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New Journal of Botany
Journal Prestige (SJR): 0.211

Number of Followers: 6

Hybrid journal (It can contain Open Access articles)
ISSN (Print) 2042-3489 - ISSN (Online) 2042-3497
Published by Taylor and Francis

[2652 journals]

Carboniferous sedimentary provenance and tectonic setting in the Darbut
region of Western Junggar (NW China): evidence from mineralogy,
geochemistry and detrital zircon U-Pb dating
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  Authors: Weng, K; Dong, Y, Xu, X, Ma, Z, Chen, B.
  Pages: jgs2020-13 - jgs2020-13
  Abstract: The Carboniferous tectonic evolution of Western Junggar is crucial to understanding the subduction–accretion process of the Central Asian Orogenic Belt, but the nature of this setting is still controversial. In this work, composite mineralogical, geochemical and detrital zircon U–Pb geochronological investigations have been conducted on Carboniferous clastic rocks in the Darbut region. The chemical compositions and sedimentary features show low sediment maturity and limited recycling, suggesting short-distance transportation and rapid accumulation. The samples contain igneous rock debris, mainly andesite and small amounts of basalt and granite, and a heavy mineral assemblage of Zr + Ap + Aug + Hbl + iron-bearing minerals (Hem-Lm, Ilm, Mag and Py). The samples feature moderate ratios of Zr/Sc (average 15.47) and Th/Sc (average 0.61), and low ratios of La/Sc, Co/Th and La/Th, as well as low Hf content, suggesting intermediate to felsic arc-related igneous provenances. Detrital zircon grains from the clastic rocks show prominent age peaks in the Devonian and Carboniferous with positive Hf(t) values, indicating a consistent provenance associated with the Tiechanggou–Halaalate island arc. Combining the petrology, geochemistry and geochronology of the sedimentary and magmatic rocks, we conclude that the Darbut Carboniferous volcanic–sedimentary strata were deposited in a back-arc basin during c. 327–311 Ma.Supplementary material: Tables S1-S6 and Figures S1-S6 are available at https://doi.org/10.6084/m9.figshare.c.5357293
  PubDate: 2021-09-28T06:35:19-07:00
  DOI: 10.1144/jgs2020-132
  Issue No: Vol. 178, No. 5 (2021)
Revised tectonostratigraphy and structural evolution of the Köli Nappe
Complex, Central Caledonides in Nordland, Norway
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  Authors: Saalmann, K; Bjerkgard, T, Slagstad, T, Sandstad, J. S, Lutro, O, Keiding, J, Snook, B, Angvik, T. L.
  Pages: jgs2020-21 - jgs2020-21
  Abstract: The nappe stack in the Røssvatnet–Hattfjelldal region in the Central Norwegian Caledonides consists of seven nappes formed at the boundary between tectonostratigraphically upper and uppermost Caledonian levels. The rocks of all nappes share a polyphase tectonometamorphic evolution that is younger than the 491 ± 10 Ma depositional (volcanic) age of parts of the succession. Early stages of deformation characterized by centimetre- to kilometre-scale folding and intense shearing accompanied by greenschist to amphibolite facies peak metamorphism are correlated with the Early Ordovician Taconian accretionary orogeny along the Laurentian margin. The Taconian structures are cut by the Krutfjellet gabbro and diorite, which yield U–Pb zircon ages of 446 ± 5 and 444 ± 4 Ma, respectively. Large-scale nappe stacking and folding post-dating the emplacement of the gabbro is related to the collision of Laurentia with Baltica (Scandian orogeny) and was followed by late- to post-orogenic extension. The revised tectonostratigraphy assigns the structurally higher nappes to the Uppermost Allochthon, whereas the lower nappes are correlated with the Middle Köli Nappe Complex (Upper Allochthon). The boundary between these nappes is marked by an imbricate zone. Taconian deformation was probably much more penetrative and widespread than hitherto thought and therefore parts of the nappe stack were probably assembled before Scandian collision.Supplementary material: Electronic Supplement 1: U–Pb zircon data and Electronic Supplement 2: Methods (laser ablation inductively coupled plasma mass spectrometry U–Pb zircon geochronology) are available at https://doi.org/10.6084/m9.figshare.c.5357255
  PubDate: 2021-09-28T06:35:19-07:00
  DOI: 10.1144/jgs2020-214
  Issue No: Vol. 178, No. 5 (2021)
Late- to post-Variscan tectonics and the kinematic relationship with W-Sn
vein-type mineralization: evidence from Late Carboniferous intramontane
basins (Porto-Satao syncline, Variscan Iberian belt)
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  Authors: Jacques, D; Muchez, P, Sintubin, M.
