Detachment tectonics in the Central Apennines, Italy

Abstract

The sedimentary cover of the investigated area consists of a thick sequence (Triassic-Miocene) of limestones and marls, overlain by turbiditic sandstones of the "Laga Formation" (Messinian- Early Pl iocene). No pre-Triassic basement is exposed. The most consipicuous structural feature is the Umbria Thrust Zone, a segment of the so-called Ancona-Anzio Line. This oblique lineament is generally taken as the boundary between the Northern and Southern Apennines, although its significance is highly controversial. But thrusting and associated folding is not confined to the Umbria Thrust Zone: it prevails allover the Central Apennines. The thrusts are mostly not single planes, but pervasive deformation zones, strongly affected by pressure solution. Slip, as indicated by slickensides, was everywhere towards the NE, regardless of the strike of the various structures. The style of deformation is "thin-skinned". Two major detachment levels - a lower, subsurface one at the base of the cover sequence, and a higher one just below the Laga Formation - are connected by a series of ramps where thrusts cut up-section. Doubling of strata above and beyond these ramps has resulted in large rootless anticlines with complicated, disharmonic forward limbs. The thrust steps developed successively, from the internal (SW) side outward. The arcuate and oblique trends of many rootless anticlines, or rather of the ramps below, have been controlled by pre-existing irregularities inside the sedimentary cover due to Jurassic block-faulting. By repeated doublings, the lower part of the cover has been shortened considerably. But very little of this shortening has affected the Laga Formation, which belongs almost entirely to the hanging wall and has been displaced as a whole. Thus the investigated area, as well as the adjacent Umbria Marche Apennines and Abruzzi including the "Ancona-Anzio Line", belong to a single allochthonous thrust complex.