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莫霍洛维奇不连续面(Mohorovičić discontinuity)是地壳与地幔的分界面,一般出现在陆壳之下约30~40km,在洋壳之下约6~7km。近60多年来,科学家从未放弃打穿莫霍面的梦想。美国于20世纪50年代末启动了莫霍面钻探计划(Mohole Drilling Project),目的是要钻透莫霍面,揭开地壳下面地幔的秘密,1961年4月美国CUSS-I钻探船在墨西哥湾西部Guadalupe island附近海域(图1)水深3600m处,首次成功钻井,在170m沉积层下取得了14m长的玄武岩岩芯(Bascom et al., 1961),迈出了向莫霍面进军的第一步。21世纪初,日本投入巨资建造了“地球”号大洋钻探船,理论上可在4000m水深的海域向海底钻进7000m,但十多年来,由于船体庞大,带来了成本、运行和管理等诸多问题,“地球”号实际上才打了3000m(Ildefonse et al., 2007, 2010)。此后,由于预算、技术和管理方面的问题,计划被迫中止。2015年,美国“JOIDES Resolution”大洋钻探船在西南印度洋中脊Atlantis Bank(图1)再次实施了莫霍钻航次,但最终仅向海底深处钻进789.7m,获得469.7m岩芯,离莫霍面相差甚远。我国正在建造全球领先的水合物钻采船(大洋钻探船),该船的首次深海钻探计划在中国南海进行(Qin Xuwen et al., 2019),有望率先实现人类钻遇莫霍面的梦想。
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图1 全球莫霍面深度变化图及已实施莫霍钻探计划位置(来自CRUST1.0模型,据Laske et al., 2013)
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Fig.1 Global Moho depth variation from the CRUST1.0model (after Laske et al., 2013) and the locations of the Moho drilling program has been implemented
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海洋岩石圈深部结构,包括莫霍面的性质(蛇纹石化蚀变界面还是火成岩地壳-地幔过渡界面),上地幔的组成、结构,壳幔的物质循环等依旧存在诸多疑问,钻透地壳依旧是解答这诸多谜团的关键。深海海底是离地球内部最近的地方,从深海海底打钻,至今还是人类直接探测地球内部无可替代的最佳选择。南海被认为是世界上最容易钻遇莫霍面的海域之一,目前已有研究显示南海海盆区洋壳最薄处远低于全球洋壳平均值(Yu Junhui et al., 2018)。因此,南海莫霍面研究具有重要的科学意义和钻探选址意义。
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南海莫霍面深度及其特征是了解该区深部地壳结构的有效途径,对认识南海构造演化及深部动力学机制有着重要意义(胡立天等, 2016;吴招才等, 2017)。南海深部地壳研究多集中在南北陆缘(如夏少红等, 2010;Niu Xiaowei et al., 2014;Lü Chuanchuan et al., 2016),相对于有大量钻井资料(包括IODP)及丰富多道地震数据的南、北陆缘,对南海深海盆的研究由于地球物理数据的缺乏,研究程度相对较弱(丁航航等, 2019)。目前对南海海盆区莫霍面的研究主要基于重力反演(秦静欣等,2011;郝天珧等,2014;胡卫剑等,2014;杨胜雄等,2015;胡立天等,2016;吴招才等, 2017),研究普遍认为南海海盆区莫霍面深度在8~14km之间,洋壳厚度在3~9km之间。但基于重力反演的莫霍面分辨率较低,无法揭示莫霍面的精细结构。
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深地震反射剖面可以直接揭示岩石圈的构造形态,包括盆地结构、壳内反射和莫霍面深度等(赵明辉等, 2011;李三忠等, 2012),是莫霍面研究的有效手段。目前研究认为“莫霍钻”的选区应满足一些必要条件,其中深反射地震剖面上存在一个强烈和单一的反射界面是最重要的前提之一(Canales et al., 2003; Ildefonse et al., 2007, 2010)。Franke et al.(2011,2014)基于多道地震数据,对南海古扩张脊南部进行了研究,但是没有明显的莫霍面反射。Li Chunfeng et al.(2015)基于跨越东部次海盆南北部的地震测线(SO49-17a),对东部次海盆的沉积与构造特征进行了研究,但该地震剖面因采集深度不够,并没有揭示东部次海盆的莫霍面反射特征;Ding Weiwei et al.(2018)通过两条多道地震测线,揭示对东部次海盆北部的洋壳基底形态和莫霍面反射特征,并发现沿残留扩张脊分布有对称的倾向扩张脊方向的下地壳反射(LCR),但是,缺乏古扩张脊南部的证据。总体上,目前南海开展的深地震探测主要集中在南北陆缘区域,海盆区则非常缺乏,无法形成对海盆区莫霍面深度特征和规律的整体认识。
