Structural and petrographic map of the Sassa gabbro complex (Dent Blanche nappe, Austroalpine tectonic system, Western Alps, Italy)

Abstract

The Sassa gabbro complex outcrops in the upper Valpelline and is part of the Permian gabbros of the Collon – Matterhorn group, set in the Dent Blanche nappe of the Western Austroalpine tectonic system. This 1:2500-scale map was constructed through the synergic use of meso- and microscopic analysis of fabrics to identify the relationships of the superposed foliations with the growth of equilibrium mineral assemblages. Throughout the foliation trajectories, a mosaic of rock volumes, in which strain is differently partitioned, is highlighted on the map for all the successive deformation stages. Therefore, this mosaic shows deformation gradients spanning from unstrained or weakly strained domains, where the primary magmatic features are still well preserved, to domains in which the Alpine fabrics are pervasive and obliterate the previous structures. The pre-Alpine history is characterized by the polyphasic emplacement of the Sassa gabbro and by a coronitic growth of successive mineral assemblages during crustal thinning-related exhumation. The Alpine history is characterized by multistage heterogeneous deformation consisting of a first D1 stage developed in blueschist facies conditions followed by D2 and D3 stages developed in greenschist facies conditions. In summary, by applying this analytical method, the geologic traces of the pre-Alpine lithospheric thinning and Alpine subduction histories have been separated.

Keywords:

• Permian gabbros
• pre-Alpine vs. Alpine evolution
• strain partitioning map
• multi-scale structural analysis

1. Introduction

The reconstruction of tectonic events and the evolution of thermal history through time in orogens requires high quality structural and petrographic mapping. The resulting geological maps should synthesize: (i) the grids of superposed foliation trajectories with related lineations; and (ii) the mineral assemblages marking successive fabrics, useful to unravel the metamorphic environment, which assists or overprints each deformation event (e.g., Austrheim, 1990; Connors & Lister, 1995; Gosso et al., 1983; Johnson & Duncan, 1992; Spalla, Siletto, Di Paola, & Gosso, 2000; Spalla, Zanoni, Williams, & Gosso, 2011; Williams, 1985; Zanoni, 2010; Zucali, Spalla, & Gosso, 2002). Such maps represent an effective correlation tool for determining structural and metamorphic regional events to identify shape and size of tectonic units (Figure 1) in polydeformed and polymetamorphic terrains (e.g., Salvi, Spalla, Zucali, & Gosso, 2010; Spalla, Gosso, Marotta, Zucali, & Salvi, 2010; Spalla, Zucali, Di Paola, & Gosso, 2005).

Figure 1. Flow chart showing how to delimit tectonic units in poly-deformed and poly-metamorphic terranes; redrawn after Spalla et al. (2005).

This contribution concerns an example from the European Alps that shows this type of mapping applied to an area where a continental crust underwent two superposed tectonic and metamorphic cycles, related to pre-Alpine rifting and Alpine subduction-collision. The map synthesizes lithologic, structural, and metamorphic data, collected at the boundary between a pre-Alpine gabbro body and its country rocks, in the Dent Blanche Nappe (DBN) of the Austroalpine tectonic system (Western Italian Alps).

2. Geologic outline

Within the continental crust of the Alps, gabbro bodies mainly occur in the Austroalpine and Southalpine (Adria plate) high temperature metamorphic rocks (Figure 2).

Figure 2. Tectonic sketch map of the Alps; 1 = crystalline massifs of the Helvetic units; 2 = continental Penninic units; 3 = Austroalpine units; 4 = Southalpine crystalline basement units; 5 = Periadriatic Oligocene intermediate to granitic magmatic bodies. Stars = Permian gabbroic bodies in the pre-Alpine continental crust: g1 = Corio – Monastero (SLZ); g2 = Sermenza (SLZ); g3 = Matterhorn – Collon – Sassa (DB); g4 = Fedoz – Braccia; g5 = Sondalo; g6 = Baerofen and Gressenberg; g7 = Bressanone - Chiusa; g8 = Predazzo - Monzoni; g9 = Val Biandino; g10 = Ivrea Val Mastallone; g11 = Ivrea - Finero. The red star locates the Sassa complex.

