Obduction Processes Research Articles

New stratigraphic and palaeontological data from carbonates related to the Vourinos–Pindos ophiolite emplacement: implications for the provenance of the ophiolites (Hellenides)

Microfacies analysis and conodont dating on carbonates associated with the Vourinos–Pindos ophiolite emplacement on the Pelagonian unit in northern Greece, combined with structural analysis, enhance our understanding of the palaeogeography and geotectonic evolution of the Hellenides. We focus on a dismembered Triassic Hallstatt Limestone succession incorporated as blocks in the Middle–Late Jurassic Avdella ophiolitic mélange, which is identical to a Triassic succession in the western Pindos (Hallstatt–Pindos succession). Initially, the Hallstatt–Pindos succession was deposited on the outer shelf along the eastern Pelagonian margin and later incorporated into the mélange during west-directed obduction processes. During the Middle to early Late Jurassic, ophiolites were obducted on the eastern Pelagonian margin, resulting in a nappe stack in front of the ophiolites in the lower plate position. Newly formed trench-like basins in front of the advancing nappe stack became filled with sedimentary mélanges, consisting of ophiolitic material mixed with blocks from the Hallstatt–Pindos succession, deposited in an outer shelf position. Subsequently, the Pindos ophiolites overthrust their foreland basin, forming a typical ophiolitic mélange (Avdella mélange). Consequently, the Triassic–Middle Jurassic Hallstatt–Pindos succession represents a far-travelled Middle–Late Jurassic nappe derived from east of the Pelagonian unit, which was bulldozed westward in front of the west-directed obducting Neotethys ophiolites. 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

Impact of Kerguelen, Marion, and Reunion plumes on the ophiolitic obduction at the northern margin of Indian plate

Geochronological data compiled for 29 major ophiolitic occurrences from the northern margin of the Indian plate precisely coincide with the three major mantle plume interactions over Indian plate (viz., Kerguelen at 130–110 Ma, Marion at 90–84 Ma, and Reunion at 65–70 Ma). We analysed i) the geological juxtapositions of these ophiolitic belts, ii) seismic tomographic imaging of the slab breakoffs, iii) equivalent deflections in the marine magnetic anomalies (MMA), iv) stratigraphic records of the Cretaceous formations and v) the paleomagnetic inclination anomalies; and propose significant influence of plume-driven forces over the subduction and obduction processes in Himalayas. The impingement of Kerguelen plume strongly influenced the ∼ 2500 km long subduction of Neo-Tethys, leading to Indus-Tsangpo-Suture-Zone (ITSZ) ophiolites with their metamorphic soles. The impact of Marion plume further led to repeated tilting of the Indian plate reinvigorating Tethyan subduction at 90 ± 5 Ma. The Reunion event at ∼ 65 Ma, led to the mega-obduction along the Tethyan margin of the Indian subcontinent. The Kerguelen plume thus played a key role in initiating the intraoceanic subductions in equatorial Neo-Tethys, and the combined slab pull and plume induced rifting separated Greater India microcontinent from the northern margin of Indian subcontinent at ∼ 120 Ma. The impact of the Reunion plume resulted in the obduction of ITSZ ophiolites at the northern margin and the Pakistan ophiolites at the northwestern margin. The mantle plume impingements caused plate tilting and triggered the subduction/obduction that is well preserved by sharp inflexions in MMA and the spreading rates. We therefore infer that the three mantle plumes have a greater role in the subduction, obduction and tilting of the Indian plate throughout the Late Mesozoic.