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张宝金等(2021)基于海盆区的地震调查资料,对莫霍面进行了解释分析,认为海盆区莫霍面地震反射强度具有“南北分带、早强晚弱”的特征,指出莫霍面地震反射在深海盆南北两侧反射较容易识别,而在残留扩张脊及其邻域反射强度较弱并不容易识别。本文在此基础之上,进一步研究了较强的莫霍面地震反射的深度分布,试图回答强反射莫霍面在南海海盆中哪里最浅的问题,为南海海盆莫霍面钻探选区论证提供重要依据。
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1 区域地质背景
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南海海盆区主要由西北、西南和东部3个次海盆组成,其中东部次海盆东临马尼拉海沟,北靠华南陆块,西南次海盆位于海盆的西南侧,介于西沙地块和南沙地块之间,西北次海盆位于中沙群岛以北、南海北部陆坡以南,是面积最小的一个次海盆(图2)。南海经历了类似大西洋的从被动大陆边缘到海底扩张的演化过程,发育了完全洋壳(Taylor and Hayes, 1980, 1983;Briais et al.,1993; Sun Zhen et al., 2006, 2009;Li Chunfeng et al., 2007, 2014),其过程极为复杂,是独有的一种模式。IODP 349航次首次精确测定了南海海盆扩张结束的年代约为15Ma(Li Chunfeng et al., 2014;Koppers,2014),而南海东北部的初始扩张时间约为32~34Ma,并且23.6Ma时东部次海盆发生了一次向南的洋中脊跃迁,同时,西南次海盆扩张开始(Li Chunfeng et al., 2014;Ding Weiwei et al., 2018; Sun Zhen et al., 2019)。南海的海底扩张由东向西推进(Li Chunfeng et al., 2015),东部大洋板块的斜向俯冲使得南海沿着走滑断层张裂(汪品先, 2019),且南海的初始拉张未受到“地幔柱”活动的影响(Yu Xun and Liu Zhifei, 2020),同时,扩张过程中扩张脊位置不断移动,南部陆缘礼乐地块向南漂移,而北部陆缘位置相对不动,形成了东宽西窄的“V”字形的海盆(图2; 林间等,2019)。
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图2 基于重力反演的南海莫霍面深度图(a)(据杨胜雄等,2015修改)和海盆区反射地震测线分布(b)
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Fig.2 Moho depth of South China Sea based on gravity inversion (a) (modified from Yang Shengxiong et al., 2015) and distribution of reflection seismic lines in the deep sea basin (b)
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COB—洋陆边界;NWSB—西北次海盆;SWSB—西南次海盆;ESB—东部次海盆;(b)中彩色圆圈为海盆区各次海盆已公开的可见或疑似莫霍面地震反射最浅的位置;红色粗线段为本文深反射多道地震测线,蓝色粗线段为图3剖面位置
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COB—Ocean-continent boundary; NWSB—Northwest sub-basin; SWSB—Southwest sub-basin; ESB—East sub-basin; the colored circles in (b) are the positions with the shallowest visible or suspected Moho seismic reflections that have been published in each sub-basin; the thick red lines are the deep reflection multi-channel seismic data in this paper, the thick blue line is the location of the section in Fig.3
