地球物理学报  2020, Vol. 63 Issue (11): 4154-4167   PDF    
青藏高原东缘新生代隆升剥露与断裂活动的低温热年代学约束
陶亚玲1, 张会平1,2, 葛玉魁1, 庞建章1, 俞晶星1, 张佳伟1, 赵旭东1, 马字发1     
1. 中国地震局地质研究所, 地震动力学国家重点实验室, 北京 100029;
2. 南方海洋科学与工程广东省实验室, 珠海 519082
摘要:青藏高原东缘作为高原生长的东边界,其新生代以来隆升剥露与扩展模式备受关注.高原内部平缓的地貌和边界构造带不显著的缩短变形被认为是下地壳流作用的重要证据.然而近年来,越来越多的低温热年代学研究结果表明,中-晚新生代以来跨不同断裂带存在显著的差异性隆升剥露,指示了断裂体系在青藏高原东缘构造变形与演化中的重要作用.本文系统收集区域内现有不同封闭温度体系的低温热年代学数据,综合分析结果表明青藏高原东缘隆升剥露及生长扩展与整个高原抬升具有准同步性.最为广泛和显著的剥露主要发生在~30 Ma以来,且高原东缘的最大侵蚀量区受控于断裂活动,快速侵蚀带的空间分布与鲜水河断裂带相一致.在区域尺度上,现有数据所揭示的剥露事件启动、持续时间及速率的显著差异性揭示了断层活动对青藏高原东缘地表剥露过程的控制作用.本文提出青藏高原向东扩展是多阶段、非均匀过程,新生代以来不同断裂带在青藏高原向东扩展过程中起到了至关重要的作用,不支持"下地壳流假说"强调的"东缘上地壳变形不显著"的认识.
关键词: 青藏高原东缘      新生代      隆升剥露      断裂活动      低温热年代学     
Cenozoic exhumation and fault activities across the eastern Tibet: constraints from low-temperature thermochronological data
TAO YaLing1, ZHANG HuiPing1,2, GE YuKui1, PANG JianZhang1, YU JingXing1, ZHANG JiaWei1, ZHAO XuDong1, MA ZiFa1     
1. State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China;
2. Laboratory of Southern Marine Science and Engineering Guangdong Laboratory, Zhuhai 519082, China
Abstract: The growth pattern of the eastern Tibet is still mysterious, where the remnant upland with low relief has been hypothesized as the response to lower crustal flow originating from the over-thickened crust of central Tibet. In recent decades, however, a growing number of low-temperature thermochronological studies have revealed a striking different sight. In this paper, we collected the apatite and zircon (U-Th)/He and fission track ages across the eastern Tibet, and found where the exhumation type is quasi-synchronized with the entire plateau uplift, the early uplift and exhumation occurred mainly in the fault zone, the most extensive and rapid exhumation process is occurred since ~30 Ma and the fault system controlled the magnitude of exhumation, the Xianshuihe fault represent the rapidest erosion zone. The significant differences in the time and duration of these events, and the exhumation rates indicate the faulting mainly controls the pattern of surface process. Moreover, the multi-stage, differential denudation process across the eastern Tibet challenge the lower crustal flow model.
Keywords: The eastern Tibet    the Cenozoic    Uplift and exhumation    Fault activity    Low-temperature thermochronology    
0 引言

青藏高原是新生代以来印度板块与欧亚板块汇聚碰撞形成的海拔最高、最年轻的高原(Tapponnier et al., 2001).印度板块持续向北俯冲使得青藏高原不断隆升且向北、东扩展,引起了强烈的陆内构造变形,奠定了现今亚洲大陆构造格局(Molnar et al., 1993).目前有关青藏高原新生代以来的隆升过程及机制一直存在争议,在青藏高原东缘主要存在两种动力学端元模型,分别为强调中下地壳增厚的地壳管道流假说(Royden et al., 1997, 2008)和刚性大陆块体逃逸假说(Molnar and Tapponnier, 1975Tapponnier et al., 1982, 2001Replumaz and Tapponnier, 2003). “下地壳流假说”认为陆内变形是以中下地壳黏塑性的均匀增厚和流变为主导,否认上地壳断裂活动对构造变形和缩短的主要调整作用;而“大陆逃逸模型”则强调刚性地块间的非连续变形特征,认为地壳缩短与断层活动吸收了板块汇聚过程中大量的形变.

