岩石学报  2018, Vol. 34 Issue (12): 3467-3478   PDF    
跳出南海看南海——新特提斯洋闭合与南海的形成演化
孙卫东1,2,3,4 , 林秋婷1 , 张丽鹏1 , 廖仁强1,4 , 李聪颖1     
1. 中国科学院海洋研究所深海研究中心, 青岛 266071;
2. 青岛海洋科学与技术国家实验室, 海洋矿产资源评价与探测技术功能实验室, 青岛 266237;
3. 中国科学院青藏高原地球科学卓越创新中心, 北京 100101;
4. 中国科学院大学, 北京 100049
摘要:本文总结了笔者参与基金委重大研究计划"南海深海过程演变"的研究成果。我们发现南海和青藏高原都是新特提斯洋闭合的产物,而非前人所说的南海是由青藏高原碰撞导致的中南半岛逃逸所形成。与青藏高原碰撞隆升机制不同,南海是新特提斯闭合后期弧后拉张的结果。新特提斯洋位于北边的欧亚大陆与南面的非洲、印度和澳大利亚板块之间,呈东宽西窄的喇叭型。在西部,新特提斯洋向北的俯冲可能在侏罗纪就开始了,局部形成了弧后盆。约130Ma前,由于凯尔盖朗等大火成岩省的喷发,新特提斯洋脊也开始向北漂移。由于新特提斯洋东部宽度较大,弧后拉张明显,形成了古南海。新特提斯洋闭合过程中一个重大事件是洋脊俯冲:从菲律宾经福建及两广到青藏高原,均有100Ma左右的埃达克岩产出,是洋脊俯冲的产物。其中,菲律宾、福建、广东埃达克岩形成了斑岩铜金矿床;而在青藏高原,埃达克岩虽有矿化,但没有形成大规模的斑岩铜金矿床。同时期,华南出现了一次短暂的大规模挤压事件,与洋脊俯冲契合。这次挤压事件可能导致了古南海闭合的开始。与此同时,青藏高原冈底斯出现高温岩石——埃达克质紫苏花岗岩;其北面有~110Ma短时间内发生的大规模花岗岩事件。考虑到板块重建的结果,这些埃达克岩和华南短时间挤压事件的时空分布显示新特提斯洋脊在约100~110Ma,近似平行于俯冲带俯冲到了欧亚大陆之下;其前片下沉,扰动软流圈,形成大规模岩浆活动;后片则缓慢后撤,于~80Ma形成了A-型花岗岩。这些A-型花岗岩多属于A2型,受到了还原性板块俯冲的影响而普遍含锡,形成了全球60%的锡矿。俯冲板片的后撤,导致了拉张,可以合理解释南海北缘的"神狐运动"。随着俯冲板片后撤,俯冲角度加大,形成新的弧后拉张,于~33Ma出现洋壳,形成了南海。青藏高原碰撞引起的物质向东、南、北等各方向逃逸,对东亚大陆的构造格局也产生了重要的影响,但是并非南海拉张的主要控制因素。到~23Ma时,东经九十度海岭的俯冲阻挡了青藏高原下方地幔物质向东南方向逃逸,改变了东亚构造格局。同时,由于该海岭俯冲产生的向北东方向的挤压,造成印支半岛向西南挠曲,导致南海洋脊产生向南的跃迁。
关键词: 南海     青藏高原     洋脊俯冲     埃达克岩     A-型花岗岩     锡矿     洋脊跃迁     碰撞    
The formation of the South China Sea resulted from the closure of the Neo-Tethys: A perspective from regional geology
SUN WeiDong1,2,3,4, LIN ChiouTing1, ZHANG LiPeng1, LIAO RenQiang1,4, LI CongYing1     
1. Center of Deep Sea Research, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China;
2. Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237, China;
3. CAS Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Science, Beijing 100101, China;
4. University of Chinese Academy of Sciences, Beijing 100049, China
Abstract: This contribution summarizes our new progresses on the formation of the South China Sea (SCS). Our results indicate that the SCS was formed during the closure of the Neo-Tethys Ocean, rather than extrusion of the Indochina Peninsula induced by the collision between Eurasian and Indian continents as previously proposed. The Neo-Tethys Ocean was located between the Eurasian, African, Indian and Australian continents. It was bell-shaped, opening towards the east. The northward subduction of the Neo-Tethys Ocean may have started as early as the Jurassic in the west. At about 130Ma, the eruption of the Kerguelen large igneous province initiated a new ridge to the south of Indian and, consequently the northward drifting of plates and the old spreading ridge of the Neo-Tethys Ocean. Back-arc extension was more developed in the east because the Neo-Tethys Ocean was wider there. The Proto-SCS was most likely one of the back-arc basins. Ridge subduction was a major event during the closure of the Neo-Tethys Ocean, which resulted east-west ward linearly distributed adakites of 100~110Ma, from the Philippines, Fujian, Canton till to the Tibetan Plateau. Adakites to the east of Canton are closely associated with porphyry Cu deposits, whereas these in the Tibetan Plateau are mineralized, but no major deposits have been discovered so far. Considering plate reconstruction results, the ridge subduction was responsible to the adakites. Meanwhile, there was a widely distributed northward compression in the South China block, which is consistent with the subduction of the Neo-Tethys ridge. We suspect that such compression was responsible to the onset of the closure of the Proto-SCS. Interestingly, adakitic charnockite formed in the Gangdese belt, while large scale magmatic flare occurred to the north of the Gangdese at~110Ma, in the Tibetan Plateau. We propose that the Neo-Tethys ridge was roughly parallel to the subduction zone and subducted underneath the Eurasian Continents at 100~110Ma, resulted in compression and adakites. Magmatic flare occurred due to the sinking of the front (north) limb of the subducted ridge. The south limb of the ridge rolled back later on, forming A-type granites of~80Ma. Most of these A-type granites are A2 type, which have been influenced by subduction of plate with reducing matters and are associated with tin deposits that account for 60% of the world's total tin reserves. Slab roll back resulted in extensions, which plausibly explains the "Shenhu" Movement. After the slab rolled back, flat subduction changed to normal subduction, the SCS formed as a result of back-arc extension during the continuous subduction of the Neo-Tethys plate, which eventually formed oceanic crust~33Ma. The collision at the Tibetan Plateau triggered mantle flows and major changes in the tectonic regime of eastern Asia. Such extension, however, was not the controlling factor for the extension of the SCS. At~23Ma, the subduction of the 90° East Ridge blocked the eastward mantle flow and southeastward extrusion of the Indochina Peninsula. Meanwhile, the subduction of the 90° East Ridge also led to the westward bending of the Indochina Peninsula and consequently the southward ridge jump of the SCS.
Key words: South China Sea     Tibetan Plateau     Ridge subduction     Adakite     A-type granite     Tin deposits     Ridge jump     Collision    

