Late Neogene horizontal and vertical displacement rates during simultaneous contraction and extension in the Southern Apennines orogen, Italy

Assessment of vertical and horizontal displacements and displacement rates within the western Adriatic orogens, where contractional and extensional deformation coexists since the Miocene (PATACCA et alii, 1990; DOGLIONI, 1991), has the potential to supply vital insight into crustal and lithospheric processes operating during continental collision. Using the tight age control on contractional motion provided by synorogenic sequences preserved in outcrop and in petroleum exploration wells, the constraints on extensional motion provided by the crustal structure of the extended hinterland, and the differential elevation of uplifted markers of ancient base level, we establish a regional pattern of vertical and horizontal motion in the Southern Apennines for the last ∼6 Ma, which points to an intricate interplay between lithospheric delamination and crustal structure (FERRANTI & OLDOW, 2005a; 2005b). During latest Miocene to Early Pleistocene, the frontal thrust of the orogen migrated toward the foreland rapidly (∼16 mm/yr) and was accompanied by subsidence with the frontal thrust belt and foredeep remaining at or below sea level. In contrast, the orogenic hinterland experienced extension, which was accompanied by uplift at -0.3 mm/yr along the eastern transition to the contractional belt but net subsidence and formation of the Tyrrhenian basin farther west. Through time, the extensional belt progressively widened toward the northeast at the same rate as the encroachment of the thrust front on the Adriatic foreland. Following a mid-Pleistocene reduction in horizontal displacement rate associated with impingement of the thrust belt on thick crust of the Adriatic interior, the frontal thrust belt and foreland experienced uplift at ∼0.5 mm/yr as contraction stepped to deeper structural levels. Uplift of the eastern margin of the extensional hinterland continued at ∼0.3 mm/yr and is followed by tectonic subsidence along the Tyrrhenian coast of southern Italy. Today, the pattern of mid-Pleistocene displacements continues, as suggested by seismicity and GPS velocities (OLDOW & FERRANTI, 2005). The similarity in migration rates of contractional and extensional fronts across southern Italy over the last 6 million years supports models of crustal delamination and roll-back of the subducted Adriatic slab (ROYDEN et alii, 1987; DOGLIONI, 1991) as a fundamental driving mechanism for deformation along the western margin of Adria. Temporal changes in the vertical and horizontal rates of deformation, however, probably reflect differences in crustal structure and are not directly related to lithospheric processes. The reduction in the horizontal displacement rate associated with the onset of rapid foreland and frontal thrust belt uplift during the Early Pleistocene corresponds to a change from thin - to thick-skinned contraction initiated with the involvement of thick continental crust in regional shortening. Unlike segments of the Apenninic chain in central Italy (LAVECCHIA et alii, 1994; CAVINATO & DE CELLES, 1999), uplift and formation of the Southern Apennine mountain chain was not primarily a response to contractional deformation. Much of the orogenic elevation, at least before the mid-Pleistocene onset of uplift in the frontal thrust belt and foreland, was accrued during the initial stages of extension related to crustal delamination.