地球物理学报  2014, Vol. 57 Issue (12): 4016-4028   PDF    
青藏高原东西向差异形变与隆升机制
姜效典1,2, 李德勇1,2, 宫伟1, 秘丛永1    
1. 中国海洋大学海洋地球科学学院, 青岛 266100;
2. 海底科学与探测技术教育部重点实验室, 青岛 266100
摘要:高精度布格重力异常约束下的三维空间域挠曲形变模拟显示,大约以90°E为界,青藏高原东、西两部分的岩石圈强度存在明显的差异.在90°E以东,岩石圈有效弹性厚度为35~45 km,该岩层厚度可使刚性的上地壳与上地幔岩石通过中下地壳柔塑性地层的黏滞流动产生构造解耦;地壳处于区域均衡状态,下地壳热物质的流动膨胀是地壳隆升的主控要素.而在90°E以西,断裂带严重削弱了该区域的岩石圈机械强度,岩石圈有效弹性厚度小于15 km,向西逐渐减小,至喀喇昆仑断裂带变为零,断裂切穿莫霍面进入地幔,发生纯剪切构造形变;这里的地壳接近局部均衡,厚皮逆冲是地形隆升的主要因素.震源深度大于80 km的地幔地震大多发生在青藏高原西部,其岩石圈深部具有的脆裂特征很好地支持了岩石圈机械强度模拟的结果.
关键词机械强度     挠曲模拟     青藏高原隆升机制     布格重力异常     地震    
Differential deformation and uplift mechanisms of the eastern and western Tibetan plateau
JIANG Xiao-Dian1,2, LI De-Yong1,2, GONG Wei1, BI Cong-Yong1    
1. Department of Marine Geosciences, Ocean University of China, Qingdao 266100, China;
2. Key Lab of Submarine Geosciences and Prospecting Techniques, Ministry of Education, Qingdao 266100, China
Abstract: Distinct mechanical strength over the eastern and western Tibetan plateau derived from flexural modeling suggests a possible different uplift mechanisms between these two portions of the plateau roughly bounced by 90°E. To the east of this longitude, the mechanical strength of the lithosphere measured by effective elastic thickness (Te) is 35 to 45 km which permits the decoupling between the strong upper crust and the strong upper mantle lithosphere by week, viscous and flowing middle and lower crust in the middle. In the crustal regional equilibrium in the eastern plateau, the flow and expansion of hot material in lower crust are the main controlling factors of crustal uplift. However, to the west of the 90°E, the strength of the lithosphere gradually decreases toward west till Te values are close to zero along the Karakorum fault. The nearly strength-less lithosphere to the west implies that the entire lithosphere is significantly faulted from surface to the mantle. The occurrence of the mantle earthquakes with focal depths larger than 80 km in the mantle lithosphere are almost all populated to the western and eastern syntaxes of the Tibetan plateau, particularly in the western end of the Himalayas. In the crustal partial equilibrium, the thick leather thrusting is the main controlling factor of topographic uplift. This evidence of deep brittle failure in the western lithosphere well supports the results from the mechanical modeling.
Key words: Mechanical strength     Flexural modeling     Uplift mechanism of Tibet     Bouguer gravity anomalies     Earthquake    

