Timing and temperature of decollement on hydrocarbon source rock beds in cyclic lacustrine successions

The generation of hydrocarbons causes lubrication of source rock bedding planes which can enhance deformation during basin subsidence and subsequent tectonic events. Deformation includes decollement, i.e. gravitational sliding of rocks above the lubricated horizon. In cyclic sequences, such as occur in many lacustrine basins, decollement is generally restricted to the organic-rich beds of the cycle. Case studies in lacustrine successions in the Devonian Orcadian Basin, Scotland, and the Triassic–Jurassic Hartford–Deerfield Basin, USA, show that movement of material associated with the decollement surface occurred in the current down-dip direction. These extensional basins experienced rapid subsidence and a high heat flow such that hydrocarbon generation, and therefore decollement, occurred early in the basin histories. During extension, half-graben rotation ensured that the beds were dipping during hydrocarbon generation, facilitating decollement. Inversion of the Orcadian Basin reactivated decollement surfaces, involving thrust motion in some cases. Minerals precipitated in tension cavities associated with decollement yield fluid inclusion homogenization temperatures in the ranges 96–139°C and 85–96°C for the Orcadian and Hartford–Deerfield basins, respectively. These data are consistent with burial into the oil window at depths of between 1.5 and 2.0 km.