VERTICAL-AXIS ROTATION ASSOCIATED WITH FOLDING AND THRUSTING - AND EXAMPLE FROM THE EASTERN SUBBENTIC ZONE OF SOUTHERN SPAIN

The differential rotation about a vertical axis across a southeast-ver gent recumbent fold and thrust fault within a single nappe from the ea stern Subbetic zone of southern Spain has been determined using paleom agnetism. The upper limb has rotated clockwise by 68-degrees +/- 22-de grees relative to the lower limb. Mapping of the structure indicates t hat the syncline is an isoclinal fold in the northeast and an open fol d in the southwest. The differential vertical-axis rotation across thi s structure appears to have been accommodated by a progressive increas e in along-strike horizontal shortening, reaching 8.5 km at 3.7 km fro m the pivot point. This differential shortening and the rotation are p robably the result of increased drag on the underlying thrust close to the pivot point compared to the farthest extent of the structure. The lateral boundary of the rotating sheet is a sinistral strike-slip fau lt, which is consistent with the rotational geometry. Pinning is proba bly enhanced by lateral heterogeneities within the stratigraphy. This result ties rotational deformation to early-middle Miocene folding and thrusting, rather than to late Miocene-Holocene strike-slip deformati on. Rotation by pinning of thrust sheets may prove to be an important aspect of deformation in many fold and thrust belts.