1 引言

青藏高原内部构造形变本质上受形成于不同时期的三条边界构造带控制,由北至南分别为:柴达木地块南缘的古生代昆仑断裂缝合带、羌塘地块北缘的三叠纪金沙江断裂缝合带和拉萨地块北缘的班公湖—怒江断裂缝合带(图 1),它们的地表形态、倾角及延伸长度、延展深度等基本要素是认识高原构造形变需要搞清的问题.其中之一是,与缝合带有关的边界断层基底在哪儿?换句话说,它们的断裂深度是多少?由于高昂的费用和二维地震成像精度的限制,它们的构造特征至今尚不清楚.然而,这些边界断裂的延展深度是理解青藏高原隆升过程和机制的重要基础.如果边界断层切穿了整个岩石圈或者说上地幔岩石圈正处于被撕裂的准缝合状态,如Airy地壳均衡理论所说的那样,岩石圈为与印度和欧亚大陆之间的持续收敛运动相协调将会发生纯粹增厚产生垂向补偿,同时导致岩石圈强度逐渐减小. Dewey和Burke(1973)的上地壳增厚模型、Tapponnier等(2001)的岩石圈整体增厚模型以及England和Houseman(1989)的整体岩石圈垂向持续应变模型均强调岩石圈强度在青藏高原变形中的作用.与上述构造模式相对应,青藏高原90°E以西部分(图 1)可能与岩石圈整体增厚到一定程度导致的机械强度减小有关,定量估计其有效弹性厚度比较低,约5~ 20 km(Braitenberg et al., 2003; Jiang et al., 2004; Jordan and Watts, 2005).

图 1 青藏高原及其邻区岩石圈有效弹性厚度(Te)和主要构造边界
图中Te等值线间距为5 km,MFT:前缘逆冲带;MCT:中央俯冲带;YZS:雅鲁藏布江缝合带;KRF:喀喇昆仑断裂;LS:拉萨地块;BNS:班公湖—怒江缝合带;QT:羌塘地块;JRS:金沙江缝合带;SG:松潘—甘孜地块;KF:昆仑山断裂; KS:昆仑缝合带;WKF:西昆仑断裂;ATF:阿尔金断裂;ATT:阿尔金逆冲带.
Fig. 1 Effective elastic thickness(Te)of the Tibetan plateau and its vicinity shown by the colored background and scaled in color bars
The interval of Te contours is 5 km. The abbreviations from south to north are Main Frontal Thrust(MFT),Main Central Thrust(MCT),Yarlung Zangbo Suture(YZS),Karakorum Fault(KRF),Lhasa Block(LS),Bangong—Nujiang Suture(BNS),Qiangtang Block(QT),Jinsha River Suture(JRS),Songpan Garzê Block(SG),Kunlun Fault(KF),Kunlun Suture(KS),West Kunlun Fault(WKF),Altun Fault(ATF), and Altun Thrust(ATT).

与西部不同,青藏高原90°E以东部分(图 1)岩 石圈强度明显增加,有效弹性厚度(Te)达30~50 km(Braitenberg et al., 2003; Jiang et al., 2004; Jordan and Watts, 2005).即使应用最简单的地热模型,青藏高原地壳也达正常厚度的两倍左右,因此仅用有效弹性厚度值(Te)难以有效描述衍生于地壳与上地幔之间流变熔融圈层的坚硬岩石圈特征(Jin et al., 2008).由相干谱证明的两种褶皱模式也均显示完整的岩石圈并非由单层结构组成(Jin et al., 1994).因此,关于青藏高原东部岩石圈的更合理解释是:其机械强度为被黏塑性下地壳所分割的坚硬上地壳和坚硬上地幔两部分岩石强度的叠加之和.青藏高原东部岩石圈的去耦模型与西部的整体增厚模型截然不同.Zhao和Morgan(1987)曾经提出一种液压注入模型来解释青藏高原的变形解耦过程.下地壳的流动性特征也已经被用来解释上地壳与上地幔变形的不一致性(Royden et al., 1997; Clark and Royden, 2000; Royden et al., 2008).

柴达木盆地同样位于90°E以东,其本是塔里木盆地原型的一部分,自渐新世开始逐渐由阿尔金断裂裂离走滑所形成(Meng and Fang, 2008).该盆地并未因逆冲推覆作用发生明显的岩石圈增厚,但已经达到了屈服应力的临界点并且正在发生挠曲变形(Meng and Fang, 2008).受塔里木盆地的阻碍,柴达木盆地的压缩量较小,说明其具有半解耦的岩石圈流变结构(Jin et al., 2008).

