Open Session - SWISS GEOSCIENCE MEETINGs

6
Symposium 1: Structural Geology, Tectonics and Geodynamics
1.3
Brittle tectonics in the Subalpine Molasse and in the Prealpes Klippen
Matzenauer Eva*, Ibele Tobias*, Mosar Jon*
* Département de Géosciences Université de Fribourg, Chemin du Musée 6, CH-1700 Fribourg (eva.matzenauer@unifr.ch)
The study of the tectonic and neotectonic structures of the Subalpine Molasse and Prealpes Klippen of the Canton Fribourg
is the scope of this ongoing PhD research. A more global aim of this study is to gain a better insight into the present and
past stress and strain fields of the transition between the Prealpes Klippen, the Subalpine Molasse and the Molasse Basin.
Observing regional structures by analysing aerial photographs and DEMs of the study area, combined with field measurements,
permits the investigation of different kinds of structures as yet unknown.
The investigation area is structurally subdivided into two major zones:
(1) the Subalpine Molasse - stacked Molasse slices which are inclined due to the Alpine orogeny, and
(2) the Prealpes, mainly the Klippen and the Gurnigel Nappe which are situated at the frontal part of the Prealpes and in
direct contact with the Alpine Foreland. The north-western part of the Prealpes Klippen, is mainly governed by several
large-scale fault-related folds whereas the south-eastern part is dominated by imbricated thrust slices dipping to the N/
NW.
Field measurements in limestone, marls and shales (Prealpes Klippen) and sandstones (Gurnigel and Subalpine Molasse) exposed
a whole suite of brittle tectonic features such as joints and fractures, slickensides, and veins, but also hitherto undescribed
features such as deformation bands. Combined these structures allow paleostress reconstructions. With the analysis
of aerial photographs and DEMs, lineaments were detected which correspond mostly to vertical - subvertical tectonic
faults.
The combination of two observation methods makes it possible to distinguish different fault hierarchies.
First kinematic analysis of the observed faults allowed us to distinguish between different fracture families and their hierarchical
order. The fault distribution and their senses of movement probably governed by a Riedel shear pattern. As shown by
previous investigations, the tectonic regime is dominated by two main fault directions: a right lateral WNW-ESE and a left
lateral NNW-SSE striking system of shear zones. The two fault families are possibly linked to a common “conjugated” fault
system with a regional extent.
1.36
Double subduction dynamics: Insight from petrological-thermomechanical
numerical models
Mishin Yury, Gerya Taras, Burg Jean-Pierre & Connolly James
Department of Earth Sciences, ETH Zurich, Schafmattstr. 30 / HPP, CH-8093 Zurich (yury.mishin@erdw.ethz.ch)
Double subduction is a geodynamic process in which two plates following each other are synchronously subducted. Double
subductions are known for both modern (Izu-Bonin-Marianas and Ryukyu arcs) and ancient (West Himalaya collision zone)
plate tectonics. However, our knowledge about this process is limited to conceptual schemes and some restricted analogue
experiments. In order to fill this gap we performed 2D numerical experiments using a coupled petrological-thermomechanical
approach based on finite differences and marker-in-cell techniques combined with thermodynamic database for the
mantle. We investigated the influence of convergence rate, intermediate plate length, activation volume of the mantle dislocation
creep and age of the lithosphere. Based on these experiments we conclude that: (A) Subduction rates at two zones
running in parallel differ and vary in time even when the total convergence rate remains constant. Supremacy of either
subduction zone depends on physical parameters such as (i) relative rates of the plates, (ii) slab ages and (iii) length of the
middle plate. (B) Subduction dynamics of the double subduction system involves several processes unknown in simple subduction
systems, such as (i) eduction (i.e. “un-subduction”), (ii) subduction re-initiation, (iii) subduction flip triggered by
shallow slab breakoff and (iv) turn-over of detached slabs to up-side-down attitudes. (C) Simulated tomographic structures
related to slab propagation account for both penetration and non-penetration of the 660 km discontinuity. Non-penetration
is favored by (i) low convergence rate, (ii) faster relative movement of the overriding plate, (iii) young age of the subducting
slab and (iv) up-side-down turn-over of detached slab.