Magmatism versus serpentinization-crustal structure along the 13°N segment at the Mid-Atlantic Ridge
Magmatism versus serpentinization-crustal structure along the 13°N segment at the Mid-Atlantic Ridge
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大西洋中脊北纬 13°段的岩浆作用与蛇纹石化地壳结构
DOI：
10.1093/gji/ggaa052
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发表时间：
2020
影响因子：
2.8
通讯作者：
Reston T
中科院分区：
文献类型：
--
作者：
Reston T
在大西洋中脊13°N处存在一个大洋核复合体（OCCs）区，被认为是一个模式产地。该地点包括两个位于13°20′N和13°30′N的OCC，被认为处于活跃和垂死的演化阶段，以及两个被称为Ashadze Complex（中心位于13°05′N）的被认为是遗迹。本文描述了沿着一条200 km长的二维样带的S波地震模拟结果，该样带从南到北穿过墨丘利和马拉松断裂带、13°N段的南外角、OCC、以13°35′N为中心的脊轴趋势偏差以及北起的东脊侧翼的最年轻洋壳。我们的反演模型和相应的Vp/Vs比值表明，13°30′N OCC下方的大部分地壳由已被挖出至最浅海底水平的变质岩性组成，而玄武岩岩性位于13°20′N OCC下方。这些截然不同的地壳结构之间的过渡发生在不到5公里的距离上，并延伸到海底以下至少0.22公里的深度。Ashadze复合体的北方和南方OCC在浅层具有对比结构，北方OCC在上地壳具有更快的S波速度。AVp/Vs比值>1.9（等效泊松比>0.3）表明在它们之间的测深凹陷之下以及在OCC南部之下的地壳之内，存在着折返和/或变质的岩性。在Marathon断裂带的北方和南翼以及Mercurius断裂带的北方翼之间，下地壳具有相对较低的Vp/Vs比值，这表明与马拉松断裂带有关的变形在下地壳内部横向延伸，有利于流体的进入。马拉松断裂带本身的下面是一个宽达120公里的低S波速度（10.2 km s-1）区，从海底到至少中地壳，反映在高Vp/Vs比和较低的密度，特别是在其测深足迹内海底以下10.1 km以下。低Vp/Vs比值<1.6（等效泊松比<0.15）突出了火山域。我们的地震和密度组合模型支持OCC演化的局部化模型。它们还显示出岩浆地壳与变质地壳在数量和分布上的相当大的山脊平行变化。研究结果表明，现今岩浆活动的焦点位于13°20′N OCC以北，在那里所观察到的岩浆增生型海底形态反映在微震活动模式中，表明其向内的中谷壁断层可能与13°20′N OCC拆离面相连。在13°30′N OCC下盘分离之后（向西）的岩浆活动和活动断层，以及微震活动集中在沿其南翼沿着的条带中，表明脊的几何形状正沿沿着轴进行重新调整。作为这一过程的一部分，一个转换偏移正在形成，最终将容纳13°30′N OCC在其北侧脊轴东侧的内角。
A region of oceanic core complexes (OCCs) exists at 13°N on the Mid-Atlantic Ridge that is regarded as a type site. This site includes two OCCs at 13°20′N and 13°30′N, thought to be in the active and dying stages of evolution, and two together called the Ashadze Complex (centred at 13°05′N) that are considered to be relict. Here we describe the results ofS-wave seismic modelling along an ∼200-km-long 2-D transect traversing, south-to-north, through both the Mercurius and Marathon fracture zones, the southern outside corner of the 13°N segment, the OCCs, the ridge axis deviation in trend centred at 13°35′N, and the youngest oceanic crust of the eastern ridge flank to the north. Our inversion model, and the correspondingVp/Vsratio, show that the majority of the crust beneath the 13°30′N OCC comprises metamorphosed lithologies that have been exhumed to the shallowest subseabed level, while basaltic lithologies underlie the 13°20′N OCC. The transition between these contrasting crustal structures occurs over a distance of <5 km, and extends to at least ∼2 km depth below seafloor. The northern and southern OCCs of the Ashadze Complex have contrasting structures at shallow depth, with the northern OCC having a fasterS-wave velocity in the upper crust. AVp/Vsratio of >1.9 (and equivalent Poisson's ratio of >0.3) indicates exhumed and/or metamorphosed lithologies beneath the bathymetric depression between them and within the crust beneath the southern OCC. Between the northern and southern flanks of the Marathon fracture zone and northern flank of Mercurius fracture zone, the lower crust has a relatively lowVp/Vsratio suggesting that the deformation associated with Marathon fracture zone, which facilitates fluid ingress, extends laterally within the lower crust. Marathon fracture zone itself is underlain by a broad zone of lowS-wave velocity (∼2.0 km s−1) up to ∼20 km wide from the seabed to at least the mid-crust, that is mirrored in a highVp/Vsratio and lower density, particularly deeper than ∼1 km below seabed within its bathymetric footprint. Volcanic domains are highlighted by a lowVp/Vsratio of <1.6 (and equivalent Poisson's ratio of <0.15). Our combined seismic and density models favour the localized model of OCC evolution. They also show a considerable ridge-parallel variability in the amount and distribution of magmatic versus metamorphosed crust. Our results suggest that the current focus of magmatism lies to the north of the 13°20′N OCC, where the magmatic accretion-type seabed morphology observed is mirrored in the pattern of microseismicity, suggesting that its inward-facing median-valley-wall fault may link to the 13°20′N OCC detachment surface. Magmatism and active faulting behind (to the west) the footwall breakaway of the 13°30′N OCC, and the microseismicity concentrated in a band along its southern flank, suggest a readjustment of ridge geometry along axis is underway. As part of this, a transform offset is forming that will ultimately accommodate the 13°30′N OCC in its inside corner on the eastern flank of the ridge axis to the north.