本文应用欧亚大陆中部、中国、蒙古及南亚的高分辨率重力数据(图 2)、DEM高程数据以及深源地震震源深度数据(图 3)对青藏高原的岩石圈结构进行研究,提出一种高原岩石圈分区差异形变模式,其东西向分区界限在90°E附近.在模型中,构造边界断裂在青藏高原90°E以西部分往往切穿整个岩石圈,而90°E以东部分在柔软、黏塑性的下地壳既已终止.差异的边界断裂深度说明青藏高原东西两部分分别具有不同的隆升机制.

图 2 青藏高原及邻区观测布格重力异常 Fig. 2 Observed Bouguer gravity anomalies(mGal)in the Tibetan Plateau and its vicinity

图 3 青藏高原及其邻区地形图和深源天然地震分布
地形高程数据来自DEM,等值线是沉积盆地的沉积物厚度,单位km,间距1 km.圆点表示地震震源中心位置,地震记录来自NEIC(National Earthquake Information Center)和USGS(United States Geological Survey)1973—2008年观测到的深度大于80 km的深源地 震,其中黑色圆点代表震源中心深度在80~100 km之间,蓝色代表100~200 km,红色代表200~300 km.
Fig. 3 Topographic image of the Tibetau Plateau and its vicinity from DEM
Contours are the basement depths in km in the sedimentary basins over Tibet and its vicinity. Solid circles show deep earthquakes acquired by NEIC and USGS from 1973 to 2008 with the focal depths larger than 80 km. Black solid circles have a depth range from 80~100 km,blue for100~200 km and red for 200~300 km
2 布格重力数据

本研究中所用的布格重力数据分别来自中国、蒙古、前苏联及印度(图 2).中国境内的原始重力数据由地质矿产部地球物理地球化学勘查研究所1989 年主编的中华人民共和国1 ∶ 2500000布格重力异常图数字化而来,其数据累计误差在中国东部(102°E以东)约为±1 mGal,在西部约±2 mGal.原始重力测点存在几个大的未覆盖空白带,特别是在西南部的青藏高原和北部边缘.为使青藏高原地区达到全覆盖,研究中补充了部分新的重力测点.新增加的 重力测点有四个来源,分别为: 中石油1994—1999年所做的测量(高瑞祺和赵政璋,2001);王谦身和安玉林于1998年在青藏高原东部测量所得的349个数据;中国海洋大学在1997年和1998年横穿青藏高原北 部测量所得的468个数据(Jiang et al., 2004)以及 在2009年横穿龙门山褶皱带测量所得的231个数据.

印度、尼泊尔、缅甸及不丹境内的布格重力数据由Balakrishnan(19972003)编译的印度次大陆及毗连区布格重力异常图数字化得到.前苏联及蒙古境内的的重力数据由俄罗斯武装部队地形服务处于 20世纪90年代测量,该重力数据分辨率达到了10 km,精度高于0.5 mGal(Kogan and McNutt, 1993).

本文使用的区域布格重力异常数据与基于EGM2008模型的卫星重力数据(BGI,http://bgi. omp.obs-mip.fr/data-products/Toolbox/Anomaly-maps-visualization)存在很大的差异.地表区域重力数据较卫星重力数据更为平滑,卫星重力明显蕴含着与地形地貌有关的短波信息,而由地表重力数据计算的长波布格重力异常则可以更好地反映了岩石圈深部结构和莫霍面起伏特征.

3 布格重力约束的三维空间域岩石圈挠曲形变模拟

岩石圈挠曲模型可以表征地球岩石圈机械强度的横向非均匀性变化,进而达到揭示板块构造活动性质的目的.目前,挠曲模拟的方法可以归纳为三类:协函数技术、小波表示及数值求解挠曲方程.Forsyth(1985)提出的协函数技术对岩石圈上部负载和下部负载具有良好的表征效果(McNutt,1983; Simons,2000),其中有效弹性厚度(Te)的计算依赖于谱窗口的最优估计(Bechtel et al., 1990; Jin et al., 1994; Ojeda and Whitman, 2002; Audet and Mareschal, 2004).小波表示方法则假设地球岩石圈挠曲谱可以用长椭球波动方程的本征函数表示(Thomson,1982; Swain and Kirby, 2006; Kirby and Swain, 2008).多窗口技术在芬诺斯堪地盾、北欧、南美以及安第斯山边缘岩石圈强度横向非均匀性研究中都取得了很好的应用效果(Pérez-Gussinyé et al., 200420072008).

