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 50 km of offset on Sinclair(1992)compared the Eocene and Oligocene the Glarus thrust(Milnes and Pfiffner, 1980). Models
1 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 synorogenic 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 50 km of offset on the Glarus thrust (Milnes and Pfiffner, 1980). Models
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 thrust
2 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 SMTfibre 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 regionalscale 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
Metamorphic grade decreases to the north, in the the blattengrat and Sardona units(Milnes and Pfiffner, direction of decreasing structural depth 1977), which is the configuration we observe today The Infrahelvetic complex describes a structural Thus the motion on the glarus thrust is" back assembly of Helvetic zone rocks that now lie beneath stepping "or out-of-sequence, since it cuts across the Glarus thrust. It is made up of four tectonic units previously accreted nappes. We return to this point which are, in ascending structural order(Figs. 2 and below Rocks of the Helvetic nappes were originally 1)the Aar Massif with its autochthonous and deposited on basement located between the aar and parautochthonous cover Gotthard Massifs and on the Gotthard Massif. They 2)the Eocene to Oligocene North Helvetic flysch were emplaced during the d, or Calanda phase of which was, at least in part, stripped off the Milnes and Pfiffner(1977). Internal imbrication of the Mesozoic cover of the Aar Massif (Schmid, 1975): Helvetic nappes was closely followed by peak 3)the Blattengrat(South Helvetic)and Sardona metamorphism in the Early Oligocene(30-35 Ma) (Ultrahelvetic or Penninic)nappes, which root Frey et al., 1973; Hunziker et al., 1986) much farther to the south relative to the helvetic Thrust faults in convergent wedges typically chop nappes(Trumpy, 1969; Schmid, 1975 Milnes and their way forward towards the foreland, which results Pfiffner, 1977; Lihou, 1996); and in the accretion of imbricate slices from the 4)the so-called Subhelvetic nappes(consisting of downgoing plate. Such behaviour is called"in- Mesozoic cover stripped from the Aar Massif) sequence" or perhaps more precisely, "forward which were also transported northwards before the stepping”.“ Out-of- sequence”or‘back- stepping Glarus thrust formed( Schmid, 1975) faults occur when a new imbricate fault splays off a Metamorphism in the Infrahelvetic complex deep part of the basal thrust and cuts up through the probably occurred between 20-25 Ma(Hunziker et al overlying thrust wedge. The result is that the front of 1986), and reached 270-300%C and 2-3 kbar in the the wedge becomes dismembered and overridden by southernmost part and 170-190oC and 1.3-1.5 kbar the rear of the wedge. In some cases, the frontal piece farther north(rahn et al., 1994, 1995) of the wedge are found re-accreted to the base of the we Paleogeography and structural relationships Lihou (1996)showed that the North Penninic described above show that the glarus thrust is a back- Prattigau flysch, the Sardona nappe, and the South stepping thrust, which means that the Infrahelvetic Helvetic Blattengrat nappe were juxtaposed and complex represents a slice that was originally accreted imbricated during an early deformational event, the D at the front of the wedge, prior to formation of the or Pizol phase of Milnes and Pfiffner(1977). The Glarus. Schmid(1975)argued that the S, Calanda- Eocene to Lower Oligocene north helvetic flysch phase cleavage in the Verrucano formed before including the Taveyannaz sandstone, was deposited in motion on the glarus thrust was initiated Evidence for front of the advancing Pizol-phase thrust wedge this timing is given by the fact that the Calanda-phase (Sinclair, 1992). The inferred tectonic transport cleavage was cataclastically reworked adjacent to the direction during the Pizol phase, as deduced from thrust plane(Schmid, 1975)(Fig. 3). We argue that calcite and quartz fibre lineations, was top-side this s, cleavage formed when the Helvetic nappes towards-340(Lihou, 1996). Lihou(1996)estimated were first overridden and accreted into the alpine that this event started in bartonian to priabonian time wedge. The Glarus thrust then cut back through the (40 Ma). According to Pfiffner(1978), the Sardona wedge and allowed the Helvetic nappes to move u and Blattengrat nappes originated more than 30 km and over a more frontal part of the alpine wedge. As a south of their present position. These nappes, together result, the already cleaved Verrucano in the hanging with all of the Helvetic nappes, were derived from the wall was juxtaposed with the relatively uncleaved south-facing carbonate margin that flanked the nfrahelvetic complex, located in the footwall southern edge of the European continent during the Mesozoic. This relationship indicates that parts of the Luchi rng the last deformational event, the D,or Ruchi phase of Milnes and Pfiffner(1977),movement Infrahelvetic units (i.e. the Blattengrat and Sardona along the glarus thrust continued and a crenulation units)were transported over the Helvetic domain cleavage developed in the Infrahelvetic complex during their collision with the European margin. Note subperpendicular to the Glarus thrust(Schmid, 1975 that younger motion on the glarus thrust allowed the Milnes and Pfiffner, 1977; Lihou, 1996). The originally more inboard Helvetic nappes to override crenulation cleavage is formed within a 300 m thick
3 Metamorphic grade decreases to the north, in the direction of decreasing structural depth. The Infrahelvetic complex describes a structural assembly of Helvetic zone rocks that now lie beneath the Glarus thrust. It is made up of four tectonic units, which are, in ascending structural order (Figs. 2 and 3): 1) the Aar Massif with its autochthonous and parautochthonous cover; 2) the Eocene to Oligocene North Helvetic flysch, which was, at least in part, stripped off the Mesozoic cover of the Aar Massif (Schmid, 1975); 3) the Blattengrat (South Helvetic) and Sardona (Ultrahelvetic or Penninic) nappes, which root much farther to the south relative to the Helvetic nappes (Trümpy, 1969; Schmid, 1975; Milnes and Pfiffner, 1977; Lihou, 1996); and 4) the so-called Subhelvetic nappes (consisting of Mesozoic cover stripped from the Aar Massif), which were also transported northwards before the Glarus thrust formed (Schmid, 1975). Metamorphism in the Infrahelvetic complex probably occurred between 20-25 Ma (Hunziker et al., 1986), and reached 270-300°C and 2-3 kbar in the southernmost part and 170-190°C and 1.3-1.5 kbar farther north (Rahn et al., 1994, 1995). 2.2. Deformation history Lihou (1996) showed that the North Penninic Prättigau flysch, the Sardona nappe, and the South Helvetic Blattengrat nappe were juxtaposed and imbricated during an early deformational event, the D1 or Pizol phase of Milnes and Pfiffner (1977). The Eocene to Lower Oligocene North Helvetic flysch, including the Taveyannaz sandstone, was deposited in front of the advancing Pizol-phase thrust wedge (Sinclair, 1992). The inferred tectonic transport direction during the Pizol phase, as deduced from calcite and quartz fibre lineations, was top-side towards ~340° (Lihou, 1996). Lihou (1996) estimated that this event started in Bartonian to Priabonian time (~40 Ma). According to Pfiffner (1978), the Sardona and Blattengrat nappes originated more than 30 km south of their present position. These nappes, together with all of the Helvetic nappes, were derived from the south-facing carbonate margin that flanked the southern edge of the European continent during the Mesozoic. This relationship indicates that parts of the Infrahelvetic units (i.e. the Blattengrat and Sardona units) were transported over the Helvetic domain during their collision with the European margin. Note that younger motion on the Glarus thrust allowed the originally more inboard Helvetic nappes to override the Blattengrat and Sardona units (Milnes and Pfiffner, 1977), which is the configuration we observe today. Thus, the motion on the Glarus thrust is “backstepping” or “out-of-sequence”, since it cuts across previously accreted nappes. We return to this point below. Rocks of the Helvetic nappes were originally deposited on basement located between the Aar and Gotthard Massifs and on the Gotthard Massif. They were emplaced during the D2 or Calanda phase of Milnes and Pfiffner (1977). Internal imbrication of the Helvetic nappes was closely followed by peak metamorphism in the Early Oligocene (30-35 Ma) (Frey et al., 1973; Hunziker et al., 1986). Thrust faults in convergent wedges typically chop their way forward towards the foreland, which results in the accretion of imbricate slices from the downgoing plate. Such behaviour is called “insequence” or perhaps more precisely, “forwardstepping”. “Out-of-sequence” or “back-stepping” faults occur when a new imbricate fault splays off a deep part of the basal thrust and cuts up through the overlying thrust wedge. The result is