TECTONIC EVOLUTION OF THE SOUTHERN APENNINES THRUST-BELT (ITALY) AS REFLECTED IN MODAL COMPOSITIONS OF CENOZOIC SANDSTONE

The composition of sandstones derived from erosion of the southern Ape nnines orogen provides constraints for Cenozoic tectonic evolution. Cl astic wedges were derived mainly from the strongly deformed Calabrian are allochthon. This key tectonic element is interpreted as a thrust s heet of European crust, moved about 300-400 km eastward during the Pal eogene to Neogene toward the Adria foreland. During the Paleogene to E arly Miocene, the Liguride Complex oceanic terrane was deposited in a remnant ocean basin, east of the Calabrian are, related to northwestwa rd subduction of Ligurian oceanic lithosphere beneath the European mar gin. Upper Paleogene quartzofeldspathic and volcanolithic Liguride san dstones testify to provenance from the subduction complex to the magma tic are, respectively. Upper Oligocene to lower Miocene quartzolithic sandstone suggests a recycled-orogen provenance (i.e., metasedimentary rocks in the growing accretionary Calabrian arc). Final closure of th e Liguride remnant ocean basin and onset of continental collision in s outhern Italy culminated in overthrusting of the Calabrian and Ligurid e thrust-sheets onto the Adriatic passive continental margin. Continen tal collision is dated as Early Miocene and was followed by formation of foreland, foredeep, and thrust-top basins. The unroofing history of the Calabrian are is documented in the detrital modes of Langhian to Tortonian foreland quartzolithic, volcanolithic, and quartzofeldspathi c sandstones. A drastic shift in composition toward ''ideal arkose'' o ccurred during late Tortonian-Messinian in both foreland and syn-rift sandstone strata, reflecting a crustal-block provenance. An oceanic ba ckarc rift in the southern Tyrrhenian area (i.e., Vavilov basin) contr ibuted to increased uplift of the deeper Calabrian are crystalline ter ranes. The Cenozoic geodynamic history of the Peri-Tyrrhenian area, wh ich included an are-trench system/continental margin collisional sutur e zone and, since late Tortonian, back-are oceanic-crust spreading (i. e., Vavilov basin), is reflected in sandstone modal composition. The t ype of sedimentary provenance analysis presented here, providing an ex ample of the changing nature of a collisional-belt source area through time, may have general application to other major orogens.