  Pages: jgs2020-22 - jgs2020-22
  Abstract: Many studies have constrained that late-Variscan buckling produced the arcuate geometry of the Ibero-Armorican belt. Nonetheless, debate remains on the associated geodynamic framework. Poorly studied Late Carboniferous intramontane basins offer an excellent framework to decipher the timing and kinematics of the late- to post-Variscan tectonics. Understanding the latter also helps constrain the structural emplacement mode of contemporaneous W–Sn–Nb–Ta–Li mineralization. In Iberia, the Porto–Sátão syncline is an example of such a Late Carboniferous intramontane basin. We present a structural analysis of the syncline, its basement and the associated W–Sn deposits. The regional structure is dictated by the Alcudian angular unconformity, caused by Cadomian tectonics (575–555 Ma) and separating tilted Ediacaran and subhorizontal Lower Paleozoic formations. Superimposed Variscan deformation led to F1–F3 folds with steep and gentle plunges, respectively. The late-orogenic D3 fabric is locally affected by post-orogenic F4 kink folds and an S4 crenulation cleavage. W–Sn-bearing vein systems occur along granite-hosted cone sheets, or exploit cross-fold joints associated with the F3 and F4 fold generations, revealing a close kinematic relationship between granite-related mineralization and the late- to post-Variscan deformation style. This structural history is interpreted as a plate-scale geodynamic change from Late Carboniferous north–south (D3) to Early Permian WNW–ESE (D4) convergence.
  PubDate: 2021-08-17T06:08:30-07:00
  DOI: 10.1144/jgs2020-223
  Issue No: Vol. 178, No. 5 (2021)
The Devonian landscape factory: plant-sediment interactions in the Old Red
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  Authors: Davies, N. S; Berry, C. M, Marshall, J. E. A, Wellman, C. H, Lindemann, F.-J.
  Pages: jgs2020-22 - jgs2020-22
  Abstract: The Devonian Period was a crucial interval in the evolution of plants. During its 60 myr duration, it witnessed the successive evolution of roots, wood, trees and forests. In addition, many of the biogeomorphic phenomena that operate in modern terrestrial environments came online for the first time. The ‘Old Red Sandstone’ of Svalbard consists of a near-continuous Silurian to Late Devonian record of sedimentary environments colonized by land plants and provides a perfect natural laboratory to aid our understanding of the facies signatures and evolution of these phenomena. We describe and illustrate a catalogue of ‘Old Red Sandstone’ features that provide evidence for the stepwise appearance of novel plant–sediment interactions, including preserved plant material and rooting structures, early large woody debris accumulations, cannel coal deposits and the oldest known vegetation-induced sedimentary structures, in addition to vegetation-influenced motifs of elevated mudrock content and complex alluvial sand bodies. These characteristics are combined to reconstruct changes to non-marine environments in this Devonian ‘landscape factory’. In addition to tectonic and climate influences, plant evolution first served as a control on the construction of the sedimentary record during this period and has persisted as a fundamental influence on Earth surface processes and landforms ever since.
  PubDate: 2021-08-17T06:08:30-07:00
  DOI: 10.1144/jgs2020-225
  Issue No: Vol. 178, No. 5 (2021)
Spatial heterogeneity of redox-sensitive trace metal enrichments in upper
Ediacaran anoxic black shales
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  Authors: Jin, C; Li, C, Algeo, T. J, Wang, G, Shi, W, Cheng, M, Zhang, Z, Wang, H, Li, N, Wang, W.
