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1 Geological Institute, University of Bergen, Allégaten 41, N-5007 Bergen, Norway
2 Geo-Recon AS., Munkedamsveien 59, N-0270 Oslo, Norway
The Sognefjord transect through the lower to middle Palaeozoic Caledonian mountain belt in southern Norway provides one of the best and most completely documented examples of late collisional tectonics in an Alpine-type orogen. It exposes a 250 km long cross-section from the cratonic foreland, in the east, through the heavily deformed continental margin (Baltica), to the remains of the Caledonian ocean (Iapetus) at the Norwegian west coast. Exceptionally detailed and complete structural data are available along the whole transect, together with good stratigraphic, radiometric, petrological, and geophysical control. In this synthesis, the structural data are analysed, in terms of the kinematics and relative age of the different deformation phases, and correlated along the whole transect. The analysis is then used, in conjunction with the other data, to carry out a retrodeformation, reconstructing the crustal geometry at different stages backward in time. The earliest of the present reconstructions (c. 410 Ma) marks the time of formation of the well-known West Norwegian eclogites, in an over-deepened root of Baltica which had developed in the ductile lower crust as a response to extreme crustal shortening. The brittle upper crust took up the shortening by the SE movement of a rigid sheet of Precambrian basement (Jotun complex) above the low-angle Jotunheimen contractional detachment, across a rigid wedge of the Baltic Shield. During the final stages of contraction (c. 410–395 Ma), the upper crust acted as an orogenic lid, against which the root ‘collapsed’ upwards by sub-vertical shortening and lateral E-W extension. During this process of inverted gravity spreading, the eclogites were carried upwards from 60–70 km to 40 km (exhumation phase 1, rate 2–3 mm a–1) and retrograded within their deforming gneissic matrix. At the end of this phase, the strain field in the upper crust changed from contraction to extension, concomitant with a broad up-doming (base Devonian unconformity) which caused a further 10 km exhumation by 385 Ma (exhumation phase 2, 1 mm a–1). This was followed by the main phase of crustal extension with the development of low-angle normal top-to-W or NW fault and shear zones, of which the Nordfjord-Sogn detachment was the most important (50 km of normal displacement). Exhumation in this phase took place by rapid uplift and erosion of the footwall of the detachment, causing the currently exposed eclogites in outer Sognefjord to rise the remaining 30 km (exhumation phase 3, 1.5 mm a–1), to become juxtaposed against Devonian conglomerates on the hanging wall. The reconstructions confirm the general picture of eclogite exhumation in western Norway, and fill out some of the details. However, they do not support the idea that the process was due to extensional orogenic collapse caused by advective or convective lithospheric thinning. Although gravity played a significant role at various stages in the process, the main phase of crustal extension seems to have been mainly related to changes in Devonian plate motion.
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