Structure and 40Ar/39Ar thermochronology of an ultrahigh-pressure transition in western Norway
Structure and 40Ar/39Ar thermochronology of an ultrahigh-pressure transition in western Norway
复制标题
挪威西部超高压转变的结构和 40Ar/39Ar 热年代学
DOI：
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发表时间：
2011
影响因子：
2.7
通讯作者：
P. Gans
中科院分区：
文献类型：
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作者：
D. Young;B. Hacker;T. Andersen;P. Gans
摘要：挪威西部的Nordfjord地区拥有一个典型的俯冲地壳剖面，其上覆有泥盆纪沉积物，上覆有超高压榴辉岩，并以地壳规模的伸展剪切带为核心。构造填图揭示了两条在该剖面的形成和折返过程中起不同作用的位移带：（1）Sandane剪切带是一条近榴辉岩壳底部的倾向北西的角闪岩相高应变带，它将外来单元与下伏结晶基底分隔开;它可能起源于早期逆冲推覆作用，但在地壳下部被自上而下的伸展组构所覆盖;（2）在构造上，Nordfjord-Sogn拆离带是一个自上而下的斜长角闪岩相至绿片岩相的拆离剪切带，位于外来单元内，确定了榴辉岩化地壳的上边界，并负责至少在中地壳深度的折返。白云母40 Ar/39 Ar年龄表明，北峡湾-松恩拆离体以下角闪岩相变形主要是在c. 397 Ma，而白云母年龄从更深的部分的超高压域表明，它冷却后390 Ma。在通过中地壳折返过程中，西向伸展伴随着南北向褶皱。中地壳至上地壳的晚期左旋压扭断层截断了早期的褶皱和剪切带。补充材料：完整的40 Ar/39 Ar数据和Nordfjord地区的简要地质图见http://www.geolsoc.org.uk/SUP18460。
Abstract: The Nordfjord region of western Norway hosts an archetypal subducted crustal section, underpinned by ultrahigh-pressure (UHP) eclogite, overlain by Devonian sediments, and cored by a crustal-scale extensional shear zone. Structural mapping reveals two distinct displacement zones that played different roles during the formation and exhumation of this section: (1) the Sandane Shear Zone is a NW-dipping, amphibolite-facies, high-strain zone near the base of the eclogite-bearing crust that separates allochthonous units from underlying crystalline basement; it may have originated during early thrusting, but was overprinted by top-to-the-west extensional fabrics at lower crustal depths; (2) structurally above this, the Nordfjord–Sogn Detachment Zone is a top-to-the-west, amphibolite- to greenschist-facies detachment shear zone within allochthonous units that defines the upper boundary of the eclogitized crust and was responsible for exhumation through at least mid-crustal depths. Muscovite 40Ar/39Ar ages suggest that amphibolite-facies deformation below the Nordfjord–Sogn Detachment was mostly finished by c. 397 Ma, whereas muscovite ages from the deeper parts of the UHP domain indicate that it cooled after 390 Ma. During exhumation through the middle crust, west-directed stretching was accompanied by north–south folding. Late sinistral transpressional faulting in the middle to upper crust truncated the earlier folds and shear zones. Supplementary material: Complete 40Ar/39Ar data and a summary geological map of the Nordfjord region are available at http://www.geolsoc.org.uk/SUP18460.