1. Introduction
The factors and dynamics governing the processes in lower continental crust rocks have long been a source of intense debate due to the lack of direct access to such lithologies on the Earth’s surface. The Ivrea-Verbano Zone (IVZ, western Southern Alps) is one of the few geological units that offers the possibility to investigate an exhumed section of the lower continental crust. Here, low- to high-grade metamorphism, magma emplacement and tectonics from the post-Variscan to Alpine have been reported (e.g. Handy et al. Reference Handy, Franz, Heller, Janott and Zurbrigg1999; Decarlis et al. Reference Decarlis, Zanetti, Ogunyele, Ceriani and Tribuzio2023). The IVZ, besides hosting the roots of a completely exposed fossil magmatic system (Quick et al. Reference Quick, Sinigoi, Peressini, Demarchi, Wooden and Sbisà2009), might also include the only available continental crust-mantle transition zone exhumed at or near the surface (Mehnert, Reference Mehnert1975; Fountain, Reference Fountain1976; Ryberg et al. Reference Ryberg, Haberland, Wawerzinek, Stiller, Bauer, Zanetti, Ziberna, Hetényi, Müntener, Weber and Krawczyk2023). These are represented by the Mafic Complex (Fig. 1), a mostly gabbroic unit that also includes metapelitic septa in the granulite facies and a variety of ultramafic lenses, the largest being the Balmuccia peridotite body (Quick et al. Reference Quick, Sinigoi and Mayer1994).
(a) Schematic geological map of the Ivrea-Verbano Zone (modified after Narduzzi et al. Reference Narduzzi, Covelli, Floreani, Pavoni, Petranich, Jantzi, Pistone, Černok, Venier, Crosera and Ziberna2025). (b) Detailed geological map showing the central portion of the Mafic Complex, Ivrea-Verbano Zone (modified after Quick et al. Reference Quick, Sinigoi, Snoke, Kalakay, Mayer and Peressini2003). The magmatic sequence studied in this paper, outlined by the yellow circle, is located approximately 1500 m east of the Balmuccia peridotite massif.
Despite decades of research, several key questions remain unresolved regarding the pre-Alpine magmatic and metamorphic evolution of the Mafic Complex. While previous studies have focused primarily on geochemical characterization of the main magmatic units (e.g. Rivalenti et al. Reference Rivalenti, Garuti and Rossi1975, Reference Rivalenti, Rossi, Siena and Sinigoi1984; Mazzucchelli, Reference Mazzucchelli1983; Pin & Sills, Reference Pin and Sills1986), detailed spatial relationships and petrography of units at scales < 100 m have received limited attention (e.g. Mariani et al. Reference Mariani, Tribuzio, Renna and Zanetti2024). This knowledge gap is particularly problematic because the complex geometries and spatial relationships of magmatic and metamorphic units cannot be easily extrapolated from sparse exposures. Large-scale lithologic and structural mapping is essential for understanding how these units were emplaced, differentiated and subsequently modified by tectonic processes. The outcrop examined in this study is located along a road cut between the villages of Isola and Mogliani, Municipality of Vocca (Fig. 1), within the Upper Zone of the layered series (Rivalenti et al. Reference Rivalenti, Rossi, Siena and Sinigoi1984) and provides an exceptional opportunity to tentatively address these questions through direct observation of well-preserved contact relationships and lithological variations.
Recent advances in digital photogrammetry, particularly Structure from Motion Multi-View Stereo (SfM-MVS) photogrammetry (e.g. Westoby et al. Reference Westoby, Brasington, Glasser, Hambrey and Reynolds2012; Carrivick et al. Reference Carrivick, Smith and Quincey2016), have revolutionized geological outcrop analysis. These methods enable the creation of Virtual Outcrop Models (VOMs; Xu et al. Reference Xu, Aiken, Bhattacharya, Corbeanu and Nielsen2000) that provide high-resolution 3D reconstructions of geological exposures. VOMs have proven particularly valuable for analysing complex spatial relationship at the outcrop scale, facilitating 3D mapping (e.g. Hodgetts et al. Reference Hodgetts, Seers, Head and Burnham2015; Burnham & Hodgetts, Reference Burnham and Hodgetts2019; Buckley et al. Reference Buckley, Ringdal, Naumann, Dolva, Kurz, Howell and Dewez2019; Aliyuda et al. Reference Aliyuda, Charlaftis, Priddy and Howell2024; Seers et al. Reference Seers, Sheharyar, Tavani and Corradetti2022), structural analysis at various scales (e.g. Corradetti et al. Reference Corradetti, Tavani, Parente, Iannace, Vinci, Pirmez, Torrieri, Giorgioni, Pignalosa and Mazzoli2018; Bistacchi et al. Reference Bistacchi, Mittempergher, Martinelli, Bistacchi, Massironi and Viseur2022; Panara et al. Reference Panara, Menegoni, Carboni and Inama2022; Cawood et al. Reference Cawood, Watkins, Bond, Warren and Cooper2023; Benedetti et al. Reference Benedetti, Casiraghi, Bertacchi and Bistacchi2025) and producing geologically meaningful photomosaics free from perspective distortion (e.g. Chesley et al. Reference Chesley, Leier, White and Torres2017; Cawood et al. Reference Cawood, Corradetti, Granado and Tavani2022). When combined with modern digital field data collection tools such as smartphone and tablet applications (Rutkofske et al. Reference Rutkofske, Pavlis and Ramirez2022; Uzkeda et al. Reference Uzkeda, Poblet, Magán, Bulnes, Martín and Fernández-Martínez2022; Tavani et al. Reference Tavani, Billi, Corradetti, Mercuri, Bosman, Cuffaro, Seers and Carminati2022; An et al. Reference An, Yong, Song, Du, Wang, Xu, Fang and Tong2024), these techniques offer unprecedented opportunities to integrate multi-scale observations and create detailed digital archives of geological exposures (Cawood & Bond, Reference Cawood and Bond2019; Buckley et al. Reference Buckley, Ringdal, Naumann, Dolva, Kurz, Howell and Dewez2019).
This study has three primary objectives: (i) to characterize the spatial relationships and petrographic variability of magmatic units at <100 m scale within an approximately 83 m wide outcrop of the Mafic Complex; (ii) to integrate digital datasets acquired through VOM construction and smartphone-based field mapping with traditional field observations and petrographic analysis; and (iii) to assess the significance of these integrated observations for interpreting magmatic processes and subsequent tectono-metamorphic overprints in the IVZ. While we acknowledge that a comprehensive understanding of regional magmatic architecture would require investigations across a broader area, this detailed single-outcrop study provides an essential methodological framework and serves as a robust starting point for establishing a systematic approach to multi-scale digital geological analysis in the IVZ. This work provides detailed constraints on magmatic evolution at a scale that bridges the gap between cm-scale thin-section observations and km-scale regional mapping (e.g. Quick et al. Reference Quick, Sinigoi, Snoke, Kalakay, Mayer and Peressini2003). Furthermore, our results will help inform the ongoing Drilling the Ivrea-Verbano zonE project (Pistone et al. Reference Pistone, Müntener, Ziberna, Hetényi and Zanetti2017), which targets the crust-mantle transition zone in this area, by providing essential ground-truth data on the complex lithological relationships and structural framework that characterize this unique exposure of deep continental crust.