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杨胜雄等(2015)基于大量船测重力数据,通过莫霍面重力异常和理论计算异常之间的多次正演、反演迭代,获得了南海全域的莫霍面深度图,结果显示南海中央海盆的莫霍面深度最浅处约为10km (图2a)。已有的深地震反射剖面显示,西北次海盆洋壳最薄处约为6km (Cameselle et al., 2017),东部次海盆北部最薄处约为6km (高金尉等, 2015a; Ding Weiwei et al., 2018);东部次海盆南部约为5km,但是莫霍面反射界面不清晰(Franke et al., 2011, 2014; Niu Xiaowei et al., 2014);西南次海盆为2.2~3.6km(表1, 图2b),但目前仅有一条测线(图2b,测线973-1),且莫霍面反射不强、不连续(Yu Junhui et al., 2018)。
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注:区域位置见图2b彩色圆圈。
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2 数据与方法
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2.1 数据来源
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本文研究区范围为111°E~120°E,9°N~22°N,以3500m水深等深线作为海盆边界。海底地形参考30arcseconds水深数据(Becker et al., 2009)。深反射多道地震数据来自广州海洋地质调查局,共计36条测线,总长度达15000km(图2b),记录时间长度为12s和14s(双程走时),所有的数据均是拖缆采集,采用单源单缆的二维观测方式,最大偏移距6200m左右,炮间距为37.5m和50m,道间距12.5m,覆盖次数60~80次。由于南海海盆区受多期构造运动影响,海底崎岖、断裂发育,地下构造复杂,深反射地震处理面临低频信号恢复难、有效反射能量弱、深部速度建模精度低的问题。笔者及其研究团队重点在低频增强、有源背景噪声衰减和精细速度建模等方面开展技术攻关,创新研发了自适应鬼波压制低频恢复、多域迭代背景噪音衰减和多信息约束三维立体建模等核心技术,大幅提高了莫霍面成像质量和识别精度。利用重新处理的结果进行了沉积基底和莫霍面的解释,其中基底界面是非常明确的,而莫霍面的解释在弱反射区存在一定困难,主要是利用重力空间的趋势结合强反射信息外推或内插得到。
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2.2 计算方法
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本文计算的莫霍面深度为水深、沉积层厚度与结晶洋壳厚度叠加得来。通过多道地震解释成果计算海底与沉积基底的时间域厚度,利用钻探井位的时深公式拟合得到沉积层厚度。Li Chunfeng et al.(2015)给出了东部海盆钻位U1431沉积层的时深公式:Z=0.000188295t2+0.695896t,其中t为双程旅行时(ms),Z为沉积层厚度(m),本次研究采用上述公式计算沉积层厚度。结晶洋壳厚度的计算方法与沉积层类似,通过全球结晶洋壳速度(Christeson,2019)拟合得时深转换公式:Z=0.00032t2+2.67047t,其中t为双程旅行时(ms),Z为地壳厚度(m)。为了得到全区的莫霍面深度分布,对解释结果进行了网格化处理并制作了二维平面图(图4)。从图4中可见,由于测网稀疏,导致控制性不足,在平面图中存在不合理的外推插值结果。为了显示整体效果,本文忽略了这些并不可靠的区域,实际分析时应当结合测网分布进行判别,这也是本研究今后需要进行改进的地方。总体上来看,本文获得的莫霍面深度图能够揭示出强地震反射条件下的较浅莫霍面分布区。
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3 结果
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3.1 海盆区莫霍面反射强度特征
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南海海盆区不同次海盆的莫霍面反射强度存在较大差异,其中东部次海盆莫霍面反射最为清晰,西北次海盆次之,西南次海盆仅零星出现清晰的莫霍面反射。南海东部次海盆、西北次海盆可识别到明显的莫霍面地震反射,尤其在东部次海盆南部及北部,莫霍面反射强且连续性较好(图3a),西北次海盆反射较强(图3b),而在西南次海盆难以识别到莫霍面地震反射(图3c)。南海海盆区南、北部有较明显的莫霍面反射,中部莫霍面反射难以识别,推测受海山分布影响。南海西南次海盆构造基底面断裂众多,且起伏较大,阻碍了莫霍面成像。整体上,较强的莫霍面反射同相轴集中于较早形成的西北次海盆、东部次海盆的北部和南部,而较晚形成的西南次海盆及东部次海盆中部,较难识别莫霍面地震反射。总体上,南海海盆区莫霍面地震反射强度呈现“南北分带、早强晚弱”的规律(张宝金等,2021)。
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图3 南海海盆区莫霍面地震反射典型特征对比(测线位置见图2b)
