sign in sign up
<<
Home , Glarus Alps, Glarus thrust, Infrahelvetic complex, Mylonite

Published in Terra Nova 12 : 281-288, 2000, 1 which should be used for any reference to this work Brittle±ductile deformation in the thrust Lochseiten (LK) calc-mylonite

Nicolas P. Badertscher* and Martin Burkhard Institut de GeÂologie, 11 Rue Emile-Argand, case postale 2, 2007 Neuchaà tel,

ABSTRACT The accommodated at least 30 km of northward footwall and expelled along the thrust, are responsible for displacement strongly localized within a 1-m layer of `Lochse- hydrofracturing and cataclastic deformation. In periods iten' (LK) calc-mylonite. This layer displays veins in various between events, deformation was ductile. In this new states of plastic deformation and a wildly refolded foliated interpretation, a substantial amount of the total thrust dis- gouge texture. Lattice- and shape-preferred orientations are placement was accommodated by numerous short-lived and observed within the ®ne-grained, recrystallized matrix. These strongly localized fracture events at the base of the Verrucano features indicate the alternate activity of brittle and ductile thrust sheet, rather than a permanently weak de collement deformation mechanisms. In contrast to the classical view that lithology. grain boundary sliding (superplasticity) is the dominant defor- mation mechanism, it is advocated that ¯uids, derived from the

strains `without necking' (Fig. 1) and the whole was Introduction as a solution to the mechanical para- intensely folded and imbricated. Large overthrusts have been recog- dox of large overthrusts (Schmid Thrusting of the Glarus (Ruchi nized since the end of the 19th et al., 1981; Schmid, 1982a). Earlier phase) postdates these deformations century, but a mechanical paradox speculations about the mechanics of (P®€ner, 1977) in an out-of-sequence quickly appeared because the motion the Glarus thrust involved lithostatic manner. of large masses along low-angle ¯uid pressures (HsuÈ , 1969). Based on ranges from anchiz- thrust faults seemed mechanically strongly altered stable isotope signa- one in the North to lower greenschist impossible, or restricted to distances tures within the LK and structural facies in the South (Frey, 1988; Rahn smaller than about 10 km (e.g. HsuÈ , observations, Burkhard and Kerrich et al., 1995). The peak of this meta- 1969). Proposals to solve this problem (1990) and Burkhard et al. (1992) morphism postdates the include: (i) large strength contrasts proposed a veiny origin for most, if phase deformations (Groshong et al., between exceedingly weak rocks in the not all of the mylonite calcite. 1984) estimated at 30±25 Ma. The de collement horizon and much stron- This contribution presents new `anchi-/epizone-boundary' is offset ger ones within the thrust sheet; structural and microstructural obser- along the Glarus thrust by about (ii) propagation of incremental slip vations relevant to the discussion of 2 km (Groshong et al., 1984, Fig. 3; domains; and (iii) close to lithostatic the origin and structural evolution of Frey, 1988) as the result of postpeak- ¯uid pressures within the de collement the LK and the mechanics of this large metamorphic thrusting between 25 horizon (e.g. Schmid, 1975; Price, overthrust. and 20 Ma (Hunziker et al., 1986). 1988; Henry and Le Pichon, 1991; Twiss and Moores, 1992). Geological setting Macroscopic observations Displacement along the Glarus at the Glarus thrust contact thrust is at least 30 km (P®€ner, Tectonic units at the front of the 1985) (see Fig. 2). This thrust is a in (Fig. 2) are The Glarus thrust is characterized by very well de®ned underlined by a subdivided into a ` complex' the presence of a continuous thin layer thin layer of the famous `Lochseiten- above and an `Infra-Helvetic complex' (20 cm to 5 m) of calc-mylonite (LK) kalk' (LK) (Heim, 1921). Classically, below the Glarus thrust (P®€ner, and a strongly asymmetrical strain the LK is interpreted to be smeared 1981; P®€ner, 1993). The Helvetic gradient away from the contact. out Mesozoic carbonates from foot- Glarus nappe comprises Thrust-related deformations are virtu- wall and/or hanging wall (Schmid, Verrucano overlain by a concordant ally absent 2±5 m below the contact, 1975; P®€ner, 1982). Superplastic ¯ow Mesozoic series. The `Infrahelvetic' whereas strong mylonitic foliations, has been proposed as the dominant complex consists of a crystalline base- subparallel to the main thrust, can be within the ment overlain by a sedimentary cover observed tens of metres above it and LK to explain the extremely high of Mesozoic carbonates and Tertiary fading out gradually into the hanging and some South Helvetic and wall. In this paper, attention is *Correspondence: Nicolas P. Badertscher, (Sardona) Flysch. The latter focused on structures within the LK; Institut de Ge ologie, 11 Rue Emile- were emplaced onto the parautochth- for a complete description of the Argand, case postale 2, 2007 NeuchaÃtel, onous carbonates in early Oligocene structures in the footwall and hanging Switzerland. Tel: +41/32 7182600; Fax: times during the `Pizol phase' wall (Fig. 3) see Schmid (1975), +41/32 7182601; E-mail: nicolas.badert- (P®€ner, 1977). In a second, main Siddans (1979), Burkhard et al. [email protected] deformation stage (Calanda phase), (1992) and Lihou (1996). Schmid 2