本研究应用3D有限差分法在空间域对强度横向非均匀性的地球岩石圈进行挠曲模拟.这种方法可以有效消除前述的频率域算法中每一个单元Te需要保持定值的局限,使结果与构造动力过程更加 贴近.但由于数值求解难度大,研究和应用实例极 少.Van Wees和Cloetingh(1994)Stewart和Watts(1997)以及 Torne等(2000)简单介绍了该方法,金煜和姜效典(2002)给出了其差分方程的完整表达,Jordan和Watts(2005)在青藏做了实验.考虑岩石圈横向非均匀性及上部地形负载的挠曲基本方程表达式为:

其中w(x,y)为3D空间域的岩石圈挠曲形变,N为单位长度上的背景构造应力,g为重力加速度,ht为高程,v为泊松比,ρc为地壳密度,Δρ地幔与地壳的密度差,D为挠曲强度,可由下式计算:

式中E为杨氏模量,h为所模拟岩石圈的有效弹性 厚度(Te).边界条件由2D挠曲模拟结果确定.模拟中所用到的参数取值为:地壳密度ρc=2670 kg·m-3,地幔密度ρm=3260 kg·m-3,杨氏模量E=1011Pa,泊松比ν=0.25,水平负载=1013N·m-1.由地形引起的 上部负载层密度和由盆地沉积及地幔热物质膨胀上涌导致的下部负载层密度分别赋值为2670 kg·m-3 、 2450 kg·m-3和3350 kg·m-3(Jiang et al., 2004; Jiang and Jin, 2005).理论布格重力异常由Parker公式(Parker,1973)计算.

地形和盆地的沉积岩被作为上部负载和内部负载加载到挠曲主控方程中.依据不同深度界面的布格异常谱函数不同,我们求解Parker公式的功率谱分布,以确定岩石圈的内部和下部负载的界面,具体模拟工作流程则如图 4所示.

图 4 三维空间域岩石圈挠曲形变模拟流程 Fig. 4 Flow chart of 3D spatial flexural modeling of lithosphere

图 5显示的是研究区布格重力异常功率谱曲线,各密度(负载)界面对应的深度被模拟.8 km界面可以解释为沉积岩底面,19 km是康氏面的平均深度,47 km是莫霍面的平均深度,220 km为岩石圈底界面.与大地电磁测深和广角地震显示的在青藏高原200 km以下有异常物质面相一致(Kind et al., 2002; Zhang et al., 2010).反演四个密度界面 产生的重力异常(图 6)显示平均深度47 km和220 km 的密度界面起伏的重力异常构成了实测重力数据(图 2)的90%.因此,我们把导致青藏高原和邻区岩石圈形变的内部和下部的异常热物质限定在大于47 km的深部.按照上下负载准相关理论(Jin et al., 1994),我们得到了两个热物质界面的起伏展布(图 7图 8),它们被作为内部负载加载到挠曲主控方程参加模拟,Airy均衡做方程的边界条件.

图 5 青藏高原及邻区布格异常功率谱随深度的变化 Fig. 5 Power spectra of Bouguer anomalies versus depth over the Tibetan Plateau and its vicinity

图 6 不同深度界面的理论布格重力异常
(a)研究区平均深度8 km密度界面产生的重力异常,界面上下密度差180 kg·m-3;(b)平均深度19 km密度界面产生的重力异常,界面上下密度差170 kg·m-3;(c)平均深度47 km密度界面产生的重力异常,界面上下密度差400 kg·m-3;(d)平均深度220 km密度界面产生的重力异常,界面上下密度差250 kg·m-3.
Fig. 6 Theoretical Bouguer anomalies at different depths
(a)Gravity anomalies at 8 km depth with density difference of 180 kg·m-3 between the upper and lower layers;(b)Gravity anomalies at 19 km depth with density difference of 170 kg·m-3;(c)Gravity anomalies at 47 km depth with density difference of 400 kg·m-3;(d)Gravity anomalies at 220 km deep depth with density difference of 250 kg·m-3.

