Mélanges: 100th anniversary of the inception of the term and concept

Editorial

Mélanges: 100

th

anniversary of the inception of the term and concept

One hundred years ago, in 1919, the British geologist Edward Greenly coined the term“mélange”, abbreviation of “autoclastic mé-lange”, in describing a tectonically disrupted and internally strained phyllite-sandstone succession in the Mona Complex (Gwna Group) in Anglesey, north Wales (Greenly, 1919). This term refers to a“lenticular strips and lumps of gritfloating in a schistose matrix” given by progressive up to complete disruption of a stratigraphic succession, differentiating these rocks from other“chaotic” units originated by sedimentary-gravitative processes (e.g., the Wildflysch Auct., largely described in the Alps after Kaufmann; inStuder, 1872;Kaufmann, 1886). Since this first definition, and afterHsü (1974), the term“mélange” has been ex-tensively used to describe the occurrence of chaotic rock assemblages in orogenic belts and ancient subduction-accretion complexes, and later extended to other geodynamic environments such as collisional and intra-continental tectonic settings, including rifting and passive margin evolution, and strike-slip tectonic settings (seeCamerlenghi and Pini, 2009;Festa et al., 2010and reference therein).

The classical descriptive and non-genetic definition of a “mélange” refers to a mappable unit (at 1:25,000 or smaller scale) or body of inter-nally disrupted and mixed rocks in, or rarely without, a pervasively de-formed matrix (seeBerkland et al., 1972;Wood, 1974;Silver and Beutner, 1980;Raymond, 1984;Cowan, 1985). Nonetheless this de fini-tion is largely accepted by“mélange workers”, the occurrence of a large number of different types of chaotic rock unit worldwide formed by dif-ferent processes (tectonic, gravitational, diapiric and their mutual inter-play and superposition), and the lack of agreement on its formal implementation (e.g.,Rast and Horton Jr., 1989;Silver and Beutner, 1980; also compareŞengör, 2003withCowan and Pini, 2001;Pini, 1999;Festa et al., 2010;Wakabayashi, 2011), have led to some confu-sion and misinterpretations in the literature. This problem particularly concerns students and researchers in the broadfield of geosciences who are not intimately familiar with mélanges, terminology, and inher-ent issues (e.g., complex internal structures and superposed origins of mélanges). In fact, in most orogenic belts and exhumed subduction– accretion complexes, a strong morphological convergence of meso- to map-scale fabric elements exists between a block-in-matrix fabric of basin-wide sedimentary (i.e., olistostromes and/or mass transport de-posits), diapiric, tectonic, and polygenetic mélanges. This resemblance is the main reason of the long-standing debate on the nature and mode of geological processes that lead to the formation of chaotic rock assemblages (i.e. gravitational vs. tectonics), particularly in areas of well-preserved, exhumed subduction–accretion complexes, such as in the Western US Cordillera, Circum-Pacific Region and Circum-Mediterranean Region. In addition, it is well-documented that the mechanisms responsible for the formation of mélanges may occur in a wide range of geological settings, spanning from relatively shallow to

deep crustal depths. In this framework, the main discussion revolves around whether the“chaotic disruption” of rock assemblages, observed in exhumed orogenic belts and subduction-accretion complexes, is a re-sult of tectonic shearing and mixing alone, achieved at different depths, or a product of tectonic or diapiric reworking and“recycling” of mass transport deposits (MTDs) during the overall geodynamic evolution of the primary formation setting (see, e.g.,Hsü, 1974;Berkland et al., 1972;Silver and Beutner, 1980;Raymond, 1984, 2015;Cowan, 1985;

Barber et al., 1986;Bettelli and Panini, 1987;Pini, 1999;Bettelli et al., 2004;Alonso et al., 2006;Festa et al., 2010, 2014, 2016;Vannucchi and Bettelli, 2010;Wakabayashi, 2011;Wakabayashi and Dilek, 2011;

Dilek et al., 2012;Barber, 2013;Balestro et al., 2015;Krohe, 2017;

Tartarotti et al., 2017). Although most of all papers focusing on mé-langes, indirectly suggested specific criteria to define and clarify the in-terpretation of both the origin and processes of their formation, these criteria were never clearly streamlined and, consequently, they are still not completely acknowledged by the majority of geoscientists. Therefore, confusion exits in the correct interpretation of mélange units with contrasting models for their origins, and consequent uncom-pleted reconstruction of the tectonic evolution of the geodynamic set-ting of their formation.