Fig. 1 Superplasticity has been proposed as an explanation for the extreme strain localization in the Lochseiten calc-mylonite (Schmid et al., 1977). (a) Superplastically deformed Pb±62%Sn alloy in a metallurgical stretching experiment from Langdon (1982). (b) Schematic cross-section of the Glarus thrust. A metre-thick calc-mylonite connects Mesozoic carbonates in the footwall and hanging wall over a total thrusting distance of about 30 km N±S; drawn to scale, the calc-mylonite would measure less than 2 lm thick in this ®gure.

Fig. 2 Tectonic overview of the central, accessible portion of the Glarus overthrust (modi®ed from Oberholzer, 1933). Some of the key localities mentioned in the text are indicated for their relative position in a NNW±SSE pro®le. North of the type locality `Lochsite' the thrust surface plunges below topography; it re-emerges some 15 km further north.

(1975) and Burkhard et al. (1992) The lower contact of the LK with ance (Fig. 5B) with a locally pervasive described the turbulent appearance the ¯ysch is strongly cuspate-lobate and well-developed stretch- of the LK owing to the refolded (Schmid, 1975) (Fig. 3). The upper ing (see also P®€ner, 1982). alternation of white, pure calcite and contact with the Verrucano, as well as In a few places, lenses of Verrucano black stylolitic layers. This banding lower contacts with Mesozoic carbon- are trapped within the LK. They can locally de®nes sheath folds and very ates (in the South) are irregular too, be interpreted either as slivers of complex 3D patterns (see Fig. 5A, but lobes and cusps have smaller tectonically emplaced Verrucano with- left-hand side). Neither pervasive amplitudes. The pattern of cusps and in the LK, or as `islands' of Verrucano schistosity nor stretching lineation lobes seems to indicate a higher vis- isolated from their surrounding coun- are observed within the classic LK cosity for the LK than for the ¯ysch or try rock by the addition of massive (Schmid, 1975). The so-called `sep- Verrucano. This observation, con- calcite veins, a€ected by strong ductile tum' is a conspicuous extremely thin ®rmed by the of LK in overprinting and folding near the (mm to cm) planar horizon crosscut- the ¯ysch, contrasts with the expected basal thrust contact. West of Riseten- ting all internal structures of the LK. weak behaviour of the calc-mylonite pass, the LK deviates signi®cantly It consists either of gouge or a very (see Schmid, 1975). from its ordinary planar con®gur- sharp zone and has been inter- In the south, where the footwall ation, describing a N-vergent of preted to have resulted from some consists of Mesozoic carbonates, LK metric amplitude with ¯ysch in the modest, late motion of the Glarus seems to be derived from the former core. This fold demonstrates nappe (Schmid, 1975). and has a generally smoother appear- clearly that ductile deformation con- 3

1 VERRUCANO 2 4 LOCHSITENKALK

3 2 6 SEPTUM

5 7 8 FLYSCH 5 9

Fig. 3 Schematic overview of macroscopically observed structures within the LK and its contacts with ¯ysch in the footwall and Verrucano in the hanging wall (modi®ed after Burkhard et al., 1992). In the ¯ysch, a steeply dipping foliation (Calanda phase) can be transposed totally by a younger (9) within the last few metres below the contact (Ruchi phase). In the Verrucano a well-developed foliation, subparallel to the thrust, is increasingly mylonitic towards the contact. Stretching lineations in the hanging wall (1) have a very constant N±S direction (compare Siddans, 1979). (3) within the Verrucano immediately above the contact have N±S-orientated fold axes, parallel to the stretching lineation. Further up, however, crenulations are orientated E±W. Abundant C¢ shear bands (4) and asymmetric strain fringes (2) on pyrite grains consistently indicate thrusting towards the North. Structures in the LK (see also text) include: 5, cuspate/lobate contacts; 6, sheath folds; 7, `septum'; 8, internal banding: strongly folded, sheared and crumpled and former veins.

