Geological Society, London, Special Publications

doi: 10.1144/SP332.11 2010; v. 332; p. 173-187Geological Society, London, Special Publications

 Francesca Salvi, Maria Iole Spalla, Michele Zucali, et al. Alpspolymetamorphic terrains: a case from the Central Italianmetamorphic reaction progress in polycyclic and Three-dimensional evaluation of fabric evolution and  

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Three-dimensional evaluation of fabric evolution and metamorphic

reaction progress in polycyclic and polymetamorphic terrains:

a case from the Central Italian Alps

FRANCESCA SALVI1,2*, MARIA IOLE SPALLA3,4, MICHELE ZUCALI3 &

GUIDO GOSSO3,4

1Dipartimento di Scienze della Terra ‘A. Desio’, Universita degli Studi di Milano,

Milano, Italy2Present address: Eni E&P Division, Via Maritano 26, 20097 S. Donato Milanese, Italy

3Dipartimento di Scienze della Terra ‘A. Desio’, Universita degli Studi di Milano,

Sezione di Geologia, Via Mangiagalli 34, 20133 Milano, Italy4C.N.R.–I.D.P.A., Sezione di Milano, Via Mangiagalli 34, 20133 Milano, Italy

*Corresponding author (e-mail: francesca.salvi@eni.it)

Abstract: The 3D reconstruction of geological bodies is an excellent tool for the representation ofcrustal structures and is applied here to understand related heterogeneities in the grain-scalefabrics; the western portion of the Languard–Tonale Alpine tectono-metamorphic unit (Austroal-pine domain, Central Alps) allows evaluation of the per cent volume of textural reworking duringpolyphase pre-Alpine and Alpine deformations. The structural and metamorphic overprintingduring the last deformation imprint involved less than 50% of rock volume; this estimate isobtained by discriminating domains that homogeneously recorded structural and metamorphicre-equilibration during crenulation–decrenulation cycles. These domains are reconstructedusing a geograhpical information system (GIS) to manipulate field data and interpretative cross-sections as a means to constrain their 3D volumes. The degree of fabric evolution is integratedat the microscale with the estimate of the reactants/products ratio to infer the progress of meta-morphic transformation related to advancing degree of mechanical reactivation. The correlationbetween degree of fabric evolution and progress of synkinematic metamorphic reactions showsthat differences between pristine mineral assemblages v. pre-existing fabrics influence the rateof reaction accomplishment. Fabric evolution and degree of metamorphic transformation increaseproportionally once above the threshold value of 60% of volume affected by fabric rejuvenation;metamorphic degree also influences the progress of metamorphic reactions.

The terrains affected by polyphase deformationand metamorphism during polycyclic tectonic evol-ution are characterized by different crustal units thatshow distinct structural and metamorphic imprints.The definition of the shape and size of these unitsis a crucial step in unravelling deep-seated mechan-isms active during tectonic processes such as crustalaccretion or consumption (Spalla et al. 2005).

The complete reconstruction of the tectono-metamorphic evolution of these terrains is possiblethrough a multidisciplinary approach based ondetailed correlation of superposed fabric elements,microstructural analysis and recognition of fabricgradients (Turner & Weiss 1963; Park 1969;Hobbs et al. 1976; Williams 1985; Passchier et al.1990; Johnson & Vernon 1995; Spalla et al. 2000;Zucali et al. 2002). The extent, degree and timingof metamorphic re-equilibrations, and associated

fabric changes, can be used to define the size andshape of rock volumes that underwent the samecrustal path during a defined time interval and con-stitute a tectono-metamorphic unit (TMU: Spallaet al. 2005).

Rocks belonging to a single TMU record aheterogeneous partitioning of the total deforma-tion, resulting in heterogeneous distribution ofmetamorphic re-equilibration due to the cataly-sing effect of deformation on the metamorphicreaction progress.

The resulting patchy distribution, at the end ofeach deformation episode, of the dominant fabricsand metamorphic assemblages provides an evalu-ation of the percentage volume of mechanicallyand chemically reacting rock portions during succes-sive stages of the tectonic evolution, in which dif-ferent crustal slices, which correspond to different

From: SPALLA, M. I., MAROTTA, A. M. & GOSSO, G. (eds) Advances in Interpretation of Geological Processes:Refinement of Multi-scale Data and Integration in Numerical Modelling. Geological Society, London, SpecialPublications, 332, 173–187. DOI: 10.1144/SP332.11 0305-8719/10/$15.00 # The Geological Society of London 2010.

TMUs, were engaged. A quantitative 3D estimate ofthe final distribution of differently re-equilibratedvolumes may give fundamental insights into theaccomplishment of structural and/or metamorphicre-equilibrations at different structural levels alongactive plate margins. This is fundamental to estimat-ing, for example, the influence of different defor-mation mechanisms, phase transitions or densityand viscosity variations in the thermomechanicalsetting of geophysical models.

In this paper we estimate the reacting rockvolumes within a single TMU, starting from a mapreporting in detail the heterogeneous distributionof textural and metamorphic transformations (mapof dominant fabric domains). Taking into accountthe degree of mechanical and mineral–chemicaltransformation of different rocks at micro- to mega-scales, every TMU includes a mosaic of domainswith an homogenous fabric evolution and degreeof metamorphic transformation, each of them show-ing a dominant fabric and metamorphic imprint.In addition, showing the low- and high-straindomains for each deformation phase, the map ofdominant fabric domains corresponds to a map ofdeformation partitioning at all scales and identifiesthe distribution of geometric patterns (homotheticmicro- to mega-structures).