that the front of the wedge becomes dismembered and overridden by the rear of the wedge. In some cases, the frontal pieces of the wedge are found re-accreted to the base of the wedge. Paleogeography and structural relationships described above show that the Glarus thrust is a backstepping thrust, which means that the Infrahelvetic complex represents a slice that was originally accreted at the front of the wedge, prior to formation of the Glarus. Schmid (1975) argued that the S2 Calandaphase cleavage in the Verrucano formed before motion on the Glarus thrust was initiated. Evidence for this timing is given by the fact that the Calanda-phase cleavage was cataclastically reworked adjacent to the thrust plane (Schmid, 1975) (Fig. 3). We argue that this S2 cleavage formed when the Helvetic nappes were first overridden and accreted into the Alpine wedge. The Glarus thrust then cut back through the wedge and allowed the Helvetic nappes to move up and over a more frontal part of the Alpine wedge. As a result, the already cleaved Verrucano in the hanging wall was juxtaposed with the relatively uncleaved Infrahelvetic complex, located in the footwall. During the last deformational event, the D3 or Ruchi phase of Milnes and Pfiffner (1977), movement along the Glarus thrust continued and a crenulation cleavage developed in the Infrahelvetic complex subperpendicular to the Glarus thrust (Schmid, 1975; Milnes and Pfiffner, 1977; Lihou, 1996). The crenulation cleavage is formed within a 300 m thick
zone beneath the glarus thrust. This spatial association Textural observations were made using thi with the thrust suggests that the cleavage is somehow sections cut in two principal planes, XY and XZ.(X,y elated to motion on the thrust (Schmid, 1975) and z refer to the maximum extension, intermediate Subhorizontal fibres in extension gashes trend 150 and maximum shortening directions. The Xr section 1600, and are thought to track the extension direction was cut parallel to cleavage, and was then used to during D, (Schmid, 1975) determine the average direction of fibre overgrowths Slip on the glarus thrust was, in part, post Unidirectional fibres were observed in Xy sections metamorphic, resulting in an inverted metamorphic The average fibre direction was thus considered to quence with higher grade rocks above lower grade mark the X direction. XZ sections were cut rocks. From this, Frey (1988)concluded that about 10- perpendicular to cleavage and parallel to X. These two 20 km of the displacement along the glarus thrust sections were then used to measure strain magnitudes postdated the Early Oligocene peak of metamorphism and internal rotations Rahn et al. (1995)revised this estimate to about 10 km Petrographic evidence indicates that SMT was the of post-metamorphic transport. K-Ar and rb-Sr white dominant deformation mechanism operating in the mica dating of the Lochseiten mylonite suggests white Taveyannaz sandstone samples below the Glarus mica growth at -23 Ma(Hunziker et al., 1986). A later thrust. These sandstones are mainly composed of first phase of slip was dated at 14-20 Ma( Frey et al cycle volcanogenic sediment Monocrystalline grains 1973). Rahn et al. (1994)showed discontinuities in of volcanic quartz and plagioclase show little to no apatite fission-track ages across the Glarus thrust undulose extinction deformation laminae. or providing strong evidence for a final increment of slip deformation twinning Polycrystalline quartz grains do in the Late Miocene. The apatite fission-track study of show undulose extinction and other evidence for Rahn et al. (1997)also showed late arching of the intracrystalline deformation. These grains are Glarus thrust(Fig 3)during the Late miocene. The interpreted to be metamorphic detritus because the Middle to Late miocene deformation coincides with intracrystalline deformation is limited to these grains enhanced subsidence in the molasse basin between and because their microstructures lack systematic 17-14 Ma, followed by the deposition of conglomerate orientations. This conclusion is further supported by units at-14-11 Ma(den Brok and Jagoutz, 2000) the fact that the metamorphic grains are commonly mantled by undeformed fibre overgrowths. The 3. Deformation Study dominance of the smt mechanism is consistent with 3.1. Evidence for SMT deformation metamorphic temperatures(see above), which were Our strain methods are designed solely for almost everywhere below the 300oC threshold needed measuring smt deformation In this context. we to activate dislocation glide -and-climb in quartz assume that all strain occurs by changes at the (Kuster and Stockhert, 1997) boundaries of grains, and that intragranular strains are As shown by Siddans(1979), the SMT mechanism negligible. We support this ption with a detailed also dominates in the verrucano above the glarus discussion of the deformation textures. and then thrust in the northern and central Glarus Alps. The follow with a description of our deformation mudstones there are red and have greyish-green measurement methods reduction spots, which Siddans(1979)used for his The units we sampled were dominated by strain analysis. Metamorphic grade increases to the iliciclastic sandstones, with quartz and feldspar south across the glarus alps The mudstones become occurring as the dominant detrital phases. About 30% green, with the colour change coinciding with the of our sandstone samples had significant secondary development of subgrains in quartz and carbonate Calcite and other carbonate minerals can recrystallization of fibre overgrowths. This transition deform by dislocation glide at relatively low mainly occurs to the south of the hinge of the broad temperatures(Schmid, 1982). Thus, these samples arch marked by the Glarus thrust(the hinge line runs were deemed unsuitable for our methods and were between Linthal and Elm, Fig. 2)(see also van Daalen excluded from the study eta.,1999 In the field the verrucano and melser sandstones Quartz and feldspar are truncated by thin selvages from above the glarus thrust and the tavayannaz composed of insoluble minerals. The selvages can be sandstone from below the thrust showed a variabl regarded as planes of finite flattening that formed developed spaced cleavage( Fig. 4). This planar fabric perpendicular to Z(Ramsay and Huber, 1983) was easy to see but linear fabrics were not visible Directed fibrous overgrowths of quartz, chlorite, and hand sample white mica mantle those grain boundary segments at a
4 zone beneath the Glarus thrust. This spatial association with the thrust suggests that the cleavage is somehow related to motion on the thrust (Schmid, 1975). Subhorizontal fibres in extension gashes trend 150- 160°, and are thought to track the extension direction during D3 (Schmid, 1975). Slip on the Glarus thrust was, in part, postmetamorphic, resulting in an inverted metamorphic sequence, with higher grade rocks above lower grade rocks. From this, Frey (1988) concluded that about 10- 20 km of the displacement along the Glarus thrust postdated the Early Oligocene peak of metamorphism. Rahn et al. (1995) revised this estimate to about 10 km of post-metamorphic transport. K-Ar and Rb-Sr white mica dating of the Lochseiten mylonite suggests white mica growth at ~23 Ma (Hunziker et al., 1986). A later phase of slip was dated at 14-20 Ma (Frey et al., 1973). Rahn et al. (1994) showed discontinuities in apatite fission-track ages across the Glarus thrust, providing strong evidence for a final increment of slip in the Late Miocene. The apatite fission-track study of Rahn et al. (1997) also showed late arching of the Glarus thrust (Fig. 3) during the Late Miocene. The Middle to Late Miocene deformation coincides with enhanced subsidence in the Molasse basin between 17-14 Ma, followed by the deposition of conglomerate units at ~14-11 Ma (den Brok and Jagoutz, 2000). 