The Rawil Depression: its structural history from Cretaceous to Neogene

In the Rawil Depression between the Aar and Mont Blanc massifs, tectonic events ranging in age from at least the Cretaceous until recent times have produced a complex puzzle of folds and faults on the published maps. This thesis is a field based study that attempts to decipher this puzzle using a multidisciplinary approach, considering thermochronometry, anisotropy of magnetic susceptibility (AMS), paleomagnetism, and clumped- and stable-isotope data in addition to structural geological techniques including field mapping, stress inversion and microstructural analysis. The overall aim of this work is to establish the succession of events that produced and exhumed the Rawil Depression, which on its southern side is still one of the most seismically active sectors of the Alps. The geological history considered ranges from Cretaceous syn-sedimentary faulting, to nappe-stacking during the late Oligocene and early Miocene, to later up-doming and related exhumation in the late Miocene to recent. New geological maps and profiles of the Rawil Depression were constructed based on existing maps and extensive fieldwork. The characterization of different vein and fault sets allowed the progressive stress and stretching history of the area to be established in detail. The NE-striking fault set dips mainly to the SE and is the oldest. These faults developed as syn-sedimentary structures active at different stages during the Cretaceous and are marked in many places by karstification and silicification of the surface, by sedimentary dykes and by the onlap of younger basinal formations. These Cretaceous faults are no longer discernible toward the basal thrust of the Wildhorn Nappe, where their possible reactivation could be responsible for the lack of any real inverted limb. However, on the upper limb they are well preserved and not markedly reactivated. Instead, they acted as buttress, promoting shorter wavelength folding in the adjacent basinal sediments. They were also not significantly reactivated during later Neogene transtensional faulting. In a profile perpendicular to the chain, the Alpine orogeny was first characterized by in-sequence thrusting and nappe-stacking of the Ultrahelvetics and Wildhorn Nappe, followed by out-of-sequence overthrusting of the Penninic Nappes, and finally by broad antiformal doming of the whole nappe sequence. However, in reality the deformation history is much more three-dimensional. After initial NW-directed stretching related to thrusting and nappe formation, the stretching direction rotates towards the WSW to SW, generally in a counter-clockwise sense. This orogen parallel stretching is developed throughout the Rawil Depression, overprints the whole nappe-stack at conditions probably close to the metamorphic peak, and is the signal generally recorded in the magnetic fabric. Based on the new thermochronometric results up-doming occurred after ca 17-15 Ma and contributed to the cooling and large scale refolding of the Helvetic units. NW- to ca. W-striking oblique normal faults crosscut nappe boundaries and constitute a dextral wrench zone that displaces the already formed axial depression. Oblique normal faults record a progressive development from (1) fault initiation on en-échelon vein sets in brittle-ductile shear zones, to (2) mylonitization of both veins and country rock, localized on these pre-existing initial fault planes, to (3) mature faults accumulating significant displacement on cataclastic fault cores and more discrete surfaces. In places, fault rocks are transitional between mylonites and cataclasites are characterized by the coexistence and/or alternation of ductile and brittle processes. The range of clumped-isotope temperatures indicates that transtensive shearing and faulting began at temperatures in the range of ~180-150°C, whereas embrittlement took place at progressively colder temperatures and shallower depths. A calculated exhumation curve based on new zircon (U-Th)/He ages and published apatite and zircon fission track ages that were used to convert clumped-isotope temperatures to corresponding estimates of time and depth, thereby proving constraints on the development of different veins and fault sets during progressive exhumation. Deformational events can be tracked in both the magnetic fabric and in the paleomagnetic directions, providing useful information about the kinematics of deformation and the thermal conditions reached during metamorphism. In a later stage, stretching direction rotates, also generally in a counter-clockwise direction, from orogen-parallel to nearly orogen-perpendicular. This SE-directed extension perpendicular to the fold axes may be associated with the Rhône Fault after ca. 5.5 Ma, when the exhumation rate increased from about 0.2 to ca. 0.8 km/Ma. During the final stage of exhumation, reactivation of the Rezli Fault Zone occurred in the ‘Alpine region’ and also at the border with the southern region in the back-limb at the Col de Puchet Fault. This event is accompanied by more open circulation of meteoric fluids together with cataclasis localized on the main slip surfaces of a few faults.