A quantitative volume estimate of the dominantfabric domains is performed by means of a 3Dmodel, for which subsurface constraints are provi-ded by carefully rendered geological cross-sections.The case of the Languard–Tonale TMU (Austro-alpine domain in the Central Alps: Zucali 2001;Spalla et al. 2005) is used as a case study tocompare the relationships between the structuraland metamorphic memories recorded at contrasting(pressure/temperature) P/T ratios [from highpressure–intermediate temperature (HP–IT), inter-mediate pressure–high temperature (IP–HT) tolow pressure–low temperature (LP–LT)] duringthe polyphase tectonic evolution of this portion ofcontinental crust involved in Permian–Triassiclithospheric thinning and subsequent Alpine sub-duction. The highly contrasting thermal regimesunder which successive tectonic imprints havebeen recorded in the Languard–Tonale TMUmake the conclusions drawn from this case studyapplicable to different geological contexts inwhich temperature can serve as catalyst to reactionkinetics or deformation mechanism activation.

Geological background

The investigated portion of the Languard–TonaleTMU (Fig. 1) is located between the upper ValCamonica and Valtellina of the Central Alps.This polydeformed and polymetamorphosed unit

belongs to the Austroalpine domain, which isconsidered to have originated from the Adriaplate based mainly on its lithological affinities,and occupies the uppermost structural level in theAlpine nappe pile. This TMU includes two litho-stratigraphical units, the Languard–Campo Nappe(LCN) and the Tonale Series (TS) (Bigi et al.1990; Schmid et al. 1996), and is bounded south-wards by the eastern segment of the Tonaledextral strike-slip fault (Stipp et al. 2004).

The mapped portion of the Languard–TonaleTMU mainly consists of low- to medium-grademuscovite, biotite- and minor staurolite-bearinggneisses, and micaschists with interlayered amphi-bolites, marbles, quartzites and pegmatites occupy-ing the northern and central sector of the map. Thesouthernmost portion of the TMU is mainly com-posed of high-grade sillimanite-bearing gneissesand micaschists, garnet- and biotite-bearing amphi-bolites, marbles and pegmatites. Post-Variscanintrusives (granitoids, diorites and minor gabbros)occur throughout the area (Ragni & Bonsignore1968; Bonsignore et al. 1971; Del Moro et al.1981; Tribuzio et al. 1999 and references therein).

The Alpine metamorphic evolution of this por-tion of the Austroalpine domain consists of ahigh-pressure imprint followed by retrogradationto greenschist-facies conditions (Spalla et al. 1995,2003, 2005; Tomaschek & Blume 1998; Gazzolaet al. 2000; Zucali 2001; Gosso et al. 2004);large-scale Alpine mylonitic belts, such as theMortirolo or Insubric Line, are often associatedwith greenschist-facies metamorphism (Werling1992; Viola et al. 2003; Stipp et al. 2004). In themapped region (Fig. 1), the rocks recorded thesame tectono-metamorphic evolution (P–T–t–dpath, where t and d are ‘relative time’ and ‘defor-mation’ respectively) during Alpine convergence(e.g. Gazzola et al. 2000; Zucali 2001; Spallaet al. 2003), even where the distribution of Alpinepolyphase deformation and metamorphic transform-ations is highly heterogeneous, and responsiblefor the localization of different dominant and dia-chronous structural and metamorphic imprints inadjacent domains (Fig. 1).

The tectono-metamorphic history of the TMULanguard–Tonale consists of six synmetamorphicdeformation phases (Fig. 2): three are pre-Alpine(D1a, D1b, D2) and three Alpine (D3, D4, D5:Gazzola et al. 2000; Zucali 2001; Spalla et al. 2003,2005; Gosso et al. 2004); the widespread Permiandiorites and granodiorites (Del Moro & Notarpietro1987) were used as markers to distinguish Alpinefrom pre-Alpine structures and metamorphic imprintsbecause they are post-Variscan and pre-Alpine.

The pre-Alpine evolution, recorded only inthe country rocks, developed under medium- tohigh-grade conditions: D1 is poorly preserved in

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Fig. 1. Simplified foliation trajectory and petrographical map of the Languard–Tonale TMU. Colour intensity gradient of the Permian meta-intrusives (metadiorites andmetagranitoides) qualitatively reproduces the increase in planar fabric intensity from coronitic (pale) to mylonitic (deep) textures (modified after Spalla & Zucali 2004). Thesquare includes the selected area for Figure 3 and the 3D modelling. Location and simplified tectonic map of the Alpine belt are included in the inset.

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Fig. 2. Sequence of the geological events recorded by the Languard–Tonale TMU. Deformation phase, structures and their associated mineralogical support, inferred metamorphicimprints and available radiometric data are reported. HT, high temperature; IT, intermediate temperature; LT, low temperature; HP, high pressure; IP, intermediate pressure;LP, low pressure; Ab, albite; And, andalusite; Bt, biotite; Chl, chlorite; Cld, chloritoid; Czo, clinozoisite; Ep, epidote, Grt, garnet; Ilm, ilmenite; Kfs, K-feldspar; Ky, kyanite;Pl, plagioclase; Qtz, quartz; Sil, sillimanite; St, staurolite; Ts, tschermakite; Ttn, titanite; Wm, white mica). *Thoni 1981. **Del Moro et al. 1981; Del Moro & Notapietro 1987.

