Qiang Z Y, Wu Q J. 2015. Upper mantle anisotropy beneath the north of northeast China and its dynamic significance. Chinese J. Geophys. (in Chinese), 58(10): 3540-3552, doi: 10.6038/cjg20151010
Upper mantle anisotropy beneath the north of northeast China and its dynamic significance
QIANG Zheng-Yang, WU Qing-Ju
Institute of Geophysics, Chinese Earthquake Administration, Beijing 100081, China
Abstract: Northeast China located in the eastern part of the Central Asian Orogenic Belt (CAOB), one of the largest Paleozoic orogens on Earth. Surrounded by the Siberian craton, North China plate and the Pacific plate, northeast China is characterized by widespread Cenozoic intraplate volcanism and lithospheric deformation caused by multiepisode extension since Late Mesozoic.As the only area developing deep earthquake in China, northeast China has suffered the subduction of the Pacific plate, which makes it become the natural laboratory of studying the lithospheric deformation, subduction and intraplate volcanos and also their interaction relationship. Seismic anisotropy determined by the splitting of shear waves, especially the core-refracted phases like SKS, is one of the most direct and effective ways to image the structure and deformation in the interior of the Earth. We measure the shear wave splitting parameters,the fast polarization direction (φ) and the splitting time between the fast and slow waves (δt), using the SplitLab analysis software package which combine both the rotation-correlation (RC) method and the transverse minimum energy (SC) method. A quality factor (good, fair, or poor) is assigned to all the shear wave splitting measurements,according to the signal to noise ratio of the initial phase, the rectilinear polarization of the particle motion after correction, and well-defined extreme value of contour plot.Only the measurements marked as 'good’ or 'fair’ are retained in the subsequent analysis and discussion, and the delay times less than 0.4 s will be marked as null measurements based on measurement error consideration. A total of 377 pair of SKS splitting measurements obtained at 147 temporary and permanent seismic stations in northeast China are used to infer the upper mantle structure of this area. The shear wave splitting results at most of stations in western part of our study area are characterized by homogeneously oriented fast directions trending N143°E to N199°E with an average close to N169°E, in agreement with the extensional orientation of the Late Mesozoic lithosphere. Additionally, the delay times vary from 0.4 s to 1.6 s with a mean value of 0.85±0.23 s and correspond to a 98±26 km thick layer if 4% anisotropy is assumed, suggesting the anisotropy mainly reside in the lithosphere. Several anisotropy with small delay times (~0.4 s) are observed in the Songliao basin and Jiamusi massif which are probably caused by the lithosphere delamination or the hot mantle upwelling that partially eroded the ancient deformation frozen within lithosphere.While in the eastern part, NNW-SSE trending fast directions with large delay times (larger than 1.0 s) are observed. As the missing of the deep seismicity of this area, we speculate that there exist a slab tear at the end of Pacific plate, whose roll-back may induce NW trending mantle flow. In addition, nearly W-E fast splitting directions are observed in Jiamusi massif only, which can be best explain by the LPO of metastable olivine within the Pacific slab.
中国东北地区位于中亚造山带的东缘,被西伯利亚板块、华北板块、西太平洋板块所夹持(Şengör et al.,1993).区域上由一系列古代增生楔和微陆块拼接而成(张兴洲等,2006),主要包括兴安陆块、松辽陆块和佳木斯陆块,并被嫩江—贺根山缝合带和牡丹江缝合带所分割(Wu et al.,2002)(图 1).普遍认为该区域经历过两个时期的构造演化:在古生代,构造演化主要受到了位于西伯利亚克拉通和华北克拉通之间的古亚洲洋的影响;然而自侏罗纪以来,太平洋板块的俯冲则成为控制该区域构造演化的主要因素.研究区广泛发育NNE-SSW向盆地构造单元,并在大兴安岭、小兴安岭和长白山形成三条火山带.目前对盆地和板内火山的形成机制仍不明确.一些学者认为板块内部断裂是软流圈上涌和太平洋板块、欧亚板块和印度板块综合作用的结果(Ren et al.,2002).通过高分辨率地震波成像研究,Zhao等(2009)提出大地幔楔模型,认为太平洋滞留板块对于板内火山的形成具有重要作用.而最新接收函数研究结果表明,长白山火山的成因主要受到地幔柱控制,而与滞留在地幔过渡带中的太平洋板块可能没有直接关系(Liu et al.,2015).