2. Geological setting
The IVZ (Fig. 1a) is 140 km long and 5–15 km wide. To the northwest, it is juxtaposed against the Austroalpine Domain by the Insubric Line. To the southeast, it is separated from the Serie dei Laghi (also known as the Strona-Ceneri Zone; Boriani et al. Reference Boriani, Burlini and Sacchi1990), comprising amphibolite-facies paragneiss and orthogneiss and late Palaeozoic volcanic and intrusive rocks, by the Cossato-Mergozzo-Brissago and Pogallo Lines (Quick et al. Reference Quick, Sinigoi and Mayer1994). During the Permian, the lower crust of the IVZ was subject to a significant magmatic event, which is more evident in the southern area, along the Sesia Valley. This is testified by the so-called Sesia magmatic system (Reference Quick, Sinigoi, Peressini, Demarchi, Wooden and SbisàQuick et al. 2009), consisting of metamorphic, intrusive and volcanic rocks of both the IVZ and Serie dei Laghi units. This area was initially thinned during the Early Permian transtensional tectonics, and more profoundly later during the Early Mesozoic rifting (Handy & Zingg, Reference Handy and Zingg1991). Later, Tertiary brittle folding and thrust faulting emplaced the IVZ into a vertical position, essentially exposing a pre-Alpine cross-section of the lower continental crust at the surface (Zingg et al. Reference Zingg, Handy, Hunziker and Schmid1990; Handy et al. Reference Handy, Franz, Heller, Janott and Zurbrigg1999).
Rocks of the IVZ are divided into two major units, the Kinzigite and Basic formations. The Kinzigite Formation consists of amphibolite- to granulite-facies metapelites and metabasites, while the Basic Formation is subdivided into the Mafic Complex and lenses of mantle peridotite (Capedri et al. Reference Capedri, Garuti, Rivalenti and Rossi1977; Rivalenti et al. Reference Rivalenti, Garuti and Rossi1975; Rivalenti et al. Reference Rivalenti, Rossi, Siena and Sinigoi1984; Sinigoi et al. Reference Sinigoi, Quick, Demarchi and Peressini2010). The Mafic Complex is about 8 km thick and made of mostly gabbroic plutonic rocks, which intruded into the lower continental crust, between ∼282 and 284 Ma (Peressini et al. Reference Peressini, Quick, Sinigoi, Hofmann and Fanning2007; Karakas et al. Reference Karakas, Wotzlaw, Guillong, Ulmer, Brack, Economos, Bergantz, Sinigoi and Bachmann2019), during the post-collisional transtensional phase of the Variscan orogeny (e.g. Rivalenti et al. Reference Rivalenti, Rossi, Siena and Sinigoi1984; Handy et al. Reference Handy, Franz, Heller, Janott and Zurbrigg1999; Decarlis et al. Reference Decarlis, Zanetti, Ogunyele, Ceriani and Tribuzio2023). Although the Mafic Complex underwent ∼90° of tilting and experienced uplift from depths exceeding 15 km during the Alpine orogeny (e.g. Brodie et al. Reference Brodie, Rex and Rutter1989), its primary internal structure and its relationship with the surrounding country rock have remained well-preserved (Quick et al. Reference Quick, Sinigoi, Negrini, Demarchi and Mayer1992, Reference Quick, Sinigoi and Mayer1994, Reference Quick, Sinigoi and Mayer1995). According to Quick et al. (Reference Quick, Sinigoi, Negrini, Demarchi and Mayer1992, Reference Quick, Sinigoi and Mayer1994, Reference Quick, Sinigoi and Mayer1995, Reference Quick, Sinigoi, Snoke, Kalakay, Mayer and Peressini2003), Sinigoi et al. (Reference Sinigoi, Quick, Clemens-Knott, Demarchi, Mazzucchelli and Rivalenti1994) and Quick & Denlinger (Reference Quick and Denlinger1993), the mafic complex is characterized by a large arcuate fold with southward concavity, giving rise to an onion-like igneous complex (also known as the ‘gabbro glacier’ model), defined by primary concentric foliation, compositional banding and the contact between the igneous complex and the Kinzigite Formation, with the core of the structure located in the Sesia Valley.
The western portion of the Mafic Complex along the Sesia Valley includes the Balmuccia peridotite body (Shervais, Reference Shervais1979; Shervais & Mukasa, Reference Shervais and Mukasa1991; Quick et al. Reference Quick, Sinigoi and Mayer1995), which crops out in an area of ca. 0.8 × 5 km and is elongated in the SSW-NNW direction. The most common interpretation is that of a fragment of mantle peridotite included in the lower crust during accretion either in the Variscan or in previous orogenesis and detached from the present-day mantle (e.g. Quick et al. Reference Quick, Sinigoi, Negrini, Demarchi and Mayer1992). While this interpretation is supported by the recent seismic model of Pasiecznik et al. (Reference Pasiecznik, Greenwood, Bleibinhaus and Hetényi2024), other models suggest a closer connection between the Balmuccia peridotite and the large geophysical anomaly lying below the IVZ (Scarponi et al. Reference Scarponi, Hetényi, Berthet, Baron, Manzotti, Petri, Pistone and Müntener2020, Reference Scarponi, Hetényi, Plomerová, Solarino, Baron and Petri2021; Ryberg et al. Reference Ryberg, Haberland, Wawerzinek, Stiller, Bauer, Zanetti, Ziberna, Hetényi, Müntener, Weber and Krawczyk2023), typically interpreted as the expression of the Adria mantle exhumed close to or at the surface (Berckhemer, Reference Berckhemer1969). While the western contact of the Balmuccia peridotite with the Mafic Complex is reported as tectonic, the eastern contact is typically inferred as magmatic (Shervais, Reference Shervais1979; Quick et al. Reference Quick, Sinigoi, Negrini, Demarchi and Mayer1992).
The first ∼1.5 km of the Mafic Complex to the east of the Balmuccia peridotite (Fig. 1b) has been defined as the ‘layered series’ by Rivalenti et al. (Reference Rivalenti, Garuti and Rossi1975, Reference Rivalenti, Rossi, Siena and Sinigoi1984), located within an area defined as the Paragneiss-bearing belt by later studies (e.g. Quick et al. Reference Quick, Sinigoi, Negrini, Demarchi and Mayer1992, Reference Quick, Sinigoi and Mayer1994, Reference Quick, Sinigoi, Snoke, Kalakay, Mayer and Peressini2003). From west to east, this layered series is further divided into the i) Basal (BZ) and ii) Intermediate Zone (IZ), both characterized by syn- and post-magmatic deformation and mainly composed of spinel websterites, peridotites and gabbros and iii) Upper Zone (UZ), made by fine-grained gabbros in contact with metapelitic septa, anorthosites, olivine gabbros and websterites without any evidence of post-magmatic deformations (according to Rivalenti et al. Reference Rivalenti, Rossi, Siena and Sinigoi1984). The abundance of ultramafic units decreases from the BZ to the UZ. Based on this subdivision, the outcrop investigated in this work is within the UZ.