  Pages: jgs2020-23 - jgs2020-23
  Abstract: The Ediacaran radiation of metazoans is widely thought to have been triggered by an increase in atmospheric and oceanic oxygen levels. Although supported by other proxies, rising oxygen levels were deduced to a significant degree from sedimentary enrichments of redox-sensitive trace elements (RSTEs). However, some organic-rich shales of this period show only minor enrichments in RSTEs, leaving the significance of RSTE data for palaeo-oxygenation interpretations in doubt. We measured and compiled proxies for marine redox conditions (Fe species, RSTEs), total organic carbon (TOC) and water mass restriction (Mo/TOC and Co x Mn v. Cd/Mo) in the intra-shelf Jiulongwan and basinal Sandu sections of the Nanhua Basin in South China. Compared with the same proxies from coeval sections in the Nanhua Basin and globally, our results document a strong spatial heterogeneity of RSTE enrichments in anoxic black shales during the late Ediacaran. We infer that RSTE enrichments were strongly influenced by local factors, such as basinal restriction, seawater RSTE concentrations, and differential elemental responses to redox conditions and other influences. The broader significance of our findings is that they highlight the difficulty of investigating global redox conditions based on an analysis of local RSTE proxies within a single depositional basin or a limited number of study sections.Thematic collection: This article is part of the Advances in the Cambrian Explosion collection available at: https://www.lyellcollection.org/cc/advances-cambrian-explosionSupplementary material: Tables S1–S3 are available at https://doi.org/10.6084/m9.figshare.c.5325047
  PubDate: 2021-08-17T06:08:30-07:00
  DOI: 10.1144/jgs2020-234
  Issue No: Vol. 178, No. 5 (2021)
Developmental capacity and the early evolution of animals
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  Authors: Erwin; D. H.
  Pages: jgs2020-24 - jgs2020-24
  Abstract: Disentangling the factors underlying the appearance of macroscopic, often skeletonized, bilaterians during the Ediacaran–Cambrian diversification of animals requires carefully parsing the contributions of ecological opportunity, environmental potential and developmental capacity. The early evolution of animals involved the introduction of genomic, developmental, morphologic and behavioural novelties, identified as the individuation of new characters, which led to the construction of new ecological networks (innovation). Here I employ a recently introduced conceptual framework for novelty and individuation that distinguishes between potentiation, novelty, innovation and adaptive adjustments to the Ediacaran–Cambrian Radiation, and focus on the roles of potentiation and novelty in the expansion of developmental capacity. Comparative developmental studies combined with molecular clock estimates and data from the fossil record suggest that developmental capacity, the potential to generate a range of morphologies, may expand rapidly through developmental novelties without leading directly to morphological novelties, or to innovation. The expected patterns from this framework are markedly different from those in adaptive radiation scenarios.Thematic collection: This article is part of the Advances in the Cambrian Explosion collection available at: https://www.lyellcollection.org/cc/advances-cambrian-explosion
  PubDate: 2021-08-17T06:08:30-07:00
  DOI: 10.1144/jgs2020-245
  Issue No: Vol. 178, No. 5 (2021)
Fuxianhuiids are mandibulates and share affinities with total-group
Myriapoda
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  Authors: Aria, C; Zhao, F, Zhu, M.
