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莫霍洛维奇不连续面(简称“莫霍面”),是地球内部一个非常重要的地震波速度突变界面,P波速度从7.6km/s跳跃到8.1km/s(周怀阳, 2017)。地质学上,莫霍面的本质是什么?什么样的物理化学特性造成了波速突变?地球的深部结构构造及运行机制如何?这些问题一直以来是地球科学界的未解之谜。迄今为止,我们对地幔的了解仅仅基于地球物理探测结果、造山带蛇绿岩套以及实验岩石学研究,缺乏原位地幔岩的研究。“莫霍钻”是唯一能够揭开地球内部连续的壳幔岩石学结构、揭示莫霍面地质意义、探索地球内部运行机制、生命生存极限、研究多圈层物质和能量循环及其资源能源和环境效应的钥匙,是我们向地球深部进军的航空母舰(汪品先, 2017, 2018)。
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1 莫霍计划的提出和发展
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“莫霍计划”是美国科学家Walter Munk和Harry Hess于1957年提出的一项堪与“登月计划”相媲美的雄心勃勃的大科学计划,旨在钻穿洋壳和莫霍面,获取原位地幔岩样品(Teagle et al.,2011),以建立完整的壳幔岩石剖面,揭示莫霍面的地质意义。
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1958年,该计划获得了美国国家科学基金会的资助,并于1961年使用CUSS1号钻探船在墨西哥瓜德罗普岛近海3558m水深处实施了5口深海钻井,钻进海底以下183m,获取了沉积盖层之下13m的玄武岩样品,标志着我们迈出了向莫霍面进军的第一步,是科学史上具有里程碑意义的钻探(Teagle et al.,2011)。之后,受政治、经济、技术和管理等因素的影响, 美国众议院于1966年取消了该计划,莫霍计划最终搁浅。尽管如此,它开创了大洋钻探的新纪元,推动了后续大洋钻探的发展,使大洋钻探从深海钻探计划(Deep Sea Drilling Project, DSDP, 1968~1983年),发展到大洋钻探计划(Ocean Drilling Program, ODP, 1985~2003年),再到综合大洋钻探计划(Integrated Ocean Drilling Program, IODP, 2003~2013年)、再发展到国际大洋发现计划(International Ocean Discovery Program, IODP, 2013~2023年)。五十多年的大洋钻探,地球科学家从未放弃 “莫霍钻”的梦想(Ildofense et al.,2010; 周怀阳, 2017),一直都在向洋壳深部进军,为莫霍钻的实现开辟道路。尤其是38口基岩进尺超过100m的钻探让我们对大洋岩石圈的组成、形成和演化有了极大的改观。
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随着科学和钻探技术的快速发展,21世纪,科学家筹划莫霍计划的热情逐渐高涨。2006年9月在美国波特兰举行了“莫霍任务”(Mission Moho)研讨会,优先确定了研究海洋岩石圈的形成和演化的科学目标(王健等, 2012)。2007年4月,Ildofense等科学家向综合大洋钻探计划(IODP)提交了“莫霍计划”建议书(Ildofense et al.,2007)。2010年6月“莫霍计划”工作组在日本金泽召开了“莫霍钻:地壳之旅和地幔探索”研讨会(Ildofense et al.,2010)。2011年出版的《照亮地球:过去,现在和未来》把“莫霍计划”列为挑战8(Bickle et al.,2011),它也是“地球号”钻探船的十年科学目标之一(汪品先, 2017)。2011年6月,综合大洋钻探计划发布了快速扩张洋中脊莫霍钻初始可行性研究报告的最终版(Pilisi et al.,2011),使莫霍计划成为未来大洋钻探的终极目标。2012年提出了“深入地幔的莫霍钻(MoHole to Mantle)”的M2M计划(Umino et al.,2012; 汪品先, 2017)。2015年12月—2016年1月,美国科学家Henry Dick牵头,在西南印度洋中脊亚特兰蒂斯滩实施了慢速扩张洋中脊莫霍计划(SloMo)的第一阶段钻探(Dick et al.,2016)。
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2 深入洋壳的探索
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自从莫霍计划开启大洋钻探的新纪元以来,科学家始终怀着钻穿莫霍面的梦想,一直向地球深部进军,不断取得革命性和创新性的成果,引领着地球科学一次又一次的重大变革和发展方向(翦知湣, 2018; 汪品先, 2018)。比如,海底最老沉积物随着远离洋中脊而逐渐变老,验证了板块构造理论中的海底扩张说;深部生物圈的发现打破了以往对生命生存极限的认知(刘志飞等, 2007; 中国大洋发现计划办公室等, 2018; Heuer et al.,2020);对不同扩张速率洋壳的钻探揭开了洋壳结构的复杂性和多变性,打破了以往对洋壳单一层状结构的认识,也为莫霍钻的实施开辟了道路(Karson et al.,2015; Dick et al.,2019; Michibayashi et al.,2019)。
