The distribution of deformation in parallel fault-related folds with migrating axial surfaces: comparison between fault-propagation and fault-bend folding

In fault-related folds that form by axial surface migration, rocks undergo deformation as they pass through axial surfaces. The distribution and inten sity of deformation in these structures has been impacted by the history of axial surface migration. Upon fold initiation, unique dip panels develop, each with a characteristic deformation intensity, depending on their histor y. During fold growth, rocks that pass through axial surfaces are transport ed between dip panels and accumulate additional deformation. By tracking th e pattern of axial surface migration in model folds, we predict the distrib ution of relative deformation intensity in simple-step. parallel fault-bend and fault-propagation anticlines. In both cases the deformation is partiti oned into unique domains we call deformation panels. For a given rheology o f the folded multilayer, deformation intensity will be homogeneously distri buted in each deformation panel. Fold limbs are always deformed. The flat c osts of fault-propagation anticlines are always undeformed. Two asymmetric deformation panels develop in fault-propagation folds above ramp angles exc eeding 29 degrees. For lower ramp angles, an additional, more intensely-def ormed panel develops at the transition between the crest and the forelimb. Deformation in the flat crests of fault-bend anticlines occurs when fault d isplacement exceeds the length of the footwall ramp, but is never found imm ediately hinterland of the crest to forelimb transition. In environments do minated by brittle deformation, our models may serve as a first-order appro ximation of the distribution of fractures in fault-related folds. (C) 2001 Elsevier Science Ltd. All rights reserved.