Extension, disruption and translation of an orogenic wedge by exhumation of large ultrahigh pressure terranes: Two examples from the Norwegian Caledonides

We propose the thrust-dominated accretion of an orogenic wedge during continental subduction can be succeeded by stretching, disruption and passive transport of the frontal part of the wedge on top of an exhuming high pressure/ultrahigh pressure (HP/UHP) metamorphic terrane. Initial thrusting occurs when cratons collide and one subducts beneath the other into the mantle. The subducted craton undergoes HP/UHP metamorphism while an accretionary orogenic wedge develops at its junction with the overlying craton. The subsequent exhumation of the HP/UHP terrane either by true extension and/or buoyancy-driven extrusion reverses the shear traction along its upper boundary from its earlier foreland-directed thrust motion to hinterland-directed normal displacement. This normal-sense shear stretches the orogenic wedge and can potentially detach a fragment of its frontal part away from the rearward part, allowing it to be carried passively towards the foreland on the exhuming plate with the length of displacement a function of the amount of exhumation of the HP/UHP terrane. The Jotun and Trondheim Basin Nappe Complexes of the Caledonide orogenic wedge in southern and central Scandinavia were thrust (sensu stricto) E/SE onto the Baltic Shield during the Scandian Orogeny when the western edge of Baltica subducted into the mantle beneath Laurentia to form the HP/UHP Western Gneiss Complex (WGC). Kinematic indicators along the basal décollements of orogenic wedge allochthons indicate a change in shear sense from top-E/SE to top-W/NW at the same time (≈415 Ma) radiometric ages indicate the WGC began exhumation from the mantle. The shear traction along the top of the exhuming WGC stretched the Jotun and Trondheim Basin allochthons, then broke them into segments, and finally separated the frontal part of some of the allochthons away from the main body, causing them to be carried passively E/SE as the WGC continued to exhume out of the mantle. The lack of fragmentation and absence of extension structures in the most external part of the Caledonian wedge is consistent with "piggyback" transport of top of this exhuming HP/UHP terrane. If this model is correct, the total displacement of the leading wedge taper could be considerably further than indicated by palinspastic reconstructions, alone.