HP-UHP exhumation during slow continental subduction: Self-consistent thermodynamically and thermomechanically coupled model with application to the Western Alps
Introduction
Most subduction-related HP-UHP metamorphic continental rocks (Liou et al., 2004; e.g., Alps, Dabie-Sulu, Himalaya, Norway) occur in zones of intracontinental collision. Although the transition from oceanic to continental subduction seems to be a continuous phenomenon (e.g., Lallemand et al., 2005), introduction of lighter continental material in the subduction zone is the source of strong buoyancy contrasts largely responsible for their exhumation, as shown by several analogue and numerical modeling studies (e.g., Burov et al., 2001, Burov and Yamato, 2007, Chemenda et al., 1995).
Interesting points about this type of metamorphism include the fact that: (1) exhumed continental material appears to be buried beyond lithospheric depths (> 100–150 km; e.g., Liou et al., 2004, Chopin, 1984, Green, 2005). (2) Continental exhumation processes are short-lived, lasting ~ 10 Myr (e.g., Guillot et al., 2005, Hacker, 2007) and burial/exhumation processes for continental material are thus transient. (3) Such exhumation generally involves only a small quantity of material, as suggested by the small surface exposures of most UHP terrains (e.g. < 2000 km2 in the internal crystalline massifs of the Western Alps, Fig. 1). These areas are almost invariably exhumed as lenses of highly metamorphosed material included in less metamorphosed terrains (e.g., Guillot et al., 2005, Avigad et al., 2003, Jolivet et al., 2005), except for the Western Gneiss in Norway (Root et al., 2005) and the Dabie-Hong'an block (Hacker et al., 2000). (4) Exhumation rates for the continental material are higher than for metamorphosed oceanic crust and sediments (Agard et al., in press, Duchêne et al., 1997), at least during the first stages of exhumation. For slow convergence zones, these rates often largely exceed not only the denudation rates but also the rock uplift rates inferred from the convergence rates and common kinematical models of accretion and subduction. This definitely implies some additional mechanisms for this exhumation stage. For example, for the Dora Maira unit (Western Alps), exhumation rates reached 34 mm yr− 1 (i.e., several times the convergence rate, Rubatto and Hermann, 2001) before later decreasing to 16 mm yr− 1 and then to 5 mm yr− 1. A similar evolution of exhumation rates was found for Norway (~ 10–11 mm yr− 1, Carswell et al., 2003, Terry et al., 2000), Himalaya (30 to 80 mm yr− 1, Hermann et al., 2001, O'Brien et al., 2001, Parrish et al., 2006), Betic Cordillera (22.5 mm yr− 1, De Jong, 2003) and Papua-New Guinea (> 17 mm yr− 1, Baldwin et al., 2004). In all cases, the later rates decrease to values lower than 10 mm yr− 1 (e.g., Terry et al., 2000, De Jong, 2003).
For these reasons, the first aim of this study is to build an unconstrained numerical thermomechanical (self-consistent) model explaining the most enigmatic first-order features of continental exhumation processes, with a particular focus on slow convergence zones. Indeed, these zones are expected to exhibit more complex behaviour and more deviations from the conceptual kinematic models of subduction since in the case of slow convergence, the system Peclet numbers (Pe, ratio of advection to diffusion time scale) may fall below 10 suggesting that continental subduction needed for burial of HP rocks is only marginally possible and may be associated with gravitational instabilities or should be highly short-lived (Toussaint et al., 2004).