To be or not to be Alpine: New petrological constraints on the metamorphism of the Chenaillet Ophiolite (Western Alps)

AbstractThe Chenaillet Ophiolite represents a very well‐preserved portion of Ligurian‐Piedmont ocean in the Western Alps. It is formed from an oceanic lithospheric succession comprising exhumed mantle, various mafic intrusives (i.e., gabbro sensu lato), and a world‐renowned sequence of pillow basalts. Apart from scarce breccias closely related to oceanic lithosphere, no sedimentary cover is exposed. Historically, the Chenaillet Ophiolite has been known for its very low temperature–low pressure Alpine metamorphism, ascribed to obduction processes. However, studies aimed at constraining the peak pressure–temperature (P–T) conditions of Alpine metamorphism are virtually lacking, the general focus having been so far on its high temperature metamorphism and geochemical features. In this paper, we investigate two kinds of rocks: gabbro and albitite/alkali syenite, whose petrographic features shed light on the complex metamorphic history of the Chenaillet Ophiolite. Detailed analyses of mineral assemblages, blastesis/deformation relationships, and mineral chemical data allow two metamorphic events to be distinguished: an earlier, high temperature event (already reported in the literature) and a second, later low temperature, high pressure event, recognized here for the first time. The low temperature, high pressure event is strikingly testified by the occurrence of lawsonite relicts in the gabbro and of interstitial omphacite in the albitite. Thermodynamic modelling (i.e., via isochemical phase diagrams) performed on a gabbro sample suggests for this unit a minimum of 9 kbar and 300°C and a maximum of 15 kbar and 450°C. Overlapping these P–T conditions with those inferred for the albitite based on the observed mineral assemblage allows the Alpine peak metamorphism to be constrained to 10–11 kbar and 340–360°C. These P–T conditions suggest a thickness of the overlying nappe stack of about 35–40 km, which is incompatible with obduction or burial processes, and instead consistent with subduction processes related to the Alpine orogeny. We argue that, opposite to the common belief that the Chenaillet Ophiolite escaped Alpine metamorphism, our new data strongly support the idea that it experienced low temperature‐blueschist‐facies metamorphism, whose evidence can still be tracked in those (few) rocks that better recorded and preserved it. This finding generates new challenging questions regarding both subduction and exhumation processes in complex orogens such as the Western Alps.

Moho topography and crustal properties in the UAE and northern Oman mountain belt from teleseismic receiver functions

SUMMARYThe United Arab Emirates (UAE)-Oman mountain belt exposes a large intact ophiolite thrust sheet that was obducted onto a rifted continental margin during the Late Cretaceous. Crustal properties of the mountain belt and the foreland region are important to better understand the mechanism of crustal deformation during the obduction process. In this study, we analysed P-wave receiver functions across the UAE and northern Oman mountain belt to determine the Moho depths, crustal properties and velocity structure beneath 53 broad-band seismic stations. Crustal thickness varies from 30.0 km in the western UAE to 46.6 km in the UAE-Oman mountain belt. The deeper Moho depth beneath the UAE-Oman mountain belt is the outcome of crustal thickening and flexure of the lithosphere during the obduction process. The eastern flank of the UAE-Oman mountain belt has relatively high-Vp/Vs ratios of 1.76–1.89. On the contrary, comparatively low-Vp/Vs ratios (1.61–1.67) are estimated in the western flank. Moreover, relatively low-Vp/Vs ratios (1.61–1.70) are observed in the foreland basin and western UAE. The high-Vp/Vs ratios and thick crust are indication of a thick ophiolite thrust sheet, while low-Vp/Vs ratios suggest that the crust is more felsic/intermediate in nature. The Vs-depth profiles in the mountain belt reveal a low-Vs zone beneath a remarkably high-Vs top layer. There is no such high-Vs top layer in the foreland area and western UAE. The high-Vs layer is correlated to a ∼10–15-km thick sequence of Semail crust and mantle ophiolite. Common Conversion Point imaging across the UAE-Oman mountain belt imaged the Moho boundary, which agrees with H-k stacking results, and intra-crustal discontinuities.

