key: cord-0067187-os20k0x4
authors: Palin, Richard M.; Roberts, Nick M.W.
title: Metamorphism at convergent plate margins: Preface
date: 2021-08-24
journal: nan
DOI: 10.1016/j.gsf.2021.101288
sha: 0ebc7576c5de9ba7db0783758f7fdace44362136
doc_id: 67187
cord_uid: os20k0x4

nan

The year 2020 will long be recognised as a period of sadness and frustration for academics worldwide, given the catastrophic impact that the spread of novel coronavirus "2019-nCoV"or COVID-19 -has had on research and teaching activities, as well as in their personal lives.

This Special Issue was designed to represent an avenue for dissemination of many studies Many geologists at all career stages attended this session and presented work that was in different stages of completion; however, just a few months later, the rapid spread of COVID-19 around the globe applied sudden brakes to many of these partially completed projects.

Several researchers who expressed interest in contributing to this Special Issue were forced to withdraw, either due to new responsibilities at home, laboratory closures at their places of work, or restrictions halting domestic or international travel. We appreciate that these changes in circumstance have resulted in fewer than expected contributions to this Special Issue, and warmly thank the authors who did manage to overcome these hurdles and produce an excellent set of papers. We hope that the academic world will be able to recover from the impact of COVID-19 so that we can -as a community -move forward once again to break new ground in our understanding of this dynamic and highly unpredictable natural world.

Convergent plate margins are sites of intense deformation, metamorphism, and magmatism, where significant heat, fluid, and mass transfer may take place. Quantifying the fluxes and drivers of orogenic processes in these regions is critical to understanding the geodynamical evolution of the lithosphere, the rates and mechanisms of metamorphism at elevated pressure and temperature conditions, and for constraining the nature of mountain building and continental growth through geological time. Investigating the drivers and petrological/tectonic implications of regional or contact metamorphism at convergent plate margins, whether in the subducting slab or overlying arc, can be achieved via many techniques, such as field mapping, petrological phase equilibrium modelling, petrochronology, geochemistry, and geodynamic and/or geophysical modelling, and which can be performed a wide range of spatial and/or temporal scales (e.g. Hossack, 1979; Bell and Johnson 1989; Li and Bebout, 2005; Roberts, 2012; St-Onge et al., 2013; Palin and White, 2016; Palin et al., 2020) . The contributions to this Special Issue exemplify innovative uses of combinations of these techniques, and address important questions related both to local geology and broader-scale geodynamic processes.

This Special Issue of Geoscience Frontiers assembles seven papers authored by a geographically diverse set of scientists at various career stages. Together, these studies focus on deformational, magmatic and metamorphic processes that occur in several continental arcs and collision zones worldwide, and spanning the Mesoproterozoic to recent.

The first study, by Avellaneda-Jiménez et al. (2021) , presents new field observations, petrological data, and results of thermobarometry from Early Cretaceous, metamorphic rocks in the Central Cordillera of the Colombian Andes. These outcrops preserve metabasites that experienced cold and steep subduction to eclogite-facies conditions, followed by thermal overprinting to amphibolite-facies conditions during buoyancy-driven exhumation. Highresolution constraints on the pressure and temperature evolution indicate a secular evolution in the thermal structure of the subduction channel, which the authors interpret as evidence for asthenospheric mantle inflow during steepening of the angle of subduction of the oceanic lithosphere through time. Such processes are hypothesized to have occurred throughout Earth history (Smith et al., 2014; Wang et al., 2019; Parsons et al., 2020) , with this contribution marking the first report of Phanerozoic slab roll-back-related metamorphism in the Caribbean.

Next, Larson et al. (2021) present new petrological analyses, results of thermobarometry, and garnet Lu-Hf age data from a sample of metapelitic schist from the Nepalese Himalaya, which reveal that the basal décollement oscillated "in and out of sequence" through time as the orogenic hinterland evolved during the Miocene. Such data may contribute to new kinematic models describing how continent-continent collisional orogenesis evolves through time. These data from east-central Nepal are correlated by the authors with similar examples in west Nepal (Soucy La Roche et al., 2018) , Bhutan (Grujic et al., 2011) , and northeast India (Warren et al., 2014) , showing that this is likely a broad-scale feature of the Himalaya, and not a localized phenomenon.

The third study, by Searle and Lamont (2021) , reviews the geological evolution of the Cyclades, and in particular, a recently proposed and highly provocative hypothesis that the entire tectonothermal history of the Cycladic islands -including Naxos, Tinos and Syroscan be explained by continent-continent collision (e.g. Lamont et al., 2020a, b; Searle and Lamont, 2020) instead of representing metamorphism in an extending back-arc region (e.g. Lister et al., 1984; Jolivet and Brun, 2010; Menant et al., 2016) . Carboniferous) high temperature/low pressure metamorphism and rift-related bimodal magmatism. The next metamorphic event is not well constrained, but evidence is presented for an Early Jurassic high pressure/low temperature event, with potentially related contact metamorphism. The late Cretaceous to early Paleogene collision of Neotethys is recorded as M3 metamorphism in the studied units. A final metamorphic stage (M4) is observed, with timing constrained to ca. 28 to 25 Ma, but the origin of which remains elusive. This study demonstrates the polymetamorphic nature of the Zagros mountains, and provides areas of interest for further detailed invesitigation with constrained P-T-t paths. The younger metamorphic stages will potentially inform about the overall evolution of the Neotethys collisional belt.

