Solid Earth, 12, 885–913, 2021 https://doi.org/10.5194/se-12-885-2021 © Author(s) 2021. This work is distributed under the Creative Commons Attribution 4.0 License. Kinematics and extent of the Piemont–Liguria Basin – implications for subduction processes in the Alps Eline Le Breton1, Sascha Brune2,3, Kamil Ustaszewski4, Sabin Zahirovic5, Maria Seton5, and R. Dietmar Müller5 1Department of Earth Sciences, Freie Universität Berlin, Berlin, Germany 2Geodynamic Modelling Section, German Research Centre for Geosciences, GFZ Potsdam, Potsdam, Germany 3Institute of Geosciences, University of Potsdam, Potsdam, Germany 4Institute for Geological Sciences, Friedrich-Schiller-Universität Jena, Jena, Germany 5EarthByte Group, School of Geosciences, The University of Sydney, Sydney, NSW 2006, Australia Correspondence: Eline Le Breton ([email protected]) Received: 18 September 2020 – Discussion started: 8 October 2020 Revised: 3 March 2021 – Accepted: 5 March 2021 – Published: 21 April 2021 Abstract. Assessing the size of a former ocean of which only 84–35 Ma, 260 km between 35–0 Ma), which greatly exceeds remnants are found in mountain belts is challenging but cru- the width of the ocean. We suggest that at least 63 % of the cial to understanding subduction and exhumation processes. subducted and accreted material was highly thinned conti- Here we present new constraints on the opening and width nental lithosphere and most of the Alpine Tethys units ex- of the Piemont–Liguria (PL) Ocean, known as the Alpine humed today derived from OCT zones. Our work highlights Tethys together with the Valais Basin. We use a regional tec- the significant proportion of distal rifted continental margins tonic reconstruction of the Western Mediterranean–Alpine involved in subduction and exhumation processes and pro- area, implemented into a global plate motion model with vides quantitative estimates for future geodynamic modeling lithospheric deformation, and 2D thermo-mechanical mod- and a better understanding of the Alpine Orogeny. eling of the rifting phase to test our kinematic reconstruc- tions for geodynamic consistency. Our model fits well with independent datasets (i.e., ages of syn-rift sediments, rift- related fault activity, and mafic rocks) and shows that, be- 1 Introduction tween Europe and northern Adria, the PL Basin opened in four stages: (1) rifting of the proximal continental margin in Over the last decades, new concepts on rifting processes, the Early Jurassic (200–180 Ma), (2) hyper-extension of the subduction initiation, and depth and exhumation of high- distal margin in the Early to Middle Jurassic (180–165 Ma), pressure rocks during subduction have emerged. The Alps (3) ocean–continent transition (OCT) formation with mantle are a natural laboratory to test ideas as they have preserved exhumation and MORB-type magmatism in the Middle–Late a detailed record of rifted continental and ophiolite units Jurassic (165–154 Ma), and (4) breakup and mature oceanic derived from the Alpine Tethys that were accreted or sub- spreading mostly in the Late Jurassic (154–145 Ma). Spread- ducted to ultra-high pressure and later exhumed at the surface ing was slow to ultra-slow (max. 22 mmyr−1, full rate) and (Froitzheim and Eberli, 1990; Froitzheim and Manatschal, decreased to ∼ 5 mmyr−1 after 145 Ma while completely 1996; Bernoulli and Jenkyns, 2009; Mohn et al., 2010; Bel- ceasing at about 130 Ma due to the motion of Iberia relative trando et al., 2014; Masini et al., 2014). Studies of hyper- to Europe during the opening of the North Atlantic. The final extended magma-poor-type continental margins such as in width of the PL mature (“true”) oceanic crust reached a max- the Alps have shed light on the tectonic complexities of dis- imum of 250 km along a NW–SE transect between Europe tal and ocean–continental transition zones, along which var- and northwestern Adria. Plate convergence along that same ious types of rocks such as granitoid basement within conti- transect has reached 680 km since 84 Ma (420 km between nental allochthons, serpentinized mantle rocks, gabbros, and basalts are put in contact by major detachment faults (e.g., Published by Copernicus Publications on behalf of the European Geosciences Union. 