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 thick3 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 “back￾stepping” 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 “in￾sequence” or perhaps more precisely, “forward￾stepping”. “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 back￾stepping 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 Calanda￾phase 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