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low-strain D2 domains, while D2 produced folds anda pervasive composite axial-planar foliation (S2).Pre-Alpine structures and related metamorphicassemblages are preserved in significant volumesmainly in the southern and northernmost parts ofthe area, whereas in the central part, dominatedby Alpine structures, they occur as small relictdomains. D1 structures are marked by contrastingmineral assemblages (Fig. 2), indicating that indifferent volumes these early structures developedunder different thermal regimes (e.g. Spalla et al.2003). The common tectonic history started withD2, marked by HT–LP mineral assemblages, indi-cating T ¼ 650–750 8C and P � 0.6 GPa (Gossoet al. 2004 and references therein). D3 occurredunder HP–IT conditions, and mineral assemblagesmarking these structures provide an estimate of500–600 8C and 1.1 + 0.2 GPa; D4 and D5 tookplace under lower-greenschist-facies conditions(T � 350 8C and P � 0.5 GPa: Gazzola et al.2000; Gosso et al. 2004). The Alpine deformation(D3–D5) is responsible for the development ofkm-scale shear zones and isoclinal folds (Fig. 1).The Alpine uplift to shallow structural levels aftersubduction, compatible with greenschist-facies con-ditions, was accomplished early during Alpine con-vergence (.78 Ma), as suggested by radiometricdata on syn-D4 micas (Del Moro & Notarpietro1987; Gazzola et al. 2000). A synthesis of pre-Alpine and Alpine mineral assemblages markingsuccessive fabric elements in metapelites andmeta-intrusives is shown in Figure 2.

During the complete structural evolution, strainpartitioning produced fabric gradients (from coroni-tic to mylonitic) at different scales during eachdeformation phase. In the Languard–Tonale TMU

it is common to observe that the more myloniticfabrics correspond to more complete metamorphicre-equilibration (Spalla et al. 2005).

Data analysis, 3D volumetric

reconstruction and discussion of

model results

The correlation of the structural and metamor-phic re-equilibration stages performed for theLanguard–Tonale TMU represents the startingpoint for the recognition of the dominant fabricdomains. Low- and high-strain domains for eachdeformation phase have been identified by a semi-quantitative estimate of the degree of fabric andassociated metamorphic transformation, both inthe host rocks and in the Permian meta-intrusives.A wide range of lithologies is generated locally, pro-viding a complex mosaic of rocks volumes withvariable mineral composition and planar fabrics.Recognition of gradients in fabric intensity andabundance of the associated metamorphic mineralproducts was the focus of field analysis andsampling strategy.

The evaluation of the dominant fabric domains,degree of metamorphic reaction progress andtheir volumetric estimate has been performed in aselected area (about 54 km2) in the southern partof the Languard–Tonale TMU (Fig. 3), in whichthe pre-Alpine (D1–D2) structures are preservedin extensive domains and the Alpine (D3–D4–D5)sequence of superposed structures are detectablewith good continuity at map scale.

The 3D model is constructed using five mainsteps, shown in the flowchart of Figure 4.

Fig. 3. Tectono-metamorphic map of the southern part of the Languard–Tonale TMU selected for the 3D volumetricmodelling of the dominant fabric domains. The number of dots and colours used for the foliation symbolsrepresent a given deformation phase and the P–T conditions associated with their formation.

3D EVALUATION OF FABRIC EVOLUTION 177

Hereinafter, the analytical input data and the modelresults are described and discussed for each step ofthe procedure.

Meso- and microstructural data input into a

GIS database

As a starting point, the numerous meso- and micro-structural data of the Languard–Tonale TMU(Gazzola et al. 2000; Spalla et al. 2003, 2005;Gosso et al. 2004) were georeferenced and storedin a GIS (Geographic Information System) data-base. The tectono-metamorphic map of theLanguard–Tonale unit was drawn at 1:10 000scale within the GIS environment. The archive ofthe data in GIS allows rapid management, represen-tation, querying and manipulation of the structuraldata. Finally, the estimates of the degree of fabricevolution and of the metamorphic reaction progress,using combined meso- and microstructural des-criptions, were organized in a georeferenced data-base in order to represent the distribution offabric domains.

Analysis of the degree of fabric evolution

and metamorphic transformation

The interpretative map of dominant fabric domainsderives from the foliation trajectory map of theLanguard–Tonale TMU (Fig. 2) (Spalla et al.2003, 2005; Gosso et al. 2004), where the foliations

(S1–S5) are chronologically distinguished on thebasis of overprinting criteria and compatibility ofmetamorphic assemblages.

Contouring of the homogeneous fabric domainsis facilitated by integrating mesostructural informa-tion with microstructural analysis to estimate thedegree of fabric evolution and metamorphic trans-formation in volume percentage; microstructuralanalysis was performed on 154 thin sections strate-gically distributed throughout the modelled area.

The mineralogical assemblages and microstruc-tures were described using a semi-quantitative esti-mate of the degree of fabric evolution (volumepercentage of planar fabric distribution) and meta-morphic transformation (modal amount of mineralassemblages expressed in %) corresponding toeach of the successive fabrics; an example of themicrostructural relationships between the degreeof fabric evolution and metamorphic reaction fororiginally foliated and isotropic rocks is shown inFigure 5.

