Ramifications of four-dimensional progressive deformation in contractionalmountain belts

Studies of progressive deformation aim to identify and establish temporal s equences of structural stages, each characterized by a suite of structures which form under specific rock and environmental conditions. Because of the natural spatial variation in rock type and environmental parameters in an orogen, several structural stages may operate concurrently in different par ts of an orogenic wedge. At any given instant in time, rocks experiencing d ifferent stages of deformation are bounded by deformation fronts. As intrin sic and extrinsic conditions of deformation vary through time, spatial migr ation of deformation fronts causes rocks to record temporal overprinting of structural stages. In convergent orogens, deformation fronts, as mapped in their finite state, are typically forelandward-dipping due to the regional orogenic wedge geometry and thermal structure. Depending on the competing deformation processes, deformation rate changes on either side of the front may cause deformation fronts to change orientation, relief and surface are a with time. Differences in migration rates of different deformation fronts could cause rearrangement of the sequence of structural stages with depth and laterally across the same mountain belt. (C) 1999 Elsevier Science Ltd. All rights reserved.