Thrust sequences

The sequential evolution of arrays of thrusts can occur by a number of geometric models: these models form the subject of this contribution. The most common of these is a simple foreland-directed (piggy-back) series which develops into either an emergent imbricate fan or, if the faults converge upwards, a duplex structure. Depending on the geometry and distribution of thrust ramps and the relative magnitude of displacements on the faults, a range of duplex geometries can result where the imbricate slices have either hinterland-dipping, antiformal stack or foreland-dipping attitudes. A frequently recognized complexity occurs when a later fault climbs across from the footwall into the hanging wall of an earlier one leading to a breached geometry. A less common generation of imbricate fans occurs by the hinterland-directed progression of thrusts in a break-back sequence. This can lead to an overstep geometry where earlier structures are truncated in the footwall of a later fault. The evolution of hinterland-directed, back-thrusts is also considered. Both internally piggy-back (regionally hinterland-directed) and break-back (regionally foreland-directed) sequences can develop, depending on the ability of the active displacement front to propagate forwards. Simple imbricate geometries can develop with an internally break-back sequence but piggy-back propagation will generally require substantial strains within the back-thrust slices or the generation of multiple detachment levels. The range of structures which can result from these fore-thrust and back-thrust sequences is illustrated using hypothetical cross-sections and natural examples from the Scottish Moine thrust belt and the western Alps. The implications of each geometry for estimates of orogenic contraction and the construction of balanced cross-sections are also emphasized.