Stratigraphy in the continental crust: lithologic and tectonic records

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Ital.J.Geosci. (Boll.Soc.Geol.It.), Vol. 128, No. 2 (2009), pp. 473-482, 9 figs. (DOI: 10.3301/IJG.2009.128.2.473)

Stratigraphy in the continental crust: lithologic and tectonic records GUIDO GOSSO (*) & MARIA IOLE SPALLA (*)

ABSTRACT Preservation of stratigraphic settings in continents is mostly confined to uppermost crustal levels, in which prevailing deformation is translational and internal strain of tectonic units is weak. This does not specially depend on association of metamorphism to tectonic processes, but rather on mechanical properties of deforming multilayers. Scattered findings of sedimentary features, not supported by structural investigation on the actual nature of lithologic layering, may lead to failure in determining size of preserved sedimentary sequences. In the intermediate and lower crust, interaction of metamorphism and deformation down to granular scale facilitates construction of new types of lithologic layers and segregation of thick differentiated mineral layerings that mimic stratigraphic sequences. Critical examples of difficulties encountered in different tectonic contexts when assessing the sedimentary origin and related stratigraphic meaning of variously deformed layered sequences are summarised. In deep subduction-collision zones similar or contrasted lithostratigraphies are of little help in definition of tectonic units; the structural and metamorphic reworking of rocks of contrasted origin constructs tectonic units that repeatedly couple and decouple from similar adjacent sequences and their actual relative mechanical paths may be disclosed by a combined structural and petrologic analytical tool delimiting volumes that experienced equivalent structural histories and metamorphic signatures (contouring of tectono-metamorphic units=TMUs); these units constitute valuable elements of correlation in metamorphic belts and for the investigation of mechanisms of lithosphere dynamics.

KEY WORDS: Tectonics, metamorphism, stratigraphy, structural correlation. RIASSUNTO La stratigrafia nella crosta continentale: impronte tettoniche e litologiche. La conservazione di rapporti stratigrafici intatti nei continenti è relegata ai livelli superiori della sovrastruttura crostale, ove la deformazione dominante è di tipo traslativo e la distorsione interna delle unità tettoniche è debole. Ciò non dipende particolarmente dall’associazione di un ambiente metamorfico alla deformazione, ma invece dalle proprietà meccaniche interne dei multistrati. Isolati ritrovamenti di strutture sedimentarie, o di fossili, non accompagnati da uno studio sistematico della reale origine dell’alternanza di composizione litologica (natura della litostratigrafia fine) portano facilmente all’indeterminatezza nella conoscenza della scala di preservazione di reali sequenze sedimentarie. Nella crosta intermedia e profonda l’interazione di ambienti metamorfici con la deformazione, diffusa sino alla scala granulare, facilita la costruzione di nuovi tipi di alternanze di composizione litologica e la segregazione di nuove alternanze di composizione mineralogica potenti sino ad alcuni centimetri, che, insieme, mimano sequenze stratigrafiche; si discutono esempi critici e tipi di difficoltà incontrate nella determinazione della natura sedimentaria di sequenze litologiche alternate. Nelle zone di subduzione e collisione profonde, i contrasti o le somiglianze nelle caratteristi-

(*) Dipartimento di Scienze della Terra «A. Desio», Università degli Studi di Milano and C.N.R.-I.D.P.A. Via Mangiagalli, 34 - 20133 Milano, Italy. [email protected], [email protected].

che litostratigrafiche sono di scarso aiuto per la definizione di unità tettoniche; la rielaborazione strutturale e metamorfica delle rocce, spesso di contrastata provenienza originale, costruisce unità tettoniche che si sono ripetutamente accoppiate e disaccoppiate da simili sequenze adiacenti. I loro reali percorsi meccanici possono essere rivelati da una nuova procedura analitica combinata, strutturale e petrologica, capace di delimitare i volumi che hanno seguito impronte strutturali e storie metamorfiche equivalenti (determinazione delle unità tettono-metamorfiche = TMU); questo tipo di untà è quindi il valido elemento di correlazione nelle catene metamorfiche, per comprendere i processi tettonici che regolano la dinamica litosferica.

TERMINI CHIAVE: Tettonica, metamorfismo, stratigrafia, correlazione strutturale. INTRODUCTION