关于南海的成因有多种模型(Taylor and Hayes, 1980, 1983; 姚伯初, 1999; Morley, 2002; 周蒂等, 2002; Sun et al., 2006, 2009; Xia et al., 2006; Xu et al., 2012; Sun, 2016),主要包括:青藏高原碰撞挤出模型(Tapponnier et al., 1982; Briais et al., 1989, 1993);弧后拉张模型(Karig, 1971; Ben-Avraham and Uyeda, 1973; Hilde et al., 1977; 郭令智等, 1983; Stern and Bloomer, 1992; Sun, 2016; Mai et al., 2018);古南海拖曳模型(Holloway, 1982; Taylor and Hayes, 1983);古南海俯冲模型(Wu and Suppe, 2017);海南地幔柱驱动模型(Flower et al., 1998; Xu et al., 2012; Zhang et al., 2018a);地幔上升流模型(Chen et al., 2017);及古南海拖曳+青藏高原挤出等多种过程联合作用的复合模型(Lee and Lawver, 1995; Morley, 2002)。

上述模型中,最著名的是Tapponnier et al. (1982)Briais et al. (1993)等人提出的青藏高原碰撞挤出模型(图 1):沙盘实验显示,印度板块与欧亚板块发生碰撞会导致中南半岛沿哀牢山-红河断裂向南逃逸超过500km(Tapponnier et al., 1990),形成了南海。但是,从哀牢山-红河断裂的发育时代和构造格局看,挤出模型并不吻合。

图 1 青藏高原挤出模型(据Tapponnier et al., 1982Sun, 2016修改) 印度板块与欧亚板块碰撞导致青藏高原东南缘向南逃逸,形成了哀牢山-红河大型走滑断裂,走滑距离达500km以上,导致南海的拉张(Tapponnier et al., 1982) Fig. 1 Extrusion model of Tibetan Plateau (modified after Tapponnier et al., 1982; Sun, 2016) Collision between Indian plate and Eurasian plate resulted in southward escape of southeastern margin of Tibetan Plateau, forming the large Ailaoshan-Red Sea strike-slip fault with a distance of more than 500km, resulting in the extension of the South China Sea (Tapponnier et al., 1982)