The Western Alps were chosen because of the wealth of available data that can be used to constrain our models. The new experimental approach is needed to circumvent typical drawbacks of previous models of continental exhumation. For example, the analogue models (e.g., Chemenda et al., 1995) predict the exhumation of crustal-scale rock volumes, which is not generally observed in natural settings. These models are not thermally coupled and thus do not allow to account for important thermally controlled properties, such as slab strength and buoyancy, or for comparisons with P–T data. Numerical models reproducing some of the major characteristics of the Alpine belt structure were published by Pfiffner et al. (2000), but these models cannot be really used to test the mechanical viability of the models or for the comparison with P–T data because their results are preconditioned by a fixed internal “boundary” condition (subduction point, or “S-point”) imposed inside the model area and maintained during the experiments. The models of Burov et al. (2001), Toussaint et al. (2004) and Burov and Yamato (2007) are free of such constraints but were not actually designed to compute synthetic thermodynamically consistent P–T–t (time) paths for continental domains, which makes it difficult to test their results against the petrologic data. Finally, Stöckhert and Gerya (2005) designed a thermodynamically coupled, yet still kinematically constrained, thermomechanical model and obtained P–T–t paths whose shapes are in a good agreement with the Alpine settings. However, in contrast with the available geological observations (Fig. 1), the exhumation of HP-UHP material implies only the overriding continental plate in their experiment.
We herein propose a new thermomechanical model that implies realistic visco-elasto-plastic rheologies, thermodynamically consistent progressive density changes, erosion-sedimentation processes and does not require any pre-imposed internal boundary conditions. The results of our best fitting experiments are then used to discuss the mechanisms of burial/exhumation.
Section snippets
Observational constraints on the continental subduction in the Western Alps
The large structural and petrological data sets available for the Western Alps were used to constrain our model. In the Western Alps, HP units also include oceanic material (e.g., Monviso, Zermatt-Saas). Exhumation of these mafic units was addressed in recent detailed modeling study by Yamato et al. (2007), and therefore we focus here on continental material only. Fig. 1 summarizes the main points concerning the existing data for the internal crystalline (ICM) Dora Maira (UHP) and the Gran
Numerical approach
We used the visco-elasto-plastic thermomechanical numerical code PARA(O)VOZ. This code incorporates the same mechanical solution kernel as the well-known F.L.A.C. (Fast Lagrangian Analysis of Continua) algorithm (Cundall, 1989) and particle-in-cell technique for particle tracing and interpolation of variables during dynamic remeshing. This code solves simultaneously the Newtonian motion equations and heat transfer equations in large-strain Lagrangian formulation:
Model setup
The model setup is based on the existing knowledge for the Western Alps (geometry, age of the lithosphere, convergence rates) and/or the results of earlier parametric study (erosion, rheology of the crust, convergence rates). Initial and boundaries conditions used in our experimental framework are described below and in Table 1.
Constraints from the parametric study
A preliminary parametric study was first carried out to test the model sensitivities to the implied convergence and erosion rates and rheological composition of the crust (Table 2). This study (see below) was necessary to justify our choice of the material parameters used in the reference experiment. For comparison, the general evolution of the reference experiment is presented on Fig. 3 and will be discussed later (Section 6.1).
Large-scale evolution of the model
Fig. 3 shows the morphologies predicted by the reference model during the first 20 Myr of the reference experiment. During the first 5 Myr, the continental lithosphere is buried down to UHP depths and exhumation begins. At the same time, the subducted oceanic slab is progressively detached from the continental slab. 15 Myr are then needed for the deepest continental material (~ 100 km) to come back to the surface at the rear of the accretionary wedge. This exhumation is fast at the first stage:
Discussion and conclusion
Our model satisfactorily reproduces the overall geodynamics of the Western Alps (i.e., morphology (Fig. 3), topography (Fig. 5b), pressure peak (Fig. 7b), P/T gradient (dP/dT, Fig. 7c), exhumation rates (Fig. 7d), timing of the processes (Fig. 7c and d)), and provides important constraints on the mechanisms controlling exhumation in a slow continental subduction context:
- (1)
The exhumation of the continental material from the subducting plate (Chopin, 2003) occurs at the rear of the accretionary
Acknowledgements
Constructive reviews by C.J. Warren and T.V. Gerya are greatly appreciated. We also thank B. Goffé for fruitful discussions and G.W. Ernst for helping us to improve the earlier draft of the manuscript.
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