Crustal Structure of the UAE‐Oman Mountain Range and Arabian Rifted Passive Margin: New Constraints From Active and Passive Seismic Methods

AbstractThe Semail ophiolite, a thick thrust sheet of Late Cretaceous oceanic crust and upper mantle, was obducted onto the previously rifted Arabian continental margin in the Late Cretaceous, and now forms part of the United Arab Emirates (UAE)‐Oman mountain belt. A deep foreland basin along the west and SW margin of the mountains developed during the obduction process, as a result of flexure due to loading of the ophiolite and underlying thrust sheets. The nature of the crust beneath the deep sedimentary basins that flank the mountain belt, and the extent to which the Arabian continental crust has thickened due to the obduction process are outstanding questions. We use a combination of active‐ and passive‐source seismic data to constrain the stratigraphy, velocity structure and crustal thickness beneath the UAE‐Oman mountains and its bounding basins. Depth‐migrated multichannel seismic reflection profile data are integrated in the modeling of traveltimes from long offset reflections and refractions, which are used to resolve the crustal thickness and velocity structure along two E‐W onshore/offshore transects in the UAE. Additionally, we apply the virtual deep seismic sounding method to distant earthquake data recorded along the two transects to image crustal thickness variations. Active seismic methods define the Semail ophiolite as a high‐velocity body dipping to the east at 40°–45°. The new crustal thickness model presented in this work provides evidence that a crustal root is present beneath the Semail ophiolite, suggesting that folding and thrusting during the obduction process may have thickened the pre‐existing crust by 16 km.

SZonal Cr-spinels from Serpentinites in North Western Part of the Greater Caucasus

Formation homogenity and geodynamic conditions for the formation of apohyperbasites is an actual problem in geological study of the Paleozoic complexes of the Greater Caucasus. Study results of chromespinelids from serpentinite fragments of Paleozoic tectonic melange in the Western part of the Greater Caucasus (outcrops in Dakhovskiy uplift and Kisha river) tested with electron microprobe methods and processing of the results of elemental analysis using non-parametric statistics lead to the conclusion that apohyperbasites belong to ophiolites of subduction environments. Their composition is close to chromespinelids’ of ophiolite complexes of supra-subduction zones (SSZ) composition. Chromespinelides from the flank of Dakhovskoye uplift are represented by subferrochromite cores with rims of magnetite; transformation of subferrochromites during obduction process (Cr# ∼0.80-0.81, Mg# ∼0.35-0.40 in the least modified cores) in to chromites occurred under the conditions of the greenshist facies and was accompanied with redistribution and removal of aluminum and magnesium, and enrichment with iron and chromium. Chromespinelides from the serpentinites of contact Agarinskiy and Kizilkol’skiy thrust sheets presented with cores of aluminumchromite with relics of chrompicotites (Cr# ∼0.50-0.70, Mg# ∼0.39-0.64), surrounded by rims of ferrochromite – chromomagnetite. This transformation occurred on the boundary of the greenschist and low-temperature zone of the amphibolite facies and was accompanied by the substitution with iron for both two-and trivalent cations.

Structure of the Offshore Obducted Oman Margin: Emplacement of Semail Ophiolite and Role of Tectonic Inheritance

AbstractThe northern Oman margin is a key area for understanding the emplacement of the Semail Ophiolite and obduction processes in general. This study uses a grid of 2D‐multichannel seismic lines tied to well data to characterize the offshore domain of the Semail Ophiolite and reappraises the obduction and post‐obduction history of the Oman margin. West of Muscat, in the Sohar basin, the late Cretaceous to Paleogene tectonic mega‐sequence records syn‐ to late‐obduction stages and the deposition of erosional products of the autochthonous Arabian sediments, including a major mass transport complex. Syn‐obduction thrusting is documented in this sector only, as a major fault emplacing a distal basement high (likely volcanic) onto Campanian sediments over >10 km. To the east, the Hatat and Tiwi basins are characterized by a less‐copious Maastrichtian‐Paleogene sequence. These basins developed above a domain characterized by the northern equivalent of the Saih Hatat dome and later extensional faults. This sector distinctively records the extensional phase associated with the exhumation and erosion of the subducted continental margin. The dichotomy between the two sectors is linked due to a structural high located offshore, in the continuation of the Semail Gap transfer fault. We propose that this transfer fault, coincident with a major Pan‐African structure, affected the architecture of the passive margin during both rifting of the Neotethys and later ophiolite emplacement, that is, during (continental) subduction and obduction.

