Thrusting and extension in the Scandian Hinterland, Norway: new U-Pb ages and tectonostratigraphic evidence

A new regional compilation map and U-Pb ages on a suite of variably deformed, Ordovician, calc-alkaline intrusive igneous rocks requires a reinterpretation of the nature of continental collision and extensional exhumation of deep-seated rocks of the Western Gneiss Region west and northwest of Trondheim. A suite of calc-alkaline plutonic rocks, in the age range 482 to 438 Ma, previously known from the region of Smøla-Hitra-Ørlandet-Froan above the Høybakken extensional detachment fault associated with the Devonian 'Old Red Sandstone' basins, is shown to extend over wide areas below the fault, commonly as strongly foliated and lineated gneisses that had been previously mistaken for parts of the Proterozoic Western Gneiss Region. At Follafoss, a member of this intrusive suite is unconformably overlain by weakly metamorphosed conglomerate and volcanogenic sedimentary rocks of probable Late Ordovician age, suggesting that both the sedimentary rocks and the underlying intrusions correlate with the Støren Nappe in the upper part of the local sequence of Caledonide nappes.

U-Pb evidence of metamorphic ages from deep-seated rocks of the Western Gneiss Region include: 1) zircon reaction rims on Proterozoic igneous baddeleyite in the Selnes Gabbro at 401 ± 2 Ma; 2) widespread development of zircon overgrowth and metamorphic zircon with omphacite and coesite inclusions from the Hareidlandet eclogite at 402 ± 2 Ma; and 3) extreme Devonian thermal resetting and neocrystallization of titanite over a wide area of the Proterozoic basement gneisses ending at 395 ± 3 Ma, here fully documented for the first time. At Kjørsvika, west of Trondheimsfjord, a ductilely deformed gneiss of the Ordovician intrusive suite contains igneous titanite dated at 455 Ma that shows little evidence for Devonian thermal resetting. This gneiss lies only 1 to 2 km northwest from a large area of Proterozoic gneisses with 100 percent Devonian reset titanite across a ductilely deformed contact that must represent a phase of extensional detachment (Agdenes detachment) much older than the more brittle detachments (compare Høybakken detachment) associated with some of the present outcrops of the Devonian clastic basins.

These and other relationships suggest the following broad sequence of Siluro-Devonian events in the region: A) Early Scandian (430 - 410 Ma) thrusting with emplacement of the composite Støren Nappe onto the relatively cool Baltoscandian margin of Baltica during contemporaneous subduction of its distal part, locally producing eclogites, possibly representing the earliest initiation of high-level 'post-orogenic' clastic basins. B) Early mid-Scandian (410 - 406 Ma) continued subduction and imbrication of a more proximal part of the Baltoscandian margin, with its heating and high-pressure metamorphism. Continued deep-level thickening by imbrication providing the gravitational potential for foreland- and hinterland-directed extension at high levels, while deposition in Devonian clastic basins probably continued. C) Late mid-Scandian (406 - 396 Ma) continued subduction of more proximal Baltican continental basement with common production of HP and UHP eclogites. Imbricate thrusting of a subducted and heated proximal continental slab provided the gravitational potential to initate hinterland backsliding and extensional emplacement of the cool high-level Støren Nappe against the cooling basement. Deposition in high-level Devonian clastic basins was active. D) Early late-Scandian (396 - 390 Ma). The extension and cooling initiated in C) brought an enormous volume of Proterozoic basement from high amphibolite facies to low amphibolite facies and through about 600°C, terminating Pb loss and neocrystallization of titanite at 395 ± 3 Ma. During this phase, ductile extensional deformation progressed into a sinistral shear field, producing folds and subhorizontal stretching lineations within the previously thrust and extensionally juxtaposed, deep and high-level packages. E) Late Scandian (390 - 375 Ma) extension in a sinistral shear field bringing the ductilely deformed package into contact with brittle, highest-level rocks along detachments associated with outcrop areas of the Devonian clastic basins. In this sequence, the brittle detachments (E) played a relatively minor role, whereas the mid-Scandian deep-crustal imbrication and related high-level collapse (B, C and early D) were the main events that brought deep metamorphic rocks relatively close to the surface.