6533b7ddfe1ef96bd1275183

RESEARCH PRODUCT

Solution-mass-transfer deformation adjacent to the Glarus Thrust, with implications for the tectonic evolution of the Alpine wedge in eastern Switzerland

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Abstract We have studied aspects of absolute finite strain of sandstones and the deformation history above and below the Glarus Thrust in eastern Switzerland. The dominant deformation mechanism is solution mass transfer (SMT), which resulted in the formation of a semi-penetrative cleavage. Our analysis indicates that the Verrucano and Melser sandstones, which lie above the thrust, were deformed coaxially, with pronounced contraction in a subvertical Z direction and minor extension in a subhorizontal X direction, trending at ∼200°. Most of the contraction in Z was balanced by mass-loss volume strains, averaging ∼36%. Below the Glarus Thrust, sandstones of the North Helvetic flysch have smaller principal strains but similar volume strains. Deformation there was also approximately coaxial. The X direction is horizontal and trends ∼160°, which is different by ∼40° from the X direction in the hanging wall. The hanging wall of the Glarus Thrust (Verrucano and Melser sandstones) was deformed first, after it was accreted deep beneath the Alpine wedge. Continued northward advance of the wedge, accomplished in part by motion on the Glarus Thrust, allowed the wedge to override and accrete the North Helvetic flysch, which then started to form an SMT cleavage. The difference in X directions may reflect a change in transport direction, but this conclusion is difficult to accept since extension was minor and was accommodated by coaxial flattening, and not simple shear. Our work indicates that mass-loss volume strains were important in sandstones of the Helvetic nappes. The missing mass cannot be accounted for at the local scale, and appears to have been transported beyond the Helvetic zone.