We present a regional synthesis of the structural architecture and tectonic evolution of the Western Alpine Ophiolites (WAO), exposed in NW Italy. The WAO represent the remnants of Alpine Tethys (Ligurian–Piedmont Ocean) that opened between Europe and Adria, and developed in four stages from the Middle Jurassic to Early Cretaceous. Emplacement of gabbroic intrusions into the extending lithospheric mantle of Europe–Adria marked the main magmatic event (Stage 1). Coalescent shear zones in the fossil upper mantle form lithospheric-scale detachment faults, which led to the exhumation of upper mantle peridotites and gabbros on the seafloor, and extensive serpentinization (Stage 2). Detachment faults, and serpentinized peridotites–gabbros in their footwalls, represent preserved fossil oceanic core complexes within the WAO. Emplacement of ophiolitic breccias and basaltic lava flows marked the syn-extensional phase (Stage 3). Radiolarian chert and limestone were deposited unconformably on this syn-extensional volcanic–sedimentary sequence, marking the post-extensional phase (Stage 4). Magmatic ages of gabbroic intrusions and mafic–felsic dykes, and depositional ages of post-extensional sequences in the WAO constrain the timing of the opening of the Ligurian–Piedmont Ocean to the Middle Jurassic ( c. 170–168 Ma), followed by a tectonic quiescence stage in the Late Jurassic–Early Cretaceous. Thematic collection: This article is part of the ‘Tethyan ophiolites and Tethyan seaways collection' available at: https://www.lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways

In the Inner Western Alps, three different types of block-in-matrix structures (BIMs) formed sequentially through time at a convergent plate margin. These show the superposition of progressive deformation from (i) subduction to eclogite-facies depths, (ii) collision, accretion, and exhumation of oceanic crust, represented by the Monviso Meta-ophiolite Complex, to (iii) collision, accretion, and exhumation of the continental Dora Maira units. The Type 1 occurs in the metasedimentary cover of the Dora Maira Unit and consists of a map-scale broken formation with boudinaged ‘native’ blocks of marble (Early Jurassic) in a calcschist matrix. It results from the tectonic overprinting of exhumation-related folding (D2-stage) on an earlier subduction-related dismembered succession (D1-stage). Type 1 also includes ‘non-mappable’ BIMs with ‘exotic’ blocks, resulting from the gravitational collapse of the Triassic carbonate platform of European Continental Margin, triggered by the Early Jurassic rifting. In the Monviso Meta-ophiolite Complex, Types 2 and 3 represent tectonically induced broken and dismembered formations, respectively. They differ from each other in the degree of stratal disruption of primary interbedded horizons of mafic metabreccia (Type 3) and mafic metasandstone (Types 2 and 3) sourced by the Late Jurassic–Early Cretaceous denudation of an oceanic core complex. Dismembered interbeds (Type 2) and isolated blocks were mixed together (Type 3) by the overlap of D2 tectonics and late- to post-exhumation extensional shearing (D3-stage). Development of these types of BIMs may be common in many exhumed convergent plate margins, where severe tectonics and metamorphic recrystallization under high-pressure conditions normally prevent the reconstruction of BIMs or mélange-forming processes. Our findings show that documenting the mode and time of the processes forming BIMs is highly relevant in order to reconstruct the oceanic seafloor morphology and composition of associated stratigraphic successions, and their control in the evolution of those convergent plate margins.

The eclogite-facies Monviso meta-ophiolite Complex in the Western Alps represents a well-preserved fragment of oceanic lithosphere and related Upper Jurassic – Lower Cretaceous sedimentary covers. This meta-ophiolite sequence records the evolution of an oceanic core complex formed by mantle exhumation along an intra-oceanic detachment fault (the Baracun Shear Zone), related to the opening of the Ligurian–Piedmont oceanic basin (Alpine Tethys). On the basis of detailed geological mapping, and structural, stratigraphic and petrological observations, we propose a new interpretation for the tectonostratigraphic architecture of the Monviso meta-ophiolite Complex, and discuss the role played by structural inheritance in its formation. We document that subduction- and exhumation-related Alpine tectonics were strongly influenced by the inherited Jurassic intra-oceanic tectonosedimentary physiography. The latter, although strongly deformed during a major Alpine stage of non-cylindrical W-verging folding and faulting along exhumation-related Alpine shear zones (i.e. the Granero–Casteldelfino and Villanova–Armoine shear zones), was not completely dismembered into different tectonic units or subduction-related mélanges as suggested in previous interpretations. The present-day architecture of the Monviso meta-ophiolite Complex results from nappe-scale folding with a significant component of shearing, and strain partitioning of the Alpine deformation, which were controlled by the inherited occurrence of (i) lateral and vertical variations of facies and thickness of sediments, (ii) an intra-oceanic fault-rock assemblage, which acted as weak horizons in concentrating deformation, and (iii) remnants of a volcanic ridge, which consists of massive metabasalt. Thus, the recognition of pre-collisional, intra-oceanic, tectonostratigraphic inheritance represents an important step in reconstructing the tectonic evolution of meta-ophiolite units in orogenic belts.

The eclogite facies Zermatt-Saas ophiolite in the Western Alps includes a composite chaotic unit exposed in the Lake Miserin area, in the southern Aosta Valley region. The chaotic unit is characterized by a block-in-matrix texture consisting of ultramafic clasts and blocks embedded within a carbonate matrix. This unit overlies massive serpentinite and ophicarbonate rocks and is unconformably overlain by layered calcschist. Despite the effects of subduction and collision-related deformation and metamorphism, the internal stratigraphy and architecture of the chaotic unit are recognizable and are attributed to different types of mass transport processes in the Jurassic Ligurian-Piedmont Ocean. This finding represents an exceptional record of the preorogenic history of the Alpine ophiolites, marked by different pulses of extensional tectonics responsible for the rough seafloor topography characterized by structural highs exposed to submarine erosion. The Jurassic tectonostratigraphic setting envisioned is comparable to that observed in present-day magma-poor slow- and ultraslow-spreading ridges, characterized by mantle exposure along fault scarps that trigger mass transport deposits and turbiditic sedimentation. Our preorogenic reconstruction is significant in an eclogitized collisional orogenic belt in which chaotic rock units may be confused with the exclusive product of subduction-related tectonics, thus obscuring the record of an important preorogenic history. © 2017 Geological Society of America