CONTROLS ON THE GENESIS OF THE APENNINE-TYRRHENIAN ARCS: MANTLE DYNAMICS VS. OROGENIC WEDGE KINEMATICS

I review here the geophysical / geological evidence helping to unravel the origin of the Apenninic arcs. The Apennines, developing in mid Miocene-Pleistocene times synchronous with back-arc spreading of the Tyrrhenian Sea, are made by two major salients: the northern Apennines and the southern Apennines-Calabria-Sicilian Maghrebides (SACSM) arcs (orogen-parallel length is ~500 and ~800 km, respectively). Paleomagnetism shows that the northern Apennine arc is properly an orocline, arising from post-Messinian bending of an originally N320° straight belt. Seismic tomography in turn reveals that the Adriatic lithosphere, subducting beneath the northern Apennines, is laterally bent, and closely mirrors the surface geological outlines. Paleomagnetism of the SACSM shows that the southern Apennines and Sicily underwent 80° CCW and 100° CW rotations, respectively, synchronous with orogenic-wedge migration. These rotations then suggest that the limbs of the SACSM opened in a “saloon-door” fashion, pushed apart by the southeastward drift of the Calabrian block. The rapid (average speed of 3 cm/yr) Calabrian migration, in turn, was driven by the southeastward roll-back of a paleo-Ionian oceanic lithospheric strand, yielding today deep mantle seismicity beneath the southern Tyrrhenian Sea. At a smaller scale, other minor curved mountain fronts are observed in the Apennines. Here I deal with the Gran Sasso arc (central Apenines), and the Gela Nappe (southern Sicily). Gran Sasso is a tight salient rotating ~90° CCW during thrust-sheet emplacement. This rotation seems to be due to the collision between two rigid carbonate paltforms during wedge propagation, and consequent lateral escape of a belt fragment towards ductile basinal sediment areas. The Gela Nappe is a 150 km-long basinal wedge salient developing between two carbonatic platform foreland obstacles. No differential rotations are observed in the limbs of this arc, confirming the results of recent physical models showing that the outer front of a salient formed between two foreland obstacles does not undergo rotations. I conclude that, although the accretionary wedge-foreland interaction may explain the origin of smaller-scale arcs, mantle dynamics undoubtedly plays a fundamental role in the genesis of the first-order Apenninic salients.