After 100 years and several papers, books and special issues, the def-inition and geological significance of mélanges are still a matter of con-troversy, but some long-debated concepts are now broadly accepted. For example, although the most important mélange forming process is thought to have taken place in subduction channels (e.g.,Cloos, 1982;

Cloos and Shreve, 1988a, 1988b), in which high degrees of mixing of rocks (including ultramafic rocks) with differing P–T–t histories and metamorphic grades may occur, the role played by gravitational pro-cesses is nowadays widely documented. Large part of mélanges recog-nized in different exhumed orogenic belts and subduction-accretion complexes are recently documented to befirstly originated from sedi-mentary mass transport processes (i.e., sedisedi-mentary mélanges or olistostromes). Subsequent deformation due to tectonic and/or diapiric processes led to the reworking and reorganization of their primary block-in-matrix fabric, with the formation of polygenetic mélanges, widely exposed in exhumed orogenic belts worldwide (e.g.,Raymond, 1984;Cowan, 1985;Alonso et al., 2006, 2015;Camerlenghi and Pini, 2009;Wakabayashi, 2011, 2017;Dilek et al., 2012;Festa et al., 2013, 2016;Platt, 2015;Ernst, 2016;Raymond, 2015, 2018;Raymond and Bero, 2015). However, a certain lack of communication still exists be-tween the different communities of geoscientists involved in MTDs and mélange studies (e.g., sedimentologists and structural geologists) and, therefore, between geoscientists working on mélanges developed at different structural levels or having different backgrounds (e.g., geologists and geophysicists).

Gondwana Research 74 (2019) 1–6

https://doi.org/10.1016/j.gr.2019.07.002

1342-937X/© 2019 Published by Elsevier B.V. on behalf of International Association for Gondwana Research. Contents lists available atScienceDirect

Gondwana Research

At larger scale, the distribution and types of ancient MTDs (i.e., sedimentary mélanges) and diapiric mélanges, has to be reconsidered in light of the increasing number of mélanges (re) interpreted as originated by sedimentary and/or diapiric processes, as clearly recognizable from the comparison between their relative abun-dance in ancient and modern subduction and collisional settings (see, e.g.,Barber et al., 1986;Maekawa et al., 1993;Fryer et al., 1999;Pini et al., 2012;Festa et al., 2014, 2016;Barber, 2013;Moscardelli and Woods, 2016;Ogata et al., 2014a, 2014b, 2019a, 2019b). Moreover, it is unclear how large-scale MTDs and diapiric mélanges would influence the overall stratigraphic-tectonic evolution of subduction-accretion complexes, as well as the temporal and spatial tectonic evolution of other geodynamic environments (i.e., passive margins, wedge-foredeep systems, back- and fore-arc systems, wedge-top basins, etc.). A complexing open question concerns the preservation potential of the sedimentary/diapiric-induced primary arrangement (or fabric) in mélanges during the subsequent tectonic deformation and recycling (see, e.g.,Festa et al., 2013, 2018), and specifically: how long (or how deep) and at which scale of observation may a sedimentary or diapiric mélange be recognized?

Within this complex scenario, the problem of a correct use of the mé-lange terminology and a complete understanding of its meaning is not a simple academic exercise. On the contrary, it has a primary geological im-portance in distinguishing the very different processes, mechanisms and strain magnitudes responsible for mélange formation (e.g., mixing vs. stratal disruption). Mélange studies may provide, not only significant con-straints to a better understanding of both the geodynamic environment in which mélanges formed, and the related tectonic evolution, but also use-ful practical implications for geohazard evaluation. For example, during the last years it has been well documented the potential role of subducted mélanges in controlling the seismic behavior of megathrusts at the sub-duction plate interface of modern convergent margins (e.g.,Fagereng and Sibson, 2010;Fagereng, 2011;Fagereng et al., 2011;Geersen et al., 2013;Bürgmann, 2018;Festa et al., 2018). The study of exhumed exam-ples of large-scale, heterogeneous MTDs (i.e., sedimentary mélanges or olistostromes), the characteristics of their block-in-matrix fabric, and the mechanisms of their emplacement, also provided significant insights on earthquake-related submarine slope failures at the front or subduction-accretion complexes, and the related devastating conse-quences in terms of natural hazards (i.e., tsunamis) and potential socio-economic loss (e.g., failure of submarine cable network) (e.g.,Harbitz et al., 2006; Chaytor et al., 2007; Hsu et al., 2008; Tappin, 2010;

Kawamura et al., 2012;Schwab et al., 2012;Yamada et al., 2012;Festa et al., 2014;Ogata et al., 2014a, 2019a, 2019b;Ortiz-Karpf et al., 2018;

Artoni et al., 2019).

