地质力学学报  2021, Vol. 27 Issue (2): 178-194
引用本文
卞爽, 于志泉, 龚俊峰, 杨蓉, 程晓敢, 林秀斌, 陈汉林. 青藏高原近南北向裂谷的时空分布特征及动力学机制[J]. 地质力学学报, 2021, 27(2): 178-194.
BIAN Shuang, YU Zhiquan, GONG Junfeng, YANG Rong, CHENG Xiaogan, LIN Xiubin, CHEN Hanlin. Spatiotemporal distribution and geodynamic mechanism of the nearly NS-trending rifts in the Tibetan Plateau[J]. Journal of Geomechanics, 2021, 27(2): 178-194.
青藏高原近南北向裂谷的时空分布特征及动力学机制
卞爽1,2, 于志泉1,2, 龚俊峰1,2, 杨蓉1,2, 程晓敢1,2, 林秀斌1,2, 陈汉林1,2    
1. 浙江大学地球科学学院, 浙江省地学大数据与地球深部资源重点实验室, 浙江 杭州 310027;
2. 教育部含油气盆地构造研究中心, 浙江 杭州 310027
摘要:青藏高原南部发育的一系列近南北向裂谷是印度-欧亚大陆持续挤压作用下的大型伸展构造,也是揭示高原后碰撞构造演化过程的重要对象。目前,关于南北向裂谷的形成机制存在多种假说模型,并对裂谷时空分布特征做出了不同的预测,这成为约束裂谷成因机制的关键条件。综合关于裂谷启动时间的已有研究成果,进一步梳理了南北向裂谷的时空分布特征,结果表明近南北向裂谷的启动时间似乎具有自西向东逐步减小的趋势,这与拉萨地体广泛出露的后碰撞岩浆作用演化过程一致。在此基础上,结合地球物理观测,推断近南北向裂谷的动力学机制与印度板片向东拆离假说最为契合。印度板片自西向东的拆离建立了向东传播的重力势能梯度,从而驱动岩石圈向东流动,最终导致南北向裂谷依次向东发育。
关键词青藏高原    近南北向裂谷    东西向伸展    时空分布特征    动力学机制    
DOI10.12090/j.issn.1006-6616.2021.27.02.018     文章编号:1006-6616(2021)02-0178-17
Spatiotemporal distribution and geodynamic mechanism of the nearly NS-trending rifts in the Tibetan Plateau
BIAN Shuang1,2, YU Zhiquan1,2, GONG Junfeng1,2, YANG Rong1,2, CHENG Xiaogan1,2, LIN Xiubin1,2, CHEN Hanlin1,2    
1. Key Laboratory of Geoscience Big Data and Deep Resource of Zhejiang Province, School of Earth Sciences, Zhejiang University, Hangzhou 310027, Zhejiang, China;
2. Research Center for Structures in Oil and Gas-bearing Basins, Ministry of Education, Hangzhou 310027, Zhejiang, China
Abstract: A set of nearly NS-trending rifts developed in the Himalayan orogen and southern Tibet, which are the large-scale extensional structures formed under the continuous compression of the Indo-Eurasia continent, playing a significant role in revealing the post-collisional evolution of the Tibetan Plateau. Different predictions have been made on the spatiotemporal distribution of these rifts through the existing hypothesis models, constituting the key factors constraining the formation system of the rifts. In this study we synthesized previous studies on the initiation time of rifting, and further clarified the spatiotemporal trend. The analysis results showed that the rifting initiated progressively earlier westward, which is consistent with the evolution of post-collisional magmatism in the Lhasa Terrain. Moreover, combined with the geophysical observations, it is inferred that the geodynamic mechanism of the nearly NS-trending rifts accords closely with the hypothesis model concerning the eastward-propagating lateral detachment of the subducted Indian slab. The Indian slab detachment resulted in asynchronous gravitational potential energy gradients, which drove the lithosphere flow eastward and eventually caused the eastward development of the rifting.
Key words: Tibetan Plateau    nearly NS-trending rifts    E-W extension    spatiotemporal distribution characteristics    geodynamic mechanism    
0 引言

Tapponnier and Molnar(1977)基于震源机制解和遥感影像解译全面识别出青藏高原内部广泛发育近南北向裂谷以来,其发育过程与形成机制就受到诸多关注。经过几十年的研究,Yin(2000)详细刻画了南北向裂谷的间距在空间上的变化,Kapp and Guynn(2004)描述了裂谷走向沿东西方向的转化,Taylor and Yin(2009)系统总结了青藏高原活动构造的展布,特别是代表东西向伸展的南北向裂谷及共轭走滑断裂带,Styron et al.(2011)对130个GPS速度场进行分析,揭示了平行于喜马拉雅弧的伸展速率变化趋势,然而,近南北向裂谷的成因机制一直众说纷纭,存在较大的争议。

