Exhuming the Sanbagawa metamorphic belt: the importance of tectonic discontinuities
Exhuming the Sanbagawa metamorphic belt: the importance of tectonic discontinuities
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挖掘三八川变质带：构造不连续性的重要性
DOI：
10.1111/j.1525-1314.1998.00072.x
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发表时间：
1998
影响因子：
3.4
通讯作者：
S. Wallis
中科院分区：
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作者：
S. Wallis
已经提出的解释区域变质带向地表迁移的构造过程可以大致分为两种类型。(i)由两个基本刚性的板块界定的收敛边缘内的角流，与浅层的伸展有关。这种类型的模型假设变形分布在整个边缘，任何不连续性都是次要的。(ii)相干变质推覆体的排出或挤压。在第二种观点中，构造不连续面是变质岩向地表迁移的基础。在日本西南部的三宝川变质带进行的各种研究中获得了丰富的地质数据，这使得它非常适合研究折返过程中连续变形与不连续变形的相对重要性。在三坝川带中，沿几个断层向下的变质压力突然下降，这表明存在一个主要的构造接触面，将两个主要的区域推覆体分隔开来：上覆的高压别子推覆体和下覆的低压Oboke推覆体。在别志推覆体中也提出了主要的构造不连续性，然而，变质温度指标、放射性年龄测定结果和显微构造研究都表明，变质后不连续性很小，该推覆体形成并保持为基本上连贯的单元。岩性组合和岩石学研究表明，两个推覆体的位置如下。别子推覆体形成于前增生楔的深处，与上覆地幔楔相邻，倾角约为30 °C。 相反，奥博克推覆体形成于增生楔内的中等深度，远离地幔楔，并且大致水平。变质作用高峰期之后的渗透变形几乎影响了整个三八川带，并在其折返过程中发挥了重要作用。然而，在低压Oboke推覆体上存在广泛的连贯Besshi推覆体，这表明浮力驱动的挤压等过程在折返过程中也很重要，并形成了三坝川变质带的结构。
Tectonic processes that have been proposed to explain the transport to the surface of regional metamorphic belts can be broadly divided into two types. (i) Corner‐flow within a convergent margin bounded by two essentially rigid plates associated with extension at shallow levels. This type of model assumes deformation is distributed throughout the margin and that any discontinuities are of secondary importance. (ii) Expulsion or extrusion of coherent metamorphic nappes. In this second idea, tectonic discontinuities are fundamental in the transport to the surface of metamorphic rocks. The wealth of geological data available from a variety of studies in the Sanbagawa metamorphic belt, southwest Japan makes it well‐suited for studying the relative importance of continuous vs. discontinuous deformation in the process of exhumation. In the Sanbagawa belt a sudden decrease in metamorphic pressure going down section of several kilobars suggests the presence of a major tectonic contact separating two major regional nappes: an overlying higher‐pressure Besshi nappe and an underlying lower‐pressure Oboke nappe. Major tectonic discontinuities have also been proposed within the Besshi nappe, however, indicators of metamorphic temperature, the results of radiometric age dating, and microstructural studies all suggest that post‐metamorphic discontinuities are minor and that this nappe formed and remained as an essentially coherent unit. Lithological associations and petrological studies suggest the following positions for the two nappes. The Besshi nappe formed deep within the former accretionary wedge, adjacent to the overlying mantle wedge, and with a dip of roughly 30 °C. In contrast, the Oboke nappe formed at moderate depths within the accretionary wedge, was distant from the mantle wedge, and was roughly horizontal. Penetrative deformation that post‐dates the peak of metamorphism has affected nearly all of the Sanbagawa belt and has played an important role in its exhumation. However, the presence of a broad coherent Besshi nappe overlying the lower‐pressure Oboke nappe suggests that some process such as buoyancy‐driven extrusion was also important in the exhumation process and in forming the structure of the Sanbagawa metamorphic belt.
Hara, I.、Shiota, T.、Hide, K.、Okamoto, K.、Takeda, K.、Hayasaka, Y. 和 Sakurai, Y.：“三波川带的推覆结构”。
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