Effects of multiple detachments in thin-skinned fold and thrust belts: insights from analogue modelling

Thin-skinned fold and thrust belts present exploration challenges in many places worldwide. The presence of multiple detachments in the stratigraphic sequence also adds to the complexity of such fold and thrust belts. This study aims to understand more about the effects of multiple detachments in thin-skinned fold and thrust belts through scaled analogue modelling experiments. Our main area of interest is Romania's prolific onshore hydrocarbon area, the foreland of the Eastern Carpathian Bend Zone. Here, one of the large uncertainties is if the Oligocene to lower Miocene strata experienced any shortening before salt deposition. If so, what would be the difference in the observed geometries?

Scaled sandbox models with layered brittle and ductile materials were used to gain critical insights into the structural evolution of this fold and thrust belt (ECBZ) and to reduce the above-mentioned uncertainties. The materials used in these experiments are: coloured dry quartz sand (for modelling brittle behaviour), silicone (for ductile behaviour of the salt), 200-300 μm glass microspheres and a mixture of silicone and granular materials (for the other detachment levels).

The experimental setup consists of a computerized deformation device that pulls a mobile plate at a constant rate beneath a fixed deformation box with one glass sidewall, one end of the box acting as a static buttress. Deformation monitoring has been achieved using top-view 3D digital image correlation techniques (DPIV- Digital Particle Image Velocimetry). The models were serially sectioned and photographed after post-experiment treatment (wetting and consolidation). The sections were used to build and interpret 3D digital models of the experiments.

Duplex structures mainly characterize the deformation in the sub-silicone. Some particular geometries observed in the sub-silicone (salt) sequence are buckle folds and lift-off folds. These mainly occur when the detachments within the sub-silicone mechanical stratigraphy consist of silicone/granular mixture. Although not traditionally interpreted and observed in the area, these results raise the possibility of alternative interpretations. The supra-silicone (salt) deformation is less complex, characterized by both fore- and backthrusts, most of them initiating as detachment folds, similar to what is seen in our area of interest.

Experimental results reduce exploration uncertainties by bringing more insights into the control and effects of multiple detachments on the structural development of fold and thrust belts. These modelling results also bring new possible interpretations in areas poorly constrained by seismic and well data.