Transcript ANTILOPE

Architecture of Colliding
Tectonic Plates in Tibet
by Passive Source
Seismology
Junmeng 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 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
The crust and mantle structure and the activities control
the formation and evolution of the entire geodynamic
regime.
Kind et al., 2002
Kumar et al., 2006
Question
Why one plateau, more
models?
Some models for Tibetan Plateau uplifting
°
ANTILOPE
(Array Network of Tibetan International Lithospheric Observation and Probe Experiments)
ANTILOPE progress till 2010
ANTILOPE-I
ANTILOPE-II
ITPCAS Array
Sections will be finished in 2009
Bad traffic condition
Donation to a Tibetan school
Instruments were carried to assigned
position by horses
Tutoring at night for a Tibetan student
(middle) in a tent at 5300m high
Seismic Wave
Propagation
Receiver Array
(seismometers)
Source
(Earthquake)
苏门答腊 2005-10-11 Ms6.0
Architecture of Colliding Tectonic Plates in Tibet
478 teleseismic
earthquakes of high
signal∕noise ratio have
been recorded and used
for receiver function
analysis.
7421 P receiver functions
from 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°
A map showing teleseismic events used for calculation of P and S receiver functions
Architecture of Colliding Tectonic Plates in Tibet
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.
Seismic P Wave receiver functions
Location and Data of ANTILOPE-I、II
ANTILOPE-I
Map of seismic
stations (black
triangles) of the two
recent profiles. Black
lines denote major
sutures and faults.
Crosses denote surface
projection of
the 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
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.
ANTILOPE-I
羚羊-I
羚羊-II
Location of ANTILOPE-I
LAB
S wave receiver
functions, sorted
by piercing
points at the 150
km conversion
depth.
(Zhao et al., 2010)
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.
LAB
羚羊-I
?
羚羊-II
ANTILOPE-II
S receiver
functions,
sorted by
piercing points
at the 150 km
conversion
depth.
(Zhao et al., 2010)
P wave receiver functions of ANTILOPE-I
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
Locations of the profiles
Upper mantle transitional zone
Behavior of upper mantle
transitional zone when disturbed
ANTILOPE-I
(Zhao et al., 2010)
P wave receiver Functions
of ANTILOPE-II
ANTILOPE-II
Locations of the profiles
ANTILOPE-II
The blue and red lines indicate the positions of
Indian and Asian plates determined by S wave
receiver functions, respectively.（Zhao et al., 2010）
Behavior of upper mantle
transitional zone when disturbed
P wave receiver functions of INDEPTH
INDEPTH
Location of the profile
Upper mantle transitional zone
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
Comparison of the transitional zones
along four lines in Tibet
ANTILOPE-I
ANTILOPE-II
INDEPTH
LMS Line
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.
Seismic anisotropy along ANTILOPE-II
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
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.
D
(Zhao et al., 2010)
Seismic anisotropy
SKS anisotropy
data at Tibet along
the two new
profiles (red) and
from a collection
of other papers
( blue). In southern 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
colliding
lithospheres, is
characterized by
strong anisotropy.
(Zhao et al.,2010 PNAS)
Architecture of Colliding Tectonic Plates in Tibet
TK Section
A
ANTILOPE-I
A
A
ANTILOPE-II+INDEPTH
A
?
LMS Section
A
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.
TK SECTION
The boundary between the Indian and
Asian 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 observations
of the upper mantle
discontinuities. Red parts of
these boxes mark
unusually 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)
Crustal Deformation Around the Tibetan Plateau
(Tibetan Plateau Fixed Reference Frame)
Wenchuan Eq
(Gan et al. 2007, JGR)
Crustal structure of ANTILOPE-II
by P wave receiver function stack
8km
4
YZS
BNS
S-wave velocity structure of the crust along
ANTILOPE-II by waveform inversion
YZS
BNS
-10
-20
-30
-40
-50
-60
-70
10
20
30
40
50
60
70
Shear wave velocity along ANTILOPE- I
by ambient noise tomography
Shear wave velocity along ANTILOPE-II
by ambient noise tomography
How to bridge the crust and mantle?
Indian crustal intrusion?
Crustal channel flow?
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。
关键科学问题（1）：亚洲岩石圈地幔的行为方式
TK Section
A
ANTILOPE-I
A
A
ANTILOPE-II+INDEPTH
A
?
LMS Section
A
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.
a
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. 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
b
（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？
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