DEEP CRUSTAL RECYCLING IN OROGENIC WEDGES FACILITATED BY MANTLE HYDRATION: NUMERICAL MODELING VS. NATURAL DATA

Studies on high-pressure (HP) and ultrahigh-pressure (UHP) rocks exposed in orogenic belts linked to collisional margin show that nappes of oceanic and continental deeply subducted crust can be exhumed to shallow structural levels. In particular, during ocean-continent-type subduction, the crustal material dragged into subduction channel is composed chiefly by oceanic crust and trench sediments, continental slices belonging to subducting plate microcontinent (Ring and Layer, 2003) or linked to early continental collision (Chemenda et al., 1995) or crustal slices tectonically eroded from the overriding plate (ablative subduction) (Tao and O’Connell, 1992, Marotta & Spalla, 2007). Several models have been developed, during last 20 years, to analyse exhumation of subducted crustal material. They can be resumed on five main mechanisms: a) crust-mantle delamination (Chemenda et al., 1995), b) slab break-off Ernst et al., 1997), c) slab retreat (Ring and Layer, 2003) and roll-back slab (Brun and Faccenna, 2008) and d) decoupling of two main ductile layers (Yamato et al., 2008), in which the exhumation is mainly driven by negative buoyancy and/or faulting and e) subduction-channel flow (Stöckhert and Gerya, 2005) in which the exhumation is driven by the upwelling flow developed a low- viscosity mantle wedge. Only channel flow takes into account recyrculation of crustal slices dragged to high depth by ablation within pre-collisional subduction zones.

The effects of the hydration mechanism on continental crust recycling are analyzed through a 2-D finite element thermomechanical model. Oceanic slab dehydration and consequent mantle wedge hydration are implemented using a dynamic method. Hydration is accomplished by lawsonite and serpentine breakdown; topography is treated as a free surface. Subduction rates of 1, 3, 5, 7.5, and 10 cm/y; slab angles of 30°, 45°, and 60°; and a mantle rheology represented by dry dunite and dry olivine flow laws have been taken into account during successive numerical experiments. Model predictions pointed out that a direct relation- ship exists between mantle rheology and the amount of recycled crustal material: the larger the viscosity contrast between hydrated and dry mantle, the larger the percentage of recycled material into the mantle wedge. Slab dip variation has a moderate impact on the recycling. Metamorphic evolution of recycled material is influenced by subduction style. T_Pmax, generally representative of eclogite facies conditions, is sensitive to changes in slab dip. A direct relationship between subduction rate and exhumation rate results for different slab dips; this relationship does not depend on the used mantle flow law. Thermal regimes predicted by different numerical models are compared to PT paths followed by continental crustal slices involved in ancient and recent subduction zones, making ablative subduction a suitable precollisional mechanism for burial and exhumation of continental crust.

Natural data and model predictions are in good agreement: the thermal states simulated for ablative subduction with a hydrated mantle wedge justify the natural PT estimates obtained on continental crust units involved in ocean-continent subduction systems. Similarly, the exhumation rates obtained from analysis of PTt paths are more compatible with natural ones than those obtained from the upwelling flow vector, which could justify only a transient exhumation stage. In general, numerical simulations and natural data show exhumation rates lower then subduction rates.

The good agreement obtained between our model and models developed using different numerical and starting approaches (e.g., Gerya and Stöckhert, 2005; Stöckhert and Gerya, 2005; Faccenda et al., 2008, 2009), except for the exhumation rates, indicates the robustness of the corner-like flow mechanism on precollisional exhumation of the continental crust. On the basis of these results, we propose ablative subduction of the overriding continental plate with corner-like flow as a good alternative, precollisional mechanism for the subduction and exhumation of continental totally decoupling. Thus, ablation decreases and a one-side subduction, characterized by an accrectionary margin, develops (Clift and Vannucchi, 2004).