Neogene-Quaternary evolution of the Central Apennine thrust front: Constraints from sequence and forward balancing of a regional cross-section/Evoluzione neogenico-quaternaria del fronte della catena centro-appenninica: vincoli dal bilanciamento di una sezione geologica regionale

In this study the outer sector of the Central Apennine of Italy and the adjacent Adriatic foreland were analysed in order to reconstruct their structural setting and the Neogene-Quaternary evolution of the fold-and-thrust belt. In the investigated area the thrust front is largely buried beneath Messinian-Pliocene siliciclastic sediments in the on-shore sector (i.e. peri-Adriatic Basin), and by Pliocene-Quaternary deposits in the adjacent Adriatic Basin (BALLY et alii, 1986; ORI et alii, 1991; CASNEDI & SERAFINI, 1994; ARTONI & CASERO, 1997; MONTEFALCONE, 2000; ARGNANI et alii, 2001; CALAMITA et alii, 2002; BOLIS et alii, 2003). In the inner sector of the chain the Mesozoic-Paleogene carbonate units crop out in the core of two anticlines (the Acquasanta and the M.gna dei Fiori Structures – KOOPMAN, 1983; MATTEI, 1987; CALAMITA et alii, 1991; CENTAMORE et alii, 1992a; INVERNIZZI & RIDOLFI, 1992; MARSILI & TOZZI, 1995; CALAMITA et alii, 1998); by contrast the top of the carbonate succession abruptly deepens toward the east where it lies at depths of more than 7000 meters below sea level. Here, the thick Pliocene-Quaternary siliciclastic succession is involved in an imbricate thin-skinned fold-and-thrust system with a leading edge located in the Central Adriatic Basin. A balanced geological cross-section based on surface geologicalstructural surveys and subsurface data (i.e. seismic reflection profiles and well-log data) was constructed with the aim to illustrate the structural setting of the outer sector of the Central Apennines of Italy. This cross-section extends through the Acquasanta anticline, the M.gna dei Fiori anticline, the peri-Adriatic Basin and the Central Adriatic off-shore, as far as the Italy-Croatia mid-line. The section allowed us to define the geometry of the Central Apennine thrust front, the foreland ramp, and the syn-tectonic basin infill; moreover, by sequential balancing and subsequent forward balancing, the tectonic evolution of the chain-foredeep-foreland system were reconstructed and the shortening rates were calculated. Based on this study, the complex structural setting of the Central Apennine fold-and-thrust belt results from the interaction between an extremely thin-skinned and a thick-skinned thrust system. The former only affects the syn-orogenic siliciclastic deposits and generally predates the deeper thrust system that cuts across the whole sedimentary cover (i.e. carbonate units and overlying siliciclastic sediments), and locally the basement. The uncoupling between thin and thick-skinned tectonics strictly controls the evolution and migration of the syn-tectonic basins, and influences the sequence of thrust propagation; this latter, with respect to the deeper stratigraphic levels, is mainly toward the foreland. Moreover, spacing and location of thrust ramps are controlled by pre-existing discontinuities that affected the foreland ramp.