沿哀牢山-红河左旋走滑断裂有大量的碱性岩以及以玉龙矿为代表的斑岩铜金矿床(Tapponnier et al., 1990; Chung et al., 1998; Hou et al., 2003; Jiang et al., 2006; Liang et al., 2006, 2007; Hou and Zhang, 2015)。锆石定年显示这些碱性岩主要形成于34~36Ma,暗示哀牢山-红河断裂至少于36Ma时就开始活动(Liang et al., 2007)。事实上,青藏高原的碱性岩并非只沿哀牢山-红河断裂分布,而且在40Ma以前就有产出(Chung et al., 1997, 1998, 2005)。这些年龄早于南海出现洋壳的时代(~33Ma)(Taylor and Hayes, 1980; Li et al., 2014a; Zhong et al., 2018),与青藏高原碰撞挤出模型不匹配。一种解释是南海形成有“滞后性”,即从大陆岩石圈被拉开到出现洋壳需要较长的时间(Liang et al., 2007)。然而,精细分析发现,位于南海西缘的哀牢山-红河断裂是左旋走滑,如果该断裂是造成南海拉张的主要原因,那么南海应该西边先拉开,而且西边比东边应该更宽阔,这与实际情况相反(Sun, 2016)。

正因为如此,有关南海形成演化有很多问题存在争论,如:南海是不是弧后扩张形成的边缘海盆?它是属于太平洋俯冲体系的弧后盆(Karig, 1971; Ben-Avraham and Uyeda, 1973; 郭令智等, 1983; Shao et al., 2017),还是新特提斯洋体系的弧后盆(Hilde et al., 1977; Stern and Bloomer, 1992; Sun, 2016)?海南地幔柱是否对南海的形成起到决定性的作用(Flower et al., 1998; Xu et al., 2012; Yan et al., 2018; Zhang et al., 2018a)?古南海的消亡对南海形成有什么作用?是其俯冲拖曳拉开了南海(Holloway, 1982; Taylor and Hayes, 1983; Hall, 1996; Morley, 2002)?还是在特提斯洋俯冲过程中被动闭合(Sun, 2016)?本文简要介绍笔者近年来在该领域的研究成果,论证南海是在新特提斯洋闭合背景下的弧后拉张盆地。

1 跳出南海看南海

南海位于太平洋板块、印度板块、澳大利亚板块和欧亚板块之间,其形成演化与上述四个板块都或多或少有关(图 1)。要找出南海形成的主控因素必须跳出南海看南海。

南海是全球最大的边缘海。从全球范围看,绝大多数边缘海都与弧后拉张有关。早期有关南海成因的弧后拉张模型认为南海是太平洋俯冲体系的弧后盆(Karig, 1971; Ben-Avraham and Uyeda, 1973; 郭令智等, 1983)。该模型的问题是南海洋壳形成于33~15Ma (Taylor and Hayes, 1980, 1983; Li et al., 2014a; Zhong et al., 2018),其扩张脊呈近东西向展布。太平洋只是在100~50Ma间向北北西俯冲,可以产生近东西向的弧后扩张,而50Ma之后开始向北西俯冲(Sun et al., 2007),弧后拉张方向近似与南海扩张脊垂直。而且在南海和太平洋之间有菲律宾海板块。因此,南海与太平洋板块俯冲之间的时空关系不匹配。

与太平洋板块不同,新特提斯洋一直向北俯冲(Hilde et al., 1977; Stern and Bloomer, 1992; Sun et al., 2018a),可以产生与南海扩张脊方向一致的弧后拉张(Sun, 2016)。但是,由于新特提斯洋壳起始俯冲的时间在早白垩世甚至更早(Sun et al., 2018a),远早于南海的形成时代(33~15Ma)(Li et al., 2014a; Zhong et al., 2018)。此外,新特提斯洋壳现在已经消亡殆尽,其俯冲闭合的细节尚不清楚,因此,新特提斯洋俯冲对南海的形成演化是否起到决定性作用受到很多质疑,弧后拉张的模型一直没有得到承认。要解决这一问题,首先需要研究新特提斯洋的闭合历史。

1.1 正常俯冲

在冈瓦纳大陆北侧与欧亚大陆之间曾经有一个新特提斯洋(Sengör et al., 1984; Stern and Bloomer, 1992; Sun et al., 2018a)。该洋为一个多岛洋并呈西窄东宽的喇叭型。喇叭口与太平洋相通,东部宽达5000~6000km。现在这个大洋俯冲消失了。众所周知,青藏高原就是新特提斯洋闭合后发生碰撞而形成的。