The Austroalpine system is the uppermost continental element tectonically overlying the stack of oceanic and continental nappes of the Alpine suture zone and in the Western Alps consists of the internal Sesia-Lanzo Zone (SLZ) and the external DBN, with related outliers (Dal Piaz, 1999). The Austroalpine system was deformed and metamorphosed from Cretaceous-Paleogene to Eocene-Lower Oligocene under different PT conditions (eclogite to greenschist facies conditions; Ernst & Liou, 2008); this metamorphic evolution, widely recorded in the Western Alps, indicates that the Austroalpine system was deeply implicated in the Alpine subduction-collision processes (Compagnoni et al., 1977; Dal Piaz, Hunziker, & Martinotti, 1972; Gosso, Messiga, Rebay, & Spalla, 2010; Meda, Marotta, & Spalla, 2010; Polino, Dal Piaz, & Gosso, 1990; Spalla, Gosso, Marotta, Zucali, & Salvi, 2010; Zanoni, Spalla, & Gosso, 2010 and refs. therein).

The pre-Alpine protoliths are high metamorphic grade paragneisses (kinzigites, felsic granulites), with interbeddings of mafic granulites, amphibolites, and marbles, Permian intrusive granitoids and gabbros, and minor Mesozoic sedimentary covers (Compagnoni et al., 1977; Dal Piaz, 1993; Manzotti, 2011; Roda & Zucali, 2011; Venturini, Martinotti, Armando, Barbero, & Hunziker, 1994). The DBN, as the rest of the Austroalpine system of Western Alps, is composed of two superposed tectonic elements: an upper element, formed by lower continental crust rocks, known as the Valpelline Series (VS) and a lower element, known as the Arolla Series (AS; Argand, 1911). AS is widespread in the North-Western DBN and is mainly formed by Permian acidic and mafic intrusives, associated with minor high-grade gneisses and metabasics. Slivers of Mesozoic metasediments are pinched along a main shear zone, separating DBN into two main tectonic units (DBN s.s. and Mont Mary) and trending NNE-SSW (Canepa, Castelletto, Cesare, Martin, & Zaggia, 1990); relics of Norian foraminifera and algae have been detected in these metasediments (Ciarapica, Dal Piaz, & Passeri, 2010). The lower continental crust rocks of the upper elements are closely similar to those of the Ivrea Zone (Southern Alps; Figure 2). This affinity suggests that Austroalpine and Southalpine domains were continuous (Adria Alpine hinterland) during Alpine convergence (Canepa et al., 1990; Compagnoni et al., 1977; Diehl, Masson, & Stutz, 1952; Stutz & Masson, 1938; Vuichard, 1989). The pre-Alpine metamorphic evolution was accomplished under a high T/P ratio, associated with gabbro and granitoid emplacement, and widely interpreted as the consequence of a Permian-Triassic lithospheric thinning, heralding the opening of the Tethyan ocean (e.g., Bussy, Venturini, Hunziker, & Martinotti, 1998; Lardeaux & Spalla, 1991; Marotta, Spalla, & Gosso, 2009; Rebay & Spalla, 2001; Roda & Zucali, 2008). Abundant gabbro bodies are mainly hosted in Permian granitoids of the AS; the main complexes are Matterhorn, Collon, and Sassa. Thick mylonitic bands mark the boundaries between the gabbro bodies and country rocks, affecting both rock types (Dal Piaz, De Vecchi, & Hunziker, 1977; Diehl et al., 1952; Monjoie, Bussy, Lapierre, & Pfeifer, 2005; Pennacchioni & Guermani, 1993).

