ANALYSIS OF KILOMETRIC-SCALE SUPERPOSED FOLDING IN THE CENTRAL COAL BASIN (CANTABRIAN ZONE, NW SPAIN)

The Carboniferous paralic sequence of the Central Coal Basin (Cantabri an zone, Hercynian NW Spain) contains an outstanding example of kilome tric-scale superposed folding that allows comparison with previous exp erimental models. First generation folds have a NS trend and mainly co nstitute 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 ob served from west to east. A second folding phase gave rise to two type s of upright, roughly E-W oriented, superposed folds. The superposed f olds generated in general by buckling, but their trend and situation a re controlled by lateral ramps of the previous thrusts in many cases. Superposed folds of the first type are the most common and have develo ped on the limbs of previous folds without folding their axial surface s. They occur in zones and usually have kilometre-scale hinge lines wi th syngenetic curved traces. These folds present wavelengths distinctl y smaller than those of the previous folds. The second type of superpo sed folds affect both the limbs and axial planes of the previous folds . These second-type folds are, in some cases, smaller than previous fo lds, 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.