图 1
Fig. 1
图 1 研究区域周边情况及台站分布图流动和固定地震台的位置分别用黑色和蓝色的圆点表示,而新生代火山的位置用红色三角表示.灰色粗线是南北重力梯度带,白色实线是太平洋俯冲板块等深线,红色的虚线是缝合线的位置.断层F1和F2是郯庐断裂的两个分支.Fig. 1 Map showing geology of the study area and surrounding regions and distribution of seismic stationsThe black and blue dots indicate stations associated with our temporary seismic stations and Chinses national and regional seismic networks, respectively. The red triangles denote the Holocene volcanoes.The grey wide line represents the North South Gravity lineament, and the red dash lines represent the suture zone. Faults F1 and F2 are the two branches of the Tanlu fault. The white lines denote the isobaths curve of the subducting Pacific plate.
受到地震资料的限制,前人在中国东北地区开展的各向异性研究工作较少,对于上地幔各向异性成因的认识还不够全面.有学者认为,中国东北地区东部W-E朝向的各向异性来源于太平洋俯冲板块中亚稳定橄榄石晶体的定向排列,西部地区近N-S朝向的各向异性还可能受到地幔流动的影响(Liu et al.,2008).Li和Niu(2010)的研究结果表明,中国东北地区NW-SE向的各向异性是由残留在岩石圈中的古老拉张变形所引起.
在本次研究中,远震SKS震相被用来测量台站下方的各向异性.虽然所用事件绝大部分来自南太平洋地区,反方位角集中分布在120°~150°之间,但是加之其他区域也有部分事件分布,所以事件方位分布总体上满足研究的要求.我们挑选震中距大于80°,震级大于5.0的地震事件,以期获得高信噪比的SKS震相.我们所使用的地震事件目录来源于美国地质调查局(USGS),IASP91地球参考模型(Kennett and Engdahl,1991)则用于计算波形的到时.最终我们选取了132个地震事件(图 2),用于地震各向异性的测量.
图 2
Fig. 2
图 2 事件震中分布图Fig. 2 The distribution map of the epicenters
为了更加有效地处理大批量剪切波数据,我们使用SplitLab软件(Wüstefeld et al.,2008)来获取剪切波分裂参数.它的优点是结合了旋转相关(RC)的方法(Bowman and Ando,2007)和切向最小能量(SC)的方法(Silver and Chan,1991),从而不仅可以验证分裂结果的有效性,而且还可以判断出无效分裂(null)的结果(Wüstefeld et al.,2008).无效分裂结果可能来源于以下三个方面的原因(Savage,1999):(1)不存在明显的水平各向异性;(2)存在水平单层各向异性结构,但是快波方向平行或垂直于事件反方位角方向;(3)存在多层或是直立倾斜各向异性层位等复杂各向异性结构.对于每一个事件,我们在测量各向异性参数之前进行滤波处理(0.02~1 Hz),同时对于数据质量比较好的事件直接使用原始数据,保证分裂结果的可靠性.图 3中所展示的是我们使用上述方法所得到的一个事件的处理结果,可以看出不同方法所得到的剪切波分裂参数还是非常一致的.
