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板块会聚边界发育岩浆弧是普遍地质现象,然而鲜见同一俯冲系统先后发育两条岩浆弧,台湾-吕宋双火山弧即是这样的实例(图1)。在台湾岛和吕宋岛之间近500km的范围内,发育两条火山弧,即西弧(WVC)和东弧(EVC)。西弧的活动时间早于第四纪,而东弧为第四纪时期的产物。当含水的大洋岩石圈进入俯冲隧道,到达一定深度后将发生脱水作用,从而降低了部分熔融的温度,形成岩浆。岩浆向上运移,侵入到上覆岩石圈或者直接喷出,进而形成岩浆弧。台湾-吕宋之间两条火山弧的俯冲系统相同,即南海-菲律宾俯冲系统。在这套俯冲系统中,什么样的特殊机制控制着岩浆房在水平方向上的位置变化呢?该问题是解答台湾-吕宋双火山弧成因的关键。
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根据火山空间分布特征、活动时限,并结合大量岩石地球化学数据(Richard, 1986; Defant et al.,1990; Yang et al.,1995),Yang et al.(1996)提出了台湾-吕宋双火山弧概念并指出了可能的演化模型。考虑到无震洋脊具有更大的浮力,能够影响俯冲板片形状 (Vogt, 1973; Kelleher et al.,1976; Vogt et al.,1976),Yang et al.(1996)认为南海洋中脊的俯冲减小了俯冲板片角度进而迫使火山弧东移,从而形成西、东两条火山弧。Bautista el al.(2001)认为南海20°N左右的板块为轻质板块,其表现为马尼拉海沟的强烈弯曲、磁静区和重力负异常(Taylor et al.,1980),由此提出了台湾-吕宋双火山弧的第二种形成模式。他同意是轻物质的俯冲导致了板片角度的减小进而使得火山弧东移,但认为是南海20°N左右的轻质物质导致俯冲角度减小而非第一种模式强调的南海洋中脊。作为南海-菲律宾俯冲系统演化的产物,台湾-吕宋双火山弧的形成过程必然受控于该俯冲系统的演化。近20年来,无论对南海扩张历史、俯冲板片的物质组成和几何样式还是上覆菲律宾板块的运动学等方面均有长足的进步,这为重新解释台湾-吕宋双火山弧形成机制这个老科学问题打开了新的窗口。
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图1 台湾-吕宋双火山弧及其邻区构造背景
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Fig.1 Tectonic setting of Taiwan-Luzon double volcanic chain and its adjacent areas
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(a)—图中红色和黑色三角形组成了东火山弧(EVC),前者为活火山,后者为第四纪死火山;黑色圆点组成了西火山弧(WVC),为前第四纪火山活动点(Yang et al.,1996)。黄色虚线表示不同作者认定的大陆和海洋分界(Continent-Ocean Boundary,COB)。黄色实线表示磁异常条带,编码为异常带标号(Li Chunfeng et al.,2014);红色虚线为推测的古洋中脊位置,由北向南经过两次跳跃(Ding Weiwei et al.,2018)。点线为推测的古洋中脊3的位置;箭头为菲律宾板块和欧亚板块的会聚方向;10Ma时,会聚方向由NNW转为NW,同时会聚速率从55mm/a增加到82mm/a(Sibuet et al.,2021);地形底图数据来源ETOPO1(https://www.ngdc.noaa.gov/mgg/global/);(b)—菲律宾板块的主要组成和洋壳年龄;DRP—大东脊省; L—吕宋; MT—马里亚纳海沟; PB—帕劳盆地; PVB—帕里西维拉盆地; SB—四国盆地; WPB—西菲律宾盆地
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(a)—The East Volcanic Chain (EVC) is showed by red and black triangles.The former are active volcanoes and the latter are Quaternary extinct volcanoes.The West Volcanic Chain (WVC) is showed by black dots, which are the places of pre-Quaternary volcanic activity (Yang et al.,1996).The dashed yellow lines indicate the inferred continent ocean boundary (COB).The solid yellow lines represent the magnetic anomaly band, and the strings are the anomaly band labels (Li Chunfeng et al.,2014).The dashed red lines are the inferred location of the paleo-mid-ocean ridge, which has undergone two jumps from north to south (Ding Weiwei et al.,2018).The dotted red line is the inferred location of paleo-mid-ocean ridge3.The arrow indicates the convergence direction of the Philippine Plate and the Eurasian plate.At 10Ma, the convergence direction changed from NNW to NW, and the convergence rate increased from 55mm/a to 82mm/a (Sibuet et al.,2021).Topographic map data source ETOPO1(https://www.ngdc.noaa.gov/mgg/global/).