We present a detailed description of the tectono-stratigraphic architecture of the eclogite-facies Internal Piedmont Zone (IPZ) metaophiolite, exposed in the Lanzo Valleys (Western Alps), which represents the remnant of the Jurassic Alpine Tethys. Seafloor spreading and mantle exhumation processes related to the Alpine Tethys evolution strongly conditioned the intra-oceanic depositional setting, which resulted in an articulated physiography and a heterogeneous stratigraphic succession above the exhumed serpentinized mantle. “Complete” and “reduced” successions were recognized, reflecting deposition in morphological or structural lows and highs, respectively. The “complete” succession consists of quartzite, followed by marble and calcschist. The “reduced” succession differs for the unconformable contact of the calcschist directly above mantle rocks, lacking quartzite and gray marble. The serpentinite at the base of this succession is intruded by metagabbro and characterized at its top by ophicalcite horizons. Mafic metabreccia grading to metasandstone mark the transition between the “complete” and “reduced” successions. The character of the reconstructed succession and basin floor physiography of the IPZ metaophiolite is well comparable with the Middle Jurassic–Late Cretaceous succession of both the Queyras Complex (External Piedmont Zone) and the Internal Ligurian Units (Northern Apennines) and with modern slow-spreading mid-ocean ridges.

New structural data have been collected in the high-pressure meta-ophiolites exposed in the Lanzo Valleys (i.e., the Lanzo Valleys Ophiolite, LVO) and tectonically overlying the continental crust of the Dora-Maira and Gran Paradiso massifs in the Western Alps. Detailed geological mapping and structural analysis carried out in the south-western sector of the LVO (i.e., the Upper Viù Valley) allow to distinguish four ductile deformation phases, corresponding to the subduction-related D1 phase, the early exhumation-related D2-phase and the late exhumation-related D3 and D4 phases. The geometry of structures and kinematics have been reconstructed, and their regional tectonic meaning is discussed in the frame of the Alpine tectonic evolution of the Internal Piedmont Zone (i.e., the remnants of the Alpine Tethyan oceanic lithosphere) in the Western Alps.

The Ligurian-Piedmont Ocean (LPO) is inferred as a relatively narrow oceanic basin in palaeogeographic restorations, but the actual amount of oceanic lithosphere generated and the timing of magmatic accretion are still debated. Magmatic ages obtained from LPO intrusive rocks predominantly range between 165 and 160 Ma, supporting the interpretation of the LPO as a magma-poor ocean. However, this relatively short timespan of magmatic accretion may also suggest that the orogen sampled older sectors of the oceanic lithosphere, while younger (and more oceanward?) sectors could have been deeply subducted without returning.We therefore focus on studying a poorly-known stack of oceanic lithosphere (i.e., the Susa and Lanzo Valley Ophiolites; SLVO), which is exposed in the inner-central sector of the Western Alps and tectonically juxtaposed with the Gran Paradiso and Dora-Maira massifs. The SLVO were metamorphosed under eclogite-facies peak conditions and consist of large volumes of serpentinite hosting up to kilometer-sized metagabbro bodies, with Fe-Ti-rich differentiated masses and rare metaplagiogranite dykes. The metaophiolite sequence also includes widespread metabasaltic rocks and a metasedimentary cover consisting of minor quartzite and marble levels overlain by calcschist.Two pairs of Fe-Ti metagabbro and metaplagiogranite s.l. sampled close to the Avigliana (lower Susa Valley) and Mondrone (middle Ala Valley) localities have been selected for zircon U-Pb dating. In each sample, the dated zircons yield magmatic ages falling within the uppermost Jurassic Period (~150 Ma). The common age, along with similar major and trace element compositions, suggests a cogenetic origin within differentiation trends for the two pairs of metagabbro-metaplagiogranite (De Togni et al., 2024). Consequently, the SLVO were sampled from a sector of the LPO characterized by magmatic activity at ~150 Ma, significantly younger than most of previously reported ages for the LPO magmatism. We argue that the SLVO represent the youngest oceanic lithosphere accreted in the Western Alps and they may provide new constraints on the structural architecture of the LPO. De Togni, M., Balestro, G., Rubatto, D., Castelli, D., Gattiglio, M., & Festa, A. (2024). Late Jurassic magmatism in the Ligurian-Piedmont Ocean constrained by zircon ages of mafic and felsic meta-intrusives. Terra Nova, 00, 1–11. doi.org/10.1111/ter.12723

1. The Alps formed through subduction- and collision-related tectonic dismemberment of the European and Adriatic passive margins and of the interposed Jurassic Ligurian-Piedmont Ocean (Handy et al....

The Western Alpine Ophiolites represent the remnants of the oceanic lithosphere of the Alpine Tethys (Ligurian–Piedmont Ocean), which separated the Europe and Adria plates at the Jurassic time. These ophiolites now consist of several tectonic units (i.e., the Piedmont Zone) exposed in the axial sector of the Alpine orogenic belt (Balestro et al., 2019). Along the inner Western Alps, the meta-ophiolites are characterized by subduction-related eclogite-facies metamorphic peak conditions (i.e., the Internal Piedmont Zone) and underwent polyphase deformation. Despite this Alpine overprint, records of the oceanic tectonostratigraphy are partly preserved.The Lanzo Valleys meta-Ophiolites (LVO) are tectonically juxtaposed with continental basement units (Dora-Maira to the South and Gran Paradiso to the North), resulting in an extended but poorly known ophiolitic complex. In the southern sector of the LVO (i.e., the Viù Valley), new geological mapping, structural analysis and lithostratigraphic observations, allowed us to reconstruct the oceanic tectonostratigraphy of this Alpine Tethys segment (De Togni et al., 2021). The LVO succession consists of serpentinite, metagabbro, metabasalt and metasediments, whose primary lithostratigraphic relationships highlight different phases of the oceanic tectonostratigraphy. Mantle peridotites were firstly intruded by small gabbroic plutons, mainly of Mg-Al-rich composition, followed by minor Fe-Ti-rich differentiated types. Mantle rocks were then exhumed at the seafloor, and partly overlain by laterally discontinuous metabasaltic lavas. The latters were interlayered with mafic breccia and sandstone layers and with calcschist (i.e., the syn-extensional succession). The whole succession was then sealed by quartzite, impure marble and calcschist, representing the post-extensional succession.The reconstructed oceanic tectonostratigraphy of LVO highlights, as documented in other sectors of the Western Alpine Ophiolites, the original occurrence of tectonically unstable reliefs and structural lows, linked to spreading-related extensional tectonics at the Jurassic time. ReferencesBalestro G., Festa A. & Dilek Y. 2019. Structural architecture of the Western Alpine ophiolites, and the Jurassic seafloor spreading tectonics of the Alpine Tethys. J. Geol. Soc. Lond., 176, 913–930.De Togni M., Gattiglio M., Ghignone S. & Festa A. 2021. Pre-Alpine tectonostratigraphic reconstruction of the Jurassic Tethys in the high-pressure Internal Piedmont Zone (Stura di Viù Valley, Western Alps). Minerals, 11 (4), 361.