现有的多数模型都对裂谷的时空分布特征进行了不同的预测,并可大致划分为三类。第一类模型强调高原内部的东西向伸展是整体上近乎一致变形的结果,要求所有裂谷起始活动时间大致相同,如重力垮塌模型(Molnar and Tapponnier, 1978)、岩石圈地幔对流移除模型(England and Houseman, 1989)、亚洲东缘边界条件改变模型(Yin, 2000, 2010)以及放射状扩展模型(Seeber and Armbruster, 1984)。第二类模型强调喜马拉雅山弧的弯曲过程与东西向伸展构造的内在关联,要求裂谷从东西两侧向中间发育,如马蹄形弯曲模型(Klootwijk et al., 1985)、印度大陆的倾斜汇聚模型(McCaffrey and Nabelek, 1998)以及印度板片双向的横向拆离模型(Webb et al., 2017)。第三类模型则强调驱动力向东的扩展,这类模型预测裂谷自西向东依次发育,如横向挤出模型(Armijo et al., 1986)、岩石圈向东流动模型(Yin and Taylor, 2011Bischoff and Flesch, 2018)以及印度板片向东拆离驱动的岩石圈流动模型(Bian et al., 2020)。上述大部分模型都能解释部分地质观测,但在不同程度上存在与实际观测不相符的问题,到目前为止仍缺乏关于南北向裂谷成因的统一的动力学机制。但无论如何,所有裂谷的时空分布特征应该是评估其成因的关键。文章通过总结南北向裂谷的启动时间,梳理了裂谷的时空分布特征,并以此为基础对其成因模型进行了系统的分析总结,进而探讨了喜马拉雅-青藏高原东西向伸展的动力学机制。

1 地质背景 1.1 近南北向裂谷的展布

新生代以来,印度-欧亚板块碰撞形成了世界瞩目的青藏高原(Dewey and Bird, 1970Yin and Harrison, 2000Blisniuk et al., 2001),其内部由不同地体组成,从南向北依次为喜马拉雅地体、拉萨地体、羌塘地体、松潘-甘孜地体和柴达木-祁连山地体,各地体之间分别以印度河-雅鲁藏布江缝合带、班公湖-怒江缝合带、金沙江缝合带和阿尼玛卿-昆仑-木孜塔格缝合带为界(图 1aYin,2000Taylor and Yin, 2009)。其中,喜马拉雅地体、拉萨地体、羌塘地体发育了广泛的东西向伸展构造,表现为一系列共轭走滑断层和近南北向裂谷(图 1bArmijo et al., 1986Taylor et al., 2003张进江和丁林,2003Taylor and Yin, 2009)。共轭走滑断层主要沿班公湖-怒江缝合带分布,缝合带以北以发育北东走向的左旋走滑断层为主,而以南以北西走向的右旋走滑断层为主。近南北向裂谷则几乎横穿了所有东西走向的构造单元,而且其南端切割了藏南拆离系到达高喜马拉雅,向北则穿过拉萨和羌塘地体,可断续延伸至金沙江缝合带附近。由于青藏高原地区野外条件困难,目前关于南北向裂谷的研究主要集中在双湖裂谷、Leo Pargil裂谷、Gurla Mandhata裂谷、隆格尔裂谷、Thakkhola裂谷、当惹雍错-孔错裂谷、申扎-定结裂谷、亚东-谷露裂谷和沃卡-错那裂谷(Harrison et al., 1995Yin et al., 1999张进江等,1999吴珍汉等,2002张进江和丁林,2003Thiede et al., 2006Zhang and Guo, 2007吴中海等, 2007, 2008曹圣华等,2009Murphy et al., 2010Lee et al., 2011Ratschbacher et al., 2011Sundell et al., 2013Styron et al., 2013McCallister et al., 2014才巴央增和赵俊猛,2018Ha et al., 2019Wolff et al., 2019Chevalier et al., 2020Wang et al., 2020张佳伟等,2020Zuo et al., 2021)。

a—喜马拉雅-青藏高原系统及其周边地区示意图;b—喜马拉雅造山带及藏南地区主要构造图(图中数字为裂谷启动年龄,揭示了自西向东变年轻的趋势,具体描述见正文以及表 1)
IYS—印度河-雅鲁藏布江缝合带;BNS—班公湖-怒江缝合带;JS—金沙江缝合带;AMS—阿尼玛卿-昆仑-木孜塔格缝合带
图 1 青藏高原伸展构造及其与后碰撞岩浆岩关系示意图(据Chung et al., 2005; Tayor and Yin, 2009; Guo et al., 2015修改) Fig. 1 Sketch map showing the extensional structures and their relationships with the post-collision magmatism in the Himalayan-Tibetan orogen (modified after Chung et al., 2005; Tayor and Yin, 2009; Guo et al., 2015). (a) Sketch of the Himalayan-Tibetan system and surrounding areas. (b) Tectonic map of the Himalayan orogen and southern Tibet with major structures. The numbers represent the initiation time of the rifts, revealing the rifting initiated gradually earlier westward. Detailed descriptions can be found in the text and Table 1.IYS—Indus-Yarlung Suture; BNS—Bangonghu-Nujiang Suture; JS—Jinshajiang Suture; AMS—Anyimagen-Kunlun-Moztagh Suture
1.2 后碰撞岩浆岩作用