(b)—seafloor age of the Philippine oceanic plate; DRP—Daito ridges province; L—Luzon; MT—Mariana Trench; PB—Palau basin; PVB—Parece Vela basin; SB—Shikoku basin; WPB—West Philippine basin
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本文在被动俯冲的动力学背景下,构建了二维数值对比模型,试图验证被动俯冲作用对减小俯冲角度的重要性。综合近年来的多学科研究成果,提出了台湾-吕宋双火山弧可能的新演化模式和主导机制,将有助于推进对该双火山弧和该俯冲系统的理解,也能够为认识其他被动俯冲过程提供参考实例。
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1 区域构造背景
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南海东部次海盆沿马尼拉海沟俯冲到菲律宾板块之下,台湾-吕宋双火山弧发育在上覆的菲律宾板块西缘。双火山弧绵延近500km近南北向展布于台湾岛和吕宋岛之间,西距马尼拉海沟100~200km不等(图1)。南海东部次海盆组成结构及其演化历史复杂。普遍认为,其北部为洋-陆过渡转换区,而南部的洋壳经历了多阶段扩张过程。面积更大的上覆菲律宾板块的生长过程和运动学更为复杂多变。下文将对这些区域展开叙述。
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1.1 南海次海盆北部洋-陆转换区
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俯冲物性的差别直接制约着俯冲过程。重的大洋壳容易发生自发深俯冲,而轻的大陆壳需要在前期俯冲大洋的拖拽作用下才能发生俯冲。所以厘定洋-陆转换边界是准确分析俯冲过程的关键。通过分析磁异常数据,前人在揭示南海次海盆扩张过程的同时认定洋-陆边界线在19°N左右(Briais et al.,1993)(图1)。从中国大陆边缘到南海次海盆北部的地震深反射剖面揭示了洋陆转换带厚度介于11~15km而宽度超过200km,其南界(即洋-陆边界)在19°N附近(Wang et al.,2006)。宽角反射数据揭示薄的洋陆转换带的南边界在20°N以南(McIntosh et al.,2013)。台湾西南部的近东西向磁异常条带揭示南海北部21.5°N以南为洋壳(Hsu, 2005)(图1)。Eakin et al.(2014)结合了前人的数据(Lester et al.,2012; McIntosh et al.,2013),校正了Briais et al.(1993)的部分认识,提出洋-陆边界应该在其偏北侧(图1)。层析成像数据进一步表明洋-陆边界在Eakin et al.(2014)所提位置的稍北侧(Wu et al.,2016; Sibuet et al.,2021)(图1)。未来必然会有更多更精确的观测数据校正目前的认识,但可能难以改变一个事实:不同作者应用不同方法得出的洋陆边界逐渐趋于统一(图1)。如此一来则意味着进入马尼拉俯冲带的大陆岩石圈可能很有限,集中在21°N左右。
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1.2 南海东部次海盆的扩张过程
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俯冲大洋岩石圈的年龄控制着其厚度、浮力,从而制约着俯冲过程的发展演化。故而有必要考察南海东部次海盆的形成年龄及相应的扩张过程。大洋扩张是板块构造的核心之一,厘定大洋扩张史的最重要手段是海底磁异常观测。当然,磁异常的实用价值不限于约束大洋扩张(Song Chengke et al.,2022)。早在20世纪70年代,人们在南海海盆就发现了磁异常条带(Ben-Avraham et al.,1973)。与其他大洋扩张过程类似,南海东部次海盆的扩张并非简单地由一条洋中脊完成,相反则是先后在多条洋中脊作用下才完成了全部扩张过程。由于数据精度偏低和采用的地磁极性年代表不统一,早年人们对南海东部次海盆洋中脊的跳跃时间有不同的观点(Briais et al.,1993; 李春峰等, 2012; Barckhausen et al.,2014)。基于GPTS2012标准,Li Chunfeng et al.(2014)认为在27Ma左右和23.6Ma左右各发生了一次洋脊跳跃事件。另外,多道地震探测所揭示的南海东部次海盆下地壳倾向洋中脊的反射特征可能由洋中脊扩张产生(Ding Weiwei et al.,2018),这进一步佐证了Li Chunfeng et al.(2014)两次跳跃的观点,相应的洋壳扩张历史如图2所示(注意,只显示出了扩张系统的一翼)。图2清楚地展示了南海东部次海盆扩张过程,32Ma开始扩张,15Ma停止扩张,其中洋中脊经历两次向南跳跃。