3. Deformation Study 3.1. Evidence for SMT deformation Our strain methods are designed solely for measuring SMT deformation. In this context, we assume that all strain occurs by changes at the boundaries of grains, and that intragranular strains are negligible. We support this assumption with a detailed discussion of the deformation textures, and then follow with a description of our deformation measurement methods. The units we sampled were dominated by siliciclastic sandstones, with quartz and feldspar occurring as the dominant detrital phases. About 30% of our sandstone samples had significant secondary carbonate. Calcite and other carbonate minerals can deform by dislocation glide at relatively low temperatures (Schmid, 1982). Thus, these samples were deemed unsuitable for our methods, and were excluded from the study. In the field, the Verrucano and Melser sandstones from above the Glarus thrust and the Tavayannaz sandstone from below the thrust showed a variably developed spaced cleavage (Fig. 4). This planar fabric was easy to see, but linear fabrics were not visible in hand sample. Textural observations were made using thin sections cut in two principal planes, XY and XZ. (X, Y and Z refer to the maximum extension, intermediate and maximum shortening directions.) The XY section was cut parallel to cleavage, and was then used to determine the average direction of fibre overgrowths. Unidirectional fibres were observed in XY sections. The average fibre direction was thus considered to mark the X direction. XZ sections were cut perpendicular to cleavage and parallel to X. These two sections were then used to measure strain magnitudes and internal rotations. Petrographic evidence indicates that SMT was the dominant deformation mechanism operating in the Taveyannaz sandstone samples below the Glarus thrust. These sandstones are mainly composed of firstcycle volcanogenic sediment. Monocrystalline grains of volcanic quartz and plagioclase show little to no undulose extinction, deformation laminae, or deformation twinning. Polycrystalline quartz grains do show undulose extinction and other evidence for intracrystalline deformation. These grains are interpreted to be metamorphic detritus because the intracrystalline deformation is limited to these grains and because their microstructures lack systematic orientations. This conclusion is further supported by the fact that the metamorphic grains are commonly mantled by undeformed fibre overgrowths. The dominance of the SMT mechanism is consistent with metamorphic temperatures (see above), which were almost everywhere below the 300°C threshold needed to activate dislocation glide-and-climb in quartz (Küster and Stöckhert, 1997). As shown by Siddans (1979), the SMT mechanism also dominates in the Verrucano above the Glarus thrust in the northern and central Glarus Alps. The mudstones there are red and have greyish-green reduction spots, which Siddans (1979) used for his strain analysis. Metamorphic grade increases to the south across the Glarus Alps. The mudstones become green, with the colour change coinciding with the development of subgrains in quartz and recrystallization of fibre overgrowths. This transition mainly occurs to the south of the hinge of the broad arch marked by the Glarus thrust (the hinge line runs between Linthal and Elm, Fig. 2) (see also van Daalen et al., 1999). Quartz and feldspar are truncated by thin selvages composed of insoluble minerals. The selvages can be regarded as planes of finite flattening that formed perpendicular to Z (Ramsay and Huber, 1983). Directed fibrous overgrowths of quartz, chlorite, and white mica mantle those grain boundary segments at a
high angle to cleavage. The fibre overgrowths are measurements but tends to be averaged out at the scale considered to record extensional strains that of the thin section accumulated during SMt deformation In XY and XZ As discussed in Ring(1996)and Feehan and sections, fibres bundles are typically straight and Brandon(1999), the formation of a SMT fabric unidirectional(Fig 4). The unidirectional geometry requires the accommodation of small motions on the indicates that strains in the y and Z directions are selvage surfaces to account for differential motion of contractional. In contrast, some studies(e. g, Ring and adjacent grains. We see no textural evidence for Brandon, 1999)have recognized multidirectional thoroughgoing slip surfaces or for oblique shearing fibres that point in all directions in the Xr plane between grains The deformation associated with Smt indicating extension in both X and Y processes is accommodated solely by shortening In XZ sections, the fibre bundles generally lie across the selvages and extension in the fibre subparallel to the trace of cleavage(Fig. 4a and b) direction Individual fibre bundles typically have a tapered Textural evidence suggests that the sandstones had geometry, with fibres converging away from the host little porosity at the start of sMT deformation. all of grain. This tapered geometry is recorded in the the space between grains is presently occupied by distribution of fibre directions, which commonly vary selvages or directed fibre overgrowth. Dissolution by as much as +15 around the average direction along selvage surfaces would quickly remove an The taper geometry has been explained as resulting initial porosity. Transient porosity might have existed from dissolution between the fibres to accommodate along the incoherent surfaces that separated the fibre shortening in the Y and Z directions(semi-deformable overgrowths from their host grains. However, the antitaxial fibre model of Ring and Brandon, 1999) porosity along this surface would have been small Extension parallel to X is accommodated solely by given that displacement-controlled fibre overgrowths growth of new fibres. The fibres are inferred to accrete only form when crack apertures are small, on the scale at the grain boundary, so that the amount of shortening of microns or less (Urai et al., 1991; Fisher and across the fibres is largest at the end of the bundles Brantley, 1992). We can think of no other textural This explanation accounts for the observation that the features that might indicate significant porosity during degree of tapering seems to increase with the amount Smt deformation We suggest that mechanical of shortening in the section for instance. fibre compaction had already removed much of the primary bundles appear more tapered in XZ sections than Xr porosity before the onset of smT deformation. This sections because shortening is greater in Z than in Y. result would be expected for a poorly sorted sediment We assume that the fibres track the incremental X where grains of different sizes could be compacted direction during the deformation history of the rock, into a tightly packed aggregate whereas cleavage records the Xr plane for the total Our observations indicate that fibre overgrowth SMT strain. Thus, parallelism between fibres and was the sole mechanism of precipitation during SMT cleavage indicates a coaxial deformation(Feehan and deformation. Alternative possibilities include: (1) Brandon, 1999; Ring and Brandon, 1999). As noted mass transfer associated with metamorphic above, most of our samples( 80%)have fibres that recrystallization of the original detrital grains, (2)the parallel the trace of cleavage. However, some samples precipitation of syntaxial overgrowths on existing (20%)have an average fibre orientation in the xz detrital grains(e.g, the overgrowth of quartz on section that is oblique to the trace of cleavage(e. g detrital quartz grains), and (3)the formation of a fine Fig. 4d), indicating a weakly non-coaxial deformation grained"matrix "around the grains In a few samples, we found individual fibres with an The following evidence indicates that the obliquity of up to 20-30 to cleavage. Nonetheless, the contribution of these other processes was minor.(1) average angle between fibres and cleavage in these The original grain boundaries of the detrital grains are samples is less than 8. We return to this topic below ell preserved and the grains themselves show little when we make specific estimates of the degree of non evidence of internal recrystallization. (2) There is coaxial evidence of alkali mass transfer. such as albitization of In some thin sections, we observed weakly curved plagioclase, but this exchange appears to have fibre bundles around large quartz or feldspar grains occurred by in-situ transfer. For instance, albitized (Fig. 4c and d). This texture appears to record plagioclase grains retain their detrial shapes, which no heterogeneous deformation around the largest grains sign of recrystallization. (3) The fibre overgrowths This localized deformation is included in our appear compositionally uniform within a thin section, which suggests that they grew from a common
5 high angle to cleavage. The fibre overgrowths are considered to record extensional strains that accumulated during SMT deformation. In XY and XZ sections, fibres bundles are typically straight and unidirectional (Fig. 4). The unidirectional geometry indicates that strains in the Y and Z directions are contractional. In contrast, some studies (e.g., Ring and Brandon, 1999) have recognized multidirectional fibres that point in all directions in the XY plane, indicating extension in both X and Y. In XZ sections, the fibre bundles generally lie subparallel to the trace of cleavage (Fig. 4a and b). Individual fibre bundles typically have a tapered geometry, with fibres converging away from the host grain. This tapered geometry is recorded in the distribution of fibre directions, which commonly vary by as much as ±15° around the average direction. The taper geometry has been explained as resulting from dissolution between the fibres to accommodate shortening in the Y and Z directions (semi-deformable antitaxial fibre model of Ring and Brandon, 1999). Extension parallel to X is accommodated solely by growth of new fibres. The fibres are inferred to accrete at the grain boundary, so that the amount of shortening across the fibres is largest at the end of the bundles. This explanation accounts for the observation that the degree of tapering seems to increase with the amount of shortening in the section. For instance, fibre bundles appear more tapered in XZ sections than XY sections