Southalpine Versus Po Plain Apenninic Arcs

Abstract

Abstract Previously known Apenninic arcs, buried under the Po Plain are compared with recently recognized arcuate systems within the Southern Alps. The three main arcuate systems of the Po Plain are all of Neogene age (Upper Miocene to Upper Pliocene) and are roughly concentric with the older, more internal Apennine arcs. They originated by ove r thrusting of the Apennines onto the Po Plain foredeep. The arcuate pattern could be related to the rheologic buttresses represented by the two more rigid Mantua and Pavia crustal blocks, characterized by positive magnetic anomalies(basic intrusions) and by repeated Pa l aeozoic(?), Triassic and Neogene volcanic bodies; such blocks have prevented lateral propagation of decollement surfaces giving rise an overall arcuate pattern. The three main arcuate systems of the Southern Alps are of different ages, show a remarkable time-space polarity to the SE and are not concentric. The western, innermost arc is of Palaeogene age; the i n termediate arc, partly buried beneath the Lombardy Plain, is of Upper Miocene age and is sharply bent around the Adamello batholith; the eastern, outermost arc has the same Upper Miocene to Upper Pliocene age as the buried Apenninic arcs and is only developed beyond the front of the first two Southalpine arcs. Such an asymmetric setting can be related to the buttressing effect provided by the Venetian (Lessini to Dolomites) Platform acting as a rigid block, owing to its peculiar magmatic and depositional history since the Upper Palaeozoic (Hercynian granitoid intrusions, Permian volcanism, mid-Triassic volcanism and tectogenesis, Palaeogene volcanism). The very close location of the Venetian Platform to the Mantua block strengthened a relevant part of the Padania crust accounting for the major time-space asymmetric distribution of the adjoining arc systems. The epidermal, roughly symmetric opposite vergence of the two arc systems does not reflect symmetrical kinematics. In fact, from the above data, the Neogene Po Plain Apenninic arcs are better explained as overthrusts upon the Padania-Adriatic foredeep. Conversely, the coeval eastermost Southalpine arc originated by underthrusting of the Adriatic-Padaniacrust towards the NNW below the Southern Alps. True, active A subduction is, therefore, suggested at the eastern Southalpine front (consistently with the Africa-Adria drift towards the N), whereas only relative, apparent or passive A subduction can be admitted for the Northapennine front.