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1.3 菲律宾板块的演化
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以大洋岩石圈为主体的菲律宾板块,最老的岩石圈年龄为60Ma(图1b)。由图1b可见菲律宾大洋岩石圈年龄分布明显呈现两阶段演化的特征,西菲律宾海盆明显早于其东部海盆的演化。马努斯地幔柱可能促使西菲律宾的开始扩张(Wu et al.,2016)。西菲律宾大洋岩石圈年龄说明其扩张持续到30Ma。30Ma后帕里西维拉盆地东侧扩张,一直持续到现今。所以南海-菲律宾俯冲系统的上覆板块最年轻年龄为30Ma左右。
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图2 南海东部次海盆扩张过程示意图
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Fig.2 Simplified schematic diagram showing the spreading history in eastern South China Sea
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黑色三角形和代码表示磁异常条带的位置和编号,与图1对应;LCR为下地壳反射面(修改自Ding Weiwei et al.,2018)
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Black triangles and numbers represent the positions and number of magnetic anomaly bands, corresponding to Fig.1; LCR is the seismic reflector of lower crust (modified from Ding Weiwei et al.,2018)
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菲律宾板块大体呈现北西向运动的特征。40Ma以来向北运动的平均速率约为6cm/a。古地磁方法是常用的约束历史纬度信息的手段,然而目前的数据整体比较离散(Wu et al.,2016),另外由于古地磁不能约束经度信息,所以目前对菲律宾板块运动历史仍然没有较统一的认识。最新的观点认为,在10Ma,会聚方向由NNW转为NW,同时会聚速率从55mm/a增加到82mm/a(Sibuet et al.,2021)。
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2 台湾-吕宋双火山弧的特征
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众多火山呈廊带状分布在台湾岛到吕宋岛近500km的范围内。这些火山的空间分布并不均匀,而是从北向南逐渐分叉,18°N处的间距可达50km(图1)。西侧分支称为西弧,东侧分支称为东弧。地貌上,东弧的火山更为陡峭,而西弧的火山地形更为平坦,说明西弧接受了更多的外动力地质作用。活动性上,东弧的南部尚有活火山,而西弧火山早已死亡。前人仔细研究了东弧和西弧的最新活动时间(Richard, 1986; Defant et al.,1990; Yang et al.,1995, 1996)。我们依据这些数据绘制了火山分布位置与年龄的关系图(图3)。尽管个别数据点的活动时间跨度较大,但依然可以清楚地看到东弧形成时间为第四纪,而西弧为前第四纪的产物。年龄分布与地貌特征可以对应。
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3 台湾-吕宋双火山弧的形成机制
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3.1 形成机制一
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Yang et al.(1996)率先提出了双火山弧的形成演化模式。该模式分为三阶段,其中前两个阶段为西弧的演化过程,第三阶段为东弧的演化过程。第一阶段(＞6Ma),伴随着沿马尼拉海沟的会聚-俯冲,南海向菲律宾板块之下俯冲,形成台湾-吕宋西弧。在此阶段,台湾-吕宋弧逐渐靠近欧亚板块的边缘,而南海古洋中脊暂未与西弧接触。第二阶段(5~4Ma),南海洋中脊与西弧接触。洋中脊更大的浮力导致其难以进入俯冲隧道,进而加积到上覆板块的边缘。与此同时,西弧的北部与欧亚板块碰撞,进而造成了地形抬升(Liew et al.,1990; Liu et al.,1990; Chen et al.,1992, 1993)。第三阶段(＜2Ma),部分洋中脊在上覆板块的刮擦作用之下加积到板块边缘,而剩余部分洋中脊在俯冲板块的持续拖拽之下克服了刮擦作用从而进入俯冲隧道。轻质的洋中脊进入俯冲隧道后将减小俯冲角度,从而导致台湾-吕宋东弧开始形成(Yang et al.,1996)。可见,导致台湾-吕宋西弧东移的直接原因是轻质洋中脊俯冲后导致的板片俯冲角度减小。另一方面,从北向南,西弧和东弧的间距逐渐增大的原因是火山弧与欧亚板块的碰撞作用导致的菲律宾板块顺时针旋转(Yang et al.,1996)。
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图3 台湾-吕宋双火山弧活动年龄分布图(短实线为火山活动时间范围;数据来源: Defant et al.,1990; Richard M, 1986; Yang et al.,1995, 1996)