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Fig.3 Comparison of typical characteristics of Moho seismic reflections in the South China Sea basin (the position of the survey lines are shown in Fig.2b)
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(a)—东部次海盆;(b)—西北次海盆;(c)—西南次海盆
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(a)—East sub-basin; (b)—Northwest sub-basin; (c)—Southwest sub-basin
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3.2 莫霍面反射空间分布特征
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基于15000km深反射地震解释剖面,结合已有研究数据,获得时间域的南海海盆区莫霍面地震反射空间分布特征(图4a)。基于沉积层时深转换公式,计算获得南海海盆区沉积层厚度(图4b)。识别南海海盆区莫霍面地震反射长度超3500km,基于结晶洋壳时深转换模型,网格化获得结晶洋壳厚度空间分布图(图4c)。综合水深数据,形成南海海盆区莫霍面深度图(图4d)。
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整体上,南海海盆区莫霍面深度较浅区域位于中南-礼乐断裂北段以西(图4d“Ⅱ区”)和南段以东(图4d“Ⅰ区”),此外在西南次海盆和东北次海盆中部(图4d“Ⅲ区”)和东部次海盆北部接近陆缘区域(图4d“Ⅳ区”)较浅。东部次海盆中央扩张脊东北部深度较大,而西南次海盆特别是“V”字形尾部因缺乏测线约束,准确深度难以判断(图4d)。在有解释数据约束且莫霍面反射强的区域,深度最浅处位于东部次海盆南部,该处莫霍面反射强、单一且较连续(图3a)。海盆区有较多海山发育,除去海山影响的区域,洋壳厚度在3~9km之间,海盆区的厚度明显小于南北陆缘。与重力反演获得的莫霍面深度相比,利用多道地震计算的莫霍面深度细节更为丰富,能反映出更精细的莫霍深度变化,受海山的影响,异常区明显多于重力反演的结果。重力反演的莫霍面深度总体趋势较为平缓,海盆中央莫霍面较浅,向南北两侧逐渐加深,但二者总体趋势一致,莫霍面深度介于8~14km之间。西北次海盆莫霍面深度均大于10km,西南次海盆部分区域莫霍面深度小于10km,但由于清晰的莫霍面反射较少,结果可信度较低;东部次海盆南部整体莫霍面较浅,最浅处仅约9.5km,其中水深4.01km,洋壳厚度仅5.54km(图4d)。
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图4 南海海盆区莫霍面深度及相关要素图
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Fig.4 Map of Moho depth and related factors in the South China Sea basin
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(a)—时间域莫霍面空间分布图;(b)—沉积层厚度图;(c)—结晶洋壳厚度图;(d)—莫霍面深度图;图中白色虚线为中南-礼乐断裂
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(a)—Spatial distribution of Moho surface in time domain; (b)—thickness of sedimentary layers; (c)—thickness of the crystalline oceanic crust; (d)—Moho depth; the white dotted line is the Zhongnan-Liyue fault
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4 讨论
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目前的地球圈层模型认识主要基于地球物理探测。莫霍面作为壳幔边界,是地球浅表层最重要的一个地球物理界面。在地震波剖面上,莫霍面所处深度表现为一个明显的强反射面,地震波纵波速度在穿过这个面的前后会发生跳跃性变化,从7.6km/s跃变为8.1km/s(Mutter and Carton, 2013)。目前为止,在打穿洋壳、获得洋壳和地幔的样品之前,人们只能试图通过研究分布在一些大陆边缘、陆地上的蛇绿岩套来为洋壳的地质结构和莫霍面的岩性地质模型提供参照物。然而,关于莫霍面的物理特性、地质学本质,上地幔顶部的物理与化学性质及其与上覆岩浆岩洋壳相互作用机制等都还是未解之谜。钻穿莫霍面,获取足够多的下洋壳岩芯样品,检验洋壳增生和熔融过程模型,进行连续的、综合的地球物理测井和井下实验测量原位物理性质,获取井下成像,并确定穿透洋壳进入上地幔的关键地球物理参数、岩性特征及转换带等,是回答洋壳结构及其形成与演化、地幔岩石组成以及深部地幔过程等前沿科学问题的最佳手段,对人类认识地球内部这一未知世界有着重大的历史意义。
