BENDING AND UNBENDING OF AN ELASTIC LITHOSPHERE - THE CENOZOIC HISTORY OF THE APENNINE AND DINARIDE FOREDEEP BASINS

The Adriatic region forms an intermediate continental foreland overthr ust along its northeastern margin by the southwest vergent Dinaric thr ust belt in Eocene-Oligocene time and along its southwestern margin by the northeast vergent Apennine trust belt in Pliocene-Quaternary time . Orogenic activity within these thrust belts was accompanied by the d evelopment of two superposed foredeep basin systems of opposite polari ty and different ages. Using well log, biostratigraphic, and seismostr atigraphic data, the geometry of this composite basin system was recon structed along three profiles at beginning of Quaternary, middle Plioc ene, beginning Pliocene, and beginning Eocene time. Modeling of recons tructed geometries using a thin elastic sheet approximation yields a r ange of acceptable effective elastic plate thicknesses for the central Adriatic region of T(e) = 5-10 km for Eocene-Oligocene flexure and T( e) = 10-15 km for Pliocene-Quaternary flexure (although an upper bound for T(e) could not be established on one of the three profiles). Thes e results are consistent with a constant effective elastsic plate thic kness of T(e) = 10 km for the Adriatic lithosphere and preclude the po ssibility that significant weakening of the Adriatic plate occurred be tween flexural events. Modeling of incremental deflections between Pli ocene and Quaternary time gives results consistent with constant value s of T(e) = 10 km in the central Adriatic and T(e) = 15 km in the nort hern Adriatic and Po Plain and shows littl evidence for weakening of t he plate during Pliocene-Quanternary time. Thus within the resolution of the data presented in this paper, there is little evidence for visc ous relxation fo the lithosphere on time scales between about 2 and 50 m.y. Analysis of bending of an idealized lithosphere with a simple br ittle-elastic-ductile rheology, and a low to moderate thermal gradient suggest that the small values of T(e) observed within the Adriatic re gion can be readily understood as the result of bending of the lithosp here to unusually high curvature (4 x 10(-6) m-1) and do not require u sually high temperatures within the foreland lithosphere. The same rhe ological model is also consistent with the absence of significant inel astic yielding for at least 50 m.y. after the cessation of loading. An apparent unbending of the Adriatic lithosphere began in early Quatern ary time, approximately coeval with the cessation of major thrusting w ithin the Apennine thrust belt. The three-dimensional pattern of Quate rnary deflection makes it difficult to attribute this phenomenon to lo cal depositional processes and suggests that unbending reflects a fund amental changes in the subductijon process in early Quaternary time. O ur preferred interpretation is that unbending is the result of a dimin ution of forces acting on the subducted Adriatic lithosphere at mantle depths.