because shortening is greater in Z than in Y. We assume that the fibres track the incremental X direction during the deformation history of the rock, whereas cleavage records the XY plane for the total SMT strain. Thus, parallelism between fibres and cleavage indicates a coaxial deformation (Feehan and Brandon, 1999; Ring and Brandon, 1999). As noted above, most of our samples (80%) have fibres that parallel the trace of cleavage. However, some samples (20%) have an average fibre orientation in the XZ section that is oblique to the trace of cleavage (e.g. Fig. 4d), indicating a weakly non-coaxial deformation. In a few samples, we found individual fibres with an obliquity of up to 20-30º to cleavage. Nonetheless, the average angle between fibres and cleavage in these samples is less than 8°. We return to this topic below when we make specific estimates of the degree of noncoaxiality. In some thin sections, we observed weakly curved fibre bundles around large quartz or feldspar grains (Fig. 4c and d). This texture appears to record heterogeneous deformation around the largest grains. This localized deformation is included in our measurements but tends to be averaged out at the scale of the thin section. As discussed in Ring (1996) and Feehan and Brandon (1999), the formation of a SMT fabric requires the accommodation of small motions on the selvage surfaces to account for differential motion of adjacent grains. We see no textural evidence for thoroughgoing slip surfaces or for oblique shearing between grains. The deformation associated with SMT processes is accommodated solely by shortening across the selvages and extension in the fibre direction. Textural evidence suggests that the sandstones had little porosity at the start of SMT deformation. All of the space between grains is presently occupied by selvages or directed fibre overgrowth. Dissolution along selvage surfaces would quickly remove an initial porosity. Transient porosity might have existed along the incoherent surfaces that separated the fibre overgrowths from their host grains. However, the porosity along this surface would have been small given that displacement-controlled fibre overgrowths only form when crack apertures are small, on the scale of microns or less (Urai et al., 1991; Fisher and Brantley, 1992). We can think of no other textural features that might indicate significant porosity during SMT deformation. We suggest that mechanical compaction had already removed much of the primary porosity before the onset of SMT deformation. This result would be expected for a poorly sorted sediment where grains of different sizes could be compacted into a tightly packed aggregate. Our observations indicate that fibre overgrowth was the sole mechanism of precipitation during SMT deformation. Alternative possibilities include: (1) mass transfer associated with metamorphic recrystallization of the original detrital grains, (2) the precipitation of syntaxial overgrowths on existing detrital grains (e.g., the overgrowth of quartz on detrital quartz grains), and (3) the formation of a finegrained “matrix” around the grains. The following evidence indicates that the contribution of these other processes was minor. (1) The original grain boundaries of the detrital grains are well preserved and the grains themselves show little evidence of internal recrystallization. (2) There is evidence of alkali mass transfer, such as albitization of plagioclase, but this exchange appears to have occurred by in-situ transfer. For instance, albitized plagioclase grains retain their detrial shapes, which no sign of recrystallization. (3) The fibre overgrowths appear compositionally uniform within a thin section, which suggests that they grew from a common
solution. In other words, they are not the product of provides a record of the original size of the grain. The local replacement of existing detrital grains by central idea behind the pds method is that principal recrystallziation. (4) There are many descriptions in directions with S Sy> and the amount of mass locally precipitated, our S, where S= final length/initial length estimates of Sp, only represent the mass-transfe Measurements were made using XZ and Xr thin omponent of the volume strain. Other sources of sections. Samples in this study have unidirectional volume strain include changes in porosity and mineral fibres, which means that Sx>I and Sy and s,< 1 density. Porosity is thought to have been small at the Thus, S, and S, were determined by the PDs method start of smt deformation and is thus ignored and Sy by the Mode method Changes in mineral density are insignificant at the low The Pds method is used to metamorphic grade in our study shortening produced by dissolution of grain The geometric relationship between the fibre boundaries. The method exploits the fact that for SMt overgrowths and the trace of cleavage was used to deformation, the dimensions of the detrital quartz and estimate the internal rotation associated with Smt feldspar grains remain unchanged in the X direction deformation. This method is described in Ring and (i.e, deformation is intergranular, not intragranular) Brandon(1999). The basic idea is that the fibre Therefore, the grain diameter in the X direction overgrowths track the incremental extension direction 6
6 solution. In other words, they are not the product of local replacement of existing detrital grains by recrystallziation. (4) There are many descriptions in the literature of a “fine-grained matrix” in low-grade immature sandstones. The “matrix problem” is at the centre of the old debate about the distinction of greywacke from arkose (see Dickinson, 1970, for a review). More recently, some of our colleagues have suggested that the matrix of the rock may account for the volume loss that we have measured. Deformed lithic grains can appear like matrix, but the outlines of such grains are usually easily recognized in plane light (pseudomatrix of Dickinson, 1970). Our experience is that the rest of the “matrix” is fibre overgrowth. The overgrowths appear as a “fine-grained matrix” when viewed in sections oblique to the X direction. XY and XZ sections are needed to see the overgrowth texture, with XZ sections providing the best view. Diagnostic features include the elongated habit of the fibre minerals (commonly quartz and mica), the consistent orientation of the fibres across the section, and the generally uniform composition of the overgrowths. (5) We found no petrographic evidence for syntaxial overgrowths, but cathodoluminscence work is needed to fully test this conclusion. 4.2. Methods Our study employs the PDS (Projected Dimension Strain), Mode, and Fiber methods for measuring strains and internal rotations in sandstones deformed by the SMT mechanism. A brief summary is provided here. Further details can be found in Feehan and Brandon (1999) and Ring and Brandon (1999). Relevant computer programs are available at www.geology.yale.edu/~brandon. Traditional methods, such as the Rf /φ method, are not suitable because the grains did not deform as passive markers but rather by truncation and precipitation along grain boundaries. In our discussion here, the principal stretches are designated as SX ≥ SY ≥ SZ, where S = final length/initial length. Measurements were made using XZ and XY thin sections. Samples in this study have unidirectional fibres, which means that SX > 1 and SY and SZ < 1. Thus, SY and SZ were determined by the PDS method, and SX by the Mode method. The PDS method is used to measure the average shortening produced by dissolution of grain boundaries. The method exploits the fact that for SMT deformation, the dimensions of the detrital quartz and feldspar grains remain unchanged in the X direction (i.e., deformation is intergranular, not intragranular). Therefore, the grain diameter in the X direction provides a record of the original size of the grain. The central idea behind the PDS method is that principal directions with S < 1, SMT deformation has reduced the average dimension of the detrital grains by a factor equivalent to the principal stretch. In contrast, the average initial dimension of a detrital grain should be preserved in the X direction because the grains lack any significant internal deformation and because the original grain surface is mantled by fibre overgrowths. Therefore, a contractive principal stretch can be determined by finding the average grain dimension parallel to a contractive principal direction and dividing it by the average grain dimension parallel to X. Dimensions are measured in a two–dimensional thin section, so a correction is needed to get the appropriate three–dimensional result (see Feehan and Brandon, 1999 for details). Our measurements were made using a petrographic microscope with a camera lucida tube and digitising tablet. Measurements are precise to better than ±3 µm. The dimension of each grain is represented by its caliper dimensions (or projected dimensions) in the principal directions lying in the section. The caliper dimensions of the grains are not affected in any significant way by grain rotations