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The Nordfjord Sogn Detachment Zone (NSDZ) in Norway is the largest known detachment in the world. The footwall was decompressed from P 3 Gpa, at 410-400 Ma, whereas the hanging wall was at greenschist facies, implying 'drastic' excision since these rocks are juxtaposed across the NSDZ. The role of the NSDZ in exhuming (U)HP rocks is not disputed; the question is: how can exhuming be quantified? Here we use two methods:
1) 'Rigid Inclusion Models' (RIMs) in the detachment mylonites and 2) Regional structural geometry and reconstructions.
(1) RIMs have reached a stage where it is possible to use them to obtain quantitative values of flow in shear zones. Data from 3 sites and analogue and numerical modeling were used to assess vorticity, strain, nature of rigid inclusion/matrix interface and confinement in the mylonites. This shows that the observed shape preferred orientation (SPO) high in the NSDZ (Site 1) is best explained by simple shear (pure shear/simple shear, Sr ≈ 0). SPO deeper in the mylonites (Site 2) can be produced by simple shear associated with shortening (Sr ≈ 1), acting on rigid inclusions in slipping contact with matrix. Site 3 close to an UHP eclogite at the base of the NSDZ, back-rotated boudins can form in confined flow associated with a considerable shortening (Sr ≈ 0.4).
(2) Structural geometry and reconstructions on small and regional scale can be used to determine the decompression history of the (U)HP. Most of the rocks in the western WGC have symplectites (bio+ plag and/or hbl + plag) after decompression of HP minerals (gar, phe, cpx). Fabrics in the WGC preserve the transition from steep HP fabrics to post-HP fabrics with flatter enveloping surface. Post-HP fabrics can be used to unravel the exhumation history and show that (U)HP rocks were initially shortened vertically/stretched horizontally in a non-rotational strain field, which affected most of the western part of the WGC. Near the NSDZ these fabrics are increasingly affected by simple shear. UHP exhumation fabrics are not preserved and there is no field data that explains exhumation of rocks from UHP- to HP-conditions.
The observed fabrics indicate a minimum of γ ≥ 20 in the NSDZ. The mylonites show evidence of partitioning from proto- to ultra-mylonites. Simple- and pure shear are heterogeneously distributed and flow in the NSDZ was heterogeneous at km and m scale. Shortening across the shear plane increases with depth and contributes to exhumation of eclogites in the footwall. The fabrics in the WGC and the NSDZ explain exhumation from depths corresponding to ca 2.5-2.7 GPa but not from the UHP conditions.
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