Genesis of tectonic inversion structures: seismic evidence for the development of key structures along the Purbeck–Isle of Wight Disturbance

The interpretation of a densely spaced and well-calibrated seismic grid sheds new light on the development and evolution of key regional and local structures in the Wessex Basin. The results help to resolve long-standing controversies concerning the tectonic significance of apparently anomalous outcrop patterns and the role of important, local ancillary structures with respect to the major monoclinal folds with which they are associated. Although the structures are entirely consistent with the effects of contractional reactivation (tectonic inversion) of normal faults, the subsurface data demonstrate the role that original extensional fault segmentation and associated relay ramps had on original depositional patterns, subsequent inversion geometries and resultant outcrop patterns. As well as illustrating regional controls on the formation of structures, the new seismic-based interpretations enable a reassessment of the Lulworth Crumple and the Ballard Down Fault. The Lulworth Crumple is interpreted as a parasitic fold complex generated by internal folding of the inverted, incompetent syn-rift fill in the immediate hanging wall to the Purbeck Fault, a reactivated major normal fault. The Ballard Down Fault’s origin is interpreted to result from the formation of a local, late-stage ‘out of the syncline’ reverse fault which propagated southwards and upwards through a Chalk succession. As the Chalk had already been rotated to form the northward-dipping steep limb of the Purbeck Monocline at Ballard Down, the structure cuts down stratigraphically. The results stress the importance of understanding the nature of original extensional fault geometries and the competence of the sedimentary units incorporated in folds in gaining a full understanding of the genesis and evolution of structural styles in inverted basins.