Sandbox models of fault-bend folding: A new investigation with a pre-existing fault ramp

Abstract

This study presents the results of analogue models of fault-bend folds (FBFs) developed over a pre-existing fault dip ramp with a non-outcropping upper fault flat. To determine the boundary conditions that control the formation of FBFs, experiments were performed with quartz sand and glass microbeads, both with and without intercalations of thin horizons of mica flakes. We varied the initial thickness, the ramp dip angle, and the basal friction coefficient of the lower fault flat. The analogue models revealed that the most important factor for the formation of FBFs was the rheological behaviour of the analogue material. The elasto-plastic frictional behaviour of the glass microbeads is fundamental for the formation of Mode I-FBFs (with interlimb angles higher than 90°), which is independent of the angle of the pre-existing fault ramp and the basal friction along the lower fault flat. In the sand models, the formation of FBFs requires a high initial stratigraphic thickness, resulting in a significant increase in the normal stress and/or in the cohesion strength. In glass microbeads, the models developed structures very similar to the FBFs described in the carbonates of the Maracaibo Basin (Venezuela) as well as those within the Appalachian thrust belt (USA).