Abstract
AbstractRadiogenic strontium isotopes (87Sr/86Sr) of vein carbonates play a central role in the tectonometamorphic study of fold-and-thrust belts and accretionary wedges and have been used to document fluid sources and fluxes, for example, along major fault zones. In addition, the 87Sr/86Sr ratios of vein carbonates can trace the diagenetic to metamorphic evolution of pore fluids in accreted sediments. Here we present 87Sr/86Sr ratios of vein carbonates from the Infrahelvetic flysch units of the central European Alps (Glarus Alps, Switzerland), which were accreted to the North Alpine fold-and-thrust belt during the early stages of continental collision. We show that the vein carbonates trace the Sr isotopic evolution of pore fluids from an initial seawater-like signature towards the Sr isotopic composition of the host rock with increasing metamorphic grade. This relationship reflects the progressive equilibration of the pore fluid with the host rock and allows us to constrain the diagenetic to low-grade metamorphic conditions of deformation events, including bedding-parallel shearing, imbricate thrusting, folding, cleavage development, tectonic mélange formation and extension. The strontium isotope systematics of vein carbonates provides new insights into the prograde to early retrograde tectonic evolution of the Alpine fold-and-thrust belt and helps to understand the relative timing of deformation events.
Highlights
Deformation at convergent plate margins depends on the presence of fluids and their strengthreducing effect
We address the Sr isotopic systematics of vein carbonates from marine foreland basin sediments of the North Alpine fold-and-thrust belt extending the dataset of Dielforder et al (2015)
We find that the 87Sr/86Sr ratios of vein carbonate correlate with the relative age of the mineral veins, where more radiogenic isotope ratios tend to be associated with mineral veins that formed at a later stage of the structural evolution
Summary
Deformation at convergent plate margins depends on the presence of fluids and their strengthreducing effect. The build-up of pore fluid overpressures reduces the effective normal stresses, which brings the rock closer to failure and allows deformation at low differential stresses (e.g. Hubbert & Rubey, 1959). Such stress and strength conditions facilitate the formation of mineral veins and are common in fold-and-thrust belts and accretionary wedges (e.g. Fisher & Byrne, 1987; Dahlen, 1990; Labaume et al 1991; Sample & Kopf, 1995; Lacroix et al 2011; Mittempergher et al 2017; Ujiie et al 2018). Mineral veins can provide information, for example, on fluid sources, fluid–rock interaction, vein formation temperatures and absolute ages of veins (e.g. Vroliijk et al 1988; Sharp & Kirschner, 1995; Tarantola et al 2007; Sample et al 2017; Beaudoin et al 2018)
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