The tectonic and lithostratigraphic evolution of continents is the long-term record of the Earth’s history. Reconstructions of steps in the structural development of the continental crust are based firstly on lithostratigraphic grounds, tracking the sedimentary, igneous and metamorphic changes, which are the basic references to the relative updating of the geologic clock. Several petrognetic and tectonic pulses may have modified previous lithostratigraphic settings at various levels of the crust. Therefore, different investigation strategies must be applied in accordance with types of processes active in the crustal environments in which the present rock association was presumably forged. At the shallowest tectonic levels, above the Conrad discontinuity, tenets of thrust tectonics (thick- and thin- skin) sufficiently help to reconstruct sedimentary sequences and solve eventual stratigraphic ambiguities affecting the deformed sedimentary record within overthrust units (e.g. MCCLAY, 1991). In the inner parts of foreland belts, close to rims of collisional sutures between continents (e.g. the external Penninic zone of the European Alps), pervasive ductile imprints following simple duplexing of sedimentary sequences impose comparison of internal strain patterns of thrust units with fold history of thrust surfaces. In such cases the comprehension of relative timing of multiple tectonic events should support correct stratigraphic reconstructions (YIN & OERTEL, 1993). This procedure becomes of general use in the intermediate to deep ductile metamorphic crust in axial zones of collisional belts. Here, any major lithostratigraphic change must be connected, at an appropriate scale, with the kinematics and timing of deformation imprints, established by overprinting criteria, and with lithologic changes induced by metamorphic reactions. With the beginning of modern structural investigation, significant events such as the addition of new rock sequences to previous units of the continental crust (intrusions of igneous rocks or unconformable sediments)

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record, see chapt. 5.4.3, in HOBBS et alii, 1976). A multiscale backward reconstruction (deconvolution) of the structural imprints, from present to past, is virtually possible in present-day evolved lithostratigraphic sequences of the intermediate and deep continental crust. It leads eventually to discrimination of earlier stratigraphic settings, in turn referred to oldest tectonic imprints, conventionally considered as ancestral reference. The basics of kinematic criteria and of mesoscopic analysis for field discrimination of complex superposed tectonic events are summarised in current textbooks (e.g. HOBBS et alii, 1976; PASSCHIER et alii, 1990). These criteria are applicable to deep seated rock associations of collision zones and old cratons, disregarding their pristine origin (igneous, metamorphic or sedimentary). Integrated analyses of structural and petrogenetic developments can virtually unravel relationships between all rock types and individuate the specific structural signatures of each rock group and those shared with other lithologies. In addition, convergent and divergent active margins are sites in which coupling and decoupling of crust and mantle tectonic slices may construct and cyclically rework new kilometrescale lithostratigraphic configurations. In axial zones of subduction-collision belts the contrasted thermomechanical evolutions of tectonic units account for multi-stage amalgamation or splitting of tectonic slices (e.g. CLOOS, 1985; ENGI et alii, 2001; SPALLA et alii, 1996). Complex dynamics of this type are helpfully predicted in recent numerical modelling of subduction systems (e.g. GERYA & STOECKHERT, 2006; MAROTTA & SPALLA, 2007). SMALL SCALE INVESTIGATIONS Fig. 1 - Preserved cross bedding (laminae and strata sets) in whiteschist facies metaquartz-arenites (with a quartz, kyanite, phlogopite, garnet, intermediate plagioclase, biotite and scapolite mineral association) from the Lower Roan sequence of the Northwestern Zambian Copperbelt, Tectonic Domes area (COSI et alii, 1992). Cliff edge is 4 m high. – Stratificazione incrociata (set di strati e set di lamine) preservata in metaquarzoareniti in facies scisti bianchi (con un’associazione mineralogica a quarzo, cianite, flogopite, granato, plagioclasio intermedio, biotite e scapolite) della sequenza Lower Roan del Copperbelt, Arco Lufiliano dello Zambia Nordoccidentale (area dei duomi, COSI et alii, 1992). Il margine della parete è alto 4m.

became fundamental for the understanding of the geologic records in terms of tectonic and lithologic history. Two milestones illustrating this approach are (i) the Torridon unconformity in the Moine-Lewisian complex (RAMSAY, 1958; RAMSAY, 1967) and (ii) the Hercynian unconformity in the Pennine zone of the Western Alps (DAL PIAZ, 1939). In these two examples it was shown for the first time that progressive tectonic modifications of geometric relationships within the whole lithostratigraphic sequence (tectonic amalgamation with the preexisting basements, re-foliation, transposition) may severely alter the successions of sedimentary layers or the lithologic relationships within an intrusive complex. This calls for resolution of the full sequence of structural imprints, indispensable for retracing the tectonic events and establishing correlation between structure and petrogenesis (ZUCALI, 2001). Obstacles are obviously manifold on this path, (e.g. mimic primary features of the sedimentary

