INDEPTH profiles and the PASSCAL experiment from 1991–1992. The TK Line is an additional profile with LAB observations used in this paper. Blue and green bars with arrows mark LAB observations of the Indian (IML) and Asian (AML) plates, respectively. Yellow and red boxes mark observationsof the upper mantle discontinuities. Red parts of these boxes markunusually late arrivals of the converted phases from the 410 discontinuity. Black bars at the northern margins of Tibet at the west line and east line denote sharp steps in Moho depth.

(Zhao et al.,2010 PNAS)

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(Gan et al. 2007, JGR)

Crustal Deformation Around the Tibetan Plateau (Tibetan Plateau Fixed Reference Frame)

Wenchuan Eq

Crustal structure of ANTILOPE-II by P wave receiver function stack

YZS BNS8km

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S-wave velocity structure of the crust along ANTILOPE-II by waveform inversion

Shear wave velocity along ANTILOPE- I by ambient noise tomography

Shear wave velocity along ANTILOPE-II by ambient noise tomography

Crustal channel flow?Indian crustal intrusion?

How to bridge the crust and mantle?

How to bridge the crust and mantle?

Are these parts with high velocity from Indian crust?

Conceptual Model

结 论1 ）印度板块和亚洲板块在青藏高原之下的碰撞边界是沿塔里木盆地西缘

到喜马拉雅东构造结一线；2 ）青藏高原的地壳缩短在高原南部通过印度地壳向亚洲地壳之下俯冲实

现调节，在高原北部为均匀的地壳增厚所吸收；3 ）青藏高原西部海拔较高、地形起伏强烈的原因是有较坚硬的岩石圈地

幔的支持，而高原北部和东部岩石圈地幔较软，故地形相对平坦、较低；4 ）在高原的北部和东部发现一个特殊的岩石圈区域（西藏板块），它夹

持于印度板块与亚洲板块之间，具有高温、低速、高 Sn 波衰减和地震各向异性，在南北挤压的应力背景下该岩石圈整体向东运动，遇到四川盆地的阻挡向四个方向运动：云贵川方向、鄂尔多斯方向；向上形成龙门山，向下进入深部地幔，汶川 地震就位于该区域的东缘。

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Lift: Simple cartoons showing the relationship of the Indian, Tibetan and Asian mantle lithospheres

Upper: Map of the Tibetan plateau and seismic profiles Transparency colors denote the locations of the Indian, Tibetan and Asian plates. The India-Tibet boundary indicates the northern edge of the Indian plate; The Tibet-Asia boundary is partly defined by the locations of Moho offsets. Blue and Green arrows denote the dominant subduction of the Indian and Asian plates, respectively.

关键科学问题（ 1 ）：亚洲岩石圈地幔的行为方式

a. 中下地壳拆离 -- 逐步生长隆升模型 (Tapponnier et al., 1990;) 。祁连山的隆起是由于阿尔金断裂的北东向走滑，在中下地壳形成了拆离面，该拆离面逐步向北东扩展冲断，依次导致南祁连、中祁连和北祁连山的隆起。如果该模型成立，在中下地壳应形成拆离面，亚洲岩石圈地幔向南插入祁连山下，祁连山早期的古亚洲岩石圈可能被彻底置换。

b. 块体拼贴 - 后期同步隆升模型（ Yin et al., 2000 ）。阿尔金断裂的北东向走滑并没有导致祁连山在中下地壳拆离，而是造成区域性的整体挤压缩短和隆升，祁连山早期的块体拼贴结构没有被破坏，仅仅是各块体沿早期拼贴缝合带再次活化。如果该模型成立，那么，南、中、北祁连山块体应该是同步隆升，幅度差别不大，早期亚洲岩石圈组成清晰可见，不需要亚洲岩石圈地幔向南插入祁连山下。那么，青藏高原向北东方向扩展是整体抬升？还是由南而北逐渐扩展？

关键科学问题（ 2）：青藏高原北东向扩展的深部 动力学机制a

b

观测系统1.在 ANTILOPE-IV 剖面上架设 100 套宽频带数字地震仪，平均点距为 5 km ，在关键构造部位加密至 3km ，记录时间为 1 年，获得约1000Gb 的高质量的地震数据；考虑到噪音成像的需要，在剖面西侧的阿尔金山西侧布设一定数量的台站，形成对研究区及周边地区有效的射线覆盖。

2. 沿剖面完成 100 个物理点的宽频带大地（ MT ）探测

用于背景噪音成像的观测系统及射线覆盖

由于在剖面的东部有固定台站，而剖面的西侧台站较少，考虑到噪音成像的需要，在剖面西侧的阿尔金山西侧布设一定数量的台站，形成对研究区及周边地区的有效射线覆盖图