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Fig.3 Distribution diagram of activity ages of the double volcanic chain between Taiwan and Luzon islands (the short bars denote the time range of volcanic activity; data from: Defant et al.,1990; Richard M, 1986; Yang et al.,1995, 1996)
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3.2 形成机制二
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注意到20°N附近的轻质高原俯冲于台湾-吕宋弧之下,Bautista et al.(2001)认为该轻质地壳对俯冲板片变缓的贡献更大,提出该轻质地壳的俯冲是形成台湾-吕宋东火山弧的直接原因。随着该轻质高原的俯冲,俯冲板片的角度逐渐变小,岩浆作用位置东移,导致西弧停止活动而在东部形成新的岩浆弧——台湾-吕宋东弧(图4)。与模型一相比较,模型二也认同板片的俯冲角度需要变小才能致使岩浆弧东移,并且俯冲角度变小均是因为某种轻质物质俯冲所导致。不同点在于,模型一中俯冲的轻质物质是洋中脊而模型二中俯冲的轻质物质是20°N左右的轻质高原。若轻质物质进入俯冲隧道势必会造成俯冲角度变小。更准确地说,模型二认为轻质高原的俯冲对俯冲角度减小的贡献较模型一中洋中脊的贡献大,且足以使得相应的岩浆弧东移。从定量的角度上,一个至关重要的科学问题是轻质物质俯冲导致俯冲角度减小的幅度是否足够将岩浆弧东移。
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3.3 轻质物质俯冲对俯冲板片角度变缓的贡献
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除了台湾-吕宋双火山弧,前人希望轻质物质的俯冲能够解释秘鲁中部(3°S~15°S)和智利中北部(27°S~33°S)的平俯冲现象(Gutscher et al.,2000; Yañez et al.,2001)。然而二维和三维数值模拟研究并不支持这样的观点(Martinod et al.,2005; Gerya et al.,2009)。二维模型意味着在第三维无限延伸,二维模型中的洋底高原在三维中意味着平行于海沟无限长,这显然比实际中的洋底高原规模大的多,如果进入俯冲隧道则势必提供更多浮力。尽管如此,轻质高原的俯冲不能明显地减小俯冲板片角度(Vogt et al.,2014; Yang Shaohua et al.,2018)。是否因为洋底高原的规模不足够大?目前并不能排除这种可能性,但是需要注意到一个基本事实:规模很大的轻质洋底高原难以进入俯冲隧道。综上,轻质物质俯冲导致俯冲板片变缓的幅度可能比较有限,需要更有效的机制减小俯冲角度进而推动台湾-吕宋西弧向东迁移而形成东弧。
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图4 轻质高原发生俯冲导致火山弧东移的概念图 (随着板片倾角减小,西火山弧停止活动而东火山弧开始形成;修改自Bautista et al.,2001)
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Fig.4 Conceptual chart of the eastward migration of volcanic chain caused by subduction of light plateau (with the decrease of slab dip angle, the activity of the West Volcanic Chain (WVC) ceased and the East Volcanic Chain (EVC) began to be formed; modified from Bautista et al.,2001)
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3.4 主动俯冲和被动俯冲对俯冲板片角度的影响
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传统俯冲模式中,人们一般将上覆板块视为固定状态,而俯冲板片沿海沟发生俯冲运动。实际上,自然界中存在相反的情况:俯冲板块位移很小,而上覆板块向海沟方向运动,例如菲律宾板块向南海方向快速运动。前者称为主动俯冲,后者称为被动俯冲。如果所有涉及俯冲过程的参数(包括会聚速率)均相同,仅仅是主动俯冲和被动俯冲的差异,相应的演化过程是否有明显差别?我们建立了一对数值模型考察主动/被动俯冲所导致的俯冲过程差异。每个初始模型包括一个大洋板块和一个上覆的大陆板块(图5a),模型大小为4000km×660km。除了会聚方向不同(会聚速率为5cm/a),所有参数均相同(其中大洋岩石圈年龄为40Ma), 物理方程和材料参数取值与前人文中取值保持一致(Yang Shaohua et al.,2018)。其中一个模型为从左侧向右推挤俯冲板片,另一个模型从右侧向左推挤上覆板块。前者的演化结果如图5b~d所示,后者的演化结果如图5e~g所示。
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图5 二维高精度数值模型的初始模型及其演化结果
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Fig.5 The configuration and evolution of 2-D high-resolution numerical model
