Geology of the Susa Shear Zone (Susa Valley, Western Alps)

The 1:10,000 scale geological map of the Susa Shear Zone (SSZ) in the inner sector of the Western Alps, aims to describe the geological setting and tectonic evolution of a ﬁ rst-order Alpine shear zone, which drove exhumation and juxtaposition of di ﬀ erent oceanic and continental margin units (i.e. the blueschist-facies External Piedmont Zone in its hanging wall and the eclogite-facies Internal Piedmont Zone and Dora Maira Massif in its footwall). The SSZ corresponds to a tectonic mélange showing a block-in-matrix structure, wherein mylonitic calcschists embed blocks of di ﬀ erent rock units. Geological mapping and structural analysis investigated overprinting relationships among shear planes and structures related to di ﬀ erent deformation phases: they show that the SSZ evolved through two tectonic events, during which apparent reverse top-to-E shear planes were superposed by extensional top-to- W ones.


Introduction
The study of geodynamic evolution of orogenic belts often focuses on first-order shear zones, which drive stacking of tectonic units. Reconstructing the geometry, inner structure and polyphase kinematics of orogenrelated shear zones is a key for better understanding subduction and exhumation processes. In the Western Alps, shear zones and tectonic mélanges bounding different tectonometamorphic units of both continental and oceanic origin, have been described within the axial sector of the belt (see Schmid, Kissling, Diehl, van Hinsbergen, & Molli, 2017, and references therein), and their detailed structural characterization as well as their role in multistage exhumation of tectonic units is an up to date topic (see e.g. Federico, Crispini, Malatesta, Torchio, & Capponi, 2015;Gasco, Gattiglio, & Borghi, 2013;Kirst & Leiss, 2017;Manzotti, Zucali, Ballèvre, Robyr, & Engi, 2014;Roda, De Salvo, Zucali, & Spalla, 2018).
In this paper, the polyphase tectonic evolution of a shear zone, the Susa Shear Zone (SSZ hereafter), exposed in the inner sector of the Western Alps (Susa Valley, NW of Italy; Figure 1), is described. The SSZ juxtaposed and exhumed different oceanic and continental margin units, which are characterized by different Alpine metamorphic P-T peaks. Together with their former northern and southern analogs (i.e. the Combin fault and the West Viso Detachment, respectively; Balestro et al., 2018;Ballevre & Merle, 1993;Tricart, Schwartz, Sue, & Lardeaux, 2004), it represents a firstorder shear zone separating two orogen-scale domains, which have been distinguished within the Alpine accretionary wedge (i.e. the 'eclogite belt' and 'frontal wedge' of Malusà, Faccenna, Garzanti, & Polino, 2011). Our geological map at 1:10,000 scale (see Main Map), aims to describe in detail the kinematics of the SSZ and the structural and lithostratigraphic setting of its footwall and hanging-wall blocks.

Methods
The geological and structural data were collected at 1:5000 scale, stored in a GIS database and represented on a topographic map compiled from the Carta Tecnica Regionale Vettoriale of the Regione Piemonte (vector_10 series, Edition 1991Edition -2005. In the geological map (see Main Map), encompassing an area of almost 30 km 2 , geological and structural data have been generalized at 1:10,000 scale. The structural architecture and geological setting of the SSZ are given in three cross sections and in a tectonic sketch map. Regional deformation phases have been distinguished through overprinting relationships among structures. Orientation of structural elements has been analyzed by equalarea lower-hemisphere stereographic projections. The geological map presented here is part of a wider project, which aims to produce detailed geological maps in tectonically meaningful sectors of the Western Alps (Balestro, Fioraso, & Lombardo, 2011Cadoppi, Camanni, Balestro, & Perrone, 2016;Fioraso, Balestro, Festa, & Lanteri, 2019;Gasco & Gattiglio, 2010;.

