geosciences Article Progressive Deformation Patterns from an Accretionary Prism (Helminthoid Flysch, Ligurian Alps, Italy) Pierre Mueller 1, Matteo Maino 1,2,* and Silvio Seno 1 1 Dipartimento di Scienze della Terra e dell’Ambiente, Università di Pavia, 27100 Pavia, Italy; [email protected] (P.M.); [email protected] (S.S.) 2 Istituto di Geoscienze e Georisorse, CNR, via Ferrata 1, 27100 Pavia, Italy * Correspondence: [email protected] Received: 16 December 2019; Accepted: 9 January 2020; Published: 11 January 2020 Abstract: This paper reports the results of a field-based structural investigation of a well-exposed paleo-accretionary prism, which experienced complex deformation in a low-grade metamorphic setting. Field analyses focused on the description of structural fabrics, with the main emphasis upon parameters like the orientation, style and kinematics of foliations, folds and shear zones. We address the research to the south-westernmost part of the Alpine chain, the Ligurian Alps, where, despite their origin as turbidite sequences deposited into the closing Alpine Tethys Ocean, the Helminthoid Flysch Nappes are presently distributed in the outer part of the chain, above the foreland. The new dataset highlights different deformation patterns related to the different spatial distribution of the flysch units. This regional-scale partitioning of strain is hence associated with progressive deformation within a two-stage geodynamic evolution. Correlations among the different orogenic domains allow the proposal of a kinematic model that describes the motion of the Helminthoid Flysch from the inner to the outer part of the orogen, encompassing the shift from subduction- to collision-related Alpine geodynamic phases. Keywords: orogenic wedge; subduction flysch; oceanic closure and continent collision; deformation phases; progressive deformation 1. Introduction The evolution of a collisional orogen entails multiple and overlapping deformation stages that reflect the physical variations during the three-dimensional spatiotemporal evolution of the incorporated rock units. Such variations derive from the effect of variable tectonic stresses affecting rocks which are heterogeneous in terms of their composition and inherited structure, and that deform under variable temperature and lithostatic pressure conditions. The resulting complex pattern is classically simplified and grouped into deformation and/or metamorphic phases characteristic of one or more tectonic domains. Each deformation phase is generally accompanied by one generation of foliation or schistosity representative of the deformational regime and the metamorphic conditions [1–3]. Changes in the orientation and/or strength of the tectonic forces, as well as in the temperature and pressure conditions, generate penetrative structures that display different orientations and shapes. In field-based studies, the determination of multiple overlapping fabrics represents the key factor to unravel the deformational history of an orogenic domain [1]. However, even within a single tectonic domain the deformation patterns may be strongly heterogeneous in response to local lithological variations or inherited geometry. Hence, the regional-scale correlation of structural fabrics among different outcrops may be difficult. Moreover, similar deformation patterns may result from different mechanisms that involve one or multiple deformation phases. Geosciences 2020, 10, 26; doi:10.3390/geosciences10010026 www.mdpi.com/journal/geosciences Geosciences 2020, 10, 26 2 of 17 Local or regional structures may be the result of distinct deformation phases, progressive deformation,Geosciences 2019 or a, 9 combination, x FOR PEER REVIEW of the two [4]. Such complexities are often underestimated, whilst2 of 17 the number of the locally observed foliations are typically uncritically appraised, and expanded to large Local or regional structures may be the result of distinct deformation phases, progressive tectonicdeformation, domains. or Consequently, a combination robustof the two regional [4]. Such interpretations complexities are may often become underestimated, challenging. whilst Therefore, the to reducenumber uncertainties, of the locally field observed analyses folia shouldtions are precisely typically uncritically indicate how appraised and where, and eachexpanded structural to large fabric occurs.tectonic Subsequently, domains. Consequently a critical evaluation, robust regional (i.e., interpretations a comparison may with become the other challenging outcrops). Therefore should, be performed,to reduce with uncertainties, the main field emphasis analyses being should put precisely upon the indicate determination how and whe ofre the each temporal structural and fabric spatial