The lithologies of the Layered Series have so far been described in terms of general petrography, whole-rock chemistry and mineral chemistry (Rivalenti et al. Reference Rivalenti, Garuti and Rossi1975, Reference Rivalenti, Rossi, Siena and Sinigoi1984; Mazzucchelli, Reference Mazzucchelli1983; Sills, Reference Sills1984; Pin & Sills, Reference Pin and Sills1986) and were interpreted to have formed as a sequence of magmatic pulses associated with distinct fractionation events during the initial growth of the Mafic Complex (Mazzucchelli, Reference Mazzucchelli1983; Rivalenti et al. Reference Rivalenti, Rossi, Siena and Sinigoi1984). The occurrence of lithological variations in the gabbroic rock (e.g. anorthosite lenses) has been suggested to be of magmatic origin, likely associated with fractional crystallization and gravitational settling. This is based on the evidence of gradual contacts between the lithological units and on the apparent similarity with the Layered complexes such as Bushveld and Skaergaard (Rivalenti et al. Reference Rivalenti, Garuti, Rossi, Siena and Sinigoi1981, Reference Rivalenti, Rossi, Siena and Sinigoi1984). However, further work on Sesia Valley outcrops (Quick et al. Reference Quick, Sinigoi and Mayer1994; Sinigoi et al. Reference Sinigoi, Quick, Clemens-Knott, Demarchi, Mazzucchelli and Rivalenti1994) showed that such similarity might be limited only to some portions of this area of the Mafic Complex, which also shows lateral variations at the scale of < 100–150 m. In this framework, more field data are required to better constrain the spatial relationships between the magmatic units and test the hypothesis of formation by cumulitic processes. In addition, our preliminary fieldwork campaigns have observed lithologies previously not reported in the Layered Series (e.g. garnetites and hornblendites), which, therefore, require a more detailed field characterization as well.
3. Methods
This study was conducted through field investigation and laboratory analyses. Field investigations involved collecting lithological and structural data, rock samples and photos to create a VOM. Laboratory analyses included assessment of field data and construction of the VOM, supported by petrographic observations of thin-sections of the sampled rocks.
3.a. Field investigation
Lithological classification was performed through basic petrographic observations with hand lenses, followed by rock sampling for subsequent thin-section petrography. Both lithological and structural data, including foliations and small faults, were collected using the StraboSpot2 Application (Walker et al. Reference Walker, Tikoff, Newman, Clark, Ash, Good, Bunse, Möller, Kahn, Williams, Michels, Andrew and Rufledt2019) for iOS installed on an iPad Pro, along with a geological classical compass (Brunton TruArc15) for rapid quality checks. The StraboSpot2 Application enabled the geotagging of data on georeferenced images, allowing each measurement to be associated with its specific structural element.
A total of 823 overlapping and georeferenced photos were collected using a DJI Air2S drone, equipped with a 20-megapixel camera featuring a 1” CMOS sensor. Most of the photos were taken at short distances (∼1 to 10 m) from the outcrop, with additional photos captured from greater distances (> 20 m) to ensure consistent georeferencing and scaling. The georeferencing of the scene was based solely on the image geotag provided by the embedded Global Navigation Satellite System receiver of the device (GPS, GLONASS and Galileo constellations), also known as direct georeferencing.
The appearance of the outcrop and the ability to identify structural elements from the images varied significantly depending on the lighting conditions, so we aimed at collecting them in one run under optimal conditions (i.e. uniform diffuse light).
3.b. 3D modelling and VOM mapping
The VOM was constructed following a well-established workflow (Corradetti et al. Reference Corradetti, Seers, Mercuri, Calligaris, Busetti and Zini2022) using Agisoft Metashape software (v. 2.1.0). From the photoset, the first step involved the photo-alignment, which simultaneously retrieved the camera parameters and reconstructed the position of the matched points among the photoset (tie points). The tie points were then densified, and from the dense cloud, a 3D mesh of the outcrop was created and textured by draping an image texture map onto it, providing a more realistic visual representation (Tavani et al. Reference Tavani, Granado, Corradetti, Girundo, Iannace, Arbués, Muñoz and Mazzoli2014). A low-resolution version of the studied VOM (∼250,000 triangles and 17.6 megapixels) is available for public viewing in the Sketchfab public repository (https://skfb.ly/pqHYG ).
The mapping of the VOM was carried out using the polyline drawing and polygon drawing tools available in Metashape. These tools allow for high-resolution interpretation by utilizing the full resolution of the photoset. Since the photoset is intrinsically linked to its 3D rendering, interpretations made on one photo are automatically reprojected onto the corresponding 3D model and linked images, ensuring a synchronized and accurate interpretation across the VOM.
The structural data collected in the field were analysed using the OpenPlot software (Tavani et al. Reference Tavani, Granado, Corradetti, Girundo, Iannace, Arbués, Muñoz and Mazzoli2014). The software has two main functionalities: structural data extraction from VOMs and data management through stereonets. In this work, only the stereonet functionality was used. All plots in this work are in the lower hemisphere, equal-area projection. OpenPlot also has the functionality to calculate slip vectors from Riedel’s shear elements.
3.c. Microscale petrographic analysis
In addition to the macroscopic identification of rocks in the field, petrographic observations were conducted using an optical microscope to identify the textures and minerals composing the lithologies. Having mapped the sampled points in the VOM, those microscopic observations were back-projected into the VOM and, thus, related to the other observations, including the structural elements.
4. Results
The studied outcrop is a road cut ∼83 m-long, with a maximum height of roughly 6 m above the ground (Fig. 2). Positioned at a road bend, its orientation to the north varies from ∼N305° at its northernmost sector to ∼N060° at its southernmost sector. The outcrop extends along a 105° arc and is described from the northern to the southern zones, detailing lithological, structural and microscopic characteristics along the entire profile. Figure 2 also shows the exact positions where samples were collected and analysed through thin-section petrography (see section 4.c). In line with previous studies (e.g. Quick et al. Reference Quick, Sinigoi and Mayer1994), we adopted an igneous nomenclature for the rocks of the Mafic Complex to highlight their plutonic origin.
The Virtual Outcrop Model (VOM) of the section studied in this work with shown positions of the collected samples. Location of the VOM is indicated in Fig. 1b.
4.a. Field lithological observations
Most of the outcrop consists of phaneritic hornblende gabbronorites with variable colour index, given by variations in plagioclase, pyroxenes, hornblende and garnet contents. These mineralogical variations are arranged as a compositional banding at the cm- to dm-scale in some portions of the outcrop, which defines the main foliation.
The first ∼27 m of the outcrop (Fig. 3; northern zone) exhibit the greatest lithological heterogeneity. The first part (∼7 m) is composed of a garnet-olivine gabbroFootnote 1 in contact with the hornblende gabbronorite. In the following part, lenses of olivine-hornblende garnetite, garnet hornblendite and anorthosite are oriented sub-parallel to the foliation and are heterogeneously distributed along the outcrop. These lenses range in thickness from 0.1 to 2.0 m. The contact between anorthosite and garnet hornblendite appears sharp in the outcrop (Fig. 4), whereas the other contacts are gradational (Fig. 5). A fractured zone characterized by grain comminution is also present but does not show evidence of displacement (see Fig. 3).
VOM lithology mapping of the northern zone of the outcrop (∼27 m). Lithology-based segmentation was manually conducted by integrating field geological observations, petrographic work on the collected samples, and observation conducted on the VOM.
Interpreted photo showing the contact between the garnet hornblendite and the anorthosite lenses, along with the variability of structural elements present in the northern zone. W-dipping low-angle (∼35°) reverse faults are represented by red lines, SSW-dipping low-angle (∼20°) normal faults by blue lines and medium-angle (∼45° to 60°) reverse faults by purple lines. The blue dot marks a fault with the same orientation of the red elements but showing an opposite sense of shear.
