Open Session - SWISS GEOSCIENCE MEETINGs

30
Symposium 1: Structural Geology, Tectonics and Geodynamics
Oxygen stable isotope investigations in progress for mineral pairs in structural and chemical equilibrium (i.e. quartz-muscovite,
quartz-hematite, calcite-muscovite, quartz-calcite) in late veins will give precise indications on their formation temperature,
while oxygen and carbon stable isotope analyses on carbonates will provide additional information on the origin of
circulating metamorphic fluids.
REFERENCES
Campani, M., Mancktelow, N.S., Seward, D. & Rolland, Y.: Thermochronological evidence for a continuous transition from
ductile to brittle deformation in the Simplon low-angle normal fault, Central Alps. 2008 EGU General Assembly, abstract
EGU2008-A-02179
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Brittle tectonics in the Swiss Molasse Basin
Ibele Tobias*, Matzenauer Eva*, Mosar Jon*
*Départment de Géosciences Université de Fribourg, Chemin du Musée 6, CH-1700 Fribourg (tobias.ibele@unifr.ch)
In an ongoing study we examine the tectonic and neotectonic structures of the Molasse Basin of the Canton Fribourg and
adjacent areas in western Switzerland. Extensive field work allows us to determine the type and spread of structures, as well
as the style and grade of deformation observed in the outcrops. The preliminary results of this work and the scope of our
future research are presented in the poster.
Due to late Tertiary alpine tectonics the region investigated is structurally subdivided into several zones: 1) The subalpine
Molasse, resembling a narrow bend of inclined and imbricated rocks at the former front of the alpine fold and thrust belt,
2) the Plateau Molasse being affected only by very flat ample folds and extending throughout most of the study area.
Within the Sandstones, mudstones and scarce conglomerates of the Lower Freshwater and Upper Marine Molasse (USM and
OMM) of the study area we observed the following brittle structures: Joints and fractures, slickensides, deformation bands
in sandstones and pitted/fractured pebbles in conglomerates. The brittle deformation is regional extended but the structures
are not uniformly distributed. Areas of undisturbed rock are present, separated by as strongly fractured areas with well developed
fault zones containing cataclasites and fault gauges.
The brittle structures observed in the field are steeply inclined and show a Riedel-type pattern. The tectonic regime is of
nearly pure strike slip nature, expressed in WNW-ESE striking right lateral and NNW-SSE striking left lateral shear zones that
are governed by an overall NW-SE orientated sub-horizontal maximum compressive stress. More detailed investigation of the
deformation bands and deformation band shear zones in the sandstone will probably allow us to determine successive stages
of deformation. Their intersection relationships seam to show cyclic stress rotations between strain increments.
The timing of deformation is not yet well determined by our field observations but since some published data of Quaternary
deformation and focal mechanisms of recent earthquakes show the same kinematics as inferred from the field data it may
be speculated that the deformation is recent and possibly still ongoing.