Geometry and kinematics of vertical-axis rotations in fold and thrust belts

Abstract

Vertical-axis rotations indicated by palaeomagnetic study are a common and important component of deformation in fold and thrust belts. The kinematics of rotating thrust sheets may be best described as a combination of a far-field rotation (or translation in a planar geometry) and a near-field, or local rotation, so that the instantaneous rotation axis lies outside the body of the sheet. This simplifies the thrust-sheet kinematics significantly, as it means that all the points on the sheet have a component of forward motion, and the bounding fault is a thrust over all its area. The consequences of a simple thrust-sheet rotation for the interpretation of fault lineation data are investigated. The lineations recorded in the footwall and the hangingwall are likely to be different, the hangingwall lineations being skewed towards the direction of rotation. The principal mechanism for local thrust-sheet rotation is differential shortening, either on folds or thrusts, or by a combination of these structures. The forward propagation of thrusting towards the foreland can accumulate large (>120°) rotations on the uppermost sheets. These rotational thrust sheets cannot extend indefinitely along strike; instead they terminate against lateral ramps or transfer faults. Sidewall duplexes may form as a consequence of rotation over a lateral ramp. Examples of rotational geometries from the Subbetic Zone of southern Spain are described. On a larger scale, obliquely convergent orogenic wedges can evolve by rotation of faults with an initial thrust geometry into progressively greater components of strike-slip motion. Reliable palaeomagnetic data are not always available for fold and thrust belts, so independent evidence for rotational deformation is of great importance. Fold and thrust geometries, particularly along-strike variability in fold geometries, highly variable axial traces of folds, evidence for near-synchronous refolding and the deviation of regional trends may all be a consequence of thrust-sheet rotation. Detailed mapping, and particularly the identification of the position of footwall and hangingwall cut-offs can also provide strong evidence for rotation.