Tectonic Structure of the Tuscany-Latium Continental Shelf (Tyrrhenian Sea)

Roberto Bartole

An old regional seismic multicoverage reflection survey (Zone "E") provides useful information on the shallow structure of the southern Tuscany-northern Latium shelf located between Elba Island and Anzio. Three main seismic units have been recognized on the grid, directly tied to the Matilde-1 offshore drilling and correlated to onshore wells and outcrops. The upper unit, characterized by continuous and generally undisturbed reflectors, is constituted by the postorogenic sedimentary cover of late Neogene-Quaternary age. This sequence unconformably lies on an intermediate unit or, when absent, a basal one.

These two units, characterized by good-to-poor seismic response, are constituted by the tectonized geologic units of the Northern and Central Apennines. The intermediate unit corresponds to the Ligurids and Sicilids allochthonous complexes, while the basal unit may be attributable to the Tuscan nappe and/or the Tuscan autochthon in the shelf sector north of the Tiber River mouth, and the Umbro-Sabina units south of the same mouth. The widespread distribution and consistent thickness of the allochthonous cover (up to 1,000 msec two-way traveltime) lying over a structured substratum might be of some interest in hydrocarbon prospecting.

The base of the postorogenic unit is characterized by numerous elongated depocenters (filled up to 1,600 msec two-way traveltime) which alternate with areas of structural highs, mostly interpreted as compressive features. Northwest-southeast trends predominate, but east-west and north-south grains are also present, the former beneath the Latial shelf, the latter beneath the Tuscan. Many strike-slip faults of anti-apennine (northeast-southwest) trend seem to cut the tectonized units, related to the differential tangential movements that occurred during the last compressive phases. Some of them are the extensions of well-known onshore transcurrent faults such as the Grosseto-Val Marecchia and the Valle Albegna trends.

Two diastrophic phases have been recognized by analyzing the seismic sections: the first phase, probably of early Miocene age, responsible for the emplacement of the allochthonous cover; and a second phase attributable to the middle-late Miocene, which strongly affected both the basal geologic units and their tectonic cover. As a consequence, the upper Neogene-Quaternary filling owes its shape and thickness mainly to this latter phase rather than to the postorogenic extensional tectonics which intensively faulted the continental slope.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.