addressing the mechanics of the Glarus thrust have deformation mechanism in siliciclastic rocks of the been proposed by Hsu (1969)and Schmid(1975) Glarus Alps, based on previous fabric work(Schmid Milnes and Pfiffner(1977), Pfiffner(1981), Sinclair 1975; Siddans, 1979)and estimates of maximum (1992)and Lihou(1996), amongst others, discussed temperature(100-350C, Burkhard et al., 1992; Rahn the tectonic evolution of the Helvetic nappes and the et al., 1994, 1995, 1997). In this paper, we report the Infrahelvetic complex of eastern Switzerland first absolute finite-strain data from the glarus alps We focus here on the deformation history below These data are used to evaluate the role of smt and above the glarus thrust and how deformation deformation and volume strains in the tectonic within the adjacent nappes relates to offset on the evolution of the glarus thrust Glarus thrust. Siddans(1979)studied relative strain in Permian mudstones of the Verrucano formation 2. Overview which lies above the Glarus thrust in the Glarus nappe 2. 1. Geologic and tectonic setting His r/o analysis on reduction spots showed a slightl Tectonic units in the alps are commonly named after prolate strain symmetry. These data lack information the paleogeographic zones from whence they were bout volume strain which is needed to estimate the derived. The Pennine zone represents a largely oceanic strain type (i.e, constrictional, plane strain or it that separated the European margin from the flattening; Ramsay and Wood, 1973: Brandon, 1995 Adriatic microcontinent the latter of which is now Feehan and Brandon (1999)and ring and brando represented by the Austroalpine nappes(e.g. Trumpy (1999)reported new methods(PDs, Mode and SMT- 1980). The Helvetic zone describes those rocks fibre methods), which allow measurement of absolute associated with the Mesozoic passive margin that strains and internal rotation in low-grade sandstone flanked the southern edge of the European plate deformed by the solution-mass-transfer(SMT Alpine orogenesis in the Helvetic zone started in the mechanism(i.e, pressure solution). The application of Late Eocene(Pfiffner, 1986) when this margin was those methods to sandstone from subduction-related overridden by the Pennine and the austroalpine accretionary wedges along the western North nappes, with the North Penninic Prattigau flysch being American margin indicates considerable mass-loss the first to be obducted onto the margin. This event is volume strains, on the order of 30-40%(Feehan and generally considered to mark the final closure of Brandon, 1999; Ring and Brandon, 1999). The volume oceanic basins in the Alps and the onset of full loss is attributed to dissolution and bulk removal of continental collision between the European and the more soluble components of the rock by regional Adriatic continental margins. Thrusting propagated scale fluid flow during SMT deformation northward, progressing from the more internal nappes The smt mechanism is thought to be a linear in the south to the more external foreland units in the viscous deformation mechanism, which operates by north(Trumpy, 1969; Milnes and Pfiffner, 1977; selective dissolution, transport, and precipitation along Pfiffner, 1986). Structural burial is recorded by grain boundaries. What remains poorly understood is metamorphism at about 30-35 Ma( hunziker et al how these grain-boundary processes operate and 986), locally culminating in amphibolite-facies interact, and what processes control the rate of SMT conditions in the southern parts of the Gotthard Massif deformation in nature. The"wet Coble creep model Frey et al., 1974). In the southern Helvetic nappes (Elliott, 1973; Rutter, 1983 )considers diffusional metamorphism locally reached greenschist facies and transport along grain boundaries to be the rate-limiting was largely coeval with the major phase of nappe step. However, the finding that Smt deformation in stacking and associated cleavage formation(D,or accretionary wedges is commonly associated with Calanda phase of Milnes and Pfiffner, 1977) considerable volume loss suggests that dissolution, The Helvetic nappes above the glarus thrust can be precipitation, or advective fluid transport could be the subdivided in ascending order into the Glarus rate controlling processes(Raj and Chyung, 1981 Murtschen, Axen and Santis nappes(Figs. 2 and 3) Mullis, 1993; Paterson, 1995) The only pre-Mesozoic unit in the Helvetic nappes is Our study here was motivated by an interest to the Permian Verrucano formation, which makes up the compare deformation of siliciclastic rocks in a typical lower two thirds of the Glarus nappe In the Glarus foreland fold-and-thrust belt with our results from Alps, metamorphism of the Verrucano reached subduction settings. We might expect some anchizonal to incipient epizonal conditions (illite similarities given that deformation in both settings crystallinity; Siddans, 1979, Frey, 1988)with peak occurred in actively accreting convergent wedges. We temperatures of 300-350@C in the area immediately knew in advance that smt was the dominant above the southernmost part of the glarus thrust2 addressing the mechanics of