Project

exhumation; deformation; fluid-rock interaction; Helvetic nappes; European Alps; Helvetic Alps; folding; faulting; extension

Lead
Lay summary
The Helvetic Alps exposed north of the Rhone valley have been the subject of many studies and are now taken as a classic example of a fold-and-thrust belt developed as the result of shortening perpendicular to the Alpine chain. However, there is increasing evidence that over the last ca. 30 million years the Alps have also undergone significant extension parallel to the chain (i.e. SW-NE), during continued convergence perpendicular the chain (i.e. SE-NW). Although the absolute timing, kinematics and regional extent is not yet determined, this extension parallel to the chain and the associated exhumation of rocks from depth appears to have been most active during two periods, concentrated around ca. 32 million years and ca. 19-17 million years. However, related movements have continued until the present time, with a zone of strongly enhanced seismicity broadly localized along the Rhone Valley. The Simplon Fault Zone is the major young tectonic structure controlling exhumation in the Central Alps. This project is the continuation of a study that until now has focused on the Simplon Fault Zone in the region where it is best defined, from the Val d’Ossola over Simplon Pass and into the Rhone Valley. The continuation project will extend these field and laboratory studies to the west along and to the north of the Rhone Valley. The geometry and kinematics of the study region is fully 3-D, involving interacting folding and thrusting related to convergence and shearing, together with coeval and subsequent faulting and vein formation related to extension and exhumation. Radiometric dating of mineral growth can provide the important 4th dimension of absolute time. The overall aim is to develop a 4D model of the progressive transition from folding, thrusting and crustal thickening to extension, faulting and exhumation in the Helvetic Alps within the footwall of the Simplon Fault Zone.

Direct link to Lay Summary

Last update: 21.02.2013

Active participation

Number	Title	Start	Funding scheme

The Simplon Fault Zone is the major Neogene tectonic structure controlling exhumation in the Central Alps. This project is the continuation of a study that until now has focused on the Simplon Fault Zone in the region where it is best defined, from the Val d’Ossola over Simplon Pass and into the Rhone Valley. The proposed continuation project will extend these field and laboratory studies to the west along the Rhone Valley and consider the deformation structures associated with Neogene orogen-parallel extension in the Helvetic nappes and underlying Aar massif (i.e. within the footwall of the Rhone-Simplon Fault Zone). Because of the smaller amount of exhumation, metamorphic conditions in this footwall segment did not exceed lower to middle greenschist facies and much deformation occurs around the brittle-ductile transition, continuing with increased exhumation into fully brittle conditions. Brittle fracture is strongly influenced by pore-fluid pressure and is now marked in the field by fibrous pressure shadows, veins and cataclasites sealed by newly precipitated quartz and calcite. These structures provide a chance to study fluid-rock interaction during deformation and exhumation and allow an assessment of the source of these fluids (e.g. metamorphic fluids from below or meteoric water from above). Fluids in small-scale structures, such as pressure shadows and veins, are likely to be effectively rock-buffered, but an isotopic signature of the infiltrating fluids may still be preserved in major faults and particularly within the immediate footwall of the Rhone-Simplon Fault Zone itself. The geometry and kinematics of the study region is fully 3-D, involving interacting folding and thrusting related to convergence, together with coeval and subsequent faulting and vein formation related to extension and exhumation. Ar-Ar and Rb-Sr dating of synkinematic mineral growth (especially white mica) can provide the important 4th dimension of absolute time, determined for specific points on the relative history, obtained from overprinting relationships in the field. The overall aim is to develop a 4D model of the progressive transition from folding, thrusting and crustal thickening to extension, faulting and exhumation in the Helvetic nappes and external massifs within the footwall of the Rhone-Simplon Fault Zone.