MESOSTRUCTURAL CONSTRAINTS ON THE FLEXURAL SLIP MECHANISM

Folding within the brittle regime is often accommodated by the flexural slip mechanism whereby shear strain is localized along discrete horizons, typically bedding-plane faults. The mechanism is well-suited for interbedded rocks of contrasting lithologies, and thus is commonly observed in folded sedimentary sequences in foreland thrust belts. Theoretical and experimental studies predict that flexural slip surfaces will lock when folds reach a critical interlimb angle, thereby requiring alternative mechanisms to facilitate continued fold tightening. We document the evolution of folding mechanisms through the analysis of mesostructures (faults, tectonic stylolites and veins) in a well-exposed syncline in the Cantabrian Zone of NW Spain. The asymmetric syncline has a shallow-dipping NW limb and a vertical to slightly overturned SE limb. Abundant slickenlines on upper and lower bedding surfaces of competent dolostones attest to the flexural slip mechanism, with lineations falling along the great circle normal to the fold axis. However, tectonic stylolites suggest that pressure solution in response to tectonic shortening accommodated fold tightening once flexural slip surfaces locked. Insight into the locking angle of flexural slip is provided by the orientations of tectonic stylolites found on opposing limbs. On the NW limb the stylolites are normal to bedding, whereas on the SE limb they are oblique to bedding. We rotated the SE limb back to the point where tectonic stylolites on both limbs are parallel to each other, yielding a limb dip of 61° at the onset of pressure solution. Based on the mesostructural analysis we envision four stages of fold development. Flexural slip accommodated the first stage until the SE limb reached a dip of ~60°, whereupon pressure solution ensued. This was followed by a phase of tangential longitudinal strain as manifested by extensional veins and normal faults in outer arcs of competent beds. A large thrust then propagated through the SE limb unlocking the bedding plane faults and initiating a second phase of flexural slip. Results from this study suggest an evolution of folding mechanisms in response to continued tectonic shortening. This in turn would lead to changes in the distribution of strain and associated structures, both in terms of stratigraphic and structural position.