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2.1 洋壳结构的复杂性和多变性
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在实施大洋钻探之前,人们对洋壳地质结构的认识主要基于陆地上蛇绿岩套综合研究提出来的Penrose模型(图1;Anonymous, 1972),模型中洋壳结构,从上到下依次为沉积岩(层1)、玄武岩和辉绿岩墙(层2)、辉长岩(层3)、递变为橄榄岩,它们的地震波速度与地球物理探测结果一致,莫霍面是辉长岩与橄榄岩的界面(Ildofense et al.,2007)。随着1961年“莫霍计划”首钻以及后续世界各大洋科学钻探的实施,科学家发现,不同扩张速率下洋壳的结构在侧向上和垂向上变化很大(图2;Dick et al.,2006; Karson et al.,2015),挑战了传统的Penrose层状洋壳结构模型。
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图1 基于Oman蛇绿岩剖面岩石地层学的Penrose模型与地震波速度(据Ildofense et al.,2007修改)
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Fig.1 Penrose model representing lithostratigraphy of the Oman ophiolite and P-wave velocity of different layered stratigraphy derived from seismic data (modified after Ildofense et al.,2007)
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超慢速(全扩张速率＜20mm/a)和慢速(全扩张速率20~50mm/a)(Karson et al., 2015)扩张速率下形成的洋壳结构,侧向差异性非常显著,层2火山岩呈明显不连续甚至呈点状或零星分布,在有的洋脊段上,大面积缺失火山岩,而且,被喻为下洋壳的层3辉长岩(Christensen et al.,1975)也很少出现,而蛇纹石化地幔橄榄岩的出露面积在一些洋脊段可达1/4~1/2(Karson et al.,2015)。有深部物质剥露的区域,缺失中上地壳的辉绿岩、玄武岩等,洋壳结构不完整(Zhang Weiqi et al.,2020),洋壳厚度变化也非常大,变化范围0~9.5km。此外,沿洋脊轴部岩浆分布变化也很大,局部可见到小型辉长岩侵入到蛇纹石化橄榄岩中(Cannat, 1993; Kelemen et al.,2007; Escartín et al.,2008, 2011; Dick, 2019)。例如,西南印度洋中脊的亚特兰蒂斯滩杂岩,在垂向上存在岩性分带,从上到下依次为氧化辉长岩、橄榄辉长岩、橄长岩或纯橄岩,以及部分发生强烈塑性变形的辉长岩(Dick et al.,2006, 2016; Zhang Weiqi et al.,2020)。
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DSDP69航次在中速扩张洋中脊(全扩张速率50~80mm/a)东太平洋Costa-Nazca洋脊实施的504B孔(图2),首次钻遇到约780m的玄武质熔岩和大于1050m的席状岩墙杂岩体,首次揭示了洋壳层状结构的地震分层2A和2B以及层2和层3边界的地质属性(Carlson, 2010, 2011)。
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快速(全扩张速率80~120mm/a)和超快速(全扩张速率＞120mm/a)扩张洋中脊的海底几乎完全由岩浆喷出海底形成的火山岩(玄武岩,图1中层2)覆盖(图2)。在构造切割或剥蚀的地方如Hess Deep裂谷(图2)以及一些转换断层处,可见到一些岩墙(辉绿岩)、深成岩(辉长岩)和地幔橄榄岩,展现出快速扩张洋中脊相对比较简单的层状洋壳结构,接近于理想的Penrose层状模型(图1)。例如,ODP Leg206, IODP309, 312和335航次在超快速扩张洋中脊Costa Rica实施的U1256D钻孔,自上而下钻遇到离轴熔岩流、席状岩墙、辉长岩以及下方的堆晶岩样品(图2)(Wilson et al.,2006; Teagle et al.,2007, 2012)。
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图2 不同扩张速率洋中脊形成的洋壳结构(据Karson et al.,2015修改)
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Fig.2 Oceanic ridge crust structures with different spreading rates (modified after Karson et al.,2015)
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DSDP—深海钻探计划;ODP—大洋钻探计划;IODP—综合大洋钻探计划;JdFR—胡安·德富卡洋脊;CRR—哥斯达黎加洋脊;EPR—东太平洋隆
