Faculté des sciences

Fluid flow pathways along the Glarus overthrust derived from stable and Sr-isotope patterns

Badertscher, Nicolas P. ; Abart, Rainer ; Burkhard, Martin ; McCaig, Andrew

In: American Journal of Science, 2002, vol. 302, p. 517-547

The Glarus thrust of the eastern Helvetic Alps has been proposed as amajor pathway for metamorphic fluids expelled from the footwall during alpinedeformation and prograde metamorphism. The stable isotope composition of calcitein a thin continuous calc-mylonite layer and gradients into the overlying Verrucanosiltstones and underlying Flysch or carbonate are analyzed in detail. A regional scalemap... More

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    Summary
    The Glarus thrust of the eastern Helvetic Alps has been proposed as amajor pathway for metamorphic fluids expelled from the footwall during alpinedeformation and prograde metamorphism. The stable isotope composition of calcitein a thin continuous calc-mylonite layer and gradients into the overlying Verrucanosiltstones and underlying Flysch or carbonate are analyzed in detail. A regional scalemap of the δ18O composition of the calc-mylonite covering 25 kilometers east-west by15 kilometers north-south is interpolated from 35 sampling sites with a total of over700 samples. This map reveals a steep northward increase in δ18O from 11 to 18 permil(SMOW), interpreted as an isotopic front within the southernmost 6 kilometers, andleveling out at about 20 permil farther north. Vertical profiles across the sharp thrustcontact show significantly different isotopic fronts in the south, where the VerrucanoFormation is thrust upon marine carbonates and in northern sites, where it rests onTertiary Flysch. Southern sites display steep continuous isotopic gradients over about 1 meterabove the thrust, where both δ18O and δ13C decrease to background, VerrucanoFormation values of 10 to 11 permil (δ18O SMOW) and –6 permil (δ13C PDB)respectively. Footwall carbonates are significantly depleted from their original value ofabout 26 permil within a zone of less than 10 meters below the thrust contact. The frontgeometry of δ13C increasing from -2 to +2 permil downward within about 1 meter ismuch steeper than the one defined by δ18O. Northern sites, in contrast, display distended isotopic alteration fronts upwardinto the Verrucano hangingwall, where higher than background values in δ18O andδ13C are found up to 15 meters and 6 meters above the thrust contact, respectively. Noisotopic alterations could be detected within the footwall Flysch up to the thrustcontact. The 87Sr/86Sr systematics has been observed on the centimeter to millimeterscalein vertical profiles across the calc-mylonite in three southern sites. In each sample(slab), a topmost zone of slightly enriched, homogeneous 87Sr/86Sr values (0.709 to0.712) is found within a zone of highly sheared and dynamically recrystallized yellowveins alternating with dark stylolite seams on the sub-millimeter scale. In two slabs,smooth vertical downward gradients to marine carbonate values (0.708) could beobserved on the centimeter-scale. Regional and local isotopic gradients are explained as exchange between thedifferent rock reservoirs through the advection/diffusion/dispersion of fluids interactingin a regime of brittle/ductile deformation associated with >30 kilometers northwardthrust translation. Different scenarios of fluid flow along and/or across the thrustplane are modeled using transport theory coupled with isotopic exchange reactions atan assumed temperature of 300°C. Considerable amounts of externally derived 18O-depleted, basement-derivedfluids are required to explain the isotopic characteristics in the southern part of thethrust. A time integrated fluid flux (TIFF) of 4500 to 9100 m3/m2 is calculated for thechannelized flow component along the thrust, far exceeding any cross-thrust componentof downward fluid infiltration of 3.45 to 5.7 m3/m2. In northern sites, however,any potential thrust parallel fluid flow is obscured by a dominant component ofupward, cross-thrust flow of fluids derived from compaction/dehydration of Flyschunits in the immediate footwall. These calcite saturated fluids left their imprint in thehangingwall Verrucano Formation in the form of increasingly 13C and 18O depletedsecondary calcite.