Open Session - SWISS GEOSCIENCE MEETINGs
Open Session - SWISS GEOSCIENCE MEETINGs
Open Session - SWISS GEOSCIENCE MEETINGs
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1.1<br />
Interaction between meteoric and metamorphic water along the Simplon<br />
fault zone: constraints from oxygen and carbon stable isotope<br />
geochemistry<br />
Guerra Ivan* ***, Vennemann Torsten**, Mancktelow Neil***, Negro François* & Kalt Angelika*<br />
*Institut de Géologie et d’Hydrogéologie, Université de Neuchâtel, Rue E. Argand 11, CH-2009 Neuchâtel (ivan.guerra@unine.ch)<br />
**Institut de Minéralogie et Géochimie, Université de Lausanne, L’Anthropole, CH-1015 Lausanne<br />
***Geologisches Institut, ETH Zürich, Leonhardstrasse 19, CH-8092 Zürich<br />
The role of fluids during faulting and fracturing associated with the Miocene to present exhumation of the Lepontine dome<br />
is examined along its south-western border, the Simplon Fault Zone (SFZ). The latest generation of tension gashes, major<br />
regional fault families and fault rocks were studied to establish their geometric relationships, the time of activity and the<br />
composition of the circulating fluids.<br />
Structural data show that the major strike orientation of late tectonic structures is NW-SE, i.e. parallel to the major crustal<br />
Simplon fault. Two other important strike directions, NE-SW and E-W, can be also highlighted. The E-W direction appears to<br />
be the latest one. Most measured fault planes are normal faults, consistent with extension and exhumation of the region<br />
since the Miocene.<br />
Oxygen stable isotope analyses on filling material of late tectonic structures mentioned above - both in the hanging wall and<br />
footwall of the SFZ - reveal two different fluid circulation systems. In the footwall, all along the SFZ, the δ 18 O values of quartz<br />
both for host rock and late veins range between 10 ‰ and 12 ‰ – which are values typical for metagranitic rocks. This supports<br />
complete buffering of infiltrating, circulating fluids by the host rock prior to fracturing and vein precipitation.<br />
In the Simplon hanging wall the situation is different: similar δ 18 O values for quartz from host rock and late veins are found<br />
at the northern and southern part of the detachment. In these two areas both hanging wall and footwall have the same<br />
metamorphic degree: greenschist facies to the north and amphibolite facies to the south. In contrast, in the central part of<br />
the SFZ the δ 18 O values for quartz from the hanging wall late veins are approximately 3.0 ‰ lower than the values observed<br />
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<br />
region there is an offset in metamorphic degree: greenschist facies in the hanging wall and amphibolite facies in the footwall.<br />
Field observations indicate that the Simplon footwall has been less affected than the hanging wall by the brittle deformation<br />
occurred starting from 8-12 Ma (Campani et al., 2008); the hanging wall is also much more pervasively fractured than the<br />
footwall. These observations, together with the distribution of the Alpine metamorphism and the isotopic values obtained,<br />
may indicate that the diffusion of meteoric water along the SFZ has probably been controlled by brittle extensional deformation<br />
style, temperature (i.e. closure temperature to diffusive exchange) and perhaps deformation/recrystallization style to<br />
the oxygen isotope exchange between an infiltrating fluid and the host rock to the veins. The isotopic data collected along<br />
the SFZ also reflect a rock-buffered system in the footwall and a meteoric fluid influenced system in the hanging wall. In<br />
this context the clay-rich fault gouge marking the detachment is acting as an impermeable barrier - if such fluid-rock exchange<br />
occurred together with or after the displacement along the fault – helping the differences in meteoric water diffusion.<br />
2<br />
Symposium 1: Structural Geology, Tectonics and Geodynamics