Lithostratigraphy
In the study area, the SSZ involves different rock units of the DM, IPZ and EPZ (see Main Map).
The DM consists of older siliciclastic metasediments and younger carbonate rocks. The siliciclastic metasediments are up to one hundred meters thick and correspond to coarse-grained massive paragneisses and medium-to fine-grained mica schists, both embedding rare lenticular bodies of metabasite. The paragneisses are interbedded with levels of quartzite and show transitional contacts with the mica schists which, in their stratigraphically upper part, are characterized by levels of quartzite and of carbonate-bearing mica schist. The carbonate metasediments are roughly one hundred meters thick and mainly consist of massive dolomitic marbles and metadolostones, which have been considered of Middle Triassic to Late Triassic age (Franchi, 1897). Medium-grained calcschists and fine-grained carbonate-rich calcschists discontinuously occur at the bottom and at the top of marbles, respectively.
The IPZ corresponds to a hundreds of meters thick meta-ophiolite and metasedimentary succession, mainly consisting of serpentinites, metabasites and calcschists. The ultrabasic rocks correspond both to massive serpentinites and serpentine schists, and locally embed meter-sized elongated bodies of metagabbro which are both of Mg-Al-rich and Fe-Ti-rich compositions. Metagabbros are massive, poorly rodingitized, and locally retain eclogitic mineral assemblages. The metabasites are rather massive and medium-grained, and derive from pillow lavas and volcanic breccia. The calcschists represent the metasedimentary cover of the meta-ophiolites; they have been attribuited to the lower part of the Late Cretaceous (Marthaler, Fudral, Deville, & Rampnoux, 1986). The calcschists are interbedded with levels of grey micaceous marble and are locally characterized by centimeter-to meter-sized horizons of impure quartzite, carbonate-bearing quartz schist and metabasite. Levels of metabreccia with clasts of dolomitic marble also occur.
The EPZ mainly consists of hundreds of meters carbonate metasediments, which vary from phyllosilicaterich calcschists to carbonate-rich schists and embed up to few decameter-sized stretched bodies of serpentinite, metabasite, prasinite and paragneiss. The ultrabasic rocks occur both as massive serpentines and serpentine schists. The metabasites are fineto medium-grained and, along the contact with the calcschists, are characterized by discontinuous centimeter-to decimeter-thick horizons of quartzite, mica schist and marble. These horizons locally occur also along the contact between calcschists and siliciclastic metasediments, which are up to few decameters thick and consist of quartz-rich paragneisses (i.e. Charbonnel Gneisses Auct.; Michel, 1953) and mica schists. The paragneisses are massive and leucocratic, and are characterized by centimeters-sized porphyroclasts of K-feldspar. Their occurrence in the oceanic succession of the EPZ likely documents a significant terrigenous input into the basin, sourced from a continental margin area.

Regional deformation phases
In the study area, four regional deformation phases (from D1 to D4) have been distinguished (see Main Map). The D1 phase is mainly defined by an early foliation (i.e. the S1), which is preserved as a structural relic in more massive rocks such as the dolomitic marbles, metagabbros and paragneisses. In the DM and IPZ, the S1 is scattered but on average it dips toward W at medium angle (Figure 2(a)); in the EPZ the S1 dips both to the N and S at high angle ( Figure  2(b)). The S1 is characterized by a N-S trending stretching lineation. At the map scale, the main D1 structure corresponds to the tectonic contact between the DM and IPZ.
The regional foliation (i.e. the S2) developed during the D2 phase and it mainly dips at low to medium angle to the W (Figure 2(c-d)). The S2 corresponds to the axial plane of non-cylindrical closed-to-isoclinal folds, which occur at all scales and pervasively deform and reorient both the tectonic contact between the DM and IPZ and the stratigraphic contacts within the DM, IPZ and EPZ. Fold axes are on average W-plunging at a low angle and are about parallel to a stretching lineation. As highlighted by related kinematic indicators (Figure 3(a)), the D2 was characterized by a westward tectonic transport. D1-and D2-related structures are partly reoriented by open-to-closed folds (Figure 3(b)), which characterize the D3 phase. These folds accommodated the exhumation-related doming of the DM and thus, in the study area (i.e. the northern to north-western flank of the DM), are northward verging. D3 fold axes are on average W-plunging at low to medium angle and axial planes mainly dip to the NNW at medium angle, although box folds with conjugated N-and Sdipping axial planes also occur. D3 folds did not develop a pervasive foliation but spaced cleavages and crenulation cleavages locally occur. Map-scale D3 axial planes have been detected both in the DM, IPZ, SSZ and EPZ. The D3 phase is also responsible for a major tectonic contact occurring within the DM and IPZ. This map-scale structure dips both toward NNW and NW at medium angle and it is defined by mesoscale discrete shear zones, which on average indicate top-to-NNW extensional shear senses.
The D4 is the latest deformation phase and is characterized by open folds, which occur at all scales in each units. D4 fold axes are both N-and S-plunging at low angle, and axial planes are steeply dipping and roughly NNW-SSE striking. Long-short D4 fold limbs highlight westward tectonic transport. Post-D4 brittle deformation was characterized by the development of different fault systems, which mainly correspond to N-S striking normal faults, and to conjugate WSW-ENE and NW-SE directed strike-slip faults showing right-lateral and left-lateral movements, respectively.