Fig. 4 Photomicrographs of Lochseiten calc-mylonites: (a) sample from Grau Berg (736.300/192.900), south; (b) foliated gouge structure in a sample from Lochsite (725.860/206.400), north. tinued after the formation of some 1998, Fig. 16; and our Fig. 4b). The latest veins are often fractured and planar LK (see also Schmid, 1975). In foliation is underlined by the alterna- sheared (Fig. 4a). some places, the LK±Verrucano con- tion of dark stylolitic and light col- tact exhibits macroscopic evidence of oured layers. The light layers can Optical microscopy on ultra brecciation and cataclasis (e.g. at often be identi®ed as former veins thin sections and Pizol) (Fig. 5A, variably fractured, folded and sheared right-hand side). (see also Burkhard et al., 1992). Sev- Microstructures on the grain scale eral generations of veins can be within the LK mylonite are observed distinguished. In the southern areas, in ultra thin sections (< 3 lm) where Microscopic observations the structure is smoother and locally a calcite appears in di€erent shades of true planar foliation is developed grey. Grain size ranges from Thin-section observations (Fig. 4a). However, former white > 100 lmto<1lm. Coarse grains At low magni®cation, typical LK veins parallel to this foliation can still clearly belong to veins and are always from the northern areas exhibits the be recognized. In both types of LK, heavily twinned (type III or IV accord- chaotic structure of a foliated and younger veins crosscut the general ing to Burkhard, 1993) (Fig. 6a,b). folded (cf. Snoke et al., structure at high angles, even these Intermediate size grains occur in 4

isolation within a matrix of very ®ne mation and associated recrystalliza- twinned, as already pointed out by grains (< 5 lm). Vein calcite grains tion causes coarse and intermediate Schmid et al. (1981). display variable degrees of disruption grains to be progressively replaced Matrix grains often show very and exhibit ample evidence for by very small `matrix' grains. In strong Lattice-Preferred Orientation dynamic recrystallization (Fig. 6a,b) contrast to the strongly sutured grain (LPO), as visualized through the use in the form of subgrains, sutured boundaries of large and intermediate of a gypsum plate (Fig. 5C,D) and grain boundaries and a mortar struc- grains, very ®ne grains have more documented by semiquantitative, ture. Grain boundary migration regular, smoother grain boundaries, photometric analyses (Price, 1973) (GBM) is observed clearly on but bulging still documents GBM (Fig. 5E). Along the ultra-thin edges thick twins (type IV according to (Fig. 6c,d). Even very small grains of of thin sections, LPOs can be seen to Burkhard, 1993). Increasing defor- less than 2 lm are occasionally be coherent on the scale of a few 5

Fig. 5 Structural and microstructural observations in LK mylonites. (A) Left-hand side: complex structures observed in the LK from northern exposures include sheath folds, shear bands and `crumpling' of the alternation of white, pure calcite and black, stylolitic layers. Sample from (738.150/197.600). Right-hand side: Contact LK-Verrucano at Ringelspitz (745.200/ 195.800), the Verrucano is brecciated and cut by the septum consisting of a cataclasite (lower contact). (B) Glarus thrust at Plaun Grond (730.3/192) with LK representative of southern localities. General banding in di€erent colours results from strongly deformed light coloured veins, dark stylolites and traces of secondary dolomite responsible for the orange/yellow alteration colour at the LK-Verrucano (green) contact. (C) Ultrathin (< 3 lm) edges of a LK thin section from KaÈ rpf (726.3/196.95) display strong lattice-preferred orientation visualized through the use of a gypsum plate (530 nm) and crossed polarizers. Note horizontal banding. (D) Close-up of the same sample as shown in (C). Average grain size of matrix calcite is less than 3 lm; note fairly straight grain boundaries and the absence of a shape preferred orientation. The large ameboid orange grain possibly represents the relic of a former . (E) Photometrically determined rose diagram illustrates the preferred orientation of calcite c-axes (projected onto the plane of observation) measured in six different spots with 200 lm diameter along the ultrathin edge of the section shown in C. (F) Cold cathodoluminescence reveals the presence of an old and sheared ghost vein in the calc-mylonite sample from the Grau Berg locality (South). Three successive generations of veins are easily distinguished in CL. Dark red grains along the second vein are quartz. (G) Same thin section as F, seen in normal light, displays a homogeneous ®ne-grained calcite matrix in the places where CL reveals the presence of ghost vein.

