Do the Ligurian Alps know that the Mediterranean dried out 6 Ma?

The Messinian Salinity Crisis (MSC) occurred between 5.96 and 5.33 Ma, and is believed to be the result of the tectonic isolation of the Mediterranean Sea from the Atlantic Ocean. It led to a drastic drop in base level and desiccation of the Mediterranean basin (e.g. Hsü et al., 1973). Consequently, rivers flowing into the Mediterranean experienced high erosion rates with knickpoints migrating upstream, incising the landscape and leaving behind deep canyons, some of which still clearly visible offshore in the Ligurian Sea. The position of the Messinian knickpoints, and whether by the end of MSC they had retreated enough to escape the subsequent flooding, is unknown. The NE-SW trending Ligurian Alps of NW Italy are onshore of the deep canyons, and some of their valleys have been suggested to be related to the MSC (Foeken et al., 2006). This paper uses apatite (U/Th)/He (AHe) thermochronology to constrain the denudation rates and thermal histories of 4 Ligurian catchments. AHe ages (24.4 ± 2 to 7.4 ± 0.4 Ma; ±1σ), indicate that a Late Neogene erosional event is present in the largest catchments that directly face the offshore canyons. Modelling these data with previously published apatite fission track data (Barbieri et al., 2003) using HeFTy (Ketcham, 2009) demonstrates that the youngest ages are consistent with increased denudation during the MSC. For a geothermal gradient of 30°C/km, this requires a maximum MSC exhumation of ~2 km at ~3.3 mm/yr. AHe ages of ~10 Ma require erosion of ~800 m during the MSC, at maximum rates of 1.3 mm/yr. AHe ages older than 20 Ma are found along the only river that flows north into the Po valley. They do not require any significant MSC erosion. These data suggest that the amount of erosion and therefore river incision is strongly correlated to the dimension of the fluvial system. As bedrock erosion is a function of stream power, a larger river of higher discharge signifies greater erosive power (Howard & Kerby, 1983). This has led to deeper incision and faster retreat of Messinian knickpoints in the Ligurian Alps. Areas where rivers were short and discharge low, or where the shoreline had retreated by hundreds of km, as in the case of the Bormida, the knickpoints were probably flooded when the Mediterranean Sea re-filled at 5.3 Ma and therefore the present landscape does not record the MSC. Barbieri, C., Carrapa, B., Di Giulio, A., Wijbrans, J., Murrell, G. 2003, Provenance of Oligocene synorogenic sediments of the Ligurian Alps (NW Italy): inferences on belt age and cooling history: Int. Journ. Earth Sci. (Geol Rundsch), v. 92, p. 758-778. Foeken, J.P.T.; Bertotti, G.V.; Dunai, T.J.; The morphology of a Messinian valley and its hinterland (Ventimiglia, NW Italy): Implications for the tectonic evolution of the Ligurian Alps; Geological Journal 41: 465-480 (2006) Howard, A. & Kerby, G. 1983, Channel changes in badlands. Geological Society of America Bulletin, v. 94: p. 739-752. Hsü, K., Cita, M., Ryan, W. 1973, Late Miocene desiccation of the Mediterranean: Nature, v. 242, p. 240-244. Ketcham, R. 2009, Apatite to Zircon, Inc. version 1.6.7, build 1.6.7.43.