Bending and unbending of an elastic lithosphere: The Cenozoic history of the Apennine and Dinaride foredeep basins

Abstract

The Adriatic region forms an intermediate continental foreland overthrust along its northeastern margin by the southwest vergent Dinaric thrust belt in Eocene‐Oligocene time and along its southwestern margin by the northeast vergent Apennine thrust belt in Pliocene‐Quaternary time. Orogenic activity within these thrust belts was accompanied by the development of two superposed foredeep basin systems of opposite polarity and different ages. Using well log, biostratigraphic, and seismostratigraphic data, the geometry of this composite basin system was reconstructed along three profiles at beginning of Quaternary, middle Pliocene, beginning Pliocene, and beginning Eocene time. Modeling of reconstructed geometries using a thin elastic sheet approximation yields a range of acceptable effective elastic plate thicknesses for the central Adriatic region of Te = 5–10 km for Eocene‐Oligocene flexure and Te = 10–15 km for Pliocene‐Quaternary flexure (although an upper bound for Te could not be established on one of the three profiles). These results are consistent with a constant effective elastic plate thickness of Te = 10 km for the Adriatic lithosphere and preclude the possibility that significant weakening of the Adriatic plate occurred between flexural events. Modeling of incremental deflections between Pliocene and Quaternary time gives results consistent with constant values of Te = 10 km in the central Adriatic and Te = 15 km in the northern Adriatic and Po Plain and shows little evidence for weakening of the plate during Pliocene‐Quaternary time. Thus within the resolution of the data presented in this paper, there is little evidence for viscous relaxation of the lithosphere on time scales between about 2 and 50 m.y. Analysis of bending of an idealized lithosphere with a simple brittle‐elastic‐ductile rheology, and a low to moderate thermal gradient suggests that the small values of Te observed within the Adriatic region can be readily understood as the result of bending of the lithosphere to unusually high curvature (4 × 10−6 m−1) and do not require unusually high temperatures within the foreland lithosphere. The same rheological model is also consistent with the absence of significant inelastic yielding for at least 50 m.y. after the cessation of loading. An apparent unbending of the Adriatic lithosphere began in early Quaternary time, approximately coeval with the cessation of major thrusting within the Apennine thrust belt. The three‐dimensional pattern of Quaternary deflection makes it difficult to attribute this phenomenon to local depositional processes and suggests that unbending reflects a fundamental change in the subduction process in early Quaternary time. Our preferred interpretation is that unbending is the result of a diminution of forces acting on the subducted Adriatic lithosphere at mantle depths.