  Pages: jgs2020-24 - jgs2020-24
  Abstract: In spite of their unrivalled ecological success, the origins of terrestrial mandibulates have long remained virtually unknown. In recent years, claims have been made based on phylogenetic results that fuxianhuiids, iconic fossils of the Chengjiang biota and purported basal euarthropods, were in fact mandibulates, allied with the problematic euthycarcinoids as the closest sister-group to Mandibulata or even stem myriapods. A re-examination of euthycarcinoid mouthparts has since brought additional support to this view. Here, we reinvestigated the anterior anatomy of the Cambrian euarthropod Fuxianhuia and its relatives. We demonstrate that the fuxianhuiid head is that of a mandibulate, sharing similarities with hymenocarines, including a limbless intercalary segment. The hypostome is a sub-triangular sclerite partially fused but anatomically independent from a large, bilobed labral plate, as observed in many extant mandibulate taxa as well as euthycarcinoids. The so-called ‘specialized post-antenn(ular) appendages' are therefore the mandibles, with a tripartite palp and large dented gnathal lobe partially covered by the hypostome-labrum complex. Under a ‘deep split' phylogenetic scenario, Fuxianhuia and its relatives are here resolved as sister-group to crown Mandibulata.Thematic collection: This article is part of the Advances in the Cambrian Explosion collection available at: https://www.lyellcollection.org/cc/advances-cambrian-explosionSupplementary material: Additional remarks on terminology, morphological interpretations and a list of modified/new characters used for the phylogenetic analysis are available at https://doi.org/10.6084/m9.figshare.c.5305042
  PubDate: 2021-08-17T06:08:30-07:00
  DOI: 10.1144/jgs2020-246
  Issue No: Vol. 178, No. 5 (2021)
The provenance of Middle Jurassic to Cretaceous sediments in the Irish and
Celtic Sea Basins: tectonic and environmental controls on sediment
sourcing
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  Authors: McCarthy, O; Fairey, B, Meere, P, Chew, D, Kerrison, A, Wray, D, Hofmann, M, Gärtner, A, Sonntag, B.-L, Linnemann, U, Kuiper, K. F.
  Pages: jgs2020-24 - jgs2020-24
  Abstract: The Jurassic and Cretaceous sedimentary infill of the Irish and Celtic Sea Basins is intimately associated with the breakup of the supercontinent Pangea, and the opening of the Atlantic margin. Previous basin studies have constrained tectonism, basin uplift and sediment composition, but sediment provenance and routing have not received detailed consideration. Current hypotheses for basin infill suggest localized sediment sourcing throughout the Jurassic and Cretaceous, despite a dynamic tectonic and palaeoenvironmental history spanning more than 100 million years. We present detrital zircon, white mica and apatite geochronology alongside heavy mineral data from five basins. Findings reveal that basin infill derived predominantly from distal sources with lesser periods of local sourcing. We deduce that tectonically induced marine transgression and regression events had a first-order control on distal v. proximal sedimentary sourcing. Additionally, tectonism which uplifted the Fastnet Basin region during the Middle–Late Jurassic recycled basin sediments into the connected Celtic and Irish Sea Basins. Detrital geochronology and heavy mineral evidence support three distinct provenance switches throughout the Jurassic and Cretaceous in these basins. Overall an integrated multi-proxy provenance approach provides novel insights to tectonic and environmental controls on basin infill as demonstrated in the Irish and Celtic Sea Basins.Supplementary material: Tables S1–S6 are available at https://doi.org/10.6084/m9.figshare.c.5343657
  PubDate: 2021-08-17T06:08:30-07:00
  DOI: 10.1144/jgs2020-247
  Issue No: Vol. 178, No. 5 (2021)
Unravelling evidence for global climate change in Mississippian carbonate
strata from the Derbyshire and North Wales Platforms, UK
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  Authors: Manifold, L; del Strother, P, Gold, D. P, Burgess, P, Hollis, C.
