Current challenges for explaining (ultra)high-pressure tectonism in the Pennine domain of the Central and Western Alps

The Pennine domain of the Central and Western (PCW) Alps, including the Dora-Maira, Monte Rosa, Gran Paradiso, Adula/Cima Lunga nappes and the Zermatt-Saas zone underwent ultrahigh- or high-pressure [(U)HP >1.5GPa] metamorphism during the Alpine orogeny. We review structural, petrological and geochronological data for the (U)HP units in the PCW Alps (i) to clarify the relationship between (U)HP metamorphism and deformation, (ii) to confront published exhumation models for the (U)HP units with the reviewed data and (iii) to evaluate consequences of different pre-Alpine paleogeographic settings (Penrose-type ocean v. hyperextended margins) on the Alpine orogeny. The review indicates that (i) peak pressures are recorded only in minor volumes of the corresponding tectonic nappes; (ii) (U)HP rocks occur within coherent and imbricate thrust sheets which show substantial pressure jumps; (iii) peak pressures are mostly associated with a top-to-the-foreland kinematics; (iv) decompression from (U)HP (4 to >1.5GPa) to greenschist or amphibolite facies (similar to 1GPa) metamorphic conditions was fast (< c. 2Ma); and (v) accreted fragments of the Alpine-Tethyan basins reflect Mesozoic hyperextended margins rather than mature Penrose-type oceans. The comparison of these characteristics with numerical exhumation models suggests that exhumation of (U)HP rocks by buoyancy-driven return flow within a subduction channel under near-lithostatic pressure is unlikely because (i) models predict volumes of (U)HP rocks that are more than an order of magnitude larger as observed in the PCW Alps and (ii) the required top-to-the-hinterland (U)HP kinematics in the hangingwall of (U)HP units has not been observed until now. Also, the buoyancy-driven exhumation of the individual crustal (U)HP units within weak (10(19)Pas) and denser (>similar to 3180kgm(-3)) rocks is questionable, because there is no evidence of such rocks around the (U)HP units. An alternative model, which could explain the main characteristics of the (U)HP units in the PCW Alps, is an orogenic wedge model that (i) involves dynamic stresses deviating from lithostatic pressure and (ii) is formed during the convergence of hyperextended margins. Deviations of dynamic stresses from the lithostatic pressure and local pressure variations cannot be excluded during the Alpine orogeny, but these deviations and variations have not been clearly identified until now.