从现有的资料看,新特提斯洋壳可能在侏罗纪就开始向北俯冲。例如在雄村有侏罗纪的斑岩铜金矿床(Qin et al., 2005; Xu et al., 2009, 2017; Tang et al., 2015; Zheng et al., 2015; Lang et al., 2017; Wang, 2017),可能与新特提斯俯冲有关。约130Ma前,由于南半球凯尔盖朗大火成岩省的喷发,新特提斯洋南面出现了新的扩张洋脊,非洲、印度和澳大利亚板块陆续向北漂移,新特提斯洋连同洋脊一起北漂(图 2)。

图 2 印度板块向北漂移速率(据van Hinsbergen et al., 2011) 在100~88Ma期间,印度板块向北运移的速度最小,与埃达克岩(Deng et al., 2017; Li et al., 2017; Ma et al., 2013; Sun et al., 2018a; van Hinsbergen et al., 2011; Wen et al., 2008; Zhang et al., 2010, 2017a)和华南短时间大规模挤压(Li et al., 2014b)近乎同时,指示新特提斯洋脊在此期间俯冲到欧亚大陆下面(据Sun et al., 2018a修改) Fig. 2 The northward drift rate of the Indian plate (after van Hinsbergen et al., 2011) During the 100~88Ma period, the Indian plate moved northward at the lowest speed. Close to the same time as the adakite (Deng et al., 2017; Li et al., 2017; Ma et al., 2013; Sun et al., 2018a; van Hinsbergen et al., 2011; Wen et al., 2008; Zhang et al., 2010, 2017a) and the short-time large-scale compression of the South China (Li et al., 2014b), indicating that the Neo-Tethys ridge subducted beneath Eurasia during this period (modified after Sun et al., 2018a)

在印度板块向北移动的最初20~30Ma间,印度板块向北漂移的速度较低。这是因为新特提斯洋脊仍然活动,而印度板块位于其南面。整个洋脊被新生洋脊推向北方,阻力较大。考虑到此时的新特提斯洋脊并未死亡,与印度板块匹配的洋脊北翼板片的漂移、俯冲速度要远大于印度板块向北漂移的速度。

需要指出的是,在新特提斯洋壳俯冲的初始阶段,俯冲板块的前端老,俯冲是高角度的,属于正常俯冲,可以形成弧后拉张。古南海可能就是这一时期形成的弧后盆。

1.2 洋脊俯冲

印度洋洋脊的发育导致了新特提斯洋脊最终俯冲消亡。要追索消失了的洋脊的运动轨迹需要从岩石学入手。活动洋脊在俯冲过程中会形成一系列特殊的岩石组合,如埃达克岩、富铌玄武岩和A-型花岗岩等(Ling et al., 2009, 2011, 2013)。最近,华南南缘阳春盆地内的石菉铜钼矿床(~105Ma)的研究认为华南南部在白垩纪应是受到了新特提斯板片俯冲的影响(Zhang et al., 2017a)。事实上,在东起菲律宾、西至青藏高原的广大地区发现了近乎同时代的埃达克岩(Qin et al., 2005; Xu et al., 2009, 2017; Tang et al., 2015; Zheng et al., 2015; Lang et al., 2017; Wang, 2017),其形成时代主要在110~100Ma左右。这暗示,特提斯洋脊约于110~100Ma期间近乎平行于俯冲带俯冲到欧亚大陆之下。随后的A-型花岗岩多形成于80Ma左右,与洋脊俯冲后伴随的板片后撤有关(Zhang et al., 2017b, 2018b; Guo et al., 2018)(图 3)。

图 3 新特提斯洋俯冲过程示意图 Fig. 3 Schematic map of the subduction process of the Neo-Tethys Ocean

值得注意的是:到110Ma时,印度板块向北漂移的速度出现小幅降低;到~100Ma时俯冲速度进一步降低;到约88Ma时,印度板块向北漂移的速度大幅度升高(图 2)。与此相对应,在青藏高原先后出现了100~86Ma的埃达克质的紫苏花岗岩和埃达克岩(Wen et al., 2008; Zhang et al., 2010; Ma et al., 2013; Sun et al., 2018a)。如此,从菲律宾、经福建及两广、到青藏高原,均有100Ma左右的埃达克岩产出(Deng et al., 2017; Li et al., 2017; Zhang et al., 2017a)。地球化学特征显示这些埃达克岩表现为俯冲洋壳部分熔融的特点(Zhang et al., 2017a; Sun et al., 2018a)。

