Isabel Edmundson

Structural geometries associated with extension, inversion and strike slip deformation; insights from field observations along the northern Somerset coast, southwest UK.

Several kilometre long sections of the northern Somerset coastline showcase excellent exposures of faults and associated structures within the Triassic and Lower Jurassic. The combination of low tides uncovering cliffs and wave cut platforms, and a complex structural history documented by multiple outcrops ensure that this coastline is a popular destination for geologists. It was for these reasons that myself, David Peacock, Dave Sanderson, Vilde Dimmen and two master’s students arranged a three day field trip to this coastline. The trip provided an opportunity for the two students to carry out reconnaissance for their projects, and for myself and Vilde to see more field examples of fault structures that could complement our own PhD projects. It is hoped that this informal presentation will give an insight into how field observations can help to improve understanding of fault geometry and associated deformation as a result of extension and subsequent reactivation.

The area has a complicated tectonic history having been affected by several structural phases beginning with N-S orientated compression during the Variscan orogeny. This event established the dominant E-W trending structural grain along which periods of later extension and compression would follow. The first of three discrete rift events occurred in the Permian, the second in the early Jurassic and the third in the late Jurassic to early Cretaceous, each indicated by wedge thickening geometries. Following this protracted period of rifting, N-S compression associated with the Alpine orogeny in the late Cretaceous reversed the sense of motion along established E-W Mesozoic faults. Tightening of roll over anticlines previously established by extension and a Lower Cretaceous regional angular unconformity provides evidence for this event. Finally, strike-slip reactivation of NW-SE trending faults during the Miocene established several pull apart basins indicating a sinistral sense of displacement. Many of the observed outcrops document this restructuring but are generally below seismic resolution. However, recent research has shown that many fault systems and associated geometries are scale independent and so small scale examples can provide good analogues for larger scale structures observed on seismic. In addition to this, many features of faults and associated damage zones are not visible on seismic, reducing the fault to a simple feature. Consequently, the complexity of the fault and fault related deformation can be overlooked, yet these features have important implications for fluid flow and should be remembered during interpretation and modelling.

Photograph showing the hangingwall anticline formed by inversion of a south dripping extensional fault in the Lower Jurassic shales and limestones. Cliff is ~50m high.