青藏高原后碰撞过程导致了一系列岩浆活动(Kelly et al., 2010),其中出露于拉萨地体内部的超钾质岩和埃达克岩被广泛认为与近南北向裂谷活动相关(图 1bChung et al., 2003, 2009Zhang et al., 2014),可成为约束裂谷成因的重要依据(侯增谦等, 2006a, 2006b丁林等,2006)。超钾质岩通常被认为来源于富集地幔的部分熔融,主要形成于25~8 Ma(Ding et al., 2003孙晨光等,2007Xu et al., 2017Wang et al., 2018),在空间上呈现两种趋势(图 1b),一部分在拉萨地体内沿近东西向展布,形成年龄自西向东减小,可能与印度板片断离相关(Guo and Wilson, 2019);另一部分则沿南北向裂谷展布,形成年龄自北向南减小,与印度板片撕裂相关(侯增谦等, 2006a, 2006b丁林等,2006Yan et al., 2019)。埃达克岩大致沿着平行于印度河-雅鲁藏布江缝合带的狭窄条带分布(图 1b),其成因尚不确定,可能是由于增厚地壳、上地幔、新特提斯洋洋壳或印度下地壳熔融引起(Zhang et al., 2014)。在藏南地区,埃达克岩具有两期活动的特征,早期>24 Ma没有明显的年龄趋势,后期20~10 Ma呈现自西向东变年轻的趋势。两期埃达克岩浆活动表现出不同的地球化学特征和成因演化,可能分别与新特提斯洋板片和印度大陆岩石圈板片的断离有关(Lu et al., 2020Lin et al., 2021)。

2 高原东西向伸展的动力学模型 2.1 近同时启动模型 2.1.1 重力垮塌模型

重力垮塌模型是最早用于解释青藏高原东西向伸展的模型(图 2aMolnar and Tapponnier, 1978Dewey,1988Harrison et al., 1992Coleman and Hodges, 1995Searle,1995Blisniuk et al., 2001)。该模型指出,由于印度-欧亚板块的挤压、地壳缩短增厚,使得青藏高原持续隆升,造成重力失稳。当高原超过最大承受高度时将发生垮塌,导致高原尺度的地壳伸展(Molnar and Tapponnier, 1978)。该模型的主要证据是南北向裂谷都集中在高海拔地区(Molnar and Tapponnier, 1978Dewey,1988)且当时认为在中—晚中新世期间近同时启动(Harrison et al., 1992Coleman and Hodges, 1995Blisniuk et al., 2001)。此外,裂谷的发育特征和地壳厚度之间具有较好的匹配性,例如:南北向裂谷在拉萨地体中发育长度较长且连续,在羌塘地体则长度变短,而在松潘-甘孜地体几乎不存在;同时,裂谷数量从南向北的减小对应于地壳厚度从南部的80 km减薄到北部的65 km(Owens and Zandt, 1997Zhao et al., 2001Kind et al., 2002)。这一匹配特征也为重力垮塌模型提供了有力的支持(Kapp and Guynn, 2004)。

a—重力垮塌模型;b—岩石圈地幔对流移除模型;c—亚洲东缘边界条件改变模型;d—放射状扩展模型;e—马蹄形弯曲模型;f—印度大陆倾斜汇聚模型;g—印度板片双向的横向拆离模型;h—横向挤出模型;i—岩石圈向东流动模型;j—板片撕裂模型 图 2 东西向伸展成因模式(据Molnar and Tapponnier, 1978England and Houseman, 1988Yin,2010Styron et al., 2011Yin and Taylor, 2011Chen et al., 2015Webb et al., 2017修改) Fig. 2 Theoretical models for the mechanism of the E-W extension (modified after Molnar and Tapponnier, 1978; England and Houseman, 1988; Yin, 2010; Styron et al., 2011; Yin and Taylor, 2011; Chen et al., 2015; Webb et al., 2017). (a) Gravitational collapse; (b) Convective thinning; (c) Change in the boundary condition; (d) Radial spreading; (e) Oroclinal bending; (f) Oblique convergence; (g) Lateral slab detachment; (h) Lateral extrusion; (i) Eastward lithospheric flow; (j) Slab tearing

随着研究的深入,重力垮塌模型受到了越来越多的挑战。首先,McCaffrey and Nabelek(1998)表明裂谷作用可以发生在任何海拔,裂谷的形成并不代表高原的隆起。其次,重力垮塌通常被认为仅仅出现在地壳范围(Bird,1991Liu and Shen, 1998),然而,在藏南和喜马拉雅地区发育了大量深(>75 km)正断层地震(Chen and Kao, 1996Zhu and Helmberger, 1996)。再次,由于重力扩散速度与地形梯度相关,而青藏高原地形梯度最大的部位在高原的南、北两侧,所以重力垮塌作用应导致南北向伸展而不是东西向伸展(Liu and Yang, 2003张进江和丁林,2003)。最后,青藏高原的主要隆升时间可能晚于东西向伸展开始的时间(Harrison et al., 1992丁林等,1995江万等,1998Zhu et al., 2017)。

2.1.2 地幔对流移除模型

基于重力垮塌,England and Houseman(1988, 1989)提出了地幔对流移除模型,认为岩石圈大规模水平缩短增厚将导致软流圈对流,增厚岩石圈地幔被整体移除。该效应可以迅速引起表面海拔(>2 km)与重力势能(5×1012~10×1012 N m-1)的增大,这足以使与重力偏应力相关的张应力大于与印度-欧亚板块碰撞相关的压应力,从而产生东西向伸展(图 2bMolnar et al., 1993van Buer et al., 2015)。该模型得到了后续不少研究的支持,例如:Molnar et al.(1993)通过东亚气候变化时间与印度板片褶皱和裂谷作用发生时间的一致性,认为在~8 Ma高原海拔出现了突然增大;西藏北部玄武质火山岩的激光40Ar/39Ar测年结果表明其形成时间大约为13 Ma,且来源于岩石圈地幔熔融,是深部岩石圈和软流圈的对流减薄所致(Turner et al., 1993);地壳捕虏体(Hacker et al., 2000)和地球物理观测(Alsdorf and Nelson, 1999Mechie et al., 2004Bai et al., 2010Yang et al., 2012)也认为高原中地壳过热(>600 ℃)、具有低速异常,可能是软流圈上涌的结果。

