Seismic imaging of deep crustal melt sills beneath Costa Rica suggests a method for the formation of the Archean continental crust
Seismic imaging of deep crustal melt sills beneath Costa Rica suggests a method for the formation of the Archean continental crust
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DOI：
10.1016/j.epsl.2015.07.062
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发表时间：
2015-11
影响因子：
5.3
通讯作者：
N. Harmon;C. Rychert
中科院分区：
文献类型：
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作者：
N. Harmon;C. Rychert
大陆地壳形成于数十亿年前，但不能用原始地幔岩浆的简单演化来解释。这需要一个多步骤的过程，可能包括在高压下重新融化湿变质玄武岩。这样的过程可能发生在因大型岩浆活动而变厚的海洋地壳深处。在中美洲，根据地质推断，弧下存在的海洋地壳厚度的变化为研究其对岩浆储存的影响、变质玄武岩的再融化以及形成大陆地壳的可能性提供了一个极好的机会。我们使用来自环境噪声层析成像的表面波对哥斯达黎加加厚的海洋高原地壳中6%的径向各向异性结构进行了成像，这些结构可能代表了深部地壳融化层。在尼加拉瓜，弧是在较薄的海洋地壳上形成的，我们没有想象到这些深层地壳融化的断层。这些深岩的存在与来自哥斯达黎加弧的更多的英质弧输出有关，表明先前的增厚地壳加速了原生玄武岩向大陆成分的加工。在太古宙，随后的水合热点火山作用或俯冲带火山作用对增厚的洋壳进行了再加工，可能同样促进了早期大陆地壳的形成。如果俯冲直到3ga才开始，这种机制可能特别重要。
Continental crust formed billions of years ago but cannot be explained by a simple evolution of primary mantle magmas. A multi-step process is required that likely includes re-melting of wet metamorphosed basalt at high pressures. Such a process could occur at depth in oceanic crust that has been thickened by a large magmatic event. In Central America, variations in geologically inferred, pre-existing oceanic crustal thickness beneath the arc provides an excellent opportunity to study its effect on magma storage, re-melting of meta-basalts, and the potential for creating continental crust. We use surface waves derived from ambient noise tomography to image 6% radially anisotropic structures in the thickened oceanic plateau crust of Costa Rica that likely represent deep crustal melt sills. In Nicaragua, where the arc is forming on thinner oceanic crust, we do not image these deep crustal melt sills. The presence of these deep sills correlates with more felsic arc outputs from the Costa Rican Arc suggesting pre-existing thickened crust accelerates processing of primary basalts to continental compositions. In the Archean, reprocessing thickened oceanic crust by subsequent hydrated hotspot volcanism or subduction zone volcanism may have similarly enhanced formation of early continental crust. This mechanism may have been particularly important if subduction did not initiate until 3 Ga.