Diagnostic criteria of salt-bearing rifted margins structures on fold-and-thrust belts: insights from modelling applied to natural case studies

Salt-detached fold-and-thrust belts have been described as having an extremely narrow cross-sectional taper, a regular structural spacing, and lack of a dominant structural vergence. However, detailed evaluation of several natural examples shows unclear structural geometries and intricate salt-sediment contacts. Geometries associated with these systems include overturned panels, large-transport thrust sheets, frequent changes in structural orientations and fold plunges, missing stratigraphic units, abrupt thickness changes and geological contacts either omitting or repeating stratigraphy marked strained evaporites and welds. The main reasons for these are: the inherent weakness of salt and the presence of early salt structures (i.e. pre-shortening) associated with a non-layer cake stratigraphy developed on salt-bearing rifted margins.

Based on both analogue and numerical models inspired in several natural case studies (Alps, Pyrenees) we here provide structural and stratigraphic templates to recognize such salt-related structures: downbuilding is represented by vertical aggradation of syn-kinematic strata, erosional truncation of megaflaps and resedimentation of salt-sediment debris. Salt-detached extension is represented by the sharp truncation of minibasin strata against triangular diapirs, while the widening of minibasins by means of shoulders, growth wedges and secondary minibasins illustrate the progressive transition from downbuilding to salt-detached extension in the evolving thermal phase. Truncation of syn-kinematic strata within the expanding wedges along with the occurrence of cusps at the salt-sediment contact also mark the transition from downbuilding into salt-detached extension.

Recognition of these features in geological maps, seismic data or through the interpretation of well intersections provide geometrical constrains to lower the uncertainty in building balanced cross-sections, and are key for reconstructing the depositional history of salt-bearing rifted margins.