The estimate of the fabric evolution is based onthe degree of grain-scale reorganization of the domi-nant fabric (Fig. 6); the successive stages of crenu-lation cleavage development proposed by Bell &Rubenach (1983; see also Passchier & Trouw2005), up to the complete transposition, were usedas a guide. Two different evolutionary schemes ofthe planar fabric were considered in accordancewith the character of the original rock (Fig. 6): thefirst for the fabric evolution of the pre-Permianhost rocks that evolved from an originally foliated

Fig. 4. Flowchart for the 3D reconstruction of the dominant fabric domains.

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Fig. 5. Example of microstructural relationships between the degree of fabric evolution and metamorphictransformation in originally foliated (left) and massive samples (right): (a) S2 foliations in garnet-, biotite- andsillimanite-bearing micaschist; (b) porphyroblast of staurolite (pre-Alpine fabric) in syn-D3 tectonitic fabric;(c) white mica, garnet- and chlorite-bearing micaschist with mylonitic fabric (S3 Alpine foliation); (d) syn-D3

coronitic fabric in a metadiorite; (e) syn-D3 differentiated crenulation cleavage in a metadiorite; (f) syn-D3

mylonitic fabric in a metadiorite showing the degree of fabric evolution and a metamorphic transformation of 100%.

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Fig. 6. Reference ranges for the comparative fabric v. mineral growth analysis of the tectono-metamorphic histories in the Languard–Tonale TMU. Schematic fabric evolutionfrom originally foliated country rocks and initially isotropic Permian intrusives. For each stage the range of the degree of fabric evolution is indicated. Only when the degreeof fabric evolution (F.E.) is �60% of the volume of the rock (thick dashed line) does the degree of metamorphic transformation increase proportionately (modified after Bell &Rubenach 1983). Asterisk indicates mean values.

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fabric; the second for the Permian meta-intrusivesthat exclusively recorded the Alpine history andare assumed to have started from an isotropicigneous fabric.

Three main evolution stages are proposed(Fig. 6) both for originally foliated and isotropicfabrics. The ranges chosen to discriminate thedifferent stages of deformation derive from themicrostructural observation. Two thresholds wereobserved: the first is approximately 20%, underwhich the new planar fabric is not persistent; andthe second is approximately 60%, under whichthe new planar fabric is developed but it is still poss-ible to recognize the enveloped previous one; overabout 60% the new fabric is pervasive and onlyrelicts of the previous fabrics are preserved.

The first stage (LD, low degree of deformation)corresponds to the early development of the newfabric (degree of fabric evolution 0–20%); thisincludes an incipient crenulation or the appearanceof a non-persistent new foliation for the countrymetapelites and for the meta-intrusives, respect-ively. The second stage (MD, medium degree ofdeformation) corresponds to a successive evolutionup to the differentiation of a new foliation, whichcan reach the stage of a differentiated crenulationcleavage or a pervasive foliation (20–60% fabricevolution degree), in originally foliated or isotropicrocks, respectively. The last stage (HD, high degreeof deformation) coincides with the progressiveobliteration of the relicts of earlier fabrics in meta-pelites and meta-intrusives (crenulated foliationremnants in microlithons or the presence of mag-matic porphyroclasts, respectively) by completegrain-scale reworking and development of newcontinuous foliations (c. 60–100% overprinting ofthe previous fabric). Ranges of volume percentageoccupied by Alpine mineral assemblages synkine-matic with D3 and D4þD5 are also indicated inFigure 6 (metamorphic transformation, M.T.),at each stage, for both originally foliated and iso-tropic rocks. The first stage is characterized by anaverage of 20%, with a maximum value of 50%,of synkinematic metamorphic transformations inoriginally foliated rocks, and by an average of40%, with a maximum of 60%, of metamorphictransformation in originally isotropic rocks; thesecond stage shows an average metamorphic trans-formation of 45 and 55% in originally foliated andisotropic rocks, respectively, with a maximumof 75% for both; in the last stage, the mineral–chemical re-equilibration can reach up to 100% ofthe volume in both cases (M.T. ranging between70 and 100%).

In addition to the catalysing effect of deforma-tion, a significant role is played by the thermalregime under which deformation and associatedmetamorphic transformations occur, with the

condition that the fabric evolution remains belowthe HD stage (�60%). For the same degree offabric evolution, the metamorphic reaction progressis more evolved during D3 (IT–HP) than during D4

or D5 (LT–LP).At the LD stage (fabric evolution (F.E.) of

0–20%, Fig. 6) the mean value of M.T. for orig-inally foliated and isotropic rocks is about 31%during the IT–HP D3 phase and 22% during theLT–LP D4þD5 phases; correspondingly, atthe MD stage (F.E. of 20–60%) the mean valueof the M.T. is 60% during D3 and 37% duringD4þD5. Generally, the degree of metamorphictransformation is dependent on the original com-position of the rocks: for example, the metadioritesshow M.T. values higher than the metagranitoidrocks at the same degree of F.E. and P–T conditions.

The resulting microstructural semi-quantitativescheme synthesized in Figure 6 highlights therelationship between fabric evolution and degreeof metamorphic transformation: the two processesdo not necessarily develop at the same rate, but anF.E. threshold of approximately 60% appears tocoincide with the transition to the HD stage, abovewhich the mechanical and mineral–chemical trans-formations of the rocks increase proportionally;only above the 60% threshold is the related synkine-matic mineral assemblage capable of reaching100% of the total volume.

