Kinematics and dynamics of tectonic nappes: 2-D numerical modelling and implications for high and ultra-high pressure tectonism in the Western Alps Stefan M. Schmalholz ⁎, Thibault Duretz, Filippo L. Schenker, Yuri Y. Podladchikov Institute of Earth Sciences, University of Lausanne, Switzerland abstract article info Article history: Received 17 October 2013 Received in revised form 31 March 2014 Accepted 12 May 2014 Available online 17 May 2014 Keywords: Tectonic overpressure Viscous heating Shear zones Inclusions Numerical modelling Western Alps We present two-dimensional numerical simulations of lithospheric shortening with a crust containing weak and strong inclusions. Thermo-mechanical coupling is included, and a crustal-scale shear zone develops self- consistently due to viscous heating and thermal softening of temperature dependent viscosities. Several tests for crustal conditions are performed showing that 1) the thickness of and strain rates within the shear zone are independent on the numerical resolution and applied numerical method (finite element and finite difference method), 2) the shear zone is stable and rotates during large strain deformation, 3) the numerical algorithm con- serves total thermal and mechanical energies, and 4) the bulk horizontal force balance is fulfilled during large strain deformation. A fold nappe develops around the shear zone in the lithospheric shortening simulation. In this simulation the stresses in the crust are limited by a friction angle of 30°. Significant tectonic overpressure (P O ) occurs in strong lower crustal rocks and in strong inclusions. Significant P O also occurs in a weak inclusion that is only partly surrounded by strong crustal rock suggesting that a continuous strong “vessel” is not required to generate significant P O in weak rocks. Maximal values of P O are ~ 2.2 GPa with corresponding deviatoric stresses ~1.5 GPa and occur in a depth of ~42 km. Maximal pressure of ~ 3.4 GPa and maximal temperatures N 700 °C occur during the formation of the fold nappe in crustal depth. Synthetic pressure–temperature paths exhibit entire cycles of pressure and temperature increase and decrease, and suggest that crustal rocks in depths b 50 km can reach the ultrahigh pressure metamorphic facies fields. Applications to tectonic nappes with high and ultra-high pressure rocks in the Western Alps are discussed, and a dynamic model for the evolution of fold nappes in the Western Alps is proposed. © 2014 Elsevier B.V. All rights reserved. 1. Introduction 1.1. Conceptual models for tectonic nappes The Western Alps are a mountain range made of tectonic nappes, and have long been a testing ground for revolutionary ideas in tectonics, such as the nappe theory (e.g. Argand, 1916; Escher et al., 1993; Handy et al., 2010; Trümpy, 1991). There is a general agreement that the overall tectonic structure of the Western Alps represents an imbricate nappe stack, and that the emplacement of the nappes happened in an ordered succession such that the stacked order from top to bottom reflects the palaeogeographic position from internal to external, respec- tively (e.g. Argand, 1916; Escher and Beaumont, 1997; Handy et al., 2010; Schmid et al., 1996). However, the discovery of high pressure and ultra-high pressure ((U)HP) metamorphism in the Western Alps (e.g. Chopin, 1984) invoked mantle depth tectonism that opened an extensive debate on the dynamics of tectonic nappes. Based on the nappe forming mechanism, the existing tectonic models of convergent orogens can be summarised by a combination of two end-members: 1) thrust and 2) intrusion models (Fig. 1). 1) Thrust models usually ap- proximate the orogen as wedge-shaped continua with a rigid buttress behind and a subducting lithospheric slab beneath (e.g. Brandon, 2004; Konstantinovskaia and Malavieille, 2005; Platt, 1986). In the thrust model the dominant process of nappe formation is accretion of tectonic units from the subducting plate into the above orogenic wedge by thrusting (brittle and/or ductile). This thrusting generates a dominant top-to-the-foreland shear sense in the nappes. In the thrust model the rocks building the orogen remain within crustal depth (say b ~ 60 km; e.g. Platt, 1986). Exhumed rocks did not subduct significantly into the mantle, and uplift and exhumation of HP rocks occurs by under- plating accompanied by isostatic uplift, extension in higher levels of the wedge and erosion (e.g. Platt, 1986). Thrust models can successfully ex- plain the coherent and imbricate nappe stacking and the first-order structural observations in the Western Alps (e.g. Platt, 1986). However, in the last decades the magnitudes of pressure estimated from observed minerals in exhumed rock in the Western Alps has increased steadily (e.g. Agard et al., 2009). It is usually assumed that the pressure estimates Tectonophysics 631 (2014) 160–175 ⁎ Corresponding author. Tel.: +41 21 692 4302; fax: +41 21 692 4305. E-mail address: stefan.schmalholz@unil.ch (S.M. Schmalholz). http://dx.doi.org/10.1016/j.tecto.2014.05.018 0040-1951/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto