Brittle faulting in the Rawil depression: field observations from the Rezli fault zones, Helvetic nappes, Western Switzerland

Abstract

The Helvetic nappes in the Swiss Alps form a classic fold-and-thrust belt related to overall NNW-directed transport. In western Switzerland, the plunge of nappe fold axes and the regional distribution of units define a broad depression, the Rawil depression, between the culminations of Aiguilles Rouge massif to the SW and Aar massif to the NE. A compilation of data from the literature shows that, in addition to thrusts related to nappe stacking, the Rawil depression is cross-cut by four sets of brittle faults: (1) NE-SW striking normal faults, (2) NW-SE striking normal faults and joints, (3) ENE-WSW striking and (4) WNW-ESE striking normal plus dextral obliqueslip faults. Fault set 1 was probably initiated during sedimentation and reactivated during nappe stacking, whereas the other fault sets formed after emplacement of the Helvetic nappes. We studied in detail two well-exposed parallel fault zones from set 4, the Rezli fault zones (RFZ) in the Wildhorn Nappe. They are SW-dipping oblique-slip faults with a total displacement across the two fault zones of *200 m vertically and *680 m horizontally. The fault zones crosscut four different lithologies: limestone, intercalated marl and limestone, marl and sandstone. The internal architecture of the RFZ strongly depends on the lithology in which they developed. In the limestones, they consist of extension veins, stylolites, cataclasites and cemented gouge, in the intercalated marls and limestones of shear zones, brittle fractures and chaotic folds, in the marls of anastomosing shear zones, pressure solution seams and veins and in the sandstones of coarse breccia, brittle faults and extension veins. Sharp, discrete fault planes within the broader fault zones cross-cut all lithologies. Fossil fault zones in the Rezli area can act as a model for studying processes still occurring at deeper levels in this seismically active region.

Fracturing and rock pulverization along an exhumed seismogenic fault zone in dolostones: The Foiana Fault Zone (Southern Alps, Italy)

“…The exploitation of preexisting anisotropies such as bedding surfaces, veins, pressure solution seams and joints is common in fault zones that cut carbonates (e.g., Graham et al, 2003;Agosta and Aydin, 2006;Gasser and Mancktelow, 2010;Hausegger et al, 2010;Cardello, 2013). At the southern termination of the FFZ, individual faults exploit overturned bedding surfaces on the eastern limb of a fault propagation anticline across the full thickness of the exposed fault zone (~300 m, Figs.…”

Veining and post-nappe transtensional faulting in the SW Helvetic Alps (Switzerland)

“…However, although the nappe stack is certainly a firstorder feature characterizing the Helvetic Alps, late faults cross-cutting the nappe stack are also an important feature on the regional scale. Locally, veins and faults can occur with different orientations and characteristics, resulting in rather complex geometries that reflect different kinematics, magnitude of displacement and intersection patterns (Burkhard 1988;Huggenberger and Aebli 1989;Gasser and Mancktelow 2010).…”

Cretaceous syn-sedimentary faulting in the Wildhorn Nappe (SW Switzerland)

“…The metamorphic grade generally increases towards the south. The nappe pile was subsequently affected by dominantly dextral transtensional faults developed or reactivated during the Neogene (Huggenberger and Aebli 1989;Burkhard 1988;Pfiffner 1993;Gasser and Mancktelow 2010;Cardello and Tesei 2013). The study area lies within the Rawil depression (Heim 1920), which preserves the highest nappes between two more deeply exhumed domal culminations, forming the Mont Blanc/Aiguilles Rouges and Aar/Gastern ''External Crystalline Massifs'' (Fig.…”

“…Accumulated displacements range up to ca. 1 km on the major structures, such as the Rezli Fault (Gasser and Mancktelow 2010), the Gletcherhorn Fault, the Iffigensee Fault (Burkhard 1988;Huggenberger and Aebli 1989), and the Mondraleche Fault (Lugeon 1914(Lugeon -1918, but are more typically on the order of a few tens of metres. More stratigraphic details are provided in Cardello (2013) and the references therein.…”

“…The field area is crossed by a large number of faults of different orientation, length, kinematics, geometry and age. As noted by Gasser and Mancktelow (2010), these can be subdivided into four sets: (1) NE-SW-and (2) NW-SE-striking normal faults and joints, and (3) ENE-WSW-and (4) WNW-ESE-striking normal plus oblique-slip normal faults. As we show here, most of the NE-SW-striking normal faults are related to Cretaceous syn-sedimentary faulting.…”