Representation of foliation in the northern zone of the magmatic sequence. a) Garnet-olivine gabbro lenses are sub-parallel to the orientation of the foliation. b) Orientation of some prominent foliation given by mineralogical banding of the hornblende gabbronorite, in turn, crosscut by brittle structures.
In the subsequent ∼20 m of the outcrop (central zone in Fig. 2) clear mineralogical banding is visible only in the southernmost part (Figs. 6 and 7f), but sharp contacts between garnet-rich and garnet-free hornblende gabbronorites are visible also in the centre of this zone (Figs. 6 and 7e). The garnet-rich portions generally appear crumbly and weakly competent, which is not the case for the garnet-free portions. The geometry of these contacts is not well defined, possibly due to the irregularity of the outcrop surface and weathering, which produced distinct alteration halo with orange-yellow colour (Figs. 6 and 7e, f). We also noted a few portions of the outcrop where boudinage-type structures with coarser grain sizes are present. They are irregularly shaped and have a maximum length of 30 cm (Fig. S1). This zone also includes mafic pegmatites and pseudotachylytes. Mafic pegmatites (Figs. 6 and 7b) are composed of cm-sized crystals of clino-, orthopyroxene and plagioclase and are found close to or cut by the pseudotachylytes (see below), either as isolated bodies or as single crystals intruded into or bordering the pseudotachylytes, suggesting a younger age for the latter. Pseudotachylytes (Fig. 6) exhibit a colour range from black to brown and can be divided into three main types: (i) injection veins branching from pools of glassy material in contact with plagioclase crystals from mafic pegmatite (Figs. 6 and 7a), (ii) networks of glass encasing blocks of mafic pegmatite (Figs. 6 and 7c) and (iii) glass layers developing along shear planes (Figs. 6 and 7d).
Detail of the central zone of the outcrop and segmented lithologies. The positions of the detailed pictures illustrated in Fig. 7 are also indicated.
Detailed representations of the most characteristic lithological features of the central zone of the outcrop. (a) Injection veins branching from pools of glassy material in contact with pegmatitic plagioclase crystals, (b) networks of glass encasing blocks of leucogabbro, (c) glass layers developing along shear planes, (d) mafic pegmatites composed of cm-sized crystals of pyroxene and plagioclase, (e) contact zone between garnet-rich and garnet-free hornblende gabbronorites, characterized by an alteration halo with orange-yellow colour and (f) accentuation of foliation in garnet-rich hornblende gabbronorites located in the final portion of the central zone of the magmatic sequence.
In the terminal part of the central zone, the outcrop transitions into a highly fractured zone approximately 1.5 m thick, as shown by the ochre-yellow polygons in Fig. 6. This is characterized by brittle deformation, yielding a substantial reduction in grain size, from large blocks to grains measuring only a few cm. No kinematic indicators suggesting possible displacements between the central and southern zones have been observed. The southern zone (Fig. 8) appears macroscopically more homogeneous, with mineralogical banding visible only in a few portions. The final part consists of a ∼6 m thick portion of garnet-olivine gabbro bordered by two fracture zones (Fig. 8). Here, lithologies are more altered (browner colours), especially at or close to the fracture zones. No pseudotachylytes were visible at the outcrop scale, except for one identified at the thin-section scale.
Detail of the southern zone of the outcrop and segmented lithologies. Three areas of intense fracturing are observed.
4.b. Field structural observations
The phaneritic texture of all lithologies often hinders the recognition of deformational features (primarily shear fractures and faults) within the outcrop, except where mineralogical banding and lithological contacts are visible (see Fig. 5). Despite this, numerous structural features have been observed and mapped.
4.b.1. Measurements of foliation
A total of 107 measurements of foliation and lithological contacts were collected in the field. Despite some variability observed along the outcrop (with foliation ranging from sub-vertical to less than 45° dipping toward E-SE or W-NW), the contour plot of poles to foliations (Fig. 9a) shows one large cluster at about 193/85 (strike/dip of the plane; equivalent to 103/05 trend/plunge of the pole) corresponding to near-vertical foliations striking NNE-SSW and, subordinately (based on data dispersion), striking N-S. A secondary cluster at 310/15 testifies the presence of NE-SW-oriented foliations dipping at about 75° toward the SE. Given that the outcrop is mostly N-S-oriented, foliation variability was emphasized and quantified by plotting foliation azimuth (RHR: right-hand rule) and dip against their northing metric position (Fig. 10a). At the northernmost part of the outcrop, the foliation azimuth exhibits values similar to those recorded in the southernmost sector. Moving southward, the azimuth initially ranges between 20° and 45° before undergoing a dip inversion. At ∼5,075,464 m northing (as indicated by the red arrow in Fig. 10a), the azimuth increases sharply to around 220°. Moving southward, the azimuth gradually decreases to ∼160° before rising again to values between 180° and 200° in the southernmost part of the outcrop (blue dots in Fig. 10a).
The equal-area, lower-hemisphere projections of planar structures collected in the field for the entire outcrop. (a) Foliation data reported in this study, compared with average of the literature data from the Sesia Valley area by Rutter et al. (Reference Rutter, Brodie and Evans1993) and Quick et al. (Reference Quick, Sinigoi, Snoke, Kalakay, Mayer and Peressini2003) (see text for details); (b) Fault and shear structures data reported in this study.
(a) Foliation azimuths (RHR: right-hand rule; green dots) and dip angles (in degrees; red dots) versus their northing Universal Transverse Mercator (UTM) metric position. (b) Fault azimuths (RHR: right-hand rule; green dots) and dip angles (in degrees; red dots) versus their northing UTM metric position. The blue arrow marks a major variation of orientation data, refer to the text for further details. Both graphs were obtained through OpenPlot.
The dip angles of the foliations show less evident trends (red dots in Fig. 10a), but it can be observed that, in the northernmost part of the outcrop, the dip angles match those in the southernmost part, similar to the observed azimuth pattern. Moving southward, the dip initially increases, reaching a maximum of 45°, before rapidly increasing again to become vertical at the point of the abrupt change in azimuth (see red arrow in Fig. 10). Beyond this point, as we continue further south, the dip decreases on average until reaching the southernmost part of the outcrop.
4.b.2. Measurements of shear fractures and faults
A total of 493 measurements of shear fractures and faults were collected. Both terms refer to rock discontinuities characterized by displacement along the plane of discontinuity. Shear fractures generally exhibit very small (often microscopic) displacements (Fossen, Reference Fossen2016), compared to faults. Since it is not always possible or practical to distinguish between the two, they are analysed together in this work. This brittle deformation, observed at a macroscopic scale, affects not only the hornblende gabbronorites and the garnet-olivine gabbros, but also the anorthosite, hornblendite and garnetite layers, resulting in offsets ranging from a few cm to several dm. Similarly, pseudotachylytes and mafic pegmatites are also subject to minor displacements. The contour plot of poles to shear fractures (Fig. 9b) highlights the presence of two clusters. The main one at 100/55 (trend/plunge) indicates the presence of N–S-oriented shear fractures dipping at about 35° toward the W. A second cluster, at about 010/25, shows shear fractures dipping at about 65° toward the south. However, both clusters exhibit some data dispersion, suggesting variability in orientation within the observed shear fractures, as also noted in the field. The dispersion of data along the outcrop can be appreciated from the scatterplot of fault azimuths (blue dots in Fig. 10b) and dip angles (red dots in Fig. 10b) plotted against their northing metric position. Despite the data dispersion, faults striking at ∼180–220° are particularly abundant and form a prominent cluster at the northernmost side of the outcrop, where low-angle dips (10–50°) are also notably common.
