Thermal evolution of a rift basin: The Tyrrhenian Sea

Abstract

Existing geological and geophysical data support the view that the opening of the Tyrrhenian Sea was by rifting of a formerly continuous continental lithosphere, which began in upper Miocene on the upper Sardinian margin and migrated southeastward to the Calabria margin. Heat flow across the Tyrrhenian Sea shows a pronounced asymmetry, from about 50–70 mW m−2 over the Sardinian margin, to more than 150 mW m−2 over its southeastern margin off southern Italy and Calabria. In this study we use the newly acquired data from Ocean Drilling Program Leg 107, heat flow, seismic velocity, and bathymetric data to constrain a group of models that supposes the Tyrrhenian Sea opened by detachment of a formerly continuous lithosphere along a southeastern facing, low‐angle normal fault. Specific models tested include (1) the Wernicke model, in which thinning and extension of the continental lithosphere are supposed to be accomplished by “simple shear” along a large‐scale, gently dipping detachment fault zone which cut through the entire lithosphere, and (2) the delamination model, in which the low‐angle detachment fault is supposed to cut only the upper and middle crust but to merge subhorizontally into the lower crust, below which concurrent pure shear may occur. A versatile, two‐dimensional finite element procedure is used to evaluate the tectonic and thermal evolution of these models, and the results are compared with observations to test the validity of the models. Delamination models with large pure shear components in the lower lithosphere and the Wernicke model predict heat flow within ±1σ of the measurements. The Wernicke model, however, predicts a basin configuration much too deep in comparison with bathymetry and a subbasinal lithospheric thickness substantially different from Rayleigh wave dispersion results. The delamination model with a large component of pure shear in the subbasinal lower lithosphere, on the other hand, predicts satisfactorily both the bathymetry and the lithospheric thickness beneath the basin.