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DSDP—Deep Sea Drilling Project; ODP—Ocean Drilling Program; IODP—Integrated Ocean Drilling Program; JdFR—Juan de Fuca Ridge; CRR—Costa Rica Ridge; EPR—East Pacific Ridge
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2.2 洋壳增生机制
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岩浆作用是地球内部物质和能量传递的主要过程,是洋壳增生的主要方式。绵延全球海底7万多千米的洋中脊是洋壳生长的主要区域,并且大部分洋壳的增生只发生在洋中脊轴部非常狭窄的范围内。
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目前对洋壳增生过程的认识,主要有3种模型:“冰川”模型,“流体”模型和“席状”模型(图3;Ildofense et al.,2006, 2010; Teagle et al.,2012; 周怀阳, 2017)。在“冰川”模型中,下洋壳辉长岩起源于很小很薄的“韭葱”状岩浆房,塑性熔体向上运移到浅层发生结晶作用,结晶作用产生的潜热贡献给上覆的热液循环系统,浅位形成的硅酸盐晶粥一边向深处沉降、一边随板块扩张向外运动(图3a)。西南印度洋洋中脊Atlantis Bank辉长岩体中锆石年龄的研究给这个模型提供了有力的支持。“流体”模型中,含岩基侵入体的塑性流体下沉并向外扩张,形成下洋壳(图3b)。“席状”模型中,下洋壳是由不同深度上的小岩浆体结晶形成的,热液活动在下洋壳广泛分布,吸收结晶潜热,并阻止形成较大的熔融区域(图3c)。地球物理资料探测到的轴部岩浆房的大范围变化,似乎给该模型提供了有力的证据。
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2.3 大洋核杂岩——钻穿莫霍面的捷径和窗口
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在探索洋壳结构的过程中,其中的一个重要发现是大洋核杂岩。它是在岩石圈伸展背景下,由拆离断层将地壳深部和/或上地幔物质拆离到洋底表面并在表面形成窗棱构造的一种特殊的穹隆状海底构造 (Cannat, 1993; Karson, 1999; Dick et al.,2000; Smith et al.,2006, 2012; 李洪林等, 2014; 于志腾等, 2014; 范庆凯等, 2018),具有不同于大陆核杂岩的独特的构造要素和岩石组合(李三忠等, 2006; Maffione et al.,2013)。大洋核杂岩主要的构造要素包括大型拆离断裂、垂直于洋中脊的大型线理或窗棱构造、穹隆状构造;大洋核杂岩的核部主要是源于地壳深部的辉长岩和地幔岩,如蛇纹石化橄榄岩、橄长岩、辉长岩、超基性岩墙和熔岩,以及少量的断层岩(如糜棱岩、绿泥石化角砾岩、断层角砾和断层泥) (Karson, 1999; Dick et al.,2000; Smith et al.,2012; Karson et al.,2015; Dick et al.,2016, 2019)。这为我们窥探地壳深部物质组成和演化提供了理想的窗口,也为钻穿洋壳,获取原位地幔岩样品提供了捷径,为莫霍钻的实施提供了理想的优选区。例如西南印度洋中脊的亚特兰蒂斯滩(Atlantis Bank)大洋核杂岩(图4),它是迄今为止唯一一个已经实施完慢速扩张洋中脊莫霍计划(SloMo)第一阶段钻探的区域(Dick et al.,2016)。
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亚特兰蒂斯滩大洋核杂岩,是迄今为止研究程度最高的大洋核杂岩。大量深潜、拖网和钻探资料揭示出,大约660km2的下地壳物质在拆离断层的作用下剥露到海底,包括枕状玄武岩、辉绿岩墙、辉长岩、橄榄岩、蛇纹岩等(图4a;Nguyen et al.,2018; Dick et al.,2019)。目前在该区域已经实施了3口科学钻探,分别是ODP118和176航次的735B孔(32°43.39′S, 57°15.96′E,钻深1508m)、ODP179航次的1105A孔(32°43.13′S, 57°16.65′E,钻深158m),以及IODP360和362T航次的U1473A孔(32°42.3622′S; 57°16.6880′E,钻深809.4m)。其中,U1473A是慢速扩张洋中脊莫霍计划的第一阶段钻探,首次钻遇到洋壳深部发生强烈变形的韧性剪切带(Dick et al.,2016, 2019;Nguyen et al.,2018)。钻探资料揭示出,该大洋核杂岩在垂向上,上部以氧化物辉长岩为主,中部以橄榄辉长岩为主,而底部以橄长岩或纯橄岩为主,部分辉长岩发育强烈的塑性变形(图4b;Dick et al.,2016, 2019)。
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图3 洋壳增生模型(据Teagle et al.,2012修改)