Recognition of correspondence of present sedimentary sequences to their original stratigraphic setting appears to be of moderate difficulty in most unmetamorphosed upper crustal tectonic contexts, where deformation has been dominantly translational. This is seldom the case in lithostratigraphic sequences of the middle and lower continental crust. Such sequences may surprisingly display scattered traces of depositional structures even in severely metamorphosed sequences (e.g. fig. 1, COSI et alii, 1992), or fossil imprints of Carboniferous-Permian pteridophyll leaf fragments in amphibolite facies rocks from the Tauern tectonic window of the eastern Alps (FRANZ et alii, 1991). Similar findings, exceptionally preserved in high grade rocks thanks to heterogeneities of strain distribution, may strongly encourage interpretation of the complete lithologic sequence as an original stratigraphic association, fully preserved at the broad scale, instead of an association of exotic rocks infolded within the sequence and amalgamated by transposition. Conversely, regional occurrences of slaty cleavage in pelitic sequences do not ensure, up to very-low grade, a coeval (syntectonic) metamorphic overprint (KISCH, 1989). Of particular interest for discriminating original stratigraphies is the recognition of the actual sedimentary origin of rocks defined as metaconglomerates in metamorphic terrains and in mylonitic fault zones, which may often be important as economic targets. A number of reported interpretations of lithologic types as metaconglomerates, therefore regarded as potential basal terms of real sedimentary sequences, may on the contrary be acknowledged as tectonic in origin. Additional structural

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Fig. 2 - Generation of a false conglomerate by sequence of increasing strain in time: a) seams of quartz-rich layers in a folded micaschist of the Lower Roan Series (Northwestern Zambian Copperbelt, Lufilian Arc) display thinned and interrupted fold limbs. Lenticular truncation of beds begins to simulate subspherical pebble-like clasts, during progressive tightening of folds; b) close up view of a tight fold hinge in a metaquartz-arenite bed, with progressing isolation of lensoidal quartz rich elements from the original sedimentary layer; c) vestigial bed of metaquartz-arenite (center, downdipping to the right), fully subdivided into subspherical pseudo-clasts of glassy quartz, within a finely foliated mylonitic schist. Bottom edge of photograph is about one metre long. – Sequenza temporale, e di strain crescente, nella generazione di un falso conglomerato: a) sciami di livelli ricchi in quarzo, in un micascisto piegato della Lower Roan Series (Copperbelt dello Zambia Nordoccidentale, Arco Lufiliano), mostrano fianchi delle pieghe assottigliati e interrotti; la troncatura lenticolare dei livelli inizia a simulare clasti simili a ciottoli subsferici durante la progressiva chiusura delle pieghe; b) ingrandimento di una cerniera di piega serrata in uno strato di metaquarzoarenite con progressivo isolamento di elementi lenticolari ricchi in quarzo dall’originario livello sedimentario; c) strato relitto di metaquarzoarenite (centro, inclinato a destra) totalmente suddiviso in pseudo-clasti subsferici di quarzo ialino, in uno scisto milonitico finemente foliato. Il lato lungo della foto è circa 1m.

Fig. 3 - (a) Pseudoconglomerate consisting of rounded pebble-like elements of granite, hydrothermal quartz and carbonates in a finely foliated granitic cataclasite-mylonite matrix (Cayre Ponciù-Lac Nègre, Mollières valley, Argentera-Mercantour massif, French Maritime Alps). Marker pen provides scale; (b) Thick vein of hydrothermal carbonates dismembered into rounded clasts in a cataclastic to mylonitic matrix of granite, (Lac Nègre Southern side). – (a) Pseudoconglomerato composto da elementi simili a clasti arrotondati di granito, quarzo idrotermale e carbonati in una matrice granitica cataclastico-milonitica finemente foliata (Cayre Ponciù-Lac Nègre, Valle di Mollières, Massiccio Argentera-Mercantour, Alpi Marittime Francesi). La penna fornisce la scala; (b) Spessa vena di carbonati idrotermali, situata in una matrice granitica da cataclastica a milonitica, suddivisa in clasti arrotondati (Lac Nègre, sponda meridionale).

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Fig. 4 - Dispersion of segmented siliceous marly layers in a tightly folded contact metamorphic silicic marble from the southern Adamello contact aureole (Val Fredda, NW of the Alpe di Bazena stables, at the intrusive rim of the Re di Castello stock). Hinge zone is not recognizable outside the photograph along the same axial plane. Parallelism of newly differentiated axial plane mineral layering with overthinned strata in fold limbs generates confusion (lower center and left side of the image) of old, sedimentary, with new, tectonic mineral layerings, of evident different origin and age; in the hinge and right limb zones a newly differentiated mineral layering is clearly oblique to strata. Sequence-top inversion and obliteration of strata are the catastrophic effects on stratigraphy in this transposition foliation. Grass stalks, gravel and coin provide scale. – Dispersione di livelli silicei e calcareo-marnosi segmentati in un marmo a silicati, con pieghe serrate, affetto da metamorfismo di contatto nell’aureola dell’Adamello meridionale (Val Fredda, NO dell’Alpe Bazena, al margine dell’intrusione Re di Castello). Non si riconosce altra cerniera, lungo lo stesso piano assiale, fuori dalla fotografia. Il parallelismo del layering mineralogico di piano assiale di nuova differenziazione, con strati sovrassotigliati ai fianchi delle pieghe, genera confusione (al centro in basso e al margine sinistro dell’immagine) tra layering mineralogici vecchi e nuovi, di evidente diversa origine ed età relativa; in alto a destra il layering di nuova differenziazione è chiaramente obliquo agli strati. L’nversione del tetto della sequenza e l’obliterazione degli strati sono gli effetti catastrofici generati sulla stratigrafia dall’imposizione di questa foliazione di trasposizione. Fili d’erba, ciottoli e una moneta forniscono la scala.

