DEFORMATION AROUND A THRUST TIP IN CARBONIFEROUS LIMESTONE AT TUTT HEAD, NEAR SWANSEA, SOUTH WALES

The Tutt Head thrust zone is the tip of a forward propagating lateral ramp which links two levels of detachment within the Langland-Mumbles Anticline. An early pressure-solution accommodated cataclastic phase of deformation is partially overprinted by later plastic deformation involving dislocation glide, dislocation climb and grain-boundary sliding mechanisms. Two- and three-dimensional strain determinations indicate that displacement reduction in the tip zone is primarily accommodated by footwall extension with subsidiary contribution from hangingwall contraction. Localized oblique movement in the tip zone is explained by the process of tip-line pinning. Geometrical considerations reveal the requirement for diachronous emergence of the tip-line from an obstacle to propagation. This enables differential displacement about a pinning point and the consequent generation of shear strain coplanar with the thrust plane. Such shear strains are necessarily localized and limited in magnitude. The stress pattern around the thrust tip is compatible with that predicted by the elastic solution to the stress distribution around a Mode II shear crack. The observation of a recognizable stress concentration suggests that quite high stress concentrations may accompany thrust-tip propagation. © 1990.