然而,岩石圈地幔被整体移除似乎是困难的。地球物理和地质学证据表明高原内部地壳和岩石圈地幔结构不均一,例如:地壳厚度从南向北减薄;南部上地壳热,下地壳和岩石圈地幔冷;北部下地壳和岩石圈地幔则较热(Nelson et al., 1996Kosarev et al., 1999Huang et al., 2000Blisniuk et al., 2001Wittlinger et al., 2004Li et al., 2008Zhao et al., 2010)。此外,Tapponnier et al.(2001)表明如果高原内部地幔比邻近克拉通地幔更冷,则不会导致热的软流圈上涌,因此,自始新世以来增厚的岩石圈地幔不会发生对流移除。

2.1.3 亚洲东缘边界条件改变模型

Yin(2000, 2010)根据高原南北向裂谷与贝加尔湖裂谷和陕西地堑间的相似性提出,如此大范围的伸展可能与亚洲东缘边界条件的改变相关。该模型指出太平洋板块在中新世出现后撤,引起亚洲大陆整体向东扩展,从而导致大规模伸展构造的发育(图 2c)。裂谷间的相似性主要表现在:①启动时间近同时,广泛开始在晚中新世到早上新世;②后碰撞岩浆作用多发生在距今约30~10 Ma,均早于裂谷作用出现的时间;③伸展都涉及到了岩石圈地幔。

尽管欧亚-太平洋板块汇聚速率减小的时间与青藏高原东西向伸展的时间相吻合(Northrup et al., 1995;李三忠等,2020),但是东亚边界条件的改变是否可以与高原内部的裂谷作用相联系仍然存在争议。因为高原内部发育的南北向裂谷距离欧亚-太平洋板块边界数千千米,似乎较难与太平洋板块俯冲相联系(Northrup et al., 1995Liu et al., 2004)。

2.1.4 放射状扩展模型

放射状扩展模型认为,响应于青藏高原自北向南的传播,喜马拉雅弧的周长呈放射状扩展,从而导致平行弧的伸展(图 2dSeeber and Armbruster, 1984Molnar and Lyon-Caen, 1989Seeber and Pêcher,1998Murphy and Copeland, 2005Murphy et al., 2009DeCelles et al., 2011Haproff et al., 2018)。支持的证据包括:①喜马拉雅造山带内部逆冲断层自北向南依次发育(DeCelles et al., 2011Long et al., 2011Bhattacharyya et al., 2015);②大量第四纪地堑趋于沿着喜马拉雅弧的径向方向分布(Ni and Barazangi, 1984Armijo et al., 1986);③震源机制解和GPS速度场表明喜马拉雅造山带相对于印度板片放射状向外逆冲(Baranowski et al., 1984Molnar and Lyon-Caen, 1989Jade et al., 2004Copley and McKenzie, 2007)。

该模型预测了喜马拉雅造山带的运动方向、裂谷发育范围及走向,要求喜马拉雅弧向南扩展(Styron et al., 2011),且裂谷每一处走向均与喜马拉雅造山带走向垂直(Yin,2006)。然而这一预测似乎与实际观测并不相符,例如,南北向裂谷广泛发育在拉萨和羌塘地体;喜马拉雅弧相对于欧亚大陆向北而不是向南移动;靠近喜马拉雅造山带东、西两端,裂谷走向与喜马拉雅弧走向并非垂直,而是分别呈50°~70°和30°夹角,等等。

2.2 从东西两侧向中间启动模型 2.2.1 马蹄形弯曲模型

马蹄形弯曲模型利用推测的线性喜马拉雅弧的弯曲来解释东西向伸展(图 2eKlootwijk et al., 1985Ratschbacher et al., 1994Schill et al,2001Li and Yin, 2008)。这一过程主要基于古地磁重建的证据,由于印度板片的旋转俯冲,西喜马拉雅发生顺时针旋转,东喜马拉雅则为逆时针旋转(Klootwijk et al., 1985Treloar and Coward, 1991Schill et al., 2001),导致喜马拉雅弧的曲率随时间不断增大,从而在喜马拉雅造山带内部产生东西向伸展。

该模型预测,响应于喜马拉雅弧的马蹄形弯曲,东西向伸展仅发育在喜马拉雅造山带内,向北至拉萨地体则变为东西向挤压,这与拉萨地体活跃的裂谷作用相矛盾(Styron et al., 2011)。此外,东喜马拉雅的运动学数据表明造山带马蹄形弯曲自4 Ma以来才开始控制喜马拉雅弧的变形(Li and Yin, 2008),这一时间远远晚于南北向裂谷的启动时间。