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目前研究认为“莫霍钻”的选区应满足以下条件:① 洋壳快速扩张形成(全扩张速率大于80mm/a);② 简单的地质构造:所选区域海底地势平坦,基底面平稳,远离断裂带、残余的相互有重叠的扩张盆地、海山和板内后期火山作用形成的其他构造体;③ 相对于目前所认识的“正常”快速扩张太平洋层状洋壳,所选区域洋壳的地震速度结构不存在异常;④ 应用深反射多道地震技术成像能够得出一个截然的、强烈的和单一的反射莫霍面;⑤ 在OBS反射数据中能够得到较强的广角莫霍面反射波(PmP),具有明显可识别的地幔顶部折射波(Pn)。满足上述条件被认为是莫霍面钻探成功的关键(Canales et al., 2003; Ildefonse et al., 2007, 2010)。
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研究表明,东部次海盆岩浆熔融程度相对较高,具有相对快速扩张洋中脊的特点(Yang Fan et al., 2019),而西南次海盆岩浆熔融程度相对较低,具有慢速扩张洋中脊的特点(Yu Junhui et al., 2018)。西南次海盆全扩张速率为35~50mm/a(Li Chunfeng et al., 2014; Yu Junhui et al., 2018),与正常海底扩张(全扩张速率＞55mm/a)产生的洋壳明显不同,扩张过程岩浆供应不足,主要表现为构造主导型海底扩张的特点,扩张中心广泛发育深断裂(Yu Junhui et al., 2018)。东部次海盆的全扩张速率为20~80mm/a,属于慢速—中速扩张区间, 与西南次海盆扩张全程处于慢速扩张(全扩张速率50~35mm/a)不同,东部次海盆经历了中速→慢速→中速→慢速的扩张过程(Li Chunfeng et al.,2012, 2014),扩张早期峰值速率可达80mm/a(图5)。
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图5 东部次海盆海底扩张速率变化过程及其与关键节点的对应关系(据赵斌等,2022修改)
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Fig.5 The changing process of seafloor spreading rate in the East sub-basin and its corresponding relationship with key nodes (modified from Zhao Bin et al., 2022)
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通过对现有OBS和深反射多道地震等资料的分析,可以发现在南海残留扩张脊两侧,均没有明显的莫霍面反射(Li Chunfeng et al., 2015; Ding Weiwei et al., 2018; Sun Zhen et al., 2019)。高金尉等(2015a)通过南海南北陆缘的地震资料研究推测,岩浆活动始终贯穿南海形成演化过程,强烈的火山活动可能发生于扩张末期或扩张停止以后,并持续数百万年。IODP钻井U1431和U1433井获得的地质记录显示,东部次海盆在经历了早中新世(约20~16Ma)的南东向扩张后,又经历了短暂的近南北向扩张(约16~15Ma),直到海盆扩张停止,扩张方向的转变导致了南北向转换断层的产生,且岩浆活动开始增强(Sun Zhen et al., 2019;Zhao Yanghui et al., 2019),中南-礼乐断裂带、残留扩张脊及其两侧的大量海山因此形成。从残留扩张脊及两侧海山的火山岩年代学研究来看,海底扩张停止后的火山活动一直持续到了上新世(约5.3Ma;杨蜀颖等, 2011)。因此推测,残留扩张脊两侧莫霍面遭到了后期岩浆活动长时间的破坏(16~5.3Ma),已不适合进行莫霍面钻探。赵斌等(2022)基于多道地震剖面,研究认为东部次海盆南部区域受扩张后岩浆活动影响较小,呈现出基底相对平坦的特点,整体上地质构造简单。
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阮爱国等(2011)基于OBS研究发现,东部次海盆南部可以追踪到来自莫霍面的反射波震相(PmP),并可以看到莫霍面的折射波震相(Pn)。速度结构方面,上地壳(不含沉积层)速度为4.8~6.8km/s,下地壳为6.8~8km/s,上地幔顶部速度从8.0km/s增至8.2km/s (阮爱国等, 2011; Niu Xiaowei et al., 2014),与具有典型洋壳结构的快速扩张型太平洋层状洋壳速度规律基本一致(Canales et al., 2003; Ildefonse et al., 2007; 图6)。因此,东部次海盆南部的洋壳地震速度结构不存在异常。
