<p>We present a new geological and structural map of the Gran Sometta -Tournalin ridge (Valle d&#8217;Aosta). In this area we have Pennidic ophiolitic units of the Combin (Co) and Zermatt-Saas (ZS) zones. In addition, in this area the continental cover sequence of the Pancherot-Cime Bianche-Bettaforca (PCB) unit crops out, close to the base of the Combin zone. The PCB and Co are characterized by Alpine greenschist facies assemblages, while the ZS is characterized by eclogitic assemblages. The greenschist and HP complexes are juxtaposed along the extensional Combin Fault Zone.</p><p>Our detailed 1:5000 map allowed reconstructing in 3D, and with a high level of detail, the spatial and crosscutting relationships between metamorphic layering (e.g. calcschists and metabasites in the Co), ductile foliations and shear zones, semi-brittle features (e.g. extensional crenulation cleavage &#8211; ECC - along the Combin Fault Zone), and post-metamorphic brittle faults.</p><p>The metamorphic layering and foliations are sub-horizontal in this area, and the ECC associated to the Combin Fault results in large components of horizontal stretching. These features are crosscut by two sets of high-angle normal faults, of Oligocene and Miocene age (according to literature), and, thanks to the favourable exposure and numerous structural data, we have been able to reconstruct these structures and their relationships in 3D.</p>
<p>3D geological modelling of complex metamorphic terrains that underwent a sequence of ductile and brittle deformation events is an extremely challenging task. Difficulties start from the input data that are frequently sparse and heterogeneous in quality and distribution. In projects based on field data only (without significant subsurface data) uncertainties are even more pronounced, but, in our project, we had the rugged topography of the Western Alps on our side, with elevations ranging from c. 1200 m to c. 3200 m and very continuous outcrops. Other problems, that we address in this contribution, arise during the modelling process. We tested different commercial software packages and some open-source research libraries and we found that no one is capable of modelling our complex structures out-of-the-box. This is not surprising since generally these codes, and particularly the commercial ones, are geared towards modelling gently deformed sedimentary sequences. However, it is possible to overcome a large range of obstacles by &#8220;fooling&#8221; implicit structural modelling algorithms, simply &#8220;cheating&#8221; on the geological meaning of model entities. This means (1) developing a conceptual model of polyphase ductile and brittle deformation, (2) finding geological/mathematical entities that are at the same time implemented in the code and able to represent the complex structures, and finally (3) carrying out the implicit modelling. For instance, tectonic contacts between large-scale tectono-metamorphic units can be treated as unconformities (and not as faults) to obtain a realistic representation. In some cases, also conformal lithological boundaries can be considered as unconformities with the goal of allowing larger thickness variations. In other situations, a &#8220;fake&#8221; stratigraphy where the same units are repeated several times can be used to model sequences of isoclinal folds and thin tectonic slices. In this contribution, some of these modelling solutions are compared in terms of (1) their straightforward implementation, and (2) their ability to generate models that properly fit the very detailed geological maps available in our study area (c. 60 km<sup>2</sup> mapped at 1:5.000-1:10.000 with a dense set of structural stations).</p>
<p>We present preliminary results on the meso- and micro-structural evolution of high-strain rocks of the Houill&#232;re Zone and Pierre-Avoi Unit outcropping along the Swiss-Italy boundary ridge, to the west of the Grand Saint Bernard Pass.</p><p>The stack of Middle and External Pennidic units is folded by polyphasic folds, developed at least partly under low-grade metamorphic conditions. Different generations of folds show isoclinal to open geometries. Fold axes are subhorizontal, trending NE-SW, and the overall fold interference pattern can be generally classified as a type 3 (Ramsay). At the microscale, an important deformation mechanism is pressure solution cleavage, consistent with relatively low-temperature conditions.</p><p>Brittle-ductile shear zones, characterized by anastomosing bands of very fine-grained fault rocks, with pressure solution seams and SCC&#8217; shear bands, exploit the weak and strongly anisotropic phyllosilicate-rich layers, particularly in the black schists of the Houill&#232;re Zone.</p><p>Brittle high-angle faults crosscut ductile and semi-brittle features and show an oblique-normal kinematics. These faults are particularly well developed in the more competent rocks of the Pierre-Avoi Unit (e.g. massive carbonates, metaconglomerates and metasandstones).</p><p>A continuous horizon, a few metres thick, with a high density of quartz veins, can be followed in the internal and upper part of the Houill&#232;re Zone. This horizon is folded, at least by the younger open folds, and constitutes a major marker to study the large-scale structure of this unit.</p>