2.2.2 倾斜汇聚模型

印度大陆倾斜汇聚模型将东西向伸展的驱动力归因于印度板块俯冲造成的基底剪切力平行于弧的分量(图 2fSeeber and Armbruster, 1984McCaffrey and Nabelek, 1998Liu and Yang, 2003Styron et al., 2011McCallister et al., 2014)。该模型最初基于喜马拉雅弧与弯曲海沟之间较强的相似性提出,随后物理模拟(McCaffrey and Nabelek, 1998)和数值模拟(Liu and Yang, 2003)实验均认为该模型可行。同时,喜马拉雅弧沿走向平滑的变化和近似平行的印度-欧亚板块汇聚速度矢量也为模型的建立奠定了坚实的基础(Bendick and Bilham, 2002Styron et al., 2011),这导致远离喜马拉雅中心部位(汇聚速率与喜马拉雅弧方向垂直),汇聚速度矢量平行于弧的分量不断增大,进而产生平行弧的伸展。

该模型要求印度板片整体性地俯冲于亚洲板片之下,这一条件必然会阻止下部幔源岩浆的喷发,即拉萨地体将不会广泛出露后碰撞岩浆岩(侯增谦等, 2006a, 2006b),这与实际情况不符。此外,地球物理观测表明印度板片向北俯冲至班公湖-怒江缝合带附近(Kind et al., 2002Tilmann et al., 2003Zhao et al., 2010),这导致倾斜汇聚模型无法解释羌塘地体内的伸展(Kapp and Guynn.,2004)。

2.2.3 横向拆离模型

横向拆离模型强调印度板片自喜马拉雅东西两端(~25 Ma)向中东部(~10 Ma)横向拆离,造成拱形喜马拉雅弧的形成,同时引起东西向伸展(图 2gWebb et al., 2017Wang et al., 2019)。该模型的提出主要基于地球物理和岩浆岩年龄趋势的观测,例如:层析成像结果揭示拆离板片与印度克拉通之间的距离向东减小;后碰撞岩浆岩从东西两侧向中东部逐渐变年轻(Guo et al., 2015Webb et al., 2017),这与印度板片的双向拆离相一致(Replumaz et al., 2010, 2014Leary et al., 2016)。然而,最近的研究表明,埃达克岩的活动分为两期:早期(>24 Ma)和晚期(20~10 Ma)分别与新特提斯洋板片和印度岩石圈板片断离相关(Lu et al., 2020Lin et al., 2021)。因此,不同成因背景下的岩浆岩整体时空分布特征是否可以统一分析值得商榷。

2.3 自西向东启动模型 2.3.1 横向挤出模型

横向挤出模型指出,西藏地壳在南北向挤压作用下沿着右旋的喀喇昆仑-嘉黎断裂带和左旋的阿尔金-昆仑断裂带向东挤出,导致两个断裂带之间的块体内部出现东西向伸展(图 2hTapponnier et al., 1982Armijo et al., 1986, 1989Peltzer and Tapponnier, 1988Molnar and Lyon-Caen, 1989)。该模型得到了多个证据的支持:①西藏中部向东移动了约1000 km(Peltzer and Tapponnier, 1988);②喀喇昆仑断裂带(Armijo et al., 1989)和阿尔金断裂带(Armijo et al., 1989Mériaux et al., 2004, 2005Cowgill et al., 2009)的滑移速率可达~30 mm/a,支持了南北西藏的解耦;③以喀喇昆仑-嘉黎断裂带为界,南北两侧的伸展构造具有显著差异,南侧裂谷系发育、走滑断层稀少,北侧裂谷不连续、多位于走滑断层末端(Armijo et al., 1989);④物理实验成功模拟了青藏高原的一级构造特征,包括南北向裂谷系(Tapponnier et al., 1982Peltzer and Tapponnier, 1988)。

该模型虽然得到了大量证据支持,但也存在争议。Murphy and Copeland(2005)Murphy et al.(2010)以及McCallister et al.(2014)表明沿喀喇昆仑断裂带的大部分滑移分量被Gurla Mandhata裂谷向南转化进入喜马拉雅造山带,只有微不足道的应变沿着印度河-雅鲁藏布江缝合带向东迁移。此外,大地测量学分析结果也表明沿喀喇昆仑断裂带和阿尔金断裂带的滑移速率远小于早期的估计(Styron et al., 2011Tian et al., 2019郑文俊等,2019),这不足以导致藏南地壳的横向挤出。

2.3.2 岩石圈下地壳向东流动模型

岩石圈向东流动模型认为,青藏高原岩石圈中软弱的下地壳可向东流动,并在西藏上地壳底部产生成对的水平剪切,从而引起沿班公湖-怒江缝合带分布的V型共轭走滑断层和与之相连的近南北向裂谷的发育(图 2iYin and Taylor, 2011Zhang et al., 2013Bischoff 20182019")。地球物理观测揭示西藏中下地壳存在大范围的低速带和各向异性(Yang et al., 2012Zhang et al., 2013Agius and Lebedev, 2017),这支持了下地壳向东流动的观点。同时,数值模拟结果也显示,低黏度(1020 Pa·s及以下)下地壳的流动可成功再现西藏南部和中部的正断层作用(Bischoff and Flesch, 20182019)。然而美中不足的是,该模型尚无法解释有关学者揭示的后碰撞岩浆岩时空分布特征(Guo et al., 2013, 2015Zhang et al., 2014Webb et al., 2017)。

