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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Four-dimensional upper crustal construction at fast-spreading mid-ocean ridges: A perspective from an upper crustal cross-section at the Hess Deep Rift
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Four-dimensional upper crustal construction at fast-spreading mid-ocean ridges: A perspective from an upper crustal cross-section at the Hess Deep Rift

机译：快速扩张的中洋海脊的四维上地壳构造：Hess深部裂谷上地壳剖面的透视图

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The faulted walls of the Hess Deep Rift in the equatorial Pacific provide one of the few significant tectonic windows into the oceanic crust created at a fast-spreading mid-ocean ridge. The 5000-m-deep rift graben exposes a slightly dismembered section of oceanic crust created at the East Pacific Rise ～ 1 Ma. The upper portion of the North Wall of the rift features a laterally extensive exposure of upper crustal rock units similar to those found in many ophiolite complexes. For tens of kilometers along the rift wall, basaltic lavas (with sparse dikes) grade down into a spectacular sheeted dike complex that is underlain locally by a heterogeneous assemblage of gabbroic rocks. The geometry of lava flows, dikes and fault zones that developed during magmatic accretion provide constraints on the cross-axis (temporal) evolution of the upper crust in this area. Outcrop-scale digital imagery from submersible vehicles provides a well-constrained geological framework for samples collected from these upper crustal rock units, and the geochemistry of lava and dike samples provides additional temporal constraints on crustal accretion and the role of along-axis magma transport during accretion. Integrated results of geological and geochemical investigations suggest a four-dimensional model for upper crustal construction whereby tectonic and magmatic processes act in concert to construct the upper oceanic crust within a few kilometers (few tens of thousands of years) of a fast-spreading ridge axis. In this model, a sub-axial melt lens, where it exists, is recharged from below with distinct parental melts, and limited along-axis mixing preserves geochemical segmentation of the melt lens. Dikes emanate from distinct portions of the melt lens and transport magmas vertically, as well as significant distances laterally along-axis, erupting lavas in the special case where magmas ascend through dikes to higher levels in the crust and intersect the seafloor. Differences in the phenocryst contents and compositions of lavas and dikes suggests that density filtering of buoyant magmas, among other factors including magma overpressure and orientations of local stresses, plays a role in the eruption of magma on the surface. During spreading and magmatic construction, subaxial subsidence accommodates thickening of the basaltic lava unit. Sub-axial subsidence, faulting, and block rotation result in an upper crust with inward-dipping (toward the axis) lava flows and outward-dipping (away from the axis) dikes. Substantial differences in upper crustal geology may result from segment-scale variations in which vertical magma delivery dominates in areas of robust magma supply (likely over sites of recently recharged melt lenses at segment centers) but lateral magma transport dominates in areas of relatively lower magma supply (possibly toward segment ends).
机译：赤道太平洋海斯深裂谷的断层壁是在快速扩张的中海山脊形成的洋壳中为数不多的重要构造窗口之一。 5000米深的裂谷地en露出了东太平洋上升〜1 Ma时形成的略微被破碎的洋壳部分。裂谷北壁的上部具有上地壳岩石单元的横向大量暴露，这与许多蛇绿岩复合体中发现的相似。沿裂谷壁数十公里处，玄武岩熔岩（带有稀疏的堤防）降级为壮观的片状堤防复合体，该复合体局部地被异质的辉长岩组合所覆盖。在岩浆积聚过程中形成的熔岩流，堤防和断层带的几何形状对该地区上地壳的横轴（时间）演化提供了限制。来自潜水器的露头规模的数字图像为从这些上地壳岩石单元收集的样品提供了一个受约束的地质框架，而熔岩和堤坝样品的地球化学为地壳增生和沿轴岩浆运移的作用提供了额外的时间限制。积聚。地质和地球化学研究的综合结果提出了上地壳构造的四维模型，通过该模型，构造和岩浆作用共同作用，在快速扩散的脊轴线的几公里（数万年）内构造了上层洋壳。在此模型中，存在的亚轴熔融透镜从下方充入不同的母体熔体，并且有限的沿轴混合保留了熔体透镜的地球化学分段。堤防从融化透镜的不同部分散发出来，并沿垂直方向以及沿轴线的横向距离运移岩浆，在特殊情况下，熔岩通过堤坝上升到地壳的较高高度并与海底相交，从而喷发熔岩。熔岩和堤坝的表晶含量和成分的差异表明，浮力岩浆的密度过滤以及岩浆超压和局部应力的取向等因素都对地表岩浆的喷发起到了作用。在铺展和岩浆施工过程中，亚轴向沉降可适应玄武岩熔岩单元的增厚。副轴向下沉，断层和块体旋转导致上地壳向内（向轴线方向）熔岩流和向外（远离轴线）方向熔岩流。上地壳地质的显着差异可能是由段规模变化引起的，其中，垂直岩浆输送在稳固的岩浆供应区域中占主导地位（可能在分段中心最近重新充注的熔融透镜的位置上），而横向岩浆输送在岩浆供应相对较低的地区占主导地位。 （可能朝着段的末端）。

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