LOCAL RELIEF, EXHUMATION RATES AND ROCK UPLIFT RATES IN THE NORTHERN APENNINE

The present, subdued relief of Northern Apennine divide depends on: i) the Basin and Range type structure of the inner sector of the Apenninic Chain ii) the prevailing flysch lithology iii) the current – mild - morphoclimatic regime

These factors hamper a high relief to set out, despite the high rock uplift rates that affect the Chain, as exhumation rates obtained from low temperature thermochronological methods have recently pointed out.

As geomorphic and thermochronologic evidences point out, the higher rock and surface uplift rates occur at present over the drainage divide of the Northern Apennine. According to Zattin et al. (2000), the pre-exhumation configuration features a 4 to 5 km thick cover (depending whether a geothermal gradient of 20°C/km or 25°C/km is assumed) of overlying Ligurian and Epiligurian Units, which was completely eroded in the last 5 M.y. at a mean rate, then, of 0,8 to 1 mm/y.

Since geological data suggest that little or no topography was present in the area between 5.0 and 2.0 Ma BP, rock uplift rates should have been quite similar, during that period, to the computed exhumation rates of approximately 0.8 mm/y. Surface uplift was therefore negligible. A residual veneer of Ligurids, presently buried under the fluvial deposits both in Mugello and in the nearby Casentino basin indicate that the 5 km thick Ligurids cover had not been completely wiped out when the basin became the site of flood plain sedimentation, that is around 2.0 Ma. The high exhumation rates occurring between 5.0 and 2.0 Ma despite the prevailing low relief was made possible by the high erodibility of most lithotipes which make up the Liguride Complex.

Despite the current much lower rock uplift rates, the present average altitude of the Apuan peaks (1500 to 1900 m) is very close to that of the Apennine divide. As a matter of fact, the higher rock uplift is here largely compensated by the higher erodibility of the turbiditic sandstones which make up the Chain backbone.

Because of the prevailing low erodibility, the local relief of the Apuan Alps is to such an extent greater than that of the typical Apennine chain, to deserve their odd name whereby a crumble of “Alps” lies well within the Apennines.