Evidence for the importance of `small' faults on block rotation

Abstract

Field and earthquake data are presented which show that small faults can be important during block rotation. `Small' here means those faults which are at least an order of magnitude smaller than the largest faults at a particular scale of observation. The NW–SE-trending dextral faults of SW England have millimetres to kilometres of displacement. The largest of these faults, such as the Sticklepath–Lustleigh fault, do not appear to cause more than a few degrees of rotation from the general E–W strike of beds. Tens of degrees of rotation are often visible, however, on faults with millimetres of displacement. Similarly, the apparent rotation of beds in the Badajoz–Cordoba Shear Zone, Spain, increases as the resolution of the faults is increased. The power-law scaling relationship of earthquake magnitudes in the San Andreas fault zone illustrates that small faults can also be important in regions of active block rotation. `Small' faults can allow deformation within rotating blocks and can allow high displacement gradients to occur on the block-bounding faults. This would reduce the need for void creation, which is a requirement of rigid block rotation models. A tentative model is presented which incorporates the concept that fault behaviour is fractal, emphasising the contribution of small faults. It is suggested that future studies of block rotation should rigorously test the contribution of `small' faults, and that particular care is needed when comparing palaeomagnetic data with regional-scale structures.