The Alpine metamorphic history of the DBN is less constrained with respect to that of the widely eclogitized SLZ. Where Alpine HP mineral assemblages have been described, both in VS and AS, they indicate metamorphic peak conditions compatible with blueschist facies and interpreted, on the basis of available radiometric data, as 48-45 Ma old (Ayrton, Bugnon, Haarpainter, Weidmann, & Frank, 1982; Canepa et al., 1990; Gardien, Reusser, & Marquer, 1994; Kienast & Nicot, 1971; Pennacchioni & Guermani, 1993; Roda & Zucali, 2008).

3. Mapping and representation techniques

The reconstruction of the sequence of structural and metamorphic imprints affecting poly-deformed and poly-metamorphic terrains is crucial to decipher the tectonics responsible for crustal accretion or consumption. Therefore, maps synthesizing rock associations, the grid of superimposed foliations, and indications of metamorphic environments in which they developed, constitute a fundamental support for deriving interpretations on tectonics of active margins. Such synthetic mapping is rarely performed and generally the tectonic subdivisions are inferred on the basis of lithostratigraphic and/or dominant metamorphic imprint affinities. Unfortunately, the strong influence of the dominant metamorphic imprint on variations of the lithostratigraphic setting (e.g., Passchier, Myers, & Kröner, 1990) makes this approach inadequate. To infer a more complete tectonic history, this map is therefore an integrated report of the field and laboratory analytical work carried out to reconstruct the relationships between the finite deformation field, metamorphic evolution, and lithostratigraphy. The Sassa gabbro, being an igneous complex recording a polycyclic metamorphic and structural history, and surfacing in a recently deglaciated Alpine area, represents an ideal terrain with a rich lithostratigraphy and superposed structural and metamorphic imprints. On the map the number of dots along trajectory traces indicates timing of superposed planar fabrics. Through micro-scale analysis, the mineral assemblages related to successive fabrics in each rock type are inferred and detailed in the map legend. They represent the basis to identify the metamorphic environment (i.e., metamorphic facies) in which the successive fabrics developed (differently colored trajectories). In this way, information on chronology of structural imprints and metamorphic environment in which they developed are located in space. In addition, strain partitioning generates contemporaneous fabric heterogeneities in adjacent rock volumes (Figure 3) that can be exploited to refine the exploration of the tectono-metamorphic memory of polydeformed rocks (e.g., Hobbs, Ord, Spalla, Gosso, & Zucali, 2010; Salvi et al., 2010 and refs therein): mineral assemblages supporting coronitic, tectonitic, and mylonitic fabrics are here described and compared to achieve this goal.

Figure 3. Schematic block diagram showing strain partitioning, generating mylonitic, tectonitic, and coronitic textures, as a consequence of deformation of a homogeneous massive rock volume. The close up shows the corresponding textures of the Sassa gabbro. Legend: 1 = coronite, 2 = tectonite, 3 = mylonite. Redrawn after Lardeaux and Spalla (1990).

The Sassa geological map used a topographic base (the CTR - Technical Regional Topography) provided by the Val d'Aosta Regional Administration. The projection coordinate system is UTM ED50; the original 1:10,000-scale was enlarged up to 1:2500 to increase the mapping detail. The map presents a synthesis of the whole data set at a scale of 1:2500 on a redrawn original topographic base. Graphical representation was performed using Adobe Illustrator.

The map plate displays equal area lower hemisphere Schmidt projections (StereoWinFull120; Allmendinger, 2002) of superposed fabric elements, such as tectonic (S1, S2, S3) and magmatic foliations, fold axes (B2, B3), and magmatic lineations (n = number of measurements).

Meso- and microstructures of the most significant stages of the geological history of the analyzed area are shown in pictures on the map.

