FORMATION OF CRENULATION CLEAVAGE AND DOWN-DIP-VERGING FOLDS DURING THE COLLAPSE OF FOLD-THRUST BELTS: EXAMPLES FROM BARABOO (WISCONSIN, USA) AND FROM A BRASILIANO OROGEN (MINAS GERAIS, BRAZIL)

We have identified two localities where fine-grained schist and phyillite are deformed by down-dip-verging (i.e., top-down) folds (F2) associated with an antithetic asymmetric crenulation cleavage (S2). Both localities occur within fold-thrust belts, and are superimposed on an earlier generation of up-dip verging (i.e., top-up) folds (F1) associated with an axial-planar phyllitic or schistose cleavage (S1). The first locality is a west-verging Brasiliano (0.58 Ga) fold-thrust belt along the eastern margin of the São Francisco Craton in eastern Brazil. Here, D2 structures occur in a west-verging thrust belt and F2 folds refold F1 folds. The second locality, the Baraboo Syncline, is a S-verging regional-scale fold within of the 1.6 Ga Mazatzal orogen. Here, D2 structures are nested within D1 structures on the upright limb of the fold. Two hypotheses have been suggested to explain the kinematics of down-dip-verging folds and antithetic crenulation: (1) The structures form during progressive top-up (thrust-related) deformation; (2) They form due to late-stage extensional collapse (vertical shortening) and/or down-dip (normal-sense) shear superimposed on earlier up-dip structures. Our observations suggest that elements of both hypotheses apply. We suggest that the down-dip verging F2 folds and the S2 crenulation initiate as micro-buckling instabilities during progressive thrust-related shortening, as soon as the S1 cleavage becomes mechanically significant. The formation of such opposite-verging structures was simulated by laboratory models in the 1970s. (Of note, the down-dip-verging folds in Baraboo are the antithetic partner of conjugate contractional kinks.) Once formed, the cleavages and the axial surfaces of the folds rotate from a steeper to shallower dip, and locally become subhorizontal, in response to vertical shortening as the vertical load increases, perhaps due to thrust stacking. In fact, the short limbs of the folds may overturn and/or buckle in the vertical direction. Progressive up-dip shear cannot explain the rotation sense that we observe—thrust-sense shear would steepen cleavages and axial surfaces. Notably, residual S1 surfaces in microlithons are sigmoidal. The shear sense on S2 cleavage domains indicated by the S1 sigmoid shape is consistently down-dip, suggesting that crenulation asymmetry reflects penetrative bookshelf-fault-like shear.