Geologic Investigations in the Cayman Trough: Evidence for Thin Oceanic Crust along the Mid-Cayman Rise
Geologic Investigations in the Cayman Trough: Evidence for Thin Oceanic Crust along the Mid-Cayman Rise
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开曼海槽地质调查：开曼中隆隆沿线薄洋壳的证据
DOI：
10.1086/628605
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发表时间：
1981
期刊：
影响因子：
--
通讯作者：
P. J. Fox
中科院分区：
文献类型：
--
作者：
J. B. Stroup;P. J. Fox
中开曼隆起是一个长110公里（约2cm/年，全速率）的吸积板块边界，以深裂谷为特征。阿尔文潜水器沿着裂谷两侧的两个区域的裂谷壁进行了18次应答器导航潜水。岩壁的起伏是由大量向内的小断层陡崖造成的，垂直位移从几十米到可能高达400米。其中辉长岩（116件）的组成范围广泛，变质历史复杂，在数量上优于其他岩石类型（玄武岩、蛇纹超镁铁质）。我们解释了悬崖上暴露的岩石组合，表明在开曼隆起中部的增生轴下存在一个浅层岩浆储层，并形成了异常薄的海洋地壳，外壳只有几百米的辉绿岩和玄武岩。这个壳层下面是一个深成组合，厚度可能只有1到2公里，由长辉长岩组成，被蛇纹化超镁铁质侵入。由于短而缓慢增生的隆升段和两条长转换断层并置，沿隆升轴形成的地壳可能很薄。紧密间隔的转变为在吸积轴下上升的软流圈楔形物充当了吸热器，导致从软流圈分离出来的熔体体积减少，因此，比正常的海洋地壳更薄。虽然中开曼隆起可能代表了这种效应的一个端元例子，但我们认为，在缓慢增生的板块边界上，薄洋壳和浅层非均质辉长岩组合的存在是脊/转换交叉点的特征。
The Mid-Cayman Rise is a 110 km long accreting plate boundary (~ 2cm/yr, full rate) characterized by a deep rift valley. Eighteen transponder-navigated dives were made by the submersible ALVIN along the rift valley walls in two areas on opposite sides of the rift valley. The relief of the walls is produced by a large number of inward facing small-throw fault escarpments with vertical displacements ranging from a few tens of meters to perhaps as much as 400 m. From these escarpments 142 samples were collected, and gabbroic rocks (116 samples), characterized by a wide range of compositions and a complex metamorphic history, predominate in number over other rock types (basalt, serpentinized ultramafic). We interpret the assemblage of rocks exposed on the escarpments to indicate that a shallow level magma reservoir exists beneath the axis of accretion at the Mid-Cayman Rise and creates anomalously thin oceanic crust with a carapace of only several hundreds of meters of diabase and basalt. This carapace is underlain by a plutonic assemblage, possibly of only 1 to 2 km thickness, composed of gabbroic rocks intruded by screens of serpentinized ultramafic. The crust created along the axis of the Rise may be thin as a result of the juxtaposition of the short, slowly accreting Rise segment and two long transform faults. The closely spaced transforms act as heat sinks for the asthenosphere wedge rising beneath the axis of accretion, resulting in a decrease in the volume of melt segregated from the asthenosphere and, hence, a thinner than normal oceanic crust. Although the Mid-Cayman Rise probably represents an end-member example of this effect, we suggest that thin oceanic crust and the existence of a heterogeneous assemblage of gabbroic rocks at shallow levels is a characteristic of ridge/transform intersections at slowly accreting plate boundaries.