  Pages: jgs2020-10 - jgs2020-10
  Abstract: The Mississippian Derbyshire and North Wales carbonate platforms were formed in similar tectonic settings within the Pennine and East Irish Sea Basin, respectively. The Derbyshire Platform was surrounded by sub-basins to the north, west and south whilst the North Wales Platform, 130 km west, had a simpler land-attached geometry. Comparison of these age-equivalent platforms allows the controls on sedimentation, at an important juncture in Earth history, to be evaluated. Both platforms are dominated by moderate-to-high-energy, laterally discontinuous facies, with weak evidence for facies cyclicity, suggesting multiple controls on deposition. Influx of siliciclastic mud on the North Wales Platform led to perturbations in carbonate accumulation; along with abundant palaeosols and coal beds this implies a more humid climate, or shallower water depths compared to the Derbyshire Platform. On both platforms, exposure surfaces can rarely be correlated over>500 m except for a regionally correlative palaeokarstic surface at the Asbian–Brigantian boundary. This exposure event appears to coincide with a significant regional facies change. Given the lack of evidence for ordering and cyclicity within the strata, the Asbian–Brigantian boundary may mark a significant event that could reflect the onset of a transitional climate, prior to the second glaciation event in the Late Palaeozoic Ice Age.
  PubDate: 2021-08-17T06:08:29-07:00
  DOI: 10.1144/jgs2020-106
  Issue No: Vol. 178, No. 5 (2021)
New vauxiid sponges from the Chengjiang Biota and their evolutionary
significance
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  Authors: Wei, F; Zhao, Y, Chen, A, Hou, X, Cong, P.
  Pages: jgs2020-16 - jgs2020-16
  Abstract: Aspiculate demosponges are rarely described in geological history due to the absence of spicules that are stable and resistant to degradation. One exception is the exquisite preservation of sponges without any mineralized skeletons discovered in Lagerstätten (e.g. the Burgess Shale). The Chengjiang Biota, an early example of a Burgess Shale-type biota in South China (Cambrian Series 2, Stage 3), is one of the only examples of convincing aspiculate sponges until now. Here, we describe Vauxia pregracilenta sp. nov. and V. paraleioia sp. nov., as well as two poorly preserved vauxiid specimens (Vauxia sp.) in open nomenclature, from the Chengjiang Biota. Vauxia pregracilenta has a fan-like holdfast and branches in various sizes, as well as a typical two-layered net-like skeleton, without spicules. The endosomal layer is hexagonal, while the dermal layer is sub-rectangular. Vauxia paraleioia is characterized by a two-layered subconical skeleton, with the dermal layer ornamented with vertical surface grooves. The openings of the dermal and endosomal layers of V. paraleioia are both hexagonal but of different sizes. These newly discovered Vauxia species indicate that the aspiculate sponges were diversified in the early Cambrian. Partial silicification of the fibres of aspiculate Vauxia are confirmed from the Chengjiang Biota.Thematic collection: This article is part of the Advances in the Cambrian Explosion collection available at: https://www.lyellcollection.org/cc/advances-cambrian-explosion
  PubDate: 2021-08-17T06:08:29-07:00
  DOI: 10.1144/jgs2020-162
  Issue No: Vol. 178, No. 5 (2021)
Terreneuvian bio- and chemostratigraphy of the South Sichuan Region (South
China)
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  Authors: Yang, B; Steiner, M.
  Pages: jgs2020-16 - jgs2020-16
  Abstract: Classical sections, such as the Maidiping and Daqiao Mine sections of South Sichuan (China), expose early Cambrian deposits that are crucial for understanding the biological and environmental evolution of Yangtze Craton. These sequences are rich in Terreneuvian small shelly fossils, which can be assigned to assemblages I and III from South China. The Anabarites trisulcatus–Protohertzina anabarica Assemblage Zone (Assemblage I) is recognized at the lower Maidiping Formation. The second assemblage (Paragloborilus subglobosus–Purella squamulosa Assemblage Zone) cannot be verified in South Sichuan, although previous reports claimed its existence based on the occurrence of Paragloborilus subglobosus. The third assemblage (Watsonella crosbyi Assemblage Zone) is confirmed in the upper Maidiping Formation. The abundant bioclasts in this interval indicate abrasions and bioerosions by winnowing or starved sedimentation. Carbon isotope values from the Maidiping section present no negative excursion at the presumed Ediacaran–Cambrian transition. A positive carbon isotope excursion is observed in the Upper Maidiping Formation (Assemblage III) which is correlated to the ZHUCE excursion in the Dahai Member of eastern Yunnan. The shallow-water deposits of South Sichuan can be correlated with the South China, western Mongolia and Siberia successions based on biozonations and carbon isotope trends.Supplementary material: Isotope data are available at https://doi.org/10.6084/m9.figshare.c.5326834Thematic collection: This article is part of the Advances in the Cambrian Explosion collection available at: https://www.lyellcollection.org/cc/advances-cambrian-explosion
  PubDate: 2021-08-17T06:08:29-07:00
  DOI: 10.1144/jgs2020-167
  Issue No: Vol. 178, No. 5 (2021)
Tectonic significance of a supra-ophiolitic sedimentary cover succession,
Unst, Shetland, Scottish Caledonides: insights from the U-Pb-Hf detrital
zircon record
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  Authors: Papapavlou, K; Strachan, R. A, Storey, C. D, Bullen, D.