analysis may implement clarification in cases when no depositional structures are available in support of their sedimentary nature. A few examples are reported here. In the lowermost levels of the presumably stratigraphic Lower Roan Series of the North-Western Zambian Copperbelt, transposition and lenticularisation of alternate metaquartz-arenite beds within micaschists generated a conglomerate-like structural association of rounded quartz elements (hosting tiny kyanite needles) incorporated within a micaschist matrix; the latter is shown to derive from the pelitic interlayers of the original quartzitic beds. The beds lenticularisation and polarity inversion processes took place in the quartzites (transposition in HOBBS et alii, 1976; LEBEDEVA, 1979) under whiteschists facies. In figs. 2a, b and c, COSI et alii, 1992) the sequence of increasingly strained kyanite-phlogopite bearing micaschists is displayed. Alternate folded sets of metaquartz-arenite beds are progressively transformed into lenticular, pebble-like, rounded strata segments and disrupted fold hinges (fig. 2b) within a micaschist marked by a mylonitic foliation (fig. 2c). Conglomeratic schists of this type, regarded as fundamental stratigraphic terms to assess the sedimentary unity of the overlying sequence, are frequent in the same region. The Rufunsa quartzite of the Kibaran belt of Zambia (UMEMURA & SUWA, 1983) is a further example of the same process. In the Western Alps, within the eclogitic micaschists of the Sesia-Lanzo zone, in the lower Aosta Valley, jadeite megablasts described by ANDREOLI et alii (1976) as outstanding subduction-related metamorphic overgrowths of high-pressure minerals after the plagioclase sites of granitoids were erroneously reinterpreted as pebbles of a monogenic Permian-Triassic conglomerate in a micaschist matrix by VENTURINI (1995). Again, at Lac Négre in the ArgenteraMercantour massif of the South Western Alps (FAUREMURET, 1955) (fig. 3) a hydrothermalised (silica and carbonates) mylonitic fault zone within a Permian granite (the poudingue de Bresses of the local stratigraphy) is another example of pseudo-conglomerate of tectonic origin. Such rocks may frequently be misunderstood in severely foliated schists if the effects of overprinting strain, giving rise to evolved tight fold systems, or to mylonite zones, are disregarded. Beyond the difficulties in determining the sedimentary origin and related stratigraphic significance of single lithologic types, deformed stratigraphic units may be analysed as multilayer sequences over thickness ranges of 100-1000 m, in which recognition of depositional local architecture and polarity is periodically hindered by local loss of stratal continuity of tectonic origin. The fluid-rich contact metamorphic aureole of the Tertiary Adamello pluton is a good example of fluidassisted intense granular scale deformation in the weakly metamorphic, fossil-bearing pelitic and siliceous carbonate rock sequences of the Mesozoic of the Southern Alps displaying sedimentological features and lithologic changes (fig. 4). In the contact aureole, syn-intrusive deformation of the country sedimentary sequences of Anisian age (Prezzo and Angolo Fm.) generated tight to isoclinal folding and up to several mm-thick differentiated mineral layerings parallel to axial planes of fold systems. Disruption of more viscous layers by boudinage, rootless folding and layering differentiation are serious obstacles to assessing dimensions of the sequence that may be regarded as a true preserved stratigraphy.

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Fig. 5 - (a) Loss of continuity of silicic strata by superposed transposition events in the metamorphic Calcare Selcifero Fm. of the Apuane Alps, at Foce Cardeto, Orto delle Donne valley, NW Apennines. First transposition surface is vertical and second surface is horizontal. In the centre an overthinned silicic layer is isoclinally folded, with horizontal axial planes, at metre scale (note hammer), after severe 10 cm scale isoclinalisation. Some confining terms of the stratigraphy are correspondingly transposed at the km scale in the surroundings; (b) Subvertical local transposition of Cambrian and Silurian horizons in the Salair Mountains, described by Ashgirei in his textbook «Strukturgeologie». Note that the up to 100 m scale transposition of lithologic layering is not matched in the regional stratigraphic distribution, from the crystalline basement (lowermost level) up to Devonian. Only at the local (100 m) scale are Silurian and Devonian horizons repeatedly mixed in a lithostratigraphic sequence. Modified from ASHGIREI (1963). – (a) Perdita di continuità di strati selciferi per eventi traspositivi sovrapposti nella Formazione metamorfica del Calcare Selcifero delle Alpi Apuane, a Foce Cardeto, Valle di Orto delle Donne, Appennino Nordoccidentale. La prima superficie di trasposizione è verticale e la seconda è orizzontale. Nel centro un livello selcifero è piegato isoclinalmente alla scala metrica con un piano assiale orizzontale (notare il martello), e a tratti sovrassottigliato, dopo un precedente intenso piegamento isoclinale alla scala decimetrica. Alcuni termini della stratigrafia sono corrispondentemente trasposti alla scala chilometrica nell’intorno; (b) Trasposizione locale subverticale di orizzonti cambriani e siluriani nei Monti Salair, descritti da Ashgirei nel suo libro di testo «Strukturgeologie». Si nota che la trasposizione sino alla scala ettometrica di livelli litologici non trova corrispondenza nella distribuzione stratigrafica regionale del basamento cristallino (livello inferiore) fino al Devoniano. Solo alla scala locale (100 m) gli orizzonti siluriani e devoniani sono ripetutamente interdigitati in una sequenza litostratigrafica. Ridisegnato da ASHGIREI (1963).