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Fig.3 Diagrams showing crustal accretion models (modified after Teagle et al.,2012)
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(a)—辉长岩“ 冰川”流动模型;(b)—“流体”流动及岩基侵入模型;(c)—“席状”岩基模型
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(a)—Gabbro glacier flow model; (b)—hybrid flow model with sill intrusions; (c)—“sheeted” sill model
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图4 西南印度洋中脊亚特兰蒂斯滩地质图(a)和剖面图(b)(据Dick et al.,2019修改)
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Fig.4 Geological map (a) and profile (b) of the Atlantis Bank, southwest Indian Mid-Ocean Ridge (modified after Dick et al.,2019)
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3 21世纪的莫霍计划
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随着科学和钻探技术的快速发展,莫霍钻的曙光也日渐清晰,提交到国际大洋发现计划的莫霍计划建议书(https://www.iodp.org/proposals/active-proposals)接憧而来,包括超慢速-慢速扩张洋中脊的莫霍计划建议书2份(Dick et al.,2018; Sanfilippo et al.,2021),快速扩张洋中脊莫霍计划建议书4份(Teagle et al.,2006; Umino et al.,2012, 2020; Morgan et al.,2014),弧前莫霍面钻探建议书1份(Michibayashi et al.,2016),弧后莫霍面钻探建议书1份(Ohara et al.,2020)(表1),共计提出了30个莫霍钻站位(图5,表1),简述如下:
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(1)超慢速-慢速扩张洋中脊莫霍面计划:① 800号建议书是超慢速扩张洋中脊莫霍计划(SloMo)多阶段钻探的第一阶段钻探建议书。由美国伍兹霍尔海洋研究所Henry J.B.Dick教授牵头,计划在西南印度洋中脊亚特兰蒂斯滩735B孔NE2.2km(图5)进行钻探,钻穿辉长岩,钻进莫霍面以下500m,揭示辉长岩层的下部组成以及壳幔过渡带,从而更好地理解洋中脊玄武岩的形成过程,检验慢速扩张洋中脊的莫霍面是否是蛇纹石化前缘(Dick et al.,2018)。② 971号建议书主题是“凯恩大洋核杂岩深钻:钻进慢速扩张洋中脊的下地壳和地幔”,由意大利Pavia大学Sanfilippo教授牵头,计划在大西洋中脊23°N的Kane Megamullion大洋核杂岩(图5)拆离断层下盘进行深钻,旨在钻穿该大洋核杂岩的地壳和地幔,检验Kane大洋核杂岩的地震地质解释、随着熔体减少地壳结构的变化,下地壳和地幔岩中热液蚀变过程随深度和温度的变化,探索Kane大洋核杂岩下洋壳和上地幔中的非自养和自养型生命类型(Sanfilippo et al.,2021)。
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图5 21世纪莫霍钻站位分布图
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Fig.5 The distribution of the21st century MoHole sites
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GM—哥斯拉Megamullion;KG—凯恩Megamullion
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GM—Godzilla Megamullion;KG—Kane Megamullion