Fates of Paleo‐Antarctic Bottom Water During the Early Eocene: Based on a Lagrangian Analysis of IPSL‐CM5A2 Climate Model Simulations

AbstractBoth deepwater formation and the obduction processes converting dense deep water to lighter surface water are the engine for the global meridional overturning circulation (MOC). Their spatio‐temporal variations effectively modify the ocean circulation and related carbon cycle, which affects climate evolution throughout geological time. Using early‐Eocene bathymetry and enhanced atmospheric CO2 concentration, the IPSL‐CM5A2 climate model has simulated a well‐ventilated Southern Ocean associated with a strong anticlockwise MOC. To trace the fates of these paleo‐Antarctic Bottom Water (paleo‐AABW), we conducted Lagrangian analyses using these IPSL‐CM5A2 model results and tracking virtual particles released at the lower limb of the MOC, defined as an initial section at 60°S below 1,900 m depth. Diagnostic analysis of these particles trajectories reveals that most paleo‐AABW circulates back to the Southern Ocean through either the initial section (43%) or the section above (31%), the remaining (>25%) crossing the base of the mixed layer mostly in tropical regions (up to half). The majority of water parcels ending in the mixed layer experience negative density transformations, intensified in the upper 500 m and mostly occurring in tropical upwelling regions, with a spatial pattern consistent with the wind‐driven Ekman pumping, largely determined by the Eocene wind stress and continental geometry. In the same way as present‐day North Atlantic Deep Water upwells in the Southern Ocean, our results suggest that the strong tropical and equatorial upwelling during the Eocene provides an efficient pathway from the abyss to the surface, but at much higher temperature, with potential implications for the oceanic carbon cycle.

Analyse d'un processus d'obduction : l'exemple de l'océan téthysien maliaque (Hellénides)

L’obduction, charriage de la lithosphère océanique sur la croûte continentale, à l’origine des ophiolites, a été défini suite à la révolution de la tectonique des plaques. Dans un premier temps, les analyses ont surtout porté sur la pétrographie et la géochimie des ensembles ophiolitiques (par ex. Vourinos en Grèce, Troodos à Chypre, nappe du Semail en Oman). Notre étude concerne principalement un secteur des Hellénides, le massif de l’Othrys, où l’on peut observer un système ophiolitique complet représenté par un empilement de nappes mises en place pendant l’obduction de l’Océan téthysien Maliaque. L’existence d’une couverture crétacée discordante sur ces nappes, permet de les attribuer, sans équivoque, au processus d’obduction du Jurassique moyen-supérieur. Le dispositif structural est constitué de trois nappes ophiolitiques recouvrant un ensemble de cinq autres nappes correspondant à l’écaillage de la marge continentale sous ces nappes ophiolitiques. Dans ce massif, les ophiolites datées du Jurassique moyen (Mega Isoma et Metalleion) présentent des caractéristiques en partie comparables (répartition, nature pétrographique, âge) à celles des ophiolites du Vourinos-Pinde du Nord, supposées être nées au sein de la plaque supérieure d’une zone de subduction. En revanche, nous avons pu mettre en évidence, en Othrys, des nappes originales : i) une nappe ophiolitique de pillow-lavas du Trias (Fourka, de type MORB) témoin de la période initiale de l’océan ; ii) des nappes issues de la marge distale permettant de reconstituer précisément cette marge. L’analyse de ces différentes nappes nous a permis de reconnaître ou de préciser certaines modalités du processus d’obduction : i) la mise en place tectonique d’une unité (Fourka) appartenant à la plaque plongeante au niveau d’une subduction (et non pas à la plaque supérieure) ; ii) la genèse de certaines nappes à partir de failles normales listriques liées au rifting triasique (processus d’inversion tectonique positive) ; iii) les modalités de déplacement des nappes sur le domaine continental (marge et plate-forme) par l’analyse des mélanges à blocs nés à l’avant des nappes. Deux événements tardi-obduction ont également été étudiés : la genèse d’un bassin d’avant-chaîne et le développement de fenêtres tectoniques au cœur des nappes.

