Early orogenic turbidite systems as tectonic tracers – examples from circum-Mediterranean orogens

Accurate understanding of the tectonic architecture from early in the evolution of orogenic belts is important for charting how they amplify into modern-day structures. However, this understanding is hampered by this subsequent deformation. But, as collision mountain belts generally initiate underwater, early synorogenic turbidite systems can reveal this architecture as represented not only by the connectivity and relative bathymetry ahead and across orogens but also by detecting deformation ahead of the main orogenic “front”. This presentation shows how sand fairway mapping is a key, but under-used, tool for understanding basin configuration in evolving convergent plate boundaries. It is underpinned by a simple statement: turbidity currents go downhill. Arguably the most useful are turbidite systems developed early in the evolution of orogenic belts where sand, derived external to the orogen, is flushed across substrates largely comprising pre-orogenic limestones and basinal marls and clays. This allows detection of potential contamination by entrained substrate particles and hence justify long-range correlations along fairways that are subsequently deformed during progressive orogenesis. Modern research has shown that turbidity currents, when confined by seafloor bathymetry, can run out for hundreds of km along rather low bathymetric gradients. The application of modern depositional understanding of confined turbidites provides a diagnostic suite of observations and facies associations to test correlations and detect active basin-floor deformation structures. Two examples are discussed from circum-Mediterranean orogens – the Numidian (early Miocene) of the Maghreb-Apennine orogen and the Annot-Champsaur (early Oligocene) of the Western Alps.

The Numidian sand fairway, derived from North Africa, extends for over 300 km in the central Mediterranean into modern-day central Italy. Turbidites deposited along confined sinuous corridors created by active submarine thrusting. Palaeobathymetry across the submarine thrust belt increased northwards into the future Apennines. The sands overlying various substrate facies, from deep-water clays to platform carbonates – indicating the designations of Mesozoic blocks and basins are unreliable guide for subsequent intra-Mediterranean palaeogeography. It is the down-system palaeobathymetry that benchmarks the water-depth back up-system. The composite Apennine–Calabrian–Maghrebian orogen with its submarine thrust belt had occluded deep-water Tethyan connections through the central Mediterranean by early Miocene times.

The Annot system can be mapped northwards from its source (Corsica-Maures-Esterel) around the Alpine arc, along what is generally interpreted to be a foredeep ahead of the orogen.  However, the turbidites are confined by active basin-floor structures, indicating their setting as thrust-top. Both the Ecrins and Argentera basement massifs were over-flowed by Annot turbidites. They are indicative of active crustal shortening partitioned ahead of the main orogen while other tracts of European continental crust were being subducted. This challenges conventional over-simplified descriptions of orogens as “foreland-migrating” and the use of transgressive unconformities in charting this migration.

Although both case-studies are classically-described orogenic “flysch” systems, their deformed segments now caught up in the orogens have, until now, been under-represented. These studies illustrate the utility of turbidite sedimentology, especially reconstructing sand fairways, in building the palaeogeographical reconstructions necessary to characterise the complex, early tectonic regimes of Mediterranean orogens. The results challenge convention.