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(2)快速扩张洋中脊莫霍计划:① 522号建议书主题是“超快速4:钻取超快速扩张洋壳的辉长岩”,由英国南安普敦大学国家海洋中心海洋与地球科学学院Teagle牵头,计划在超快速扩张洋中脊东太平洋隆科克斯(Cocos)板块(图5)的1256D孔继续深钻,钻取辉长岩,采集一个完整的从熔岩到岩墙再到辉长岩的洋壳岩石剖面(Teagle et al.,2006)。通过ODP206航次,IODP309和312航次的多次钻探,1256D孔已经钻进海底以下1521m,基岩进尺＞1250m,在海底以下1407m已经钻遇到辉长岩类岩石(Wilson et al.,2006; Teagle et al.,2006, 2012),是迄今为止仅次于504B孔(钻进海底以下2111m) (Dick et al.,1992; Michibayashi et al.,2019)的深钻。该建议书的主要任务是钻进地震反射剖面上显示熔融透镜体的区域,验证它是否是辉长岩,确定扩张速率和深度是否与轴脊熔融透镜体的发育存在反向关系,确定快速扩张洋中脊上洋壳的构造和组成。② 805号建议书的主题是“深入地幔的莫霍钻(M2M)”,由日本仕左卡大学Katsuyoshi Michibayashi教授牵头,计划在快速扩张洋中脊Cocos板块、加利福尼亚Baja高原以及夏威夷岛弧(图5)实施超深钻(＞6000m),钻穿洋壳,钻进岩石圈地幔500m,获取新鲜的上地幔岩石样品,确定地幔的组成和结构、莫霍面的性质以及洋壳的形成和演化等(Umino et al.,2012)。③ 876号建议书主题是“弯曲断层的蛇纹石化:从洋中脊到海沟的洋壳和地幔的演化”,由英国伦敦大学皇家哈洛威学院地球科学系J.Morgan教授牵头,计划在快速扩张的科克斯板块中美洲海沟附近的弯曲断层附近(图5) 进行钻探,钻穿洋壳,获取完整的壳幔岩石样品,研究俯冲过程中再循环洋壳和地幔的物质组成以及洋中脊、海底扩张和俯冲板片弯曲过程中洋壳和地幔的演化。④ 951号建议书主题是“钻探夏威夷近海North Arch的成熟洋壳”,由日本金泽大学Susumu Umino教授牵头,计划用地球号钻探船在夏威夷近海(M2M计划的优选站位之一)(图5)实施钻探,钻取~80Ma成熟洋壳的堆晶辉长岩,为未来莫霍钻设计实施一个先导孔,积累“地球号”钻探船钻取深部硬岩的经验(Morgan et al.,2014)。
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(3)弧前莫霍计划:898号建议书的主题是“深入地幔的弧前莫霍钻(Fore Arc M2M)”,也是由日本仕左卡大学Katsuyoshi Michibayashi教授牵头,计划在西北太平洋博宁海沟的弧前(图5)开展钻探,钻穿洋壳和莫霍面,钻取上地幔岩石样品,以揭示超深俯冲带地壳起源和演化,莫霍面性质以及岩石圈地幔增生过程和地球动力学演化(Michibayashiet al.,2016)。
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(4)弧后盆地莫霍计划:941号建议书主题是“Godzilla Megamullion弧后盆地岩石圈结构”,由日本水文与海洋学部Yasuhiko Ohara教授牵头,计划在菲律宾海Parece Vela盆地中世界上最大的大洋核杂岩Godzilla Megamullion(Ohara, 2016)(图5)沿拆离断裂走向布设钻孔,实施钻探,获取弧后盆地洋壳和上地幔的组成、大洋核杂岩的结构,探索热液停止活动后洋壳中的生命,检验生命是否适应、如何适应生存环境的突变(Ohara et al.,2020)。
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在上述提交的建议书中,莫霍钻的选址位于大洋核杂岩(如亚特兰蒂斯滩大洋核杂岩、Kane Megamullion大洋核杂岩和Godzilla Megamullion大洋核杂岩),或者位于薄洋壳区(如Cocos板块、加利福尼亚Baja高原以及夏威夷岛弧、科克斯板块中美洲海沟附近的弯曲断层)。采取的策略既有薄洋壳区单孔的多次重复钻进,如522号建议书,也有在大洋核杂岩的多阶段钻探,如800号建议书。
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4 莫霍计划的展望
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“莫霍计划”一直以来都是科学家梦寐以求的科学目标。目前国际上在快速和慢速扩张洋中脊都已钻至下洋壳辉长岩,而且慢速扩张洋中脊莫霍计划(SloMo)的第一阶段钻探已经执行完(Dick et al.,2016),弧前、弧后盆地以及快速扩张洋中脊的莫霍计划建议书也已提交到国际大洋发现计划,而边缘海的莫霍计划尚未提出。我国南海海域是太平洋西缘环形俯冲体系下形成的最大最典型最具有代表性的边缘海,南海莫霍面钻探对边缘海的研究具有非常重大的意义。
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21世纪,中国正经历着科学发展的黄金时期。我们应当抓住机遇,围绕“建设海洋强国”的目标,借鉴大洋钻探五十多年来的成功经验,聚焦地球内部如何运行这一国际前沿热点问题,提出以“钻穿南海莫霍面”为远大目标的科学计划,选择南海莫霍钻优选站位,迈出“南海莫霍钻”的第一步,逐步钻穿南海莫霍面,获取新鲜的地幔岩样品,建立南海完整的壳幔岩石剖面,揭开南海岩石圈壳幔结构,揭示莫霍面的地质意义以及南海独特的深部过程和动力学机制。如若我国能够率先实现“南海莫霍钻”,必将问鼎国际学术界的高峰。
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我国要率先实现“莫霍钻”,南海莫霍钻的选址则是重中之重。鉴于国际上为了实现钻穿洋壳钻入上地幔的宏伟目标,采取了两种策略:一种是选取洋壳最薄地方的一个钻孔进行重复钻取,最终钻进莫霍面;第二种策略是在下洋壳和地幔物质剥露区(如大洋核杂岩)实施多阶段钻探,最终连成一个完整的剖面。我国南海海域是否有类似的区域呢?
