Subduction-collision Processes Controlled by Decoupling at the Plate Interface and the Rheology of the Colliding Plates

Abstract:

Continent-continent collision follows subduction of oceanic lithosphere and is expected to result in doubly verging orogens with well-developed retro-wedges on the overriding plate. We question this simple view and argue that retro-wedge formation is restricted to specific rheological conditions within the lower and upper plates as well as the plate contact; thus being the exception rather than the rule during collision.

Two series of physical analogue experiments have been deployed to infer favourable rheological conditions for the transfer of strain to the upper plate and thus the development of retro-wedges. Subduction of continental lithosphere occurs along a pre-scribed zone of weakness in the mantle lithosphere (series I) or follows subduction of oceanic lithosphere (series II). In either case the contact between the subducting and overriding plate s weak. The degree of plate coupling however is not constant and is together with the rheological structures of the lower and upper plates, in particular the presence of decoupling horizons, key variable in this study.

Analogue experiments with strong decoupling at the plate boundary and at the Moho or the brittle-ductile transition of the incoming plate lead to outward propagating mountain belts by successive imbrication of upper crustal thrust sheets independent as to whether collision was preceded by subduction of oceanic lithosphere or not. Under these conditions, which are typical for subduction-dominated orogens like the Carpathians, the Dinarides or the Apennines, no significant retro-wedges with large-displacement retro-shears develop.

Transfer of strain to the upper plate is favoured when the degree of plate coupling is high, the crust of the colliding and in particular the subducting plate is strong and when the upper plate contains decoupling horizons (e.g. at the Moho or the brittle-ductile transition). Under such conditions large-scale retro-shears develop and deformation propagates outward on the upper plate to form a retro-wedge.

These analogue experiments provide insight in past rheological conditions of doubly verging mountain belts, revealing that retro-wedge formation is only possible under restricted rheological conditions.