青藏高原在向东扩展过程中,受扬子地块的阻挡,强烈变形隆起,在高原东缘形成了大型逆冲断裂带——龙门山—雅砻江逆冲带,构成了青藏高原的东边界(图 1),同时控制着区域中强地震的发生及其次生地质灾害(Xu et al., 2009; Zhang et al., 2010).在北段龙门山断裂带内,一系列断层的逆冲活动导致其相对于四川盆地的显著隆起(Kirby et al., 2002; Jia et al., 2006),导致龙门山与四川盆地间在水平距离不到50 km的范围,海拔从~600 m上升到6500 m.而在南段的雅砻江逆冲带内,地形对比并不像龙门山地区强烈,形成一条宽50~200 km的地形过渡带(刘静等,2008),包括以锦屏山为主体的构造体系,且这种长波地形一直被下地壳流模型作为上地壳均匀增厚的证据(图 1).

图 1 青藏高原东缘地质构造简图 Fig. 1 Major Cenozoic faults and topography of the eastern Tibet

然而近几年来,陆续有研究报道龙门山—雅砻江逆冲带不同构造部位在中-晚新生代以来发生了显著的地壳缩短及差异抬升(Wang et al., 2012a; Zhang et al., 2016; Cao et al., 2019).龙门山逆冲带沿走向存在显著剥蚀差异,中、南段经历了更快的隆升(李智武等,2010Li et al., 2012);从四川盆地向龙门山腹地也存在明显的剥蚀差异,中段断层上盘的剥露速率可达下盘的2倍(Tian et al., 2013; Shen et al., 2019).

上述一系列构造变形及差异剥露特征,暗示着青藏高原东缘不同构造带内断层活动在高原边界扩展中起着重要调节作用.系统梳理和分析现有的低温热年代学研究成果能够为深入理解和认识区域性断裂构造在高原东缘隆升及地貌演化中的作用.鉴于此,本文收集整理了青藏高原东缘670个低温热年代学年龄,结合地形参数特征,分析了断层活动与地表剥露的时空相关性及其构造地貌意义,相关结果可为进一步深入研究青藏高原东缘新生代隆升的时空扩展模式及其动力学机制提供参考和基础数据.

1 区域地质背景

青藏高原东缘主要由松潘—甘孜、义敦、羌塘和康滇等地体(地块)拼接而成(Burchfiel and Chen, 2013).晚三叠纪以来,随着古特提斯洋闭合,各地体之间相互拼贴造山,形成了金沙江、怒江和澜沧江缝合带等大型构造缝合带(Roger et al., 2010; Ding et al., 2013Jian et al., 2019);松潘—甘孜、义敦地块逐渐进入隆升剥蚀过程(Jackson et al., 2018).与此同时,在扬子板块西缘的四川—楚雄一带(今龙门山—雅砻江逆冲带下盘)发育一个近南北向的龙门山古前陆盆地(Xu et al., 2013, 2015; Li et al., 2018Yan et al., 2018),发育巨厚的陆相碎屑地层,直到晚白垩纪-早新生代,可能受新特提斯洋闭合的远程效应影响,除四川盆地,该前陆盆地逐渐萎缩直至消失(Van der Voo et al., 1999; Kapp et al., 2005; Deng et al., 2018; Huang et al., 2019),大部分地区逐渐向缓慢的剥蚀状态转换(Tian et al., 2014a, b; Yang et al., 2017).

始新世以来,在印度—欧亚板块碰撞作用下,青藏高原向东挤出,受华南块体的阻挡,使高原东部发生了显著的构造变形(Replumaz and Tapponnier, 2003; Royden et al., 2008),主要表现为一系列NNW向的大型走滑断层和褶皱带,以及NNE向的褶皱逆冲带(Chen and Wilson, 1996Wang et al., 1998Roger et al., 2004),如龙门山—雅砻江逆冲带、鲜水河—小江断裂和哀牢山—红河断裂等(图 1),这些大型断裂带控制着青藏高原东缘的构造活动空间特征.