4. Original field data

Structural mapping has been performed in two key subareas of the Sassa complex characterized by different rock associations and degree of Alpine fabric development: ‘Berger de la Sassa’ and ‘Bivouac de la Sassa’. The first subarea (Berger de la Sassa) comprises a gabbroic intrusive complex (Sassa metagabbros) only partially deformed during Alpine convergence. Deformation partitioning allowed the preservation of large coronitic domains in which the pre-Alpine igneous textures and intrusion relationships can be easily inferred. The second subarea (Bivouac de la Sassa) is characterized by AS metagranitoids with Alpine tectonitic or mylonitic textures, in which only rare mm- to m-sized coronitic domains occur. The degree of fabric evolution (Salvi et al., 2010) during Alpine deformation stages is represented on the map and on Figure 4 by different densities of foliation trajectories and by the shapes of igneous enclaves. Tectonitic fabrics mainly consist of secondary foliations (S-tectonites of Passchier & Trouw, 2005).

Figure 4. Detail of the map (southern sector of Berger de la Sassa subarea) showing hectometer-scale deformation partitioning: poorly deformed domains preserving the magmatic structures are wrapped by S1 or S2 foliated domains. Legend as in the ‘Structural and petrographic map of the Sassa gabbro complex’.

4.1. Rock types

Protoliths of mapped metaintrusive rocks consist of gabbros, leucogabbros, anorthosites, melanocratic gabbros, granitoids, tonalites, quartzdiorites, and porphyritic, plagiogranitic, and aplitic dykes (Figure 5a–h). The following synthetic description of different lithotypes is integrated with details on mineral composition and degree of Alpine fabric reworking (coronitic, tectonitic or mylonitic texture as represented in Figure 3) that are reported in Table 1.

Table 1. Textural and mineralogical features of different lithotypes (a = acidic protoliths. b = basic protoliths) in the two mapped subareas.

Table 2. Summary of the intrusion relationships in the Sassa gabbroic complex.

Table 3. Pre-Alpine and Alpine superposed structure and related mineral assemblages as inferred by microstructural analysis. Mineral abbreviations after Whitney and Evans (2010) except for white mica (Wm).

Figure 5. (a) leucocratic/melanocratic alternating layers (from cm- to m-thick) affected by D3 folds in metagranitoids; close to Bivouac de la Sassa; (b) mesocratic metagabbro with dark green clinopyroxene rimmed by dark amphibole, showing grain-size variation; scale bar in centimeters; (c) mm-grained metamorphic microgabbro cut by a meta-plagiogranitic vein with undulate contacts; red scale bar is 1 cm long; (d) mm-grained leucocratic metagabbro with less than 30% of amphibole and biotite content; red scale bar is 1 cm long; (e) melanocratic metagabbro mainly consisting of amphibole (dark green), opaque minerals, and chlorite (light green); red scale bar is 1 cm long; (f) mm-grained meta-anorthosite consisting of 95% of plagioclase (dark portions are fresh plagioclase grains); red scale bar is 1 cm long; (g) contact between metagabbros (deep green color) and metagranitoids (yellowish color) is frequently marked by a thick mylonitic layer as in the case of upper Vallone d'Otemma; (h) meta-tonalite rich in porphyritic enclaves in Vallone d'Otemma.

4.1.1. Acidic and intermediate protoliths (Arolla metagranitoids and meta-aplites)

Acidic and intermediate rocks, belonging to AS metagranitoids, show a very heterogeneous texture and composition and three groups are recognized: leucocratic metagranitoids (Figure 5a), melanocratic metagranitoids, and meta-tonalites and –quartzdiorites. Textural features and mineral assemblages of different lithotypes are described in Table 1a.