Forearc Crustal Structure of Ecuador Revealed by Gravity and Aeromagnetic Anomalies and Their Geodynamic Implications

AbstractAlong the Western Cordillera of Ecuador, fault-bounded ophiolites derived from the Late Cretaceous Caribbean Large Igneous Province (CLIP) have provided key petrotectonic indicators that outline the nature and the mechanism of continental growth in this region. However, most of the forearc basement across Western Ecuador is buried under sediments impairing its crustal structure understanding. Here, we propose a first crustal model throughout the spectral analysis of gravity and aeromagnetic data, constrained by observations made both at the surface and at the subsurface. Three main geophysical domains, within the North Andean Sliver in Western Ecuador, have been defined based on spectral analysis and augmented by 2D forward models. An outer domain, characterized by magnetic anomalies associated with mafic rocks, coincides with evidence of a split intraoceanic arc system. An inner domain is governed by long-wavelength mid to deep crust-sourced gravity and magnetic anomalies possibly evidencing the root of a paleoisland arc and the residuum of a partial melting event with subsequent associated serpentinization, the latest possibly associated with an obduction process during the middle Eocene-Oligocene. In addition, our model supports the presence of a lithospheric vertical tear fault, herein the southern suture domain, inherited from an oblique arc-continent interaction. Our interpretation also brings new insights and constraints on the early geodynamic evolution of the Ecuadorian forearc and provides evidence on the structural style and preservation potential of the forearc basement, most likely the roots of a mature island arc built within an oceanic plateau.

Heat and salinity transport between the permanent pycnocline and the mixed layer due to the obduction process evaluated from a gridded Argo dataset

It is necessary for air–sea interaction and climate research to evaluate heat and salinity transport between the mixed layer (ML) and the permanent pycnocline and its contribution to the ML properties. The authors have calculated the obduction and subduction rates with the Eulerian definition using a 1°-grid Argo dataset and incorporated the obduction rate into the ML budget analysis in order to assess the impact of the entrained permanent pycnocline water on the ML temperature and salinity. They have successfully separated the effect of obduction on the ML from the entrainment of the seasonal pycnocline, and specified the regions where the obduction impact is noticeable. The ML heating rate due to obduction was smaller than ± 0.1 K/yr on average over most of the domain, but it was larger than − 0.5 K/yr in places around the Circumpolar Current, in the northeastern tropical Pacific, the southwestern tropical Indian Ocean, and the northeastern North Atlantic. The salinizing rate due to obduction exceeded ± 0.02 psu/yr in these areas, and in the subarctic North Pacific and the eastern South Pacific. The waters entrained from the permanent pycnocline warm and salinize the ML in the high latitudes due to the dichothermal structure. The method applied in this study also enabled us to assess the residence time and route of obducted and subducted water parcels below the ML. Furthermore, the authors examined the changes of obduction and subduction related with the marine heatwave that occurred in the northeastern North Pacific around 2014.