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图6 蛇绿岩剖面、层状地壳和典型快速扩张洋壳P波速度模型(a) (据Ildefonse et al., 2007修改); 东太平洋扩张脊西侧洋壳岩性地层解释图(b) (数据来自平行于扩张脊的广角地震折射剖面;据Canales et al., 2003); 目标区西侧的P波速度模型(c) (OBS973-2剖面,测线位置见图2b,速度单位为km/s;据阮爱国等,2011修改)
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Fig.6 P-wave velocity model of ophiolite profile, layered crust and typical rapidly expanding oceanic crust (a) (modified from Ildefonse et al., 2007); lithostratigraphic interpretation of the oceanic crust on the west side of the eastern Pacific spreading ridge (b), with data from a wide-angle seismic refraction profile parallel to the spreading ridge (after Canales et al., 2003); P-wave velocity model on the west side of the target area (c) (OBS973-2profile, the location of the survey line is shown in Fig.2b, the velocity unit is km/s; modified from Ruan Aiguo et al., 2011)
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虽然研究显示南海西南次海盆的洋壳仅厚2.3~3.9km(于俊辉等, 2017),但是已有的地震资料仅能看到很少的疑似莫霍面反射,且可靠程度不高。此外,西南次海盆属于慢速扩张形成的洋壳,并不是理想的莫霍面钻探场所。西北次海盆只经历了南海早期短暂的扩张过程,面积非常小,且莫霍面反射可靠性不强。东部次海盆除扩张脊两侧区域莫霍面遭受后期岩浆活动影响成像困难之外,南北两缘均可见清晰的莫霍面反射(图3)。总体上,东部次海盆北部莫霍面深度大于海盆南部,南部的莫霍面深度最浅处为9.5km左右(图4d)。两个区域的水深都在4000m左右,且海底地形平坦,少有海山和丘陵发育;基底之上沉积物厚度稍有区别,北部靠近陆缘区域的沉积厚度为2000m左右,而南部仅1000m左右(图4)。从深反射多道地震成像效果看,东部次海盆南部莫霍面反射比较强烈和单一,界面之上无其他壳内反射,而北部的莫霍面反射虽然清晰,但是附近壳内反射较多。综合来说,东部次海盆南部区域是南海比较理想的莫霍面钻探备选区(图7)。
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5 结论
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本文利用深反射多道地震解释数据,针对莫霍面深度进行了初步分析,明确了对应较强莫霍面地震反射的较浅莫霍面分布区域。结合前人有关南海海盆扩张过程、扩张速率、海盆沉积与基底特征、扩张停止后岩浆活动特征和洋壳地震速度结构等研究,对南海海盆区莫霍面钻探备选区进行了初步判断:
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(1)南海海盆不同区域的莫霍面反射强度存在较大差异,东部次海盆莫霍面反射最为强烈且清晰,西北次海盆次之,西南次海盆仅有零星出现的清晰莫霍面反射且可信度不高。东部次海盆南部局部区域呈现莫霍面地震反射清晰、强烈和单一的特点,有利于莫霍面钻探。
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(2)南海海盆区莫霍面地震反射强烈和可信度高的区域中,东部次海盆南部是莫霍面深度较浅的区域之一,最浅处约为9.5km,其中洋壳厚度仅5.54km,可作为南海莫霍面钻探研究备选区。
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图7 南海海盆区莫霍面钻探选址建议备选区(黑色方框内)
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Fig.7 Proposed drilling area for the Moho in the South China Sea basin (in the black box)
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致谢:衷心感谢广州海洋地质调查局深部地球物理研究团队成员的有益讨论和辛勤付出!感谢匿名审稿人提出的宝贵修改意见建议和鼓励,让本文的科学性得到极大提高,也让笔者研究团队受益匪浅!
