The Band-e-Zeyarat ophiolite (BEZO) sedimentary cover in the Makran Accretionary Prism (SE Iran) records a complex tectono-sedimentary evolution, extending from its formation at a mid-ocean ridge (MOR) setting to deformation in an accretionary prism. Stratigraphic and biostratigraphic data indicate the occurrence of a Valanginian transition zone sequence separating the volcanic sequence and pelagic sedimentary cover. The latter consists of lower Hauterivian cherty limestone passing upwards to upper Hauterivian–Barremian marl and limestone. The pelagic sedimentary cover continues with post-Barremian–Cenomanian (?) marls. Arenites are interlayered in the sedimentary cover; they are composed of rock fragments derived from volcanic-arc and continental margin settings. The BEZO sedimentary cover is intruded by dykes and sills showing enriched MOR basalt chemical affinity. Structural analysis indicates a polyphase deformation history that involved faulting and folding. Our multidisciplinary results indicate that the BEZO formed in a MOR setting and that it was subsequently overprinted by off-axis and within-plate magmatism as it spread away from this MOR. The Band-e-Zeyarat oceanic crust was incorporated into the Makran prism in the latest Late Cretaceous–Paleocene and was further deformed via strike-slip faulting along the dextral Minab-Sabzevaran fault during the Miocene–Pliocene. Supplementary material: A geological sketch map of the Band-e-Zeyarat ophiolite, photomicrographs of arenites, microphotographs of selected calcareous nannofossil taxa, a geochemical discrimination diagram, a table with semiquantitative estimation of calcareous nannofossil abundance, and a text with detailed descriptions of analytical methods, accurancy, and detection limits for whole-rock geochemical analysis are available at https://doi.org/10.6084/m9.figshare.c.6843835 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists
We document that the undifferentiated chaotic Ligurian Units of the Monferrato–Torino Hill sector (MO-TH) at the Alps–Apennines junction consist of three different units that are comparable with the Cassio, Caio and Sporno Units of the External Ligurian Units of the Northern Apennines. Their internal stratigraphy reflects the character of units deposited in an ocean–continent transition (OCT) zone between the northwestern termination of the Ligurian–Piedmont oceanic basin and the thinned passive margin of Adria microcontinent. The inherited wedge-shaped architecture of this OCT, which gradually closed toward the north in the present-day Canavese Zone, controlled the Late Cretaceous–early Eocene flysch deposition at the trench of the External Ligurian accretionary wedge during the oblique subduction. This favoured the formation of an accretionary wedge increasing in thickness and elevation toward the SE, from the MO-TH to the Emilia Northern Apennines. Our results therefore provide significant information on both the palaeogeographical reconstruction of the northwestern termination of the Ligurian–Piedmont oceanic basin and the role played by inherited along-strike variations (stratigraphy, structural architecture and morphology) of OCT zones in controlling subduction–accretionary processes. Supplementary material: A spreadsheet with X-ray fluorescence spectrometry and inductively coupled plasma mass spectrometry whole-rock major and trace element composition of mantle peridotites, and photomicrographs of mantle peridotites are available at https://doi.org/10.6084/m9.figshare.c.4519643
preadsheet with X-ray fluorescence spectrometry and inductively coupled plasma mass spectrometry whole rock major and trace element composition of mantle peridotites.
Upper Lutetian–Bartonian sedimentary mélanges, corresponding to ancient mud-rich submarine mass transport deposits, are widely distributed over an area c. 300 km long and tens of kilometres wide along the exhumed outer part of the External Ligurian accretionary wedge in the Northern Apennines. The occurrence of methane-derived carbonate concretions (septarians) in a specific tectonostratigraphic position below these sedimentary mélanges allows us to document the relationships among a significant period of regional-scale slope failure, climate change (the Early and Mid-Eocene Optimum stages), the dissociation of gas hydrates and accretionary tectonics during the Ligurian Tectonic Phase (early–mid-Lutetian). The distribution of septarians at the core of thrust-related anticlines suggests that the dissociation of gas hydrates was triggered by accretionary tectonics rather than climate change. The different ages of slope failure emplacement and the formation of the septarians support the view that the dissociation of gas hydrates was not the most important trigger for slope failure. The latter occurred during a tectonic quiescence stage associated with a regressive depositional trend, and probably minor residual tectonic pulses, which followed the Ligurian Tectonic Phase, favouring the dynamic re-equilibrium of the External Ligurian accretionary wedge. Our findings provide useful information for a better understanding of the factors controlling giant slope failure events in modern accretionary settings, where they may cause tsunamis.