4.1.2. Basic protoliths (Sassa metaintrusives)

Mesocratic gabbros, leucogabbros, melagabbros, and anorthosites constitute the Sassa mafic complex protoliths, which are intruded by porphyritic, plagiogranitic, and aplitic dykes. Fine-grained meta-plagiogranites occur in thin unmappable dykes and intruded the gabbros before Alpine deformation and metamorphism. Grain-size in mesocratic metagabbros (Figure 5b, c, microphotographs 6 and 7 on the map) varies from pegmatitic gabbros to microgabbros and they are rich in porphyritic enclaves. Leucocratic metagabbros (Figure 5d) are medium- to fine-grained rocks. Melanocratic metagabbros (Figure 5e) are medium- to fine-grained rocks and form m-sized lenses within the leucocratic and mesocratic metagabbros. At Vallone d'Otemma meta-anorthosites (Figure 5f and photomicrograph 1 on the map) outcrop as m-sized lenses in leucogabbros and meta-porphyrites form dykes in basic meta-intrusives and in meta-tonalites/diorites, close to the contact with mesocratic metagabbro. Details on textures and variations in mineral compositions are listed in Table 1b.

5. Structural relationships

The two mapped subareas are characterized by different rock types and structures: (i) the Berger de la Sassa subarea with metagabbroic rocks weakly affected by the Alpine deformation and primary magmatic features detectable in hectometer-sized volumes; (ii) the Bivouac de la Sassa subarea with metagranitoids pervasively affected by Alpine tectonitic and mylonitic textures.

The intrusion sequence can be inferred by the structural relations among different magmatic rocks (Table 2).

Hereafter we synthetically describe the pre-Alpine and Alpine structures in the two subareas; a more detailed description is given in Table 3, where the complete list of superposed structures and the mineral support of successive fabrics are itemized.

5.1. Berger de la Sassa subarea

5.1.1. Pre-Alpine magmatic structures

The contact between metagabbros and metagranitoids is regularly marked by a thick mylonitic band, affecting both rock types (Figure 5g and panoramic photo 2 on the map).

Mesocratic metagabbros are prevalent and contain melanocratic m-sized tabular or lenticular layers with transitional contacts. Meta-plagiogranites and pegmatitic metagabbros form ramified vein and dykelet nets within both metagabbros. The soft contact between meta-plagiogranitic veins, pegmatitic metagabbro dykes, and mesocratic metagabbros suggests that the former represent residual melts that were injected into the embodying material when it was still a viscous crystal mush.

Three types of meta-porphyrites have been distinguished: (i) type Ia are enclosed in mesocratic metagabbros; enclaves are stretched and concentrated in m-thick layers, defining a magmatic foliation; (ii) type Ib meta-porphyrites form m-thick dykes, which are disrupted within mesocratic metagabbros, displaying a soft contact with them (outcrop photo 4 on the map). These dykes contain enclaves of the country rock. Porphyrite enclaves in the country rock are concentrated at the dyke margins; (iii) type II meta-porphyrites of Vallone d'Otemma form 10 cm-thick dykes sharply intruding metagabbros and meta-aplites and locally showing chilled margins (Figure 6a and outcrop photo 1 on the map). These features suggest that type Ia and Ib porphyrites intruded a gabbro crystal mush, whereas type II was emplaced in totally solidified country rocks.

Except for type II porphyrites and meta-aplites, all of these rocks underwent pre-Alpine subsolidus re-equilibrations under amphibolite facies conditions (Table 3), followed by greenschist re-equilibration. The lack of subsolidus pre-Alpine amphibolite facies mineral assemblages in type II porphyrites and meta-aplites, together with their sharp intrusive contacts and chilled margins, suggests that these two kinds of dykes intruded the country rocks at the end of the pre-Alpine evolution.

5.1.2. Alpine tectonic structures

D1 deforms and transposes all the magmatic contacts and features, including aplitic and type II porphyritic dykes, and is concentrated along E-W trending sub-vertical bands. The foliation intensity gradient increases from the rim to the core of these deformation bands, where the S1 foliation is mylonitic. S1 is generally a spaced foliation; where it is mylonitic, boundaries of dykes are transposed and metaporphyritic enclaves are elongated and boudinaged (outcrop photos 5 and 10 on the map).