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摘要
钻遇莫霍面是人类一直以来的梦想。深海海底是地球上离莫霍面最近的地方,目前有研究推测南海是世界上莫霍面深度最浅的海域之一,但缺乏足够的直接证据。深反射地震探测可以直接揭示岩石圈的构造形态,是莫霍面探测的重要手段。本文基于长达15000 km的深反射多道地震剖面的解释、处理、制图和分析,结合前人的研究,形成了南海海盆区莫霍面反射特征和空间分布的初步认识。① 南海东部次海盆南部早期经历了较快速扩张,岩浆供应充足,受扩张停止后岩浆活动影响较小,基底平坦,地质构造相对简单,同时洋壳地震速度结构不存在异常,且有较强的广角莫霍面反射波和可识别的地幔顶部折射波,具备莫霍面钻探的基本条件。② 南海海盆不同区域的莫霍面反射强度存在较大差异。其中东部次海盆莫霍面反射最为强烈且清晰,西北次海盆次之,西南次海盆仅有零星出现的清晰莫霍面反射且可信度不高。③ 识别南海海盆区莫霍面地震反射长度超过3500 km,首次形成了海盆区深度域莫霍面地震反射空间分布图。与重力反演的莫霍面深度相比,利用深反射多道地震计算的莫霍面深度细节更为丰富,并且可以在垂向上清晰刻画莫霍面的结构。整体上,南海海盆区莫霍面地震反射强烈和可信度高的区域中,深度较浅的区域之一是东部次海盆南部,最浅处仅约9.5 km,其中水深4.01 km,洋壳厚度仅5.54 km。综合判断,东部次海盆南部是南海重要的莫霍面钻探备选区,这对南海莫霍面钻探选址具有重要意义。
Abstract
Drilling into the Mohorovičić discontinuity (Moho) has always been a human dream. The deep seabed is the closest place to the Moho on Earth. At present, some studies speculate that the South China Sea is one of the shallowest part of the Moho in the world, but there is a lack of sufficient direct evidence.Deep reflection seismic detection can directly reveal the structural form of the lithosphere and is an important means of Moho detection.Based on interpretation, processing, mapping and analysis of the 15000 km deep-reflection multi-channel seismic profiles and existing research, this paper has formed a preliminary understanding of the reflection characteristics and spatial distribution of the Moho in the South China Sea basin (SCSB). (1) The southern part of the eastern sub-basin experienced rapid expansion in the early stage, with sufficient magma supply and little influence of magmatic activity after the expansion stopped. The basement is flat, and the geological structure is relatively simple. At the same time, there is no abnormality in the seismic velocity structure of the oceanic crust, and there are strong wide-angle Moho reflection waves and identifiable mantle top refracted waves, which meet the basic conditions for Moho drilling. (2) The intensity of Moho reflection in different regions of the SCSB varies greatly. Among them, the Moho surface reflection in the East sub-basin is the strongest and clearest, followed by the Northwest sub-basin, while the South west sub-basin has only sporadic clear Moho reflections, and the reliability is not high. (3) This study identified the Moho seismic reflection over 3500 km in the SCSB, and formed the spatial distribution map of the Moho seismic reflection in the depth region of the basin for the first time. Compared with the Moho depth obtained by gravity inversion, the Moho depth calculated by deep reflection multi-channel seismic is richer in details, and the structure of the Moho can be clearly depicted in vertical direction. On the whole, among the areas with strong Moho seismic reflection and high reliability in the SCSB, the southern part of the East sub-basin is one of the areas with shallower Moho, with the shallowest area only about 9.5 km (among them, the water depth is 4.01 km and the thickness of the ocean crust is only 5.54 km). Based on comprehensive judgment, the southern part of the East sub-basin is an important Moho drilling candidate area in the South China Sea, which is of great significance to the selection of the Moho drilling site.
