Syn-thrusting, near-surface flexural-slipping and stress deflection along folded sedimentary layers of the Sant Corneli-Bóixols Anticline (Pyrenees, Spain)
Muñoz, Josep A.
In the Spanish Pyrenees the Sant Corneli-Bóixols thrust-related anticline displays an outstandingly preserved growth strata sequence. These strata lie on top of a major unconformity exposed at the anticline’s forelimb that divides and decouples a lower pre-folding unit from an upper syn-folding one. The former consists of steeply-dipping to overturned strata with widespread bedding-parallel shears indicative of folding by flexural-slip, whereas the syn-folding strata above define a 200-m amplitude S-shaped fold. In the inner and outer sectors of the forelimb, both pre- and syn-folding strata are near-vertical to overturned and the unconformity angle ranges from 10° to 30º. In the central portion of the forelimb, syn-folding layers are shallowly-dipping, whereas the angular unconformity is about 90° and the unconformity surface displays strong S-C shear structures, which provide a top-to-the foreland shear sense. This sheared unconformity is offset by steeply-dipping faults which are at low angles to the underlying layers of the pre-folding unit. Strong shearing along the unconformity surface also occurred in the inner sector of the forelimb with S-C structures providing an opposite, top-to-the hinterland, shear sense. Cross-cutting relationships and shear senses along the pre-folding bedding surfaces and the unconformity indicate that regardless of its orientation, layering in the pre- and syn-folding sequences of the Sant Corneli-Bóixols anticline was continuously sheared. This shearing promoted an intense stress deflection, with the maximum component of the stress tensor remaining at low angles to beds during most of the folding process.