地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
ISSN-L : 0022-135X
論説
ハワイ沖上部地殻掘削から海洋プレートの形成過程に迫る
海野 進草野 有紀
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ジャーナル フリー

2021 年 130 巻 4 号 p. 599-614

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 The style of crustal extension is governed by M, which is the ratio of magma consumed to relax the crustal strain caused by plate spreading. Bathymetric profiles across ridges are reproduced well by changing M. However, what determines the value of M has not been explained. Fast-spread oceanic crust comprises dense sheet flows underlain by thin dense sheeted dikes compared to magmas. This density structure increases the magma extruded, allowing the crust to be extended solely by magmatic accretion; whereas, the intermediate-spread crust consists of less dense pillow lavas, yielding an apparent level of neutral buoyancy that traps magma to develop the sheeted dikes below. Consequently, the crust extends through dike intrusions in the lower levels and faults at shallow levels. Thus, the density structure of the oceanic crust determines the style of plate spreading, or the value of M. Because the spreading rate or the strain rate does not vary within the same ridge segment, intrasegment variations in crustal structure depend directly on the supply rate of magma, or the value of M, which decreases with the thinning of extrusive layers and the thickening of sheeted dikes along the Galapagos Spreading Center and the East Pacific Rise. This tendency is supported by the crustal architecture observed in holes 504B and 1256D, the Hess Deep and the Oman Ophiolite. The density structure of the upper crust can be discerned by the proportion of sheet flows among extrusive rocks on the ridge axis, which is drastically reduced with spreading rates from 10 cm/a to 7 cm/a. This spreading rate interval coincides with the change in axial magma chamber depth. Throughout this rate interval, the key observation is whether the style of crustal extension from magmatic accretion-dominant to fault displacement-dominant changes gradually, or changes abruptly across a certain threshold that divides these two. To understand how the crustal extension varies through the spreading rate interval, it is proposed to drill into the 78-81 Ma old crust spread at 7 cm/a at one of the candidate sites of MoHole on the North Arch off Hawaii. Crust of this age and spreading rate interval has never been drilled in the history of ocean drilling. This drilling will provide a reference section for the upper oceanic crust and a pilot hole to inform the design of future mantle drilling.

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