886 E. Le Breton et al.: Kinematics and extent of the Piemont–Liguria Basin Florineth and Froitzheim, 1994; Müntener and Hermann, Adria to ca. 900 km between Sardinia–Adria in Handy et al. 2001; Ferrando et al., 2004; Manatschal, 2004; Manatschal (2010; their Fig. 8b). However, those estimates do not dis- and Müntener, 2009; Epin et al., 2019). These zones of in- tinguish between extended continental and oceanic domains. herited weakness and thermal anomalies may represent ideal Thus, the aim of this paper is to provide robust kinematic candidates for the localization of subduction initiation when constraints and address the question of how wide the PL plate motion becomes convergent (Beltrando et al., 2010; Tu- Ocean and its rifted margin were, which is crucial to un- gend et al., 2014; Stern and Gerya, 2018; Kiss et al., 2020; derstanding slab pull forces, rollback, and the exhumation of Zhou et al., 2020). high-pressure rock in the Alps. We furthermore discuss the Moreover, the Alpine chain is enigmatic due to its very ar- style of rifting and extent of hyper-extended rifted margins cuate plate boundaries and the switch of subduction polarity versus mature oceanic domain (i.e., “true” oceanic, not tran- along strike (Fig. 1), the lack of a well-developed magmatic sitional crust) that went into subduction during the formation arc (e.g., McCarthy et al., 2018), and episodes of slab break- of the Western and Central Alps. For this, we present one off (e.g., Wortel and Spakman, 2000; Handy et al., 2015). possible kinematic scenario for this area, based on a compi- Seismic tomography models beneath the Alps are thus diffi- lation of previously published models and an updated model cult to interpret (see Kästle et al., 2020, for a recent review). for the past motion of Corsica–Sardinia. This regional re- Thermo-mechanical modeling is a key tool to gain a better construction goes back to Triassic times and is implemented understanding of orogenic processes in the Alps (e.g., Gerya within the recent global plate model of Müller et al. (2019). et al., 2002; Yamato et al., 2007; Duretz et al., 2011; Ruh et Furthermore, we test this kinematic scenario for geodynamic al., 2015; Reuber et al., 2016; Spakman et al., 2018; Dal Zilio consistency with thermo-mechanical modeling of the rift- et al., 2020) and potentially a better interpretation of seismic ing phase and compare our results with existing geological tomography models. These geodynamic models, however, records from the Alps. require quantitative input such as estimates of the former extent and thickness of the continental margins and oceanic domains involved in subduction, the paleo-location of plate 2 Geological setting boundaries through time, and the direction and rate of plate convergence. Kinematic reconstructions are thus crucial but The Alpine–Mediterranean belt results from a complex tec- challenging in such a tectonically complex area. First, re- tonic evolution that involved the closure of two oceans gional geological reconstructions need to be globally con- (Alpine Tethys and Neo-Tethys) and the subduction– nected and brought into a plate–mantle reference frame in or- collision of several continental domains (Briançonnais, der to assess mantle–plate–surface interactions through time Sesia, AlCaPa (standing for Alpine–Carpathians–Pannonian (e.g., Müller et al., 2019). Second, there is growing debate unit), Tisza, Dacia; Fig. 1) between two major plates – Eura- that maximal burial depth of metamorphic rocks in subduc- sia and Africa – that have been converging since Cretaceous tion zones, as estimated from mineral phase equilibria and times (e.g., Dewey et al., 1989). The Adriatic plate (Adria) assuming lithostatic pressure, may be overestimated due to is a key player due to its central position between Eurasia local overpressure and change in tectonic regime (Ford et and Africa. Today, Adria is surrounded by orogens: the Alps al., 2006; Schmalholz and Podladchikov, 2013; Pleuger and to the north, where Adria is the upper indenting plate; the Podladchikov, 2014; Reuber et al., 2016; Yamato and Brun, Dinarides to the east and the Apennines to the west, where 2017; Moulas et al., 2019). This has significant implica- Adria is the subducting lower plate (Fig. 1). It is bounded to tions for regional tectonic reconstructions that use an inferred the south by the accretionary prism of the Calabrian Arc and amount of subduction based on peak depths of (ultra-)high- the Kefalonia Fault (KF in Fig. 1). pressure metamorphism (e.g., Schmid et al., 1996; Handy et The Alps contain the remains of two Jurassic- to al., 2010, 2015) and for conceptual models of the exhumation Cretaceous-age basins (Piemont–Liguria and Valais) to- of high-pressure rocks (e.g., Brun and Faccenna, 2008). gether referred to as the Alpine Tethys (e.g., Stampfli et The past extent and size of the Piemont–Liguria (PL) al., 1998; Fig. 1) or, more recently, also as the “Alpine At- Ocean and its rifted margins, prior to the formation of the lantic” to emphasize its kinematic link with the Atlantic Alps, remain poorly constrained. Studies based on the age Ocean (Gawlick and Missoni, 2019). Here, however, we range of mafic rocks derived from the PL Ocean in the Alps adopt the well-established term Alpine Tethys. In Cretaceous and inferring ultra-slow spreading rates, propose a final ex- and Cenozoic times, the former Adriatic continental mar- tent ranging between 300 km (Li et al., 2013; Manzotti et al., gin was accreted to the upper plate of the Alpine Orogen, 2014) and 500 km (Froitzheim and Manatschal, 1996; their represented today by the Austroalpine (part of “AlCaPa”; Fig.