Assessment of the scale of preservation of stratigraphic sequences in cases of severe deformation is better supported with tests of the spatial persistence of deformation patterns, by means of continuous mapping of fabric elements, and of fabric gradients in the total strain field, at scales ranging from outcrop to map size. In fig. 5 two examples illustrate alteration of stratigraphic order of layers at various scales. The correspondence of stratal reorganisation from m to km scale (i.e. normally from a single to a few chronostratigraphic levels) should be checked by structural mapping over the range that connects the two scales. Lack of structural analysis in the first case led DALLAN NARDI & NARDI (1972) to propose a second formation for the Hettangian Carrara Marble (Marmo del Roccandagia Fm.) in an area of the Northwestern Apuane Alps, where closer structural investigation demonstrates manifold repetition of the original 100 m scale stratigraphy as a result of transposition (fig. 5a) and, consistent with this, the whole Mesozoic stratigraphy may be shown, by continuous mapping of mesoscopic fabric elements, to be duplicated at the km scale. LARGE SCALE INVESTIGATIONS

A simpler case of transposition at a similar scale to that of fig. 5a is represented in fig. 5b. The geologic profile illustrates the regional stratigraphy (ASHGIREI, 1963), obviously reconstructed on sedimentologic and paleontologic grounds, in an area of central Europe once offering significant economic targets. The local, detailed scale lithologic layering (obliquely oriented in the lower right

of fig. 5b) provides misleading information if layer orientation at outcrop scale (10-100 m) is taken as a reference for a larger scale attitude (1-10 km). Absence of local stratigraphic records in the tightly infolded Cambrian-Silurian horizons, or of detailed lithologic and structural reconstruction, may easily lead to erroneous interpretations of the overall stratigraphy. Mapping of the detailed lithostratigraphy, by adding all fabric elements (particularly the mineral-scale foliations grid), across areas including some thick stratigraphically reliable horizons (on the basis of sedimentological characters) located just side of evidently transposed horizons (fig. 6), is definitely the key to reconstructing the full stratigraphy within such a type of structurally contrasted sequence (HOBBS et alii, 1976). The scale of pervasiveness of the transposition altering the original stratigraphy appears to depend much on the relative thickness between differently resistant layers and on layer viscosity contrast (RAMBERG, 1955). Therefore, at a broader scale, lithostratigraphic members of a sequence, 2 or 3 orders of magnitude thicker (up to hundreds of metres) than those of the Adamello country rocks quoted above, although deformed by successive kilometre-scale fold systems of Alpine age, are highly valuable guides to a stratigraphic reconstruction. A previous recognition of the actual structural frame described by continuous lithologic layerings is the pre-requirement of the reconstruction. This structural frame may be constructed through an accurate mapping technique, combining: (I) macro- and micro-scale analysis of strain imprints, (II) lithological surface contouring and (III) the tracing of mineral foliation trajectories (form surface maps, fig. 7a). Even in weakly metamorphic terrains,

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Fig. 6 - Kilometre-scale tight fold system affecting the Monte Marguaréis sequence of the Mesozoic carbonate Ligurian Briançonnais in the Conca delle Carsene deglaciated cirque (external Pennine zone, alta Valle Pesio, Maritime Alps). The huge refolded fold system, with bent subvertical axial surfaces, strikes about NorthSouth. Ladinian calcareous dolomitic levels in the center-left; gray Dogger and whitish Malm limestones, in the centre and upper right; Cretaceous-Eocene marls, in the two synclines, marked by central grassy stripes. This interference pattern of crescent type was generated by a second open folding with EW striking axial surfaces. View to the North from northern spur of Bric dell’Omo, near Colla Piana, see CARMINATI & GOSSO (2000). Diagonal of image is one kilometre long. – Sistema di pieghe isoclinali a scala chilometrica nella successione stratigrafica carbonatica del Brianzonese Ligure del Monte Marguaréis, sull’altipiano della Conca delle Carsene (zona Pennidica esterna, alta Valle Pesio, Alpi Marittime); le enormi pieghe ripiegate hanno superfici assiali ricurve subverticali, dirette circa Nord-Sud. Nel centro, a sinistra, vi sono calcari dolomitici ladinici; nel centro e a destra in alto calcari grigi del Dogger e calcari bianchi del Malm; calcari marnosi del Cretaceo-Eocene occupano due sinclinali segnate da sottili striscie erbose. La serie di strutture di interferenza a mezzaluna è generata da pieghe con piano assiale diretto E-W (la diagonale dell’immagine è lunga un chilometro); ripresa fotografica verso Nord, dallo sperone N di Bric dell’Omo, presso la Colla Piana (vedi CARMINATI & GOSSO, 2000).

