Solution-mass-transfer deformation adjacent to the glarus thrust, with implications for the tectonic evolution of the alpine wedge in eastern Switzerland Uwe Ring*Mark t. Brandon. Alexander ramthun *Corresponding author. E-mail address: ring( @mail. uni-mainz de u. ring) Institut fur Geowissenschaften, Johannes-Gutenberg Universitat, Becherweg 21, 55099 Mainz, Germany Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, CT 06520, USA January, 2001: Final revised manuscript, to be published in Journal of Structural Geology 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-1600, 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 Helevtic nappes. The missing mass cannot be accounted for at the local scale, and appears to have been transported beyond the Helvetic zone 1. Introduction turbidites of the infrahelvetics to trench-fill turbidites The glarus thrust is the sole thrust of the helvetic formed at ocean-continent subduction zones. An nappes and a conspicuous feature in the landscape of important distinction is that both the Infrahelvetics and the Glarus Alps(Fig. 1). In 1841, Arnold Escher von Helvetics were originally underlain by european der linth discovered the glarus thrust but was continental crust, and not by oceanic crust reluctant to publish his observations: " No one would The Glarus thrust itself is marked by a <l-m thick believe me, they would put me into an asylum"(p. 195 layer of highly sheared calc-mylonite(Lochseitenkalk) in Greene, 1982). Further work by Marcel Bertrand (e.g. Trumpy, 1969; Hsu, 1969; Schmid, 1975),which 1884), Edward Suess(1904, 1909), and Albert Heim separates Eocene-Oligocene turbidites below the (1919)established the geometry and origin of this thrust from conglomerate and mudstone of the impressive structure. Bertrand's publication in 1884 is Permian Verrucano formation above(Heim, 1919 fig widely regarded as marking the birth of Alpine nappe 33 in Trumpy, 1980). The Lochseitenkalk itself theory (trumpy, 1998), with the Glarus thrust probably derived from Jurassic limestone(Schmid 1975)and was carried up and over the footwall of the thrust fault Glarus thrust. Ductile shearing was certainly important The Glarus thrust separates the Infrahelvetic during nappe transport, but significant brittle slip must complex in its footwall from the Helvetic nappes in its have occurred as well. The reason is that the break hanging wall. Both units were derived from the beneath the Lochseiten marks a major stratigraphic Helvetic zone, which refers to the Mesozoic passive discontinuity, with Jurassic limestone and Permian margin that bordered the southern side of the Verrucano overlying Eocene North Helvetic flysch European continent. The underlying Infrahelvetic Thus, localized slip is needed to account for the complex is distinguished by a thick sequence of syn- observed stratigraphic offset orogenic turbidites, and underlying Mesozoic platform The Glarus thrust is folded into a broad east-west carbonates of the European margin The turbidites are trending antiform with gently dipping limbs(Schmid locally volcanoclastic(. g, Taveyannaz sandstone) 1975). Geologic estimates require >50 km of offset on Sinclair(1992)compared the Eocene and Oligocene the Glarus thrust(Milnes and Pfiffner, 1980). Models1 Solution-mass-transfer deformation adjacent to the Glarus thrust, with implications for the tectonic evolution of the Alpine wedge in eastern Switzerland Uwe Ring*1 , Mark T. Brandon2 , Alexander Ramthun1 *Corresponding author. E-mail address: ring@mail.uni-mainz.de (U. Ring) 1 Institut für Geowissenschaften, Johannes-Gutenberg Universität, Becherweg 21, 55099 Mainz, Germany 2 Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, CT 06520, USA January, 2001: Final revised manuscript, to be published in Journal of Structural Geology 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 Helevtic nappes. The missing mass cannot be accounted for at the local scale, and appears to have been transported beyond the Helvetic zone. 1. Introduction The Glarus thrust is the sole thrust of the Helvetic nappes and a conspicuous feature in the landscape of the Glarus Alps (Fig. 1). In 1841, Arnold Escher von der Linth discovered the Glarus thrust, but was reluctant to publish his observations: "No one would believe me, they would put me into an asylum" (p. 195 in Greene, 1982). Further work by Marcel Bertrand (1884), Edward Suess (1904, 1909), and Albert Heim (1919) established the geometry and origin of this impressive structure. Bertrand’s publication in 1884 is widely regarded as marking the birth of Alpine nappe theory (Trümpy, 1998), with the Glarus thrust recognized as the type example of an orogen-scale thrust fault. The Glarus thrust separates the Infrahelvetic complex in its footwall from the Helvetic nappes in its hanging wall. Both units were derived from the Helvetic zone, which refers to the Mesozoic passive margin that bordered the southern side of the European continent. The underlying Infrahelvetic complex is distinguished by a thick sequence of syn￾orogenic turbidites, and underlying Mesozoic platform carbonates of the European margin. The turbidites are locally volcanoclastic (e.g., Taveyannaz sandstone). Sinclair (1992) compared the Eocene and Oligocene turbidites of the Infrahelvetics to trench-fill turbidites formed at ocean-continent subduction zones. An important distinction is that both the Infrahelvetics and Helvetics were originally underlain by European continental crust, and not by oceanic crust. The Glarus thrust itself is marked by a <1-m thick layer of highly sheared calc-mylonite (Lochseitenkalk) (e.g. Trümpy, 1969; Hsü, 1969; Schmid, 1975), which separates Eocene-Oligocene turbidites below the thrust from conglomerate and mudstone of the Permian Verrucano formation above (Heim, 1919; fig. 33 in Trümpy, 1980). The Lochseitenkalk itself probably derived from Jurassic limestone (Schmid, 1975) and was carried up and over the footwall of the Glarus thrust. Ductile shearing was certainly important during nappe transport, but significant brittle slip must have occurred as well. The reason is that the break beneath the Lochseiten marks a major stratigraphic discontinuity, with Jurassic limestone and Permian Verrucano overlying Eocene North Helvetic flysch. Thus, localized slip is needed to account for the observed stratigraphic offset. The Glarus thrust is folded into a broad, east-west trending antiform with gently dipping limbs (Schmid, 1975). Geologic estimates require >50 km of offset on the Glarus thrust (Milnes and Pfiffner, 1980). Models