associated with compaction. For instance, PDS measurements on undeformed sandstones gave undeformed results (i.e. S ~ 1) (Ring and Brandon, 1999). The Mode method is used to determine the extensional strain recorded by the fibre overgrowths. The modal percentage of fibres in a rock is directly related to the absolute extensional stretch in the rock. Fibre modes are most easily measured in the XZ section. For unidirectional fibres, SX = (1 - m) -1, where m is the modal fraction of fibre. Given absolute strains, the volume stretch SV (= final volume/initial volume) is equal to the product of the principal stretches (SX ⋅ SY ⋅ SZ). Because our methods focus entirely on the loss of mass from grains and the amount of mass locally precipitated, our estimates of SV only represent the mass-transfer component of the volume strain. Other sources of volume strain include changes in porosity and mineral density. Porosity is thought to have been small at the start of SMT deformation and is thus ignored. Changes in mineral density are insignificant at the low metamorphic grade in our study area. The geometric relationship between the fibre overgrowths and the trace of cleavage was used to estimate the internal rotation associated with SMT deformation. This method is described in Ring and Brandon (1999). The basic idea is that the fibre overgrowths track the incremental extension direction
during the deformation, whereas the cleavage using the Hencky method where only the stretch approximates the XY plane of the finite deformation tensor is needed(see Appendix B of Brandon, 1995) Internal rotation was estimated using the FIBer program to model the shape of about 30-50 fibres 3.3. Results The internal rotation axis is assumed to parallel o digitised in the XZ section(Ring and Brandon, 1999) bles i and 2 list our deformation measurements for the Verrucano and melser sandstones from the Tables I and 2 report internal rotation and average Helvetic nappes above the glarus thrust, and for the kinematic numbers for Smt deformation Internal Taveyannaz and north helvetic flysch sandstones rotation is represented by a right-handed rotation axis from the Infrahelvetic complex below the thrust defined by a trend and plunge, and a rotation angle, Sample locations are shown in Fig. 2. Note that most Q2j. Wn and Wm are the average kinematic vorticity of our samples from the Helvetic nappes an e trom numbers and Am the average kinematic dilatancy more than 1 km above the thrust plane The stereograms(Fig. 5)show that the z directions number(Means et al., 1980; Passchier, 1991; Means are clustered around a steeply plunging maximum, and 1994; Ring and Brandon, 1999). Definitions and other the X and y directions are scattered in a weakly details are given in Ring and Brandon(1999). A brief defined subhorizontal girdle. The average for Z(Table review is provided here. The m subscript for the 2)defines the average flattening plane, which has a kinematic numbers indicates a path-averaged value strike of 30 and a dip of 10 to the SE. The average X assuming a steady three-dimensional deformation. If direction is close to horizontal, although its trend SMt deformation were unsteady, then Wm would changes from 200 above the glarus thrust to 160% have no direct relationship to the time history of the below the thrust. The Nadai plot(Fig. 6)shows a instantaneous kinematic vorticity number Wk. The scatter of both prolate and oblate strain symmetries simple geometry of the overgrowths in our samples The strain type(Fig. 7)is generally weakly suggests that Smt deformation was fairly steady, at constrictional, as indicated by S, S 1991; Ring and Brandon, 1999). The deformation is S,)and approximately plane strain(Sr= 1). This isochoric if A=0. dilatant if A>0 and surprising result, with S, =1, stems from the variable orientations of X and y in the flattening plane, which compactive if Am <0. For example, a deformation means that local extensional strains are averaged out at involving uniaxial shortening and an equal loss of the regional scale volume, would have A=-l because the rates of Below the Glarus thrust, individual samples show haller strain magnitudes in X and y. For instance. X olume strain and deviatoric strain would be equal but strains range from +3 to +12%. In contrast, the tensor opposite in sign average indicates absolute principal stretches of 0.99 Table 2 reports tensor averages for our deformation 0.88 and 0.73, which is nearly identical to the tensor measurements As discussed in Brandon(1995) average above the thrust Thus the low strain deformation data must be averaged in tensor form to magnitudes in X and y are only manifested at the local ensure that the magnitudes and directions of the cale. At the regional scale, sandstones above and I stretches and rotations are correctly below the fault record a similar smt deformation associated. If the rotational component of the deformation is small. then one can average the stretch involving constrictional plane strain The absolute strain data indicate pronounced mass- tensor and the internal rotation tensor separately, without introducing significant errors(Brandon, loss volume strains ranging from-9 to-54% in the 1995). In this study, tensor averages were calculated sandstones above the glarus thrust. and -8 to -49% for sandstones below the thrust. The average is the same for both 36%. At the outcrop
7 during the deformation, whereas the cleavage approximates the XY plane of the finite deformation. Internal rotation was estimated using the FIBER program to model the shape of about 30-50 fibres digitised in the XZ section (Ring and Brandon, 1999). The internal rotation axis is assumed to parallel Y. Tables 1 and 2 report internal rotation and average kinematic numbers for SMT deformation. Internal rotation is represented by a right-handed rotation axis, defined by a trend and plunge, and a rotation angle, Ωi. Wm and * Wm are the average kinematic vorticity numbers and * Am the average kinematic dilatancy number (Means et al., 1980; Passchier, 1991; Means, 1994; Ring and Brandon, 1999). Definitions and other details are given in Ring and Brandon (1999). A brief review is provided here. The m subscript for the kinematic numbers indicates a path-averaged value assuming a steady three-dimensional deformation. If SMT deformation were unsteady, then * Wm would have no direct relationship to the time history of the instantaneous kinematic vorticity number * Wk . The simple geometry of the overgrowths in our samples suggests that SMT deformation was fairly steady, at least in its orientation. An asterisk indicates that the kinematic number is based on the deviatoric stretching rate rather than the absolute stretching rate. Thus, a coaxial deformation is indicated by * Wm = 0 and a noncoaxial simple-shear deformation by * Wm = 1, regardless of the amount of volume strain. * Am describes the average ratio of the volume-strain rate relative to the deviatoric stretching rate (Passchier, 1991; Ring and Brandon, 1999). The deformation is isochoric if * Am = 0, dilatant if * Am > 0, and compactive if * Am SY > SZ) and approximately plane strain (SX ≈ 1). This surprising result, with SX ≈ 1, stems from the variable orientations of X and Y in the flattening plane, which means that local extensional strains are averaged out at the regional scale. Below the Glarus thrust, individual samples show smaller strain magnitudes in X and Y. For instance, X strains range from +3 to +12%. In contrast, the tensor average indicates absolute principal stretches of 0.99, 0.88 and 0.73, which is nearly identical to the tensor average above the thrust. Thus, the low strain magnitudes in X and Y are only manifested at the local scale. At the regional scale, sandstones above and below the fault record a similar SMT deformation involving constrictional plane strain. The absolute strain data indicate pronounced massloss volume strains ranging from −9 to −54% in the sandstones above the Glarus thrust, and −8 to −49% for sandstones below the thrust. The average is the same for both groups, −36%. At the outcrop scale