Based on the characteristics of the described foliations, faults’ orientation and the morphology of the outcrop, data were divided into three groups reflecting their northing position, which are presented separately here to highlight the structural characteristics and faults’ clustering of these three zones of the outcrop. Data collected in the northern zone reveal a well-clustered fault assemblage (Fig. 11a) that, as recognize in the field, is made up of: (1) a majority of W-dipping low-angle (∼35°) reverse faults (red poles in Fig. 11b; see Fig. 4); (2) SSW-dipping low-angle (∼20°) normal faults (blue poles in Fig. 11b; see Fig. 4), generally confined between the previous red-shear faults and occasionally dissecting them with very minor displacements (<5 cm); and (3) medium-angle (∼45° to 60°) reverse faults (purple poles in Fig. 11b; see Fig. 4), which sometimes occur as isolated segments and sometimes connect with ramp geometries to the red-shear elements. Additionally, a few fault elements were collected that are oblique to normal to the main fault assemblage (green poles in Fig. 11b; see Fig. 4).
The equal-area, lower-hemisphere projections of the structural elements collected in the outcrop, divided into the three studied areas. (a) Cumulative density contour of poles to faults and shear structures of the northern zone. (b) Poles to faults and shear fractures of the northern zone. W-dipping low-angle (∼35°) reverse faults are represented by red poles (red lines in Fig. 4), SSW-dipping low-angle (∼20°) normal faults are represented by blue poles (blue lines in Fig. 4), medium-angle (∼45 to 60°) reverse faults are represented by purple poles (purple lines in Fig. 4) and normal faults oblique to the main fault assemblage are represented by green poles. (c) Cumulative density contour of poles to faults and shear structures of the central zone. (d) Great circles of the faults and shear fractures of the central zone with slip vectors from Riedel’s shear elements (red arrows). (e) Great circles of shear fractures of the central zone with slip vectors observed in the field (red arrows). (f) Low-angle (∼28°) SSE-dipping fault and shear fracture surfaces (blue great circles) and moderate-angle (40–55°) north-dipping fault and shear fracture surfaces (red great circles), both displacing foliations with a normal sense of shear. (g) Nine fault surfaces with dip angles similar to the north-dipping set in (f) characterized in the field by the presence of pseudotachylytes fillings (green great circles). (h) Cumulative density contour of poles to faults and shear structures of the southern zone. (i) Great circles of the faults and shear fractures of the southern zone with slip vectors (red arrows).
The central zone and the southern zone are separated by a pronounced bend in the outcrop. Data from the central zone of the outcrop are characterized by a prominent clustering of poles to shear fractures and faults at ∼010/20 (Fig. 11c), corresponding to steep-dipping (> 70°) and predominantly south-dipping faults. Many of these shear fractures and faults exhibit oblique-slip striations (Fig. 11d), though their sense of movement was not distinct. Some of the other clusters observed were differentiated in the field and hence plotted separately. Two fault surfaces striking about 72/83 (RHR) having normal (rake about 80°) slip (Fig. 11e). Shear fractures, orthogonal to the previous, are oriented at a high angle (> 60°) from N-S to NNW-SSE and dipping either to the west or east. The surfaces dipping to the east exhibit dip-slip to oblique-slip (average rake of ∼110°), while those dipping to the west show predominantly left-lateral slip (Fig. 11e). Strike-slip kinematics were also observed on surfaces dipping at a low angle (∼20°) toward the SW (Fig. 11d). Fault and shear fracture surfaces dipping at a low angle (∼28°) toward SSE displace foliations with a normal sense of shear (blue great circles in Fig. 11f) and surfaces dipping at a moderate angle (between 40 and 55°) toward the north also displace foliations with a normal sense of shear (red great circles in Fig. 11f). Finally, nine surfaces dipping similarly to the last set of faults were characterized in the field by the presence of pseudotachylytes fillings (green great circles in Fig. 11g).
In the southern zone of the outcrop, the faults collected exhibit clusters and slip directions similar to those observed in the central zone, with a prominent clustering of poles to shear fractures at ∼002/30 (Fig. 11h) reflecting the presence of steeply dipping, predominantly E-W oriented surfaces (Fig. 11i). However, this zone lacks of any N-S-striking elements and has no clearly identifiable kinematics, likely due to the almost 90° difference in exposure orientation compared to the central zone.
4.c. Petrography
Hornblende gabbronorites show medium-grained, granoblastic to polygonal textures. Mineralogical banding is sometimes visible at the scale of the thin-sections (e.g. Fig. S12a, b) and is defined by alternation of plagioclase- and pyroxene/hornblende-rich layers. Garnet exhibits large variability in modal abundance (0–25 mod%,Footnote 2 see Figs. 12a, S11 and S13a, b; Figs. 12b and S5; and Figs. 12c and S10, respectively), and only in one sample seems to be associated with the mineralogical banding (see Fig. S12a, b). Where garnet is present in low modal abundance (e.g. Fig. 12d) it forms coronitic textures around oxides (i.e. magnetite-ilmenite-hercynite). When it is more abundant, it either forms poikiloblastic textures (see Figs. 12c and S10), often including oxides, or nearly polygonal grains associated with hornblende, pyroxene and/or plagioclase, or sometimes surrounding oxides. Hornblende (10–40 mod%) often exhibits triple junctions with plagioclase and pyroxenes. Orthopyroxene (5–20 mod%) is usually more abundant than clinopyroxene and occasionally contains rutile inclusions. Clinopyroxene (5–15 mod%) is always present but, in some cases, shows smaller grain size. Plagioclase (25–50 mod%) often shows optical zoning. In one sample (13.8B; Figs. 12a and S11), the cores of the crystals contain dark green inclusions identified as hercynite. Oxides and sulphides, usually less than ∼5 mod%, are present as stubby crystals associated with the granoblastic texture formed by silicates and are often enclosed within garnet crystals (Figs. 12c and S10). The portions close to the fracture zones also include biotite occurring associated with hornblende or oxides, and chlorite, which mostly substitutes for pyroxene (Fig. S16a, b).
Petrographic characteristics of the hornblende gabbronorites representative of the entire outcrop (parallel Nicols). A variability in garnet abundance is observed, ranging from 0 mod% (b) to ∼25 mod% (a). Moreover, different textures can be distinguished, including coronitic around oxides (c), granoblastic (d) and poikiloblastic (a). Mineral abbreviations are after Whitney & Evans (Reference Whitney and Evans2010).
Garnet-olivine gabbros exhibit a granoblastic to polygonal, medium-grained texture and are generally devoid of any preferred orientation, with the exception of some portions where a slight mineralogical banding defined by plagioclase and mafic minerals is present. Olivine (5–10 mod%) crystals, despite retaining their euhedral prismatic habit, appear partially to completely altered. Garnet is abundant (20–25% modal) and highly fractured, displaying coronitic textures mostly around olivine and oxide (i.e. magnetite-ilmenite-hercynite) crystals (Figs. 13a and S2). Both hornblende and pyroxene are associated with olivine and garnet, and their content is ∼15–20 mod%. Plagioclase (40–50 mod%) frequently exhibits a polygonal texture, concentric zoning and often shows inclusions of hercynite in the cores. Stubby oxide and sulphide minerals represent ∼5 mod% of the rock (see rock sample 13.2B in Fig. S3a, b). The most altered samples collected near the fractured zones (e.g. sample 13.2B in Fig. S3a, b) show chloritized clinopyroxene and biotite replacing hornblende and are often associated with oxides. In two samples collected in the final portion of the southern zone (Fig. S15a, b), olivine has been completely replaced by secondary minerals.
