Divergence in subduction zones and exhumation of high pressure rocks (Eocene Western Alps)
Divergence in subduction zones and exhumation of high pressure rocks (Eocene Western Alps)
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DOI：
10.1016/j.epsl.2011.08.002
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发表时间：
2011-10
影响因子：
5.3
通讯作者：
M. Malusà;C. Faccenna;E. Garzanti;R. Polino
中科院分区：
文献类型：
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作者：
M. Malusà;C. Faccenna;E. Garzanti;R. Polino
高压岩石的折返一直是地球科学中有争议的问题。在这篇文章中，我们分析了西阿尔卑斯地区的构造变质，地层和板块运动的限制，在最早的造山阶段的折返机制和构造演化提供了新的见解。西阿尔卑斯山脉的始新世榴辉岩在造山带的上板块一侧形成了一个20- 25 km宽的带（榴辉岩带），暴露在低压增生楔后部的伸展剪切带之下。榴辉岩带各单元是俯冲带内最年轻的峰压组合，经历了45- 40 Ma的超快速构造折返。在整个这一阶段，侵蚀的作用可以忽略不计。始新世前陆盆地仍然处于饥饿状态，而轴向带碎屑的大量到达在折返完成后很久就开始了。因此，基于固定边界剥露模型（如河道流）和/或暗示地表快速侵蚀（如板块断裂）的构造重建与地质证据不一致。在缺乏侵蚀的情况下，通过覆盖层的折返需要俯冲带内的分歧。我们证明，这不是通过下板块（欧洲）的回滚实现的，而是通过上板块（阿德里亚-非洲）远离西阿尔卑斯海沟的NNEward运动实现的。这种运动导致了上板块薄弱部分的局部扩张，在增生楔的后部，并允许上地壳中的地壳运动带以比俯冲速率快得多的速度构造就位。构造折返在渐新世停止，当时沿西阿尔卑斯山脉走向的伸展-发散沿着变为伸展-会聚。开始缓慢侵蚀去顶同步记录的压力-温度路径在所有主要构造单元的西阿尔卑斯山，并到达造山碎屑沉积盆地。这项工作表明，在大陆俯冲带中，上板块和海沟之间的分歧是一种可行的机制，以发掘大型和连贯的榴辉岩单元。我们的发掘模型可以应用于显示类似发掘记录的其他榴辉岩带，包括西部片麻岩地区、达比-苏禄和东部巴布亚新几内亚。
Exhumation of high-pressure rocks has long remained a controversial issue in the Earth sciences. In this article, we analyze the tectono-metamorphic, stratigraphic and plate-motion constraints from the Western Alps region, providing new insights on exhumation mechanisms and tectonic evolution during the earliest orogenic stages. Eocene eclogites of the Western Alps form a 20–25km wide belt on the upper-plate side of the orogen (Eclogite belt), exposed beneath extensional shear zones at the rear of a lower-pressure accretionary wedge. Units of the Eclogite belt show the youngest peak-pressure assemblages within the subduction zone, and experienced superfast tectonic exhumation since 45–40Ma. The role of erosion was negligible during the whole of this stage. Eocene foreland basins remained starved, and the massive arrival of axial-belt detritus began well after exhumation was completed. Tectonic reconstructions based on fixed-boundaries exhumation models (e.g. channel flow), and/or implying fast erosion at the surface (e.g. slab breakoff), are thus not consistent with geological evidence. In the lack of erosion, exhumation through the overburden requires divergence within the subduction zone. We demonstrate that this was not attained by rollback of the lower plate (Europe), but was instead attained by NNEward motion of the upper plate (Adria-Africa) away from the Western Alps trench. Such motion induced localized extension within the weak portion of the upper plate, at the rear of the accretionary wedge, and allowed tectonic emplacement of the Eclogite belt in the upper crust at rates much faster than subduction rates. Tectonic exhumation ceased in the Oligocene, when oblique-divergence along the Western Alps traverse changed into oblique-convergence. The onset of slow erosional unroofing was synchronously recorded by pressure–temperature paths in all major tectonic units of the Western Alps, and by arrival of orogenic detritus in sedimentary basins. This work demonstrates that divergence between upper plate and trench is a viable mechanism to exhume large and coherent eclogite units in continental subduction zones. Our exhumation model can be applied to other eclogite belts showing a similar exhumational record, including the Western Gneiss Region, the Dabie-Sulu, and eastern Papua New Guinea.