Internal structure and salt flow in the Les Avellanes allochthonous salt sheet: insights from field observations and analogue modelling comparison

Salt sequences usually contain interbedded, non-saline, sedimentary layers (carbonates, sulphates, and siliciclastics) which behave as brittle, competent layers entrained within the weak, viscous salt. These layers become fragmented, and further stretched and folded as the host rock salt is mobilized. In diapirs reaching the surface, fragments of these brittle layers (stringers) can be transported upwards along the diapir stem from their source layer and then laterally and sometimes gravitationally downwards in the allochthonous salt sheets, becoming embedded in the diapir caprock as the salt is dissolved by unsaturated fluids. Therefore, the stringers arrangement may serve as a proxy to understand salt flow. To test this hypothesis, we have examined the internal structure of the Les Avellanes diapir rocks (South-Central Pyrenean fold-and-thrust belt) which represents a syn-orogenic laterally advancing salt sheet, early Oligocene in age. To understand the internal structure, the diapir exposure has been mapped in detail and projected in a cross-section along the expected flow direction. Then, to evaluate this structure in terms of flow kinematics and dynamics, we have reproduced the Les Avellanes lateral salt sheet with analogue models equipped with a stereographic system of strain quantification (LaVision GmbH).

The Les Avellanes Diapir exposes a gypsum rich caprock with numerous Triassic carbonate and subvolcanic stringers, which were carried along within the diapir stem and salt sheet. The carbonate stringers show contrasting bottom and top facies (laminated vs. tabular) constraining their stratigraphic polarity. The stringers are mainly subvertical in the diapir stem and around the probable crestal/feeder area, flat lying stringers are disrupted by several extensional faults. Towards the allochthonous salt body, they are obliquely or vertically imbricated with some of the stringers overturned. This suggests that stringers were carried through the feeder conduit to the surface, then became stretched horizontally in the feeder area and imbricated and stacked within the laterally advancing salt sheet.

This hypothesis has been evaluated using analogue models. The modelling setup consisting of a box with a moving wall to simulate shortening, and two silicone layers (polydimethylsiloxane) separated by two thin, colored granular layers (simulating carbonate layers disrupted into stringers). The host rock overburden, represented by colored sand, is continuously sieved around a rectangular vertical conduit of the diapir. During shortening, caprock made of cohesive material (glass beads) is added on top and syn-kinematic sand layers are added adjacent to the laterally advancing silicone extrusion. In the cross-sections of the models, stringers are verticalized in the diapir conduit, parallel to the walls, and distorted into isoclinal folds with downflow vergence in the advancing allochthonous extrusion. The surface strain pattern revealed extension around the crestal area, and belts of contraction downslope in the advancing body developing in sequence backwards from the front as caprock rafts and stringers continuously became imbricated, blocking and decelerating the flow. As the internal structure of the deformed stringers is compatible with field observations, similar strain patterns visible in the model may be attributed to the development of this salt sheet.