  Pages: jgs2020-16 - jgs2020-16
  Abstract: The tectonic significance of the Muness Phyllite, which overlies the Unst–Fetlar ophiolite in Shetland, Scottish Caledonides, is poorly understood. U–Pb analyses of detrital zircons show that it was deposited after c. 469 Ma. Early Paleozoic grains have Hf values of –0.3 to +12.3 and were probably derived from the extension of the Midland Valley arc. Psammite clasts and the matrix of the Muness Phyllite contain Proterozoic and Archean detrital zircons with age peaks of c. 1, 1.4–1.5, 1.6–1.7, 1.8–1.9 and 2.7 Ga. These are consistent with ultimate derivation from NE Laurentia sources and were probably recycled from the Neoproterozoic East Mainland Succession that underlies the Mesozoic East Shetland Basin. The Muness Phyllite is interpreted to have been deposited soon after the Grampian I orogeny in a successor basin that overstepped and received detritus from the Midland Valley arc, the East Mainland Succession and the Unst–Fetlar ophiolite. It was then deformed and metamorphosed, probably at c. 450 Ma during the Grampian II orogenic event. The Muness Phyllite therefore provides a record of middle to late Ordovician tectonic events along the Scottish sector of the Laurentian margin following ophiolite obduction.Supplementary material: Analytical details and instrumentation parameters and U–Pb and Lu–Hf isotopic data are available at https://doi.org/10.6084/m9.figshare.c.5324986
  PubDate: 2021-08-17T06:08:29-07:00
  DOI: 10.1144/jgs2020-169
  Issue No: Vol. 178, No. 5 (2021)
Nature of the Cuvier Abyssal Plain crust, offshore NW Australia
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  Authors: Reeve, M. T; Magee, C, Bastow, I. D, McDermott, C, Jackson, C. A.- L, Bell, R. E, Prytulak, J.
  Pages: jgs2020-17 - jgs2020-17
  Abstract: Magnetic stripes have long been assumed to be indicative of oceanic crust. However, continental crust heavily intruded by magma can also record magnetic stripes. We re-evaluate the nature of the Cuvier Abyssal Plain (CAP), offshore NW Australia, which hosts magnetic stripes and has previously been defined as oceanic crust. We show that chemical data from a basalt within the CAP, previously described as an enriched mid-ocean ridge basalt, could equally be interpreted to contain evidence of contamination by continental material. We also recognize seaward-dipping reflector sequences in seismic reflection data across the CAP. Borehole data from overlying sedimentary rocks suggests that these seaward-dipping reflectors were emplaced in a shallow water (500 km further offshore NW Australia than currently thought. This would impact plate tectonic reconstructions, as well as heat flow and basin modelling studies. Our work also supports the growing consensus that magnetic stripes cannot, by themselves, be used to determine crustal affinity.Supplementary material: Enlarged and uninterpreted versions of the magnetic data and seismic reflection lines are available at https://doi.org/10.6084/m9.figshare.c.5332172
  PubDate: 2021-08-17T06:08:29-07:00
  DOI: 10.1144/jgs2020-172
  Issue No: Vol. 178, No. 5 (2021)
The spatial distribution of igneous centres along the Norwegian Atlantic
Margin (More and Voring) and their relationship to magmatic plumbing
systems
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  Authors: Kilhams, B; Chedburn, L, Schofield, N, Lokling Lunde, I, Romain, H, Jolley, D, Haug Eide, C.