Most ophiolitic mélanges and chaotic rock units in exhumed subduction zone complexes and orogenic belts are commonly interpreted as the products of tectonic processes (e.g., underplating and return flow) acting at intermediate to great depths (depth > 10–15 km, T > 250 °C) at convergent margins. Conversely, observations from modern and ancient, non- to poorly metamorphosed subduction–accretion complexes (recognized as mélanges and chaotic rock units) around the world show that these rock associations: (1) likely formed at shallow structural levels first, and (2) were later subducted and became tectonically reworked. As such, they mainly consist of broken formations (> 21.5%), and sedimentary (c. 20%), polygenetic (> 13.7%) and/or diapiric (c. 6.7%) mélanges. Tectonic mélanges are limited to <3.0% (in surface distribution), suggesting that tectonic processes do not make efficient mixing mechanisms at shallow structural levels. Subduction of structural inheritances (e.g., ocean-continent transition zones, and lithological and structural heterogeneities in ocean plate stratigraphy – OPS – assemblages) plays a more significant role in forming mélanges and chaotic rock units at shallow depths; it can also control the origin and location of plate interface and the dynamics of the wedge front (i.e., tectonic accretion vs. erosion). However, not all chaotic rock units that formed at shallow structural levels may become subducted; but, if subducted, their fate might be different depending on whether they become part of the plate interface or if they become part of the lower plate. Our global field observations, suggesting that most mélanges and chaotic rock units form at shallow depths, have significant implications for the tectonic evolution of subduction zone complexes and orogenic belts.
<p>In the upper mantle, volatiles control its composition, partial melting conditions, as well as the ascent rate of the formed melts. As consequence, volatile composition of the mantle is, in turn, recorded in the melts and, therefore, in the erupted basaltic rocks. Despite their importance, origin, budget, and fluxes of the volatiles in the upper mantle are poorly constrained. It is well known that the main input of mantle volatiles, such as carbon (C) and sulphur (S), represents components released from the subducting slab, <em>e.g.</em>, oceanic rocks and sediments, whose have characteristic isotopic signatures. In this view, studies of isotopic ratios of volatiles of subduction-related magmatic rocks could be used to identify the chemical components released by the subducting slab metasomatizing the upper mantle. To confirm this hypothesis, we investigated the major and trace element composition, as well as the C and S elemental contents and isotopic ratios of subvolcanic and volcanic rocks of the Vardar ophiolites of North Macedonia, which represent remnants of the Mesozoic Tethyan oceanic lithosphere formed in supra-subduction zone tectonic settings.</p><p>The ophiolites were sampled at Lipkovo and Demir Kapija localities, in the northern and southern part of North Macedonia, respectively. Based on whole-rock major and trace element composition, two main groups of rocks can be distinguished: i) Group 1 rocks, which are subalkaline basalts with backarc affinity and ii) Group 2 rocks, which are calc-alkaline basalts with arc affinity. The petrogenetic modelling based on trace and Rare Earth Elements, indicates that Group 1 mantle sources were affected by limited metasomatic processes by slab-released components, in particular aqueous fluids and sediment melts, whereas the Group 2 mantle sources were strongly metasomatized by sediment melts and adakitic melts. Accordingly, the Group 1 rocks exhibit C-enriched and S-depleted isotopic signature, indicating a minor involvement of melts from the subducting sediments. On the other hand, the C-depleted and S-enriched isotopic signatures of the Group 2 rocks suggest a major involvement of melts derived from the subducting sediments rich in organic matter and sulphate phases Therefore, both geochemical and isotopic data of the subvolcanic and volcanic samples of the North Macedonia ophiolites show that the sub-arc mantle sources are more affected by slab-released fluids than those of the backarc basin, which are more distal from the trench. Thus, combining the geochemical and isotopic data of subvolcanic and volcanic samples of complex geological framework can contribute to reconstruct the geodynamic scenarios, such as that of the Vardar ophiolites in the Dinaric-Hellenic belt. In addition, this approach may be useful to better understand the global geodynamic cycles of volatiles reconstructing their origin, budget, and isotopic composition, and understand the impacts on climate and environment from local to global scale.</p>
Most of mélanges occurring in exhumed subduction complexes and orogenic belts are commonly interpreted as the product of tectonic processes (e.g., underplating and return flow) acting at intermediate to great depths (T > 250 °C, depth >10–15 km). Conversely, observations on modern and ancient non- to poorly metamorphosed subduction complexes around the world, clearly show that the largest part (c. 64.7%) of mélanges and chaotic rock units are already formed at shallower structural levels (T < 250 °C, depth <10–15 km). They mainly consist of broken formations (>21.5%), sedimentary (c. 20%), polygenetic (>13.7%), and diapiric (c. 6.7%) mélanges. Tectonic mélanges are limited to about 2.7%, suggesting that tectonics is not an efficient mixing process at shallow structural levels. We document that the subduction of structural inheritances (e.g., ocean-continent transition zones, and ocean plate stratigraphy) plays a significant role in forming and differentiate the different types of chaotic units at shallow depths, also controlling the location of the plate interface and the dynamics of the wedge front (i.e., tectonic accretion vs. erosion). However, not all chaotic units that formed at shallow structural levels can be subducted and, as subducted, their fate could be very different if they become part of the plate interface or if they share the fate of the lower plate. Our findings demonstrate that the evidence that the larger part of mélanges and chaotic units form at shallow depths has significant implications for a better understanding of the tectonic evolution of subduction complexes and orogenic belts, ranging from the mode and time of Precambrian Earth evolution and the onset of plate tectonics to the role of mélanges in controlling the seismic behavior.
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