Petro-tectonic evolution of metamorphic sole of the Semail ophiolite, UAE

The Semail ophiolite located in the eastern part of the Arabian platform preserves remnants of ocean plate stratigraphy and related metamorphic sole. To understand the petro-tectonic evolution of a metamorphic sole during subduction to obduction processes, here we investigate the garnet metagabbros from the metamorphic sole and the tonalites which intruded the mantle section of the Khor Fakkan Block. We present results from petrology, geochemistry, zircon U-Pb, Hf and O isotope analyses and phase equilibria modeling. The garnet metagabbro samples have E-MORB-type enriched-mantle compositions with zircon dates of ca. 89–96 Ma, and positive εHf(t) values ranging from 5.6 to 10.0. The tonalite is peraluminous with those range of ca. 87–92 Ma, and a range of positive εHf(t) values of 5.1–10.0. The similarity in εHf values from both the garnet metagabbro and tonalite samples suggests a strong relevance to their mantle source, indicating the role of subducted material during their formation. In contrast, the δ18O(zircon) values show distinctly different values of high δ18O(zircon) of ~13–16‰ for the tonalite and ~ 5–8‰ for the metagabbro samples, reflecting variations in the role of surface-derived source materials. The phase equilibria modeling of the garnet metagabbro shows high-pressure amphibolite facies metamorphism that preceded the peak granulite facies metamorphism, followed by lower pressure hydration and decompression. This clockwise P-T path might reflect partial melting and differentiation of mantle wedge section above subducted slab. Our results provide insights into the complex processes within a supra-subduction zone, implying differences in degree of partial melting of the ocean plate stratigraphic sequences including recycled oceanic slab and surface-derived marine sediments that were subsequently interacted with hydrothermally altered mantle at a mantle wedge during subduction to obduction processes that formed the Semail ophiolite during the Upper Cretaceous.

Sm-Nd geochronology and petrologic investigation of sub-ophiolite metamorphic sole from the Dinarides (Krivaja-Konjuh, Ophiolite Complex, Bosnia and Herzegovina)

The Dinaridic segment of Neotethys was affected by a widespread shortening and related subduction-accretion-obduction processes that commenced in the middle Jurassic. In the Dinarides, the Krivaja-Konjuh Ophiolite Complex (KKOC) stands as the largest ophiolite complex with a well-exposed metamorphic sole which is the key to understanding the dynamics of intraoceanic subduction initiation in this part of Neotethys. In this contribution we present Sm-Nd geochronology on a granulite facies amphibolite from the KKOC, as well as a detailed petrological description. A five-point isochrone age calculated from clinopyroxene, plagioclase, garnet, amphibole and whole rock is 162 ± 14 Ma (MSWD = 6.2), whereas garnet and whole rock yield 160 ± 7 Ma. Ages calculated from all data points except clinopyroxene are 162 ± 5 Ma (MSWD = 1.09). Petrographic investigations suggest that these ages date granulite facies metamorphic conditions (i.e. peak metamorphism of Grt-Cpx amphibolite) rather than post-peak exhumation or obduction processes. Phase textural relationships are in line with previous research, which indicated a peak metamorphism equilibration pressure and temperature of ~1 GPa and ~800 °C, respectively. Granulite facies conditions are elucidated for an igneous precursor, which underwent a multi-stage metamorphism that gave rise to recrystallization of igneous clinopyroxene and plagioclase, epitaxial growth of amphibole, and garnet blastosis. Taking into account the age of gabbronorite from the youngest segment of the KKOC oceanic crust (Taorcian to Bathonian) and ages of radiolarian assemblages from the KKOC melange (Bajocian to Bathonian), it may be inferred that within ~25 Ma the Dinaridic segment of Neotethys evolved rapidly from active ridge spreading through a stage of intraoceanic subduction and arc magmatism toward sub-ophioliteexhumation and further obduction along the Adria passive margin at the end of the Jurassic era.

Ventilation in the South China Sea: Subduction versus obduction

We investigate ventilation in the South China Sea (SCS) based on a recently developed framework for annual subduction/obduction in monsoon-dominated oceans. It is found that subduction occur in winter, and it was greatly weakened by ~58% due to the summer monsoon. Furthermore, there is a second obduction window opened up during the summer monsoon period, besides the winter obduction window.The winter subduction is concentrated in two narrow regions: one is located in the northern SCS and the other in the eastern SCS from 10°N to 15°N. The winter obduction distributes widely in two areas: the northeastern SCS and the area south of 10°N, while the summer obduction occurs primarily near the Vietnam. The key factors regulating subduction/obduction and their variability at different areas are different: the lateral induction term is the key contributor for subduction in the northern SCS; however, for the subduction in the eastern SCS and the obduction process, the vertical pumping term is dominant. Furthermore, the subducted (obducted) water in the SCS would penetrate into (come from) the subsurface ocean at the depth ~ 100m after (before) one year.

