Analysis of natural tectonic systems coupled with numerical modelling of the polycyclic continental lithosphere of the Alps

ISSN 0020-6814 print/ISSN 1938-2839 online© 2010 Taylor & FrancisDOI: 10.1080/00206814.2010.482737http://www.informaworld.com

International Geology ReviewVol. 52, Nos. 10–12, October–December 2010, 1268–1302

TIGR0020-68141938-2839International Geology Review, Vol. 1, No. 1, Apr 2010: pp. 0–0International Geology Review Analysis of natural tectonic systems coupled with numerical modelling of the polycyclic continental lithosphere of the Alps

International Geology ReviewM.I. Spalla et al. Maria Iole Spallaa,b, Guido Gossoa,b*, Anna Maria Marottac, Michele Zucalia and Francesca Salvid

aDipartimento di Scienze della Terra ‘A. Desio’ Sezione di Geologia, Università degli Studi di Milano, Milano, Italy; bCNR-IDPA, Milano, Italy; cDipartimento di Scienze della Terra ‘A. Desio’

Sezione di Geofisica, Università degli Studi di Milano, Milano, Italy; dEni E&P Division, San Donato Milanese, Italy

(Accepted 19 March 2010)

Subduction–collision zones are characterized by the amalgamation and disaggregationof lithospheric slices; such processes work in competition in constructing the tectonicarchitecture of metamorphic belts. Determination of contours for thermally and struc-turally characterized units is crucial to define the variations in sizes of such slicesinvolved in the dynamic evolution of an active margin. The dimensions of these entitieschange over time and must be reconstructed using the structural and metamorphicevolution of the basement rocks as tracers, rather than by simply relying on lithologicassociations. They constitute tectono-metamorphic units (TMUs) and representdiscrete portions of the orogenic crust influenced by a sequence of metamorphic andtextural changes. Their translational trajectories and shape changes during deformationcannot simply be derived from the analysis of the geometries and kinematics oftectonic units but from a joint reconstruction of quantitative P-T-d-t paths. The TMUinvestigation tool bears a marked thermo-tectonic connotation and, through modelling,offers the opportunity to test the physical compatibilities of interconnected variables,such as density, viscosity, and heat transfer, with the interpretative geologic history.Comparison between modelling predictions and natural data obtained by this analyticalapproach has helped solve longstanding ambiguities on the pre-Alpine and Alpinegeodynamic evolution of the different continental units of the Central and WesternAlps and explore the crustal levels of protolith derivation. Three-dimensional estimationof structurally and chemically re-equilibrated volumes aids in the evaluation of physicalparameters chosen for the numerical modelling.

Keywords: tectono-metamorphic units; pre-Alpine rifting; Alpine subduction;geophysical modelling; P-T-d-t paths

IntroductionNappe theory originated in the deformed Alpine foreland belt more than a century ago, butthe detailed structure acquired during deep lithospheric processes and the kinematics ofthe axial collision zone remain unsolved. The long-lasting literature and the tectonicassociation of geologic units that manifest rifting, subduction, collision, and collapsesignatures make this belt a case of great interest. A historical account of the kinematic

*Corresponding author. Email: [email protected]

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International Geology Review 1269

interpretations of nappe stacking mechanisms in the Central and Western Alps spans morethan a century and reveals how slow progress was in such a remarkably exposed chain,with an alternate dominance of investigation methods until the 1960s.

Stratigraphic-palaeontological investigations in the marginal sedimentary zones of theexternal Western Alpine arc first made clear to Heim (1878) and Haug and Kilian (1894)that regional overthrust folds caused orogenic crustal thickening in these sedimentarysequences of the inner European margin. At the beginning of the next century, Lugeon(1901), Argand (1911), and Staub (1917) admirably applied the nappe theory to the entiremetamorphic axial zone. Recognizing an approximate separation of pre-Alpine contin-ental crust lithostratigraphies from oceanic and marginal Mesozoic rock associations, theydelineated the architecture of the axial suture zone between the African and Europeanmargins (Figures 1 and 2). A shallow, low-angle ductile upthrusting, similar to the simpleoverthrust nappe mechanisms ascertained in the foreland, was envisaged to have formed –even in the metamorphic crust of axial belt – the thick tectonic pile of the Pennines. Thesenappes were thought to have originated from huge embryonic folds within a contractingocean, thinly floored by pre-Alpine continental rocks. Actually, nappe margins were

Figure 1. Historical tectonic diagram (Argand 1911) that first displayed the axial suture zone ofthe Central–Western Alps as a stack of recumbent overthrust folds (Pennine nappes); the structure isdelineated by the boundaries of thin pre-Alpine continental rocks (unornamented) with respect toMesozoic green stones and metasediments (shelf, slope, trench, and oceanic) (stippled), originallylocated between the European and African continents (Argand 1911 shared by Staub 1917).Geometric complexities in the front cross section originate from about a 20-km down-dip projectiontowards the Northern Aosta Valley (SW) of regional-scale infoldings mostly mapped in the Ossolaand Tessin Alps (NW rim); average regional trend of continental recumbent fold nappe axes issupposed to plunge towards the front cross section (as indicated on the right face, i.e. axes culminateto the northeast). The topmost tectonic levels of the pile (Austroalpine of Dent Blanche) lie abovethe Mesozoic oceanic associations. Historical names of nappes consisting of pre-Alpine continentalcrust are from 0 to VI, respectively: Verampio, Antigorio, Lebendun, Monte Leone (Ossola–Tessinlower Pennine nappes), Grand St Bernard, Monte Rosa, Dent Blanche (upper Pennine nappes); theintervening sheets, structurally less explored and consisting of Mesozoic sediments and ophiolites,bear less celebrated tectonic names (as for instance Monviso, Avic, Zermatt-Saas ophiolites). Dottedsquare locates block diagram on Figure 2 map; front face base length is 90 km long and height is25 km; modern scripts are additions to original.


1270 M.I. Spalla et al.

traced in the axial Alpine zone by simply contouring contrasted lithologic associations orrock sequences of presumably different age (i.e. continental vs. oceanic, respectively, pre-Permian or pre-Triassic vs. Mesozoic; Argand 1911, 1916; Staub 1917, 1924; see Masson1976 and Dal Piaz 2001 for exhaustive reviews).

This interpretation benefited from the new chronostratigraphic distinctions betweenpre-Mesozoic and Mesozoic sequences [findings of Liassic ammonites by Franchi (1898)in carbonate beds of the outer margin of the Penninic Ocean, at Alpe di Narbona in ValGrana, Southwest Alps] and by the publication of the 1 : 400,000 scale synthetic map ofthe Western Alps by Regio Ufficio Geologico (1908). This new map distinguishedpre-Alpine continental rock associations from Mesozoic ocean-derived metaophiolites(green stones) and coeval sediments (calcschists or schistes lustrés) over the huge WesternAlpine area from the Tyrrhenian Sea to the Tessin. The kinematics inferred to explain theaxial nappe stack illustrated in the famous 3D block diagram of Figure 1 implied a timesequence of recumbent fold overthrusts from SE to NW (from top to bottom in the presentpile, i.e. from the African – or Adriatic – to the European margin). Noteworthy was thestratigraphy of the Penninic ocean floor, invariably envisaged as a ubiquitous thin layer ofcontinental crust, locally transected by mafic–ultramafic syntectonic extrusions and flowson the overlying Mesozoic sediments. Such supposed pre-orogenic stratigraphy was inter-preted as shortened into the Penninic recumbent megafolds pile through a continuousprocess of ocean suturing, terminating by collision of the main impinging continents.

After three decades of testing this structural and lithostratigraphic configuration, suchstimulating fold nappe interpretations resulted in numerous inconsistencies in the tectono-stratigraphy and related structural tenets. Discontinuities in the detailed lithostratigraphicsequences did not accord with the upright and overturned limbs of many recumbent fold

Figure 2. Tectonic map of the Alps, with location of the three case histories discussed in the text(sites 1 and 2 = black diamonds; 3 = arrow, for the whole SLZ). Legend: (1) Southalpine basement,(2) Austroalpine basement, (3) Penninic basement, (4) Helvetic-Dauphinois-Provençal basement, (5)Tertiary intrusive stocks. Dotted line square locates the Argand’s (1911) block diagram of Figure 1.



nappes (e.g. Hermann 1925, 1938; Stutz and Masson 1938; Diehl et al. 1952). The newresults reoriented kinematic interpretation of crustal thickening towards a diffuse sheetoverthrusting process of more rigid style (gleitbrettertektonik) in the upper Pennine andAustroalpine crusts. Similarly, the absence of continental crust on the ocean floor and theactualistic recognition of the purely mantle-derived lithologic associations of the oceancrust (Gass 1968; Reinhardt 1969) definitely disproved the basic premise of an oceanicrealm between Africa and Europe floored by a continental crust lithostratigraphy inherentin the recumbent fold overthrust interpretation (see Bernoulli et al. 2003, for a review).

