What is the real front of orogens? The Pyrenean orogen as a case study

The concept of 'orogenic front' is discussed theoretically and on the basis of the Pyrenees example. Depending on the geological criterion, a morpholo gic front (mountain front), a thrust-wedge front, a reactivation front and a deformation front can be distinguished. The classical, critical thrust we dge made of cover material, moving above a shallow decollement, is consider ed as the upper pan of a larger, steady-state wedge above a shallow basal d ecollement connected to a deeper one (lower crust?) and where both sediment ary and basement units are involved: its front corresponds to both the moun tain front and the thrust-wedge front, that is the front of the allochthono us nappes. Transmission of orogenic stresses (which decrease exponentially with increasing distance to the belt), responsible for intraplate deformati ons, especially inversion of preexisting extensional features, over a dista nce greater than 1 000 km away from the belt, require both a relatively rig id lithosphere and a crustal decoupling far in the foreland. The outermost reactivated and/or inverted structure thus locates the reactivation front, which limits a low-slope and low-strength foreland area that is not yet at steady-state and therefore cannot be discussed in terms of critical taper w edge. An outermost deformation front, limiting the part of the foreland whe re orogenic stresses hale only been recorded by microstructures without sig nificant displacements, can also be considered. ((C) 1999 Academic des scie nces / Editions scientifiques et medicales Elsevier SAS.).