洋壳的基本问题与人类的莫霍钻梦想

doi:10.11867/j.issn.1001-8166.2017.12.1245

一般认为,地球物理上的莫霍面是地幔和地壳的分界面。被海水覆盖的洋壳通常只有6~7 km厚,相对于较厚的陆壳来说,洋壳不仅年轻得多,也活跃得多。然而,洋壳的地质结构以及莫霍面的地质意义到底是什么,至今仍是固体地球科学领域最大的悬疑之一。现有的结果表明,依据分布在大陆边缘的蛇绿岩套确定的Penrose模型基本不适用于慢速和超慢速扩张洋中脊的实际洋壳结构,对快速扩张洋中脊的适用性也有一定的疑问。解答洋壳结构及其莫霍面地质意义最有效的手段仍是大洋钻探。20世纪60年代初,试图挺进莫霍面的“莫霍钻”计划揭开了地球科学最伟大的大洋钻探计划的序幕。新世纪以来,随着各种研讨和计划的切实开展,人类真正实现莫霍钻梦想的日子似乎正在来临。

关键词: 洋壳结构, 莫霍面, 下洋壳, 岩浆房, 蛇绿岩套

Knowledge of ocean crust is one of bases to understand the deep and the surface of our planet. Since the definition of the Earth crust based on the geophysical discovery of Moho, marvelous efforts have been made to understand the geological significance of the Moho and the structure of the ocean crust. Up to date, it becomes clear that the Penrose model built up on ophiolite is unsuitable for the explanation of the ocean crust structure along slow and ultraslow spreading ridges, and probably also questionable for that of fast spreading ridges. The only effective way to solve the problem is to drill into the geophysical detected Moho and get samples. With the development of modern technology and more logic scientific strategy, that largely improved from the milestone Mohole projects carried out about half a century ago. The time to realize the Mohole dream seams coming.

Key words: Ocean crust, Moho, Lower ocean crust, Magma chamber, Ophiolite.

中图分类号:

P756.5

图1 基于Oman蛇绿岩剖面岩石地层学的Penrose模型与地震数据比较

Fig.1 Penrose model of oceanic crust based on Oman ophiolite and its comparison to seismic data

图2 平行轴向横穿慢速扩张洋中脊的洋壳剖面简化图 ^{[

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洋脊段末端之下的浅莫霍标注在图中,反映沿着大西洋洋中脊北部内高角之下的地震观察

Fig.2 Simplified, interpreted, axis-parallel section through slow-spread crust ^{[

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]}
Note the shallower Moho beneath segment ends, reflecting seismic observations beneath inside high corners along the northern Mid-Atlantic Ridge

图3 2种洋壳增生的端元模型
(a)“ 冰川”模型,塑性熔体是从洋壳轴部岩浆房下沉并扩张向外,形成下洋壳;(b)“席状”模型,在洋壳中有不同深度的多个熔融体,除了浅层岩浆房岩体之外,深部熔体通过水力破碎传输,结合基底岩床塑性流动;红色箭头代表热液活动范围与方向

Fig.3 Tow types of oceanic crust accretion models (“Glacial”(a) and “sheet”(b) )
(a) A “gabbro-glacier” model, in which melt crystallizes in a small sill at the base of the sheeted dike complex from which cumulates subside down; (b) A “sheeted sill” model, in which the lower oceanic crust forms through the crystallization of multiple sills. Arrow in red represents hydrothermal fluid activities

图4 洋中脊剖面的理想结构图解(未按比例)(据参考文献[ 26 ]修改)
列举了可能影响贯穿过洋脊剖面热液循环的强度和类型因素,比如断层,海山, 基底地形和不透水沉积物以及可能的生命;箭头表示热(红色)和流体(蓝色)流动

Fig.4 Schematic architecture of a mid-ocean ridge flank (not to scale) (modified after reference[26])
Parameters illustrated that may influence the intensity and style of hydrothermal circulation through the ridge flanks, such as faults, seamounts, basement topography, and impermeable sediments, which isolate the crust from the oceans. Hypothetical change in microbial community structure with the depth limit of life increases with crustal age is also shown on the right. Arrows indicate heat (red) and fluid (blue) flow

图5 太平洋莫霍钻3个候选海区 ^{[

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A:科克斯板块区;B:近南部和下加利福尼亚;C:夏威夷外区

Fig.5 Bathymetric map showing the three selected areas for large-scale MoHole site ^{[

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A:Cocos plate region; B: Off Southern/Baja California region; C:Off Hawaii region

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摘要

Fundamental Questions of Ocean Crust and the Dream for Mohole