Structural Features and Basin Tectonics of the Tyrrhenian Sea

Abstract

The principal geological and geophysical features of the Tyrrhenian Sea are described and discussed in order to determine a correct kinematic picture. Its bilateral symmetry, which gives it the character of an interarc basin, is also manifested by the deep crustal configuration that should indicate opposite-vergent arcuate overthrusting (obduction) of the Tyrrhenian oceanic crust onto the Alpine (modern inactive front) and the Apennine (active front) foreland basements, respectively. The eastern, seismically active front is believed to have resulted from a process of Neogene Apenninization of the previous Alpine fold structures, triggered by the eastward migration of domal uplifts, transient in time and space. The Miocene regional updoming of the Tyrrhenian crust, producing the Tyrrhenidmegadome and the northern Tyrrhenian (Etruscan) dome, originated episodes of extensional rifting expressed by the formation of crestal three-armed fault troughs, crudely delineated by magnetic lineations. The domal uplift belts provided the adjacent troughs both with terrigenous turbidite wedges and with thrust sheets, olistostromes, and basement nappes, detached by delamination from the flanks of the ridges. After the crust was supracrustally thinned by erosion, delamination, and lateral gravitational spreading, the axial collapse and subsidence of the rift belts took place in Plio-Quaternary time. It was accompanied by differential cauldronlike collapses, creating the ring-structures (krikogens) and the volcano-tectonic depressions now observed on the Tyrrhenian seafloor. The limited areal distribution of the oceanic basement, indicated both by the recent seismic crustal studies and by magnetic anomalies (by AGIP), precludes the possibility of a large horizontal separation and relative rotation required by plate tectonics. The origin of the Tyrrhenian Basin is explained by the occurrence of intraplate, postorogenic, essentially vertical tectonic movements (termed krikogenesis), activated by localized flowage in the "plastic" mantle. Its regional structural configuration is considered to have resulted from the interaction of two contiguous topographic arches (i.e., the hinterland and foreland swells) in different stages of structural evolution.