Upslope-verging back thrusts developed during downslope-directed slumping of mass transport deposits

While much research has recently been focussed on downslope-verging systems of gravity-driven fold and thrust belts within mass transport deposits (MTDs), rather less attention has been paid to back thrusts, which are defined as displaying the opposite vergence to the main transport direction in thrust systems. A fundamental question arises over whether back thrusts in downslope-verging MTDs record actual movement back upslope. In order to address this issue, we have examined exceptional outcrops of Pleistocene fold and thrust systems developed in MTDs around the Dead Sea Basin. Back thrusts can be interpreted in terms of a ‘downslope-directed underthrust model’, where material moves down slope and is driven into the footwall of the back thrust, resulting in the ‘jacking up’ of the largely passive hangingwall. Our data support this underthrust model and include the observation that stratigraphic units may be markedly thickened (up to 250%) in the footwall of back thrusts. This thickening is a consequence of pure shear lateral compaction as the ‘wedge’ of sediment is driven into the footwall to create an underthrust. In addition, back thrusts may be rotated as new back thrusts form in their footwalls, ultimately resulting in overturned thrusts. The observation that steeper back thrusts typically accommodate less displacement than gently-dipping back thrusts suggests that steepening occurred during back thrusting, and is therefore a consequence of ‘footwall wedging’. Contrary to some recent interpretations, we demonstrate that back thrusts can develop in gravity-driven systems and cannot therefore be used to distinguish different emplacement mechanisms for MTDs.