Geodynamic conditions of oil and gas accumulation in the fore-Urals and fore-Novaya Zemlya belt

A brief tectonic and geological overview of the fore-Urals and fore-Novaya Zemlya belt of oil and gas accumulation and the results of studying of the conditions for hydrocarbon deposits formation in uplift-thrust structures of its eastern side are presented. It is shown that the early stages of the belt formation are associated with Paleozoic subduction and obduction processes occurring in the transition zone between the Paleo-European continent and Uralian paleo-ocean located eastward (in modern coordinates) the continent. The intensive thrust-folded dislocations of the western verergency are fixed into the eastern side of the fore-Urals and fore-Novaya Zemlya belt of oil and gas accumulation. We associate their formation with the continental collision that occurred during the closure of the Uralian paleo-ocean at the Latest Paleozoic, and in the most northern segments of the belt, probably, at the Earliest Mesozoic. The structural paragenesis of the eastern side of the fore-Urals and fore-Novaya Zemlya belt of oil and gas accumulation includes uplift-thrusts and thrusts, as well as various scaled structural units formed under the conditions of the latitude compression.

Detrital signatures of impending collision: The deep-water record of the Upper Cretaceous Bordighera Sandstone and its basal complex (Ligurian Alps, Italy)

Despite intensive research efforts and significant advances in the understanding of subduction and obduction processes that affected several units which at the present day compose the Western Alps, the paleogeographic evolution of the Alpine Tethys represents a debated topic in Alpine geology. The role of the opposing continental margins (passive European margin and active Adriatic margin) as source regions for Cretaceous siliciclastic turbidites bordering the convergent system remains disputed. To address this question along the Ligurian Alps transect, a multi-proxy provenance analysis is applied to the two terrigenous superimposed units (Hauterivian-Campanian San Bartolomeo Fm. and Campanian-Maastrichtian Bordighera Sandstone) of the San Remo-Monte Saccarello Unit of the Western Ligurian Flysch complex. Petrographic analyses characterize the basal San Bartolomeo Fm. as quartz-rich mature sandstones. By contrast, the overlying Bordighera Sandstone represents texturally and compositionally immature first-cycle arkosic arenites. This change records the evolution of the sediment provenance from a stable craton into a continental basement uplift setting, reflecting erosion of granitoid plutons and the low-grade metamorphic basement. Geochronological data (UPb detrital zircon ages) indicate that virtually the same source terranes provided the source for both formations. The detrital age spectra display age peaks are compatible with well-documented magmatic and metamorphic pulses that affected the Southern Variscides in the Paleo-European margin. The strong affinity of clastic detritus with the Paleo-European margin basement rocks underlines the importance of the lower plate passive continental margin in supplying sand-rich turbidite systems prior to the arrival of the passive margin in the subduction zone.

Extent of the Middle Cretaceous Orogen in Eastern Asia and the Geodynamic Causes of Its Transformation

An extensive Middle Cretaceous orogenic belt of East Asia, which occupies the Okhotsk–Koryak–Western Kamchatka territory, encompasses thrust-and-fold structures. It formed over an interval of 130–110 Ma as a result of perioceanic accretion–obduction processes involving marine allochthonous complexes up to the Barremian inclusive, which experienced large amplitude displacements. Under the remote impact of subsequent (end of the Cretaceous and Cenozoic) perioceanic tectonic events, the primary structures of the Middle Cretaceous belt underwent compression-related superimposed deformations and fragmentation of allochthons with negligible displacement amplitudes. This led to the formation within the belt of a mosaic pattern of alternating fragments of neo-autochthons and allochthons bounded by young thrusts and strike–slip faults, often suggested as independent heterochronous terranes.