Fig. 6 (a) Heavily twinned coarse calcite grains of veiny origin. Recrystallized `matrix' grains nucleate along twin- and grain- boundaries resulting in a mortar texture. Sample from KaÈ rpf (726.250/197), thin section view, crossed polarizers. (b) idem, SEM view. (c) Shape-preferred orientation in an LK sample from Pign (732.25/192.525), southern part of the Glarus thrust, thin section view, crossed polarizers. (d) idem, SEM view. millimetres across domains with very orange and yellow as a consequence of geochemical vein signature despite ®ne-grained matrix and coarser minute substitutions of Ca2+ by Mn2+ intense folding and shearing and grains. No Shape-Preferred Orienta- and the presence of other trace associated dynamic recrystallization. tions (SPO) could be detected in the elements (Barbin and Schvoerer, Diffuse boundaries (Fig. 5F,G) of such very ®ne-grained matrix in the north, 1997). CL microscopy reveals the pres- ghost veins are thought to be the result whereas weak SPO with mean axial ence of an unexpected composite layer- of chemical diffusion associated with ratios of up to 2 are developed in the ing within optically homogeneous GBM (Hay and Evans, 1987). south (Fig. 6c,d). veins. Most interestingly, CL provides evidence for completely recrystallized Discussion and conclusions `ghost' veins within the ultra®ne- Cathodoluminescence on thin sections grained matrix (Fig. 5F,G). Optically Based on experimental data and the In cold cathodoluminescence (CL), nondetectable, such ghost veins absence of clear LPOs within LK calcite appears in various shades of apparently preserved some subtle mylonite samples, Schmid proposed 6

