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Tectonic History |
Department of Earth Science and Engineering, Royal School of Mines, Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW7 2BP, UK
A field study of the orientation of surface features on joint planes has been carried out on the north Somerset coast along the southern margin of the Bristol Channel Basin (BCB), with the aim of inferring the palaeostress orientations. Surface features are very delicate structures on joint surfaces and when preserved give important clues regarding the direction of propagation of a joint and the interaction between the remote and local stresses.
In this chapter surface features (i.e. plumose structures and twist-hackle fringes) observed on joints in Liassic limestone beds interbedded with shales from the BCB are described and their implications regarding the evolution of the stress field operating during their formation considered. The results are then compared with the generally accepted model for the region of an anticlockwise rotation of the stress field. The incompatibility between N-S-directed remote compression and continually changing local stress caused by the local structures (folds and faults), inhomogeneities and the anisotropy created by rapidly alternating limestone and shale beds resulted in the master joints breaking down into en echelon cracks (twist-hackle fringes). Joints in thicker limestone beds show well-developed plumose structures and twist-hackle fringes, whereas in thinner beds the fringe zones extend from the top to the bottom of the bed. It is found that regardless of the orientation of the parent joint in which the twist-hackle fringes are contained, they generally strike E-W and show both right- and left-stepping arrangements.