图 7 加载至岩石圈挠曲形变主控方程模拟的内部负载分布,来自下地壳的融熔膨胀的热物质 Fig. 7 Surface loading distribution stemmed from molten and exp and ing hot materials of the lower crust,which are loaded to the lithosphere flexural modeling

图 8 加载至岩石圈挠曲形变主控方程的内部负载分布,来自上地幔热物质上涌 Fig. 8 Surface loading distribution associated with upwelling hot materials of upper mantle,which are loaded to the lithosphere flexural modeling

求解板块弹性形变模型的主控方程,实测布格重力异常用于约束,得到莫霍面深度的理论计算值(图 9).当岩石圈处于弹性形变时,莫霍面深度的变化反映了岩石圈挠曲形变的强度和形态.假定印度大陆和欧亚岩石圈的初始弹性厚度分别是80 km和40 km(图 10),用Parker公式计算理论重力异常,由实测布格重力(图 2)做约束,根据剩余重力异常调整岩石圈弹性强度、进行叠代.剩余布格重力异常最小化会导致上部负载函数的收敛速度减小,用其反演替代下部负载会使模拟迭代过程明显收敛.当布格异常观测值和理论模拟值之间的均方根差最小时即为有效弹性厚度(Te)的最佳拟合值.据此,我们得到了青藏高原与邻区的岩石圈有效弹性厚度分布(图 1).

图 9 实测布格重力约束的青藏高原及邻区理论莫霍面挠曲形变 Fig. 9 The oretical flexure of Moho constrained by observed Bouguer gravity anomalies over the Tibetan Plateau and its vicinity

图 10 初始岩石圈有效弹性厚度模型 Fig. 10 Initial model of lithosphere effective elastic thickness

明显的岩石圈机械强度的差异展现在图中:在经度90°以西的区域,Te大多小于30 km,在西昆仑断裂、喀喇昆仑断裂和雅鲁藏布江缝合带数值减小到大约5 km,部分接近于0,预示断裂切穿莫霍面进入上地幔,地壳达到局部均衡.而在经度90°以东的区域Te多在40~50 km,反映了中等刚度的岩石圈强度,尽管青藏高原的地壳厚度在60~75 km,是平均地壳的两倍,但其有效弹性厚度仅40~50 km,只能解释为塑性柔软的下地壳使刚度大的上地壳和上地幔解耦,减弱了整个岩石圈的机械强度.反射地震和大地电磁测深得到的地层波速(图 11)支持我们的模拟结果:经度小于90°的高原西南部,纵波速度明显小于其他地区,当地层深度大于50 km后这个特征更加显著.

图 11 青藏高原地壳纵波速度(李松林等,2002;李秋生等,2003;Jiang et al., 2006;刘明军等,2008; Zhang et al., 2011a2011b2013; Teng et al., 2013; Wang et al., 2013)
(a)40 km深度地壳纵波速度;(b)50 km深度地壳纵波速度;(c)60 km深度地壳纵波速度;(d)70 km深度地壳纵波速度.
Fig. 11 Compressional velocity of crust over the Tibetan Plateau(Li et al., 2002; Li et al., 2003; Jiang et al., 2006; Liu et al., 2008;Zhang et al., 2011a2011b2013; Teng et al., 2013; Wang et al., 2013)
(a)Compressional velocity at 40 km depth of crust;(b)Compressional velocity at 50 km depth of crust; (c)Compressional velocity at 60 km depth of crust;(d)Compressional velocity at 70 km depth of crust.
4 剩余布格重力异常讨论

