Basement-involved shortening and deep detachment tectonics in forelands of orogens: Insights from recent collision belts (Taiwan, Western Alps, Pyrenees)



[1] New combined structural and seismotectonic analyses demonstrate basement-involved shortening in forelands of recent collisional orogens (Taiwan, Western Alps, Pyrenees). Basement thrusts documented by seismicity (e.g., the 21 September 1999, Chi-Chi earthquake in Taiwan) and/or structural data are triggered and localized by preexisting basement faults which constitute crustal weakness zones available for reactivation under low stress levels. Reactivation of basement faults may induce localization of folds and thrusts in the shallow thrust wedge, development of crystalline thrust sheets, out-of-sequence basement thrusting and late basement uplift, deformation/refolding of shallow thin-skinned nappes, and development of accommodation structures such as transfer faults leading to a kinematic segmentation of foreland thrust belts. Reactivation of preexisting basement faults also occurs in the far foreland in response to the far-field transmission of orogenic stresses depending on the amount of the changing-through-time mechanical coupling between the orogen and its foreland. Displacements related to basement shortening in forelands are accommodated at the scale of the upper crust, which requires that it is partially decoupled from the deeper lithospheric levels by a crustal detachment. This detachment presumably occurs along the midcrustal, thermally weakened brittle-ductile transition. It may either ramp toward the surface into a shallow, upper crustal detachment beneath the fold-thrust belt and/or extend beneath the foreland and accommodate basin inversion far away from the orogen. Occurrence and relative timing of shallow and deep detachment tectonics in forelands seem to be dependent on mechanical boundary conditions such as the presence of ductile horizons within the cover sequence or of preexisting weakness zones in the underlying basement.