2D dominant fabric domain map

The rock volumes that reached an F.E. of at least60% are shown in a map (Fig. 7a, b) that isdivided into different HDi domains (high degree ofgrain-scale deformation during the i-phase of defor-mation), each corresponding to the most intenselyreacting rock volumes during each deformationphase. The size of the HDi domains is related tothe interpreted geological map scale (1:10 000).According to the previous microstructural obser-vations of HDi domains, the metamorphic reactionscoeval with the i-phase of fabric development werenearly totally accomplished and related mineralassemblages occupy a mean volume �75%(Fig. 6). The representation of the dominant fabricdomains has been simplified by grouping thepre-Alpine structures (D1þD2) and the twogreenschist-facies Alpine deformation phases(D4þD5). D1 structures are, in fact, poorly pre-served mainly as metre-scale relict fold hingesin D2; similarly, D5 structures are rare andextremely localized.

Boundaries between different domains thathomogeneously recorded deformation were tracedbased on estimates of the fabric evolution andof the degree of metamorphic transformation atthe meso- and micro-scale for each deformation

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phase, according to the subdivision of Figure 6.Bounding surfaces of each domain are envelopingsurfaces, compatible with the structural patternrepresented on the foliation trajectory map. The pre-cision of the boundaries ranges from 5 m, where thestructures are widely exposed, to 100 m, wherethe outcrops are scanty and the degree of interpret-ation is higher.

Three dominant fabric domains were recognizedin the southernmost portion of the Languard–Tonale TMU (Fig. 7a, b): southwards a pre-Alpinedominant fabric prevails (D1þD2); the centralpart is characterized by a D3 Alpine dominantfabric; and the northern part by the greenschistD4þD5 fabrics. A few 10 m-scale HD(1þ2) andHD3 relicts are preserved in the HD3 and in theHD(4þ5) domains, respectively (Fig. 7a, b).

The HD(1þ2) domain is characterized by a perva-sive differentiated S2 foliation, in places continuous,

developed under HT–LP metamorphic conditions.This domain includes the low-strain D3 domains,where S2 is locally folded without the developmentof an axial-planar S3 foliation. In the HD(1þ2)

domain the Alpine mineralogical assemblagesrepresent a low volume percentage (�25% meanvalue), showing incomplete metamorphic trans-formations such as pseudomorphic replacement ofthe pre-Alpine minerals or fine-grained reactionrims. In HD3 domains the earlier HP Alpine foli-ation (S3) is dominant and associated with a foliatedto mylonitic fabric. The HD(4þ5) domain is charac-terized by the prevalence of D4 with minor D5

Alpine structures developed at LT–LP conditions:the dominant S4 differentiated foliation withmylonitic fabric was locally folded during D5. Inthe Alpine domains the relict pre-Alpine fabricsare scarce or absent. Moreover, in the northernHD(4þ5) and in the HD3 domains some lenticular

Fig. 7. 3D prospective of the map of dominant fabric domains. (a) Downwards view and (b) from the south. Three maindomains were recognized: the pre-Alpine HD(1þ2) domain, corresponding to the high concentration of pre-AlpineD1þD2 deformation at the grain-scale; the HD3 domain corresponding to the high concentration of D3 (Alpine) at thegrain-scale; and the HD(4þ5) domain corresponding to the high concentration of D4þD5, greenschist-facies,Alpine-related deformation at the grain-scale. In the HD3 and HD(4þ5) domains a few lenticular relicts are preserved inwhich syn-D1þD2 and syn-D3 tectonitic–mylonitic fabrics are dominant, respectively. (c) Subsurface construction ofthe surfaces bounding HDi domains based on their intersection with the topographic surface (boundary) and theprojected traces of the buried surface on each cross-section. (d) Triangulated buried surface obtained by interpolatingthe constraints shown in (c). (e) and (f) Structural model of the map of the dominant fabric domains showing thebounding buried surfaces: 3D prospective view from (e) the west and (f) the SW.

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low-strain domains ranging from 1 to 100 m-scalewere recognized in which syn-D3 and syn-D2 struc-tures are dominant, respectively (Fig. 7a, b).

Comparison of the tectono-metamorphic map(Fig. 3) with the map of the dominant fabricdomains (Fig. 7) clearly shows that the distributionof the high- and low-strain domains is not controlledby lithology as HDi domains cross-cut lithologicalboundaries. This may be related to strain softeningthat resulted from strain localization during suc-cessive deformation stages, making the originallycompetent rocks, such as diorites or granulites,softer by grain-size reduction along deformationzones that may develop independently of pre-existing lithological boundaries.

3D dominant fabric domain model

The map of dominant fabric domains (Fig. 7a, b)makes the 3D reconstruction of the different HDi

domains feasible, considering only the surfacedata, when this is made possible by using a DTMof the topographical surface (Digital ElevationModel, from www.cartografia.regione.lombardia.it),

and the lithological and structural field data storedin the georeferenced database. The main constraintsused for the subsurface reconstruction are the bound-aries between the HDi domains projected from thetopographical surface onto a set of parallel north–south cross-sections, chosen perpendicular to theregional trend of Alpine fold axes.

