Geo-petrographic map of the southern termination of the Simplon ductile shear zone (Swiss Alps) through quartz c-axis patterns

The study area is located in the Central-Western Swiss Alps in the canton of Valais, 30 km from Domodossola (Italy) and about 30 km southeastward from the Swiss town of Brig. This area has undergone a complex deformation history from brittle to plastic regime as testified by the mylonitic rocks outcropping within a well-exposed shear zone. In the geological literature the Simplon fault is considered as a major extensional structure of the Central-Western Swiss Alps, and its origin is still a matter of open debate. Many authors attribute to it a fundamental role of tectonic release, framing it in a model of lateral extrusion activated as a result of continental collision (Miocene) of the European plate with Adrian microplate, and guided by the geometry of indentation between the respective plate boundaries. This regional scale tectonic lineament, following the estimates of some authors, should have accommodated a total slip of ca. 70 ± 35 km through a ductile shearing but about 10 ± 5 km through brittle activity. The highest frequency of earthquakes in the vicinity of the Rhône-Simplon line between Brig and Martigny, and focal mechanisms suggest dextral transcurrent movements. The high uplift, estimated above 15 mm/year in the region of Brig show us that the fault is still active.The purpose of this study, focusing on the ductile activity of the fault, is to reconstruct via petrographical and structural analyses the tectono-thermal evolution of metamorphic rocks outcropping near the village of Simplon (CH) in the 5-km wide and NW-SE trending shear zone.In the Central-Western Alps previous authors recognized at least five different stages of ductile deformation of which only the first four are found pervasively in the Simplon area generating four different sets of lineation systems. These mesoscale structures are essential evidence which allow us to delineate a correct reconstruction of the tectono-metamorphic history. Indeed, four different types of ductile episodes and related stretching lineations (Ln) has been identified in the study area. The Meso-Alpine continental collision-subduction (D1) towards SE of the European plate below the Adria, produced NW verging nappes developing syn- deformational ductile structures at temperatures above 300°C, which in the granitic crustal rocks under retrograde metamorphic conditions (greenschist-facies) generates a system of SE dipping lineation (L1). The second phase of deformation (D2) transposes the early schistosity together with L1 lineations producing tight isoclinal folds at all scales. The most impressive structure is the ductile "Simplon Shear Zone", which generates a system of lineation (L2) SW oriented, related to the extension in NE-SW direction along the shear zone. The overlapping geometric structures of D1-D2 gives rise to a Type 3 interference pattern (i.e. coaxial) even at large scale. D2 produced a pervasive planar fabric, S2, that represents the dominant regional foliation throughout the Lepontine Dome. The third NW-oriented system of lineation (L3) is related to a SE verging backthrusting along the Canavese line. The "Simplon Shear Zone" during its ductile deformation history has been interpreted as a backfolding (Antiform of Vanzone) toward SE that has given shape to a typical "Z". In the Lepontine region, “backthrusting” begin under amphibolite-facies conditions and continues during retrograde metamorphic conditions with the formation of Verzasca and Salmone anticlines, Maggia syncline and the Wandfluhhorn antiformal syncline. The fourth system of lineation (L4), from EW to NE-SW oriented, generated after the Oligocene backfolding as the result of a dextral transpression is coeval to the movements occurred along the Rhône-Simplon Line. On all scales, these folds are characterized by a poly- to disharmonic geometry and by predominantly steep to sub vertical fold axial planes generating only locally a new axial plane cleavage. The folds associated with the fifth phase of deformation (D5), are typically chevron or kink folds, with wavelengths and amplitudes up to metric scale. In the "Simplon Shear Zone" the ductile deformation followed by brittle movement along the low angle dextral normal fault (dipping 30° SW) of the Rhône-Simplon line (slip of ca. 14 km). The structural-geological field survey was conducted over an area of about 120 km2 using as the base the topographic map of Switzerland (scale 1:25.000) which allowed us to realize the geological map here presented. About 80 oriented samples were collected with the purpose to cover any variations of deformation and to be representative of the entire range of fault-related rocks developed within the shear zone, from cataclasites to mylonites.On some representative oriented samples, chosen according to different lithologies (marbles, micaschists, amphibolites, gneiss, and ophiolites) and increasing amount of strain, 11 thin-sections were prepared, cut orthogonally with respect to the mylonitic foliation and parallel to the stretching lineation. A petrographic and microstructural study aimed to establish the timing of deformation and crystallization episodes was performed. The thermal range associated with the main deformational events experienced by these rocks was estimated on some representative samples by means of quartz c-axis orientation patterns with the aid of a computerized image analysis technique (AVA).The survey of accurate field data with GPS positioning (Global Position System) has enabled the creation, thanks to a GIS software (Geographical Information System), of a database containing petrographic observation as well as all the structural elements which spatial orientation has been measured in the field by a geological compass (stretching lineation, mylonitic foliations, fold axes, and fracture planes) and then allowed us to obtain further derivative thematic structural or geo-petrographical maps.