extreme thinning, down to local discontinuities of some stratigraphic marker horizons may therefore be explained in accordance with periodicity of the mosaic-pattern of the strain field typical of large scale fold interference patterns (fig. 6; see BRIZIO et alii, 1983; MENARDI-NOGUERA, 1988; 1990; CARMINATI & GOSSO, 2000). At the Alpine continent-ocean boundary in the Western Alps (southern Sesia-Lanzo Zone), several rock types, metamorphosed during subduction and exhumation, are repeatedly deformed by subduction tectonics (fig. 7a, b, c; SPALLA, 1983; SPALLA et alii, 1983). In this regime, interfingering between lithologic types is generated at all scales during repeated folding and translation of slices sampled from the continental margin and the subducting ocean crust. Such processes give rise to new lithostratigraphic units corresponding to a mélange of protoliths with contrasted origin. In this context of periodic coupling and decoupling of crustal slices in tectonic regimes of accretion, subduction and collision, the exploitation of the full structural and metamorphic memory of rocks represents a new tool able to unravel the common or contrasted tectonic paths of each structural unit.

Multi-scale configuration and size of lithologic differences generated by the tectonic mélange are illustrated by fig. 7a, b and c. In this case of dramatic reworking and mixing of rock sequences from different original environments, the reliable marker of the common evolution is the sequence of structural and metamorphic imprints; a simple lithostratigraphic analysis aiming at reconstruction of pristine environments, is actually unable to determine timing of coupling or decoupling of crustal slices of different provenance. Deciphering the full history of metamorphic and structural imprints allows reconstruction of a new type of unit by determination of the common structural and metamorphic evolution. Tectonothermal histories revealed by this analysis bear evident geodynamic implications, since they trace the transit of units throughout different levels of the lithosphere displaying at the same time their progressive changes in mechanical behavior. The definition of their contours is obtained by discriminating crustal volumes carrying distinct structural and metamorphic records (tectono-metamorphic units=TMUs). Size definition of TMUs and careful reconstruction of their tectono-thermal histories is crucial to infer detailed kinematics of tectonic processes at deep-seated structural levels in active margins. The analytical procedure to define tectono-metamorphic units consists of: i) construction of maps of the total deformation field, reporting the configuration of lithostratigraphic and fabric elements (combined surface shape of lithostratigraphic units and foliation trajectories); ii) analysis of superposition sequence of all structures by testing their kinematic compatibilities; iii) individuation of mineral assemblages constituting the mineralogical support of each of the superposed fabrics; iv) correspondence between equivalent mineral assemblages developed in adjacent heterogeneously deformed volumes (low and high strain fabrics). New maps representing superposed fabrics and their supporting mineral assemblages, graphically overlapping the deformed lithostratigraphy, have been a useful tool to individuate different TMUs in homogenous lithostratigraphic units or to incorporate different lithostratigraphic units in a single TMU (e.g. SPALLA et alii, 2000; GOSSO et alii, 2004; SPALLA et alii, 2005). After orogenic continental collision and detachment of subducted lithospheric roots the lowermost continental crust rock associations may experience partial melting. If these lowermost levels of the thickened crust are contemporaneously subjected to an extensional tectonic regime, driven by asthenosphere upwelling (VISSERS et alii, 1995; VON BLANCKENBURG & DAVIES, 1995; PLATT, 1998) or by plate divergence, the high grade lithologic layering will be severely disaggregated and recomposed into a renewed lithostratigraphy. Under such active petrogenetic and tectonic conditions, local deformation patterns of rocks containing migrating melt fractions may be apparently chaotic (migmatite mesoscopic fabrics). However, the typical kilometre scale new lithostratigraphic rearrangement is regularly oriented over thick sequences in which melt migration has been active. Actually orientations of lithologic layering stable over the km-scale are frequent in migmatite belts tectonically uplifted at the innermost roots of thick skin foreland belts (e.g. the HelveticDauphinois-Provençal zone of the Alps). During this latecollisional uplift and upthrusting, partially molten high grade gneisses, resident in the intermediate crust and

