The geology of the central Apennines and foreland basin, Italy. (Volumes I and II)
Cooper, J. Calvin
Bally, Albert W.
Doctor of Philosophy
Seismic, surface, and well data from the mid-Adriatic Sea and the Umbria-Marche region of central Italy suggest a shallowly detached decollement thrust geometry for the central Apennines and its foreland basin with a substantial amount of shortening. The front of the Apennine overthrust system trends NW-SE in the Adriatic Sea. Palinspastically restored balanced cross sections suggest that shortening and the number of thrust sheets increase significantly from north to south. Much of the increased shortening is due to three north-south trending duplex structures in the foreland basin, which lose their identity to the north. Paleozoic basement does not appear to be involved in the thrusting east of the Tevere River, and the Triassic evaporites act as the principal detachment surface. Seismic profiles in the Umbria-Marche region do not permit the identification of the basement. However, magnetic and gravimetric interpretations indicate that the basement dips gently westward beneath the central Apennines to depths of 12-14 kms near the Tevere River. Onlap relationships in the Adriatic Sea suggest that thrust structures were formed predominantly in the middle Pliocene, with additional growth in the upper Pliocene. The Pleistocene shows no folding. The structures in the western foreland seem to have been formed in the later part of the lower Pliocene. The youngest rocks beneath the Sibillini thrust are of middle Messinian age, and thrusting gets progressively older to the west. Normal faulting occurred shortly after thrusting, and a later extensional phase continues in the Present. A Jurassic extensional event probably was dominated by moderate to low angle, east dipping normal faults, with high angle antithetic faults. Tertiary thrusting inverted some of the Jurassic extensional structures, apparently taking advantage of pre-existing discontinuities in the Triassic evaporites. In the Sibillini mountains, modern normal faults listrically sole into the pre-existing thrust detachment surface, and do not cut the (footwall) sediments of the Laga Basin. The Miocene-Pliocene clastic sequence is often detached from the Jurassic to Early Tertiary carbonate sections, and thus the structural style of shallow units in the foreland seems more complicated than deeper or more westerly units. Prominent reflections on a seismic profile are not offset by surface lineaments identified in Landsat Thematic Mapper data.