ISTerre : Institut des Sciences de la Terre

PhD subject : Structure and evolution of crustal duplex and nappe stacking

Supervisors : Jean BRAUN, Peter VAN DER BEEK

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Summary

The objectives of this PhD are to explore the structure and the dynamics of crustal scale duplexes and nappe stack in a compressive area. This PhD will be a multidisciplinary study on these objects and we plan to investigate these structures through fieldwork, 3D numerical modeling and thermochronologic dating.

During this PhD, I will try to clarify some aspect of continental collision dynamics amongst them :

Fig. 1 – East West variations in the detachement dip and ramp position on the MBT for the Himalayan belt. Figure modified from X. Robert 2008.

 The size, the orientation and the rheology of the heterogeneity which leads to crustal-scale duplex formation. This part will mainly deal with the mechanical and rheological aspect of the problem as well as with the reactivation of pre-existing structures. Some previous work have been done in 2D at the crustal scale for the French Pyrenees by Beaumont et al. 2000 and by Stockmal et al. 2007 for thin-skined structures. The 2D numerical modeling is performed by using Sopale Nested, an arbitrary Eulerian-Lagrangian numerical model (Fullsack 1993). Some preliminary results are presented on figure 3.

 This PhD will also investigate the impact of the regional shortening, in interaction with surface processes, on the crustal ramp geometry. This part will deal with several concepts such as underthrusting versus otherthrusting, underplating, out of sequence thrusting, control and localization of the deformation by surface processes, intern deformation of the duplex This part will be constrain by thermochonological and structural data (see Robert et al. 2009and Herman et al. 2010 for example).

 I will also try to quantify three dimensional impact of a lateral change of the ramp dip (Figure 1) on the tectonic and exhumation style (figure 2).

This work will be applied to the himalayan mountain belt. Indeed, X. Robert 2008 showed that the Main Boundary Thrust (MBT), which is the main detachement for the himalayan mountain belt, presents some important East West dipping variation as well as changes in the ramp position (figure 1). These variations are linked with huge differences in the overlaying thrust stackgeometry (cf figure 2 and F. Jouanne et al 2004 for example). During this PhD, we will try to link numerical models and fieldwork datas in order to identify the causes of theses changes in detachement geometry and tectonic style (rheology, erosion, temporal evolution of independent segments...).

Fig. 2 – E-W variation in the thrusting geometry along the Himalayan mountain belt. These changes can be correlate with the variations in the detachement geometry showed by X. Robert 2008. Figure modified from F. Jouanne et al 2004.

Fig. 3 – Preliminary results on the impact of the rheologie on the thrusting geometry. Upper : Materials (Blue : continental crust, Green : Tibet, Red : mantel lithosphere, Yellow : detachment layer), Lower : total strain. In the figure 3(a), the Tibet is stronger than the colliding Indian plate and we can notice the development of 5 major crustal scale thrust after 10 Myr of shortening. On the figure 3(b), the Tibet is weaker than the colliding indian plate. In this case, only 3 major thrust develop during the same time, but the underplating is much more important and lead to the formation of an synclinal shape klippe.