there is no evidence of where this missing mass went areas adjacent to the Glarus thrust, but the details of The volume fraction of veins in outcrops is generally their measurements and the localities are not no greater than a few percent. Thus, we conclude that discussed. Neither study provides any information SMt deformation was influenced by a large flux of about principal directions. By themselves, strain-ratio fluid that was able to dissolve the sandstones and to data have limited utility, but they do provide transport the dissolved load over a scale larger than information about the symmetry and magnitude of the our study area. deviatoric component of the strain. Thus, comparisons Figure 8 shows that volume strain and deviatoric with our data are limited to the Nadai plot( Fig. 6) strain are uncorrelated, which implies that these strains all data sets show a similar clustering along the are controlled by different processes. Based on our prolate/oblate boundary, so they share the same strain previous work, we have found that a strong correlation symmetry. In contrast, they have very different strain between deviatoric strain and volume strain only magnitudes In Figure 6, the strain ratio R(=S/s) occurs where fibre overgrowths are small or absent provides a useful measure of deviatoric strain (e.g. Feehan and Brandon, 1999). The reason is that magnitude(note that R is independent of volume volume and deviatoric strains are solely a function of strain). Our measurements have Rz ranging from 1.4 shortening strains in Y and Z. Deviatoric strains to 2.3, whereas Siddans'(1979)measurements range become uncorrelated with volume strain when there from 1.5 to 9.5. However the highest strain are variations in extensional strain(as indicated by magnitudes for Siddans' data are those from localities variations in the modal abundance of fibre near the Glarus thrust(grey triangles in Fig. 6b). The overgrowth). Uncorrelated volume strain and remaining localities(black triangles in Fig. 6b)show a deviatoric strain are observed in the eastern belt of closer correspondence to our results. The"averages the franciscan Complex of California (Ring and given in Milnes and Pfiffner(1977) have an Rx of-4 Brandon, 1999), as well as in our Glarus study here for rocks below the Glarus thrust(open triangle in Fig The variations in fibre overgrowth means that some of 6b)and -13 for rocks directly above the thrust(open the dissolved grain mass is re-precipitated locally in circle in Fig. 6b) the rock. The rock must extend in at least one These results suggest that differences between direction to be able to accommodate the locally these studies is due, at least in part, to difference in precipitated mass the heterogeneity influenced, at least in part, by the s verage of a heterog strain field wi Only 7 out of the 18 samples have sufficient tensional strain to determine the rotational proximity to the glarus thrust. a denser and more component of the deformation, Q2i and wm(table 1) uniform coverage would be needed to test this Small extensional strains means short fibres As the interpretation. Grain-size effects might also be fibres get shorter, so does the resolution of the important. Our study focused exclusively on medium- incremental extension path. S, must be greater than grain sandstones, whereas Siddans'(1979) study was 1. 10 to get reliable estimates of Q2i and Wm(ring restricted to Verrucano mudstones where the reduction spots are found. Milnes and Pfiffner(1977) and Brandon, 1999). Our measurements indicate did not report what they sampled, but we suspect that minor non-coaxiality during smt deformation. One they also focused on the Verrucano mudstone sample has Wm=0. 29, but the rest are less than 0.18 because of the availability of reduction sp a9. The rotational component of the deformation may be elatively small, but the internal rotation axes show 4. Discussion consistent orientation and shear sense(Fig 9). The 4.1. Mass loss average rotation axis (Table 2, asterisk in Fig 9)is A surprising result of our study is the large mass loss, horizontal and indicates a general top- north sense of about 36%. in sandstones above and below the glarus shear. which is similar to the shear -sense direction thrust The amount of dissolved mass is large, and determined for the glarus thrust( schmid, 1975 there is no obvious repository for this dissolved mass Milnes and Pfiffner, 1980; Lihou, 1996) in the region around the Glarus thrust. The volume strain can be considered as a form of internal erosion 3.4. Previous regional strain work within the Alpine wedge. SMT deformation included Siddans (1979) presents principal strain ratios both closed and open exchange, involving local measured from reduction spots in mudstones at 12 precipitation of fibre overgrowths and wholesale loss localities in the Verrucano formation milnes and of mass from the rock. The open-system behaviour Pfiffner(1977)report some"average" strain ratios for was probably driven by dissolution and bulk removal
8 there is no evidence of where this missing mass went. The volume fraction of veins in outcrops is generally no greater than a few percent. Thus, we conclude that SMT deformation was influenced by a large flux of fluid that was able to dissolve the sandstones and to transport the dissolved load over a scale larger than our study area. Figure 8 shows that volume strain and deviatoric strain are uncorrelated, which implies that these strains are controlled by different processes. Based on our previous work, we have found that a strong correlation between deviatoric strain and volume strain only occurs where fibre overgrowths are small or absent (e.g. Feehan and Brandon, 1999). The reason is that volume and deviatoric strains are solely a function of shortening strains in Y and Z. Deviatoric strains become uncorrelated with volume strain when there are variations in extensional strain (as indicated by variations in the modal abundance of fibre overgrowth). Uncorrelated volume strain and deviatoric strain are observed in the Eastern Belt of the Franciscan Complex of California (Ring and Brandon, 1999), as well as in our Glarus study here. The variations in fibre overgrowth means that some of the dissolved grain mass is re-precipitated locally in the rock. The rock must extend in at least one direction to be able to accommodate the locally precipitated mass. Only 7 out of the 18 samples have sufficient extensional strain to determine the rotational component of the deformation, Ωi and * Wm (Table 1). Small extensional strains means short fibres. As the fibres get shorter, so does the resolution of the incremental extension path. SX must be greater than ~1.10 to get reliable estimates of Ωi and * Wm (Ring and Brandon, 1999). Our measurements indicate minor non-coaxiality during SMT deformation. One sample has * Wm = 0.29, but the rest are less than 0.18. The rotational component of the deformation may be relatively small, but the internal rotation axes show a consistent orientation and shear sense (Fig. 9). The average rotation axis (Table 2, asterisk in Fig. 9) is horizontal and indicates a general top-north sense of shear, which is similar to the shear-sense direction determined for the Glarus thrust (Schmid, 1975; Milnes and Pfiffner, 1980; Lihou, 1996). 3.4. Previous regional strain work Siddans (1979) presents principal strain ratios measured from reduction spots in mudstones at 12 localities in the Verrucano formation. Milnes and Pfiffner (1977) report some “average” strain ratios for areas adjacent to the Glarus thrust, but the details of their measurements and the localities are not discussed. Neither study provides any information about principal directions. By themselves, strain-ratio data have limited utility, but they do provide information about the symmetry and magnitude of the deviatoric component of the strain. Thus, comparisons with our data are limited to the Nadai plot (Fig. 6). All data sets show a similar clustering along the prolate/oblate boundary, so they share the same strain symmetry. In contrast, they have very different strain magnitudes. In Figure 6, the strain ratio RXZ (= SX/SZ) provides a useful measure of deviatoric strain magnitude (note that RXZ is independent of volume strain). Our measurements have RXZ ranging from 1.4 to 2.3, whereas Siddans’ (1979) measurements range from 1.5 to 9.5. However, the highest strain magnitudes for Siddans’ data are those from localities near the Glarus thrust (grey triangles in Fig. 6b). The remaining localities (black triangles in Fig. 6b) show a closer correspondence to our results. The “averages” given in Milnes and Pfiffner (1977) have an RXZ of ~4 for rocks below the Glarus thrust (open triangle in Fig. 6b) and ~13 for rocks directly above the thrust (open circle in Fig. 6b). These results suggest that differences between these studies is due, at least in part, to difference in sample coverage of a heterogeneous strain field, with the heterogeneity influenced, at least in part, by the proximity to the Glarus thrust. A denser and more uniform coverage would be needed to test this interpretation. Grain-size effects might also be important. Our study focused exclusively on mediumgrain sandstones, whereas Siddans’ (1979) study was restricted to Verrucano mudstones, where the reduction spots are found. Milnes and Pfiffner (1977) did not report what they sampled, but we suspect that they also focused on the Verrucano mudstones because of the availability of reduction spots. 4. Discussion 4.1. Mass loss A surprising result of our study is the large mass loss, about 36%, in sandstones above and below the Glarus thrust. The amount of dissolved mass is large, and there is no obvious repository for this dissolved mass in the region around the Glarus thrust. The volume strain can be considered as a form of internal erosion within the Alpine wedge. SMT deformation included both closed and open exchange, involving local precipitation of fibre overgrowths and wholesale loss of mass from the rock. The open-system behaviour was probably driven by dissolution and bulk removal
