Open Session - SWISS GEOSCIENCE MEETINGs

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effect on the core formation time, Earth Planets and Space, 59, 1035-1045.
Schmeling, H. et al. 2008: A benchmark comparison of spontaneous subduction models - Towards a free surface, Physics of
the Earth and Planetary Interiors, doi:10.1016/j.pepi.2008.06.028, in press.
Stevenson, D.J. 1981: Models of the Earth’s core, Science, 214, 611–619.
Ziethe, R. & Spohn, T. 2007: Two-dimensional stokes flow around a heated cylinder: A possible application for diapirs in the
mantle, Journal of Geophysical Research, 112, B09403, doi:10.1029/2006JB004789.
1.1
Interaction between meteoric and metamorphic water along the Simplon
fault zone: constraints from oxygen and carbon stable isotope
geochemistry
Guerra Ivan* ***, Vennemann Torsten**, Mancktelow Neil***, Negro François* & Kalt Angelika*
*Institut de Géologie et d’Hydrogéologie, Université de Neuchâtel, Rue E. Argand 11, CH-2009 Neuchâtel (ivan.guerra@unine.ch)
**Institut de Minéralogie et Géochimie, Université de Lausanne, L’Anthropole, CH-1015 Lausanne
***Geologisches Institut, ETH Zürich, Leonhardstrasse 19, CH-8092 Zürich
The role of fluids during faulting and fracturing associated with the Miocene to present exhumation of the Lepontine dome
is examined along its south-western border, the Simplon Fault Zone (SFZ). The latest generation of tension gashes, major
regional fault families and fault rocks were studied to establish their geometric relationships, the time of activity and the
composition of the circulating fluids.
Structural data show that the major strike orientation of late tectonic structures is NW-SE, i.e. parallel to the major crustal
Simplon fault. Two other important strike directions, NE-SW and E-W, can be also highlighted. The E-W direction appears to
be the latest one. Most measured fault planes are normal faults, consistent with extension and exhumation of the region
since the Miocene.
Oxygen stable isotope analyses on filling material of late tectonic structures mentioned above - both in the hanging wall and
footwall of the SFZ - reveal two different fluid circulation systems. In the footwall, all along the SFZ, the δ 18 O values of quartz
both for host rock and late veins range between 10 ‰ and 12 ‰ – which are values typical for metagranitic rocks. This supports
complete buffering of infiltrating, circulating fluids by the host rock prior to fracturing and vein precipitation.
In the Simplon hanging wall the situation is different: similar δ 18 O values for quartz from host rock and late veins are found
at the northern and southern part of the detachment. In these two areas both hanging wall and footwall have the same
metamorphic degree: greenschist facies to the north and amphibolite facies to the south. In contrast, in the central part of
the SFZ the δ 18 O values for quartz from the hanging wall late veins are approximately 3.0 ‰ lower than the values observed
in the footwall (8.0 ± 0.4 ‰ and 11.1 ± 0.8 ‰ respectively; modern meteoric water has a δ 18 O value of around -13 ‰). In this
region there is an offset in metamorphic degree: greenschist facies in the hanging wall and amphibolite facies in the footwall.
Field observations indicate that the Simplon footwall has been less affected than the hanging wall by the brittle deformation
occurred starting from 8-12 Ma (Campani et al., 2008); the hanging wall is also much more pervasively fractured than the
footwall. These observations, together with the distribution of the Alpine metamorphism and the isotopic values obtained,
may indicate that the diffusion of meteoric water along the SFZ has probably been controlled by brittle extensional deformation
style, temperature (i.e. closure temperature to diffusive exchange) and perhaps deformation/recrystallization style to
the oxygen isotope exchange between an infiltrating fluid and the host rock to the veins. The isotopic data collected along
the SFZ also reflect a rock-buffered system in the footwall and a meteoric fluid influenced system in the hanging wall. In
this context the clay-rich fault gouge marking the detachment is acting as an impermeable barrier - if such fluid-rock exchange
occurred together with or after the displacement along the fault – helping the differences in meteoric water diffusion.
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Symposium 1: Structural Geology, Tectonics and Geodynamics