观测数据与理论模型数值之间的差异可以帮助我们评估本文模拟结果的有效性.图 12展示了布格异常观测值(图 2)与理论模拟数值之间的剩余重力异常特征,中国境内的剩余异常约为±15 mGal,与实测重力异常值的绝对误差约为3%,该剩余异常精度印证了我们模型的可靠性,完全满足本次构造动力学研究的需求.剩余异常的极大和极小值主要分布在东印度板块和中亚中国大陆西部,这些地区往往缺少重力测点数据的控制,只有对以上地区做一些实地测量以填补其数据空白才可以提高模拟精度.

图 12 观测布格异常和3D空间域岩石圈挠曲模拟的理论布格异常间的剩余重力异常 Fig. 12 Residual gravity anomalies between the observed Bouguer anomalies and the theoretical Bouguer anomalies derived from 3D spatial flexural modeling of the lithosphere
5 来自挠曲模拟的地球模型及深源地震震源解释

应用实测布格重力异常数值约束的岩石圈挠曲形变模型显示了青藏高原及邻区莫霍面的起伏变化(图 9).NEIC(National Earthquake Information Center)和USGS(United States Geological Survey)观测到的位于研究区内的深部地幔地震的震源深度均大于80 km,表明地震主要发生在上地幔岩石圈内.为说明青藏高原东西区域隆升机制的差异,本文建立了一个新的覆盖青藏高原及其邻区的3D地球岩石圈结构模型.

图 13展示的即为青藏高原及其邻区的3D地球模型,模型垂向幅度较水平方向放大了15倍.模型顶层为地形;中层为莫霍面,彩色圆点表示了1973年至2008年间发生的、深度介于81~288 km的1125次地幔地震的震源中心位置;底部的高倾角层面系震源中心的拟合面,在这个由震源中心展布约束的区带以下岩层发生了脆裂形变.

图 13 青藏高原及其邻区的3D地球模型(垂向比例幅度较水平方向放大15倍)
模型顶层为地形高程,中部为莫霍面,由南至北的高角度倾斜面代表印度岩石圈俯冲,俯冲带分布受莫霍面以下深源地震震源 中心深度(彩色圆点)的控制.1125个地震来自NEIC和USGS记录的1973年—2008年间震源深度在81~288 km的深地震.
Fig. 13 3D Earth model of the Tibetan plateau and its vicinity(The vertical exaggeration is 15 times the horizontal scale)
The top horizon is topography and the middle horizon is Moho. The high angle diving horizon from south to north represents the subducting Indian mantle lithosphere, and the diving horizon is constrained by the earthquake focal depths below Moho(balls in color). There are 1125 deep earthquakes from 1973 to 2008 with focal depths from 81 to 288 km. The earthquake data are from NEIC and USGS.

我们的模拟显示,青藏高原内部的昆仑缝合带、金沙江缝合带和班公湖—怒江缝合带处的断裂活动仅仅局限于地壳内部,没有深切至地幔.同时,深部地震的分布(图 3)清楚地显示:绝大多数深源地 震分布在青藏高原的西部,少数深度介于80~100 km 的地震事件发生于雅鲁藏布江缝合带(图 3),在经度大于90°的高原东部没有地幔地震发生.

6 结论

据高精度重力约束的岩石圈挠曲模拟和深源天然地震观测结果,得出如下结论:连接青藏高原西构造结的主要边界断裂切穿莫霍面进入地幔,严重削弱了该区域的岩石圈机械强度,并发生纯剪切构造形变;这里的地壳接近局部均衡,厚皮逆冲是地形隆升的主要因素.而与西部不同,青藏高原中东部缝合带边界断层的构造形变仅仅局限于地壳内部,这里的岩石圈由于塑性柔软下地壳的作用发生了构造解耦;地壳处于区域均衡状态,下地壳热物质的流动膨胀是地壳隆升的主控要素.

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