Convergence-related ‘dynamic spreading’ in a mid-crustal ductile thrust zone: a possible orogenic wedge model

Collisional orogenic belts usually form as approximately wedge-shaped dynamic units irrespective of their bulk rheology. Quantitative models suggest that if the boundary conditions and mechanical properties of a wedge remain unchanged, it will undergo internal yielding (shortening or extension) until a stable, unchanging shape (or ‘taper’) is established. It follows that geological processes periodically leading to changes in wedge shape, or mechanical properties, will perturb the system, generating internal deformation as the wedge strives to re-establish a stable taper. Geological applications of wedge models to upper-crustal features such as accretionary prisms and foreland thrust belts have highlighted the importance of internal, thrust-parallel shortening strains. The Caledonian, mid-crustal ductile thrust zone of Sutherland, N Scotland, displays many geometric and kinematic similarities to shallow-crustal thrust belts. In contrast to such belts, however, deformation fabric studies suggest a predominance of thrust-parallel extension strains which developed synchronously with large bulk shortening due to thrust telescoping. Using a viscous wedge model, it is suggested that the deformation occurs due to a process of dynamic spreading , possibly triggered by intermittent thrust-slice accretion, towards the foreland, at depth (‘underplating’) and/or strain softening in amphibolite-facies mylonite zones along the ductile thrusts. Similar deformation fabrics are recognized in many regions of ductile thrusting which suggests that a dynamic spreading model may be applicable to mid-crustal deformation regimes in collision zones worldwide.