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首页> 外文期刊>Tectonophysics: International Journal of Geotectonics and the Geology and Physics of the Interior of the Earth >Magmatic-tectonic effects of high thermal regime at the site of active ridge subduction: the Chile Triple Junction model
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Magmatic-tectonic effects of high thermal regime at the site of active ridge subduction: the Chile Triple Junction model

机译：活跃脊俯冲部位高热态的岩浆构造效应：智利三结模型

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摘要

High thermal gradients are expected to be found at sites of subduction of very young oceanic lithosphere and more particularly at ridge-trench-trench (RTT) triple junctions, where active oceanic spreading ridges enter a subduction zone. Active tectonics, associated with the emplacement of two main types of volcanic products, (I) MORE-type magmas, and (2) calc-alkaline acidic magmas in the forearc, also characterize these plate junction domains. In this context, MORE-type magmas are generally thought to derive from the buried active spreading center subducted at shallow depths, whereas the origin of calc-alkaline acidic magmas is more problematic. One of the best constrained examples of ridge-trench interaction is the Chile Triple Junction (CTJ) located southwest of the South American plate at 46 degrees 12'S, where the active Chile spreading center enters the subduction zone. In this area, there is a clear correlation between the emplacement of magmatic products and the migration of the triple junction along the active margin. The CTJ lava population is bimodal, with mafic to intermediate lavas (48-56% SiO2) and acidic lavas ranging from dacites to rhyolites (66-73% SiO2). Previous models have shown that partial melting of oceanic crust plus 10-20% of sediments, leaving an amphibole- and plagioclase-rich residue, is the only process that may account for the genesis of acidic magmas, Due to special plate geometry in the CTJ area,a given section of the margin may be successively affected by the passage of several ridge segments. We emphasize that repeated passages will lead to the development of very high thermal gradients allowing melting of rocks of oceanic origin at temperatures of 800-900 degreesC and low pressures, corresponding to depths of 10-20 km depth only. In addition, the structure of the CTJ forearc domain is dominated by horizontal displacements and tilting of crustal blocks along a network of strike-slip faults. The occurrence of such a deformed domain implies that an important tectonic coupling may exist between the upper and the lower plates leading to the partitioning of the continental lithosphere and to the tectonic underplating of very young oceanic lithosphere below the continental wedge. We assume that in the case of the CTJ, the uncommon situation of three successive ridge segments entering the trench at 2-3 Ma intervals only resulted in a strong and finally long-lived thermal anomaly. This anomaly caused remelting of underplated portions of very young, still hot oceanic lithosphere. Only particular geometrical RTT configurations are able to produce such features. These include linear continental margin, short ridge segments slightly oblique to the trench and short transform faults. Finally, the CTJ example shows that a possible scenario for the origin of calc-alkaline acidic rocks in the near-trench region involves coeval tectonic coupling and repeated passage of thermal anomalies due to successive subduction of short ridge segments. Therefore, the local abundance of calc-alkaline acidic rocks, associated with MORE-type lavas in ancient series, could be the tracer of plate tectonic configurations involving the subduction of short ridge segments in a relatively short duration. (C) 2000 Elsevier Science B.V. All rights reserved. [References: 44]
机译：预计在非常年轻的海洋岩石圈俯冲点，尤其是在脊-海沟-海沟（RTT）三重交界处会发现高热梯度，那里活跃的海洋扩张脊进入俯冲带。活跃的构造学与火山产物的两种主要类型的放置有关，（I）岩心类型的岩浆和（2）前臂的钙碱性酸性岩浆也表征了这些板块交界处。在这种情况下，更多类型的岩浆通常被认为是从俯冲的浅埋深度活跃的活动扩散中心衍生而来的，而钙碱性酸性岩浆的成因则更为棘手。脊-沟相互作用的最佳约束例子之一是智利三结（CTJ），它位于南美板块西南部46°12'S处，活跃的智利扩散中心进入俯冲带。在该区域，岩浆产品的位置与三重结沿活动边缘的迁移之间存在明显的相关性。 CTJ熔岩群是双峰的，镁铁质至中层熔岩（48-56％SiO2），酸性熔岩范围从达西到流纹岩（66-73％SiO2）。以前的模型表明，由于CTJ中特殊的板块几何形状，洋壳的部分融化加上10-20％的沉积物会留下富含闪石和斜长石的残留物，这是唯一可解释酸性岩浆成因的过程。区域，边缘的给定部分可能会受到几个脊段的通过的影响。我们强调，反复通过将导致非常高的热梯度的发展，从而使海洋起源的岩石在800-900摄氏度的温度和低压（仅对应于10-20公里深度）下熔化。此外，CTJ前臂区域的结构主要由沿走滑断层网络的水平位移和地壳块的倾斜引起。这种变形区域的出现意味着上，下板之间可能存在重要的构造耦合，从而导致大陆岩石圈的分隔以及大陆楔以下非常年轻的海洋岩石圈的构造底板。我们假设在CTJ情况下，三个相继的脊段以2-3 Ma的间隔进入沟槽的罕见情况只会导致强烈的并最终长寿命的热异常。这种异常现象导致非常年轻但仍很热的海洋岩石圈的底板下部分重熔。只有特定的几何RTT配置才能产生这种特征。这些包括线性大陆边缘，短脊部分（略微倾斜于沟槽）和短转换断层。最后，CTJ实例表明，在近海沟地区，钙碱性酸性岩石的成因可能是由于短脊段的连续俯冲，导致了古时代的构造耦合和热异常的反复通过。因此，与古代系列的MORE型熔岩有关的钙碱性酸性岩石的局部丰度可能是板块构造构造的示踪剂，涉及在相对较短的时间内俯冲短脊段。 （C）2000 Elsevier Science B.V.保留所有权利。 [参考：44]

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