Brittle±ductile deformation in the Glarus thrust Lochseiten(LK) calc-mylonite

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

Introduction

Large overthrusts have been recog-nized since the end of the 19thcentury, but a mechanical paradoxquickly appeared because the motionof large rock masses along low-anglethrust faults seemed mechanicallyimpossible, or restricted to distancessmaller than about 10 km (e.g. HsuÈ ,1969). Proposals to solve this probleminclude: (i) large strength contrastsbetween exceedingly weak rocks in thede collement horizon and much stron-ger ones within the thrust sheet;(ii) propagation of incremental slipdomains; and (iii) close to lithostatic¯uid pressures within the de collementhorizon (e.g. Schmid, 1975; Price,1988; Henry and Le Pichon, 1991;Twiss and Moores, 1992).Displacement along the Glarus

thrust is at least 30 km (P®�ner,1985) (see Fig. 2). This thrust is avery well de®ned fault underlined by athin layer of the famous `Lochseiten-kalk' (LK) (Heim, 1921). Classically,the LK is interpreted to be smearedout Mesozoic carbonates from foot-wall and/or hanging wall (Schmid,1975; P®�ner, 1982). Superplastic ¯owhas been proposed as the dominantdeformation mechanism within theLK to explain the extremely high

strains `without necking' (Fig. 1) andas a solution to the mechanical para-dox of large overthrusts (Schmidet al., 1981; Schmid, 1982a). Earlierspeculations about the mechanics ofthe Glarus thrust involved lithostatic¯uid pressures (HsuÈ , 1969). Based onstrongly altered stable isotope signa-tures within the LK and structuralobservations, Burkhard and Kerrich(1990) and Burkhard et al. (1992)proposed a veiny origin for most, ifnot all of the mylonite calcite.This contribution presents new

structural and microstructural obser-vations relevant to the discussion ofthe origin and structural evolution ofthe LK and the mechanics of this largeoverthrust.

Geological setting

Tectonic units at the front of the Alpsin Eastern Switzerland (Fig. 2) aresubdivided into a `Helvetic complex'above and an `Infra-Helvetic complex'below the Glarus thrust (P®�ner,1981; P®�ner, 1993). The HelveticGlarus nappe comprises PermianVerrucano overlain by a concordantMesozoic series. The `Infrahelvetic'complex consists of a crystalline base-ment overlain by a sedimentary coverof Mesozoic carbonates and TertiaryFlysch and some South Helvetic andPenninic (Sardona) Flysch. The latterwere emplaced onto the parautochth-onous carbonates in early Oligocenetimes during the `Pizol phase'(P®�ner, 1977). In a second, maindeformation stage (Calanda phase),

the whole Infrahelvetic complex wasintensely folded and imbricated.Thrusting of the Glarus nappe (Ruchiphase) postdates these deformations(P®�ner, 1977) in an out-of-sequencemanner.Metamorphism ranges from anchiz-

one in the North to lower greenschistfacies in the South (Frey, 1988; Rahnet al., 1995). The peak of this meta-morphism postdates the Calandaphase deformations (Groshong et al.,1984) estimated at 30±25 Ma. The`anchi-/epizone-boundary' is offsetalong the Glarus thrust by about2 km (Groshong et al., 1984, Fig. 3;Frey, 1988) as the result of postpeak-metamorphic thrusting between 25and 20 Ma (Hunziker et al., 1986).

Macroscopic observationsat the Glarus thrust contact

The Glarus thrust is characterized bythe presence of a continuous thin layer(20 cm to 5 m) of calc-mylonite (LK)and a strongly asymmetrical straingradient away from the contact.Thrust-related deformations are virtu-ally absent 2±5 m below the contact,whereas strong mylonitic foliations,subparallel to the main thrust, can beobserved tens of metres above it andfading out gradually into the hangingwall. In this paper, attention isfocused on structures within the LK;for a complete description of thestructures in the footwall and hangingwall (Fig. 3) see Schmid (1975),Siddans (1979), Burkhard et al.(1992) and Lihou (1996). Schmid

