Mass transfer from the Alps to the Apennines: volumetric constraints in the provenance study of the Macigno-Modino source-basin system, Chattian-Aquitanian, north-western Italy.

ABSTRACT: A quantitative provenance approach is applied to the Late Oligocene–Early Miocene Macigno–Modino turbidite complex which accumulated in the foreland basin developed in front of the growing Northern Apennines, but was supplied from the rapidly rising Western Alps, at the northwestern end of the basin. As well as the traditional recognition of source rock lithologies and their gross paleogeographic location, the provenance approach adopted here evaluates the relative contributions to the clastic complex from different types of source rock, and estimates the volume of material transferred from the Alps to the Apennines within the Macigno–Modino source–basin system, and the overall volume of rock assumed to have undergone erosion in order to supply it. These elements are used to constrain the possible source of the clastics and to infer the main paleogeological features of the source area. It is concluded that an estimated volume of ca. 17–20 ð 103 km3 of basement (mainly metamorphic) rocks, together with ca. 103 km3 of intermediate composition volcanic rocks was transferred as detritus from the Western Alps area to the Northern Apennines foreland basin, creating the 250–300 km long, 50 km wide and up to 2.7 km thick Macigno–Modino clastic complex. Lithology and paleogeography, combined with volume estimates and current knowledge of the regional geology of the Western Alps, consistently suggest that the bulk of the sediments funnelled into the Macigno–Modino complex were derived from the uplifted Ivrea crustal block, which formed a crystalline massif some 5000 km2 in extent, that rose ca. 3 km during the Oligocene, forming one of the main elements of Western Alpine geology during that time.