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

Nicolas P. Badertscher^*, Rainer Abart^**, Martin Burkhard^*, and Andrew McCaig^***

^* Université de Neuchatel, Institut de Géologie, 11 Emile-Argand KP2, 2007 Neuchâtel, Switzerland
^** Institut für Mineralogie und Petrologie, Universitaetsplatz 2, 8010 Graz, Austria
^*** School of Earth Sciences, The University of Leeds, Leeds LS2 9JT, United Kingdom

The Glarus thrust of the eastern Helvetic Alps has been proposed as a major pathway for metamorphic fluids expelled from the footwall during alpine deformation and prograde metamorphism. The stable isotope composition of calcite in a thin continuous calc-mylonite layer and gradients into the overlying Verrucano siltstones and underlying Flysch or carbonate are analyzed in detail. A regional scale map of the ¹⁸O composition of the calc-mylonite covering 25 kilometers east-west by 15 kilometers north-south is interpolated from 35 sampling sites with a total of over 700 samples. This map reveals a steep northward increase in ¹⁸O from 11 to 18 permil (SMOW), interpreted as an isotopic front within the southernmost 6 kilometers, and leveling out at about 20 permil farther north. Vertical profiles across the sharp thrust contact show significantly different isotopic fronts in the south, where the Verrucano Formation is thrust upon marine carbonates and in northern sites, where it rests on Tertiary Flysch.

Southern sites display steep continuous isotopic gradients over about 1 meter above the thrust, where both ¹⁸O and ¹³C decrease to background, Verrucano Formation values of 10 to 11 permil (¹⁸O SMOW) and –6 permil (¹³C PDB) respectively. Footwall carbonates are significantly depleted from their original value of about 26 permil within a zone of less than 10 meters below the thrust contact. The front geometry of ¹³C increasing from –2 to +2 permil downward within about 1 meter is much steeper than the one defined by ¹⁸O.

Northern sites, in contrast, display distended isotopic alteration fronts upward into the Verrucano hangingwall, where higher than background values in ¹⁸O and ¹³C are found up to 15 meters and 6 meters above the thrust contact, respectively. No isotopic alterations could be detected within the footwall Flysch up to the thrust contact. The ⁸⁷Sr/⁸⁶Sr systematics has been observed on the centimeter to millimeter-scale in 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 to 0.712) is found within a zone of highly sheared and dynamically recrystallized yellow veins alternating with dark stylolite seams on the sub-millimeter scale. In two slabs, smooth vertical downward gradients to marine carbonate values (0.708) could be observed on the centimeter-scale.

Regional and local isotopic gradients are explained as exchange between the different rock reservoirs through the advection/diffusion/dispersion of fluids interacting in a regime of brittle/ductile deformation associated with >30 kilometers northward thrust translation. Different scenarios of fluid flow along and/or across the thrust plane are modeled using transport theory coupled with isotopic exchange reactions at an assumed temperature of 300°C.

Considerable amounts of externally derived ¹⁸O-depleted, basement-derived fluids are required to explain the isotopic characteristics in the southern part of the thrust. A time integrated fluid flux (TIFF) of 4500 to 9100 m³/m² is calculated for the channelized flow component along the thrust, far exceeding any cross-thrust component of 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 of upward, cross-thrust flow of fluids derived from compaction/dehydration of Flysch units in the immediate footwall. These calcite saturated fluids left their imprint in the hangingwall Verrucano Formation in the form of increasingly ¹³C and ¹⁸O depleted secondary calcite.

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