This Special Issue of Gondwana Research, which is published to cele-brate the 100th_{anniversary of the inception of the term“mélange” and}

concept in the geological literature (Greenly, 1919), is aimed at explor-ing and documentexplor-ing such open problems and further ways of research revolving around mélanges, at various scales and in different tectonic settings and ages, through multi-disciplinary case studies from around the world.

The papers of this Special Issue present the most up-to-date observa-tions and interpretaobserva-tions on various typeset classical mélanges from around the world, their forming processes, diagnostic criteria to differ-entiate among different types, and the related terminology. The major-ity of the papers in this Special Issue are based on the contributions presented at a dedicated Mélange session that we have organized and convened at the GSA Meeting in Seattle, WA-USA in October 2017. The geographic locations of the case studies covered in this Special Issue are shown inFig. 1. This Special Issue represents, therefore, a sig-nificant contribution to the contemporary literature on mélanges and related problems in their recognition, classification and understanding through the Earth history, providing also a modern and updated archive on well-documented mélange occurrences from different geodynamic environments. Detailed multidisciplinary studies of mélanges, their

related terminology, and processes of formation, are insightful for a bet-ter understanding and constraining of their tectono-sedimentary evolu-tion, and the interplay and superposition of different processes (tectonic, sedimentary, and diapiric) during different stages of orogenic buildup and crustal exhumation, independently from their age (i.e., from Proterozoic to Phanerozoic).

We have organized the papers of this Special issue in three sections. Thefirst part includes three papers which are aimed to provide useful diagnostic criteria to differentiate between mélanges formed by differ-ent processes, mechanisms, and related diagnostic features. The second part includes four case studies defining the different distribution of tec-tonic, sedimentary, diapiric and polygenetic mélanges in the geologic evolution of ancient and modern subduction-accretionary complexes and orogenic belts. They investigate the role of these mélanges in con-trolling the repartition of deformation and the dynamic equilibrium of the frontal wedge of subduction complexes, as well as the characteris-tics and meaning of their tectonic reworking and reorganization. The third and last section contains six papers documenting the primary im-portance of the study of different types of mélange, including ophiolitic mélanges, in better constraining and reappraising the regional tectonic evolution of their geodynamic setting.

1. Part I: Diagnostic features to recognition of the tectonic, sedimen-tary or diapiric origin of mélanges

Commonly, the internal fabric of a mélange is the result of different processes (e.g., tectonic disruption, metamorphic transformation, mass-transport, diapirism,fluid expulsion, etc.) and their interplay and super-position, which are strongly controlled by different factors, such as the consolidation degree of the primary bedded succession, the rheological contrast between competent layers, and the kinematics and strain rate of deformational processes. In addition, multiple deformation events during the tectonic evolution of orogenic belts and ancient subduction-accretion complexes ultimately obliterate the primary block-in-matrix fabric of mélanges, thereby complicating their correct recognition. The three papers presented in this Section provide detailed descriptions of diagnostic features and criteria to practically differenti-ate mélange types, and reldifferenti-ated structures, formed by different pro-cesses. These papers provide efficient tools to be applied at different scales of observation (from micro- to meso- and macro-scales) during field work on mélange rock units, regardless of their location, age, and tectonic history.

After several years of study on most of the notable examples of mé-langes throughout the world,Festa et al. (2019-in this issue)present a comprehensive overview and synthesis of a diverse set of_field-based stratigraphic and structural criteria, which are at the base of geological mapping rules, to differentiate between various mélange types, devel-oped by disparate geological processes and mechanisms in orogenic belts and exhumed subduction-accretion complexes. After the rede fini-tion of the current concepts of mélange and mélange nomenclature, the Authors describe the most diagnostic features of tectonic, sedimentary and diapiric mélanges at different scales, and discuss some of the main issues complicating the application of these diagnostic criteria. The Au-thors introduce two new additional criteria (the“tectonic environment” and the“deformation” criteria) to the other ones in approaching these complexities, and in recognizing different processes of polygenetic mé-langes formation in thefield when primary diagnostic fabrics were reworked by multiple deformational events. The proposed diagnostic criteria can be applied to allfield-based investigations of mélanges and broken formations in orogenic belts and exhumed subduction-accretion complexes around the world, regardless of their location, age, and tectonic history.

