Sequential balancing of growth structures, a late Tertiary example from the central Apennines.

During the late Tertiary, the external front of the Central Apennines migrates towards the Adriatic foreland basin of the Marche and Abruzzi regions. The outermost synsedimentary growth structure, i.e. the buried Tortoreto-Campomare imbricate fan, was analysed using a seismic reflection line and borehole data. The growth structures record the interaction of tectonic and sedimentary processes at a scale of a few kilometres. The syntectonic sedimentary units form progressive unconformities along the Tortoreto-Campomare thrust panel and are used to unravel the geometric evolution of this buried external front of the Apennines. A sequential balancing method is used to restore unconformity surfaces to their most plausible initial configuration. This methodology allows the reconstruction of nine evolutionary stages for the studied structure since the Pliocene (≈ 4 Ma). These stages define the local geometry of the foredeep basin during and after each unconformity. As each unconformity surface represents a palaeotopography, nine successive topographic surfaces have been restored. The progressive unconformity formed close to the hinge of the Tortoreto-Campomare thrust panel. The cut/seal relationships between the tip of the thrusts and the stratigraphie units constrain the timing of thrusting, and define a local break-back sequence. However, synchronous and out-of-sequence thrusting also occurred at different stages of the structure growth. The time interval of activity and the thrust displacements calculated by cross section restorations allow the estimation of compressional rates, these ranging between 0.12mm/y and 4.25 mm/y. The former value is comparable to displacement rates in previous literature. Despite limitations, this sequential balancing technique is promising and useful for the study of growth folds.