grain boundary sliding (superplastic ®ne-grained matrix, should not allow (Kennedy and Logan, 1997; Kennedy ¯ow) as the dominant deformation the preservation of strong LPOs. et al., 1998), and the Gavarnie thrust mechanism to account for the extreme Alternatively, when measured inte- in the Pyrenees (McCaig et al., 1995). strain localization observed at the grally on large (cm-size slides) by The paradox of the lobate±cuspate base of the Glarus thrust (Schmid XRD techniques, LPOs may be contact, which suggests a competent et al., 1977; Schmid et al., 1981; `diluted' by the rotation of mm- to LK sandwiched in between less Schmid, 1982a). In this interpretation, cm-size rock fragments, which are comptetent Verrucano and ¯ysch, the LK is considered a very weak apparent in the form of `chaotic' ®nds an elegant solution in the fault- de collement layer on which the Glarus foliated gouge textures (Figs 4b, 5A). valve/seismic failure scenario. Frac- nappe was translated as a rigid block. The Glarus thrust roots at mid- turing and calcite mineralizations Burkhard et al. (1992) provided stable crustal levels (P®€ner, 1985). Dewa- along the base of the Verrucano thrust isotope evidence for considerable ¯uid tering by and prograde sheet are triggered by ¯uctuations in advection during thrusting and there- metamorphism in the footwall pro- ¯uid pressure, because ¯uids are fore speculated about the role of such duce considerable amounts of ¯uids stored below this `permeability bar- ¯uids in deformation (cf. Bowman with a general tendency to escape rier'. The planar septum horizons, et al., 1994). Meso- and microstruc- upward towards the foreland (Oliver, and veins re¯ect some of tural observations document the 1986; Marquer and Burkhard, 1992). the latest brittle events. Background alternating activity of brittle and crys- The role of such ¯uids in deformation deformation of the entire thrust zone tal-plastic deformation processes and depends critically on the ¯uid produc- in the ductile regime is responsible for the omnipresence of dissolution-cry- tion rate and permeability in the the repeated shearing and folding of stallization processes. These observa- surrounding rock masses (Connolly older septums, veins and the LK±wall tions are dif®cult to reconcile with the and Thompson, 1989; Gueguen et al., rock contacts, as well as mylonitiza- idea of a single, dominant deforma- 1991). It is proposed herein that tion of the Verrucano and folding of tion mechanism such as the superplas- an abrupt change in permeability the topmost metres of ¯ysch. The ticity proposed by Schmid et al. (1977, between the footwall and hanging wall relative contributions of seismic and 1981) and Schmid (1982a). Brittle lead to ¯uid channelling at this par- plastic deformation to the total thrust deformation features have been noted ticular thrust contact. In this scenario, translation are dicult to evaluate. In by earlier authors (Heim, 1921; Sch- ¯uids produced in the footwall perco- the present interpretation, the thrust mid et al., 1981; P®ffner, 1982) but late continuously upward, preferen- translation was accommodated mostly generally such observations were dis- tially along the steeply inclined by numerous seismic slip events. In carded as unimportant, late overprint- pre-existing (Calanda phase) foliation. summary, the Glarus nappe with its ing. However, there is no evidence The clay-rich Verrucano thrust sheet, enigmatic LK, is a good candidate for for an evolution from plastic to brit- with actively forming subhorizontal a major which owes its tle deformation with time. Foliated foliation, represents a permeability localization and apparent softening gouge textures and overprinted vein barrier to this percolation. In analogy to transiently near lithostatic ¯uid bands are more obvious in northern to the fault-valve scenario (Sibson, pressures (Etheridge et al., 1984; than southern localities. This, how- 1990), ¯uid pressures would increase Carter and Dworkin, 1990; Carter ever, can be interpreted in terms of a below this contact up to the threshold et al., 1990; Henry and Le Pichon, temperature gradient up-dip, rather for hydrofracturing associated with 1991), rather than to the presence of than re¯ecting an evolution through seismic slip (Sibson, 1990; Petit et al., a permanently weak de collement time. Ductile deformation and 1999). As a result of dilatency, ¯uid lithology. dynamic recrystallization were merely pressures would drop abruptly to more ef®cient in wiping out the evi- values lower than within wall rock. Acknowledgements dence for brittle deformations further Healing of the fracture network and south, rather than being more import- formation of veins would progres- We would like to thank F. Gainon, ant in terms of their contribution to sively seal the thrust fault, allowing N. Challandes and A. Aubry for their help total strain and thrust translation. ¯uid pressures to build up again. with ®eld work and sampling; K. Ramseyer The absence of LPOs was a central Between fracture events, ductile intra- for CL imagery. A. M. McCaig and argument used by Schmid et al. crystalline deformation and dynamic R. Abart are thanked for stimulating discussions and active collaboration. We (1977) in favour of superplasticity. recrystallization would result in the acknowledge thorough and constructive The present observations of strong formation of a microcrystalline matrix reviews by S. M. Schmid and B. den Brok. LPOs seen in optical microscopy within the LK and of LPOs within this This study is supported by the Swiss (Fig. 5C±E) contrast with the lack matrix. Thrust faults do not have National Science Foundation Grants of LPOs in the XRD-samples ana- the ideal orientation for such fault- 20-50535.97 and 20-56920.99. lysed by Schmid et al. (1981). It valve behaviour (Sibson, 1990; Nguyen could be argued that these optically et al., 1998). However, similar struc- References visible LPOs are a local phenomenon, tural observations and interpretations inherited from other LPOs resulting have been reported from other thrusts Barbin, V. and Schvoerer, M., 1997. from twinning of some coarse vein in the Apennines (Coli and Sani, Cathodoluminescence et ge osciences. grains that have totally recrystallized. 1990), the McConnell thrust in the Earth Planet. Sci., 325, 157±169. However, according to Walker et al. Canadian Rockies and the Hunter Bowman, J.R., Willett, S.D. and Cook, S.J., 1994. Oxygen isotopic transport (1990), grain boundary sliding in the Valley thrust in the Appalachians 7