variationsoccurs of. Subsequently, the deformation a critical and theevaluation generating (i.e., mechanismsa comparison [ 1with,2,4]. the other outcrops) should be Inperformed this paper,, with we the presentmain emphasis a field-based, being put structural upon the determin analysisation of the of the uppermost temporal portionand spatial of the paleo-accretionaryvariations of the wedge deformatio preservedn and the in genera the Ligurianting mechanisms Alps. The [1,2, Ligurian4]. Alps are the southernmost portion ofIn the this Alpine paper, chain. we present They a preservefield-based a complete, structural stratigraphic analysis of the record uppermost of the portion European of the and paleo oceanic- accretionary wedge preserved in the Ligurian Alps. The Ligurian Alps are the southernmost portion (Alpine Tethys) paleogeographic domains (Figure1;[ 5–12]). In particular, the turbiditic covers of of the Alpine chain. They preserve a complete stratigraphic record of the European and oceanic the ophiolite sequences, namely the Helminthoid Flysch units, are well exposed in the outermost (Alpine Tethys) paleogeographic domains (Figure 1; [5–12]). In particular, the turbiditic covers of the part (SW)ophiolite of thesequences chain,, thrustnamely directly the Helminthoid onto the Flysch foreland units (Figure, are well2)—despite exposed in the the fact outermost that they part were detached(SW) fromof the their chain, innermost thrust directly position onto (NE) the in foreland close vicinity (Figure to 2) the—despite subduction the fact trench. that they Although were the absencedetached of high-grade from their metamorphisminnermost position is (NE clear) in evidence close vicinity of thin-skinned to the subduction thrusting, trench. theAlthough kinematic the of the translationabsence of high and- emplacementgrade metamorphism of the Helmintoidis clear evidence Flysch of thin nappes-skinned is still thrusting poorly, the understood kinematic of [8 ,10]. Theyt showhe translation heterogeneous and emplacement structural of fabrics the Helmintoid and both Flysch longitudinal nappes andis still traversal poorly understood geometric variations,[8,10]. and therewithThey show preventheterogeneous a trivial structural kinematic fabrics interpretation. and both longitudinal The data and presented traversal geo in thismetric study variations allow, the linkageand of therewith the fabric prevent and geometric a trivial kinematic heterogeneities interpretation to regional. The data variations presented which in this can study be integrated allow the into an evolutionarylinkage of the path fabric that and incorporates geometric heterogeneities their variations to regional in space variations and time. which can be integrated into an evolutionary path that incorporates their variations in space and time. FigureFigure 1. (a 1.) Location(a) Location of of the the study study area. area. ((bb)) TectonicTectonic sketch sketch of of the the Western Western and and Ligurian Ligurian Alps Alps [12]. [12]. Tectonic units are grouped into Internal (Piedmont, Piedmont–Ligurian and Briançonnais) and Tectonic units are grouped into Internal (Piedmont, Piedmont–Ligurian and Briançonnais) and External External (Provençal–Dauphinois, Helvetic and External Massifs) domains. The yellow box shows the (Provençal–Dauphinois, Helvetic and External Massifs) domains. The yellow box shows the location of location of the Helminthoid Flysch nappes (HF) above the Penninic Basal Contact (PBT). DB: Dent the Helminthoid Flysch nappes (HF) above the Penninic Basal Contact (PBT). DB: Dent Blanche nappe Blanche nappe (Austroalpine domain); E-U: Embrunais-Ubaye nappes; TH-M: Torino hill and (AustroalpineMonferrato domain); high; TPB: E-U: Tertiary Embrunais-Ubaye Piedmont Basin. nappes; TH-M: Torino hill and Monferrato high; TPB: Tertiary Piedmont Basin. Geosciences 2020, 10, 26 3 of 17 Geosciences 2019, 9, x FOR PEER REVIEW 3 of 17 Figure 2. Tectonic setting of the Ligurian Alps [66,,88,12,12],], showing the main paleogeographic domains (Piedmont(Piedmont–Ligurian,–Ligurian, Prepiedmont, Briançonnais and Dauphinois). The The Helminthoid Flysch nappes rest above the Briançonnais Briançonnais–Dauphinois–Dauphinois boundary boundary through through the the Penninic Penninic Basal Basal Contact Contact (PB (PBC).C). PF PF:: MioceneMiocene-age-age Pietra Pietra di di Finale deposits; deposits; Voltri Voltri indicates the Voltri Massif of the Piedmont–LigurianPiedmont–Ligurian oceanic basement. 2. Geological Geological S Settingetting TThehe Helminthoid Helminthoid Flysch nappes represent the structurally topmost units of the Ligurian Ligurian segment segment of the Western Alps (Figure(Figure1 1;;[ [6,8,6,8,1010]]).). The The Alpine Alpine edifice edifice is is formed