Solution-mass-transfer deformation adjacent to the glarus thrust, with implications for the tectonic evolution of the alpine wedge in eastern Switzerland Uwe Ring*Mark t. Brandon. Alexander ramthun *Corresponding author. E-mail address: ring( @mail. uni-mainz de u. ring) Institut fur Geowissenschaften, Johannes-Gutenberg Universitat, Becherweg 21, 55099 Mainz, Germany Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, CT 06520, USA January, 2001: Final revised manuscript, to be published in Journal of Structural Geology Abstract: We have studied aspects of absolute finite strain of sandstones and the deformation history above and below the glarus thrust in eastern switzerland the dominant deformation mechanism is solution mass transfer (SMT), which resulted in the formation of a semi-penetrative cleavage Our analysis indicates that the verrucano and Melser sandstones, which lie above the thrust, were deformed coaxially, with pronounced contraction in a subvertical Z direction and minor extension in a subhorizontal X direction, trending at -200%. Most of the contraction in Z was balanced by mass-loss volume strains, averaging -36%. Below the Glarus thrust, sandstones of the North Helvetic flysch have smaller principal strains but similar volume strains. Deformation there was also approximately coaxial The X direction is horizontal and trends-1600, which is different by -40 from the X direction in the hanging wall The hanging wall of the Glarus thrust( verrucano and Melser sandstones)was deformed first, after it was accreted deep beneath the alpine wedge. Continued northward advance of the wedge, accomplished in part by motion on the Glarus thrust, allowed the wedge to override and accrete the North Helvetic flysch, which then started to form an SMT cleavage. The difference in X directions may reflect a change in transport direction, but this conclusion is difficult to accept since extension was minor and was accommodated by coaxial flattening, and not simple shear. Our work indicates that mass-loss volume strains were important in sandstones of the Helevtic nappes. The missing mass cannot be accounted for at the local scale, and appears to have been transported beyond the Helvetic zone 1. Introduction turbidites of the infrahelvetics to trench-fill turbidites The glarus thrust is the sole thrust of the helvetic formed at ocean-continent subduction zones. An nappes and a conspicuous feature in the landscape of important distinction is that both the Infrahelvetics and the Glarus Alps(Fig. 1). In 1841, Arnold Escher von Helvetics were originally underlain by european der linth discovered the glarus thrust but was continental crust, and not by oceanic crust reluctant to publish his observations: " No one would The Glarus thrust itself is marked by a <l-m thick believe me, they would put me into an asylum"(p. 195 layer of highly sheared calc-mylonite(Lochseitenkalk) in Greene, 1982). Further work by Marcel Bertrand (e.g. Trumpy, 1969; Hsu, 1969; Schmid, 1975),which 1884), Edward Suess(1904, 1909), and Albert Heim separates Eocene-Oligocene turbidites below the (1919)established the geometry and origin of this thrust from conglomerate and mudstone of the impressive structure. Bertrand's publication in 1884 is Permian Verrucano formation above(Heim, 1919 fig widely regarded as marking the birth of Alpine nappe 33 in Trumpy, 1980). The Lochseitenkalk itself theory (trumpy, 1998), with the Glarus thrust probably derived from Jurassic limestone(Schmid 1975)and was carried up and over the footwall of the thrust fault Glarus thrust. Ductile shearing was certainly important The Glarus thrust separates the Infrahelvetic during nappe transport, but significant brittle slip must complex in its footwall from the Helvetic nappes in its have occurred as well. The reason is that the break hanging wall. Both units were derived from the beneath the Lochseiten marks a major stratigraphic Helvetic zone, which refers to the Mesozoic passive discontinuity, with Jurassic limestone and Permian margin that bordered the southern side of the Verrucano overlying Eocene North Helvetic flysch European continent. The underlying Infrahelvetic Thus, localized slip is needed to account for the complex is distinguished by a thick sequence of syn- observed stratigraphic offset orogenic turbidites, and underlying Mesozoic platform The Glarus thrust is folded into a broad east-west carbonates of the European margin The turbidites are trending antiform with gently dipping limbs(Schmid locally volcanoclastic(. g, Taveyannaz sandstone) 1975). Geologic estimates require >50 km of offset on Sinclair(1992)compared the Eocene and Oligocene the Glarus thrust(Milnes and Pfiffner, 1980). 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 <1-m thick layer of highly sheared calc-mylonite (Lochseitenkalk) (e.g. Trümpy, 1969; Hsü, 1969; Schmid, 1975), which separates Eocene-Oligocene turbidites below the thrust from conglomerate and mudstone of the Permian Verrucano formation above (Heim, 1919; fig. 33 in Trümpy, 1980). The Lochseitenkalk itself probably derived from Jurassic limestone (Schmid, 1975) and was carried up and over the footwall of the Glarus thrust. Ductile shearing was certainly important during nappe transport, but significant brittle slip must have occurred as well. The reason is that the break beneath the Lochseiten marks a major stratigraphic discontinuity, with Jurassic limestone and Permian Verrucano overlying Eocene North Helvetic flysch. Thus, localized slip is needed to account for the observed stratigraphic offset. The Glarus thrust is folded into a broad, east-west trending antiform with gently dipping limbs (Schmid, 1975). Geologic estimates require >50 km of offset on the Glarus thrust (Milnes and Pfiffner, 1980). Models
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