Publications

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  • Publication
    Accès libre
    Fluid flow pathways along the Glarus overthrust derived from stable and Sr-isotope patterns
    (2002)
    Badertscher, Nicolas P.
    ;
    Abart, Rainer
    ;
    Burkhard, Martin
    ;
    McCaig, Andrew
    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.
  • Publication
    Accès libre
    Oxygen, carbon and strontium isotope systematics in two profiles across the Glarus thrust : implications for fluid flow
    (2002)
    Abart, Rainer
    ;
    Badertscher, Nicolas P.
    ;
    Burkhard, Martin
    ;
    Povoden, Erwin
    The Glarus thrust is a prominent tectonic feature in the eastern Helvetic Alps. It has been recognized as a potential major pathway for syntectonic crustal scale fluid flow. The oxygen, carbon and strontium isotope patterns obtained from two vertical profiles across the thrust indicate fundamentally different flow regimes in the southern section of the thrust, where the footwall is represented by Mesozoic limestones, and in the northern section, where the footwall is represented by Tertiary flysch. At the Grauberg locality in the south, the observed isotope patterns give evidence of a net mass transport component from the hanging wall Verrucano to the footwall limestone with a maximum time-integrated volumetric fluid flux of 6.1 m3/m2 In the south, the hydration of the lowermost 10 to 20 m of the hanging wall Verrucano requires introduction of an aqueous fluid by subhorizontal flow along the thrust with a minimum time integrated flux of 240 m3/m2. At the Lochseite locality in the north, the isotope patterns indicate a vertical mass transport component from the footwall flysch to the hanging wall Verrucano with a time-integrated fluid flux of 2.6 m3/m2. In the north, the fluids were probably derived from compaction and dehydration of the footwall flysch during thrusting. The ascending fluids were ponded below the Verrucano and 'lubricated' the thrust. Short-term pressure drops associated with seismic motion along the thrust led to the precipitation of calcite in veins at the thrust surface contributing material to the Lochseiten calc-mylonite, a thin calc-mylonite layer at the thrust contact. Although cross thrust fluid flow may have been two to three orders of magnitude smaller than flow along the thrust, it had a major impact on the isotopic composition of the Lochseiten calc-mylonite. In particular, it buffered the oxygen isotope composition of the calc-mylonite towards the relatively 18O-depleted composition of the hanging wall Verrucano in the south and towards the relatively 18O-enriched compositions of the footwall flysch in the north. By this mechanism a regional south to north 18O-enrichment trend was simulated within the Lochseiten calc-mylonite.