Ogawa (2019-in this issue)describes different diagnostic features formed at early stages of deformation of sand- and mud-rich chaotic de-posits (mélanges) in modern and ancient accretionary complexes around the world, providing comparative considerations from results

of deformation under un- and semi-lithi_{fied conditions. The Author} stressed on the significance of pore-fluid pressure, mud pressure, and frequency of stress period in order to interpret the meaning of these dif-ferent types of chaotic features. He points out the contrasting or mutual relationships between deformation of sand- and mud-rich chaotic de-posits in relation to sudden earthquake-like, high-frequency, shaking periods. Results of this paper can be successfully applied to correctly in-terpret various chaotic deposits (mélanges) throughout the world, and to discuss their origin in both onshore and offshore case studies.

Wakabayashi (2019-in this issue)compares primarily outcrop and petrographic scale features of disputed, polygenetic examples, of serpentinite mélange from the Franciscan subduction complex, to un-disputed sedimentary serpentinite mélange of the Great Valley Group, to sheared serpentinite and metaserpentinite in shear zones in the Fran-ciscan Complex, and northern Sierra Nevada, for which the serpentinite does not have a clastic history (tectonic serpentinite mélange unlikely to be disputed). Through outcrop and petrographic observations, the Author emphasizes the textural differences in block-matrix relationship between deformed“sedimentary” and “tectonic” serpentinite mélanges as well as generalizing these criteria to siliciclastic mélanges and pointing out some differences between the two. The paper will also re-late the formation of the various serpentinite mélange types to specific settings within the convergent plate margin orogen.

2. Part II: Tectonic, sedimentary, diapiric, and polygenetic mélanges in ancient subduction-accretionary complexes and orogenic belts

During the evolution of modern subduction-accretionary com-plexes, different types of mélange may form at different structural levels to maintain the dynamic equilibrium of the wedge, and therefore acting as significant markers of deformation events. However, in ancient oro-genic belts and exhumed subduction–accretion complexes with a re-cord of multiple deformational events, sedimentary mélange fabrics are commonly overprinted and significantly reworked by tectonic (and/or diapiric) processes, extremely complicating their distinction

from tectonic mélanges. This is still the core problem of a long-standing debate, especially in the Western US Cordillera and in all the exhumed subduction–accretion complexes and orogenic belts world-wide. It revolves around the main mechanisms of incorporation and mixing of exotic blocks within the host matrix. The four papers in this Section present case studies of sedimentary, tectonic, diapiric, and poly-genetic mélange types, and their detailed comparison, to discuss the na-ture and processes of mélange formation at different structural levels.

Raymond (2019-in this issue)describes the three principal, overlap-ping roles played by different mélange types in the origin and architec-ture of subduction accretionary complexes. First, tectonic mélanges serve as zones of concentrated deformation within and below the ac-creted rocks, facilitating preservation of inter-mélange, less deformed, accretionary units. Tectonically dismembered formations (without “ex-otic” blocks) or thinner units of scaly rocks and breccia formed along the decollement at the top of the down-going plate. Olistostromes (sedi-mentary mélanges) may be incorporated into the decollement here. In the mid-arc to inner-arc regions, exotic block-bearing mélanges de-velop in zones of tectonic fragmentation and mixing of accreting ocean plate stratigraphy, in mud diapirs, and along out-of-sequence faults. Second, after accretion, the sedimentary, tectonic, diapiric and polygenetic mélange units serve as single blocks of sheet architectural accretionary units of the subduction accretionary complex. Diapiric mé-langes and sedimentary mémé-langes may become incorporated into the subduction accretionary complex during ongoing deformation. Third, mélanges serve as post-subduction stress guides that focus shear strain during continuing and post-accretion deformation of the subduction ac-cretionary complex, allowing its ongoing modification during progres-sive deformation of the orogeny.

Wakita (2019-in this issue)describes the tectonic setting required to preserve sedimentary mélanges without tectonic overprinting in Paleo-zoic and MesoPaleo-zoic accretionary complexes in southwest Japan. The Au-thor documents that the accretion of volcanic arcs and oceanic plateau might play an important role in interrupting the processes of subduc-tion and accresubduc-tion during the formasubduc-tion of accresubduc-tionary complexes, Figure 1. - World map showing the lithospheric plates, their boundaries (modified fromFesta et al., 2018), and the case studies of mélanges covered by the papers in this Special Issue (marked by light blue boxes).

allowing to preserve sedimentary mélanges which mainly consist of ba-salt and limestone clasts within a mudstone matrix. Therefore, the Au-thor outlines that the occurrence of sedimentary mélanges without a tectonic overprint in ancient accretionary complexes, suggests that the site of oceanic plate subduction jumped ocean-wards because of the buoyancy of the incorporated oceanic plateau.

