Passive continental margins must be viewed as tectonically alive; they undergo thinning, subsidence, and possibly extension, and their form influences and is influenced by the pattern of the overlying sedimentary cover. Mesozoic and lower Tertiary carbonates deposited on the South Tethyan continental margin during opening of the central Atlantic and Tethys Oceans carry a useful record of subsidence. Since a very large area subsided for more than 150 m.y. with apparently no tectonic activity other than normal faulting, the subsidence was probably an isostatic response to crustal thinning. Isostatic considerations, using reasonable densities, indicate that crustal thickness t′c at a particular time is given by
t′c = tc − 4.6t′w − 1.8 t′s,
where t′c is the original crustal thickness, and t′w and t′s are the water depth and sedimentary thickness, respectively. Calculations based on Apennine Stratigraphic sequences indicate thinning of the Italian continental crust by approximately 40% beneath pelagic basins and 20% beneath carbonate platforms.
We suggest that this thinning may have been due to extension of the crust resulting from a mechanism, proposed by Bott in 1971, which indicates, that unbalanced lateral forces should be present at the contact between crusts of different thickness, even when they are in isostatic equilibrium. Extension and thinning of the South Tethyan margin affected a belt several hundred kilometres wide; we suggest that this was due to the effect of the Triassic intracontinental rifting that preceded Jurassic oceanic rifting. During the earlier, intracontinental rifting, numerous basins of Newark type were formed, and the associated crustal discontinuities would have provided a number of loci where Bolt's lateral flow mechanism could have acted.
The Apennine crust is now of at least normal thickness, indicating that it has been rethickened; this probably occurred during Tertiary collisional orogeny, as was suggested for the Eastern Alps by Helwig in 1976. An important aspect of Helwig's re-thickening model is that it removes the need for subduction of large volumes of buoyant continental crust during collisional orogeny.