Normal fault localization controls during syn- and post-orogenic extension affecting thin-skinned architecture

Abstract

Balanced cross sections through thrustbelts affected by post-orogenic extension reveal that normal faults are mostly developed in the backlimbs of pre-existing, asymmetric, fault-propagation and detachment folds. Outcrop study, geological cross section balancing, reflection seismic interpretation and numerical modeling in the Eastern Balkans indicate that the nucleation of these normal faults is affected by the occurrence of plastic strain zones in backlimbs, represented by clusters of small-scale dilatant shear fractures. Thrustbelt segments where these zones did not evolve into thrust faults and became passively rotated into steeper geometries are prone to normal fault development during post-orogenic extension. Instead of developing its own precursor fracture clusters, each normal fault of this type nucleates using pre-existing clusters as a shortcut in its development. Rare occurrences of post-orogenic extension-driven faults, which reactivate entire pre-existing thrust fault ramps or develop in fold forelimbs indicate the existence of other parameters that co-control the development of normal faults in this setting. These parameters include thrustbelt topography as well as variations in décollement geometry and frictional properties.