Hinge-parallel fluid flow in fold-thrust belts: how widespread?

Evidence exists in both active and ancient fold-thrust belts for extensive hydraulic connectivity over distances of kilometres to tens of kilometres along-strike during fold growth. Components of 3D structural permeability contributing to along-strike flow include fold closures with interlayered high and low permeability beds. continually resheared bedding planes, saddle reef structures with incrementally created hinge-line gapes, and various thrust flat-ramp and fault-fracture intersections. Structurally controlled directional permeability may arise both in 'normally' pressured crust (i.e. where fluid pressures are approximately hydrostatic) and at depth in strongly overpressured sub-metamorphic environments where fluid pressures approach lithostatic. Along-strike flow parallel to fold hinges is particularly likely where non-uniform shortening occurs along a fold-thrust belt during compressional inversion or continental collision. Interpretation of fluid redistribution from cross-strike sections coupled with 2D numerical modelling may then be highly misleading.