Tectonics of the Préalpes Klippen and the subalpine molasse (canton Fribourg, Switzerland)

This thesis deals with a structural analysis of the préalpine nappe stack with major focus on the frontal part of the Préalpes Romandes and the Subalpine Molasse. The préalpine nappes underwent a complex paleo- and Alpine tectonic deformation history to attain the present-day position to the NW of the Helvetic nappes and to the SE of the Western Molasse basin. In contrast to most of the previous studies, the emphasis lays in the structural and geodynamical aspect, rather than in surface geology and stratigraphy. Diverse structural features of different scales, ranging large-scale fault-related faults, faults, and fault zones to small-scale fractures, secondary faults, veins, stylolites, and brittle shear bands witness an eventful history of the Préalpes Klippen. These tectonic features were carefully analysed and investigated in order to unravel the tectonic history of the Préalpes Romandes - mainly of the Préalpes Médianes nappe - and to attribute the observed structures to a corresponding deformational event pre-, syn-, or post-dating the préalpine nappe emplacement (30Ma - present-day). Extensive field work in the Préalpes Médianes and its adjacent structural units allowed measuring and interpreting characteristic structural features, mostly fracture planes with fault-slip indicators, but also veins, stylolites and brittle shear bands. These observations were complemented with analyses of aerial photographs, digital elevation models, and already existing geological maps to define the extent of the large-scale structures. The observed local fracture pattern is mainly related to the fold-and-thrust development during the nappe emplacement. The majority of the folds are faultpropagation folds verging towards NW. However, some hinterland verging backthrusts were identified associated probably to the eastward prolongation of the Ultrahelvetic lens of the Massif de Montsalvens underneath the Préalpes Médianes. In the Schopfenspitz area an important backthrust, the Maischüpfen thrust exposes a large fault zone with brittle shear bands indicating an unambiguous thrusting movement towards SE. Probably associated to the folding, normal faults develop both parallel and perpendicular to the fold axis leading to a strong segmentation of the fold limbs. However, the reactivation of these fault planes as strike-slip faults attests an on-going deformational evolution post-dating the nappe emplacement. As an example for these common structural elements, a closer look is given at the Dent de Broc and the Tzintre area, where the complexity of the structures due to reactivation is clearly visible. The emplacement of the préalpine nappes onto the Alpine foreland announced a final period of thrusting. Uplift rates, earthquakes, and out-of-sequence thrusts witness an on-going deformation of the Alpine wedge trying to readjust its instable wedge geometry by interplay of erosion and the formation of crustal imbricates. Within the Préalpes Médianes, several thrusts hint out-of sequence thrusts, especially along important paleofaults, cutting through the whole préalpine nappe pile. The Schopfenspitz thrust - interpreted as an out-of-sequence fault - corresponds to the Rianda-Stockhorn paleofault outcropping in the Jaun valley and thrusting the Schopfenspitz mountaintop. Post-emplacement thrusting is also affecting the Gurnigel nappe by a late thrusting of the Préalpes Médianes nappe on top of the initially superimposing nappe. As well as Ultrahelvetic lenses, outcropping within the Gurnigel nappe are indicating a late stage of thrusting. Additionally, an ubiquitous strike-slip facture pattern consisting mostly of two fault directions - a N-S oriented sinistral and a WNW-ESE oriented dextral fracture set - prove another a neotectonic to on-going deformation. Acting together as conjugated fault zones, the préalpine fault system coincides on a larger scale with conjugated fault systems common in the Jura mountains and the Molasse basin. Moreover, fault kinematic analyses and paleostress reconstructions allow a better insight into the evolution of the stress field of the frontal part of the Préalpes Romandes. Fault slip data collected at more than 50 measurement sites, mostly in the Préalpes Médianes, but also in the Gurnigel nappe and the Subalpine Molasse expose a heterogeneous dataset influenced by local tectonic structures, but also by reactivation of inherited structures. Overprinting relationships of fault-striation with opposed slip directions indicate the influence of several stress regimes belonging to different tectonic events. Careful data separation allowed the reconstruction of homogeneous subsystems. Hereby, the results show two different deformation phases. The first one seems to be related to the folding and the thrusting of the préalpine nappe displaying mostly a fold axis parallel extension. While the second one, a strike-slip stress regime, is prevailing throughout the entire investigation area characterised by N-S trending sinistral and WNW-ESE oriented dextral strike-slip faults. Within different structural entities ranging from the Préalpes Médianes towards the Molasse basin, indications for both a strike-slip and a compressional stress regime related to the out-of-sequence thrusting were observed. Even if the origin of these two stress regimes differs, a mutual interaction between them is possible. Maintaining the identical orientation of the compressional stress axis, a permutation of the extensional and intermediate stress axes defines the one or the other stress regime. Furthermore, a 3D modelling approach allowed representing the complex structures of the allochthonous nappe stack of the Préalpes Romandes, as well as the fold-and-thrust structures of the Préalpes Médianes. These models give a better insight into the spatial continuation of the geology at depth than common 2D cross-sections. Additionally, the 3D models help to validate the existing geological maps, cross-sections, and our interpretations, as well as to prove the consistency amongst these different data inputs. Taking into account available data and personal interpretations, the established 3D models do not intend to give a precise reproduction at depth, but rather a suggestion of a possible solution.