For each cross-section the topographical profile,its intersection with the dominant fabric domainboundaries and the projection of structural measure-ments onto the plane of section were automaticallyobtained (Zanchi et al. 2009). As much as possible,the geological cross-sections honour the mesostruc-tural data, the fold geometry and the deformationstyle of each of the superimposed deformationphases. A test of the geometric consistency of thegeological structures in the cross-sections is per-formed with an iterative process through 2D and3D. This procedure consists of: (i) interpretationof the 2D parallel geological sections; (ii) importof the 2D sections in the 3D modeller software;(iii) a check of the geometrical coherence betweenthe adjacent sections (e.g. axes or axial planesurfaces continuity); (iv) export from 3D to 2D

Fig. 7. Continued.

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environment for editing the possible mistakes; and(v) return to the 3D software. Sometimes, this pro-cedure induces a check of the interpretation of thegeological boundaries on the map.

Buried surfaces are reconstructed by projectingthe boundaries between HDi domains on the topo-graphic surface onto each cross-section, as well asimportant structural elements such as the foldhinges; successively, the geometry of the buriedsurfaces fitting these constraints are interpolated(Fig. 7b, c).

Reconstruction of buried surfaces is an excellenttool for the 2D geological interpretation test: the3D representation of the geological structure high-lights the geometric inconsistencies of the inter-pretation. This may require the rigorous revisionof the 2D interpretative tectono-metamorphic map,the map of the dominant fabric domain and thecross-sections.

In detail, the contact surface striking NE–SWbetween the northern HD(4þ5) and the HD3

domains dips southwards at approximately 608(Fig. 7e, f ). The boundary shows a wide, gentle anti-cline northwards, coherent with that described byViola et al. (2003). The HD(1þ2) domain isbounded to the north by a surface that strikes

east–west and has a steep dip southwards thatdecreases with depth, and to the south by a surfacethat strikes NE–SW and dips 708 towards thesouth; this surface also bounds the southernHD(4þ5) domain (Fig. 7e, f).

In summary, the modelled portion of theLanguard–Tonale TMU is bounded northwardsand southwards by two HD(4þ5) domains, while inthe central zone D3 structures are dominant and alenticular pre-Alpine HD(1þ2) domain is preserved(Fig. 7e, f ).

Volumetric estimate of the dominant

fabric domain

The reconstructed buried surfaces, correspondingto the boundaries between HDi domains, constitutethe 3D topological model. As already pointed outin the third phase, we assume that the degree ofF.E. for any particular deformation phase is homo-geneously distributed at the map-scale, and istherefore strongly influenced by the chosen rep-resentation scale. From this, the reconstruction ofthe volumetric model is possible in which thewhole volume is represented through a discretegrid, made up of 3D equal volume cells (Fig. 8).

Fig. 8. (a) 3D volumetric model of the map of the dominant fabric domains of the southern part of the Languard–Tonale TMU (about 53 km3). (b) HD(4þ5) dominant domains corresponding to about 52% of the whole volume;(c) the HD3 domain corresponding to about 37%; and (d) the pre-Alpine HD(1þ2) domain corresponding to about 12%.

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The grid is intersected by the reconstructed buriedsurfaces, which allows computation of thevolumes, corresponding to volumes of the differentdominant fabric domains. The total volume of theLanguard–Tonale TMU considered in this studyequates to approximately 53 km3. The volumesestimated for the three domains are: 12% for theHD(1þ2) domain, of which 0.2% is representedby the lenticular relics preserved within the HD3

domain; 37% for the HD3; and 51% for theHD(4þ5) domains.

Using the fabric evolution v. metamorphic trans-formation relationships synthesized in Figure 6, itis possible to estimate the minimal volumes occu-pied by the mineral assemblages synkinematicwith each group of structures characterizing thethree HD domains: pre-Alpine assemblages (syn-D1þD2) occupy a minimal volume ranging from7 to 12%; early Alpine assemblages (syn-D3)occupy a minimal volume of 22–37%; and thelate Alpine assemblage (syn-D4þD5) represents aminimal volume of 30–51%.

Conclusions

From the micro- and mesostructural analysis perfor-med in this volume of Alpine continental crust,which was repeatedly tectonized in an activemargin during Permian extension and Alpine sub-duction, it appears that the dominant metamorphicimprint corresponds to that associated with themost pervasive strain fabric. This is in agreementwith other results from the Alpine belt (Spallaet al. 2005) where the recognition of tectonic unitsthat record a common structural and metamorphicevolution (TMU) has been similarly based ondetailed structural mapping and microstructuralanalysis.

The construction of the map of dominant fabricdomains, based on the degree of fabric evolution,has demonstrated that the localization of the high-strain domains is not controlled by the lithologicalsetting, because high-strain domains of successivedeformation stages appear to be discordant withlithological boundaries.

The correlation between the degree of fabricevolution and the progress of synkinematic meta-morphic reactions in different lithologies hasshown that differences in original mineral assem-blages and fabrics (i.e. originally foliated or isotro-pic) exerts more of an influence on the degree ofreaction accomplishment. For instance, originallyisotropic meta-intrusives achieved a greater degreeof mineral transformation, at low and mediumdegree of deformation (Fig. 6), than the originallyfoliated country rocks at the same degree of defor-mation. At a high degree of deformation (HD) the

related metamorphic transformations are similar inspite of their different original texture or mineralcomposition. Fabric evolution and degree of meta-morphic transformation do not increase proportion-ally at LD and MD stages. There is a threshold at thetransition to the HD stage (c. 60% evolution) abovewhich mechanical and chemical transformationsincrease proportionally and the synkinematic min-erals can achieve total replacement of pre-existingmineral phases. These results highlight the influenceof strain energy as a catalyst on metamorphic trans-formation, as discussed by Hobbs et al. (2010).