而后,Bian et al.(2020)对岩石圈向东流动模型进行了改进,表明印度板片自西向东发生横向拆离,造成西侧地形早于东侧隆升,从而建立了自西向东的重力势能梯度,进一步驱动岩石圈向东流动,最终导致近南北向裂谷和共轭走滑断层依次向东发育。

2.4 其他模型

上述模型都对裂谷的整体时空分布特征进行了预测,但有一个例外是板片撕裂模型,该模型并不要求藏南发育的不同裂谷之间存在联系。板片撕裂模型表明向北俯冲的印度板片被撕裂成几个具有不同宽度、不同俯冲角度的部分,并发生分段式差异俯冲,最终导致在藏南及喜马拉雅造山带发育近南北向裂谷(图 2jYin,2000贺日政和高锐,2003侯增谦和李振清,2004侯增谦等, 2006a, 2006bXiao et al., 2007Chen et al., 2015Li and Song, 2018)。该模型得到了地球物理探测和后碰撞岩浆岩证据的支持。地球物理探测表明,俯冲板片具有东西向差异,这被认为是板片存在撕裂的证据(Xiao et al., 2007Chen et al., 2015Pei et al., 2016Liang et al., 2016Duan et al., 2017Wang et al., 2017Li and Song, 2018Wu et al., 2019a, 2019bSi et al., 2019Liu et al., 2020Shi et al., 2020)。同时,板片撕裂为软流圈上涌提供了通道,合理解释了拉萨地体内广泛出露的后碰撞岩浆岩(丁林等,2006侯增谦等,2006a赵志丹等,2006孙晨光等, 2007, 2008Guo et al., 2018)。尤其是近南北向裂谷系、板片撕裂位置以及超钾质岩浆岩在时间和空间上具有较好的对应关系(侯增谦等,2006b丁林等,2006Chen et al., 2015Li and Song, 2018Yan et al., 2019)。

3 近南北向裂谷时空分布特征

在羌塘地体中,双湖裂谷(图 1b中的A)是唯一获得启动时间约束的裂谷。40Ar/39Ar和Rb/Sr年代学结果表明双湖地堑主边界正断层内的矿化年龄为~13.5 Ma,该年龄被解释为裂谷作用的下限(Blisniuk et al., 2001)。由于羌塘地体中约束近南北向裂谷启动时间的年代学数据稀少,因此双湖裂谷并不考虑纳入裂谷时空分布格局统计中。

相比于羌塘地体,拉萨地体中近东西向伸展的时间得到了更多的约束,自西向东描述如下:①在西部的隆格尔裂谷北段(图 1b中的B),变形的糜棱状淡色花岗岩中锆石U-Pb定年结果表明裂谷在~15 Ma启动(Kapp et al., 2008),这略早于磷灰石、锆石(U-Th)/He热年代学(Woodruff et al., 2013)以及热模拟(Sundell et al., 2013)结果揭示的10~8 Ma的启动时间;②在隆格尔裂谷南部(图 1b中的C),锆石(U-Th)/He年龄的PECUBE模拟结果显示正断层活动始于16~12 Ma(Styron et al., 2013);③东侧的Lopukangri裂谷(图 1b中的D)中,黑云母40Ar/39Ar热年代学数据表明东西向伸展开始在15~14 Ma(Sanchez et al., 2013);④再往东,磷灰石和锆石(U-Th)/He年龄表明当惹雍错裂谷(图 1b中的E)在~13 Ma和~6 Ma发生两期活动(Dewane et al., 2006),裂谷在~13 Ma启动的结果与数值模拟揭示的~15 Ma的裂谷启动时间(Wolff et al., 2019)近似一致;⑤在申扎裂谷(图 1b中的F)中,锆石和磷灰石(U-Th)/He年龄约束裂谷作用启动在14 Ma,随后在10~6 Ma发生加速活动(Hager et al., 2009);⑥进一步往东,磷灰石(U-Th)/He数据表明谷露裂谷(图 1b中的G)在7~5 Ma开始活动(Stockli et al., 2002);⑦西南部的念青唐古拉裂谷(图 1b中的H)中下盘岩体的云母和钾长石40Ar/39Ar数据以及磷灰石裂变径迹结果都记录了该地区在~8 Ma的快速冷却,进而约束了裂谷的启动时间(Harrison et al., 1995吴珍汉等,2002Kapp et al., 2005)。