ABSTRACT

The Glarus thrust accommodated at least 30 km of northwarddisplacement strongly localized within a 1-m layer of `Lochse-iten' (LK) calc-mylonite. This layer displays veins in variousstates of plastic deformation and a wildly refolded foliatedgouge texture. Lattice- and shape-preferred orientations areobserved within the ®ne-grained, recrystallized matrix. Thesefeatures indicate the alternate activity of brittle and ductiledeformation mechanisms. In contrast to the classical view thatgrain boundary sliding (superplasticity) is the dominant defor-mation mechanism, it is advocated that ¯uids, derived from the

footwall and expelled along the thrust, are responsible forhydrofracturing and cataclastic deformation. In periodsbetween fracture events, deformation was ductile. In this newinterpretation, a substantial amount of the total thrust dis-placement was accommodated by numerous short-lived andstrongly localized fracture events at the base of the Verrucanothrust sheet, rather than a permanently weak de collementlithology.

*Correspondence: Nicolas P. Badertscher,

Institut de Ge ologie, 11 Rue Emile-

Argand, case postale 2, 2007 Neuchaà tel,

Switzerland. Tel: +41/32 7182600; Fax:

+41/32 7182601; E-mail: nicolas.badert-

scher@unine.ch

Published in Terra Nova 12 : 281-288, 2000,which should be used for any reference to this work

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(1975) and Burkhard et al. (1992)described the turbulent appearanceof the LK owing to the refoldedalternation of white, pure calcite andblack stylolitic layers. This bandinglocally de®nes sheath folds and verycomplex 3D patterns (see Fig. 5A,left-hand side). Neither pervasiveschistosity nor stretching lineationare observed within the classic LK(Schmid, 1975). The so-called `sep-tum' is a conspicuous extremely thin(mm to cm) planar horizon crosscut-ting all internal structures of the LK.It consists either of gouge or a verysharp shear zone and has been inter-preted to have resulted from somemodest, late motion of the Glarusnappe (Schmid, 1975).

The lower contact of the LK withthe ¯ysch is strongly cuspate-lobate(Schmid, 1975) (Fig. 3). The uppercontact with the Verrucano, as well aslower contacts with Mesozoic carbon-ates (in the South) are irregular too,but lobes and cusps have smalleramplitudes. The pattern of cusps andlobes seems to indicate a higher vis-cosity for the LK than for the ¯ysch orVerrucano. This observation, con-®rmed by the boudinage of LK inthe ¯ysch, contrasts with the expectedweak behaviour of the calc-mylonite(see Schmid, 1975).In the south, where the footwall

consists of Mesozoic carbonates, LKseems to be derived from the formerand has a generally smoother appear-

ance (Fig. 5B) with a locally pervasivefoliation and well-developed stretch-ing lineation (see also P®�ner, 1982).In a few places, lenses of Verrucano

are trapped within the LK. They canbe interpreted either as slivers oftectonically emplaced Verrucano with-in the LK, or as `islands' of Verrucanoisolated from their surrounding coun-try rock by the addition of massivecalcite veins, a�ected by strong ductileoverprinting and folding near thebasal thrust contact. West of Riseten-pass, the LK deviates signi®cantlyfrom its ordinary planar con®gur-ation, describing a N-vergent fold ofmetric amplitude with ¯ysch in theanticline core. This fold demonstratesclearly that ductile deformation con-

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 footwalland hanging wall over a total thrusting distance of about 30 km N±S; drawn to scale, the calc-mylonite would measure less than2 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 thekey 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.

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tinued after the formation of someplanar LK (see also Schmid, 1975). Insome places, the LK±Verrucano con-tact exhibits macroscopic evidence ofbrecciation and cataclasis (e.g. atRingelspitz and Pizol) (Fig. 5A,right-hand side).