印度板块漂移速度的减慢、埃达克岩的出现等现象指示新特提斯洋脊在~100Ma时俯冲到欧亚板块之下了。这种推断得到了构造变形方面的支持。在~105Ma时,华南出现了一次短暂的近南北向大规模挤压事件(Li et al., 2014b)。挤压和埃达克岩的时空分布均与洋脊俯冲契合。古南海是一个很小的洋盆。正常情况下很难产生自发俯冲。新特提斯洋脊俯冲所产生的挤压事件可能导致了古南海闭合的开始。

通常只有年轻(< 25Ma)的洋壳俯冲才能发生部分熔融(Defant and Drummond, 1990),而俯冲洋壳部分熔融往往有利于斑岩铜金成矿(Thieblemont et al., 1997; Sun et al., 2010, 2011, 2012, 2013, 2015b, 2017; Zhang et al., 2017a)。在菲律宾(Deng et al., 2017)、福建(Li et al., 2017)和广东(Zhang et al., 2017a),埃达克岩形成了斑岩铜金矿床。在青藏高原,虽然没有形成大规模的斑岩铜金矿床,但是埃达克岩也有矿化。

成矿作用这种由东向西减弱的分布特点与新特提斯洋向东开口的喇叭型一致。由于新特提斯规模较小,是一个喇叭型的,其间有过多次大洋缺氧事件,其洋壳上有大量富含有机物的沉积物(Jenkyns, 2010),造就了新特提斯构造域这一全球最大的油气储库,油气丰富(甘克文, 2000; Meulenkamp and Sissingh, 2003; 邹才能等, 2015)。因此,俯冲带的氧逸度偏低,洋脊俯冲没有普遍形成大规模的铜金矿床,只是在其东部喇叭口附近,由于比较开阔且与广阔的太平洋联通,有机物含量较低,氧逸度足够高,所以有铜金矿床产出。整体呈现由东向西,斑岩铜金成矿逐渐减弱。

在洋脊俯冲的时候,由于俯冲角度小,并不出现弧后拉张,而是形成挤压。挤压和岛弧火山的缺失是洋脊俯冲的一个重要特征(Sun et al., 2010)——洋脊处洋壳年轻,比老的洋壳含水少、温度高,因此洋脊俯冲通常俯冲角度很小。小角度俯冲导致俯冲板片与上覆板片间的相互作用加强,往往形成造山带。最典型的例子是美洲西海岸。东北太平洋的洋脊自6千多万年起自北向南陆续俯冲到北美大陆之下(Coney and Reynolds, 1977),但是迄今为止,美洲仍然没有出现弧后盆,更没有在俯冲带后面出现玄武质洋壳。取而代之的是先挤压后拉张的构造过程(Sun, 2015; Zhu et al., 2015)。

洋脊俯冲的另一个重要的特征是会出现板片窗,产生高温岩石。在板片断离、下沉等过程中,会在短时间内出现岩浆岩爆发。这些都在青藏高原出现:在约100~86Ma期间,冈底斯出现埃达克质紫苏花岗岩,正是典型的高温岩石(Wen et al., 2008; Zhang et al., 2010; Ma et al., 2013; Sun et al., 2018a),很可能是俯冲板片在扰动的软流圈内发生部分熔融。同时,在冈底斯北面的那曲等一线,有~110Ma的短时间内发生的大规模花岗岩和火山岩事件(Zhu et al., 2009; Sun et al., 2015a; 孙赛军等, 2015)。最合理的解释是当洋脊平行俯冲带完全俯冲下去时,洋脊的前翼板片会下沉,导致板片窗被拉开,扰动软流圈,在短时间内形成大规模岩浆活动。扰动的软流圈导致俯冲洋壳的进一步熔融,形成高温埃达克岩,即埃达克质紫苏花岗岩。

洋脊平行俯冲的情况下,一般在板片窗打开20Ma左右,其后翼板片开始缓慢后撤,形成A-型花岗岩。这些A-型花岗岩往往属于A2型,显示受到了板块俯冲的影响。A-型花岗岩,为洋脊俯冲的典型产物(Li et al., 2012a)。在新特提斯构造域,有大量~80Ma的A-型花岗岩(Cheng et al., 2013, 2016; Xu et al., 2015; Zhang et al., 2017b, 2018b; Guo et al., 2018)。如前所述,新特提斯构造域油气丰富(甘克文, 2000; Meulenkamp and Sissingh, 2003; 邹才能等, 2015),俯冲带的氧逸度低。正是由于氧逸度低,A-型花岗岩普遍含锡,形成了全球60%的锡矿。

