THE BACKBONE OF OROGENS: WHY A REALISTIC PALAEOGEOGRAPHY MATTERS

The type of lithosphere reaching subduction zones has profound implications for the evolution of orogenic belts and for mass balance calculations in subduction zones. Numerical and conceptual models of subduction generally distinguish between only two types of lithosphere: (1) oceanic and (2) continental, with the latter consisting of subcontinental mantle and a 20-30 km thick crust, overlain by pre-, syn- and post- rift sediments. However, recent studies of present day rifted margins provided compelling evidence of the existence of a third type of transitional lithosphere between typical ‘oceanic’ and ‘continental’ lithosphere. Such domains, which can be up to 200 km wide, have been labeled Ocean-Continent Transition Zones (OCTZ). They are characterized by the presence of windows of exhumed mantle between slivers of hyper-extended and hydrated continental crust resting upon serpentinized mantle. Pre-rift sediments are present only rarely as extensional allochthons and syn- and post- rift sediments seal the extension-related lithostratigraphy.

Lithological associations compatible with distal continental margins as described above are frequently found in the high-pressure parts of Alpine orogens. However, the pervasive deformation that affected these heterogeneous units during plate convergence leads them to be often discarded as tectonic mélanges, where the juxtaposition of seemingly unrelated rocks is ascribed exclusively to the orogenic event. Our study indicates that several high pressure units of the Western Alps derive from OCTZ’s, which were located along the margins of the Jurassic Tethys. These results have profound implications for understanding the dynamics of orogens and the processes of burial and exhumation of (U)HP rocks.

Here we present two examples from the Zermatt-Saas Zone, which is part of the Eclogitic Piemonte Units, and from the Sesia Zone. The Zermatt-Saas Zone consists of abundant serpentinized mantle, intruded by Jurassic gabbros and locally overlain by slivers of continental basement rocks and by syn- to post- rift sediments. Our study on zircons from Permian plutonic rocks of the Etirol-Levaz continental basement slice shows that a distinctive phase of zircon growth occurred at ca. 170-160 Ma. High U/Th ratios and zoning patterns suggest that zircons grew as a result of melt infiltration related to the intrusion of mafic magmas, also dated at ca. 170-160 Ma, in the underlying serpentinites. Therefore, the continental basement slices and the oceanic basement rocks were already juxtaposed in the Jurassic and they were probably part of an Ocean-Continent Transition Zone (OCTZ). Alpine tectonics resulted only in minor reworking of the Jurassic contacts, generally preserving the original geometry.

Differently from the Eclogitic Piemonte Units, the Sesia Zone (SZ), which underwent Alpine metamorphism in blueschist to eclogite facies conditions in the Cretaceous, consists largely of Palaeozoic continental basement and Jurassic/Cretaceous metasediments, while Jurassic serpentinites are rare. Evidence of pre-Alpine cataclasis along the serpentinite-continental basement contact is locally preserved. Palaeozoic basement and exhumed serpentinites are locally overlain by Mesozoic sediments, consisting of syn-rift meta-arkose, and post-rift Jurassic metachert and calcschist. Therefore, the different units of the Sesia Zone preserve a relationship between continental basement, exhumed peridotites and sedimentary cover typical of the distal part of extended continental margins and of OCTZ’s.

The partial preservation on a regional scale of the rift-related relationships between rock units that underwent subduction to (U)HP conditions indicates that (1) the association of serpentinites and continental basement does not necessarily derive from chaotic counter-flow in a subduction channel, but may also be an inherited feature from the rifting history and (2) the process of tectonic burial and exhumation is not necessarily chaotic but large coherent blocks can behave relatively rigidly, while well-defined movement zones accommodate most of the deformation.

Furthermore, this study indicates that large gneissic terranes found in metamorphic belts may sample extensional allochthons derived from hyper-extended continental crust. Therefore, in those circumstances, gneissic terranes do not represent small slivers scraped off larger microplates of continental lithosphere that have gone into subduction. These results have important implications for the geochemical and geodynamic evolution of convergent plate margins, implying that large masses of continental crust are not necessarily recycled in the mantle during orogenesis.