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 removal8 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 medium￾grain 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