2 南海形成演化

洋脊俯冲之后的俯冲板片后撤,导致了弧后拉张,产生A-型花岗岩。这种构造体制可以很好地解释南海北坡的拉张盆地。中生代晚期至始新世(K3-E1)期间,欧亚大陆华南一带向南海北坡延伸的陆缘发生一系列的NE-SW走向构造断裂与岩浆作用(Holloway, 1982; Taylor and Hayes, 1983; Hinz and Schlüter, 1985; Ru and Pigott, 1986; Lee and Watkins, 1998),这个地质事件被命名为“神狐运动”(姚伯初, 1993)。此事件于南海北坡主要发展出三道断陷带,而后逐步发展为一系列的地堑与半地堑盆地,分别位于北部湾、珠江口、琼东南及台西南一带(Li, 1984; Liu, 1986; 姚伯初, 1993; Yao, 1999; 秦国权, 2000; Yan et al., 2001)。与此同时,华南-南海北部陆缘的岩浆活动从钙碱性玄武岩及岛弧安山岩,自晚中生代起至新生代期间逐步转化为双峰式火山岩、拉班玄武岩与碱性玄武岩的过程,也显示拉张的特点。这种拉张与洋脊俯冲后的板片后撤在时空上是契合的。

古地磁资料显示(Lee and Lawver, 1995),礼乐滩在65Ma时位于南海北部、海南岛以东,~19°N处;45Ma时已经向东南方向移动了数十千米;到32Ma时到达17°N,向南漂移了>200km(图 4)。而现今爪哇海沟与加里曼丹之间存在的一系列边缘海盆,包括苏禄海(47~41Ma, Lee and McCabe, 1986; 45~30Ma, Müller et al., 1991)、西里伯斯海(72~65Ma, Lee and McCabe, 1986; 44~42Ma, Shyu et al., 1991; Nichols and Hall, 1999)、班达海(6~3Ma, Honthaas et al., 1998; Hinschberger et al., 2001)等。这里是太平洋与新特提斯俯冲体系交汇处。太平洋向西的俯冲导致了复杂的山湾构造。把一些海盆从东面推过来了。这些海盆都有不同程度的旋转。在35~40Ma以前这些边缘海尚未完全扩张并逆时针旋转到现今的位置。我们推测白垩纪时期,南海及周缘边缘海盆形成之前加里曼丹岛的位置相当靠近中国华南,当时新特提斯洋向北俯冲后曾发生多次后撤,加里曼丹岛南侧保存着白垩纪的蛇绿岩可能就是这个事件的证据之一。亦即现今俯冲带的位置并不能代表当时新特提斯洋俯冲带的位置。显然,南海在~33Ma出现洋壳之前,曾经经历了长时间的拉张,与“神狐运动”拉张特征相符。只是,拉张的速度低于南海扩张期。

图 4 古地磁显示礼乐滩65Ma时位于南海北部~19°N,到32Ma时到达~17°N,向南漂移了>200km(据Lee and Lawver, 1995) Fig. 4 Paleomagnetism shows that Liyuetan was located at ~19°N in the northern part of the South China Sea at 65Ma, reaches ~17°N at 32Ma, and drifts to the south more than 200km (after to Lee and Lawver, 1995)

这些均与我们前期提出的模型(Sun, 2016)一致:即古南海和南海均为新特提斯洋闭合过程中产生的弧后盆。其中古南海是早期俯冲的产物,在洋脊俯冲过程中开始消亡。新南海则是随着俯冲板片后撤,俯冲角度进一步加大,新特提斯板块俯冲再次转变为高角度俯冲,在地幔楔角流的作用下形成新的弧后拉张,最终于~33Ma出现洋壳(Sun, 2016)。

值得指出,印度板块于约60~55Ma时与欧亚大陆发生碰撞(Wang et al., 2008; Ding et al., 2016; Hu et al., 2016; Wang, 2017),碰撞导致了深达地幔尺度的高原物质逃逸(Sternai et al., 2014; Tapponnier et al., 1982),可能也对南海的弧后拉张事件,包括“神狐运动”,有所影响。即地幔流动可能促进南海东部的扩张;但中南半岛向南的逃逸却很可能阻碍了南海西部次海盆的扩张,致使西南次海盆扩张滞后。

3 讨论 3.1 洋脊跃迁

扩张脊向南的跃迁是弧后拉张的重要特征。由于是弧后拉张,扩张脊两侧的受力是不同的。随着拉张的进行,扩张脊距离俯冲带越来越远,地幔楔角流的影响随之逐渐南移,可以导致洋脊向俯冲带的跃迁。

