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

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–Rhône 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–Rhône 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 Rhône Valley.