low temperatures and low permeabilities thought to typify this setting Burkhard et al. (1992)argues for advection of externally derived fluids, associated with large fluid- 4.2. Regional tectonic evolution to-rock ratios, during motion of the Helvetic thrust There is evidence in the Helvetic Alps that Carbonate-bearing veins in the taveyannaz sandstone deformation propagated northward with time, starting which is otherwise free of carbonate. also indicate in the structurally highest and most internal units to mass transfer over distances greater than the outcrop the south( Sardona and Blattengrat nappes )and scale moving outward to more external and structurally Our results here are similar to those from other deep units to the north. Lihou(1996)related cleavage studies we have done on sandstones in convergent formation in the Sardona and Blattengrat nappes to the wedge settings. These include the late cretaceous San Late Eocene D, Pizol phase and reported a-160- Juan-Cascade nappes(Feehan and Brandon, 1999), the trending extension direction. We concur with Milnes Late Cenozoic Olympic subduction complex( Brandon and Pfiffner(1977)that the Glarus thrust formed and Kang, 1995), the mesozoic franciscan subduction during the subsequent D, Calanda phase and complex(ring and Brandon, 1999; Bolhar and ring 2001), and the Permian-Cretaceous Torlesse o. commodated early transport of the Helvetic nappes nto the Infrahelvetic complex. Cleavage in the subduction complex in the South island of New Verrucano is thought to have formed early during the Zealand(Maxelon et al., 1998). All examples show Calanda phase. The average X direction for the significant mass loss in association with an Verrucano is -200 at that time(Siddans, 1979, this approximately plane-strain coaxial deformation at the study ) We tie cleavage formation in the taveyannaz regional scale sandstone and the north Helvetic Flysch to the late Our study highlights an interesting contrast in the Calanda phase and the D, Ruchi phase(Schmid character of mass transfer with increasing grade. The 1975). The average X direction is -160%at that time mass loss that we have documented in the low-grade and identical to that reported by Schmid(1975)for the sandstones tends to be pervasive throughout the rock D Ruchi phase. Therefore, we suggest that the In contrast, mass transfer in greenschist-and Taveyannaz sandstone and associated North Helvetic mphibolite-facies rocks tends to be more localized Flysch, which lie beneath the Glarus thrust,were occurring in association with veins and ductile shear deformed after the verrucano and melser sandstone zones(e.g. Selverstone et al., 1991; Newman and which lies above the thrust Mitra, 1993; Ague, 1994; Bailey et al, 1994; O Hara Overall. the data suggest that the downward 1994;Ring,1999 propagation of deformation during the Oligocene There has been recurring evidence that smt Calanda and Ruchi phases was associated with a deformation is commonly associated with large mass loss volume strains(e.g, Wright and Platt, 1982 However, the problem with interpreting this changing curving x direction that changed from -200 to -1 Wright and Henderson, 1992). This result, however, X direction is that our strain work indicates that little has been difficult to reconcile with the fact that silicate extension was associated with x above and below the minerals have very low solubilities in typic Glarus thrust. In other words, the regional-scale metamorphic fluids. Thus, large mass-loss volume constrictional plane-strain deformation associated with strains seem to require large fluid-to-rock ratios One D, and D, was produced by shortening in Y and z way out of this dilemma, proposed by Etheridge et al directions. We also note the interpretation of X as a (1983), is that fluid flow in low-grade settings direction of tectonic transport is only valid for a driven by thermal convection, which would allow re simple-shear deformation, which is not supported by circulation of a small volume of fluid. This our strain data. The observation that the smt strains interpretation is supported by theoretical models are, in general, only slightly non-coaxial also discussed by Wood and Hewitt(1982, 1984)and Criss challenges the interpretation that the ductile and Hoffmeister(1991), which show that there is no deformation in the nappe is related to shear coupling threshold for the onset of thermal convection when on the glarus thrust. In contrast, we argue that the isotherms are inclined. Convection will always occur, generally coaxial deformation away from the glarus although the flow velocities may become quite small if thrust indicates that the thrust was very weak. In fact, the thermal gradients are small. We speculate that experimental work by Hsu(1969)and Schmid(1975 porous-medium convective flow may be an under showed that the lochseitenkalk is very weak appreciated process in convergent wedges, despite the Likewise, very low shear coupling on the subduction
9 of the more soluble components of the rock, due to flow of a solvent fluid phase on a regional scale. Burkhard et al. (1992) argues for advection of externally derived fluids, associated with large fluidto-rock ratios, during motion of the Helvetic thrust. Carbonate-bearing veins in the Taveyannaz sandstone, which is otherwise free of carbonate, also indicate mass transfer over distances greater than the outcrop scale. Our results here are similar to those from other studies we have done on sandstones in convergent wedge settings. These include the Late Cretaceous San Juan-Cascade nappes (Feehan and Brandon, 1999), the Late Cenozoic Olympic subduction complex (Brandon and Kang, 1995), the Mesozoic Franciscan subduction complex (Ring and Brandon, 1999; Bolhar and Ring, 2001), and the Permian-Cretaceous Torlesse subduction complex in the South Island of New Zealand (Maxelon et al., 1998). All examples show significant mass loss in association with an approximately plane-strain coaxial deformation at the regional scale. Our study highlights an interesting contrast in the character of mass transfer with increasing grade. The mass loss that we have documented in the low-grade sandstones tends to be pervasive throughout the rock. In contrast, mass transfer in greenschist- and amphibolite-facies rocks tends to be more localized, occurring in association with veins and ductile shear zones (e.g. Selverstone et al., 1991; Newman and Mitra, 1993; Ague, 1994; Bailey et al., 1994; O’Hara, 1994; Ring, 1999). There has been recurring evidence that SMT deformation is commonly associated with large massloss volume strains (e.g., Wright and Platt, 1982; Wright and Henderson, 1992). This result, however, has been difficult to reconcile with the fact that silicate minerals have very low solubilities in typical metamorphic fluids. Thus, large mass-loss volume strains seem to require large fluid-to-rock ratios. One way out of this dilemma, proposed by Etheridge et al. (1983), is that fluid flow in low-grade settings is driven by thermal convection, which would allow recirculation of a small volume of fluid. This interpretation is supported by theoretical models discussed by Wood and Hewitt (1982, 1984) and Criss and Hoffmeister (1991), which show that there is no threshold for the onset of thermal convection when isotherms are inclined. Convection will always occur, although the flow velocities may become quite small if the thermal gradients are small. We speculate that porous-medium convective flow may be an underappreciated process in convergent wedges, despite the low temperatures and low permeabilities thought to typify this setting. 4.2. Regional tectonic evolution There is evidence in the Helvetic Alps that deformation propagated northward with time, starting in the structurally highest and most internal units to the south (Sardona and Blattengrat nappes) and moving outward to more external and structurally deep units to the north. Lihou (1996) related cleavage formation in the Sardona and Blattengrat nappes to the Late Eocene D1 Pizol phase and reported a ~160°- trending extension direction. We concur with Milnes and Pfiffner (1977) that the Glarus thrust formed during the subsequent D2 Calanda phase and accommodated early transport of the Helvetic nappes onto the Infrahelvetic complex. Cleavage in the Verrucano is thought to have formed early during the Calanda phase. The average X direction for the Verrucano is ~200° at that time (Siddans, 1979; this study). We tie cleavage formation in the Taveyannaz sandstone and the North Helvetic Flysch to the late Calanda phase and the D3 Ruchi phase (Schmid, 1975). The average X direction is ~160° at that time and identical to that reported by Schmid (1975) for the D3 Ruchi phase. Therefore, we suggest that the Taveyannaz sandstone and associated North Helvetic Flysch, which lie beneath the Glarus thrust, were deformed after the Verrucano and Melser sandstone, which lies above the thrust. Overall, the data suggest that the downward propagation of deformation during the Oligocene Calanda and Ruchi phases was associated with a curving X direction that changed from ~200° to ~160°. However, the problem with interpreting this changing X direction is that our strain work indicates that little extension was associated with X above and below the Glarus thrust. In other words, the regional-scale constrictional plane-strain deformation associated with D2 and D3 was produced by shortening in Y and Z directions. We also note the interpretation of X as a direction of tectonic transport is only valid for a simple-shear deformation, which is not supported by our strain data. The observation that the SMT strains are, in general, only slightly non-coaxial also challenges the interpretation that the ductile deformation in the nappe is related to shear coupling on the Glarus thrust. In contrast, we argue that the generally coaxial deformation away from the Glarus thrust indicates that the thrust was very weak. In fact, experimental work by Hsü (1969) and Schmid (1975) showed that the Lochseitenkalk is very weak. Likewise, very low shear coupling on the subduction