and exchange during ¯uid ¯ow, one- calcite; temperature dependence, phe- Petit, J.-P., Wibberley, C.A.J. and Ruiz, dimensional models and applications. nomenology, and possible applications G., 1999. `Crack-seal' slip: a new fault- Am. J. Sci., 294, 1±55. to geologic systems. Contr. Miner. Pet- valve mechanism. J. Struct. Geol., 21, Burkhard, M., 1993. Calcite twins, their rol., 97, 127±141. 1199±1207. geometry, appearance and signi®cance as Heim, A., 1921. Geologie der Schweiz. Band P®€ner, O.A., 1977. Tektonische Unter- -strain markers and indicators of 2: die Schweizer Alpen 1. HaÈlfte. Tauch- suchungen im Infrahelvetikum der tectonic regime; a review. J. Struct. niz, Leipzig. Ostschweiz. Translated title: Tectonic Geol., 15, 351±368. Henry, P. and Le Pichon, X., 1991. Fluid studies of the Infrahelvetic Complex, Burkhard, M. and Kerrich, R., 1990. ¯ow along a decollement layer; a model eastern Switzerland. Techn Hochsch. Fluid±rock interactions during thrusting applied to the 16 degrees N section of the Geol. Inst. Mitt., 217, 164. of the Glarus Nappe; evidence from Barbados . J. Geo- P®€ner, O.A., 1981. Fold- and -thrust geochemical and stable isotope data. phys. Res. B, 96, 6507±6528. in the Helvetic Schweiz. Miner. Petrol. Mitt. ( ˆ Bull. HsuÈ , K.J., 1969. A preliminary Anlysis of (E. Switzerland). In: Thrust and Nappe Suisse Min. PeÂt.), 70, 77±82. the Statics and Kinetics of the Glarus Tectonics (R. Price, ed.), pp. 319±327. Burkhard, M., Kerrich, R., Maas, R. and Overthrust. Eclog. Geol. Helv., 62, The Geological Society of London, Fyfe, W.S., 1992. Stable and Sr-isotope 143±154. London. evidence for ¯uid advection during Hunziker, J.C., Frey, M., Clauer, N. et al., P®€ner, O.A., 1982. Deformation mech- thrusting of the Glarus nappe (Swiss 1986. The evolution of illite to musco- anisms and ¯ow regimes in limestones Alps). Contr. Miner. Petrol., 112, vite; mineralogical and isotopic data from the Helvetic Zone of the Swiss 293±311. from the , Switzerland. Alps. J. Struct. Geol., 4, 429±442. Carter, K.E. and Dworkin, S.I., 1990. Contr. Miner. Petrol., 92, 157±180. P®€ner, O.A., 1985. Displacements along Channelized ¯uid ¯ow trough shear Kennedy, L.A. and Logan, J.M., 1997. The thrust faults. Eclog. Geol. Helv., 78, zones during ¯uid-enhanced dynamic role of veining and dissolution in the 313±333. recrystallization, Northern Apennines, evolution of ®ne-grained mylonites; the P®€ner, O.A., 1993. The structure of the . Geology, 18, 720±723. McConnell Thrust, Alberta. J. Struct. and its relation to the Carter, N.L., Kronenberg, A.K., Ross, J.V. Geol., 19, 785±797. mechanical stratigraphy. J. Struct. Geol., and Wiltschko, D.V., 1990. Control of Kennedy, L.A., Logan, J.M., Chester, 15, 511±521. ¯uids on deformation of rocks. F.M.E., Engelder, T.E. and Shimamoto, Price, G.P., 1973. The photometric method In: Deformation Mechanisms, Rheology T.E., 1998. Microstructures of catacla- in microstructural analysis. Am. J. Sci., and Tectonics (R.J. Knipe and sites in a limestone-on-shale thrust fault; 273, 523±537. E.H. Rutter, eds). Spec. Publ. Geol. Soc. implications for low-temperature recrys- Price, R.A., 1988. The mechanical paradox London, 54, 1±13. tallization of calcite. Rock deformation; of large overthrusts. Bull. Geol. Soc. Coli, M. and Sani, F., 1990. Vein distri- the Logan Volume. Tectonophysics, 295, Am., 100, 1898±1908. bution in a thrust zone: a case history 167±186. Rahn, M., Mullis, J., Erdelbrock, K. and from the Northern Apennines, Italy. In: Langdon, T.G., 1982. The mechanical Frey, M., 1995. Alpine metamorphism in Deformation Mechanisms, Rheology and properties of superplastic materials. the North Helvetic Flysch of the Glarus Tectonics (R.J. Knipe and E.H. Rutter, Mettalurg. Trans., 13A, 689±701. Alps, Switzerland. Eclog. Geol. Helv., 88, eds). Spec. Publ. Geol. Soc. London, 54, Lihou, J.C., 1996. Structure and deforma- 157±178. 