Shortly later, studies of the complex structures of the Penninic-style nappes displayedorogenic deformation sequences and revealed that many of the presumed frontal hinges ofthe ‘Pennine-style’ recumbent fold nappes were in fact late generation structures in theAlpine tectonic sequence (Higgins 1964; Hall 1972); this let the early nappe-formingmechanism remain unknown and confined honestly to a tectonic episode weakly definedas ‘earliest Alpine deformation phase of isoclinal rootless folding and overthrusting’(Milnes and Schmutz 1978; Huber et al. 1980). Such cautious conclusion reinforced thegleitbrett interpretation (sheet overthrusting) of collisional crustal thickening, whichremained still unrelated to specific crust or mantle structural levels and thermal regimes.

Special critical attention may usefully be paid to advances achieved from the 1970s;they offer an evaluation of real gains and a retrospective discrimination of sterile opinionsfrom the valid bases on which irrevocable changes were progressively acquired. A firmestablishment of the metamorphic conditions assisting plasticity of geologic units duringorogeny was becoming possible just in this period when experimental results of mineralequilibria made petrologic estimates routinely applicable at the regional scale. Since thenestimates of physical conditions of metamorphic assemblages, from continental andoceanic rocks, suggested subduction-related P/T ratios in the suture zone (e.g. Ernst 1971;Dal Piaz et al. 1972; Compagnoni and Maffeo 1973; Kienast 1983). Consequently, 60years after Argand’s and Staub’s brilliant ideas, deformation style and petrogenesisimposed interpretations of Alpine orogenic thickening of the axial zone in agreement withnew lithosphere-scale tectonic processes, dramatically different from simple recumbentfolding of the shallowest crust, as that taking place in the thrust and fold belt of the fore-land. In addition, a vast existence of metastable assemblages (Ernst 1963) within unitsexhumed from depth reinforced reasons to undertake regional tectonic studies as a multi-scale investigation on the full petrostructural orogenic history recorded by natural rocks. Afirst wealth of PT estimates, geophysically relevant to orogenic studies, called soon forupdating the petrostructural exploration of the entire axial nappe system of the Alps by astrategic change in the scale of structural mapping. Such a time-consuming task readilyheaded alpine geologists to selecting key areas, facilitated by a stimulating secular databaseon mineral compositions or on typical subduction-related textural equilibria. Very attract-ing rocks were already known, as for instance the micascisti argentei of Dora-Maira, theeclogitic micaschists of Sesia-Lanzo Zone (Novarese 1903), the glaucophane-bearingassemblages in the Adula nappe (Van Der Plas 1959), the Alpine jadeite in theBriançonnais zone basement (Lorenzoni 1965; Michard 1967), and the eclogitized pillowlavas of the Zermatt ophiolites underlying the Dent Blanche continental sheet of theMatterhorn mountain (Bearth 1959).

The narrow axial zone of the belt, where ocean suturing apparently generated the highesttectonic complexity, paradoxically provided in the petro-structural memory of polyde-formed tectonites, forged in a full subduction–collision cycle the tectonothermal data crit-ical to undertake on a new physical base the reconstruction of nappe trajectories. This willchange and support testing the kinematic interpretations of Alpine nappes in the ocean



suture zone, based, over nearly a century, on predetermined palaeogeographic premisesreproducing those useful to solve the shallow crustal tectonics of the two symmetricbelts of the collided Alpine forelands. During the last 20 years, structural petrologists rec-ognized in the axial Alpine suture that units contoured on the basis of their lithostrati-graphic similarities do not coincide with those including rocks sharing the same tectonicand metamorphic evolution (e.g. Caron et al. 1984; Pognante 1989a, b; Spalla et al. 1996).Actually, subduction–collision zones are characterized by repeated coupling and decou-pling of lithospheric slices, acting in competition when building up tectonic units of ametamorphic belt (e.g. Platt 1986, 1987; Polino et al. 1990). During plate convergence,contours of these mobile units are transient (e.g. Spalla et al. 1996, 2005) and can beinvestigated integrating structural and petrologic analysis. In an actualistic view, the deepoceanic-continental infoldings may be generated by tectonic erosion of the upper platecontinent during subduction (e.g. Lallemand and Le Pichon 1987; Pellettier and Dupont1990; Von Huene and Lallemand 1990) and by the successive ductile interplay of thinocean and continent slices from the sliding plates, at depth within the supra-subductionorogenic wedge (e.g. Polino et al. 1990). Numerical simulations are able to verify thermaland mechanical parameters and to refine qualitative geologic models of this second type,implying tectonic circulation (convection) of heterogeneous materials from crust andmantle. This material is tectonically sampled by ablation at various structural levels andexhumed, after subduction, within the upper plate mantle wedge (e.g. Cloos 1982; Geryaet al. 2002; Stoeckhert and Gerya 2005). The understanding of natural cases through acomparison with predictions of geophysical models needs the evaluation of the real size ofcrust and mantle-derived slices that coherently shared, after amalgamation, the sametectonic and thermal history. In this light, the structural and metamorphic evolutions ofbasement rocks, rather than the purely lithologic associations, trace their transit through-out different levels of the lithosphere and sub-lithospheric mantle. Therefore, the refine-ment of a method for the individuation of contours of thermally characterized andstructurally distinct units became crucial (Spalla et al. 2005). Comparison of natural dataoriented to these scopes with modelling predictions can help to solve structural settingsand ambiguities on the geodynamic evolution of polycyclic metamorphic terrains.

In this article, we show that the integration of structural, petrologic, and geophysicalmethods may contribute to a multi-scale investigation of orogenic processes of crustalthickening in the collisional belt of the European Alps. A petrostructural analyticalprocedure is shown to delimit a new type of tectonic units that better fits the physicalrequirements needed to construct naturally based numerical simulations of lithospherebehaviour. This method may lead to quantitative geodynamic interpretations and generatenatural parameters to be tested by geophysical modelling of lithosphere-scale tectonics.

Analytical procedureAn effective working method is needed to individuate the volumes recording the samestructural and metamorphic evolution and consequently identify tectono-metamorphicunits (TMUs of Spalla et al. 2000, 2005) by reconstruction of the detailed structural andmetamorphic evolutions of basement rocks, which are the tracers of their path acrossdifferent lithospheric levels. The investigation requires joint use of structural correlationof mesoscopic deformation patterns and microstructural analysis to decipher equilibriummetamorphic assemblages supporting transient fabrics, possibly related to absolute agedata (Turner and Weiss 1963; Park 1969; Hobbs et al. 1976; Williams 1985; Passchieret al. 1990; Johnson and Duncan 1992; Spalla et al. 2000; Di Vincenzo and Palmeri 2001;



Zucali et al. 2002b; Vernon 2004; Passchier and Trouw 2005; Villa 2010). Unfortunatelydeformation heterogeneity makes a field structural correlation exclusively grounded ongeometric criteria poorly reliable, because of marked variations in deformation style, asfor example where mylonitic textures develop at the side of domains that remain weaklydeformed or non-deformed (e.g. Spalla and Zucali 2004; Hobbs et al. 2010).

However, the abrupt breaks in the mesoscopic structural record created by deformationheterogeneity may be favourably utilized, once scaled in time. They consist in the closegeneration, during incremental deformation steps, of discrete volumes that escape defor-mation and in which metamorphic reactions take place without formation of a neworiented fabric (coronitic textures) and of volumes in which, during progress of reactions,normal planar/linear tectonites (tectonitic textures) or mylonites (mylonitic textures)develop. Therefore, an analytical approach able to obtain a more confident tectonic corre-lation is provided by (i) structural maps adding a grid of the foliation trajectories to lithos-tratigraphic data; (ii) checking of kinematic compatibilities between all superposedstructures (mainly shear zones, fold systems, and related differentiation of new granular-scale layerings) and of their relative chronology at the regional scale, to separate successivedeformation stages; (iii) a sampling strategy selecting sites critical to identifying themineralogical support of superposed fabrics in different lithologies and to recognize com-patibilities between parageneses formed in different chemical systems; (iv) cross controlof fabric sequences and metamorphic assemblages in adjacent volumes that recordedheterogeneously the deformation; (v) evaluation of mineral chemical variations in micro-structural sites located along fabric gradients, from the low- to the high-strain zones; (vi)thermobarometric calculations on assemblages marking successive fabrics of knownrelative chronology; and (vii) reconstruction of P-T-t or P-T-d-t (PT-relative deformationtime) paths, which constitute the basic reference for the individuation of TMUs, defined asthe volumes that underwent the same PT history, although rich of contrasting petrostructuraltypes.

Case histories(a) This analytical approach has been applied to a portion of the Southalpine basement inthe Como Lake region (location 1 in Figure 2), with the aim of understanding in moredetail the pre-Alpine metamorphic and structural conditions of the African (or Adriatic)margin of the Alpine belt, from which many of the Alpine nappes most probablyoriginated.

The Southalpine basement, which extends from the Dinarides to the Western Alps,represents the portion of Adria plate that constituted the orogenic lid of the Alpine chainduring its Mesozoic–Tertiary evolution. The central portion of this basement lies betweenthe Como Lake and the Adamello Massif and is bounded by the Insubric-Tonale Line tothe North and by the Po Plain to the South. The pre-Alpine metamorphic basement iscapped by Permian–Mesozoic sedimentary cover sequences and is locally weakly meta-morphosed during Alpine times. It consists of prevailing gneisses and micaschists withinterlayered metagranitoids, metabasics, marbles, quartzites, and minor pegmatites. Thelithostratigraphic homogeneity earlier facilitated the individuation of two wide lithostrati-graphic units, the Edolo Schists and the Morbegno Gneisses, considered as homogeneousalso in their metamorphic evolution over a long time.