thrusts have been proposed by Brandon and Ring of the Helvetic nappes. We regard this option as (1998), who summarized five quantitative studies of unrealistic because of the regionally low strains and ductile deformation from deeply exhumed he lack of evidence for pronounced non-coaxial accretionary wedges. They concluded that flow there deformation in large parts of the Verrucano. We is almost always coaxial because the subduction thrust propose that the flat-lying foliation in the verrucano is was too weak transmit a significant shear tractio basically a result of pronounced vertical shortening These findings bring up the question of how intra that accompanied nappe translation. This would imply nappe deformation relates to nappe translation in that early Calanda-phase nappe stacking in the Glarus orogens. In general, there are two different ideas in Alps was not by simple shear. It also implies that the thrust belts. The first is that ductile strain in nappes is exhumation of the Glarus thrust was not solely due to distinctly non-coaxial with stretching lineations erosion but has also been aided by coaxial vertical parallel to thrust transport at the base of the nappes. A shortening. Assuming an initial depth of 12-15 km and certain amount of coupling at faults is needed to a residence time within the ductile crust of about 25 internally shear adjacent thrust nappes. In the western Myr, we calculate, using the one-dimensional Helvetic nappes for example, Ramsay and Huber numerical model of Feehan and Brandon(1999), that (1983)showed that carbonates of the Morcles nappe vertical ductile shortening contributed about 1.5 km supply strong evidence for shear. Studies in the (at rates of 0. 2 km/Myr) to the exhumation of the strongly ductilely deformed Penninic nappes also Glarus thrust. i. e. about 10% appear to support such a view(e.g. Merle et al., 1989; Ring, 1992). The second idea is that faults are weak Acknowledgements as suggested for the Lochseitenkalk at the glarus This study was funded by the deutsche thrust(Hsu, 1969, Schmid, 1975). The two different Forschungsgemeinschaft through the concepts may, at least in part, depend on the strengt Graduiertenkolleg" Stoffbestand von Kruste und contrast between the mylonite between nappes and the Mantel at Mainz University and a us/German interior of the nappe. The viscosities of the carbonates exchange program funded by the National Science of the Morcles nappe and the Lochseitenkalk are Foundation and the deutscher Akademischer different. In the Morcles nappe, limestones are Austauschdienst. We thank meinert rahn bas den deformed by dislocation creep(ramsay and Huber Brok and Oliver Jagoutz for numerous discussions and 1983). For the Lochseiten mylonite, Schmid (1982) Stefan Schmid and Kyuichi Kanagawa for careful proposed deformation by superplastic flow. We have reviews and Richard Lisle for editorial handling shown that deformation in the Glarus nappe and the North Helvetic flysch is approximately coaxial, as expected for a weak thrust. Therefore, the Lochseitenkalk was probably considerably weaker than the limestone in the Morcles nappe and this rheologic contrast might have controlled the different structural styles in the Helvetic nappes of western and eastern switzerland caused vertical thinning of the overlying wedge a g) We propose that deep accretion(ie. underpl recorded by the formation of a subhorizontal clear in the hanging wall of the glarus thrust. Vertical shortening of 30% appears to be balanced by mass loss Our rotation data show that the foliation formed during a weakly non-coaxial deformation within large parts of the Glarus nappe. In a typical foreland fold- and-thrust belt, one would expect the foliation in a thrust sheet to be at a high angle to the underlying thrust and then curving asymptotically into subparallelism with the thrust plane theoretically the flat-lying foliation in the Verrucano could be due to rotation as a result of large non-coaxial strains, i.e. that the Verrucano represents a huge shear zone at the base
10 thrusts have been proposed by Brandon and Ring (1998), who summarized five quantitative studies of ductile deformation from deeply exhumed accretionary wedges. They concluded that flow there is almost always coaxial because the subduction thrust was too weak transmit a significant shear traction. These findings bring up the question of how intranappe deformation relates to nappe translation in orogens. In general, there are two different ideas in thrust belts. The first is that ductile strain in nappes is distinctly non-coaxial with stretching lineations parallel to thrust transport at the base of the nappes. A certain amount of coupling at faults is needed to internally shear adjacent thrust nappes. In the western Helvetic nappes for example, Ramsay and Huber (1983) showed that carbonates of the Morcles nappe supply strong evidence for shear. Studies in the strongly ductilely deformed Penninic nappes also appear to support such a view (e.g. Merle et al., 1989; Ring, 1992). The second idea is that faults are weak, as suggested for the Lochseitenkalk at the Glarus thrust (Hsü, 1969; Schmid, 1975). The two different concepts may, at least in part, depend on the strength contrast between the mylonite between nappes and the interior of the nappe. The viscosities of the carbonates of the Morcles nappe and the Lochseitenkalk are different. In the Morcles nappe, limestones are deformed by dislocation creep (Ramsay and Huber, 1983). For the Lochseiten mylonite, Schmid (1982) proposed deformation by superplastic flow. We have shown that deformation in the Glarus nappe and the North Helvetic flysch is approximately coaxial, as expected for a weak thrust. Therefore, the Lochseitenkalk was probably considerably weaker than the limestone in the Morcles nappe and this rheologic contrast might have controlled the different structural styles in the Helvetic nappes of western and eastern Switzerland. We propose that deep accretion (i.e. underplating) caused vertical thinning of the overlying wedge, as recorded by the formation of a subhorizontal cleavage in the hanging wall of the Glarus thrust. Vertical shortening of 30% appears to be balanced by mass loss. Our rotation data show that the foliation formed during a weakly non-coaxial deformation within large parts of the Glarus nappe. In a typical foreland foldand-thrust belt, one would expect the foliation in a thrust sheet to be at a high angle to the underlying thrust and then curving asymptotically into subparallelism with the thrust plane. Theoretically the flat-lying foliation in the Verrucano could be due to rotation as a result of large non-coaxial strains, i.e. that the Verrucano represents a huge shear zone at the base of the Helvetic nappes. We regard this option as unrealistic because of the regionally low strains and the lack of evidence for pronounced non-coaxial deformation in large parts of the Verrucano. We propose that the flat-lying foliation in the Verrucano is basically a result of pronounced vertical shortening that accompanied nappe translation. This would imply that early Calanda-phase nappe stacking in the Glarus Alps was not by simple shear. It also implies that the exhumation of the Glarus thrust was not solely due to erosion but has also been aided by coaxial vertical shortening. Assuming an initial depth of 12-15 km and a residence time within the ductile crust of about 25 Myr, we calculate, using the one-dimensional numerical model of Feehan and Brandon (1999), that vertical ductile shortening contributed about 1.5 km (at rates of 0.2 km/Myr) to the exhumation of the Glarus thrust, i.e. about 10%. Acknowledgements This study was funded by the Deutsche Forschungsgemeinschaft through the Graduiertenkolleg “Stoffbestand von Kruste und Mantel” at Mainz University and a US/German exchange program funded by the National Science Foundation and the Deutscher Akademischer Austauschdienst. We thank Meinert Rahn, Bas den Brok and Oliver Jagoutz for numerous discussions and Stefan Schmid and Kyuichi Kanagawa for careful reviews and Richard Lisle for editorial handling