Structural Relationships Between the Nordfjord-Sogn Detachment Zone and Ultrahigh- Pressure Rocks in the Nordfjord Region, Western Norway

The Nordfjord area of Western Norway hosts an archetypal prograde metamorphic transition within the root of the Scandian collisional orogen, from amphibolite facies to ultrahigh-pressure (UHP) eclogite facies. Although previous studies have correlated amphibolite-facies mylonites across the Nordfjord area with the regional, normal-sense Nordfjord-Sogn Detachment Zone (NSDZ), the large-scale structural relationships of the eclogite-bearing rocks, basement-allochthon tectonostratigraphy, and major zones of deformation remain poorly defined. We carried out a detailed structural, metamorphic and thermochronological analysis of the greater Nordfjord area, examining the structural character of the transition from lower pressure rocks westward to the UHP province. A >5 km thickness of shallowly W-dipping, normal-sense shear fabrics pervades structurally higher allochthonous units and fades downward into the Baltica basement; these mylonites are capped by greenschist-facies detachments. Eclogite and coesite-eclogite isograds cut across the tectonostratigraphy and were attenuated by mylonitic shearing and layer-parallel thinning; no evidence of a discrete high-strain contact with significant pressure difference was found within the UHP boundary zone. Top-W amphibolite-facies mylonitization occurred across a broad section of crust in the Nordfjord area, but was localized along two shear surfaces: 1) a major shear zone within the structurally- higher allochthons that defines the upper boundary of eclogitized crust, which we correlate with the NSDZ; and 2) a localized high-strain zone-informally named the Sandane Shear Zone-that follows an earlier thrust contact between the allochthonous units and the underlying basement. No structural features illuminate the mechanisms responsible for exhumation of the Nordfjord UHP rocks through the mantle; exhumation through lower- to mid-crustal depths was accomplished by normal-sense shear on the NSDZ. Muscovite 40Ar/39Ar cooling ages suggest regional amphibolite-facies shearing was finished by 399-395 Ma; subsequent unroofing was accomplished by higher-level greenschist-facies detachments.