475±482. tional history of the Infrahelvetic Flysch Schmid, S.M., 1975. The Glarus over- Connolly, J.A.D. and Thompson, A.B., units, Glarus Alps, eastern Switzerland. thrust; ®eld evidence and mechanical 1989. Fluid and enthalpy production Eclog. Geol. Helv., 89, 439±460. model. Eclog. Geol. Helv., 68, 247±280. during regional metamorphism. Contr. Marquer, D. and Burkhard, M., 1992. Schmid, S.M., 1982a. Microfabric studies Miner. Petrol., 102, 347±366. Fluid circulation, progressive deforma- as Indicators of Deformation Mech- Etheridge, M.A., Wall, V.J., Cox, S.F. and tion and mass-transfer processes in the anisms and Flow Laws Operative in Vernon, R.H., 1984. High ¯uid pressures upper crust; the example of basement- Mountain Building. In: Mountain Build- during regional metamorphism and cover relationships in the External ing Processes (K.J. HsuÈ , ed.), pp. 95±110. deformation; implications for mass Crystalline Massifs, Switzerland. Academic Press, London. transport and deformation mechanisms. J. Struct. Geol., 14, 1047±1057. Schmid, S.M., Boland, J.M. and Paterson, J. Geophys. Res., 89, 4344±4358. McCaig, A.M., Wayne, D.M., Marshall, M.S., 1977. Superplastic ¯ow in ®ne- Frey, M., 1988. Discontinuous inverse J.D., Banks, D. and Henderson, I., 1995. grained limestone. Tectonophysics, 43, metamorphic zonation, Glarus Alps, Isotopic and ¯uid inclusion studies of 257±291. Switzerland; evidence from illite `crys- ¯uid movement along the Gavarnie Schmid, S.M., Casey, M. and Starkey, J., tallinity' data. Schweiz. Miner. Petrol. Thrust, central Pyrenees; reaction fronts 1981a. The microfabric of calcite tecto- Mitt. ( ˆ Bull. Suisse Min. PeÂt.), 68, in carbonate mylonites. Am. J. Sci., 295, nites from the Helvetic Nappes (Swiss 171±183. 309±343. Alps). In: Thrust and Nappe Tectonics Groshong, R.H., P®€ner, O.A. and Prin- Nguyen, P.T., Cox, S.F., Harris, L.B. and (R. Price, ed.), pp. 151±158. The Geolo- gle, L.R., 1984. in the Powell, C.M., 1998. Fault-valve beha- gical Society of London, London. Helvetic thrust belt of eastern Switzer- viour in optimally oriented shear zones; Sibson, R.H., 1990. Conditions for fault- land from the leading edge to the inter- an example at the Revenge gold mine, valve behaviour. In: Deformation Mech- nal zone. J. Struct. Geol., 6, 5±18. Kambalda, Western Australia. J. Struct. anisms, Rheology and Tectonics (R.J. Gueguen, Y., David, C., Gavrilenko, P. Geol., 20, 1625±1640. Knipe and E. H. Rutter, eds). Spec. and Torgersen, T.P., 1991. Percolation Oberholzer, J., 1933. Geologie der Glarner Publ. Geol. Soc. London, 54, 15±28. networks and ¯uid transport in the crust. Alpen. Beitr. Geol. Karte Schweiz., F28. Siddans, A.W.B., 1979. Deformation, In: Crustal-Scale Fluid Transport; Oliver, J., 1986. Fluids expelled tectonically metamorphism and texture development Magnitude and Mechanisms, vol. 18, from orogenic belts; their role in hydro- in Permian mudstones of the Glarus pp. 931±934. Snowbird, UT. carbon migration and other geologic Alps (eastern Switzerland). Eclog. Geol. Hay, R.S. and Evans, B., 1987. Chemically phenomena. Geology, 14, 99±102. Helv., 72, 601±621. induced grain boundary migration in 8

Snoke, A.W., Tullis, J. and Todd, V.R., E.M. Moores, eds), pp. 206±213. Free- (R.J. Knipe and E.H. Rutter, eds). Spec. 1998. Fault-Related Rocks: a Photogra- man, New York. Publ. Geol. Soc. London, 54, 259±284. phic Atlas. Princeton University Press, Walker, A.N., Rutter, E.H. and Brodie, NJ. K.H., 1990. Experimental study of grain- Received 19 November 1999; revised version Twiss, R.J. and Moores, E.M., 1992. The size sensitive ¯ow of synthetic, hot- accepted 27 February 2001 Mechanics of Large Overthrusts. pressed calcite rocks. In: Deformation In: (R.J. Twiss and Mechanisms, Rheology and Tectonics