南海扩张的一个重要的特色是洋脊跃迁。目前古地磁数据已识别出南海存在两次洋脊跃迁事件,第一次跃迁事件使东次海盆扩张脊从17°N向南跃迁到15°N附近,跃迁距离达到200km,但是扩张方向未发生明显改变。对于这一次跃迁事件发生的时间,还存在争议,如27Ma (Briais et al., 1993)、24.8Ma (李春峰和宋陶然, 2012)、25Ma (Barckhausen et al., 2014),主要原因是选取的地磁极性年代表(geomagnetic polarity time scale)标准不统一。基于最新的古地磁数据及地磁极性年代表(GPTS2012)(Gradstein et al., 2012),Li et al.(2014a)认为第一次跃迁发生在27Ma左右;第二次跃迁发生在23.6Ma左右。跃迁的距离约20km。但是跃迁使洋脊扩张方向发生改变,由原来的近S-N向变为NW-SE向(Li et al., 2014a)。此时洋脊扩张速率最大,约75km/Myr,这次跃迁时间也代表了西南次海盆洋脊扩张开始的时间(Li et al., 2014a)。最近,Ding et al. (2018)通过对南海北部洋壳内部结构特征的分析,发现下地壳内存在两组共轭的、倾向相向的反射层事件(Low Crustal Reflector Events),结合古地磁特征,认为和南海扩张脊往南发生两次跃迁有关。前人研究表明,洋脊跃迁主要受控于地幔物质活动(Hey and Vogt, 1977; Deschamps and Fujiwara, 2003; Carbotte et al., 2004; Han et al., 2016),洋脊向地幔柱方向跃迁。但是南海地幔柱时代上晚于南海拉张,而且位于南海北部。如果跃迁是由地幔柱活动引起的,应该是向北,而不是向南。南海洋脊向南跃迁与海沟后撤在力学上是吻合的。

3.2 “神狐运动”

“神狐运动”的成因涉及南海北部陆缘的张裂成因、触发机制等,目前仍存在诸多争议。此前主流观点认为华南-南海北部陆缘的岩浆活动,自晚中生代起至新生代期间逐步转化为双峰式火山岩、拉班玄武岩与碱性玄武岩的过程,认为该事件可能与燕山运动太平洋板块后撤有关(Li et al., 2012b; Yan et al., 2014; Zahirovic et al., 2014)。

如前所述,东起菲律宾西至青藏高原的广大地区都发现了~100Ma的埃达克岩(Wen et al., 2008; Zhang et al., 2010, 2017a; Ma et al., 2013; Deng et al., 2017; Li et al., 2017; Sun et al., 2018a)。结合印度板块漂移速度在此时降到最低(图 2),可以推动特提斯洋脊于约100Ma前近乎平行于俯冲带俯冲到欧亚大陆之下。洋脊俯冲过程中,俯冲角度低,俯冲板片可以延伸到内陆。随后,俯冲板片会后撤,俯冲角度增大。这一过程可以合理解释神狐运动,也得到了欧亚大陆南缘东部~80Ma的大规模锡钨成矿的支持(Cheng et al., 2016; Zhang et al., 2017b, 2018b; Guo et al., 2018)。

3.3 碰撞挤出模型

关于南海的形成有很多模型。其中占主导的是青藏高原挤出模型(图 1)。印度与欧亚大陆的碰撞不仅造成了青藏高原的隆升,而且造成了整个东亚地区构造格局的巨变,影响范围北到贝加尔湖,东到太平洋,南到中南半岛(Tapponnier and Molnar, 1976)。沙盘实验显示,青藏高原碰撞可能造成了中南半岛沿哀牢山-红河断裂向南逃逸,从而导致南海的拉张(Tapponnier et al., 1982)。但是,沙盘模型采用的是刚性的材料,而大尺度的变化则是韧性的(Sun, 2016)。

事实上,哀牢山-红河断裂的时代与南海的拉张时代也并不一致。例如,沿哀牢山断裂广泛发育的碱性岩锆石U-Pb年龄在36~34Ma之间,显示哀牢山-红河断裂的活动起始时间比南海打开(玄武岩出现)的时间早约3Myr左右(Liang et al., 2007)。值得注意的是,青藏高原的碱性岩并非只沿哀牢山断裂分布,而且至少40Ma前就有碱性岩(Chung et al., 1998, 2005)。而位于哀牢山-红河断裂上的玉龙等斑岩铜矿最早的形成时代也是42~41Ma (Liang et al., 2006; 陈喜连等, 2016; Lin et al., 2018),比南海洋壳出现的时间早8~9Myr。

