Mechanisms of mélange formation: Examples from SW Japan and southern Scotland

The formation of synthetic Riedel shears either as discrete faults or ductile shear-zones is important in the fragmentation of layers to form mélanges. The magnitude of longitudinal and shear strain necessary for this fragmentation has been assessed using a simple rigid-domino model for layer-parallel shear. The strain required for layer fragmentation is a function of block aspect ratio and initial fracture dip relative to layering. Layer stretching can occur during simple shear if folds are developed but a component of extension parallel to the shear zones reduces the strain required for fragmentation. Simple models of this type break down when there is a component of dilation across the mélange block bounding fractures allowing shale matrix infill or mineral vein growth. Some mélanges show evidence, in the form of symmetrical structures, of a more coaxial deformation history and this may in part reflect deformation partitioning between sandstone and shale layers. Other mechanisms of mélange formation include the development of out-of-sequence thrusts cutting through already dipping beds. The causes of strike-parallel extension are tentatively related to large-scale evidence of arc-parallel forearc stretching.