Temperature can also catalyse the progressof metamorphic reactions: D3 took place underepidote–amphibolite-facies conditions (IT–HP)that yielded a greater degree of synkinematic meta-morphic transformation than during D4 and D5,corresponding to lower greenschist-facies condi-tions (LT–LP).

The 3D modelling permits us to obtain the volu-metric estimate of the degree of fabric evolutionand associated metamorphic recrystallization ofrock volumes within a TMU. Only half (51%) ofthe total rock volume was mechanically and chemi-cally re-equilibrated during the late stages of thetectono-metamorphic evolution (D4 and D5 underLT–LP metamorphic conditions); one-tenth of thetotal volume preserves the structural and meta-morphic imprints related to the earlier stages (D1

and D2 pre-Alpine deformation phases), presumablydue to a very poor mechanical reactivation duringthe whole Alpine orogeny.

The diffuse heterogeneity of textural and meta-morphic imprints requires that the characterizationof TMUs be based on detailed field mapping overan area reaching a critical size, which depends, inpart, on the scale of strain partitioning in the region.

Results from this type of detailed field andlaboratory procedure are useful to refine, constrainand verify geophysical modelling that simulatesthe mechanical behaviour at active plate margins,assuming the changes of continental or oceanicrheology on the basis of a full accomplishment ofthe predicted phase transitions that drive changesof the dominant active deformation mechanisms.Our 3D modelling allows an estimation of thevolumes preserving textural and mineral relictsafter phase transitions, and may help to evaluatethe potential influence that relict domains have onthe choice of the physical parameters for thermo-mechanical modelling, such as density or viscosity.

The authors thank D. Gibson and R. Trouw for their con-structive criticisms and helpful suggestions. This workwas developed within the gOcad Consortium; A. Zanchiis also thanked. Funding by FIRST07-08 of the Universitadegli Studi di Milano and CNR-IDPA. America JournalExperts Association provided the English revision ofthe text.

3D EVALUATION OF FABRIC EVOLUTION 185

References

BELL, T. H. & RUBENACH, M. J. 1983. Sequentialporphyroblast growth and crenulation cleavage devel-opment during progressive deformation. Tectono-physics, 92, 171–194.

BIGI, G., CASTELLARIN, A., COLI, M., DAL PIAZ, G. V.,SARTORI, R., SCANDONE, P. & VAI, G. B. 1990.Structural Model of Italy, Sheets 1–2. CNR, ProgettoFinalizzato Geodinamica. SELCA, Firenze.

BONSIGNORE, G., CASATI, P. ET AL. 1971. Note Illus-trative della Carta Geologica d’Italia alla scala1:100.000, Fogli 7 e 18: Pizzo Bernina e Sondrio.Nuova Tecnica Grafica, Servizio Geologico Italiano.

DEL MORO, A. & NOTARPIETRO, A. 1987. Rb–Srgeochemistry of some Hercynian granitoids over-printed by eo-Alpine metamorphism in the UpperValtellina, Central Alps. Schwizerische Mineralo-gische und Petrographische Mitteilungen, 67, 295.

DEL MORO, A., NOTARPIETRO, A. & POTENZA, R. 1981.Revisione del significato strutturale delle masse intru-sive minori dell’alta Valtellina, risultati preliminari.Rendiconti Societa Italiana Mineralogia e Petrologia,38, 89–96.

GAZZOLA, D., GOSSO, G., PULCRANO, E. & SPALLA,M. I. 2000. Eo-Alpine HP metamorphism in thePermian intrusives from the steep belt of the CentralAlps (Languard–Campo nappe and Tonale Series).Geodinamica Acta, 13, 149–167.

GOSSO, G., SALVI, F., SPALLA, M. I. & ZUCALI, M.2004. Map of deformation partitioning in the polyde-formed and polymetamorphic Austroalpine basementof the Central Alps (Upper Valtellina and Upper Valca-monica, Italy). In: PASQUARE, G. & VENTURINI, C.(eds) Mapping Geology in Italy. APAT, Roma,295–304.

HOBBS, B. E., MEANS, W. D. & WILLIAMS, P. F. 1976.An Outline of Structural Geology. Wiley, New York.

HOBBS, B. E., ORD, A., SPALLA, M. I., GOSSO, G. &ZUCALI, M. 2010. The interaction of deformationand metamorphic reactions. In: SPALLA, M. I.,MAROTTA, A. M. & GOSSO, G. (eds) Advances inInterpretation of Geological Processes: Refinementof Multi-scale Data and Integration in NumericalModelling. Geological Society, London, Special Publi-cations, 332, 189–222.

JOHNSON, S. E. & VERNON, R. H. 1995. Inferring thetiming of porphyroblast growth in the absence of con-tinuity between inclusion trails and matrix foliations;can it be reliably done? Journal of StructuralGeology, 17, 1203–1206.

PARK, R. G. 1969. Structural correlations in metamorphicbelts. Tectonophysics, 7(4), 323–338.

PASSCHIER, C. W. & TROUW, R. A. J. 2005. Microtec-tonics. Springer, Berlin.

