Development of systematic joints in response to flexure-related fibre stress in flexed foreland plates: the Apulian forebulge case history, Italy

Starting from Pliocene times, the Apulian foreland in southern Italy has undergone a flexural process underneath the Apennines thrust-fold belt. Mesozoic carbonate beds from the exposed Apulian forebulge are intensely dissected by systematic vertical joints striking parallel to the NW-trending flexure hinge. The sedimentary cover involved in the flexure consists of a 5.5 km thick succession of Mesozoic-Cenozoic carbonate beds strongly interpenetrated along bedding surfaces and overlying Triassic anhydrites and dolomites. On the assumption of the applicability of the linear elastic theory, the flexural curve of the Apulian foreland is reconstructed by best fitting a set of data derived from logs of oil wells and referring to the height of the base of Pliocene sediments. A set of flexural parameters is obtained from the reconstructed flexural curve. By using these parameters in solving the flexure equations for the fibre stress, we obtained a flexure- related hinge-perpendicular fibre stress in the outer arc of the Apulian forebulge in excess of -100 MPa. This value is far greater than the tensile strength of the exposed carbonate rocks. This result supports the hypothesis that the flexure-related fibre stress in the Apulian forebulge can have driven the initiation of the observed regional systematic joints. This model may be applied to any flexed foreland plate and constitutes an alternative to the classical model of elastic response of rocks to the variations of lithostatic loads through time that has been often invoked to explain sets of systematic joints over large areas of foreland regions.