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

International audienceIn the Rawil Depression of the south-westernHelvetic Alps, oblique (normal plus dextral strike slip) faultsare common but their relative age, regional role and theprocesses leading to their development are not yet fullydetermined. This field study establishes the orientation anddistribution of these faults and associated veins, the faultgeometries and kinematics, and the relationship betweenveining and faulting. Three post-nappe sets of faults can bedistinguished on the basis of their strike: (1) NNW/NWstriking;(2) WNW/W-striking; and (3) WSW-striking ones.Faults sets (1) and (2) generally dip at moderate angle to theSW and typically develop domino-like structures, with aspacing of around 1 km. Fault set (3) is steeper, the strikeslipcomponent is larger, and it is directly associated with themain regional-scale branch of the Simplon–Rhoˆne Fault.Although these faults are broadly coeval, there are clearexamples of set (2) cross-cutting set (1), and set (3) crosscutting(1) and (2), which establishes, at least locally, a relativechronological succession. This transtensional faultinglargely post-dates folding related to nappe formation becausefold geometry can be matched across the obliquely crosscuttingfaults. Regional dextral-transtensional fault developmentis related to differential exhumation of the External Crystalline Massifs over the last 15–17 Ma, coeval withrelated movement on the Simplon–Rhoˆne Fault. Locally there is a transition from an initial more ductile myloniticfabric to cataclasite, accompanied by brittle-ductile veiningand intense pressure solution. This progressive embrittlementduring faulting is due to exhumation and cooling duringfaulting, higher strain rates, or increased pore-fluid pressures.Faults of sets (1) and (2) developed across the brittleductiletransition and may represent fossil seismogenic zonesin rocks with high pore-fluid pressure, providing exposedexamples of seismic faults in similar rocks currently active atdepth north of the Rhoˆne Valley