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Fig. 7 - (a) Form surface map of the multiply deformed and transposed tectonic horizon corresponding to the contact between the preAlpine continental margin of the Adria plate (Sesia-Lanzo Zone) and the Mesozoic calcschists at the border of the Tethyan ocean (Piemonte Zone), E slope of Salvini locality (above the Zanai-Gias Vej ridge, Valle di Lanzo, alpage road from Chiaves to Lago Monastero). The gneissic layer in the centre of the map, thinned and interrupted by transposition within the Mesozoic rocks is reproduced in fig. 7b. Piemonte Zone: 1) calcschists, 2) quartzites and marbles, 3) metabasics; Sesia-Lanzo Zone: 4) metagranitoids and micaschists; (b) Infolding of Mesozoic metasediments (dark grey) in the gneisses of the continental Adria margin (white and light gray). Note a geologist sitting on the second generation fold hinge. The whole fold pattern developed under a subduction regime; the older structures are marked by eclogitic assemblages and are refolded and re-equilibrated under blueschist to greenschist facies conditions during exhumation; (c) General view, originally drawn at the 1:25.000 scale (SPALLA, 1983) of the tectonic infoldings of figs. 7a and b, generated at continent-ocean boundary during the Alpine subduction cycle. The set of cross sections shows the effects of transposition of the continental and oceanic crust and associated sediments boundary; km scale interfingerings correspond to smaller scale effects depicted in figs. 7a and b. Redrawn from SPALLA et alii (1983). – (a) Carta delle superfici di forma di un orizzonte tettonico polideformato e trasposto, corrispondente al contatto tra il margine continentale della placca Adria (Zona Sesia-Lanzo) e i Calcescisti mesozoici al bordo dell’oceano tetideo (Zona Piemontese) sul versante E delle baite Salvini (al di sopra della cresta Zanai-Gias Vej, Valle di Lanzo, strada da Chiaves al Lago Monastero). In fig. 7b è rappresentato il livello gneissico al centro della carta, assottigliato e interrotto dalla trasposizione all’interno delle rocce mesozoiche. Zona Piemontese: 1) calcescisti, 2) quarziti e marmi, 3) metabasiti; Zona Sesia-Lanzo: 4) metagranitoidi e micascisti; (b) Implicazioni di metasediemnti mesozoici (grigio scuro) negli gneiss del margine continentale della placca Adria (bianco e grigio chiaro). Notare il geologo seduto sulla cerniera della piega di seconda generazione. L’intero sistema di pieghe sovrapposte si è sviluppato in un regime di subduzione; le strutture più antiche sono marcate da paragenesi eclogitiche e sono ripiegate e riequilibrate durante l’esumazione in condizioni metamorfiche di facies scisti blu e scisti verdi; (c) Vista d’insieme, disegnata in originale alla scala 1:25.000 (SPALLA et alii, 1983) delle implicazioni tettoniche delle fig. 7a e b, generate al contatto tra rocce continentali e oceaniche durante il ciclo di subduzione alpino. Il gruppo di sezioni geologiche rappresenta gli effetti della trasposizione del limite tra crosta continentale ed oceanica coi sedimenti associati; le implicazioni a scala chilometrica corrispondono agli effetti di deformazione a scala minore di fig. 7a e b. Ridisegnato da SPALLA et alii (1983).

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Fig. 8 - Experimental deformation of rock analogues (paraffin wax layers and silicone films) under layer-normal compression (KIDAN & COSGROVE, 1996). The melting point of the white layers is lower than that of black layers. Boudinage in conjugate sets allows way-up channelling of the molten fraction. Rotation into tight parallelism of segmented strata is highly facilitated. Redrawn from COSGROVE (1997). – Deformazione sperimentale analogica per compressione normale ai livelli (livelli a cere di paraffina e film di silicone) da KIDAN & COSGROVE (1996); nell’esperimento, il punto di fusione dei livelli bianchi è inferiore a quello dei livelli neri, i quali si segmentano (budinano) in sistemi coniugati, rendendo così possibile la canalizzazione della frazione fusa verso l’alto. La rotazione verso il parallelismo degli strati precedentemente segmentati per boudinage è in questo modo estremamente facilitata. Ridisegnato da COSGROVE (1997).

Fig. 9 - Multilayer of Mesozoic metacarbonates, previously boudinaged and later shortened by inversion of the stress field. Continuation of progressive shortening would produce a catastrophic effect of mixing of the left- and right-sided boudins on the limbs of the folds. Low-grade metasedimentary cover of the Argentera massif at Sblana locality, along the Gesso river right bank, in the plane West of Entracque, Italian Maritime Alps. Photograph by courtesy of Janos Urai. Lower image edge is three metres long. – Multistrato di rocce marmoree mesozoiche, precedentemente budinato e in seguito raccorciato a causa dell’inversione del campo di stress. La continuazione del raccorciamento progressivo arriverebbe a produrre un effetto catastrofico di mescolamento dei segmenti boudinati destri e sinistri sui fianchi delle pieghe. Copertura sedimentaria di basso grado del Massiccio dell’Argentera, in località Sblana, lungo la riva destra del torrente Gesso, nella piana a Ovest di Entracque, Alpi Marittime italiane. Immagine gentilmente fornita da Janos Urai. Il lato lungo della fotografia misura tre metri.