Petrographic characteristics of all lenses of different lithologies representative of the entire outcrop (parallel Nicols). (a) Note the coronitic garnet surrounding the olivine crystal, which exhibits a euhedral prismatic habit. (b) The olivine-hornblende garnetite displays a granoblastic to polygonal texture and a high abundance of oxides. (c) The hornblendite shows a significant garnet abundance and strong evidence of alteration. (d) The anorthosite exhibits a polygonal texture. Mineral abbreviations are after Whitney & Evans (Reference Whitney and Evans2010).
Olivine-hornblende garnetites (Figs. 13b and S4) also show a granoblastic to polygonal texture, but no evidence of mineralogical banding. Garnet (∼50 mod%) forms the main texture, while the remaining mineralogical assemblage consists of partially altered olivine (∼15 mod%), hornblende (15–20 mod%) and pyroxene (∼10 mod%), often showing triple junctions. Oxides (magnetite-ilmenite-hercynite) exhibit an anomalously high modal content (5–10 mod%). Plagioclase is a minor phase (5–10 mod%) and often contains hercynite inclusions in its core.
Garnet hornblendites (Figs. 13c and S6) are highly fractured and show a fine- to medium-grained granoblastic texture. Hornblende and garnet are the most abundant phases (30–40 mod% each). Oxides (<5 mod. %; i.e. magnetite-ilmenite-hercynite) are sometimes completely rimmed by garnet, which recalls the coronitic texture observed in the previous lithologies. Orthopyroxene is very fractured (∼10 mod%), and clinopyroxene (∼10 mod%) is often replaced by secondary minerals. Plagioclase is <10 mod%.
Anorthosites (Figs. 13d and S7) have a polygonal texture and are medium-coarse grained. They are primarily composed of plagioclase (∼90 mod. %) with partial concentric zoning, albite-type twinning and triple junctions. Hornblende is interstitial, and its contact with plagioclase forms triple junctions. Notably, the contact between anorthosites and garnet hornblendites, which appears to be sharp at the scale of the outcrop, is gradational at the scale of the thin-section (Fig. S6a, b), suggesting an igneous origin also for this sequence.
5. Discussion
5.a. Methodological insights on digital field techniques and VOM usage
In this work, we implemented remote sensing and digital field techniques for data collection and analysis. Data collection using the StraboSpot2 Application enables geotagging of each structural element on georeferenced images (also known as spot mapping). Once the user becomes sufficiently familiar with the app, this significantly speeds up both the acquisition process and the organization of data. However, as of now, the app does not allow users to retrieve specific coordinates of structural elements from the image coordinates. Consequently, all elements linked to a particular image share the same coordinates as the location from which the image was taken. This limitation contrasts with another commonly used digital data acquisition app, FieldMove by Petroleum Experts Limited, which assigns a specific geotag to each measurement. However, FieldMove does not allow for image spot mapping, which can be very useful, particularly when multiple data are collected within short distances. In fact, when several data are collected at distances shorter than the typical accuracy of the devices’ localization capabilities (Tavani et al. Reference Tavani, Billi, Corradetti, Mercuri, Bosman, Cuffaro, Seers and Carminati2022), spot mapping is extremely useful for keeping data organized in digital form.
The availability of a georeferenced VOM, combined with image spot mapping of measured structural planar features, enabled us to retrace these elements within the 3D environment of Metashape, thereby obtaining their precise geotags. The relative accuracy of these geotags is below the cm-scale, as it depends on the resolution of the VOM, while the absolute accuracy of the geotags is linked to the accuracy of the VOM georeferencing, which analysis is beyond the scope of this work. Nevertheless, without this step, it would not have been possible to analyse data based on their coordinates (see Fig. 10), a common practice in studying large areas, such as along regional transects. Additionally, the VOM enabled precise mapping and the integration of multi-scale structural and petrographic observations. This digital representation allows for the integration of further observations without requiring multiple field visits. VOMs typically also allow for the extraction of planar orientations, for example, using OpenPlot software, which could lead to other valuable insights. However, this aspect falls beyond the scope of this work, and only field data were used in this analysis.
5.b. Interpretation of structural and lithological data
The study of this outcrop in the Sesia Valley has revealed various structural complexities and lithological relationships that are summarized in Fig. 14.
Interpreted VOM showing some of the main lithological and structural features of the study site, highlighting the relationships between the bodies and lithological contacts. The red arrow indicates the position where a sharp change in the strike and dip of the mineralogical banding and of the faults and shear structures is observed (see Fig. 10).
5.b.1. Magmatic lithologies and structures
Several pieces of evidence show that the main lithological variability is of magmatic origin. These include (i) the mineralogical banding, which can be interpreted as the result of gravitational settling during magmatic crystallization (i.e. Rivalenti et al. Reference Rivalenti, Rossi, Siena and Sinigoi1984), (ii) the lithological contacts of the olivine-hornblende garnetite, garnet hornblendite and anorthosite lenses, which are gradational and systematically parallel to the mineralogical banding (see Fig. 5a), (iii) the occurrence of olivine, which sometimes shows euhedral prismatic habit, testifying crystallization from a liquid. These observations support the hypothesis of Rivalenti et al. (Reference Rivalenti, Garuti and Rossi1975, Reference Rivalenti, Garuti, Rossi, Siena and Sinigoi1981, Reference Rivalenti, Rossi, Siena and Sinigoi1984) and Pin & Sills (Reference Pin and Sills1986) of a stratified magmatic sequence. This outcrop, in fact, is part of the UZ of the Layered Series, defined by the same authors as an alternation of gabbros, websterites, olivine gabbros and minor metapelites and anorthosites. The detailed mapping and sampling used in this work also allowed us to identify the occurrence of olivine-hornblende garnetite and garnet hornblendite, which, to our knowledge, have never been reported in the literature for this area. Hornblendites have already been identified in other parts of the Mafic Complex, but the presence of garnet has never been reported (e.g. Berno et al. Reference Berno, Tribuzio, Zanetti and Hémond2020). Notably, in the field, these garnet-rich lithologies appear as dark grey to black crystalline rocks, which at hand specimen scale can be confused with the pyroxenites typically observed in the lower parts of the Layered Series.
Another lithology that has not been previously reported is the mafic pegmatite (see Fig. 7b). Although an in-depth investigation is required to decrypt its origin, the mineralogical composition and contact relationships with the hornblende gabbronorite suggest late-stage crystallization in the Permian Mafic Complex at melt-poor and fluid-rich conditions. The confirmation that they pre-date the more recent structures of the outcrop is given by the fact that they are cut by the pseudotachylytes (see Fig. S9a, b), which include their fragments in the glass matrix (see Fig. S8a, b).