In the Como Lake metamorphic basement, a single lithostratigraphic unit was previ-ously known (e.g. Crespi et al. 1980; Mottana et al. 1985). New structural maps showingthe foliation trajectory grid superposed on the structural setting of the lithostratigraphy



(form surface maps in Hobbs et al. 1976) have been produced (Figure 3). On these maps,the sequence of superposed fabrics has been related to the metamorphic conditions underwhich successive deformation stages took place (Figures 3–5; Spalla et al. 2000, 2005).By microstructural analysis, the degree of new planar fabric development has been evalu-ated, using the six-stage decrenulation model as a reference (Bell and Rubenach 1983;Passchier and Trouw 2005) to explore the relationships between progress of structural and



metamorphic transformations. In addition, the foliation trajectory map allows constrainingwith confidence location, thickness (∼7 km), and extent of each TMU.

This procedure allowed contouring three TMUs, bounded by two main syn-metamorphicfault zones (MFZ and LVGFZ in Figures 3 and 6). The P-T-d-t evolution inferred in eachTMU (Figure 6 and Table 1) suggests that the rocks have been deformed in an evolvinggeodynamic scenario in which the successive structural and metamorphic re-equilibrationstages can be interpreted as the records of the subduction (DCZ in Figures 3 and 6) andsubsequent collision (DCZ and MMZ in Figures 3 and 6) during the Variscan convergenceor the lithospheric thinning (DOZ in Figures 3 and 6) developed during Permian–Meso-

Figure 3. Southalpine pre-Alpine basement in the Como Lake region (Spalla et al. 2000, 2002; diPaola and Spalla 2004): the three tectono-metamorphic units Domaso-Cortafò, Dervio-Olgiasca,and Monte Muggio (DCZ, DOZ, and MMZ, respectively, on the map) are separated by greenschistmylonitic bands defining the Musso Fault Zone (MFZ) and the Lugano-Val Grande Fault Zone(LVGFZ). Legend: 1 = Dolomia Principale (Norian): re-crystallized massive dolostones, locallycataclastic; 2 = massive medium to fine-grained and calcareous breccias at the contact with themicaschists (sedimentary slices along the Insubric Line). DCZ: 3 = metapelites with BtII, MsII, Pl,Grt, ±St, and ±Ky defining the S2 foliation reactivated during D3; Chl, opaque minerals, MsIII,BtIII, and Ab define S3. Relics of S1 are underlined by BtI, MsI, and Cld; 4 = amphibolites withHblII, Pl, ±Ilm, ±Qtz, defining that S3 is underlined by Ep, Chl, and Ttn. HblI and Grt occur as por-phyroclasts in S2. MFZ: 5 = mylonites with Chl, Ms, Ab, and ribbon Qtz, locally with ultramylo-nitic texture. s-c structures and extensional crenulation cleavages are widespread. DOZ:6 = micaschists and gneisses with Chl and Ms underlining S3, in places with mylonitic textures ors-c cleavages; Grt and Bt relics are preserved; the modal amount of Chl increases towards theLVGFZ; 7 = Grt-St-bearing micaschists and minor gneisses containing MsI, BtI, GrtI, and St con-temporaneous with S1. S2 crenulation cleavage is defined by BtII, MsII, GrtII, ±Sil. Chl and MsIIIgrow during D3; 8 = Sil-Bt-bearing micaschists and minor gneisses with Ms, BtII, GrtII, Sil, Pl,±Kfs mark S2; BtII and Sil define s-c structures and extensional crenulation cleavages; relics ofGrtI, Ky, and St are locally preserved. Centimetre-sized And poikiloblasts grow during late D2;9 = amphibolites with Pl, HblII, ±Qtz, ±Grt, ±Bt, ±Ep, and relics of HblI; amphibolites with Hbl, Di,Pl, ±Ttn; S2 is defined by the HblII and Pl compositional layering or by the SPO of Hbl and Di; rarehornblendites with Chl, Ttn, Ilm show coronitic textures; 10a = metagranitoids with Ms and Chl,containing relics of Bt and Grt. Compositional layering defines S2; 10b = fine-grained myloniticmetagranitoids with Ms and Chl, containing millimetre-sized Kfs porphyroclasts; 11 = Qtz, Kfs,Ms, tourmaline, ±Grt-bearing pegmatites. Generally with undeformed cores and foliated (S2) mar-gins; 12 = quartzite layers of centimetre to metre thickness, containing Chl, Bt, and Ms; 13 = fine-to medium-grained, white to light grey marbles locally containing amphibole and pyroxene; silicate-rich layers are constituted by Zo, Tr, Tlc, Chl. LVGFZ: 14 = mylonitic micaschists and gneisseswith Qtz, Ms, Chl, and Ab underlining S3; shear zones with s-c structures and extensional crenulationcleavages and dark-coloured ultramylonites are widespread; cataclastic feldspar, sericite-bearinggneisses, with granular texture; S3 is well developed at the boundaries with the mylonites. MMZ:15 = metapelites with Chl, Ab, MsII, ±Mrg underlining S2 foliation. Relics of Grt, Bt, St, Pl, and Kydeveloped during D1. Grt-amphibolites rarely occur as metre-sized lenses elongated in S2; rarequartzite layers of centimetre to metre thickness, containing Chl, Bt, and Ms, have compositionallayering parallel to S2; 16 = metagranitoids with Ms, Chl, ±Bt; mineralogical layering defined byalternating quartz-feldspar and sheet silicate layers is parallel to S2. Foliation trajectories are distin-guished on the basis of relative chronology (relative ages of successive foliations) and of the associ-ated metamorphic imprint. The information on the relative chronology of superposed foliation andon the metamorphic environments in which they developed is specified, respectively, by relativeages and metamorphic imprint symbols. In the DCZ the D2/D3 foliation symbol is used to discrimi-nate S2 foliation reactivated during D3. 4A–4F and 5A–5H locate the photographs of Figures 4 and5, respectively.



zoic extension (Diella et al. 1992; Bertotti et al. 1993; Gosso et al. 1997; Marotta andSpalla 2007; Marotta et al. 2009).

The individuation of volumes with matching sequences of structural and metamorphicimprints (TMU) becomes immediate using the described working technique (Figures 3and 6) and clarifies that the dominant metamorphic imprint does not coincide with the

Figure 4. Mesoscale structural patterns of the Southalpine Basement in the Como Lake region.Domaso-Cortafò tectono-metamorphic unit: (A) interference pattern because of superimposition of aD3 open fold upon a D2 isoclinal fold, marked by S1 discontinuous foliation in the micaschists; (B) S3shear bands crosscutting S2 reactivated foliation in micaschists; Dervio-Olgiasca tectono-metamor-phic unit: (C) S1 foliation, well marked in mica-rich layers of St and Grt-bearing micaschists, at ahigh angle with layering, is crenulated during D2; (D) shear and foliation planes developing duringD2, in sillimanite and biotite gneiss, at pegmatite margins; Monte Muggio tectono-metamorphicunit: (E) leucocratic layer transposed and deformed by D2 tight folds in Monte Muggio micaschists;fault rocks and fault zones: (F) D3 extensional crenulation cleavage in the greenschist mylonitic beltalong the Lugano-Val Grande Fault zone (DOZ southern boundary).



Figure 5. Microstructural features of Domaso-Cortafò metapelites: (A) S1 foliation of dusty parti-cles and with aligned chloritoid is preserved as an internal foliation within a staurolite porphyroblastgrown during D2 microfolding; red square locates enlargement of (B) – plane-polarized light, longside of photomicrograph = 7 mm; (B) close-up of (A) on syn-D1 chloritoid enclosed in staurolite,together with tourmaline and opaque minerals – plane-polarized light, long side ofphotomicrograph = 0.5 mm; (C) S3 microshear plane, marked by chlorite, biotite III, plagioclase III,and opaque minerals – plane-polarized light, long side of photomicrograph = 0.4 mm; microstructural



Tmax-PTmax reached during each P–T loop and, therefore, the regional distribution of thedominant metamorphic imprints cannot be used to individuate the ‘metamorphic fieldgradient’ (e.g. England and Richardson 1977; Peacock 1989; Spear 1993). The latter cannotsupport the discrimination of TMUs in crustal volumes affected by polyphase deformationand metamorphism. On the contrary, in this case, the dominant metamorphic imprintcorresponds to that of the more evolved fabric (e.g. Spalla et al. 2000, 2005) and adjacentportions of a single TMU can be characterized by different dominant metamorphicimprints. The non-coincidence between dominant metamorphic imprint and Tmax–PTmaxcharacterizes a TMU (DOZ in Figure 6), in which different dominant metamorphicimprints affect adjacent portions of this unit characterized by different stages of D2 fabricevolution or where S3 prevails. Actually, where S2 is less evolved (up to stages 3 and 4 ofBell and Rubenach 1983), the syn-D1 intermediate pressure amphibolite-facies mineralsare dominant in volume. On the contrary, where S2 foliation is more evolved (up to stages5 and 6) or is mylonitic, the HT-LP metamorphic mineral assemblages prevail and only inthis case the dominant metamorphic imprint coincides with the Tmax−PTmax.

