What steers the “folding to faulting” transition in carbonate-dominated seismic fold-and-thrust belts? New insights from the Eastern Southern Alps (Northern Italy)

Several parameters steer the modes of shortening of carbonate-dominated fold-and-thrust belts from incipient- (layer parallel shortening, buckle folds) to evolved deformation stages (verging folds, discrete thrusts). In this study, we address the spatial and temporal evolution of contractional structures within carbonate-dominated fold-and-thrust belts by documenting the geometry, kinematics and structural architecture of the San Donato-Costa Thrust Zone, a splay of the regional Belluno Thrust of the seismically active Eastern Southern Alps (Northern Italy). Deformation is there accommodated by a variety of features ranging from open and upright to tight and verging folds cut by later thrusts. An integrated structural analysis indicates inherited primary features to have effectively steered the deformation style of the thrust and its immediate hanging wall and footwall. We propose an evolving deformation scenario initially governed by the inherited lithological features and localised pressure solution, then by the geometry of folds accommodating progressive shortening and, finally, by thrusting. The folding-faulting transition occurs when fold forelimbs dip ∼80° and the ratio between the dip angle of fore- and back limbs becomes ∼3.3. These geometric boundary conditions control the mechanical behaviour of carbonate multilayer successions during orogenic shortening in fold-and-thrust belts, assisting the partitioning between seismic and aseismic deformation.