在地貌形态上,中-新生代以来形成的东北走向的龙门山和近南北走向的锦屏山造山带,延伸600 km(许志琴等,2007),相比北段龙门山,雅砻江逆冲带南北两侧地形高差较小.Liu-Zeng等(2008)研究认为雅砻江逆冲带内峡谷深切,指示了一个正在遭受侵蚀的古老高原边界.地质地貌分析及地球物理数据揭示雅砻江逆冲带很可能是龙门山逆冲带向南的延伸段,这一连续的线性构造带由于后期(~13 Ma)活动的NNW向左旋走滑的鲜水河—小江断裂错开,达~60 km(许志琴等,2007Wang et al., 1998Burchfiel et al., 1995He et al., 2006Liu-Zeng et al., 2008; Yan and Lin, 2015; Li et al., 2017).哀牢山—红河断裂从滇西北向南穿过越南直达中国南海,在青藏高原向东挤出过程中的走滑量被认为可达约700 km(Leloup et al., 1995; Chung et al., 1997; Li et al., 2017),并被作为地壳逃逸模型的重要证据(Tapponnier et al., 1982, 2001Leloup et al., 1995Li et al., 2017).

青藏高原内部地形起伏小,且这一平坦面向东南缘延伸(Clark et al., 2006),被认为是下地壳管道流模型的重要依据.早期的低温热年代学研究结果显示,青藏高原东缘的快速剥露过程主要发生在15~10 Ma以来(Xu and Kamp, 2000; Kirby et al., 2002; Clark et al., 2005; Godard et al., 2009; Ouimet et al., 2010),这为下地壳管道流变形开启于晚中新世的论断提供了重要的地质证据.Wang等(2012a)在龙门山中段的研究揭示出了渐新世(30~25 Ma)和晚中新世(~10 Ma)两期显著的快速剥露事件,表明青藏高原东缘早期发生了明显的缩短变形过程(Oskin, 2012),后续研究进一步揭示出区域性断层构造在高原东缘的早期变形中具有重要的控制作用(Zhang et al., 2016; Liu-Zeng et al., 2018Cao et al., 2019, 2020).

2 高原东缘新生代隆升剥露的低温热年代学记录

地表过程可以系统地反映气候与构造的变化,活跃造山带的地表过程约束已经成为揭示和判别构造活动的重要途径.相比其他方法,低温热年代学测年体系的突出优点是对温度变化的灵敏性,可以很好的约束中浅地表的冷却过程,因此成为研究造山期后山脉隆升剥露的重要手段(Farley, 2002Ehlers and Farley, 2003Reiners and Brandon, 2006Reiners and Shuster, 2009).在青藏高原生长及扩展历史的研究中,除了高温的锆石U-Pb测年体系对基底成岩年龄的限定,高原后期演化更多的是利用云母和长石的K/Ar、40Ar/39Ar,磷灰石和锆石的裂变径迹(FT)及(U-Th)/He等低温热年代学测年体系,综合重建70~300 ℃(~10 km)中上地壳冷却历史.

2.1 快速剥露事件的时间特征

本文收集了近十几年发表的青藏高原东缘低温热年代学年龄结果,对新生代以来的年龄(670个)进行了归类分析,统计结果以直方图和核密度估计(KDE)的形式显示(图 2).结果显示,AHe、AFT、ZHe和ZFT年龄主要集中在约30~25 Ma以来,其中在约15~10 Ma至今高原东缘经历了最为广泛的剥露;另外,AFT和ZHe年龄显示约40~30 Ma和>40 Ma之前也存在快速剥露事件,与整个青藏高原的剥露过程具有一定的一致性(钟大赉和丁林, 1996; 王国灿等, 2011; 张克信等, 2013).