(b) The following investigation is a useful essay to generalize the significance of theseresults; it is conducted as in the previous case in a polyphase, but monocyclic metamorphicterrain, with the same analytical approach of a portion of the Alpine basement lying in theaxial part of the belts; here the Alpine tectono-metamorphic evolution generally overprintsa polycyclic Variscan or Permian–Triassic structural and metamorphic history. This is thecase of the Languard-Tonale nappe (Location 2 in Figure 2), which is the uppermost unitin the Austroalpine nappe pile of the Central Alps and is constituted by polymetamorphicrocks displaying a steeply dipping attitude immediately north of the Insubric-Tonaletectonic line (Schmid et al. 1996). This portion of the Austroalpine continental crustallows us to compare the relationships between the structural and metamorphic memoriesrecorded at contrasting P/T ratios (from high pressure–intermediate temperature, interme-diate pressure–high temperature to low pressure–low temperature), during the polyphasetectonic evolution related to the Permian–Triassic lithospheric thinning and subsequentAlpine subduction (Table 2; Gazzola et al. 2000; Zucali 2001; Spalla et al. 2005). Thehighly contrasting metamorphic environments under which successive tectonic imprintshave been recorded attribute to these results a general value for different geologicalcontexts, characterized by a wide range of thermal regimes, as the variations in T, remaining

patterns of Dervio-Olgiasca metapelites: (D) staurolite porphyroblasts, bent during S2 development,contain inclusion trails at a high angle with S2 (Corenno Plinio locality) – thin section negativeprint, long side of photomicrograph = 3 mm; (E) garnet porphyroblasts replaced by sillimanite–biotite intergrowths. Fibrous sillimanite and biotite mark both shear and foliation planes and occurin asymmetric garnet pressure shadows. Granoblastic domains contain mainly quartz and plagi-oclase, with minor K-feldspar and red-brown biotite – plane-polarized light, long side ofphotomicrograph = 15 mm; (F) shear and foliation planes marked by sillimanite and biotite II inter-growths. Biotite II has a Ti content higher than that of relict biotite I, indicating T-increment duringD2 (Diella et al. 1992) – plane-polarized light, long side of photomicrograph = 2.5 mm. Microstruc-tural patterns of Monte Muggio metapelites: (G) chlorite and minor white mica replace stauroliteduring D3; microfracture crosscutting staurolite is filled by Fe-rich chlorite – plane-polarized light,long side of photomicrograph = 8 mm; microstructural patterns of fault rocks: (H) extensionalcrenulation cleavage marked by chlorite and fine-grained white mica, developing in a greenschistmylonite of LVGFZ – plane-polarized light, long side of photomicrograph = 8 mm.



a fundamental catalyst on reaction kinetics or deformation mechanisms activation, influ-ence the effectiveness of the analytical approach.

In this central Alpine area, three lithostratigraphic units were individuated, on theground of lithologic associations and dominant metamorphic imprints (Ragni andBonsignore, 1966; Bonsignore et al. 1971): (i) a low- to medium-grade metapelites andmetaintrusives unit (Scisti di Pietra Rossa Series = SPRS in Figure 7), (ii) a medium-grade

Figure 6. Map showing the boundaries of the TMU in the Southalpine basement of the Como Lakeregion (Spalla et al. 2005 and refs therein). DCZ TMU is coloured in light yellow (northern sector);DOZ TMU is coloured in violet, lavender, and light blue; and the MMZ TMU is in orange (southernsector). P-T-d-t paths display the relative ages of metamorphic imprints associated with successivedeformation stages in each TMU, together with the radiometric ages from the literature. Coloursshow the correspondence with the dominant metamorphic imprint, which is homogeneous for theDCZ and MMZ TMUs, but different in adjacent portions of the DOZ TMU, dominated by variousdegrees of fabric evolutions.



metapelites unit (Scisti della Cima Rovaia Series = SCRS in Figure 7), and (iii) a high-grade gneisses unit (Scisti del Tonale Series = STS in Figure 7). Permian intrusives (gran-itoids, diorites, and minor gabbroids) variably reworked by Alpine tectonics during Creta-ceous commonly occur in all these units (Figure 7; Table 2) (e.g. Del Moro et al. 1981;Tribuzio et al. 1999) and represent the markers that most effectively separate the Alpinefrom the pre-Alpine tectonic and metamorphic history, synthesized in the P-T-d-t path ofFigure 7 (Gazzola et al. 2000; Zucali 2001).

In addition to a structural map displaying the foliation trajectories related to themetamorphic conditions under which successive deformation stages occurred, in thiscase also fabric gradients have been evaluated (Figure 7) and related to the rate of meta-morphic transformation (Figures 8 and 9), to better constrain the relationship betweenthe degree of mechanical and mineral chemical transformations in the different rocktypes (Gosso et al. 2004; Spalla and Zucali 2004; Spalla et al., 2005; Salvi et al. 2010).To this purpose, the degree of fabric evolution is integrated with the microstructuralanalysis by the semi-quantitative estimate of the products/reactants ratio (Figure 8), char-acterizing each superposed fabric development, to infer the progress of metamorphictransformation related to the degree of mechanical reorganization (Salvi et al. 2010). Asemi-quantitative estimate of the degree of fabric evolution (volume percentage of planarfabric distribution) and metamorphic transformation (percentage of modal amount ofmineral assemblages), corresponding to each of the successive fabrics, has been used; anexample of the microstructural relationships between the degree of fabric evolution andmetamorphic reaction is shown in the insets of Figure 7. The successive stages of crenula-tion cleavage development proposed by Bell and Rubenach (1983; see also Passchier andTrouw 2005) have been used by Salvi et al. (2010) as a guide to evaluate the degree of

Table 1. Assemblage sequences in the single lithostratigraphic unit (column) constituting the threedifferent tectono-metamorphic units (rows) of the Como Lake Southalpine basement. PT estimates,related to the different deformation stages (D1, D2, . . .), together with age data, facilitate the readingof the P-T-d-t paths shown in Figure 10. The successive mineral assemblages refer to meso andmicrostructures represented in Figures 8 and 9. (TMUs – tectono-metamorphic units; LSUs – lithos-tratigraphic units.).

TMUs LSUs

‘Morbegno (or Stabiello) Gneisses’

PT estimates Ages

MMZ Syn-D1 = Ms, Bt, Grt, Pl, Qtz, ±St, ±Ky

T = 560–600°C; P = 0.8–1.1 GPa

≥ 330 Ma

(Gosso et al. 1997; Spalla et al. 1998)

Syn-D2 = MsII, Chl, Ab, Qtz, ±Mrg, ±Ep

T < 500°C; P <4 GPa

≥ 260 Ma

DCZ Pre-D2 = Cld, BtI, MsI, GrtI, Pl, Qtz

− ∼ 385 Ma

(di Paola and Spalla 2000; di Paola et al. 2001)

Syn-D2 = BtII, MsII, GrtII, Pl, Qtz, ±St, ±Ky

T = 560–650°C; P = 0.7–1.1 GPa

≥ 330 Ma

Syn-D3 = MsIII, Chl, Ab, Qtz, ±Ep, ±BtIII, ±GrtIII

T < 550°C; P <0.6 GPa

Pre-Permian

DOZ Syn-D1 = Ms, Bt, Grt, Pl, Qtz, ±St, ±Ky

T = 550–630°C; P = 0.7–0.9 GPa

≥ 330 Ma

(Diella et al. 1992; Spalla et al. 1998)

Syn-D2 = BtII, Sil, Pl, Qtz, ±GrtII, ±Kfs

T = 650–750°C; P = 0.4–0.55 GPa

≥ 226 Ma

Syn-D3 = MsII, Chl, Ab, Qtz, ±Ep

T < 500°C; P < 0.3 GPa

Liassic



Tabl

e 2.

Sequ

ence

of s

truct

ural

and

met

amor

phic

impr

ints

reco

rded

by

the

Lang

uard

-Ton

ale

Aus

troal

pine

bas

emen

t of t

he S

outh

ern

Stee

p B

elt o

f the

Cen

tral

Alp

s. Th

e st

ruct

ural

and

met

amor

phic

evo

lutio

n be

com

es u

nifo

rm s

tarti

ng f

rom

D2,

dur

ing

pre-

Alp

ine

times

, sin

ce th

e en

d of

the

Alp

ine

evol

utio

n, m

akin

g th

eLa

ngua

rd-T

onal

e ba

sem

ent f

orm

ed b

y a

sing

le T

MU

dur

ing

this

tim

e in

terv

al. D

efor

mat

ion

stage

s, th

e re

late

d m

iner

al a

ssem

blag

es, i

nfer

red

PT e

stim

ates

, and

radi

omet

ric a

ges a

re re

porte

d.