PASSCHIER, C. W., MYERS, J. S. & KRONER, A. 1990.Field Geology of High-grade Gneiss Terrains.Springer, Berlin.

RAGNI, U. & BONSIGNORE, G. 1968. Contributoalla conoscenza del Cristallino dell’Alta Valtellina edell’Alta Val Camonica (Alpi Retiche). Geologicalmap.

SCHMID, S. M. & BERGER, A. 1996. The Bergell Pluton(Southern Switzerland–Northern Italy), overview

accompanying a geological–tectonic map of the intru-sion and surrounding contry rocks. SchweizerischeMineralogische und Petrographische Mitteilungen,76, 329–355.

SPALLA, M. I. & ZUCALI, 2004. Deformation vs. meta-moprhic re-equilibration in polymetamoprhic rocks:a key to infer quality P–T–d– t path. Periodico diMineralogia, 73(2), 249–257.

SPALLA, M. I., MESSIGA, B. & GOSSO, G. 1995.LT-alpine overprint on the HT-rifting related meta-morphism in the steep belt of the Languard–Camponappe. The Cima Rovaia and Scisti del Tonaleunits represent two different extents of alpinere-equilibration. In: International Ophiolite Sym-posium, Pavia, Abstract Volume, 148.

SPALLA, M. I., SILETTO, G. B., DI PAOLA, S. & GOSSO,G. 2000. The role of structural and metamorphicmemory in the distinction of tectono-metamorphicunits; the basement of the Como Lake in the southernAlps. Journal of Geodynamics, 30, 191–204.

SPALLA, M. I., ZUCALI, M., DI PAOLA, S. & GOSSO, G.2005. A critical assesment of the tectono-thermalmemory of rocks and definition of the tectonometa-morphic units: evidence from fabric and degreeof metamorphic transformations. In: GAPAIS, D.,BRUN, J. P. & COBBOLD, P. R. (eds) DeformationMechanisms, Rheology and Tectonics: From Mineralsto the Lithosphere. Geological Society, London,Special Publications, 243, 227–247.

SPALLA, M. I., ZUCALI, M., SALVI, F., GOSSO, G. &GAZZOLA, D. 2003. Structural Map of the Languard-Campo – Serie Del Tonale Nappes (Passo DelMortirolo – Valtellina–Valcamonica Divide,Austroalpine Domain, Central Italian Alps).Memorie di Scienze Geologiche, 55.

STIPP, M., FUGENSCHUH, B., GROMET, L. P., STUNITZ,H. & SCHMID, S. M. 2004. Contemporaneous pluton-ism and strike-slip faulting: a case study from theTonale fault zone north of the Adamello pluton(Italian Alps). Tectonics, 23, TC3004.

THONI, M. 1981. Degree and evolution of the Alpine meta-morphism in the Austroalpine unit west of the HoheTauern in the light of K/Ar and Rb/Sr age determi-nations on micas. Jahrbuch Geologische Bundesan-stalt – A, 124, 111–174.

TOMASCHEK, F. & BLUMEL, P. 1998. Eo-Alpine mediumgrade metamorphism in the Austroalpine Campo base-ment at Passo Gavia (northern Italian Alps). TerraNostra, 98(1), 78–79.

TRIBUZIO, R., THIRLWALL, M. F. & MESSIGA, B. 1999.Petrology, mineral and isotope geochemistry of theSondalo gabbroic complex (Central Alps, NorthernItaly): implications for the origin of post-Varisicanmagmatism. Contributions to Mineralogy and Petrol-ogy, 136, 48–62.

TURNER, F. J. & WEISS, L. E. 1963. Structural Analysis ofMetamorphic Tectonites. McGraw-Hill, New York.

VIOLA, G., MANCKTELOW, N. S., SEWARD, D., MEIER,A. & MARTIN, S. 2003. The Pejo fault system; anexample of multiple tectonic activity in the ItalianEastern Alps. Geological Society of America Bulletin,115(5), 515–532.

WERLING, E. 1992. Tonale-, Pejo- und Judicarien-Linie:Kinematic, Mikrostrukture und Metamorphose von

F. SALVI ET AL.186

Tektoniten aus raumlich interferierenden, aberverschiedenaltrigen Verwerfungszonen. PhD thesis,ETH Zunch.

WILLIAMS, P. F. 1985. Multiply deformed terrains –problems of correlation. Journal of StructuralGeology, 7, 269–280.

ZANCHI, A., SALVI, F., ZANCHETTA, S., STERLACCHINI,S. & GUERRA, G. 2009. 3D reconstruction of complexgeological bodies: Examples from the Alps. Compu-ters & Geosciences, 35, 49–69.

ZUCALI, M. 2001. La correlazione nei terreni metamor-fici: due esempi dall’Austroalpino occidentale (ZonaSesia-Lanzo) e centrale (Falda Languard-Campo/Serie del Tonale). PhD thesis, Universita di Milano.

ZUCALI, M., SPALLA, M. I. & GOSSO, G. 2002. Fabricevolution and reaction rate as correlation tools, theexample of the Eclogitic Micaschists complex in theSesia–Lanzo Zone (Monte Mucrone–Monte Mars,Western Alps Italy). Schweizerische Mineralogischeund Petrographische Mitteilungen, 82, 429–454.

3D EVALUATION OF FABRIC EVOLUTION 187