图 2 青藏高原东缘新生代低温热年代学数据统计(根据Vermeesch, 2018, IsoplotR KDE模块) Fig. 2 Histogram of the low-temperature thermal chronological ages since the Cenozoic collected from the eastern Tibet (Vermeesch, 2018)

不同封闭温度的测年体系揭示的剥露事件具有一致性,每一个测年体系记录了其对应的地壳深度经历的剥露历史,但也存在年龄结果并不代表快速剥露事件的情况,例如,Replumaz等(2020)在澜沧江河谷的高程剖面磷灰石年龄(3~1.2 Ma)的热史模拟结果显示,快速剥露事件开始于河谷底部(~1.5 Ma),因此我们对年龄峰值与其来源文献进行了核对,确认了年龄与热历史的一致.虽然>40 Ma的数据量较少,但与热史模拟结果揭示的快速剥露事件具有很好的一致性(Liu-Zeng et al., 2018Replumaz et al., 2020Cao et al., 2020).整体而言,区域上的剥露事件是准同步的,但由于构造活动的空间差异性及早期剥露历史记录较少,使得区域上年龄峰值界限模糊(图 2),因此,年龄的空间分布特征分析显得尤为重要.

2.2 快速剥露事件的空间特征

根据青藏高原东缘的低温热年代数据揭示的峰值特征(图 2),对年龄结果进行分阶段显示(图 3).整体来看,以鲜水河断裂为界,南北两侧区域低温热年代学记录存在较大差异.北侧的龙门山及其西部地区,夷平面不发育(Clark et al., 2006, 图 3),低温热年代学研究比较集中,结果显示龙门山断裂带内~10 Ma以来经历重要的抬升变形(Kirby et al., 2002; Godard et al., 2009; Wang et al., 2012a; Tan et al., 2017; Tian et al., 2015; Shen et al., 2019);而在其南侧地区,夷平面保存较为完整,且低温热年代学研究相对较少,研究结果更多的是揭示出了早期的剥露过程(>15~10 Ma)(Wang et al., 2012b; Tian et al., 2015; Shen et al., 2016; Zhang et al., 2016; Wang et al., 2017; Cao et al., 2019)(图 3).夷平面上的年龄普遍偏老,如川滇块体内部的稻城—理塘—甘孜一带,岩体冷却年龄为50~150 Ma(Clark et al., 2005; 来庆洲等, 2006; Lai et al., 2007; Wilson and Fowler, 2011; Tian et al., 2014b),若尔盖高原为50~130 Ma(Arne et al., 1997; Tian et al., 2014a),揭示出了比较稳定的剥露过程;而较快的剥露过程主要发生在高起伏地形区(图 135).其中,30~40 Ma和>40 Ma的年龄主要来自于龙门山—雅砻江逆冲带及鲜水河断裂以南地区,红河断裂带也记录了同期的构造活动事件(Leloup et al., 1995, 2001);而~30 Ma以来的年龄分布除了龙门山—雅砻江褶皱逆冲带以外,更多的是集中于河流峡谷内(如澜沧江、金沙江、雅砻江、大渡河等),以及鲜水河—小江和理塘走滑断裂带(Clark et al., 2005; Ouimet et al., 2010; Wilson and Fowler, 2011; Wang et al., 2012a, b; Zhang et al., 2016, 2017; Cao et al., 2019, 2020).

图 3 青藏高原低温年代学数据分阶段显示 Fig. 3 Step-displaying of low-temperature thermal chronological ages across the eastern Tibet
图 4 青藏高原东缘地形起伏度和坡度 Fig. 4 Topographic relief and slope across the eastern Tibet
图 5 青藏高原东缘~30 Ma以来的侵蚀速率空间特征(基于表 1现有速率的克里金插值结果) Fig. 5 Spatial distribution of exhumation rates since 30 Ma across the eastern Tibet (Kriging interpolation results based on table 1)

另外,根据不同封闭温度年龄的空间特征(图 3),~30 Ma以来青藏高原东缘高起伏地区上地壳的剥露量普遍可达1~4 km,尤其是雅砻江—龙门山逆冲带的上盘地区剥露量较大,可达5~7 km,与大量发育的背斜隆起等地层变形特征吻合(Roger et al., 2004许志琴等,2007; Burchfiel and Chen, 2013);而川滇地块内部区域在新生代以来剥露量很小.

