Abstract
In the Rawil Depression of the south-western Helvetic Alps, oblique (normal plus dextral strike slip) faults are common but their relative age, regional role and the processes leading to their development are not yet fully determined. This field study establishes the orientation and distribution of these faults and associated veins, the fault geometries and kinematics, and the relationship between veining and faulting. Three post-nappe sets of faults can be distinguished on the basis of their strike: (1) NNW/NW-striking; (2) WNW/W-striking; and (3) WSW-striking ones. Faults sets (1) and (2) generally dip at moderate angle to the SW and typically develop domino-like structures, with a spacing of around 1 km. Fault set (3) is steeper, the strike-slip component is larger, and it is directly associated with the main regional-scale branch of the Simplon–Rhone Fault. Although these faults are broadly coeval, there are clear examples of set (2) cross-cutting set (1), and set (3) cross-cutting (1) and (2), which establishes, at least locally, a relative chronological succession. This transtensional faulting largely post-dates folding related to nappe formation because fold geometry can be matched across the obliquely cross-cutting faults. Regional dextral-transtensional fault development is related to differential exhumation of the External Crystalline Massifs over the last 15–17 Ma, coeval with related movement on the Simplon–Rhone Fault. Locally there is a transition from an initial more ductile mylonitic fabric to cataclasite, accompanied by brittle-ductile veining and intense pressure solution. This progressive embrittlement during faulting is due to exhumation and cooling during faulting, higher strain rates, or increased pore-fluid pressures. Faults of sets (1) and (2) developed across the brittle-ductile transition and may represent fossil seismogenic zones in rocks with high pore-fluid pressure, providing exposed examples of seismic faults in similar rocks currently active at depth north of the Rhone Valley.
Highlights
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; Ustaszewski and Pfiffner 2008; Gasser and Mancktelow 2010)
Total fault displacement was calculated along several geological cross-sections, the most important ones being shown in profiles AB and CD profiles of Fig. 2, which is modified after Cardello and Mancktelow (2014)
In the ‘high alpine region’, two main folding events are discernible whereas in the ‘southern region’ up to four deformational phases related to folding can be recognized (D1–4)
Summary
The Helvetic Alps are one of the most studied fold-and-thrust belts in the world and are well known as the classic example where far-travelled ‘‘nappes’’ were initially recognized (Bertrand 1884; Schardt 1893; Lugeon 1902; Argand 1916; Heim 1920; Trumpy 1980; Masson 1980a, b; Ramsay et al 1981, 1983; Dietrich 1989; Escher et al 1993; Pfiffner 1993, 2011). 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; Ustaszewski and Pfiffner 2008; Gasser and Mancktelow 2010).
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