在喜马拉雅造山带中,有学者对裂谷启动时间的约束也开展了许多研究。①在Leo Pargil裂谷中(图 1b中的I),独居石U-Pb数据表明韧性剪切开始在23 Ma(Langille et al., 2012),被认为代表了裂谷作用的启动时限;②在该裂谷东侧的Gurla Mandhata裂谷(图 1b中的J)中,淡色花岗岩的独居石Th-Pb定年结果表明东西向伸展开始在15 Ma(Murphy and Copeland, 2005),与锆石(U-Th)/He年龄的模拟结果所认为的14 Ma的启动时间一致(McCallister et al., 2014);③在Thakkhola裂谷(图 1b中的K)中,同伸展变形的淡色花岗岩(Larson et al., 2020)和南北向热液岩脉(Coleman and Hodges, 1995)的白云母40Ar/39Ar定年结果分别表明东西向伸展开始在17 Ma和14 Ma;④再往东,通过锆石和磷灰石(U-Th)/He数据的反演模拟,孔错裂谷(图 1b中的L)的活动时间被约束在13~12 Ma(Lee et al., 2011);⑤根据云母40Ar/39Ar热年代学年龄,定结裂谷(图 1b中的M)启动在13~10 Ma(Kali et al., 2010);⑥进一步往东,亚东裂谷(图 1b中的N)切割了藏南拆离系,因此,裂谷的启动时间晚于藏南拆离系终止活动的时间,即 < 10 Ma(Edwards and Harrison, 1997)或~12 Ma(Xu et al., 2013);⑦在裂谷系最东侧的错那裂谷(图 1b中的O)中,黑云母、钾长石40Ar/39Ar和锆石、磷灰石(U-Th)/He热年代学数据表明裂谷作用启动在~3 Ma(Bian et al., 2020),这与根据断层带内硅质膜年龄约束的5 Ma的启动时间大体一致(吴中海等, 2007, 2008哈广浩等,2018)。错那裂谷的重要性表现在,它是唯一发育于喜马拉雅东段的裂谷,其活动年龄可区分裂谷究竟是自西向东依次启动,还是从东西两端向中间启动(Bian et al., 2020)。

总的来说,高原内部南北向裂谷的启动时间被限制在中新世—上新世,存在较大的年龄差异(表 1图 1b)。这些年龄的获得主要基于裂谷与藏南拆离系的交切关系、与东西向伸展相关的同构造变形现象、伸展岩脉、磁性地层以及热年代学等方法。其中,交切关系和伸展岩脉方法约束裂谷启动时间的上限;同构造变形和磁性地层方法约束裂谷启动时间的下限;而热年代学方法能够记录引起区域快速剥蚀的构造事件,因此基于一定的假设,在裂谷发育区域可约束裂谷的启动时间(哈广浩等,2018张佳伟等,2020)。这是目前较为可靠的约束方法,也是国际上普遍接受的方法。为了获得较为准确的裂谷启动时间趋势,文章尽可能选取热年代学方法约束的裂谷启动年龄,进一步汇编了近南北向裂谷的时空分布特征图(图 1b)。假设已报道数据能够真实反映裂谷形成时间,且能够代表整个裂谷,那么近南北向裂谷的启动时间表现出自西向东逐渐变年轻的趋势,即,自最西侧Leo Pargil裂谷的23 Ma,减小到中部的17~10 Ma,再减小到东侧谷露裂谷、念青唐古拉裂谷的8~5 Ma,直到最东侧错那裂谷的5~3 Ma。

表 1 青藏高原南北向裂谷启动时间 Table 1 Initiation time of the NS-trending rifts in the Tibetan Plateau
4 讨论 4.1 青藏高原中南部的近东西向伸展变形机制

作为青藏高原重要的后碰撞构造系统,近南北向裂谷的形成机制受到了诸多关注,所提出的模型也被反复讨论和验证,但是,很多问题至今仍处于争论之中。特别是这些模型都对裂谷的时空分布特征进行了不同的预测,因而这可作为约束裂谷成因机制的关键条件。

综合目前关于裂谷启动时间的已有数据,可初步刻画了整个高原近南北向裂谷的时空分布特征(图 1)。结果显示,近南北向裂谷的启动时间似乎具有自西向东逐步减小的趋势。如果这一时空分布特征确实存在,则可在较大程度上排除那些预测裂谷近同时启动或从东西两侧向中部发育的模型,从而使得横向挤出和岩石圈向东流动模型成为最可能的模型。然而,该模型无法合理地解释与近南北向裂谷紧密相联系的后碰撞岩浆岩的发育趋势。

Bian et al.(2020)基于上述的近南北向裂谷时空发育特征,并重新分析后碰撞岩浆作用演化过程后,提出了“印度板片向东拆离模型”来解释青藏高原中南部的近东西向伸展变形(图 3)。该模型认为,向北俯冲的新特提斯洋板片在大约50~40 Ma发生断离(DeCelles et al., 2002Zhu et al., 2015Garzanti et al., 2018),并导致软流圈上涌,拉萨地体下地壳发生重熔作用,产生了早期(>24 Ma)埃达克岩(Lu et al., 2020Lin et al., 2021)。洋壳断离后,印度大陆岩石圈持续俯冲并固定在地幔中,大约25 Ma开始自西向东逐步拆离(Replumaz et al., 2010, 2014Leary et al., 2016),这得到了地球物理观测证据的支持。层析成像结果表明现今印度-欧亚碰撞带之下存在一个高速带,解释为拆离的印度板片的存在,该残余板片的长度向东增加,并且与印度克拉通之间的距离向东减小,即拆离的印度板片向东不断变浅,与板片横向拆离向东的传播趋势相匹配(Replumaz et al., 2010, 2014)。软流圈物质沿印度板片拆离窗上涌,并逐步向东迁移,从而形成了自西向东变年轻的后期(20~10 Ma)埃达克岩和沿东西向分布的超钾质岩(25~8 Ma)。