The orientation of the mineralogical banding and lithological contacts is consistent with the general ‘onion-like’ structure of this area of the Mafic Complex, explained by the ‘gabbro glacier’ model of Quick et al. (Reference Quick, Sinigoi, Negrini, Demarchi and Mayer1992, Reference Quick, Sinigoi and Mayer1994, Reference Quick, Sinigoi and Mayer1995, Reference Quick, Sinigoi, Snoke, Kalakay, Mayer and Peressini2003), Quick & Denlinger (Reference Quick and Denlinger1993) and Sinigoi et al. (Reference Sinigoi, Quick, Clemens-Knott, Demarchi, Mazzucchelli and Rivalenti1994). This has been interpreted by these authors as the result of gradual magmatic accretion through underplating during Permian crustal extension. The deformation of the foliation is also interpreted to be of syn-magmatic origin and part of the km-scale folding of the foliation of the Mafic Complex (Quick et al. Reference Quick, Sinigoi, Negrini, Demarchi and Mayer1992, Reference Quick, Sinigoi and Mayer1994, Reference Quick, Sinigoi, Snoke, Kalakay, Mayer and Peressini2003; Sinigoi et al. Reference Sinigoi, Quick, Clemens-Knott, Demarchi, Mazzucchelli and Rivalenti1994). Excluding the southernmost measurements, the strike direction and dip of the foliation in the southern and central zones (Figs. 10 and 14) gradually shift from north to south, ranging from ∼220° to 170°. Since the change is gradual rather than abrupt, it suggests a degree of continuity in the observed succession, likely reflecting folding on a larger scale than the outcrop itself. It is worth noting that, despite Rivalenti et al. (Reference Rivalenti, Rossi, Siena and Sinigoi1984) reporting no deformation structure for the UZ of the Layered series, such structures have been later reported for various areas of the Paragneiss-bearing belt (e.g. Quick et al. Reference Quick, Sinigoi, Negrini, Demarchi and Mayer1992, Reference Quick, Sinigoi and Mayer1994, Reference Quick, Sinigoi, Snoke, Kalakay, Mayer and Peressini2003), in which this outcrop is included. These typically are isoclinal folds and boudinage structures at the scale of a few decimetres to tens of metres, which are interpreted to be syn-magmatic according to Quick et al. (Reference Quick, Sinigoi, Negrini, Demarchi and Mayer1992, Reference Quick, Sinigoi and Mayer1994, Reference Quick, Sinigoi, Snoke, Kalakay, Mayer and Peressini2003). Notably, we also identified a boudinage structure within the hornblende gabbronorite in the outcrop (see Fig. S1). Quick et al. (Reference Quick, Sinigoi, Negrini, Demarchi and Mayer1992, Reference Quick, Sinigoi and Mayer1994, Reference Quick, Sinigoi, Snoke, Kalakay, Mayer and Peressini2003) link these structures to a general km-scale folding of the foliation of the Mafic Complex, particularly well shown by the shape of the Paragneiss-bearing belt. Based on these previous interpretations, we interpret the observed folding at our outcrop as related to these structures and, therefore, also as evidence of syn-magmatic deformation. The only available measurements of foliation close to this outcrop are from Rutter et al. (Reference Rutter, Brodie and Evans1993) and Quick et al. (Reference Quick, Sinigoi, Snoke, Kalakay, Mayer and Peressini2003) (Fig. 1). To compare our results on this outcrop with their measurements, we selected only the literature data that fall within 1 km distance from our outcrop. This comparison is included in Fig. 9a and shows that there is a slight discrepancy, likely due to local variability in the foliation at the scale of hundreds of metres. Again, this local variability might be related to the small- to large-scale folding of the Mafic Complex, which could be captured only with observations at different scales.
5.b.2. Post-magmatic metamorphic overprint
As already reported for the gabbroic rocks of the Mafic Complex, a metamorphic overprint likely occurred after the magmatic crystallization was completed and possibly lasted from Permian to Jurassic times (Handy et al. Reference Handy, Franz, Heller, Janott and Zurbrigg1999; Decarlis et al. Reference Decarlis, Zanetti, Ogunyele, Ceriani and Tribuzio2023 and references therein). This metamorphic overprint is mainly represented by the granoblastic to polygonal textures, often exhibiting triple junctions between grain boundaries and testifying granulite-facies equilibration during the slow cooling of the magmatic rocks at the base of the crust (Rutter et al. Reference Rutter, Brodie, James and Burlini2007). The presence of garnet could also be related to the same cooling event. The coronitic textures of garnet around olivine, and particularly around oxides (see Fig. 12d), as well as their frequent association with plagioclase and pyroxenes (see Figs. 12 and 13), suggest an origin through metamorphic reactions between these phases. These textures may have formed during subsolidus cooling at high pressure, as previously suggested for gabbroic rocks of the Mafic Complex (Rivalenti et al. Reference Rivalenti, Garuti and Rossi1975, Reference Rivalenti, Rossi, Siena and Sinigoi1984; Mazzucchelli, Reference Mazzucchelli1983, Reference Mazzucchelli, Rivalenti, Vannucci, Bottazzi, Ottolini, Hofmann, Sinigoi and Demarchi1992; Sills, Reference Sills1984; Pin & Sills, Reference Pin and Sills1986), and from other localities (Berger et al. Reference Berger, Caby, Liégeois, Mercier and Demaiffe2009). The irregular distribution of garnet textures and abundance (see Figs. 12 and 13) does not appear to be directly related to structural features or lithological variations. We suggest that its formation could have been influenced by possible compositional heterogeneity of the rock, which could stem from original local differences in either the magma composition or in the crystallized cumulate assemblage. This has already been suggested by Sills (Reference Sills1984), but the source of this compositional difference at such a small scale still needs to be investigated. Notably, the spatial association with metapelitic septa has been suggested to be related to the presence of garnet in the gabbronorites (e.g. Mazzucchelli et al. Reference Mazzucchelli, Rivalenti, Vannucci, Bottazzi, Ottolini, Hofmann, Sinigoi and Demarchi1992), but such an association has never been supported by quantitative field data that relate the distance from the metapelitic septa to the garnet content.
Additional evidence of metamorphic overprint is given by the rutile exsolution in orthopyroxenes and hercynites exsolution in plagioclases, which could also have occurred during cooling of the Mafic Complex. The formation of hercynite exsolutions might depend on the original Fe content of the plagioclase. Notably, their presence appears to be associated with the presence of olivine, with the exception of sample 13.8B (Fig. 12a). The interpretation of these inclusions is subject to another currently ongoing investigation and does not fall within the scope of this work.
5.b.3. Pre-alpine and alpine deformation and exhumation
The alpine faults and shear structures that have been mapped crosscut all the lithologies composing the magmatic sequence. There is no evidence of displacements larger than a few tens of cm, and therefore, it can be stated that the original spatial relationships between the magmatic units have been largely maintained. The northern zone of the outcrop is characterized by the change of the dip direction (Fig. 10b) and a considerably different fault assemblage with respect to the rest of the outcrop (Figs. 9b and 10b). The fault assemblage depicts a classical Riedel shear assemblage characterized by an overall top-to-the-southeast sense of shear. In agreement with Quick et al. (Reference Quick, Sinigoi, Snoke, Kalakay, Mayer and Peressini2003) and Menegoni et al. (Reference Menegoni, Panara, Greenwood, Mariani, Zanetti and Hetényi2024), all minor faults could represent Riedel structures related to the movement of the Insubric Line and are, therefore, compatible with Alpine orogenesis. In this framework, the west-dipping low-angle reverse faults (red in Fig. 11b) represent the main shear planes or Y-shear, the blue shear fractures are the synthetic R-planes and the purple shear fractures are the synthetic P-planes. A few antithetic R’-planes were also observed but not measured because they were not accessible in the field. In a few places, the main shear planes present an opposite sense of movement (blue dot in Fig. 4). We interpret this as a post-seismic stress relaxation (Meneghini & Moore, Reference Meneghini and Moore2007), likely controlled by fluid overpressure conditions associated with pre-/co-seismic phases (Curzi et al. Reference Curzi, Aldega, Billi, Boschi, Carminati, Vignaroli, Viola and Bernasconi2024 and references therein).