Def

orm

atio

nM

etam

orph

ic

impr

int

Min

eral

ass

embl

ages

Mag

mat

ism

Age

Met

apel

ites

Met

aint

rusi

ves

D1a

D1b

HT-

IPIT

-IP

Bt+

Grt

+Pl

+Si

l+K

fs+

Qtz

+Ilm

Bt+

Wm

+St

+G

rt+

Pl±

Ky

+Q

tz+

Ilm

Pre/

syn

D2

HT-

LPG

arne

t-bea

ring

pegm

atite

s31

3±

3M

aapr

e-A

lpin

e

D2

HT-

LPB

t+G

rt+

Pl+

Sil+

Kfs

+Q

tz+

IlmD

iorit

es, g

rani

toid

es,

and

pegm

atite

s28

0–29

0M

ab

Post

D2

IT-L

PA

nd+

Mb

< 26

0M

ab

D3

IT-H

PW

m+

Grt

+A

b+

Czo

+C

ld±

Ky

±Ts

+Q

tzW

m+

Grt

+A

b+

Czo

+Ts

+Q

tz±

MgC

hl+

Ilm>

78M

abA

lpin

e

D4-

D5

LT-L

PW

m+

Chl

+Ep

+Q

tz+

Ttn

Wm

+C

hl+

Act

+Ep

+Q

tz

+B

t+Tt

n≥

78M

ab

a Thoe

ni (1

981)

; b Del

Mor

o an

d N

otar

piet

ro (1

987)

; Del

Mor

o et

al.

(198

1); G

azzo

la e

t al.

(200

0).



grain-scale reorganization of the dominant fabric (Figure 8) during Alpine deformations inthe Permian intrusives and in the surrounding metapelites, evolving from an isotropicigneous fabric and from an originally foliated fabric, respectively. Three main evolution-ary stages are proposed for both original fabrics that coincide with different deformationdegrees (low, medium, and high degree of deformation), corresponding to differentdegrees of fabric evolution and matching different extents of metamorphic transformationin Figure 8.

Figure 7. Simplified geological map of kilometre-scale strain gradients and related structureswithin Permian-age metaintrusives here redrawn after Spalla and Zucali (2004). Legend: Colour gra-dients identify fabric gradients, from coronitic to mylonitic fabrics, within metadiorites (blue gradi-ent) and metagranitoids (yellow gradient); (grey) Grt-bearing pegmatites; (dotted pattern)undifferentiated country rocks of Permian intrusives; foliation trajectories are shown for S1 and S2(blue trajectories = HT/IT-IP metamorphic assemblages), S3 (red trajectories = IT-HP metamorphicassemblages), and S4 and S5 foliations (green trajectories = LT-LP metamorphic assemblages);axial plane trajectories are also reported (green trajectories with synform or antiform symbols). Thepartially hidden red rectangle is the perimeter of the 3D modelled area of Figure 10. The P-T-d-tpath synthesizes the Alpine (red) and pre-Alpine (blue) evolution in which the common trajectorystarts from D2 pre-Alpine deformation stage (Gazzola et al. 2000; Zucali 2001; Gosso et al. 2004).D1a and D1b paths refer to the pre-D2 PT evolution characterizing the northernmost and the south-ernmost sectors, respectively. Mineral assemblages relative to the successive re-equilibration stages,synkinematic with D1, D2, D3, and D4, are listed in Table 2. Photographs exemplify meso- andmicrostructural features of coronitic to mylonitic domains within metadiorites: (A) mylonitic S1foliation in a metadiorite is well marked by the orientation of quartz- and feldspar-rich ribbons,alternated with amphibole-rich layers, and by a stronger grain-size reduction with respect to the nor-mal tectonitic fabric; (B) poorly deformed metadiorite with a well-preserved igneous texture over-grown by metamorphic minerals (coronitic texture), such as millimetric garnets (reddish round-shaped grains), that formed at the boundary between igneous plagioclase, amphibole, and biotite;(C) coronitic tschermakitic amphibole developed at the boundary of dark igneous hornblende andbiotite in an undeformed metadiorite – plane-polarized light, long side of photomicrograph = 2 mm;(D) mylonitic foliation in metadiorite, wrapping garnet porphyroblasts and igneous amphibole por-phyroclasts, is defined by fine-grained aggregate of AmpII, white mica, and clinozoisite. AmpI isrimmed by AmpII – crossed polars, long side of photomicrograph = 4 mm.



The results show that differences between pristine mineral assemblages and pre-existing fabrics influence the rate of reaction accomplishment and indicate that fabricevolution and metamorphic transformation degrees increase proportionally once a criticalthreshold of 60% in the volume affected by fabric rejuvenation is attained (Salvi et al.2010). It may be noted that the metamorphic degree exerts also a significant influence onthe progress of metamorphic reactions if the fabric evolution remains below the high degreeof deformation threshold (HD which has ≥ 60% of F.E. in Figure 8); actually, given thesame degree of fabric evolution, metamorphic transformations are more evolved underintermediate-temperature than under low-temperature conditions.

With the aim of making microstructural analysis more reliable, quantitative textureanalysis (QTA) using neutron diffraction data has been performed on deformed metadioritesdisplaying coronitic to mylonitic fabrics (Figure 9). QTA using diffraction relies on themeasurement of pole figures used as input data for the determination of the orientationdistribution function (ODF) of crystallites (Bunge and Esling 1982; Wenk 1985). Thecrystallographic recalculation of several pole figures from different crystallographicplanes, according to their crystal structure in the ODF, allows a complete QTA of the

Figure 8. Reference ranges for the comparative fabric versus mineral growth analysis of thetectono-metamorphic histories in the Languard-Tonale TMU after Salvi et al. (2010) showing theschematic fabric evolution from originally foliated country rocks and initially isotropic Permianintrusives. For each stage, the range of the degree of fabric evolution is indicated, together with themean values of metamorphic transformation (M.T.) degree. Only when the degree of fabric evolu-tion (F.E.) is ≥60% of the rocks volume (thick dashed line) does the degree of metamorphic trans-formation increase proportionately (modified after Bell and Rubenach 1983).



Figure 9. Comparison of microstructural and chemical evolutions and crystallographic preferredorientations within adjacent rock volumes of metadiorites that were mechanically and chemically re-equilibrated during the Alpine HP-IT stage. From low-strain (i.e. coronitic) to high-strain domains(i.e. mylonitic) the mechanical, chemical, and crystallographic imprints tend towards a high homog-enization through an intermediate stage (i.e. tectonitic), where both mechanical and chemical re-equilibrations are incomplete. Neutron diffraction experiments were carried out at the Institut LaueLangevin (ILL, Grénoble, France) high-flux reactor using the position-sensitive detector of the D20beamline. The detector spans a 2q range of 153.6° with a resolution of 0.1°, and the neutron wave-length is monochromatized to l = 2.41 Å. An Eulerian cradle allows the c and j angle rotations.Scans were operated from c = 0 to 90° at steps of 5° using a fixed incidence angle w of 10° and fromj = 0 to 355° (steps of 5°). Diffraction data were collected for 4 s. A Rietveld texture analysis (Lut-terotti et al. 1997) was performed for all patterns with the software package MAUD (Lutterotti et al.1999). MAUD uses a Rietveld core routine to compute spectra and a so-called Le Bail algorithm(Matthies et al. 1997) to extract the differences between random and textured intensities for eachcomputed peak. These spectra are the basis of computing the ODF using the eWIMV algorithm(Morales et al. 2002; Lutterotti et al. 2004). The obtained ODF was then introduced in the cyclicRietveld refinement. mrd = multiple of random distribution. Crystallographic orientations can berecalculated from the ODF and shown as a pole figure or an inverse pole figure: each pole figurerepresents the density distribution of the selected crystallographic plane orientation within the inves-tigated space, in terms of m.r.d. (Figure 9), with respect to the fabric axes (e.g. X, Y, Z) and struc-tures (e.g. mesoscopic foliation, XY plane, and/or lineation, X direction). Here pole figures show theorientation of (001) crystallographic planes of amphibole in coronitic, tectonitic, and myloniticmetadiorites. The intensities of mrd depend not only on the relative number of crystallites diffractingin a particular orientation in the three-dimensional fabric space but also on the diffracted intensity,which consequently depends on the grain size and lattice parameters. The variation in lattice param-eters occurring from pre-Alpine igneous to Alpine metamorphic amphiboles is negligible, whereasthe grain-size variation from coronitic to mylonitic texture may be significant. The samples used inthis texture analysis were selected minimizing the grain-size differences, to allow comparison ofresults.



specific mineral phase (e.g. Kocks et al. 1998; Leiss et al. 2000; Zucali et al. 2002a, 2010;Wenk 2006; Zucali and Chateigner 2006), which is not possible by measuring single polefigures (e.g. by a Universal Stage). From the ODF any crystallographic orientation can berecalculated and displayed as a pole figure or an inverse pole figure, as in Figure 9, wherethe orientation of (001) crystallographic planes of amphibole in coronitic, tectonitic, andmylonitic metadiorites is shown.

Within coronitic domains, amphiboles show an incipient preferred orientation of themain axis [001] with respect to the foliation plane (XY plane), associated with maximumvalues of 1.62 multiple of random distribution (mrd) (see Figure 9 caption). Thesemaxima are generally more localized than smoothed, probably because of the occurrenceof single large igneous grains, likely re-oriented towards the XY foliation plane, during theAlpine deformation phases.

