Abstract

The Central Apennine foreland basin system in the Marche and Abruzzi regions of Italy preserves the eastern leading edge of the Apennine orogenic wedge, formed from late Miocene to Pleistocene. Four thrust stacks make up this external portion Central Apennine orogenic wedge; they progressively propagated towards the east, accreted the foredeep basin deposits and simultaneously shifted the depocenters of foredeep and wedge-top basins. An updated chronostratigraphic scheme, integrating recent numerical ages and new GSSP of the Mediterranean area, made it possible to quantify the following rates averaged on time interval of hundreds of thousands years: 1) migration rates of the leading edges of the orogenic wedge (21.47 mm/y); 2) depocenter migration rates of foredeep (9.4 mm/y) and wedge-top (5 mm/y) basins; 3) sedimentation rates in foredeep (2.13 mm/y) and wedge-top (1.1 mm/y) depocenters. The values obtained show that the late Miocene-Pleistocene growth of this orogenic wedge can be envisaged as a cyclic repetition of two accretion modes. A fast-accretion mode, or paroxismal phase, associated with creation of a new thrust front, widespread erosion and widening of the orogenic wedge. Since the late Miocene, three paroxismal phases occurred (around 5.6 M.a., 4.35 M.a and 3.59 M.a.). In the slow-accretion mode, the thrust front did not advance and the orogenic wedge was shortened by out-of-sequence thrusts distributed over the entire wedge; sedimentation rate overcame tectonic deformation and the height of the wedge was increased by the deposition of kilometers-thick marine and continental sediments. This two-mode accretion, common to other orogenic wedges on longer-term periodicity, can be considered typical of orogens and might help in foreseeing the evolution and growth rates of other outer orogenic wedges.

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