图 3
Fig. 3
图 3 使用SplitLab软件获取SKS震相分裂参数的示例左上方显示的是各向异性校正之前的初始地震波形:实线代表切向分量,虚线代表径向分量;垂直的虚线是理论震相到时,阴影区域为计算时窗.右上角显示的是结果的水平投影.上方中间显示事件信息以及三种方法所得的分裂参数.中间一行是旋转相关法(RC)所得结果:(a)快波和慢波波形对比图;(b)经过校正的径向分量和切向分量;(c)校正前后的质子运动轨迹;(d)相关系数随延迟时间和快波方向变化的极值图.最后一行展示的是最小能量法(SC)的结果:(e)快波和慢波波形对比图;(f)经过校正的径向分量和切向分量;(g)校正前后的质子运动轨迹;(h)切向分量最小能量图.Fig. 3 Example of a SKS splitting measurement using the SplitLab packageThe upper left panel shows the initial seismograms before anisotropy correction: solid line is transverse component, dashed line is radial component, vertical dashed lines are theoretical phase arrival times and the shaded area represents the selected calculation window. The upper right panel represents a stereoplot of the result. Header shows the specification of teleseismic event as well as splitting parameters resulting from the three techniques. Center panels display the results from the Rotation-Correlation (RC) technique: (a) normalized components after rotation in RC anisotropy system; (b) corrected radial (dashed) and transverse (solid) components; (c) particle motion before (dashed) and after (solid) RC correction and (d) contour plot for the maximum value of correlation coefficient as function of delay time and fast polarization angle. Lower panel displays the results from the minimum energy (SC) technique: (e) normalized components after rotation in SC anisotropy system; (f) corrected radial (dashed) and transverse (solid) components; (g) SC particle motion before and after correction and (h) map of minimum energy on transverse component.
表 1 中国东北地区北部台站SKS震相分裂测量结果Table 1 The SKS splitting parameters for stations beneath the north of northeast China
台站
纬度/°N
经度/°E
事件数目
旋转相关法
最小能量法
φ/(°)
Δφ/(°)
δt/s
Δδt/s
φ/(°)
Δφ/(°)
δt/s
Δδt/s
松辽盆地中西部及兴安岭地区(Part A)
ARS
47.17
119.95
7
-9.4
8.2
0.7
0.1
-12.4
6.7
0.7
0.2
EH28
48.23
127.24
1
23.7
0.0
0.8
0.0
15.0
0.0
0.7
0.0
EH29
48.33
126.99
4
5.7
9.3
0.6
0.1
8.0
7.6
0.6
0.1
EH30
48.42
126.75
3
-6.3
10.1
0.6
0.1
-5.3
4.2
0.6
0.2
EH31
48.53
126.55
4
4.5
6.7
0.6
0.2
-6.0
9.1
0.6
0.1
EH33
48.72
126.03
3
0.9
1.7
0.6
0.1
-6.7
2.3
0.6
0.1
EH34
48.83
125.90
2
-14.5
4.8
0.6
0.0
-26.5
3.5
0.8
0.1
EH35
48.89
125.62
5
-10.4
8.6
0.5
0.1
-16.4
11.6
0.6
0.1
EH36
49.00
125.41
3
15.1
7.9
0.6
0.1
13.7
4.2
0.6
0.0
EH41
49.51
124.22
3
-24.1
2.6
1.0
0.1
-24.7
6.7
1.0
0.2
EH42
49.55
124.02
1
-28.6
0.0
1.0
0.0