Ogata et al. (2019-in this issue)discuss the role and mechanism of incorporation of“exotic” and “native” blocks during gravitational pro-cesses, and their progressive deformation, by presenting examples from the northern Apennines of Italy and the northwestern Dinarides in Slovenia. The Authors document that important information, such as the kinematics of processes and internal strain partitioning, can be re-constructed from the study of gravitational-related features in sedimen-tary mélanges, providing fundamental paleogeographic and paleophysiographic constraints, as well as consolidating the basis for a possible updated reappraisal of some classic mélanges. The study of mechanisms of incorporation of such blocks can thus provide significant constraints to distinguish tectonic and sedimentary mélanges in oro-genic belts and exhumed subduction-accretion complexes.

Moore et al. (2019-in this issue)document the diverse structure of tectonic, sedimentary and diapiric chaotic rock bodies exposed along the west coast of Myanmar and formed during different stages of tec-tonic evolution of a subduction zone. The Authors document that the tectonic mélanges are shear zones with incorporated“exotic” blocks of Cretaceous ophiolites. Adjacent to the shear zones, the degree of shear fabric diminishes and tectonic broken formations (with no “ex-otic” blocks) occur. The sedimentary broken formations (endolistostromes) are mass transport deposits within folded, but not sheared, Eocene-Oligocene turbidite sequences. The diapiric mélanges are deposits of active mud volcanoes that carry fragments of older units, with“exotic” blocks, from depths of a few kilometers.

3. Part III: Mélanges and regional geology

Mélanges are intimately linked with tectonically-induced geological processes that characterized all the diverse geodynamic environments, including passive and active margins, intra-continental deformational settings and strike-slip settings, constituting a significant component of the Earth geological record. Therefore, the detailed study and the un-derstanding of their tectono-stratigraphic and petrological-geochemical relationships with surrounding rocks is crucial to provide new con-straints on the geodynamic evolution of the considered area. The six pa-pers of this third and last Section address notable case studies, discussing the significance of mélanges in the definition of the tectonic evolution of their geodynamic setting of formation, with regional-scale perspective.

To this end, on the basis of a regional scale comparison of Middle to Late Jurassic sedimentary mélanges formed due to ophiolite obduction onto the western (Adriatic) Neo-Tethys margin,Gawlick and Missoni (2019-in this issue)propose the existence of a single Neo-Tethys Ocean in the Western Tethyan realm, instead of multi-ocean and multi-continent scenarios. These thrust-related mélanges, which occur in the Alpine-Carpathian-Dinaridic Mountain chain, consist of reworked Triassic to Middle Jurassic oceanic crust with its sedimentary cover. Re-sults of this paper show that the analysis of ancient Neo-Tethys mé-langes along the Eastern Mediterranean mountain ranges allows both a facies reconstruction of the outer western passive margin of the Neo-Tethys and conclusions on the processes and timing of Jurassic orogeny.

Hajná et al. (2019-in this issue)report a case example of a chert -graywacke sedimentary mélange exhumed and exceptionally well pre-served within the Neoproterozoic to early Cambrian accretionary wedge in the west-central Bohemian Massif. Based on major and trace element geochemistry, combined withfield and structural mapping and U–Pb detrital zircon geochronology, the Authors develop a new model for the formation of the chert– graywacke association along

active margins. This model invokes a key role for tensile fracturing asso-ciated with the outer swell to explain formation of blocky debrisflows and their subsequent mixing with the terrigenous trench-_{fill, up to} pro-duce sedimentary mélanges composed of mixed oceanic and terrige-nous material. The studied type of sedimentary mélange composed of pelagic/hemipelagic chert blocks and terrigenous, arc-derived gray-wacke matrix, represents a rarely documented case of submarine, outer trench slope, mass-wasting deposit and may be considered a new type of subduction-related mélange, defined by the Authors as “outer-trench-slope mélange”.