印度板片的横向拆离过程可使得被拆离部分由于下伏板片拖曳力的释放而反弹,产生垂直运动,进而造成地表的地形抬升(Wortel and Spakman, 2000),同时,重力势能在已拆离板片的上方迅速累积。随着印度板片横向拆离向东扩展,地形隆升与重力势能累积现象也不断向东传播,从而形成自西向东的重力势能梯度。重力势能向东的迁移也受到了多个因素的影响。如,随着印度板片持续向北挤压,喜马拉雅-青藏高原造山带西侧由于具有较窄的变形带,从而发育了比东侧更高的应变率继而更快速的地形隆升(Yang and Liu, 2013)。此外,最近的古地磁观测结果表明印度-欧亚板块碰撞前大印度的几何形状可能导致西侧较东侧碰撞时间更早且具有更大的大陆汇聚量(Meng et al., 2020),这使得西侧重力势能的积累速度比东侧快。随后,重力势能梯度驱动中下地壳向东流动,进而拖曳上地壳产生V形共轭走滑断层和南北向裂谷(Yin and Taylor, 2011Zhang et al., 2013Bischoff and Flesch, 2018)。

这一过程的另一结果是俯冲板片的纵向撕裂。随着板片横向拆离的发展,印度岩石圈以不同的方式向前俯冲:在西部,被拆离所释放的板片由向下俯冲变为向上隆起;在中部和东部,未拆离板片在下伏板片拖曳力作用下继续向下俯冲。拆离和未拆离板片之间的运动学差异导致在它们的连接点处产生剪应力,当其累积至超过俯冲板片的极限强度时(Rosenbaum et al., 2008),板片自北向南产生撕裂(图 3)。围绕着板片撕裂边缘,顺时针运动的地幔环流进一步加强了中下地壳的向东流动(Zandt and Humphreys, 2008)。与此同时,软流圈在板片撕裂处上涌,产生了沿南北向裂谷分布的超钾质岩浆岩(Guo et al., 2018Guo and Wilson, 2019)。这一过程与在当惹雍错裂谷附近观测到的低速异常相一致(Liang et al., 2016)。随后,印度大陆岩石圈在大约10 Ma完全拆离。印度板片在之后的向北俯冲过程中,一方面导致藏南软流圈上涌的板片窗口关闭(Chen et al., 2018),使得8 Ma以来的岩浆活动消失(Guo and Wilson, 2019);另一方面导致西藏地壳增厚,可能达15~20 km,这足以使得下地壳继续向东流动(Decelles et al., 2011Styron et al., 2015),导致运动学上相连的共轭走滑断层和近南北向裂谷向东扩展。

MCT—主逆冲断层;STD—藏南拆离系;IYS—印度河-雅鲁藏布江缝合带;BNS—班公湖-怒江缝合带 图 3 青藏高原东西向伸展的地球动力学过程示意图 Fig. 3 Schematic diagram showing the geodynamic process of the E-W extension in the Himalayan-Tibetan orogen MCT—Main Central Thrust; STD—Southern Tibet Detachment; IYS—Indus-Yarlung Suture; BNS—Bangong-Nujiang Suture

综上,印度板片的向东拆离模型可合理地解释青藏高原中南部的近东西向伸展特征及相关的岩石圈变形现象。第一,模型符合近南北向裂谷作用自西向东逐步发育的时空分布特征。第二,模型表明近南北向裂谷与相连的V形共轭走滑断层的发育是可以兼容的,它们之间存在耦合的机制。第三,模型合理地解释了后碰撞岩浆岩的发育趋势。第四,模型可综合解释已有研究所揭示的印度板片拆离、撕裂、回撤以及岩石圈向东流动等过程。

4.2 关于板片撕裂模型

板片撕裂模型由于不要求藏南发育的不同裂谷之间存在联系,因此无法通过所有裂谷的时空分布特征来验证。这使得板片撕裂究竟对东西向伸展有没有贡献这一问题依然得不到确认。详细的地球物理观测和后碰撞岩浆岩证据表明板片撕裂现象可能是存在的,但它与南北向裂谷之间是否存在因果关系,目前还难以定论。因为板片撕裂既可能是南北向裂谷的主要驱动机制(Chen et al., 2015Li and Song, 2018),也可能仅起到辅助作用(Bian et al., 2020)。由于板片撕裂是从北往南发生的,正如超钾质岩自北向南变年轻的趋势(Guo et al., 2013Bian et al., 2020),如果板片撕裂是南北向裂谷的主要成因机制,则要求同一裂谷带从北往南发育,即从单条裂谷的发育过程来看,北部早而南部晚。也就是说,进一步详细刻画单条裂谷的发育过程可对板片撕裂模型提供新的制约。

5 结论

(1) 关于青藏高原近南北向裂谷的形成机制存在多种假说模型,根据模型对裂谷时空分布特征的预测,可将其大致划分为近同时启动、从东西两侧向中部启动以及自西向东启动三类,这成为约束裂谷成因机制的关键条件。

(2) 通过综合研究关于近南北向裂谷启动时间和形成机制的已有成果发现,裂谷的启动时间具有自西向东逐步减小的趋势;裂谷单调向东的发育模式可在较大程度上排除那些预测裂谷近同时启动或从东西两侧向中部启动的模型;近南北向裂谷的形成可能受控于印度板片自西向东的拆离驱动的岩石圈向东流动。

致谢: 感谢吴中海研究员和另一位匿名审稿人对本文提出的建设性修改意见。

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