the Glarus thrust have been proposed by Hsü (1969) and Schmid (1975). Milnes and Pfiffner (1977), Pfiffner (1981), Sinclair (1992) and Lihou (1996), amongst others, discussed the tectonic evolution of the Helvetic nappes and the Infrahelvetic complex of eastern Switzerland. We focus here on the deformation history below and above the Glarus thrust, and how deformation within the adjacent nappes relates to offset on the Glarus thrust. Siddans (1979) studied relative strain in Permian mudstones of the Verrucano formation, which lies above the Glarus thrust in the Glarus nappe. His Rf /φ analysis on reduction spots showed a slightly prolate strain symmetry. These data lack information about volume strain, which is needed to estimate the strain type (i.e., constrictional, plane strain or flattening; Ramsay and Wood, 1973; Brandon, 1995). Feehan and Brandon (1999) and Ring and Brandon (1999) reported new methods (PDS, Mode and SMT￾fibre methods), which allow measurement of absolute strains and internal rotation in low-grade sandstone deformed by the solution-mass-transfer (SMT) mechanism (i.e., pressure solution). The application of those methods to sandstone from subduction-related accretionary wedges along the western North American margin indicates considerable mass-loss volume strains, on the order of 30-40% (Feehan and Brandon, 1999; Ring and Brandon, 1999). The volume loss is attributed to dissolution and bulk removal of the more soluble components of the rock by regional￾scale fluid flow during SMT deformation. The SMT mechanism is thought to be a linear viscous deformation mechanism, which operates by selective dissolution, transport, and precipitation along grain boundaries. What remains poorly understood is how these grain-boundary processes operate and interact, and what processes control the rate of SMT deformation in nature. The “wet” Coble creep model (Elliott, 1973; Rutter, 1983) considers diffusional transport along grain boundaries to be the rate-limiting step. However, the finding that SMT deformation in accretionary wedges is commonly associated with considerable volume loss suggests that dissolution, precipitation, or advective fluid transport could be the rate controlling processes (Raj and Chyung, 1981; Mullis, 1993; Paterson, 1995). Our study here was motivated by an interest to compare deformation of siliciclastic rocks in a typical foreland fold-and-thrust belt with our results from subduction settings. We might expect some similarities given that deformation in both settings occurred in actively accreting convergent wedges. We knew in advance that SMT was the dominant deformation mechanism in siliciclastic rocks of the Glarus Alps, based on previous fabric work (Schmid, 1975; Siddans, 1979) and estimates of maximum temperature (100-350°C, Burkhard et al., 1992; Rahn et al., 1994, 1995, 1997). In this paper, we report the first absolute finite-strain data from the Glarus Alps. These data are used to evaluate the role of SMT deformation and volume strains in the tectonic evolution of the Glarus thrust. 2. Overview 2.1. Geologic and tectonic setting Tectonic units in the Alps are commonly named after the paleogeographic zones from whence they were derived. The Pennine zone represents a largely oceanic unit that separated the European margin from the Adriatic microcontinent, the latter of which is now represented by the Austroalpine nappes (e.g. Trümpy, 1980). The Helvetic zone describes those rocks associated with the Mesozoic passive margin that flanked the southern edge of the European plate. Alpine orogenesis in the Helvetic zone started in the Late Eocene (Pfiffner, 1986) when this margin was overridden by the Pennine and the Austroalpine nappes, with the North Penninic Prättigau flysch being the first to be obducted onto the margin. This event is generally considered to mark the final closure of oceanic basins in the Alps and the onset of full continental collision between the European and Adriatic continental margins. Thrusting propagated northward, progressing from the more internal nappes in the south to the more external foreland units in the north (Trümpy, 1969; Milnes and Pfiffner, 1977; Pfiffner, 1986). Structural burial is recorded by metamorphism at about 30-35 Ma (Hunziker et al., 1986), locally culminating in amphibolite-facies conditions in the southern parts of the Gotthard Massif (Frey et al., 1974). In the southern Helvetic nappes, metamorphism locally reached greenschist facies and was largely coeval with the major phase of nappe stacking and associated cleavage formation (D2 or Calanda phase of Milnes and Pfiffner, 1977). The Helvetic nappes above the Glarus thrust can be subdivided in ascending order into the Glarus, Mürtschen, Axen and Säntis nappes (Figs. 2 and 3). The only pre-Mesozoic unit in the Helvetic nappes is the Permian Verrucano formation, which makes up the lower two thirds of the Glarus nappe. In the Glarus Alps, metamorphism of the Verrucano reached anchizonal to incipient epizonal conditions (illite crystallinity; Siddans, 1979; Frey, 1988) with peak temperatures of 300-350°C in the area immediately above the southernmost part of the Glarus thrust