Analysis of kilometric-scale superposed folding in the Central Coal Basin (Cantabrian zone, NW Spain)

The Carboniferous paralic sequence of the Central Coal Basin (Cantabrian zone, Hercynian NW Spain) contains an outstanding example of kilometric-scale superposed folding that allows comparison with previous experimental models. First generation folds have a N-S trend and mainly constitute a fold train of fault propagation folds in which an increase of interlimb angle and an increase in the dip of the axial plane is observed from west to east. A second folding phase gave rise to two types of upright, roughly E-W oriented, superposed folds. The superposed folds generated in general by buckling, but their trend and situation are controlled by lateral ramps of the previous thrusts in many cases. Superposed folds of the first type are the most common and have developed on the limbs of previous folds without folding their axial surfaces. They occur in zones and usually have kilometre-scale hinge lines with syngenetic curved traces. These folds present wavelengths distinctly smaller than those of the previous folds. The second type of superposed folds affect both the limbs and axial planes of the previous folds. These second-type folds are, in some cases, smaller than previous folds, and have developed on closed early folds, but in other cases they are large wavelength folds related to the tightening of lateral ramps of the thrusts.