generated in previous orogenic cycles, are brought to the surface. These migmatites, poorly reactivated at the granular scale during final upthrusting and mostly passively translated in the rear of the foreland belt, often preserve an extensional tectonic imprint as a dominant conjugate boudinage pattern, developed under a generally subvertical crustal load, as explained by COSGROVE (1997), after confrontation of analogue modelling and conjugate boudinage of natural rocks (fig. 8). Lenticular penetrative deformation of the more viscous layers (melanosomes) of the migmatite multilayers by conjugate boudinage and rotation to parallelism is frequently observed in structural mapping of boudinaged lithologic units. In these cases the process of stratal segmentation and rotation is catastrophic, at the expenses of the pre-migmatitic lithologic assemblage. Further alteration of the whole sequence by thinning is generated during upwards pulsating expulsion of melts, channelled by the same conjugate shearing promoting active boudinage of lithologic layering in the deep crust. Extreme regularity of extensional reorientation of layers of many migmatite belts and frequent apparent viscosity inversion in the deforming multilayer, promoted by cyclic melt expulsion, add support to this interpretation. Consequently, a multi-scale lithostratigraphic rejuvenation takes place during conjugated boudinage, which is flanked by a relevant secondary effect of transposition to horizontality in the partially molten multilayer sequences of the lower crust. This last process, although evident in COSGROVE’s interpretation, has not yet been considered as a cause for lamellar horizontal reflection structures in the lowermost crust, resulting from deep seismics experiments. Significant melt extraction during boudinage and coalescence of adjacent restitic layers induce severe thinning and rock sequence renewal, purely by gravity-driven layer-normal compression. Finally, it must be taken into account that further complexity may be introduced in a boudinaged lithologic sequence if the segmented viscous levels of the multilayer are later subjected, by stress field inversion, to layer-parallel shortening. An example of initial stage of shortening of a boudinaged set of weakly metamorphic strata is illustrated in fig. 9. The only strata that simply display folds are the ones that never experienced boudinage, thanks to favourable viscosity and thickness, and they appear as better markers of the new strain field superimposed upon the previously boudinaged levels. They are mostly bending at the margins of the boudin neck fills and start the packing of boudinaged segments into tight to parallel fold limbs, adding further disorder in the lithostratigraphic arrangement. The overprint of a severely boudinaged layer set, up to tight folding, will impose a further general reorganisation of the segmented lithostratigraphic units. This process, induced by a simple stress field inversion, is quite frequent in polydeformed sequences. Therefore, reconstruction of the pre-deformational setting will face serious difficulties, as it is the case in old migmatite terrains, retrogressed and structurally reworked within younger orogens. CONCLUSIVE REMARKS

In the continental crust, tracing of a complete geologic history, as a backward reconstruction of successive tectonic imprints and their related lithostratigraphies,

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with timing of progressive amalgamation of rocks from various petrogenetic environments, necessitates widely different procedures, in accord with changes of crustal level and lithospheric pathways of rock associations. A valuable investigation strategy, of general utility in polydeformed continental crust, is based on detailed structural and petrographic analysis, reported in accurate maps. The construction of such maps must take into account the following steps: – Check for fossil sedimentary, igneous and metamorphic signatures. – Distribution map of scale and frequence of these signatures. – Map of fabric (and possibly strain) gradients and of main strain domain boundaries. – Test of kinematic compatibilities between all structures and of their superposition sequence, to separate areally the successive deformation imprints (structural correlation). – Evaluation of lenticularisation effects on lithologies of each kinematically coherent structural imprint. – Solid lithologic map, with fine scale orientation of lithologic layerings (shape of lithologic units). – Map of trajectories of mineral scale foliations, tracking or cutting across lithostratigraphy (form surface map). – Linkage of map scale structures with planar and linear microstructures and of their mineralogical support, necessary for the interpretation of thermo-baric evolution. – Cross-control of metamorphic equivalence of superposed mineral transformations displaying different textures in the superposed fabric sequences of adjacent heterogeneously deformed volumes (coronites, S-L tectonites, mylonites). – Tracing contours of volumes that share corresponding tectonic and metamorphic histories (TMUs=tectonometamorphic units). ACKNOWLEDGEMENTS Thanks are due to G. Chiodi for providing quality elaboration of the images. Ivan Mercolli and Bruno Lombardo much improved conceptual and form clarity of the text by constructive reviewing. Support from C.N.R.-IDPA Milano, and FIRST 2007 are acknowledged.

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Received 29 April 2008; revised version accepted 23 October 2008.