Next, Walczak et al. (2021) present new geochronological data for felsic orthogneiss from the Seve Nappe Complex, Scandinavian Caledonides, using a zircon age depth-profiling method. The careful analysis of thin zircon rims and overgrowths is shown by the authors to document two discrete subduction and (partial) exhumation events separated by c. 20 Myr;

affectionately referred to as double-dunking! Such yo-yo tectonics at convergent margins have been reported in rocks from Turkey (Umhoefer et al., 2007) and the Italian Alps (Rubatto et al., 2011) , and require high-spatial resolution geochronology to identify; thus, they may be a common feature of metamorphism at convergent margins, but have been formerly missed in studies that have not had access to these newly developed analytical techniques.

The final study by Zuza et al. (2021) documents critical field observations and petrological data supporting a highly contentious theory that has arisen in recent years -that tectonic pressure within deforming continental crust can significantly exceed that expected solely induced by the volume of overburden (Petrini and Podladchikov, 2000; Moulas et al., 2013; Gerya, 2015) . This contentious phenomenon is termed 'overpressure' and has been proposed based on the results of crustal-scale numerical modelling and first-principles calculations of mechanical/elastic interactions between different minerals at the grain scale (Schmalholz and Podladchikov, 2013; Wheeler, 2014; Tajčmanová et al., 2015) ; however, evidence of overpressure in the natural world has been difficult to discover. Recent workers have reported such occurrences in the Alps (Luisier et al., 2019) and potentially also the Himalaya (Marques et al., 2018) . The authors provide a valuable addition to this list with evidence from the North American Cordillera.

We thank all the contributors to this Special Issue and the referees who spared their valuable time to provide insightful and helpful comments. Dr. L. Wang, Editorial Assistant at Geoscience Frontiers, and Prof. Santosh, Editor-in-Chief, are also thanked for their support throughout the production process. We hope that the papers assembled herein will be of interest to many researchers worldwide for many years to come.

Metamorphic gradient modification in the Early Cretaceous Northern Andes subduction zone: A record from thermally overprinted high-pressure rocks

Porphyroblast inclusion trails: the key to orogenesis

Tectonic overpressure and underpressure in lithospheric tectonics and metamorphism

Rapid synconvergent exhumation of Mioceneaged lower orogenic crust in the eastern Himalaya

The use of balanced cross-sections in the calculation of orogenic contraction: A review

Cenozoic geodynamic evolution of the Aegean

The age, origin, and emplacement of the Tsiknias Ophiolite

Structure, petrography, and thermobarometry

The P-T-t-D evolution the Mahabharat, east-central Nepal: The out-of-sequence development of the Himalaya

Carbon and nitrogen geochemistry of sediments in the Central American convergent margin: Insights regarding subduction input fluxes, diagenesis, and paleoproductivity

Metamorphic core complexes of Cordilleran type in the Cyclades

Metamorphic pressure variation in a coherent Alpine nappe challenges lithostatic pressure paradigm

Channel flow, tectonic overpressure, and exhumation of high-pressure rocks in the Greater Himalayas

Kinematic reconstructions and magmatic evolution illuminating crustal and mantle dynamics of the eastern Mediterranean region since the late Cretaceous

The problem of depth in geology: When pressure does not translate into depth

Secular change and the onset of plate tectonics on Earth

Emergence of blueschists on Earth linked to secular changes in oceanic crust composition

Geological, geophysical and plate kinematic constraints for models of the India-Asia collision and the post-Triassic central Tethys oceans

Lithospheric pressure-depth relationship in compressive regions of thickened crust

Increased loss of continental crust during supercontinent amalgamation

Yo-yo subduction recorded by accessory minerals in the Italian Western Alps

Tectonic overpressure in weak crustal-scale shear zones and implications for the exhumation of high-pressure rocks

Compressional metamorphic core complexes, low-angle normal faults and extensional fabrics in compressional tectonic settings

Compressional origin of the Aegean Orogeny

The~ 1.4 Ga A-type granitoids in the "Chottanagpur crustal block" (India), and its relocation from Columbia to Rodinia

Paleogeographic record of Eocene Farallon slab rollback beneath western North America

Preservation of the early evolution of the Himalayan middle crust in foreland klippen: insights from the Karnali klippe, west Nepal

Integrated pressure-temperature-time constraints for the Tso Morari dome (Northwest India): implications for the burial and exhumation path of UHP units in the western H imalaya

Grain-scale pressure variations in metamorphic rocks: implications for the interpretation of petrographic observations

Yo-yo tectonics in a wrench zone, Central Anatolian fault zone, Turkey

Zircon age depth-profiling sheds light on the early Caledonian evolution of Seve Nappe Complex in west-central Jämtland

Neoproterozoic magmatism in the northern margin of the Yangtze Block, China: Implications for slab rollback in a subduction-related setting

Timing and conditions of peak metamorphism and cooling across the Zimithang Thrust

Dramatic effects of stress on metamorphic reactions

Geologic field evidence for non-lithostatic overpressure recorded in the North American Cordillera hinterland

Nick has co-authored over 100 papers, edited several special issues, and currently sits on the editorial board for Geoscience Frontiers and Minerals.

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