In tectonitic domains, the maximum values of mrd are similar to those observed inthe coronitic domains (1.68 mrd), but here the distribution of the (001) planes shows acloser relation with the meso- and microscopic fabrics (Figure 9), and (001) polesdescribe sharp maxima at an angle of about 15° with the X direction within the XZplane (pole figure plane). Moreover, maxima are more smoothed than those previ-ously described for the same crystallographic plane in the coronitic domains; thismore homogeneous distribution of the (001) planes may be attributed to the weight ofthe newly formed grains with respect to the overall texture, likely implying an activedeformation mechanism of these grains such as oriented growth and slide mostlyalong [001] axes. This texture reflects the meso- and microscopic pattern, character-ized by a monoclinic symmetry between (001) poles and the XY foliation plane, typi-cal for a monoclinic-type shear zone.

The (001) poles, within mylonitic domains, reinforce the tendency to distribute parallelto a single maximum at 2.56 mrd. In this sample, the density distribution is sharp and canbe easily identified in parallelism with the X axis. Highest density values may be related toan almost complete grain-size reduction and homogenization at higher strain values, aswell as to the complete development of the texture, leading to an amplification of thesignals, along the textured planes. At this stage, the texture is almost completely definedby the newly formed amphiboles characterized by a different chemical composition,reflecting the newly attained Alpine PT conditions (see Ti vs. Na compositionaldiagrams in Figure 9). The preferred growth and sliding is now parallel with the X direction,showing an orthorhombic instead of a monoclinic symmetry. This apparent transitionfrom monoclinic to orthorhombic symmetry within the same shear zone, from intermedi-ate to high strains, may be explained here by the contribution given by the old igneous andthe new metamorphic grains to the crystallographic preferred orientation. Larger igneousporphyroclasts mainly passively rotate towards the X direction, reaching an equilibriumposition at an angle of about 15°, whereas the new metamorphic grains dynamically growcloser to the X direction and the intracrystalline sliding along [001] direction developsparallel to the X fabric axis.

Texture pole figures quantitatively show which lattice-scale mechanisms are activeduring the amphibole syn-metamorphic deformation, with respect to the heterogeneousstrain field within the shear zone, and allow separating contribution of the two amphibolegenerations to the overall texture, quantifying their respective mechanical behaviour.These results make the interaction between progress of metamorphic reactions (e.g. modalamount of igneous and metamorphic amphiboles) and strain rate more quantitative andmake the individuation of the contribution of single deformation mechanisms to themacroscopic fabric possible. Therefore, fabric heterogeneity may indicate the dominance



of different deformation mechanisms and may show different physical properties in asingle rock type.

Also in this last Alpine case history, dealing with a polyphase and polycyclic meta-morphic terrain, as in the previous one the discrimination of the size and boundaries of aTMU was obtained by a regional-scale detailed structural map recording foliation trajecto-ries and supported by an accurate microstructural analysis of the fabrics marking a numberof progressive metamorphic re-equilibration steps, completed by a sequence of pressureand temperature estimates and interpreted in terms of P-T-d-t paths.

The structural and metamorphic history inferred in this part of the Austroalpinedomain demonstrates that the three lithostratigraphic units described in the old literatureform a single Alpine TMU and that they reflect an Alpine strain gradient with its relatedreaction progress (Gosso et al. 2004). Actually, the three units coincide with zones of dif-ferent intensity of Alpine deformation: where the dominant fabrics are pre-Alpine, wherelow grain scale Alpine deformation domains occurred, or where Alpine metamorphictransformations are mainly coronitic. The dominant metamorphic imprint always corre-sponds to that of the most pervasive fabric, where the degree of granular-scale reorgan-ization is high. The different sequence of prevailing structural and metamorphicimprints indicates that the rock memory is contrasted in adjacent portions of this singleAlpine TMU.

3D ModellingThe two case histories show that a single TMU records a heterogeneous partitioning ofthe total deformation, which is responsible for a heterogeneous distribution of metamor-phic re-equilibration, consequent to the catalysing effect of deformation on reactionprogress. By the end of each deformation episode the result is a patchy distribution ofdominant fabrics and metamorphic assemblages, and the volume of mechanically andchemically reacting rock portions during successive stages of the tectonic evolution canbe evaluated.

An estimate of the reacting rock volumes within a single TMU has been performed forthe last example starting from a map that reports in detail the heterogeneous distribution oftextural and metamorphic transformations (map of dominant fabric domains: Salvi et al.2008, 2010). The rock volumes that reached the critical threshold of fabric evolution(≥60%) are represented on this kind of map, in which different HD volumes are mademanifest. They correspond to the most intensely reacting rock volumes during each defor-mation phase. In accordance with the meso- and microstructural observations on theLanguard-Tonale rocks, in the HD domains the metamorphic reactions coeval with thedevelopment of successive fabrics were nearly totally accomplished, and the related min-eral assemblages occupy a mean volume ≥65% (Figure 8). Salvi et al. (2010) synthesizeda map of the dominant fabric domains of part of the Languard-Tonale TMU, by groupingthe pre-Alpine structures (D1 + D2) and the two greenschist-facies Alpine deformationphases (D4 + D5) and therefore obtaining three HD domains: D1 relics are as rare as theextremely localized D5 structures. This map of the dominant fabric domains supported thequantitative estimate of the highly mechanically and thermally re-equilibrated volumes,which has been performed by means of a 3D model (Figure 10). Here subsurface con-straints are provided by carefully rendered geological cross sections, as discussed in Salviet al. (2008, 2010), on a total volume of ∼53 km3.

The size of HD domains depends on the scale of the interpreted structural map, there-fore influencing the volume estimate (Figure 10). Computation of volumes occupied by



the three HD domains are 12% for the HD(1 + 2) domain, of which 0.2% is represented bythe lenticular relics preserved within the HD3 domain; 37% for the HD3; and 51% for theHD(4 + 5) domains. Using the fabric evolution versus metamorphic transformation rela-tionships (Figure 8), Salvi et al. (2010) estimate the minimal volumes occupied by themineral assemblages synkinematic with each group of structures characterizing the threeHD domains: the pre-Alpine assemblages (syn-D1 + D2) occupy a minimal volumeranging from 7 to 12%, the early Alpine assemblages (syn-D3) occupy a minimal volumeof 22–37%, and finally, the late Alpine assemblage (syn-D4 + D5) only covers a minimalvolume of 30–51%.

If the tectono-metamorphic map (Figure 7) is compared with the 3D model of thedominant fabric domains (Figure 10), it clearly appears that the distribution of the high-and low-strain domains is not controlled by lithology, as HD domains crosscut litho-logic boundaries. The resulting pattern most probably derives by strain softening, afterstrain localization during successive deformation stages and therefore making the origi-nally resistant rocks (e.g. diorites or granulites) softer by grain-size reduction alongdeformation zones, which in turn may develop independently on pre-existing lithologicboundaries.

Figure 10. Different views of the 3D volumetric model of the map of the dominant fabric domainsof the southern part of the Languard-Tonale TMU (about 53 km3), located in Figure 11. HD4domains correspond to about 52% of the whole volume, HD3 domains occupy about 37%, and thepre-Alpine HD2 domains correspond to about 12%. The occurrence of small-scale lenses of D3 andD2 relicts in HD4 and HD3 zones points out the influence of the scale on the estimate of totallymechanically and mineralogically re-equilibrated volumes.



Numerical modelling and geodynamic interpretationsIn both the Alpine examples, a regional metamorphic imprint developed under HT-LPconditions during Permian–Triassic times (e.g. Diella et al. 1992; Spalla et al. 1995;Zucali 2001). This Permian to Triassic HT-LP metamorphism affected wide portions ofthe Southalpine and Austroalpine crust and postdates the Variscan subduction and collision(Ledru et al. 2001; Spalla and Marotta 2007 and refs. therein). Anomalously high thermalregimes are consequent to the late orogenic collapse of a collisional belt or to lithosphericthinning, thereby promoting continental rifting (e.g. Thompson 1981; Wickham andOxburg 1985; Sandiford and Powell 1986; Platt 1998; Beardsmore and Cull 2001). Thisinterpretative ambiguity is enhanced when continental rifting does not develop on a stablecontinental lithosphere, but follows the thermal and mechanical instabilities induced by asubduction–collision process, in which the rifting precursor signals overprint the markersof a late orogenic extension as it is the case in the Alps. Here the occurrence of relictVariscan structural and metamorphic imprints indicates that before the Pangaea break-up,the continental lithosphere was thermally and mechanically perturbed by Variscansubduction and collision. To reduce this ambiguity, numerical geodynamic models havebeen implemented using finite-element techniques for analysing the effects of activeextension during the Permian–Triassic period (from 300 to 220 Ma), overprinting a previoushistory of Variscan subduction–collision up to 300 Ma (Marotta and Spalla 2007; Spallaand Marotta 2007; Marotta et al. 2009). The model predictions on thermal state and strainlocalization have been compared with metamorphic data, time interval of plutonic andvolcanic activity, and coeval onset of sedimentary environments and indicated that (i) dur-ing Palaeozoic, the pre-Alpine crust was part of an active ocean–continent convergentmargin that evolved into an intracontinental suture zone; (ii) the Permian–Triassic meta-morphic, igneous, and sedimentary imprints cannot result from the thermal relaxationsuccessive to lithospheric unrooting characterizing the late orogenic extension, becausethe simulated thermal detachment occurs too early (360 Ma) with respect to the naturalthermal records (290–225 Ma; Marotta and Spalla 2007; Spalla and Marotta 2007); (iii) arelatively high rate of active extension (≥2.0 cm a-1) is required to achieve the thermal fitwith the maximal number of natural PT data from different tectonic units and to reach thethermal conditions allowing partial melting of the crust accompanying gabbroic intrusionsand the HT-LP metamorphism (Marotta et al. 2009); (iv) during the late orogenic evolu-tion a decoupling of the deformation regime occurs between the upper crust and the lowercrust/mantle coherent system, whereas in the active extensional models the extensionalregime is widespread through the crust, allowing strain localization that in natural systemsmay correspond to faulting that generates basin opening and provides pathways formagma uprising; and (v) strain localization shows a periodicity, comparable as order ofmagnitude with that of episodic faulting and related volcanic pulses, basin deepening, andfacies fluctuations in natural systems. Agreement between model predictions and Permianto Triassic natural data, in terms of age, thermal gradient, and compositional affinity (oce-anic or continental crust, lithospheric mantle), appears to be justified by the progression oflithospheric thinning, later evolving into formation of new oceanic lithosphere.

