| Abstract: | Fault planes propagate radially from a point source and this failure propagation process is very similar to the movement of a dislocation through a crystal lattice. An elastic strain represented by an extra half plane is necessarily imparted to the lattice at an edge dislocation, and this is equivalent to the ductile bead which accompanies the propagating tip of a thrust fault. This ductile bead migrates with the fault tip, and imparts a characteristic internal strain to the thrust sheet. A two-dimensional model is presented to illustrate the inter-relationship between fault plane slip, fault tip propagation and internal strain. In multilayered sequences, internal strain is usually represented by asymmetric folds verging in the thrust transport direction. A simple technique, the displacement/distance plot utilizes the fact that displacement dies out towards the fault tip. This plot can be used to quantify the relative stretch, measured parallel to the fault movement, which is dependent on the slip/propagation rate, and it may be used to define exactly the position of the fault tip on a cross-section. Examples of fold-thrust structures from Devon (England) and Pembrokeshire (Wales) are used to illustrate the technique. |
