Rift–raft tectonics: examples of gravitational tectonics from the Zechstein basins of northwest Europe

Abstract

The concept of rift–raft tectonics was developed by Penge et al (1993) to describe structures formed above a salt substrate along the shallow margins of the East Central Graben. These structures were formed initially during the late Triassic–early Jurassic basement extension and modified by later Jurassic and Cretaceous phases. Rift–raft tectonism is characterized by the presence of relatively undeformed blocks (rafts) separated by zones of intense deformation (rifts) associated with reactive, active and passive halokinesis. This tectonic style is part of a continuum of such styles produced by gravity sliding, which can be observed in the Zechstein basins of northwest Europe, where near continuous sheets of Triassic sediments have been deformed in a dominantly extensional regime. Down-dip movement of these sheets caused compression in the deeper parts of the basins, thereby triggering a number of diapirs. Examples from the Fisher Bank Basin, East Central Graben and South Central Graben are illustrated and compared to those of the Irish Sea. Variation in forms are described and empirically related to tectonic environments as a result of the interplay of the controls identified. These include salt thickness, lithology of the Mesozoic overburden and regional dip. Physical and finite element modelling have been used to demonstrate the validity of the interpretation and thus demonstrate the importance of gravity sliding in forming structures in Zechstein basins.