整体而言,青藏高原拉萨地块、松潘—甘孜地块以及昆仑地块在新生代早期(>40 Ma)快速剥露的记录较少.在羌塘中部,作为唐古拉逆冲带前陆盆地的可可西里(72~51 Ma)和沱沱河盆地(~52 Ma)记录了在唐古拉山早期的快速隆升过程(Li et al., 2012Jin et al., 2018);冈底斯弧后的林周盆地在69~45 Ma的林子宗火山岩喷发及其与下覆白垩纪红层的地层间断(72~69 Ma)(Mo et al., 2007; Sun et al., 2012)揭示了青藏高原早期强烈的构造活动与变形.另外,在高原东部的玉树—囊谦地区,北以甘孜—玉树断裂为界,南以新生代玉树—囊谦逆冲带为界,发育白垩纪—古新世红层(青海省区域地质志,1991),且在~51 Ma发生显著缩短变形(Horton et al., 2002; Spurlin et al., 2005);挤压构造背景的贡觉盆地在69~64 Ma和52~48 Ma经历了快速的沉积过程(Tang et al., 2017Li et al., 2020);扬子地块西缘楚雄—西昌一带(今雅砻江—玉龙逆冲带前缘地区)在晚白垩纪-早古新世发育巨厚河湖相沉积(Deng et al. 2017, 2018),以及高原东南部澜沧江峡谷以及鲁甸地区花岗岩体的快速剥露(60~40 Ma)(Liu-Zeng et al., 2018; Cao et al., 2020)都很好地记录了青藏高原早期向东扩展的多阶段、不均匀性特征,揭示区域性大断裂对高原东部变形过程具有重要调节作用.

3 侵蚀速率与地形特征相关性分析

构造活动和地表过程共同控制着侵蚀作用.就整个高原而言,由于高原内部地势平坦且以内流水系为主,侵蚀过程较弱.而在边缘造山带,较大地形梯度及众多的外流水系使得侵蚀作用强烈.但是在地形梯度较小的东缘,侵蚀过程的空间特征应该能够揭示重要的信息.

基于现有的研究成果,我们重点分析了新生代以来青藏高原东缘的快速剥露与地貌特征的空间相关性.地形起伏度分析的结果显示(图 4a),龙门山—雅砻江逆冲带及鲜水河断裂与最大地形起伏带重合,且大渡河、雅砻江、金沙江、澜沧江和怒江等流域的深切河谷段也对应高地形起伏区;同样,地形坡度分析结果也证实了这一点(图 4b).

基于低温热年代学的剥露速率估算主要来自高程剖面数据,通过对现有的侵蚀速率进行整理和分析发现(表 1图 5),虽然在~40 Ma以来青藏高原东缘开启了显著的阶段式抬升,但在~30 Ma之前,除了龙门山—雅砻江—玉龙逆冲带(0.5~0.7 km·Ma-1) (Wang et al., 2012a; Zhang et al., 2016; Cao et al., 2019)之外的区域侵蚀速率十分缓慢,为~0.1 km·Ma-1;而在~30 Ma之后,侵蚀速率明显普遍增加,可达到0.8~1.4 km·Ma-1(图 5).在整个研究区内,鲜水河断裂带侵蚀速率最大(图 35).表明晚新生代以来,鲜水河断裂的启动及后期活动可能控制着高原东缘构造变形的差异性以及剥露作用的空间特征,这和我们所观察到的区域内夷平面分布及热年代学数据空间分布特征相吻合(图 3).

表 1 基于低温热年代学高程剖面的侵蚀速率 Table 1 Exhumation rates constrained by low-temperature thermal chronological ages

为进一步探讨青藏高原东缘新生代以来不同地质历史时期的剥露速率变化,我们依据Herman和Brandon(2015)提出的低温热年代数据反演法,将不同封闭温度的冷却年龄数据转换为剥露速率的时空分布,结果显示(图 6),剥露速率变化经历了三个阶段:1)在60~30 Ma之间,整个区域上剥露速率较低;在此期间剥露主要发生在龙门山—雅砻江逆冲带和乔后断裂一带,到始新世-渐新世剥露区域不断扩大;2)在约30~10 Ma之间,剥露速率显著增加,部分区域的剥露速率可达~0.8 mm·a-1;3)在~10 Ma前后,整个高原东缘进入普遍的快速剥露阶段,尤其以澜沧江流域、雅砻江—龙门山逆冲带以及鲜水河断裂带最为显著.其中,鲜水河断裂一带侵蚀速率最大(图 36),这与现有侵蚀速率的简单插值结果基本一致(图 5).这种显著的差异性剥露模式揭示了断层活动对高原东缘构造变形及剥露过程空间特征的重要控制作用.