In the central zone, only a smaller number of faults match the structural assemblage observed in the northern zone (Fig. 11c). The less visible foliation in this area, compared to the well-foliated northern zone, may have hindered the recognition of those elements. Nevertheless, this area exhibited much greater variability in fault orientations (Fig. 11c), including ENE-WSW-oriented faults with various dip angles, displaying normal to oblique left-lateral transtensive kinematics (Fig. 11e, f) and high-angle NNW-SSE-oriented west-dipping left-lateral faults with east-dipping oblique-slip right-lateral transtensive kinematics. A similar fault assemblage (Fig. 11h, i) was observed in the southern zone, where high-angle E-W-striking and south-dipping shear fractures with striations oriented toward the southeast and SW-dipping faults with strike-slip kinematics (Fig. 11i) are similarly abundant. Also in this case, these structural features are compatible with those observed in the adjacent areas (e.g. Brodie & Rutter, Reference Brodie and Rutter1987; Rutter et al. Reference Rutter, Brodie and Evans1993; Rutter et al. Reference Rutter, Brodie, James and Burlini2007) and likely represent brittle tectonic events that affected the IVZ during the Alpine orogeny. Notably, no structural or lithological changes are visible at the outcrop scale at the boundary between the northern and central zones.
The central zone is characterized by a wide variety of pseudotachylyte structures (Fig. 7a, c, d), which exhibit different relationships with the faults and shear fractures. Whether this might indicate multiple generations of pseudotachylytes, as previously suggested for nearby areas by Techmer et al. (Reference Techmer, Ahrendt and Weber1992) and Souquière & Fabbri (Reference Souquière and Fabbri2010), would require further investigations. Based on the observations of this work, it could be speculated that one generation consists of pseudotachylyte structures cut by Alpine faults and shear fractures (Fig. 7a, c), indicating that pseudotachylytes formation occurred during a brittle event predating Alpine deformation, as also suggested for pseudotachylytes identified in association with the Balmuccia peridotite (Souquière & Fabbri, Reference Souquière and Fabbri2010) and the Premosello metamorphic sequence (Pittarello et al. Reference Pittarello, Pennacchioni and Di Toro2012). A second type of pseudotachylyte, oriented parallel to Alpine faults and shear fractures, as shown in Fig. 7d, could represent a later generation associated with Alpine deformation; similar evidence has also been analysed in other areas of the IVZ (Techmer et al. Reference Techmer, Ahrendt and Weber1992; Rutter et al. Reference Rutter, Brodie and Evans1993; Obata & Karato, Reference Obata and Karato1995). Notably, Rutter et al. (Reference Rutter, Brodie, James and Burlini2007) reported the occurrence of pseudotachylytes associated with faults showing evidence of retrograde metamorphism. Retrograde metamorphism is also highly evident in this study area, with advanced alteration features, including biotite forming reaction textures with hornblende and oxides, as well as chloritization of clinopyroxenes. Such processes represent typical products of fluid circulation promoted by the Alpine fracturing and faulting system under greenschist-facies conditions (Zingg et al. Reference Zingg, Handy, Hunziker and Schmid1990).
6. Summary and conclusions
We conducted a detailed study of one outcrop within the UZ of the layered series in the IVZ, combining one of the most recent remote sensing technologies (VOM) and structural data collection with petrographic descriptions. This allowed us to reconstruct a digital model of the outcrop, where lithologies and their spatial relationships can be clearly distinguished and quantified. This, in turn, allowed us to investigate the possible relationships among the units of the magmatic sequence and establish a relative chronological framework. The most significant findings can be summarized as follows:
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(1) Hornblende gabbronorite lithology constitutes most of the outcrop. The variability of the foliation along the outcrop, defined by mineralogical banding, is consistent with the ‘gabbro glacier’ model (e.g. Quick et al. Reference Quick, Sinigoi, Negrini, Demarchi and Mayer1992). New lithological units have been identified, including olivine-hornblende garnetite and garnet hornblendite lenses. These are often associated with anorthosite lenses and are characterized by gradual contacts with hornblende gabbronorite, which suggests an igneous origin. Garnet-olivine gabbros are larger units that do not show clear relationships with the host gabbronorite, due to the systematic occurrence of fracture zones at or close to the contacts. The absence of kinematic indicators suggests that the fracture zones may have formed at contacts where lithological contrast is favoured. Mafic pegmatites suggest a late-stage crystallization in the Permian Mafic Complex under melt-poor and fluid-rich conditions.
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(2) Granoblastic to polygonal textures characterized by frequent triple junctions, along with coronitic textures of garnet surrounding oxides and olivine, as well as the presence of rutile exsolution in orthopyroxenes and hercynite exsolution in plagioclase, provide clear evidence of a metamorphic overprint. This might have occurred during subsolidus cooling at high pressure, as previously highlighted in studies on the Mafic Complex by Rivalenti et al. (Reference Rivalenti, Garuti and Rossi1975, Reference Rivalenti, Rossi, Siena and Sinigoi1984) and Mazzucchelli (Reference Mazzucchelli1983). The irregular distribution of garnet textures and abundance suggests that their formation may have been influenced by compositional heterogeneity of the rock, possibly stemming from local differences in magma composition or in the crystallized cumulate assemblage.
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(3) Brittle deformations, mostly related to Alpine tectonics, are testified by localized fracture systems, shear structures and a variety of pseudotachylytes. Overall, these deformations did not significantly change the original spatial relationships between magmatic units. Detailed measurements showed a sharp change in the fault arrangements from the northern to the central and southern zones of the outcrop. The reason for this change may be related to evident lithological heterogeneities, such as the lack of evident foliation in the central and southern zones, leading to a different strain response and the development of different brittle fracture assemblage. Further investigations on other outcrops in the area, as well as numerical modelling, may help to validate this hypothesis. Additional hypotheses for the observed variation can be: (i) a sampling bias due to the turning orientation of the outcrop surface, or (ii) a recognition bias related to the greater difficulty of identifying certain fracture orientations in non-foliated lithologies.
The detailed petrographic study, combined with the application of the VOMs methodology, has proven effective in providing an in-depth understanding of the geometry of magmatic units, as well as in demonstrating limited metamorphic overprinting from subsequent tectonic events. However, this is a geographically very restricted study, and further investigations on both the geochemical composition of the lithologies and field data are required to construct a detailed petrological and geodynamic model of the lower crust and its role in the evolution of the Alpine orogeny.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/S001675682510023X
Acknowledgements
L. Z. acknowledges the ‘Fondazione CRTrieste’, project ref. 24112-2024.0028. F.N. acknowledges support from Project 101066580 - STECALMY - Horizon-MSCA-2021-PF-01, founded by European Research Executive Agency (REA) – Marie Skłodowska-Curie Actions & Support to Experts A.2 – MSCA European Postdoctoral Fellowships. A.Č. acknowledges Rita Levi Montalcini fellowship by the Italian Ministry for University and Research (MUR). Philippe Turpaud is gratefully acknowledged for the preparation of the thin-sections. The manuscript benefited of the constructive review of Fabrizio Tursi and Adam Cawood. We thank associate editor Eleanor Jennings for handling the manuscript.
Competing interests
The authors declare none.
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