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(1)南海薄洋壳区或残余洋中脊已有钻探区:① 我国南海洋壳已经于15~16Ma前终止扩张,沉积物厚度2~3km(杨胜雄等, 2015)。洋壳厚度具有很大的不确定性,杨胜雄等(2015)通过重磁异常反演,提出南海中央海盆(或东部次海盆)的地壳厚度8~14km,海盆区莫霍面抬升;而其他学者提出南海地壳厚度为3~9km之间,其中东部次海盆的莫霍面深度＞12km,地壳厚度＞6km (吴招才等, 2017);还有学者认为南海的莫霍面深度10~12km,中央海盆的地壳厚度不到7km(Gozzard et al.,2019)。南海海盆沉积层较厚的区域,如果钻探,需要钻穿较厚的沉积层才能钻遇洋壳基底岩石,因此,不作为南海莫霍钻选址的优先考虑范围。南海莫霍钻优先考虑沉积物较薄的区域和/或薄洋壳区、洋中脊已有钻探区,如南海残余洋中脊及其周缘区域。② 南海海盆残余洋中脊是洋壳增生的地方,沉积层相对较薄,是南海莫霍面钻探的优选区。南海东部次海盆残余洋中脊附近的磁异常和重力异常资料反演给出的莫霍面埋深~15km,洋壳厚度~10km(Li Chunfeng et al.,2015)。张莉等(2013)通过对三维OBS数据的处理,初步估算了南海东部次海盆残余洋中脊黄岩海山下方的地壳厚度约为8km。Zhao Minghui et al.(2018)通过南海东部次海盆中部OBS地震反射资料和广角地震反射折射资料的P波速度模型的正演和反演,识别出了薄洋壳(＜5km)区、典型洋壳(5~6km)区和厚洋壳(＞6km)区,其中薄洋壳区位于N55°E残余扩张洋中脊区,大约80km宽,莫霍面埋深10~11km(图6d)。由我国科学家李春峰教授和林间教授牵头在南海东部次海盆残余洋中脊实施的IODP349航次U1431站位(图6a),钻进1008.8m,钻穿972m沉积层之后成功钻遇到基底玄武岩(图6c;Li Chunfeng et al.,2014, 2015)。玄武岩的矿物组成和地球化学特征证实它们是典型的洋中脊型玄武岩(Li Chunfeng et al.,2015),镁同位素δ26Mg值-0.27‰±0.06‰与地幔镁同位素值相似,还具有与地幔相似的Sr-Nd-Pb-Hf同位素(Zhong et al.,2021)。南海残余洋中脊洋壳结构如何,还需钻穿洋壳钻取原位岩石样品才能得以证实。考虑到南海洋壳沉积层2~3km厚,相对于其他尚未开展钻探的区域而言,残余洋中脊附近的U1431孔已经钻穿沉积层。因此,可以优选考虑在该钻孔或者附近沉积层相对较薄的区域进行重复钻进,最终钻穿莫霍面。
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图6 南海地貌图及南海莫霍钻优选站位(a);南海管事海山大洋斜长花岗岩样品(b)(位置见图6a);U1431钻孔岩性剖面图(c)(据Li Chunfeng et al.,2015);南海东部次海盆广角地震反射/折射剖面P波速度结构(d)(剖面位置见图6a, 改自Zhao Minghui et al.,2018)
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Fig.6 Topography of the South China Sea and preferable holes for drilling Mohole (a); oceanic plagiogranite dredged at the Guanshi Seamount in the South China Sea (b) (location is shown in Fig.6a); lithologies of site U1431 (c) (after Li Chunfeng et al.,2015); P wave crustal structures of wide-angle seismic reflection/refrection profile (d) (location of profile is shown in Fig.6a, modified after Zhao Minghui et al.,2018)
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(2)南海深部物质剥露区——南海管事平顶海山?南海东部次海盆的管事平顶海山具有异常高的空间重力异常和磁异常(陈洁等, 2012; 杨胜雄等, 2015; Gozzard et al.,2019),表明该区域可能有深部高密度高磁性的物质出露。此外,中国地质调查局广州海洋地质调查局在该海山上拖网到了大洋斜长花岗岩(图6a、b),它主要由长石、石英、角闪石、阳起石组成,SiO2含量61.56%,MgO含量2.34%,含有相对较高的Al2O3(14.86%)和CaO(7.10%),相对较低的Na2O(2.08%),亏损K2O(0.09%)、大离子亲石元素(Rb、Ba、Th、K)和轻稀土元素,稀土元素配分模式和微量元素组成与含水辉长岩部分熔融形成的大洋斜长花岗岩相似,还具有与洋中脊玄武岩相似的Sr-Nd-Pb-Hf同位素组成。Zhong Lifeng et al.(2018)根据这些特征,结合大地构造背景提出,它可能是下洋壳辉长岩在剪切作用下部分熔融形成的。这意味着该区域有可能有下洋壳辉长岩剥露到海底,那么,它将有可能是离莫霍面最近的区域,则有望成为南海莫霍钻的潜在优选区,可以采取多阶段钻探,钻进地幔。但是该区域地球物理资料匮乏,壳幔结构如何,还需进一步深入研究。