-34.0
0.0
1.1
0.0
EH44
49.93
123.56
1
-14.9
0.0
0.7
0.0
-24.0
0.0
0.8
0.0
EH45
50.07
123.27
5
-26.2
8.2
1.2
0.3
-27.0
4.5
1.2
0.2
EH46
50.20
123.01
5
-9.9
5.1
0.8
0.2
-1.0
4.9
0.9
0.2
EH56
50.98
120.76
1
-20.1
0.0
0.6
0.0
-22.0
0.0
0.6
0.0
EH57
51.09
120.60
2
-20.7
10.2
0.8
0.1
-24.5
3.5
0.9
0.1
EH59
51.18
120.04
5
-7.5
5.5
0.6
0.2
-0.2
11.8
0.7
0.1
GNH
50.78
121.46
2
-53.5
21.3
0.7
0.0
-53.0
22.6
0.7
0.0
IDR
46.73
122.89
5
-13.0
5.3
0.9
0.3
-22.6
6.7
1.0
0.3
MDG
51.27
120.78
9
-12.9
7.2
0.7
0.2
-7.0
6.9
0.8
0.2
NEH
48.48
124.56
1
1.5
0.0
0.6
0.0
1.0
0.0
0.6
0.0
NZN
47.54
122.88
5
-13.7
8.0
0.7
0.2
-25.6
4.2
0.8
0.2
WLT
46.04
122.03
1
-10.7
0.0
0.5
0.0
-6.0
0.0
0.6
0.0
ZLT
48.03
122.65
14
-7.9
8.7
0.7
0.2
-7.6
11.6
0.8
0.1
SM01
49.56
117.44
1
-29.0
0.0
0.6
0.0
-21.0
0.0
0.6
0.0
SM02
49.52
117.70
8
-16.9
4.7
0.7
0.2
-24.4
4.9
0.8
0.2
SM03
49.37
117.86
8
-14.2
9.1
0.7
0.2
-17.8
9.1
0.7
0.1
SM04
49.35
118.20
5
-15.8
7.2
0.9
0.2
-15.0
4.5
0.8
0.2
SM05
49.20
118.38
5
-10.1
5.1
0.7
0.2
-16.6
7.7
0.8
0.1
SM06
49.20
118.61
5
-10.6
7.4
0.8
0.2
-11.0
10.6
0.9
0.2
SM07
49.09
118.78
4
1.1
6.7
0.8
0.0
4.0
4.3
0.8
0.0
SM08
49.02
119.00
3
-4.3
2.2
0.9
0.2
-2.3
2.1
0.9
0.2
SM09
48.91
119.29
2
-10.2
3.4
0.6
0.1
-18.5
2.1
0.7
0.1
SM10
48.82
119.54
8
-8.2
9.3
0.8
0.2
-9.6
5.0
0.8
0.2
SM11
48.70
119.70
2
-9.7
3.4
0.7
0.0
-10.0
1.4
0.7
0.0
SM12
48.62
119.98
3
-14.1
9.4
0.8
0.1
-13.3
2.5
0.8
0.1
SM13
48.48
119.98
5
-10.3
4.6
0.7
0.1
-10.8
3.3
0.7
0.1
SM14
48.45
120.41
1
-10.2
0.0
0.8
0.0
-9.0
0.0
0.8
0.0
SM15
48.37
120.67
3
-13.9
5.0
0.9
0.2
-14.0
5.6
0.9
0.3
SM16
48.30
120.83
1
-4.4
0.0
1.1
0.0
-4.0
0.0
1.1
0.0
SM17
48.16
121.23
4
0.5
9.8
0.9
0.1
-2.3
9.3
0.9
0.1
SM19
48.01
121.56
4
-8.7
2.2
0.7
0.2
-11.5
3.1
0.7
0.2
SM20
47.99
121.86
13
-10.5
4.5
1.0
0.2
-9.5
3.7
1.0
0.2
SM21
47.86
122.05
9
-11.2
4.6
1.1
0.1
-11.3
2.2
1.0
0.1
SM22
47.77
122.25
25
-6.4
5.0
1.0
0.1
-7.2
4.2
1.0
0.1
SM23
47.69
122.46
12
1.0
6.6
0.9
0.1
0.8
5.4
0.9
0.1
SM24
47.61
122.70
1
-8.1
-8.1
0.6
0.6
-15.0
-15.0
0.6
0.6
SM26
47.45
123.16
5
-32.9
3.2
1.2
0.2
-33.6
2.5
1.2
0.2
SM27
47.34
123.35
4
-31.7
3.3
1.3
0.2
-34.0
0.8
1.3
0.1
SM28
47.34
123.35
1
-22.7
-22.7
1.5
1.5
-26.0
-26.0
1.5
1.5
SM29
47.20
123.83
5
-16.6
5.9
0.9
0.2
-18.6
12.5
1.0
0.2
SM30
47.17
124.12
1
-13.0
0.0
0.9
0.0
-8.0
0.0
0.9
0.0
SM31
47.00
124.30
8
12.2
4.9
0.9
0.1
8.1
3.0
0.8
0.1
SM32
46.92