图 6 青藏高原东缘新生代以来的侵蚀速率时空变化特征,图中黄圈标示图 3中的样本位置 Fig. 6 Temporal and spatial distribution of exhumation rates since the Cenozoic across the eastern Tibet. The yellow circles are sample locations as in Fig. 3
4 讨论 4.1 青藏高原东缘新生代以来的隆升剥露模式

新生代以来,随着印度—欧亚板块碰撞的持续,高原逐渐向东扩展,形成了地形梯度较小的高原东边界,与其他边界形成鲜明对比,因此成为探讨高原扩展及变形过程的焦点区域.钟大赉等(1996)对东喜马拉雅构造结区域进行了系统的AFT年代学分析,年龄结果主要集中在~25 Ma以来,并识别出3期显著的冷却事件(25~17 Ma,13~8 Ma和3 Ma);王国灿等(2011)对青藏高原造山带新生代以来的AFT数据进行整合并分区对比,结果显示除了高原东北缘及个别区域,西昆仑、喜马拉雅以及高原东南缘的冷却年龄主要集中在25~20 Ma以来;Wilson和Fowler(2011)在高原东缘川滇地块及龙门山地区进行了AFT年代学分析,所有数据的综合高程-年龄剖面显示快速剥露发生在30~20 Ma以来.本文综合分析了不同封闭温度的AHe、AFT、ZHe、ZFT年龄,结果显示,青藏高原东缘在~30 Ma以来经历了阶段性隆升剥露,这与以上的AFT年龄结果具有很好的一致性.说明伴随青藏高原多期变形,高原东边界也曾经历了同期变形(Horton et al., 2002; Spurlin et al., 2005; Tang et al., 2017; Liu-Zeng et al., 2018; Li et al., 2020; Cao et al., 2020).

龙门山—雅砻江逆冲带以龙门山和锦屏山为主体,围限了青藏高原的东边界.龙门—锦屏造山带的雏形在中生代就已经出现(许志琴等,2007),经历了新生代中晚期的强烈变形后,在中中新世抬升到现今高度(图 3).在新生代中-晚期,龙门山—雅砻江—玉龙逆冲带和哀牢山—红河断裂带的活动特征具有很好的时空一致性(图 3, 6).NNE走向的龙门山—雅砻江—玉龙逆冲断裂带在约35~30 Ma快速剥露(Wang et al., 2012a; Zhang et al., 2016; Cao et al., 2019),同期,NW走向的哀牢山—红河剪切带从南向北的逐渐形成(37~17 Ma)(Leloup et al., 1995, 2001; Zhang and Schärer, 1999; Gilley et al., 2003),这一变形模式揭示了该时期青藏高原向东扩展作用加强以及阶段性隆升剥露历史的开启.

哀牢山—红河断裂作为川滇块体的西边界,其韧性剪切特征指示了早期(36~17 Ma)的左行塑性变形,在中新世晚期(~5 Ma)转为右旋(Leloup et al., 1995, 2001; Replumaz and Tapponnier, 2003; Schoenbohm et al., 2006; Wang et al., 2016),指示了东缘应力场的变化;而在17~5 Ma期间较弱的活动性与川滇块体东边界鲜水河断裂—小江断裂在约13~5 Ma的开启(Roger et al., 1995; Zhang et al., 2004; Zhu et al., 2008; Wang et al., 2009; Li et al., 2015)也具有很好的空间一致性.鲜水河—小江断裂从高原东部地区一直向南延伸到云南南部与哀牢山—红河断裂相接(图 1),它的形成进一步加强了高原东缘的差异剥露过程(图 356).

4.2 断裂活动与差异性隆升剥露

青藏高原东缘的主要变形都发生在中-新生代以来,但是这些变形的时间一直以来都没有得到很好的约束(Burchfiel and Chen, 2013).龙门山—雅砻江—玉龙逆冲断裂带作为东南缘主要的边界断裂,其变形特征对高原东缘扩展特征具有重要的指示作用.刘静等(2008)通过分析地貌与地层特征,认为雅砻江—玉龙逆冲带南北两侧经历了显著的差异性剥蚀,尤其是北侧广泛出露的深成大型花岗岩体,而南侧发育侏罗纪-白垩纪沉积物盖层,剥露量差异可达~10 km,不支持下地壳流模型(Clark and Royden, 2000)提出的青藏高原东南缘上地壳变形对地形演化影响较小的推论.