Microscopic observations

Thin-section observations

At low magni®cation, typical LKfrom the northern areas exhibits thechaotic structure of a foliated andfolded cataclasite (cf. Snoke et al.,

1998, Fig. 16; and our Fig. 4b). Thefoliation is underlined by the alterna-tion of dark stylolitic and light col-oured layers. The light layers canoften be identi®ed as former veinsvariably fractured, folded and sheared(see also Burkhard et al., 1992). Sev-eral generations of veins can bedistinguished. In the southern areas,the structure is smoother and locally atrue planar foliation is developed(Fig. 4a). However, former whiteveins parallel to this foliation can stillbe recognized. In both types of LK,younger veins crosscut the generalstructure at high angles, even these

latest veins are often fractured andsheared (Fig. 4a).

Optical microscopy on ultrathin sections

Microstructures on the grain scalewithin the LK mylonite are observedin ultra thin sections (< 3 lm) wherecalcite appears in di�erent shades ofgrey. Grain size ranges from> 100 lm to < 1 lm. Coarse grainsclearly belong to veins and are alwaysheavily twinned (type III or IV accord-ing to Burkhard, 1993) (Fig. 6a,b).Intermediate size grains occur in

SEPTUM

VERRUCANO

LOCHSITENKALK

FLYSCH

12

23

7

4

5

5

8

9

6

Fig. 3 Schematic overview of macroscopically observed structures within the LK and its contacts with ¯ysch in the footwall andVerrucano in the hanging wall (modi®ed after Burkhard et al., 1992). In the ¯ysch, a steeply dipping foliation (Calanda phase) canbe transposed totally by a younger crenulation cleavage (9) within the last few metres below the contact (Ruchi phase). In theVerrucano a well-developed foliation, subparallel to the thrust, is increasingly mylonitic towards the contact. Stretching lineationsin the hanging wall (1) have a very constant N±S direction (compare Siddans, 1979). Crenulations (3) within the Verrucanoimmediately 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 consistentlyindicate 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 stylolites and former veins.

Fig. 4 Photomicrographs of Lochseiten calc-mylonites: (a) sample from Grau Berg (736.300/192.900), south; (b) foliated gougestructure in a sample from Lochsite (725.860/206.400), north.

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isolation within a matrix of very ®negrains (< 5 lm). Vein calcite grainsdisplay variable degrees of disruptionand exhibit ample evidence fordynamic recrystallization (Fig. 6a,b)in the form of subgrains, suturedgrain boundaries and a mortar struc-ture. Grain boundary migration(GBM) is observed clearly onthick twins (type IV according toBurkhard, 1993). Increasing defor-

mation and associated recrystalliza-tion causes coarse and intermediategrains to be progressively replacedby very small `matrix' grains. Incontrast to the strongly sutured grainboundaries of large and intermediategrains, very ®ne grains have moreregular, smoother grain boundaries,but bulging still documents GBM(Fig. 6c,d). Even very small grains ofless than 2 lm are occasionally

twinned, as already pointed out bySchmid et al. (1981).

Matrix grains often show verystrong Lattice-Preferred Orientation(LPO), as visualized through the useof a gypsum plate (Fig. 5C,D) anddocumented by semiquantitative,photometric analyses (Price, 1973)(Fig. 5E). Along the ultra-thin edgesof thin sections, LPOs can be seen tobe coherent on the scale of a few

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millimetres across domains with very®ne-grained matrix and coarsergrains. No Shape-Preferred Orienta-tions (SPO) could be detected in thevery ®ne-grained matrix in the north,whereas weak SPO with mean axialratios of up to 2 are developed in thesouth (Fig. 6c,d).

Cathodoluminescence on thin sections

In cold cathodoluminescence (CL),calcite appears in various shades of

orange and yellow as a consequence ofminute substitutions of Ca2+ byMn2+

and the presence of other traceelements (Barbin and Schvoerer,1997). CL microscopy reveals the pres-ence of an unexpected composite layer-ing within optically homogeneousveins. Most interestingly, CL providesevidence for completely recrystallized`ghost' veins within the ultra®ne-grained matrix (Fig. 5F,G). Opticallynondetectable, such ghost veinsapparently preserved some subtle

geochemical vein signature despiteintense folding and shearing andassociated dynamic recrystallization.Diffuse boundaries (Fig. 5F,G) of suchghost veins are thought to be the resultof chemical diffusion associated withGBM (Hay and Evans, 1987).