一种观点认为,如此大的年代差异说明哀牢山-红河断裂活动不是造成南海打开的原因(Chung et al., 2008)。另一种观点则认为,南海打开晚于哀牢山-红河断裂是”滞后效应”(Liang et al., 2007)——从开始拉张到出现洋壳需要很长的时间。但是,问题的关键在于哀牢山-红河断裂大规模走滑与南海的拉张格局并不配套。

值得注意的是哀牢山-红河断裂停止活动的时间为~23Ma (Tapponnier et al., 1990)。这个年龄明显老于南海停止扩张的时代。哀牢山-红河断裂局部有少量的样品给出了17Ma的Ar-Ar年龄,前人将其解释为显示该断层活动的时期有可能持续到23Ma之后(Leloup et al., 1993, 2001)。但是,Ar-Ar年龄是冷却年龄,并非排他性的断层活动证据。相反,由于只有局部有此年龄,说明至少此时已经没有全断层的剧烈运移。即使是17Ma也老于南海扩张结束的时代。更重要的是,哀牢山-红河断裂位于南海西缘,是左旋走滑,其走滑的结果是造成南海西面先拉开且应该比东边更宽阔。事实上,南海东部次海盆先拉张,西南次海盆后拉张,因此红河-哀牢山断裂并非是南海形成的主控因素(Sun, 2016)。

3.4 23Ma构造事件

~23Ma是一次有全球意义的重要地质事件:(1)中新世与渐新世的界限;(2)哀牢山-红河断裂的活动停止(Tapponnier et al., 1990);(3)喜马拉雅构造带快速隆升(Ding et al., 2017);(4)冈底斯构造带的埃达克岩由陆壳部分熔融型的转变为洋壳部分熔融型的、形成斑岩铜矿的起始时间也是~23Ma (Hu et al., 2015; Sun et al., 2018a);(5)南海洋脊发生了第二次向南的跃迁;(6)这次跃迁伴随着沉积物物源的巨大变化(Li et al., 2003; Wu and Suppe, 2017),与青藏高原的隆升有关。

值得注意的是,虽然发生于23Ma的南海扩张脊第二次跃迁的距离很短(20km),但是造成了洋脊扩张方向的改变(由近S-N向变为NW-SE向)。同时,西南次海盆洋脊开始扩张(Li et al., 2014a)。

所有这些很可能都是一个构造体制下的产物:东经九十度海岭俯冲(Sun et al., 2018b)。洋脊俯冲首先导致冈底斯带出现洋壳部分熔融,形成埃达克岩(Hu et al., 2015; Sun et al., 2018a)。洋壳部分熔融形成的埃达克岩有两个显著的特色:高氧逸度和成矿(Sun et al., 2013, 2015b; Zhang et al., 2017a)。冈底斯埃达克岩的时代和地球化学特征均支持东经九十度海岭的俯冲。从变形的角度,正如沙盘模拟实验显示的(Tapponnier et al., 1982),东经九十度海岭的碰撞,可以导致碰撞处欧亚板块向北挠折;相应地,印支半岛向南西方向挠曲。由于哀牢山-红河断裂没有延伸到南海扩张脊南缘,即南海洋脊南缘与中南半岛是相连的。因此,中南半岛向南西的挠曲可以合理解释南海扩张脊的小距离跃迁和扩张方向的改变,也可以解释西南次海盆开始拉张等事件。

东经九十度海岭俯冲也可以解释~23Ma青藏高原南缘快速隆升事件:厚大的刚性洋脊俯冲到软流圈,阻挡了青藏高原下方地幔物质逃逸,导致青藏高原南缘快速抬升(Sun et al., 2018a)。

这种区域的事件与渐新世-中新世界线同时,是偶然还是必然,值得深入研究。一个最可能的原因是23Ma事件导致了印尼水道的深水区关闭,西太平洋暖池形成。

4 结论

青藏高原和南海都是新特提斯洋闭合的产物。青藏高原是印度板块与欧亚板块碰撞的产物,而南海则是新特提斯洋向北俯冲所形成的弧后盆。新特提斯洋脊俯冲形成了埃达克岩,导致了古南海的闭合。印度板块与欧亚大陆碰撞引起的东南亚半岛向南逃逸,并非南海形成的主要原因。一些青藏高原的重大事件在南海有反应。其中23Ma,青藏高原快速隆升和南海洋脊跃迁及物源的改变均与东经九十度海岭的俯冲有关。

致谢      感谢李曙光院士和朱日祥院士提出的建设性意见。

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