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(a)—初始模型配置(局部);(b、d)—大洋板块在左侧推挤下以5cm/a的速率向右会聚-俯冲的演化过程,其中右侧的上覆板块固定不动; (e、g)—大洋板块固定不动,上覆大陆板块在右侧推挤下以5cm/a的速率向右发生会聚-俯冲的演化过程
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(a)—Initial model; (b, d)—the subduction process of the oceanic plate at the rate of 5cm/a under the left-lateral push, with the fixed overriding plate;(e, g)—with the fixed oceanic plate, the subduction process at the rate of 5cm/a under the right-lateral push
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两个模型在初始阶段的演化样式非常相似(图5b、e),然而后续的演化过程发生急剧变化。当上覆板块固定,大洋板块向右会聚的过程中,俯冲角度整体大于45°,板块发生了陡俯冲(图5c、d)。而当大洋板块固定,大陆板块向左会聚的过程中,俯冲角度很小,板块发生了平俯冲(图5f、g)。前者伴随发生大规模水化地幔的部分熔融,并发育有弧后盆地。后者没有部分熔融的发生,而是表现为冷结构,从而抑制了熔融的发生。可见,会聚方向(或者主动/被动俯冲)是俯冲系统至关重要的因素,被动俯冲作用能够有效减小俯冲角度,在南海-菲律宾俯冲系统中,有必要进一步分析被动俯冲作用。
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3.5 被动俯冲背景下大洋岩石圈年龄和会聚速度对俯冲过程的控制作用
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如前文所述,被动俯冲背景下可以减小俯冲板片角度,故而有必要考察此过程的细节。有两个重要因素控制着被动俯冲时俯冲角度的变化:① 俯冲大洋岩石圈的年龄;② 会聚-俯冲的速率。俯冲大洋岩石圈年龄越小,则俯冲角度越小,其中平俯冲普遍出现的大洋岩石圈年龄为40Ma;会聚-俯冲速率越大,则俯冲角度越小,若会聚速率大于等于4cm/a、大洋岩石圈年龄为40Ma,则普遍出现平俯冲(Yang Shaohua et al.,2018)。
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海底磁异常条带研究揭示南海东部次海盆大洋岩石圈年龄小于33Ma(Li Chunfeng et al.,2014),显然该年龄使得东部次海盆容易发生小角度俯冲。尽管由于洋中脊跳跃导致大洋岩石圈年龄并非线性变化,但是总体趋势依然是从最新的洋中脊(图2中古洋中脊3)向北大洋岩石圈年龄越来越老。所以越靠北越难以发生小角度俯冲。
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目前,沿马尼拉俯冲带的会聚速率可达8cm/a(Sella et al.,2002; Kreemer et al.,2003)。结合大洋岩石圈年龄特征来看,南海东部次海盆容易发生小角度俯冲。地质历史上的会聚速率往往借助古地磁观测进行约束,比如古地磁方法可以很好地约束印度-欧亚会聚速率(Torsvik et al.,2012; van Hinsbergen et al.,2012)。古地磁研究认为菲律宾向北运动的平均速率约为6cm/a(Queano et al.,2007; Yamazaki et al.,2010; Wu et al.,2016)。但是注意到古地磁方法本身的限制(只能约束纬度信息),同时由于菲律宾板块东西向运动分量不可忽视,使得较准确地得到沿马尼拉俯冲带的会聚速率比较困难。但可以肯定的是,会聚速率的变化过程是从零开始波动变化到目前的大小。
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3.6 被动俯冲背景下菲律宾板块北西向加速运动——东火山弧的形成原因
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控制台湾-吕宋双火山弧形成的因素可分为两部分:① 导致俯冲角度减小的因素;② 导致东弧、西弧向南逐渐分叉的因素。由1.1节可知,进入马尼拉俯冲带的大陆岩石圈可能很有限,参与主要俯冲过程的为大洋岩石圈。被动俯冲初期,菲律宾板块以较低的速率与欧亚板块逐渐会聚,南海东部次海盆发生俯冲。低会聚速率下发生高角度俯冲,诱发地幔部分熔融,进而导致台湾-吕宋西火山弧的形成。
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随着会聚速率迅速增大,俯冲角度变缓致使热源东移,西弧在冷结构的抑制下停止活动,而东移的热源诱发地幔部分熔融,进而形成新的火山弧——台湾-吕宋东弧。由于从南海次海盆最新的洋中脊向北,大洋岩石圈年龄整体越来越大,相应地将导致俯冲角度减小的幅度越来越小,这意味着热源东移的距离越来越小,从而使得形成的东弧与西弧呈分叉特征,即南部间距大而北部间距小(图6)。与此同时,由于从南到北俯冲角度减小的幅度越来越小,那么俯冲板片将表现出南缓北陡的俯冲形态,这一推断得到了地震层析成像结果的印证(Fan Jianke et al.,2016)。
