Intraoceanic subduction of “heterogeneous” oceanic lithosphere in narrow basins: 2D numerical modeling.

We provide 2D thermomechanical numerical models to constrain the subduction process of narrow (~ 600 km wide) oceanic basins resulting in formation and exhumation of serpentinite-bearing high-pressure (HP) and ultrahigh-pressure (UHP) complexes presently exposed as dismembered massifs in orogenic belts. We simulated subduction of “heterogeneous” (i.e. non layered) oceanic lithosphere testing different serpentinite rheologies and varying other major parameters (initial slab dip, convergence rate, age and structure of oceanic lithosphere, distance subduction zone/continental margin, weak zone geometry). We show that serpentinite rheology is the most important parameter affecting both the geometry and behavior of serpentinite channels that form in the mantle-wedge as the result of upward migration of slab fluids. A strong serpentinite power-law rheology is responsible for the continuous accretion and underthrusting of slab slices at the base of mantle-wedge to form planar- or wedge-shape serpentinitic channels; the great part of sediments is buried only during continental crust subduction but is never exhumed; moreover slab slices do not mix with mantle wedge rocks. Exhumation of mafic/ultramafic rocks occurs from pressure not exceeding 15 kbar. On the other hand, a weak Newtonian behavior of serpentine best describes subduction and exhumation dynamics. This rheology produces channels that usually widen to depth; slab and sediment slices are easily incorporated in mantle wedge serpentinite during ongoing convergence; in this case, slab and mantle-wedge serpentinite are mixed together and can be closely associated in the channel. The models running a Newtonian rheology highlight that slices of the oceanic overriding plate can be eroded and dragged in the channel till great depth. A weak power-law serpentinite rheology produces a transitional behavior between that of strong power-law and Newtonian rheology. The serpentinitic mélange, produced using a Newtonian rheology, is finally exhumed from pressure > 20 kbar during the final stages of continental crust subduction and collision. The rise to shallow depth of the HP mélange is driven by concurrent serpentinite buoyancy, slab roll-back and asthenospheric mantle upwelling. Moreover, we propose that the subduction/exhumation dynamics observed in the model acted during closure of the narrow Alpine Tethys ocean; in particular, we suggest that some slices presently outcropping in HP ophiolitic massifs of the Western Alps (Voltri and Monviso massifs) were tectonically coupled at slab/mantle interface and exhumed during collision in a soft, low-viscosity serpentinite channel. We finally investigated intensity and dynamics of magmatic processes during subduction. Our numerical experiments suggest that even subduction of narrow oceanic basins may indeed allow the development of a magmatic arc. In such relatively small short-living arcs, steeper, faster and younger slabs produce earlier onset of the volcanism after the beginning of subduction.