Discussion and conclusions

Based on experimental data and theabsence of clear LPOs within LKmylonite samples, Schmid proposed

Fig. 5 Structural and microstructural observations in LK mylonites. (A) Left-hand side: complex structures observed in the LKfrom northern exposures include sheath folds, shear bands and `crumpling' of the alternation of white, pure calcite and black,stylolitic layers. Sample from Piz Sardona (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 PlaunGrond (730.3/192) with LK representative of southern localities. General banding in di�erent colours results from stronglydeformed light coloured veins, dark stylolites and traces of secondary dolomite responsible for the orange/yellow alteration colourat the LK-Verrucano (green) contact. (C) Ultrathin (< 3 lm) edges of a LK thin section from KaÈ rpf (726.3/196.95) display stronglattice-preferred orientation visualized through the use of a gypsum plate (530 nm) and crossed polarizers. Note horizontalbanding. (D) Close-up of the same sample as shown in (C). Average grain size of matrix calcite is less than 3 lm; note fairlystraight grain boundaries and the absence of a shape preferred orientation. The large ameboid orange grain possibly represents therelic of a former vein. (E) Photometrically determined rose diagram illustrates the preferred orientation of calcite c-axes (projectedonto the plane of observation) measured in six different spots with 200 lm diameter along the ultrathin edge of the section shownin C. (F) Cold cathodoluminescence reveals the presence of an old and sheared ghost vein in the calc-mylonite sample from theGrau Berg locality (South). Three successive generations of veins are easily distinguished in CL. Dark red grains along the secondvein are quartz. (G) Same thin section as F, seen in normal light, displays a homogeneous ®ne-grained calcite matrix in the placeswhere 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, SEMview. (c) Shape-preferred orientation in an LK sample from Vorab Pign (732.25/192.525), southern part of the Glarus thrust, thinsection view, crossed polarizers. (d) idem, SEM view.

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grain boundary sliding (superplastic¯ow) as the dominant deformationmechanism to account for the extremestrain localization observed at thebase of the Glarus thrust (Schmidet al., 1977; Schmid et al., 1981;Schmid, 1982a). In this interpretation,the LK is considered a very weakde collement layer on which the Glarusnappe was translated as a rigid block.Burkhard et al. (1992) provided stableisotope evidence for considerable ¯uidadvection during thrusting and there-fore speculated about the role of such¯uids in deformation (cf. Bowmanet al., 1994). Meso- and microstruc-tural observations document thealternating activity of brittle and crys-tal-plastic deformation processes andthe omnipresence of dissolution-cry-stallization processes. These observa-tions are dif®cult to reconcile with theidea of a single, dominant deforma-tion mechanism such as the superplas-ticity proposed by Schmid et al. (1977,1981) and Schmid (1982a). Brittledeformation features have been notedby earlier authors (Heim, 1921; Sch-mid et al., 1981; P®ffner, 1982) butgenerally such observations were dis-carded as unimportant, late overprint-ing. However, there is no evidencefor an evolution from plastic to brit-tle deformation with time. Foliatedgouge textures and overprinted veinbands are more obvious in northernthan southern localities. This, how-ever, can be interpreted in terms of atemperature gradient up-dip, ratherthan re¯ecting an evolution throughtime. Ductile deformation anddynamic recrystallization were merelymore ef®cient in wiping out the evi-dence for brittle deformations furthersouth, rather than being more import-ant in terms of their contribution tototal strain and thrust translation.The absence of LPOs was a central

argument used by Schmid et al.(1977) in favour of superplasticity.The present observations of strongLPOs seen in optical microscopy(Fig. 5C±E) contrast with the lackof LPOs in the XRD-samples ana-lysed by Schmid et al. (1981). Itcould be argued that these opticallyvisible LPOs are a local phenomenon,inherited from other LPOs resultingfrom twinning of some coarse veingrains that have totally recrystallized.However, according to Walker et al.(1990), grain boundary sliding in the