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与台湾-吕宋双火山弧发育时期有时间重叠的另一重要事件是台湾岛与西弧的碰撞。弧-陆碰撞大约发生在9~6.5Ma(Lin et al.,2003; Tensi et al.,2006; Sibuet et al.,2021)。前人认为该弧-陆碰撞导致上覆的菲律宾板块顺时针旋转(Yang et al.,1996)。台湾-西弧碰撞的体量显然与广阔的菲律宾板块难以比较,单纯台湾-西弧碰撞是否能直接导致菲律宾板块旋转仍然存疑,然而菲律宾板块运动学特征确实表现出了顺时针旋转特征(Suppe, 1981; Mesalles et al.,2014; Lee et al.,2015; Sibuet et al.,2021)。如果以弧-陆碰撞点为轴菲律宾板块发生可观的顺时针刚性旋转,则可能促进形成东弧与西弧的分叉。原因是,如果以弧-陆碰撞点为轴发生刚性旋转,那么距碰撞点远的地方会聚速率大而近的地方会聚速率小(即双火山弧南部会聚速率大北部会聚速率小),会聚速率越大则俯冲角度越小,即意味着南部间距大而北部间距小。从而形成东弧与西弧的分叉特征。
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图6 台湾-吕宋双火山弧形成机制概念图 (V为被动俯冲的会聚速率)
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Fig.6 Conceptual chart of the formation mechanism of Taiwan-Luzon Island double volcanic Chain (V denotes the convergence rate of passive subduction)
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目前难以进一步确定大洋岩石圈年龄向北增加和菲律宾板块旋转这两个因素对东弧-西弧分叉的形成哪个占主导或者作用相当。可能需要更定量的手段评价二者对东弧和西弧分叉的贡献。基本确定的是,菲律宾板块北西向加速运动导致了东弧的形成。
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4 结论
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台湾岛与吕宋岛之间发育两条火山弧,其成因尚有争论。本文综合了前人多学科的研究成果,结合高精度二维数值对比模型,得出以下结论:
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(1)前人提出的轻质物质(包括洋中脊和轻质高原)沿马尼拉海沟向菲律宾板块之下的俯冲作用可能不足以推动俯冲板片角度减小进而形成台湾-吕宋双火山弧中的东弧。
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(2)与主动俯冲相比,适当条件下的被动俯冲作用能够有效减小俯冲角度,并不需要俯冲的轻质物质提供额外浮力。南海-菲律宾俯冲系统的特征(南海东部次海盆的年龄、会聚-俯冲速率)满足发生低角度俯冲的要求。
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(3)在被动俯冲背景下,菲律宾-欧亚板块的低速会聚形成高角度俯冲从而导致西弧的形成;菲律宾板块北西向加速运动导致俯冲角度减小,西弧停止活动而东弧开始形成。
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(4)导致西弧和东弧分叉的可能原因是大洋岩石圈年龄向北增加和(或)菲律宾板块旋转。
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摘要
台湾-吕宋双火山弧绵延近500 km,南北向展布于台湾岛和吕宋岛之间,其成因尚无定论。前人将其归因于轻质物质(南海洋中脊或南海北部轻质高原)的俯冲,然而数值模拟研究并不能有效支持该观点。随着欧亚-菲律宾板块的会聚,南海东部次海盆被动俯冲于菲律宾板块之下。为了验证被动俯冲和主动俯冲对俯冲角度的控制作用,本文构建了二维高精度数值对比模型,在其他因素完全相同的情况下,考察会聚方向的差异对俯冲角度的影响。模拟结果显示,仅仅改变会聚方向,俯冲角度就能够发生巨大变化。在被动俯冲背景下如果条件合适则易于发生低角度俯冲。综合前人研究成果可以发现,南海-菲律宾俯冲系统的条件满足发生低角度俯冲的要求,表现在两方面:① 南海东部次海盆的年龄从最新的洋中脊向北波动增加,最老为33 Ma,小于低角度俯冲洋盆年龄要求的40 Ma;② 欧亚-菲律宾板块的会聚速率大于低角度俯冲速率要求的4 cm/a。由此,本文认为菲律宾-欧亚板块在被动俯冲背景下的低速会聚形成了高角度俯冲从而导致西弧的形成;之后菲律宾板块北西向加速运动导致俯冲角度减小,西弧停止活动而东弧开始形成。本文模型不需要俯冲的轻质物质提供额外的浮力进而促使板片角度减小。西弧和东弧存在分叉现象,我们认为原因可能是洋壳年龄向北增加和(或)菲律宾板块旋转。本文提出了台湾-吕宋双火山弧另一种可能的演化机制,有助于推进对该双火山弧和该俯冲系统的理解,也能够为认识其他被动俯冲过程提供参考实例。
Abstract