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致谢:真诚感谢中国科学院南海海洋地质研究所阎贫研究员和中国科学院深海科学与工程研究所吴时国研究员给予的建设性意见和建议,也衷心感谢编委和审稿人的细心审阅和宝贵意见及建议。
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摘要
“莫霍计划”是20世纪50年代末提出来的堪与“登月计划”相媲美的最宏伟的科学计划之一。它开启了大洋钻探的新篇章,拉开了探索大洋岩石圈的序幕。历经半个多世纪的深海探索,研究学者逐渐揭开了洋壳结构的复杂性和多变性,其中代表下洋壳层3的辉长岩乃至地幔岩剥露的大洋核杂岩,为莫霍钻的实施提供了捷径和优选区。本文综述了“莫霍计划”以来在洋壳结构、洋壳增生机制和大洋核杂岩方面的研究进展,通过分析国际上莫霍计划的选址和策略,结合我国南海洋壳的特征,初步提出“南海莫霍计划”的两个站位及策略:① 南海残余洋中脊附近的U1431孔,该孔已钻到玄武岩层,可通过重复钻进钻穿莫霍面;② 南海东缘残余扩张脊的管事海山,可能有下洋壳辉长岩剥露,可在该区域采取多阶段钻探钻穿莫霍面。
Abstract
The project Mohole was one of the most ambitious scientific plans proposed in the late 1950s, comparable to the “Moon Landing Ambition”. It initiated and accelerated the scientific ocean drilling, through which, researchers have gradually revealed the complexity and variability of the oceanic crust after exploring the deep sea for more than half a century. One special structure as the oceanic core complex, exposed the lower crust and even mantle rocks onthe seafloor, providing a shortcut for drilling through the Moho. We here summarize the progress on oceanic crust, accretion mechanism and oceanic core complex since the initiation of the project Mohole. By analyzing the preferred sites and strategies proposed in the IODP proposals on Mohole, we aim to shortlist two possible sites for the future project Mohole in the South China Sea based on the crust characteristics. One possible site is the U1431 hole, near the relic mid-ocean ridge in the eastern sub-basin of the South China Sea. This hole has penetrated into the basalt of the oceanic crust. Thus, reentry into this hole could be taken into account in drilling through the Moho of the South China Sea. Another preferred site is the Guanshi seamount in the eastern margin of the South China Sea, where previous studies have suggested that the lower crust was probably exposed. Therefore, it could be potentially selected for multi-phase drilling to penetrate into the Moho.