Katopody and Oldow (2019-in this issue)describe the structural relations between the Vendovi assemblage and arc-sourced sedi-mentary rocks in the northwest Cascades (northwestern Washington State) that are structurally overlain by a Late Jurassic ophiolitic sedimentary mélange. The Authors document that the sed-imentary mélange, which consists of blocks of peridotite, basalt, an-desite, banded chert, plagiogranite, gabbro, and silicified argillite in a matrix of argillite and volcanoclastic-derived breccias, formed dur-ing mass wastdur-ing events. The mélange underwent phases of brittle and soft sediment deformation prior to the Late Jurassic, when it was overlapped by an arc-sourced volcaniclastic sedimentary se-quence. Together, the mélange and the overlap sequence were tec-tonically deformed during subsequent accretionary tectonic stages. The Authors conclude that the depositional and structural history of the Late Jurassic ophiolitic sedimentary mélange and its Jura-Cretaceous stratigraphic overlap succession require a tectonic set-ting where arc and ophiolitic affinity blocks are in proximity. Exten-sion within a nascent accretionary wedge, resulting in the exhumation of arc and ophiolitic affinity blocks, provides a viable tectonic setting for the formation of the Vendovi assemblage.

Throughout map relationships, outcrop and thin-section scale obser-vations,Lacombe et al. (2019-in this issue)de_{fine the tectonic evolution} of the Western Newfoundland Appalachians in which an early, Taconian, West Bay Thrust Sheet was emplaced onto the Laurential margin. The Authors document the initial contribute of gravitational processes, forming sedimentary mélanges (debrisflows), which con-tributed igneous blocks to the allochthon. However, the majority of fragmentation and mixing, forming broken formations and mélanges, took place in an environment of horizontal tectonic extension, pro-moted by the occurrence of highfluid-pressure. The Authors also pro-pose a method of mapping that allows the distinction of broken formations from mélanges in a sector previously described as consisting of“undivided mélanges”, and makes it easier to identify map-scale structures amongst sparse outcrops.

Zheng et al. (2019-in this issue)report the description and interpre-tation of a new ophiolitic mélange with blocks of serpentinite, gabbro, basalt and chert in the northern West Junggar in China. Throughout de-tailedfield work, zircon SHRIMP U-Pb analyses, oxygen isotopic and whole-rock geochemical data of basalt and gabbro from the ophiolitic mélange, the Authors discuss the early Paleozoic tectonic setting of the northern West Junggar, providing new evidence of the timing of subduction initiation and distribution of ophiolitic mélanges. They doc-ument that the E’min ophiolitic mélange formed in a forearc setting dur-ing subduction initiation in the northern West Juggar.

Fuentes et al. (2019-in this issue)present a structural analysis of the Chañaral tectonic mélange in northern Chile, which represents part of the late Paleozoic accretionary complex in the southwestern margin of Gondwana. Through the evaluation of a kinematic model of triclinic transpression with inclined extrusion, the Authors document that the orientation, arrangement and verging of the tectonic structures and fab-rics measured in the mélanges conform a robust dataset for the applica-tion of the model. This model allows to obtain a three-dimensional perspective of the complex relationship between the oblique plate mo-tion and thefinal structural configuration of the tectonic mélanges. The results of this work highlight the importance of 3D studies in tectonic mélanges.

Acknowledgments

We thank all the contributors to this Special Issue for their time and effort, and express our sincere gratitude to a large number of colleagues who provided valuable and timely reviews of the papers in it. We also extend our thanks to the Editor-in-Chief, Prof. Santosh, for his careful editorial help and guidance during the preparation of this Special Issue, and to the Elsevier staff in the Gondwana Research journal office. This study isfinancially supported by research grants from the Università di Torino (Ricerca Locale“ex 60%” 2014–2018) to AF; from the Italian Ministry of Education, University and Research (“Finanziamento annuale individuale delle attività base di ricerca” 2017) to AF and (PRIN 2010/2011“GEOPROB” – Geodynamic Processes of Oceanic Basins, n. 2010AZR98L_002) to AF and GAP, the Università di Trieste (FRA 2013 and 2018) to GAP.

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Andrea Festa Dipartimento di Scienze della Terra, Università di Torino, Via Valperga Caluso 35, 10125 Torino, Italy Corresponding Author. E-mail address:andrea.festa@unito.it

Kei Ogata Department of Earth Sciences, Vrjie Universiteit, De Boelelaan 1085, 1081 Amsterdam, the Netherlands E-mail address:k.ogata@vu.nl

Gian Andrea Pini Dipartimento di Matematica e Geoscienze, Università di Trieste, Via E. Weiss 2, 34128 Trieste, Italy E-mail address:gpini@units.it