  Pages: jgs2020-19 - jgs2020-19
  Abstract: A series of offshore intra-basinal igneous centres have been documented across the North Atlantic Igneous Province including the UK, Ireland and Greenland. However, inconsistent cross-border terminology implies that similar features are not present in the Norwegian offshore, which, in turns, leads to misperceptions of cross-border geological differences. This paper presents evidence for a series of Norwegian igneous centres and suggests a consistent non-genetic cross-border terminology. In the Møre Basin, several igneous centres sit close to the continent–ocean boundary (COB), which have previously been identified as seamounts and/or ‘outer highs’. To provide cross-border consistency these features are consolidated under umbrella terms: igneous centres or volcanic fissures. Further centres are probably present within the Møre Basin (east of the COB) where 3D seismic data were not available. In the Vøring Basin two new igneous centres, one intrusive and one extrusive, are identified within the continental domain. Additionally, a possible deep magmatic upwelling associated with the regionally significant T-Reflector is identified. These igneous centre end-members represent the complexity of the magmatic plumbing across the Norwegian margin. With further data it is likely that further igneous centres will be identified offshore mid-Norway.
  PubDate: 2021-08-17T06:08:29-07:00
  DOI: 10.1144/jgs2020-192
  Issue No: Vol. 178, No. 5 (2021)
Quo vadis Zeus: is there a Zas shear zone on Naxos Island, Aegean Sea,
Greece' A review of metamorphic history and new kinematic data
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  Authors: Peillod, A; Tehler, E, Ring, U.
  Pages: jgs2020-21 - jgs2020-21
  Abstract: Detecting zones of considerable early-orogenic displacement in rather monotonous rock sequences which have undergone a late and pervasive tectonometamorphic overprint is challenging. It has been proposed that the alleged Zas shear zone in the passive margin sequence of the Cycladic Blueschist Unit on Naxos Island, Greece, separates amphibolite facies, non-high-pressure rocks (Koronos Unit) below the shear zone from Eocene high-pressure rocks (Zas Unit) above the shear zone. We review existing pressure–temperature data from the Koronos Unit and present new kinematic data from the anticipated Zas shear zone to evaluate the tectonic significance of this recently proposed structure. This has implications for unravelling the subduction history of rock units from tectonometamorphic datasets. Common to all pressure–temperature data from the Koronos Unit is a well-defined amphibolite facies equilibration stage at 8–11 kbar and 600–700°C, followed by initial near-isothermal to slightly prograde decompression and subsequent pronounced cooling. This segment of the high-temperature pressure–temperature loop is associated with top-to-the-NNE extensional deformation in the footwall of the Miocene Naxos–Paros detachment. Little is known about the metamorphism preceding the amphibolite facies overprint in the Koronos Unit. Our review shows that it is likely that the rocks experienced a prior high-pressure metamorphic overprint that is typical of rocks of the Cycladic Blueschist Unit. Our kinematic data show that the Zas shear zone contains variably deformed rocks with dominantly top-to-the-NNE shear sense indicators that developed under greenschist facies metamorphism in the footwall of the Naxos–Paros detachment. No significant offset can be detected across the Zas shear zone and the geology on either side of it does not support large-scale movement across the shear zone. We discuss a model in which the Zas shear zone is considered to be a minor zone of deformed schist near the biotite-in isograd of Miocene high-temperature metamorphism. We conclude that there is no need to tectonically subdivide the passive margin sequence of the Cycladic Blueschist Unit on Naxos.
  PubDate: 2021-08-17T06:08:29-07:00
  DOI: 10.1144/jgs2020-217
  Issue No: Vol. 178, No. 5 (2021)