The Susa Shear Zone
The SSZ separates the DM and IPZ (i.e. the footwall) from the EPZ (i.e. the hanging-wall) and it is about 500 meters thick (Figure 4). It corresponds to a tectonic mélange (sensu Festa, Pini, Ogata, & Dilek, 2019), showing a block-in-matrix structure, with mylonitic calcschists embedding elongated blocks of different rock units. Blocks are mainly meters-to decameterssized and consist of paragneiss (Charbonnel Gneisses Auct.), metabasite, grey marbles and serpentinite, which were tectonically sampled from the EPZ and IPZ. A major hectometers-thick block of dolomitic marbles sliced from the DM, occurs in the southern side of the Susa Valley (see Main Map).
The SSZ was characterized by a polyphase kinematic evolution and two superposed tectonic events (T1 and T2 hereafter) have been recognized. The T1 is defined by a pervasive mylonitic foliation, which dips at low angle both toward NW and SW (Figure 2(e)). T1related stretching lineations are on average westward plunging and kinematic indicators (i.e. S-C fabrics and sigma-shaped porphyroclasts and quartz veins) provide reverse top-to-E sense of shears (Figure 2(e) and Figure 3(c-d)). The block-in-matrix structure of the SSZ mainly developed during the T1 and, at the map scale, T1 tectonic contacts correspond to the boundaries of the SSZ and of the embedded blocks ( Figure 4). The occurrences of both T1 tectonic contacts and T1-related mylonitic foliation deformed by D3 folds (Figure 3(b)), highlight that the T1 occurred between the D2 and D3 regional deformation phases, which developed under greenschist-facies conditions (see . The T2 is defined by discrete shear bands, which are on average W-dipping at medium-angle (Figure 2(f)). T2-related SC fabrics are consistent with extensional top-to-W sense of shears (Figure 3(e-f)). Superposition between T1 and T2 structures is locally highlighted by T2 shear planes wrapping blocks containing T1-related structures, and by T2 C-planes crosscutting T1 Cplanes. At the map scale, T2 tectonic contacts occur within the SSZ or close to its boundaries, within the EPZ and IPZ (see Main Map), and their relative movements caused the downward bending of both previous structures and lithological contacts. The T2 was coeval to the D4 phase, whose folds accommodated the same westward tectonic transport. The latter was likely driven by a late stage of doming of the DM, during which pre-D4 structures were likely reoriented (Lardeaux et al., 2006). As a consequence, the top-to-E sense of shear of the T1 shear planes would correspond to an apparent reverse kinematic, and T1-related structures would have actually originated as extensional E-dipping structures (see also Ghignone & Gattiglio, 2013).
Geological mapping and structural analysis highlight that the SSZ evolved through two different tectonic events, over which early developed top-to-E shear planes (i.e. the T1) were superposed by late top-to-W ones (i.e. the T2). Along the former southern analog of the SSZ (i.e. the West Viso Detachment; Tricart et al., 2004), top-to-E shear zones have been described and interpreted either as conjugate sets of extensional top-to-W/E shear planes (Ballevre, Lagabrielle, & Merle, 1990) or as evidence of a large-scale buck-thrusting phase (Philippot, 1990). It is here pointed out that T1 structures developed between the D2 and D3 and thereby they can not be coeval to T2 structures, which developed during the D4. We also suggest that, as a result of D3-to D4-related regional doming of the DM and westward tilting of the overlying IPZ and EPZ, the top-to-E shear planes were likely reoriented giving apparent reverse kinematics.

Software
The map has been drawn using the software QGIS (v. 3.4.2-Madeira) and Adobe Illustrator® 10. Structural data have been projected with the software StereoNett © .