The Taiwan-Luzon double volcanic chain stretches for nearly 500 km and locates between Taiwan Island and Luzon Island in a north-south direction. Its origin has not been determined. It is attributed to the subduction of light materials (mid-ocean ridge of the South China Sea or light plateau in the northern part of the South China Sea). However, numerical modeling does not effectively support this viewpoint. Along with convergence of the Eurasian-Philippine Plate, the eastern sub-basin of the South China Sea was passively subducted beneath the Philippine Plate. In order to verify the control of passive subduction and active subduction on the subduction angle, two-dimensional high-precision numerical models are constructed in this study. The influence of convergence direction on the subduction angle is investigated. The simulation results suggest that the subduction angle can change dramatically only by transferring convergence direction. Under passive subduction, low-angle subduction occurs readily if conditions are appropriate. Based on a large number of previous multidisciplinary studies, it can be determined that the conditions of the South China Sea-Philippines subduction system meet the requirements of low-angle subduction, which is manifested in two aspects: ① the age of the eastern South China Sea sub-basin decreases northward from the latest mid-ocean ridge, with the oldest of 33 Ma, less than the 40 Ma required for low-angle subduction; ② the convergence rate of Eurasian-Philippine plate is higher than 4 cm/a required by low-angle subduction. Therefore, this study proposes that the low-rate convergence of the Philippine-Eurasian plate against the background of passive subduction formed a high-angle subduction, which led to the formation of the western arc. After that, the northwestward accelerated motion of the Philippine Plate resulted in the reduction of the subduction angle, thereby the activity of the western arc stopped and the eastern arc began to form. The new proposed model does not need the subducted low-density materials to provide additional buoyancy to reduce the subduction angle. The two arcs are separated in the south by 50 km. We suggest that the potential reasons are that the oceanic crust age increases northward and (or) the Philippine plate rotates about Taiwan. Follow up work is needed to further confirm, falsify or quantitatively investigate their contributions. In this study, a evolution mechanism of Taiwan-Luzon double volcanic chain is proposed, which may be helpful to facilitate the understanding of the double volcanic chain and the subduction system, and can also provide an example for understanding other passive subduction processes.