124.52
7
13.0
6.8
0.6
0.1
8.3
7.4
0.5
0.2
SM33
46.80
124.75
3
-2.4
2.7
0.6
0.2
-7.3
3.5
0.6
0.2
SM35
46.66
125.20
2
-22.4
6.0
0.9
0.1
-22.5
10.6
0.9
0.1
SM36
46.58
125.43
2
-28.0
1.8
1.2
0.0
-29.5
4.9
1.3
0.2
SM37
46.48
125.72
1
-28.2
0.0
1.6
0.0
-26.0
0.0
1.6
0.0
松辽盆地东部及佳木斯地块(Part B)
BEL
46.97
127.16
1
-86.4
0.0
0.8
0.0
85.0
0.0
0.9
0.0
BJS
48.69
126.08
1
-0.1
0.0
0.7
0.0
-6.0
0.0
0.7
0.0
JIY
49.04
129.87
8
-1.0
7.6
0..8
0.2
-10.8
4.9
0.9
0.2
BNX
45.74
127.40
1
79.8
0.0
1.4
0.0
87.0
0.0
1.3
0.0
JMS
46.73
130.32
2
88.2
19.0
0.8
0.3
98.5
19.1
0.9
0.1
LBE
47.30
130.83
3
-20.3
1.6
1.6
0.2
-22.7
2.3
1.6
0.1
MDJ
44.62
129.59
40
-86.8
9.0
0.7
0.2
-80.9
29.0
0.7
0.2
QAN
46.58
127.62
1
69.9
0.0
1.5
0.0
75.0
0.0
1.4
0.0
SYS
46.66
131.09
1
-15.0
0.0
0.8
0.0
-18.0
0.0
0.9
0.0
TOH
46.08
128.65
1
84.6
0.0
1.2
0.0
84.0
0.0
1.2
0.0
XUK
49.08
128.92
7
-7.8
6.3
1.2
0.2
-6.1
3.5
1.3
0.1
YCH
47.63
128.57
1
77.5
0.0
1.0
0.0
73.0
0.0
1.1
0.0
YIL
46.48
129.29
3
5.1
2.2
0.4
0.1
2.3
10.3
0.4
0.1
EH08
46.32
131.88
2
-16.7
16.9
1.0
0.5
-16.5
12.0
1.0
0.4
EH09
46.43
131.59
2
-9.8
7.6
0.8
0.1
-17.0
0.0
1.0
0.1
EH13
46.81
130.69
1
-10.8
0.0
0.7
0.0
-20.0
0.0
0.8
0.0
EH14
46.89
130.44
1
-11.0
0.0
0.8
0.0
-16.0
0.0
0.8
0.0
EH15
46.98
130.21
3
-16.8
13.9
1.2
0.3
-8.0
10.0
1.0
0.2
EH17
47.15
129.77
1
8.6
8.6
0.7
0.7
1.0
1.0
0.6
0.6
EH19
47.36
129.31
1
18.7
18.7
0.7
0.7
16.0
16.0
0.6
0.6
EH24
47.85
128.21
1
1.4
1.4
0.5
0.5
-5.0
-5.0
0.5
0.5
SM45
45.80
127.51
1
-76.2
0.0
1.3
0.0
-85.0
0.0
1.3
0.0
SM49
45.44
128.42
1
87.5
0.0
0.9
0.0
79.0
0.0
1.0
0.0
SM54
44.97
129.56
4
72.1
3.5
1.1
0.1
67.5
3.7
1.2
0.1
SM55
44.92
129.76
2
81.8
1.9
1.0
0.2
77.5
6.4
1.1
0.1
SM57
44.72
130.26
1
-3.2
0.0
0.7
0.0
9.0
0.0
0.8
0.0
SM58
44.65
130.48
4
6.4
5.5
1.1
0.1
11.5
3.0
1.2
0.1
SM60
44.50
130.97
3
1.3
10.8
0.8
0.1
-5.0
5.0
0.8
0.1
两种各向异性结果(Part B)
HEG
47.35
130.24
7
-14.4
24.0
0.7
0.1
-24.6
19.2
0.8
0.1
MIH
45.63
131.81
3
-40.6
39.7
1.0
0.1
-42.3
36.1
1.0
0.1
SM47
45.61
127.95
3
-34.7
46.9
0.8
0.5
24.7
56.2
0.8
0.5
BAQ
46.33
132.15
2
-41.6
37.3
1.2
0.6
-46.5
41.7
1.2
0.6
表 1 中国东北地区北部台站SKS震相分裂测量结果Table 1 The SKS splitting parameters for stations beneath the north of northeast China
图 4
Fig. 4