®ne-grained matrix, should not allowthe preservation of strong LPOs.Alternatively, when measured inte-grally on large (cm-size slides) byXRD techniques, LPOs may be`diluted' by the rotation of mm- tocm-size rock fragments, which areapparent in the form of `chaotic'foliated gouge textures (Figs 4b, 5A).The Glarus thrust roots at mid-

crustal levels (P®�ner, 1985). Dewa-tering by compaction and progrademetamorphism in the footwall pro-duce considerable amounts of ¯uidswith a general tendency to escapeupward towards the foreland (Oliver,1986; Marquer and Burkhard, 1992).The role of such ¯uids in deformationdepends critically on the ¯uid produc-tion rate and permeability in thesurrounding rock masses (Connollyand Thompson, 1989; Gueguen et al.,1991). It is proposed herein thatan abrupt change in permeabilitybetween the footwall and hanging walllead to ¯uid channelling at this par-ticular thrust contact. In this scenario,¯uids produced in the footwall perco-late continuously upward, preferen-tially along the steeply inclinedpre-existing (Calanda phase) foliation.The clay-rich Verrucano thrust sheet,with actively forming subhorizontalfoliation, represents a permeabilitybarrier to this percolation. In analogyto the fault-valve scenario (Sibson,1990), ¯uid pressures would increasebelow this contact up to the thresholdfor hydrofracturing associated withseismic slip (Sibson, 1990; Petit et al.,1999). As a result of dilatency, ¯uidpressures would drop abruptly tovalues lower than within wall rock.Healing of the fracture network andformation of veins would progres-sively seal the thrust fault, allowing¯uid pressures to build up again.Between fracture events, ductile intra-crystalline deformation and dynamicrecrystallization would result in theformation of a microcrystalline matrixwithin the LK and of LPOs within thismatrix. Thrust faults do not havethe ideal orientation for such fault-valve behaviour (Sibson, 1990;Nguyenet al., 1998). However, similar struc-tural observations and interpretationshave been reported from other thrustsin the Apennines (Coli and Sani,1990), the McConnell thrust in theCanadian Rockies and the HunterValley thrust in the Appalachians

(Kennedy and Logan, 1997; Kennedyet al., 1998), and the Gavarnie thrustin the Pyrenees (McCaig et al., 1995).The paradox of the lobate±cuspate

contact, which suggests a competentLK sandwiched in between lesscomptetent Verrucano and ¯ysch,®nds an elegant solution in the fault-valve/seismic failure scenario. Frac-turing and calcite mineralizationsalong the base of the Verrucano thrustsheet are triggered by ¯uctuations in¯uid pressure, because ¯uids arestored below this `permeability bar-rier'. The planar septum horizons,cataclasites and veins re¯ect some ofthe latest brittle events. Backgrounddeformation of the entire thrust zonein the ductile regime is responsible forthe repeated shearing and folding ofolder septums, veins and the LK±wallrock contacts, as well as mylonitiza-tion of the Verrucano and folding ofthe topmost metres of ¯ysch. Therelative contributions of seismic andplastic deformation to the total thrusttranslation are di�cult to evaluate. Inthe present interpretation, the thrusttranslation was accommodated mostlyby numerous seismic slip events. Insummary, the Glarus nappe with itsenigmatic LK, is a good candidate fora major thrust fault which owes itslocalization and apparent softeningto transiently near lithostatic ¯uidpressures (Etheridge et al., 1984;Carter and Dworkin, 1990; Carteret al., 1990; Henry and Le Pichon,1991), rather than to the presence ofa permanently weak de collementlithology.

Acknowledgements

We would like to thank F. Gainon,N. Challandes and A. Aubry for their helpwith ®eld work and sampling; K. Ramseyerfor CL imagery. A. M. McCaig andR. Abart are thanked for stimulatingdiscussions and active collaboration. Weacknowledge thorough and constructivereviews by S. M. Schmid and B. den Brok.This study is supported by the SwissNational Science Foundation Grants20-50535.97 and 20-56920.99.

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Received 19 November 1999; revised versionaccepted 27 February 2001

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