D2 structures are not pervasive in metagabbros and D2 is concentrated in NE-SW trending bands in which S2 foliation may be mylonitic (photomicrographs 2 and 8 on the map). F2 folds show subvertical B2 axes and are associated with S2 crenulation cleavage (outcrop photo 6 on the map). D1 structures and lithologic boundaries may be isoclinally folded (outcrop photo 3 on the map) and transposed into S2. In the upper Vallone d'Otemma, the contact between melanocratic metagabbros and leucocratic metagranitoids is defined by a pervasive S2 mylonitic foliation (panoramic photo 2 on the map), which dips 45° toward the NNE.

Figure 6. (a) metagabbro cut by meta-aplites and type II meta-porphyritic dykes. Porphyritic dykes show chilled margins. Black Brunton compass for scale; (b) S3 crenulation cleavage develops where S2 is more pervasive in mylonitic metagranitoids (Bivouac de la Sassa subarea); (c) View of the base of the Glacier de la Sassa. Valpelline Series and Arolla Series rocks are on the right side and left side of the picture respectively. The contact between the two Series is mylonitic, parallel to the S2 foliation recorded in the metagranitoids, and affected by meter-scale D3 folds; S2 trace in orange; (d) interference pattern between D2 and D3 folding in melanocratic metagranitoids; S1, S2, and AP3 are in blue, orange, and green, respectively; 4 cm long eraser as scale; (e) Ramsay's type 3 interference pattern between D2 and D3 folding in leucocratic metagranitoids; S1, S2, and AP3 are in blue, orange, and green, respectively.

D3 structures consist of open folds with tens of meters wavelength. Where foliations S1 and S2 are more penetrative or mylonitic, mm- to cm-sized asymmetric D3 crenulation, locally associated with a differentiated new mineral layering (S3), developed.

In the upper Vallone d'Otemma the boundary between metagabbros and metagranitoids shows a sinistral offset of tens of meters and in its central part, meta-leucogabbro and meta-anorthosite layers show the same offset, suggesting that a fault is buried by talus and glacial deposits.

5.2. Bivouac de la Sassa subarea

5.2.1. Pre-Alpine magmatic structures

Magmatic structures are basically preserved only in meta-tonalites and -quartzdiorites as the cm- to dm-sized porphyritic enclaves (photomicrograph 3 on the map) in the lower Vallone d'Otemma (more abundant toward the boundary with metagabbro). Contact between leucocratic and melanocratic metagranitoids and between meta-aplites and country rocks are generally parallelized to S1 or S2.

5.2.2. Alpine tectonic structures

In metagranitoids S1 frequently transposes lithologic boundaries and is in small-scale relics. In leucocratic metagranitoids S1 is a spaced foliation, in melanocratic metagranitoids it is a continuous foliation whereas in meta-aplites it is only locally preserved.

D2 are the most pervasive structures and partially or fully obliterate previous structures. S2 varies from a spaced to mylonitic foliation and frequently is a subvertical transposition foliation, generally associated with isoclinal F2 folds (outcrop photo 8 on the map) with subvertical axis.

D3 structures are pervasive and produce asymmetric folds where previous fabrics are more evolved (photomicrograph 5 on the map and Figure 6b) and S3 crenulation cleavage (photomicrograph 4 on the map) mainly developed in mylonitic leucocratic metagranitoids; F3 open folds display a 10-meter scale wavelength (Figure 6c and panoramic photos 7 and 10 on the map). In Lower Bec de la Sassa area Ramsay's type 2 and 3 overprinting patterns result by the superposition of F2 and F3 folds (Figures 6d, e).

Microstructural analysis of different lithotypes from both subareas (Table 3) indicates that mineral associations supporting S1 foliation developed under blueschist facies conditions, whereas S2 and S3 are marked by greenschist facies minerals.

6. Summary and conclusions

This detailed structural and petrographic mapping facilitated the reconstruction of the deformation and metamorphic history of DBN throughout the removal of the Alpine tectonic imprints back to a few undeformed igneous-textured cores of Permian gabbros and granitoids.