图 4 (a) 各向异性方向和延迟时间展示图,短线的方向与长短分别代表各向异性方向和延迟时间,黑色圆点表 示台站的位置;(b)无效分裂结果展示图,平行或垂直于事件的反方位角的方向用十字线来表示,而红色的十字 线代表只有无效分裂结果的台站Fig. 4 (a) Individual splitting measurement plotted at each temporary station is characterized by their azimuths and the delay time (orientation and length of the segment, respectively). Location of each station is represented by black dot. (b) Null measurements plotted at the station location as crosses whose bars parallel and perpendicular to the back-azimuth of the analyzed events. The red crosses represent the results of solely null measurements. Black dots are the location of each station
图 6 SM测线部分台站下方各向异性参数随经度的变化趋势(a) 各向异性延迟时间; (b) 快波极化方向.Fig. 6 Variations of the mean anisotropy parameters obtained along the latitude beneath several stations from SM seismic line(a) The delay times; (b) Fast polarization directions.
此外,刚性的岩石圈通常被认为漂浮在软流圈之上,由于岩石圈的运动拖曳软流圈地幔而产生的各向异性通常会平行于板块运动的方向(Vinnik et al.,1992).基于HS3-Nuvel1A板块运动模型所得出的研究区欧亚板块运动速度(相对于热点参考系)为2 cm·a-1,方向N62°W(Gripp and Gordon,2002)(图 8).显然,绝对板块运动方向(APM)与我们测量所得到的绝大多数各向异性方向有较大差别,所以这种机制也可能不是产生各向异性的主要因素.
图 8
Fig. 8
图 8 研究区台站下方各向异性参数综合分析图小圆圈代表台站的位置.黑色、绿色和红色的短棒代表各向异性参数,其中红色的短棒是NNW-SSE朝向的并且延迟时间大于1.0 s 的各向异性,而绿色的短棒则代表近W-E向的各向异性.紫色的箭头为绝对板块运动速度.橙色的实线和虚线分别代表断层与缝合带的位置.灰色宽线代表南北重力梯度带.实心圆点代表了深部地震活动性(紫色: 300~400 km; 粉红色: 400~500 km; 红色:>500 km).蓝色阴影区域是太平洋俯冲板块可能存在撕裂的地方. 右上角的图是我们提出的太平洋板块撕裂模型:蓝色和红色分别表示太平洋俯冲板块和地幔流.绿色的箭头表明了俯冲板块的移动方向,而红色的箭头表明了地幔可能流动的方向.Fig. 8 Comprehensive analysis diagram of the measured fast directions and splitting timesLocations of stations are represented by little circles.Black, green and red bars represent the anisotropy parameters, of which the red bars represent the NNW-SSE trending anisotropy with delay time lager than 1.0s and the green bars represent the nearly W-E trending anisotropy. The purple arrow represents the absolute plate motion vector of the Eurasian plate (Gripp and Gordon, 2002).The orange solid and dash line indicate branches of the Tanlu fault and the suture zone, respectively.The grey wide line represents the North South Gravity lineament. Deep seismicity is shown as solid dots (purple: 300~400 km; pink: 400~500 km; red:>500 km).The shaded area indicates a speculative slab tear. The upper right shows our proposed slab-tear model for the origin of anisotropy under this area:the subducting Pacific slab and the mantle flow are shown in blue and red, respectively. The green arrow indicates the subduction of the Pacific slab and the red arrow indicates the possible direction of the mantle flow.