本文基于低温热年代学的分析了青藏高原东缘侵蚀速率的空间分布(图 356),结果显示较快速剥露主要发生在深切的河流峡谷及主要断裂带,而远离断层的区域剥露过程缓慢,保存了古夷平面(图 3).这表明断层活动在青藏高原东缘扩展变形过程中具有重要的调节作用.Zhang等(2016)在玉农希断裂上盘的AHe和ZHe年龄分析结果显示,~35 Ma以来,该区域经历了阶段性快速隆升剥露,速率达0.5~0.7 km·Ma-1,剥露量约4~5 km;且在断裂北侧约100 km内,剥露速率向南(接近断层)增加了近10倍.随后,Cao等(2019)也报道了玉龙断裂带上盘~30 Ma以来的快速隆升剥露.这与龙门山逆冲带的快速隆升剥露(~30 Ma以来)及断层上下盘间显著的差异性剥露具有很好的一致性(Li et al., 2012; Tian et al., 2013; Shen et al., 2019).因此,龙门山—雅砻江—玉龙逆冲断裂带在青藏高原向东挤出过程调节了大量的形变量,不支持“下地壳流假说”强调的“东缘上地壳变形不显著”的论断.

另外,根据10Be约束的流域尺度侵蚀速率,雅砻江和大渡河的侵蚀速率向上游逐渐减小,流经雅砻江逆冲带及鲜水河断裂带一带时可达~5 mm·a-1(Ouimet et al., 2009).Henck等(2011)Yang等(2016)通过10Be和低温热年代学方法对怒江、湄公河和长江流域长时间尺度的侵蚀速率研究发现,在空间上侵蚀量和侵蚀速率向东逐渐减小, 且速率向北(向高原腹地)减小.区域性差异隆升能够很好地解释侵蚀速率的这一空间变化特征.

整体而言,相比龙门山及其西侧地区,鲜水河断裂以南区域保存了较老的年龄(图 3),因此,我们推测这种显著的差异性隆升剥露是对高原东部断裂带活动强弱的反应.在青藏高原向东扩展过程中,由于受扬子地块的阻挡,龙门山逆冲带强烈活动,其东西两侧地块经历了差异性演化过程,龙门山地区强烈缩短隆起,地表剥露显著(Burchfiel and Chen, 2013Wang et al., 2012aTian et al., 2015),早期构造变形记录难以保存;而鲜水河断裂带以南地区(川滇地块)整体经历了相对缓慢的隆升变形过程(图 7),在高原向东扩展过程中大型走滑断裂,如鲜水河断裂和哀牢山—红河断裂以及川滇块体的顺时针旋转调解了大量的变形,夷平面及较老的构造变形历史在块体内部更多的保存了下来(图 3, 7).

图 7 鲜水河断裂带南北两侧的差异变形示意图 Fig. 7 Differential deformation between north and south sides of the Xianshuihe fault
5 结论

青藏高原东缘~40 Ma以来开启了幕式剥露过程,与整个青藏高原抬升具有准同步性.~30 Ma之前的快速剥露主要集中在大型断裂带,如龙门山—雅砻江—玉龙逆冲带和哀牢山—红河剪切带内,而区域广泛的快速剥露事件主要发生在~30 Ma之后; 快速剥露事件及速率的时空特征揭示了区域性大断裂的活动控制着整个高原东缘的最大侵蚀量区, 其中鲜水河断裂带一带为最快侵蚀区.

鲜水河断裂的启动及后期活动控制了高原东缘的剥露过程.以鲜水河断裂为界,南侧区域剥露量较低,夷平面广泛保存,记录了较为完整的高原边界演化历史.

青藏高原向东扩展是多阶段、非均匀过程.新生代以来, 断裂带活动在青藏高原向东扩展过程中调节了大量形变,逆冲带上盘隆升剥露量可达~4 km,不支持“下地壳流假说”强调的“东缘上地壳变形不显著”的认识.

致谢  感谢期刊编辑和审稿人提供的修改建议及意见.
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