This example indicates that a thermomechanical model helps to discriminatebetween ambiguous tectonic interpretations on the base of compatibility between PTestimates from natural rocks and the thermal states predicted by a set of numerical sim-ulations, allowing individuation of a geodynamic scenario based on physical reliability.Numerical modelling has been critical to support the geodynamic interpretation on thesubduction mechanism invoked to justify the subduction and exhumation of continental



crust units of the Western Alps, characterized by a very high P/T ratio (Meda et al.2010). To verify if this significantly high P/T ratio can result from tectonic erosion,ablative subduction, and recycling in a mantle wedge, a 2D numerical model to simulateoceanic subduction beneath a continent has been implemented, focusing on the roleplayed by mantle hydration in the recycling of continental crust in the wedge region. Asit is well known (e.g. Spalla et al. 1996; Berger and Bousquet 2008), in the WesternAlps HP-LT metamorphism widely affects not only units derived from the Tethyanoceanic lithosphere (i.e. ophiolites and related sedimentary sequences), but also largevolumes of the pre-Alpine continental crust, among which the Austroalpine Sesia-LanzoZone (SLZ; 3 in Figure 2) is the largest fragment with the Late-Cretaceous (earlyAlpine) eclogite-facies imprint (e.g. Bearth 1959; Ernst 1973; Compagnoni et al. 1977;Dal Piaz and Ernst 1978; Pognante 1989a, 1989b; Zucali et al. 2004). The Alpine evolu-tion of the latter has been interpreted as compatible with a cycle of deep burial andexhumation during an active subduction of oceanic lithosphere (e.g. Spalla et al. 1996;Zucali et al. 2002b and refs. therein). This tectonic interpretation has been suggested bythe peculiar high P/T ratio characterizing the Pmax-TPmax (Table 3) and the retrogradeexhumation-related path.

Modelling predictions (see detailed discussion in Meda et al. 2010) indicate thathydration is fundamental to allow recycling of crustal material, buried in-depth into themantle, at depths ≤150 km, making the uprising and exhumation of buried crustal materialduring active subduction possible [Figure 11(a) and (b)]. Another significant result is thatthe recycled crustal material can be sampled from any crustal level of the continentalupper plate (Figure 12). As also indicated by the Pmax and ΔPmax final distributions(Figure 13), the crustal recycling induces the coupling of volumes that reached differentdepths during their corner flow paths, justifying the juxtaposition, at the end of the sub-duction–exhumation process, of slices that underwent different PT trajectories, to assem-ble a continental crust agglomerate which can correspond to an Alpine nappe.

In addition, the relationships between natural PT estimates, from the AustroalpineSLZ rocks, and predicted PT values (Table 3 and Figure 14) show that the simulatedgeodynamic scenario generates a thermal regime coherent with that affecting the sub-ducted continental crust of the SLZ. This regime may have remained stable for a long timeduring Alpine subduction, allowing the SLZ rocks to accomplish their burial and exhumationpath under an active subduction regime, before continental collision.

Discussion and conclusionThe discussed Alpine examples highlight that tectonic analysis devoted to geodynamicreconstructions, in terrains affected by polyphase deformation and metamorphism,requires a new approach merging petrologic and structural investigations, based ondetailed correlation of superposed fabric elements, microstructural analysis, and fieldrecognition of fabric gradients. The extent, degree, and timing of metamorphic re-equili-brations and associated fabric changes can be used to define the size and shape of TMUs,which are the rock volumes that underwent the same tectonic path during a time interval.

TMUs are very different in shape from units contoured on the base of dominantmetamorphic imprints or of pure lithostratigraphic settings, in different metamorphicenvironments from HP-LT to HT-LP. Our results indicate that

− there is no correspondence between the dominant metamorphic imprint and theTmax–PTmax imprint of each P-T-d-t path. The dominant metamorphic imprint is



Tabl

e 3.

Rev

iew

of t

he m

etam

orph

ic a

ssem

blag

es, P

T es

timat

es o

f clim

ax c

ondi

tions

, and

radi

omet

ric a

ges

in th

e Se

sia-

Lanz

o Zo

ne ro

cks.

Key

s ar

e re

porte

d in

Figu

re 1

4.

Key

PT c

limax

ass

embl

age

P max

(GPa

)T m

ax (°

C)

PT c

limax

age

(Ma)

Ref

eren

ces

1M

etag

abbr

os G

ln, C

ld, E

p, G

rt±

Ph±

Pa1.

75±

0.25

500

±50

60–7

5R

ebay

and

Mes

siga

(200

7)2

Mar

bles

Cal

, Dol

, Qtz

, Na-

Cpx

, GrtI

, PhI

, ZoI

, Al-T

tn1.

5±

0.1

570

±50

/C

aste

lli (1

991)

3M

etap

elite

s Cpx

, Grt,

Ky,

Ph,

Qtz

±G

ln±

Cld

; m

etag

rani

toid

s Grt,

Cpx

, K

fs, P

h, Z

o, Q

tz

1.45

±0.

1562

5±

2571

.2±

3.2

Hy

(198

4)

4M

etag

rani

toid

s Jd,

Qtz

, Grt,

Zo,

Qtz

, Ph

1.25

±0.

1551

0±

3012

9±

15; 1

14O

berh

änsl

i et a

l. (1

985)

5M

etap

elite

s Grt

±O

mp,

Ph

±Pa

±G

ln±

Ep, Q

tz;

met

abas

ics G

rt, O

mp,

Rt±

Ph±

Gln

±Ep

; mar

bles

Cal

(D

ol, A

nk)±

Grt

±Ph

±Zo

±Q

tz

1.6

±0.

155

0±

5090

–67

Com

pagn

oni e

t al.

(197

7)

6G

rt, O

mp

or Jd

, Zo,

Rt,

Cld

1.5

±0.

260

0±

5090

–70

Will

iam

s and

Com

pagn

oni (

1983

)7

Grt,

Om

p, Z

o, P

h, R

t, Q

tz2.

4±

0.2

600

±50

65±

3Sp

alla

et a

l. (1

997)

8O

mp,

Grt,

Zo

±G

ln±

Bar

1.55

±0.

1552

5±

25/

Lard

eaux

et a

l. (1

982)

9Jd

, Qtz

, Grt;

Om

p, G

rt1.

40±

0.15

450

±50

/A

ndre

oli e

t al.

(197

6)10

Qtz

, Ph,

Om

p/Jd

, Gln

, Rt±

Pa±

Ctd

±Zo

1.65

±0.

3552

5±

25/

Pogn

ante

(198

9a, 1

989b

)11

Jd, P

h, G

rt±

Kfs

±Zo

1.15

±0.

0542

5±

25/

Pogn

ante

(198

9a, 1

989b

)12

Gl,

Lws,

Ep, S

ph, Q

tz1.

0±

0.2

350

±50

/Po

gnan

te (1

989a

, 198

9b)

13G

rt, O

mp,

Pa

1.6

±0.

157

5±

25/

Rei

nsch

(197

9)14

Grt,

Om

p, G

ln1.

18±

0.08

503

±28

/D

esm

ons a

nd G

hent

(197

7)15

Eclo

gitiz

ed g

ranu

lites

Om

p, G

rt, Q

tz, A

mp;

ecl

ogiti

c gn

eiss

es Jd

,Grt,

Ph,

Ky;

met

abas

ics O

mp,

Bar

±G

ln1.

4±

0.1

525

±25

/G

osso

et a

l. (1

982)

16M

etag

rani

toid

s Grt,

Jd, P

h, Z

o, Q

tz; m

etap

elite

s Om

p, G

rt,

Am

p, P

h, Q

tz; m

etab

asic

s Grt,

Om

p, A

mp,

Qtz

1.65

±0.

1557

5±

2565

±5

(Muc

rone

); 65

±3

(Aos

ta V

alle

y); 6

8±

7 (B

onze

)R

ubat

to e

t al.