4.1 各向异性与岩石圈中的古老形变
通常认为各向异性会记录岩石圈最后一次构造变形,无论这次变形发生在远古或是现今(Silver,1996).从晚中生代以来,中国东北地区岩石圈经历了多期拉张作用(Ren et al.,2002).在侏罗纪时期,太平洋板块向西的持续俯冲,导致中国东北地区岩石圈挤压增厚.而在早白垩纪时期,由于俯冲板块的转向,岩石圈从挤压转为拉张并伴随着岩石圈地 幔的拆沉(Wang et al.,2006; Zhang et al.,2010). 中国东北地区这一时期的拉伸环境得到了岩石年代学研究的支持(Wu et al.,2005).此外,变质核杂岩的研究也表明,早白垩纪时期的岩石圈有NW-SE向的拉伸构造(Allen et al.,1997).前人通过实验和地质调查发现,在伸展构造区域,矿物的定向排列往往平行于构造伸展方向,从而使得各向异性的方向与伸展方向一致(Savage,1999).另一方面,各向异性层位的厚度是影响各向异性延迟时间的主要因 素.中国东北地区岩石圈的拆沉以及活跃的热地幔物质上涌都会破坏保留在岩石圈中的古老形变,可能是造成松辽盆地部分台站和佳木斯地块NW-SE或NNW-SSE朝向各向异性延迟时间明显小于研究区平均水平的原因.体波成像结果显示,松辽盆地下方呈现以高速为主、高低速异常的混合,可能存在岩石圈拆沉过程;而佳木斯地块岩石圈下方表现为局部低速区,说明有热地幔物质上涌(张风雪等,2013,2014).接收函数的研究结果表明,松辽盆地下方的岩石圈比兴安地块减薄近50 km(Zhang et al.,2014).如果假设4%的各向异性程度,那么这将对应着各向异性延迟时间减少约0.4 s,与我们观测的结果所吻合.因此我们认为,中国东北上地幔各向异性反映了岩石圈拉张形变,这种古老形变自晚中生代以来就一直被保存在岩石圈内,并在后期经历了不同程度的改造.
另一方面,我们认为只出现在佳木斯地块中的近W-E向的各向异性,可能是太平洋板块在地幔过渡带中俯冲所留下的痕迹.Liu等(2008)在分析SKS分裂的基础上通过对比近震S波分裂时间与事件震源深度的关系,认为MDJ台下方的各向异性主要来源于地幔过渡带.而数值模拟和岩石物理实验表明,地幔过渡带中两种主要矿物(瓦兹利石和尖晶橄榄石)都不能在SKS路径上产生明显的各向异 性(Tommasi et al.,2004; Li et al.,2006).因此,我们所观察到的近W-E向的各向异性就很可能来自于太平洋俯冲板块中的橄榄石.橄榄石—瓦兹利石相变的数值模拟研究发现,在俯冲板块中其相变发生的深度可以超过地幔过渡带深度上百公里(Devaux et al.,1997; Schubert et al.,2001).接收函数研究结果表明(Liu et al.,2015),在纬度40°N—46°N之间,660 km 间断面产生下沉,暗示着太平洋俯冲板块在地幔过渡带的滞留,这与我们所观测到的W-E方向的各向异性区域相符合.因此,我们认为太平洋板块的西向俯冲,造成了板块中亚稳定状态的橄榄石矿物晶体定向排列,从而产生W-E向的各向异性.
致谢 感谢所有参与到中国东北流动地震观测工作中的同志,感谢中国地震局地球物理研究所“国家数字测震台网数据备份中心”提供的固定台地震波形数据.与张瑞青副研究员和李永华研究员的讨论对本文十分有益.匿名审稿专家的宝贵修改意见使得本文更加严谨、流畅.文章大部分图件使用GMT软件绘制(Wessel and Smith,1998).
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