Strain partitioning during Alpine deformation allows the identification of domains where the magmatic features are still well preserved, even though igneous minerals are replaced by pre-Alpine or Alpine metamorphic minerals (e.g., south of Berger de la Sassa, at the lower southeast slope of Grand Epicoune, and in the central part of Vallone d'Otemma). Where the Alpine deformation is weak, the analysis of intersection relationships between the different magmatic rocks (Table 2) allowed us to infer the pre-Alpine magmatic history: the Sassa complex is the result of multiple intrusions of heterogeneous gabbroic magmas at intermediate crustal levels; however, sharp dyke contacts and chilled margins testify that protoliths of type II meta-porphyrites and meta-aplites emplaced at shallow crustal levels, into the main gabbro body when this was totally crystallized. Pre-Alpine coronitic textures developed under amphibolite and later greenschist facies metamorphic conditions replacing magmatic assemblages.

Within gabbroic rocks, S1 foliation dominates east of Berger de la Sassa, whereas northwest of Berger de la Sassa and in the upper Vallone d'Otemma, S1 is preserved only in the S2 lithons. Between Berger de la Sassa and the lower southeast slope of Grand Epicoune, S2 obliterates S1, rotating and transposing dykes and magmatic contacts. Generally S2 is localized in shear zones at the boundaries between gabbros and metagranitoids and along bands crosscutting the gabbro. S3 foliation is the less pervasive and is recorded mainly where D1 and D2 deformation-enhanced mineral transformations generated phyllosilicate richer rocks (lower part of Vallone d'Otemma and Bivouac de la Sassa).

This analytical field approach supports a more objective link of the time sequence of mineral re-equilibrations with fabric evolution. In particular, this study allowed the discrimination of geologic traces related to the pre-Alpine magmatic and post-magmatic evolution and Alpine history as synthesized in the sequence of geologic events in Table 4. The pre-Alpine metamorphic coronitic mineral assemblages suggest a decompression during the cooling of the Sassa gabbro complex. The Alpine history is characterized by the localization of deformation within metagabbros and metagranitoids during both subduction and exhumation as testified by blueschist facies mineral assemblages marking S1 in metagabbros and by the syn-D2 and -D3 mineral assemblages characteristic of greenschist facies conditions.

Table 4. Summary of pre-Alpine and Alpine evolution from the magmatic emplacement of the Sassa gabbro to its final reworking during the Alpine cycle. A = amphibolite; GS = greenschist; BS = blueschist. Ages for magmatic emplacement are related to Matterhorn and Collon gabbro (Dal Piaz et al., 1977; Monjoie et al., 2005); D1 ages after Ayrton et al. (1982).

Software

The Sassa geological map used a topographic base (the CTR – Technical Regional Topography) provided by the Val d'Aosta Regional Administration. The projection coordinate system is UTM ED50; the original 1:10,000-scale was enlarged up to 1:2500 to increase the mapping detail. The map presents a synthesis of the whole data set at a scale of 1:2500 on a redrawn original topographic base. Graphical representation was performed using Adobe Illustrator. The map plate displays equal area lower hemisphere Schmidt projections (StereoWinFull120; Allmendinger, 2002) of superposed fabric elements, such as tectonic (S1, S2, S3) and magmatic foliations, fold axes (B2, B3), and magmatic lineations (n = number of measurements). Meso- and microstructures of the most significant stages of the geological history of the analyzed area are shown in pictures on the map.

Acknowledgements

R. Berg, C. Malatesta, and M. Shand are thanked for the accurate revisions. C. Malinverno provided thin sections. M. Zucali, G. Rebay, and M. Roda are thanked for stimulating discussions. Funding by PRIN 2008 ‘Tectonic trajectories of subducted lithosphere in the Alpine collisional orogen from structure, metamorphism and lithostratigraphy’. D.Z. acknowledges funding from the project: ‘Studio petrologico e strutturale delle rocce di alta pressione delle Alpi Occidentali’, Università di Pavia.