(199

9)

17Jd

, Zo,

Qtz

±K

y±

Kfs

1.7

±0.

155

0±

3065

Com

pagn

oni (

2003

)18

Om

p, G

rt, Z

o±

Gln

±B

ar+

Qtz

1.65

±0.

3555

0±

5066

±1;

55–

48C

aste

lli a

nd R

ubat

to (2

002)

(Con

tinue

d)



Tabl

e 3.

(Con

tinue

d).

Key

PT c

limax

ass

embl

age

P max

(GPa

)T m

ax (°

C)

PT c

limax

age

(Ma)

Ref

eren

ces

19Ph

±O

mp,

Grt,

Ttn

1.5

±0.

257

5±

45∼6

5C

aste

lli a

nd R

ubat

to (2

002)

20O

mp,

Czo

±Zo

, Ky,

Qtz

, Rt

2.0

±0.

161

0±

40/

Gio

rgie

tti e

t al.

(200

0)21

Par,

Gln

, GrtI

I1.

5±

0.1

550

±50

69.4

±0.

7; 1

40.5

±0.

6R

uffe

t et a

l. (1

995)

22G

rt, Jd

, Zo,

Ph

1.85

±0.

1560

0±

50/

Trop

per e

t al.

(199

9)23

Om

p, C

zo, G

rt, K

y, R

t, Q

tz±

Zo1.

9±

0.2

600

±50

/Tr

oppe

r and

Ess

ene

(200

2)24

Met

apel

ites P

h, Q

tz, G

rt, O

mp,

Am

p, R

t; m

etab

asic

s Ph,

A

mp,

Grt,

Rt±

Om

p±

Zo±

Cc;

met

aint

rusi

ves P

h,

Am

p, G

rt, R

t±O

mp

±Zo

/Czo

±C

c; q

uartz

ites P

h, G

rt,

Rt,

Ky,

Cld

±C

c

2±

0.3

550

±50

65±

5Zu

cali

et a

l. (2

002b

)

25M

etap

elite

s Jd,

Wm

, Qtz

, Kfs

; met

abas

ics G

l, G

rt, R

t, O

mp

±Zo

±Tc

±W

m1.

6±

0.2

525

±25

/Po

gnan

te e

t al.

(198

7)

26M

etap

elite

s Wm

±Zo

±C

ld±

Gl±

Ky;

met

abas

ics G

l/Act

, Zo

0.9

±0.

135

0±

50/

Pogn

ante

et a

l. (1

987)

27Jd

, Qtz

, Wm

, Kfs

1.35

±0.

1543

0±

20/

Pogn

ante

et a

l. (1

987)

28Fe

-Om

p, A

b, P

h, G

rt1.

3±

0.1

540

±10

/La

rdea

ux e

t al.

(198

3)29

Om

p, K

y, E

p, G

rt2.

1±

0.1

610

±20

60–7

0Zu

cali

et a

l. (2

004)



instead that associated with the most pervasive fabric, where its degree of evolutionreached a critical threshold;

− the ‘metamorphic field gradient’ does not match the spatial distribution of dominantmetamorphic imprints that, on the contrary, depends on heterogeneity of the defor-mation and therefore TMUs can be contoured only with the support of a detailedregional structural map. Therefore, different metamorphic imprints may dominatewithin a single TMU, or equivalent dominant metamorphic imprints can character-ize different TMUs;

− correlated P-T-d-t evolutions allow to trace TMU boundaries, which differ withrespect to contours of lithostratigraphic units; in fact the three lithostratigraphic sub-divisions proposed in the Central Austroalpine basement simply correspond tozones of Alpine deformation partitioning characterizing a single Alpine TMU.

Figure 11. Model predictions on the upper plate continental crust during a numerical simulationof subduction implying the hydration of the mantle wedge (Meda et al. 2010). In (a) and (b) thethermomechanically predicted configurations at 10 Ma (a) and 30 Ma (b) are shown from thebeginning of the numerical simulation. Continental crustal markers are in brown and oceanicmarkers in black; the background, grading from blue to violet (see the legend), represents the tem-perature field and white arrows the velocity field. Axial coordinates are in kilometres (modifiedafter Meda et al. 2010).



The comparison between a tectono-metamorphic map and a 3D model of the dominantfabric domains shows that high- and low-strain domains distribution is independent oflithology, as they transect lithologic boundaries: this is probably because of strain soften-ing, after strain localization during successive deformations.

The quantitative 3D estimate of the distribution of differently re-equilibrated volumesoffers fundamental insights into the accomplishment, at different structural levels alongactive plate margins, of structural and/or metamorphic re-equilibrations within a TMU.Generally only a half of the total rock volume was mechanically and chemically re-equili-brated during the late stages of the tectonic and metamorphic evolution and up to one-tenth of the total volume preserves the structural and metamorphic imprints related to theearlier stages.

Results from the type of field and laboratory procedure proposed here are useful torefine, constrain, and verify geophysical modelling that simulates the mechanicalbehaviour at active plate margins, assuming the changes of continental or oceanic rheol-ogy on the base of full accomplishment of the predicted phase transitions that drivechanges of the dominant active deformation mechanisms. A widespread individuationof TMUs across the belt and the associated use of 3D modelling will allow estimates ofvolumes preserving textural and mineral relicts after phase transitions and will help toevaluate the potential influence that relict domains will have on the choice of the phys-ical parameters for thermomechanical modelling, such as, for example, density orviscosity.

Although contouring of TMUs is still very localized across the Alps, an advantage isthat high-quality P-T-t or P-T-d-t paths, frequently based on accurate petrostructural field-work, are already diffused in this belt and are referred to a wide span of time, oftenextending beyond the Alpine history. The integrated use of these metamorphic data

Figure 12. The thermomechanical evolution of markers from the continental crust in the hydratedmantle wedge is shown after 60 Ma from the beginning of subduction; the marker colours indicatetheir initial structural levels: red = upper crust (0–10 km); yellow = middle crust (10–20 km);green = lower crust (20–30 km); black lines are isotherms every 200 K, until 1500 K (modified afterMeda et al. 2010).



together with the other natural igneous and sedimentary markers turned out to be funda-mental to infer ancient geodynamic settings through numerical simulations.

The described examples show the effectiveness of numerical modelling in discriminat-ing between ambiguous tectonic interpretations, allowing the choice of the most compati-ble geodynamic scenario on the base of physical reliability, of age coincidence, ofcompositional affinity (oceanic or continental crust, lithospheric mantle), and of compati-bility between PT estimates from natural rocks and the thermal states predicted duringsuccessive numerical simulations. It is worth noting that this has been possible also wherethe natural data derive from a fragmented setting, carrying reminiscences of rifted marginseven after a subduction and collision, as in the case of the Alpine orogeny. Therefore thejoint use of natural data and numerical modelling may help to infer the puzzled riftingevolution even in a relict passive margin fossilized within a mountain belt and this inte-grates palaeogeographic reconstructions.

Figure 13. (a) Pmax (GPa) recorded by each continental crustal marker; (b) ΔPmax (GPa) calculatedfor each continental crustal marker. Each marker is represented in its final configuration, after70 Ma from the beginning of the subduction (modified after Meda et al. 2010).



On the contrary, the numerical modelling on ocean–continent subduction systemsindicates that continental lithosphere slices can be ablated from the upper plate, bur-ied and exhumed during active subduction, before continental collision. The simu-lated thermal regimes predict metamorphic evolutions, accomplished under high P/Tratios, coherent with those affecting some subducted Alpine continental slices as theSLZ.

In addition, modelling predictions suggest that recycling of subducted crustalslices is possible in a hydrated wedge and that the recycled continental crust can besampled from any structural level of the upper plate. Simulated crust recycling pro-duces a configuration, at the end of the subduction/exhumation process, in whichslices that underwent different PT trajectories are juxtaposed, testifying the couplingof volumes that reached different depths during their corner flow paths. This mecha-nism allows envisaging an Alpine continental nappe of the axial zone as a crustalagglomerate of regurgitated deep material, assembled at shallow depth within thesupra-subduction wedge.

Results from the analytical procedure, applied in the axial Alpine belt, and predictionsfrom numerical simulations drive to a new nappe concept; in addition, they make clearthat the comprehension of mechanisms active at different structural levels in convergentsystems must be further investigated before adopting palaeogeographic reconstructionsexclusively inferred from considerations on the shallow crust tectonics.

Figure 14. Comparison between PT conditions reached by the subducted continental crustal mate-rial (coloured markers) at different times from the beginning (0 Ma) of the numerical simulation; thetime reference is given in the vertical legend. The light blue and red lines represent the minimal andmaximal P/T ratios, respectively; the black line represents the maximal T/depth ratio during the latestage of evolution; black squares plot the natural PT values of Pmax TPmax conditions estimated forthe Sesia-Lanzo Zone rocks according to the data review of Meda et al. (2010).



AcknowledgementsThe critical reading by Bernhard Stoeckhert greatly improved the text. Patient editing and advicefrom Y. Dilek and W.G. Ernst are warmly acknowledged. Funding from PUR 2008 of Universitàdegli Studi di Milano and from CNR-IDPA were utilized.

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Analysis of natural tectonic systems coupled with numerical modelling of the polycyclic continental lithosphere of the Alps

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