Résultat de la recherche
Oxygen, carbon and strontium isotope systematics in two profiles across the Glarus thrust : implications for fluid flow
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.
Brittle-ductile deformation in the Glarus thrust Lochseiten (LK) calc-mylonite
The Glarus thrust accommodated at least 30 km of northward displacement strongly localized within a 1-m layer of 'Lochseiten' (LK) calc-mylonite. This layer displays veins in various states of plastic deformation and a wildly refolded foliated gouge texture. Lattice- and shape-preferred orientations are observed within the fine-grained, recrystallized matrix. These features indicate the alternate activity of brittle and ductile deformation mechanisms. In contrast to the classical view that grain boundary sliding (superplasticity) is the dominant deformation mechanism, it is advocated that fluids, derived from the footwall and expelled along the thrust, are responsible for hydrofracturing and cataclastic deformation. In periods between fracture events, deformation was ductile. In this new interpretation, a substantial amount of the total thrust displacement was accommodated by numerous short-lived and strongly localized fracture events at the base of the Verrucano thrust sheet, rather than a permanently weak décollement lithology.
Finite strain has no influence on the illite crystallinity of tectonized Eocene limestone breccias of the Morcles nappe, Swiss Alps
The relationship between illite crystallinity (IC) and finite strain as well as lattice strain and crystallite size of illite is examined in a series of 27 deformed breccia samples from the inverted limb of the Morcles nappe. The IC is determined independently for limestone components and a red/green clay-silt matrix. The finite strain varies widely (D = 0.8ÿ2.5). The IC values vary from diagenetic to epizonal, but no correlation with finite strain could be established. The spread of IC values is explained by heterogeneities within the protolith which have not been homogenized/ obliterated by the anchi- to epizonal metamorphic overprint. While limestone pebbles display IC values in agreement with the regional metamorphic conditions, the clay-silt matrix has anomalously high IC values, even after deconvolution of the 10 A Ê peak to correct for the presence of paragonite/ muscovite. No correlation could be established between finite strain, lattice strain and/or crystallite size of illite.
Glarus overthrust: A major pathway for the escape of fluids out of the Alpine orogen
Thrust-related fluid flow coupled with isotopic exchange between fluid and rock is simulated in a three-dimensional finite-element model of the Glarus nappe, eastern Swiss Alps. Numerical simulations are matched against well-established oxygen isotope gradients on the kilometer scale along the thrust. At internal southern locations, strongly channelized thrust-parallel fluid flow requires a high permeability contrast of >100:1 between the mylonite zone and country rocks and a high hydraulic head in the hinterland and footwall. In contrast, isotopic patterns ∼5–10 km farther north indicate a predominantly vertical, upward drainage of fluids. We propose a situation in which the Glarus thrust evolved northward across the boundary between the lithostatic and hydrostatic fluid- pressure regimes—the “impermeable cap.” A cyclic behavior of fluid-pressure buildup, fracturing, channelized fluid escape, and sealing explains structural and geochemical observations and the best-fit three-dimensional fluid-flow model parameters.