TIMING AND TEMPERATURE OF DECOLLEMENT ON HYDROCARBON SOURCE-ROCK BEDSIN CYCLIC LACUSTRINE SUCCESSIONS

The generation of hydrocarbons causes lubrication of source rock beddi ng planes which can enhance deformation during basin subsidence and su bsequent tectonic events. Deformation includes decollement, i.e. gravi tational sliding of rocks above the lubricated horizon. In cyclic sequ ences, such as occur in many lacustrine basins, decollement is general ly restricted to the organic-rich beds of the cycle. Case studies in l acustrine successions in the Devonian Orcadian Basin, Scotland, and th e Triassic-Jurassic Hartford-Deerfield Basin, USA, show that movement of material associated with the decollement surface occurred in the cu rrent down-dip direction. These extensional basins experienced rapid s ubsidence and a high heat flow such that hydrocarbon generation, and t herefore decollement, occurred early in the basin histories. During ex tension, half-graben rotation ensured that the beds were dipping durin g hydrocarbon generation, facilitating decollement. Inversion of the O rcadian Basin reactivated decollement surfaces, involving thrust motio n in some cases. Minerals precipitated in tension cavities associated with decollement yield fluid inclusion homogenization temperatures in the ranges 96-139 degrees C and 85-96 degrees C for the Orcadian and H artford-Deerfield basins, respectively. These data are consistent with burial into the oil window at depths of between 1.5 and 2.0 km. (C) 1 998 Elsevier Science B.V. All rights reserved.