STRESS FIELD EVOLUTION IN THE FRONTAL PORTION OF FOLD-AND-THRUST BELTS: EVIDENCE FROM VEIN SYSTEMS

Mohr-Coulomb failure criteria closely constrain the possible stress fields in thrust sheets at the time of their formation and accretion to the base of an advancing orogenic wedge. However, vein systems observed in the outer portions of ancient fold-and-thrust belts suggest that the subsequent history of the stress field is much more complex. In particular, vein systems suggest that thrusting is followed by vertical extension and then by strike-parallel horizontal extension. A model is proposed wherein the fluid pressures are low during initial due to dilation of the rock mass associated with the failure process. Subsequent flow of high-pressure water from the base of the orogenic wedge along the newly connected thrust system lowers the effective frictional resistance allowing rapid displacement of the new thrust sheet and reduced tectonic stress. The high fluid pressure gradient created by the wedge geometry ultimately raises the local fluid pressure to the level that it exceeds local confining stress (i.e. lithostatic stress), thereby depositing vein material along the thrust surfaces; very little of this vein material shows any evidence of syn- or post-depositional fault movement. The loss of most of the frictional resistance causing the tectonic stress reduces the local tectonic stress to the level that the strike-parallel stress (i.e., the combined Poisson contributions of vertical and tectonic stresses) is less than the vertical stress, so the continuing high fluid pressures open the previously created cross-strike joints and deposit the vertical strike-normal veins so commonly observed in the frontal portions of fold-and-thrust belts.