Basement controls on structural evolution in thin-skinned thrust belts: implications for migration of deformation into orogenic forelands

Fold and thrust belts on the outer fringes of contractional mountain belts are classically interpreted as forming in sedimentary rocks that have detached from the underlying continental crust that acts as a rigid, undeformed basement. This is “thin-skinned tectonics” and the Jura fold and thrust belt of northern Switzerland is an archetype. We analyse published cross-sections and interpret new high resolution seismic imagery, to show that it is not simply thin-skinned. Faults in the underlying basement coincide with folds and thrusts in the overlying cover. We argue that, by off-setting the basal evaporite detachment, the basement faults form steps that
nucleate structures in the otherwise thin-skinned thrust belt. However, the basement faults are not simply inherited structures, as previously thought, but were locally activated during thrusting. impacting the evolutionary sequence of these thrusts in the cover. In 3D, the folds that nucleate on basement steps can propagate laterally into areas where basement faulting is only weakly developed. It is the availability of exceptional seismic imagery that permits these deductions. Most other fold and thrust belts worldwide do not have this image quality and interpretations can rely on single profiles. Therefore, the role of basement faults may have gone un-noticed.