Extension, disruption, and translation of an orogenic wedge by exhumation of large ultrahigh-pressure terranes: Examples from the Norwegian Caledonides
Far-traveled allochthons (greater than 100 km) within collisional orogenic wedges may have undergone significant lateral movement by passive transport (in addition to thrusting) where they lie tectonically above large, exhumed, high-pressure/ultrahigh-pressure (HP/UHP) metamorphic terranes. Continental collision results in the subduction of one craton beneath another into the mantle. The subducted craton undergoes HP/UHP metamorphism, while an accretionary orogenic wedge develops simultaneously at its junction with the overlying craton. The subsequent exhumation of a large HP/UHP terrane by either far-field extension or buoyancy-driven extrusion, or both, reverses the shear traction along its upper boundary from foreland-directed thrust motion to hinterland-directed normal displacement. This normal-sense shear can stretch, thin, and fragment the overlying wedge and even carry a detached frontal fragment passively toward the foreland on top of the exhuming plate. The total “piggyback” displacement would be a function of the amount of exhumation of the HP/UHP terrane and the timing of its breakoff from the hinterland portion of the wedge. This model is applied to the Trondheim and Jotun nappe complexes of the Caledonides of southern Scandinavia, which were translated greater than 300 km to the E and SE, respectively, during the 430–385 Ma Scandian orogeny. Their hinterland boundaries rest on top of the HP/UHP Western Gneiss Complex. Kinematic indicators along their basal décollements indicate a change in shear sense from top-E/SE to top-W/NW at the same time (ca. 405 Ma) that radiometric ages indicate the Western Gneiss Complex began to exhume from the mantle. Displacements of tens of kilometers along these décollements stretched and thinned the Trondheim nappe complex and fragmented the Jotun nappe complex. Ultimately, this basal traction led to the breakaway of the frontal segments of the allochthons, allowing them to be carried passively to the E/SE as the Western Gneiss Complex continued to exhume. Top-W/NW shear continued between the Western Gneiss Complex and the stranded rearward segments of the allochthons, resulting in the opening up of the Western Gneiss Region tectonic window between the E/SE-translating nappes and their relatively “fixed” equivalents in the W/NW. The total displacement of the traveled frontal allochthons could have been considerably farther than that accomplished by thrusting alone.
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