From ocean depths to mountain tops: Uplift of the Troodos ophiolite (Cyprus) constrained by low‐temperature thermochronology and geomorphic analysis

AbstractThe timing and mode of uplift of the Troodos ophiolite are constrained by low‐temperature thermochronology combined with geomorphic analysis. Zircon (U‐Th)/He and apatite fission track cooling ages in the Troodos plutonic sequence are all Cretaceous (83–106 Ma) and within error of published zircon U‐Pb crystallization ages. This indicates early cooling of the oceanic crust and termination of spreading axis magmatism at ~90 Ma. Apatite (U‐Th)/He ages decrease with reconstructed crustal depths from ~40 Ma near the top of the sheeted‐dike complex to ~4 Ma within the mantle sequence. A prominent inflection point in the age versus depth curve defines the bottom of the exhumed helium partial retention zone and records the onset of rapid exhumation of the main Troodos massif at 6 ± 2 Ma. Inverse thermal modeling supports this conclusion, indicating that the timing of uplift is earlier than previously estimated. The boundaries of the mantle sequence exposed in the core of the Troodos structure closely overlap the boundaries of a concentric zone delineated by high local relief and higher channel steepness indices, indicating differential exhumation and uplift of this area relative to its surroundings. This zone also overlaps with a prominent negative Bouguer gravity anomaly. The timing and pattern of the Troodos ophiolite uplift suggest that it is driven by serpentinite diapirism, possibly triggered by Miocene reactivation of subduction along the Cyprean Arc. The worldwide ubiquity of suprasubduction zone ophiolites may thus reflect the importance of extensive serpentinization at the overthrusting mantle wedge in obduction processes.

Obduction of old oceanic lithosphere due to reheating and plate reorganization: Insights from numerical modelling and the NE Anatolia – Lesser Caucasus case example

The ophiolites of NE Anatolia and of the Lesser Caucasus (NALC) evidence an obduction over ∼200km of oceanic lithosphere of Middle Jurassic age (c. 175–165Ma) along an entire tectonic boundary (>1000km) at around 90Ma. The obduction process is characterized by four first order geological constraints:(1)Ophiolites represent remnants of a single ophiolite nappe currently of only a few kilometres thick and 200km long. The oceanic crust was old (∼80Ma) at the time of its obduction.(2)The presence of OIB-type magmatism emplaced up to 10Ma prior to obduction preserved on top of the ophiolites is indicative of mantle upwelling processes (hotspot).(3)The leading edge of the Taurides-Anatolides, represented by the South Armenian Block, did not experience pressures exceeding 0.8GPa nor temperatures greater than ∼300°C during underthrusting below the obducting oceanic lithosphere.(4)An oceanic domain of a maximum 1000km (from north to south) remained between Taurides-Anatolides and Pontides-Southern Eurasian Margin after the obduction.We employ two-dimensional thermo-mechanical numerical modelling in order to investigate obduction dynamics of a re-heated oceanic lithosphere. Our results suggest that thermal rejuvenation (i.e. reheating) of the oceanic domain, tectonic compression, and the structure of the passive margin are essential ingredients for enabling obduction. Afterwards, extension induced by far-field plate kinematics (subduction below Southern Eurasian Margin), facilitates the thinning of the ophiolite, the transport of the ophiolite on the continental domain, and the exhumation of continental basement through the ophiolite. The combined action of thermal rejuvenation and compression are ascribed to a major change in tectonic motions occurring at 110–90Ma, which led to simultaneous obductions in the Oman (Arabia) and NALC regions.

Structure and metamorphism beneath the obducting Oman ophiolite: Evidence from the Bani Hamid granulites, northern Oman mountains

The Cretaceous Semail ophiolite (northern Oman and the United Arab Emirates) includes an intact thrust slice of Tethyan oceanic crust and upper mantle formed above a northeast-dipping subduction zone that was the site of initiation of obduction. The normal metamorphic sole of the Semail ophiolite comprises a highly condensed sequence of hornblende + plagioclase ± garnet amphibolites with small enclaves of garnet + clinopyroxene granulites immediately beneath the mantle sequence peridotites, tectonically underlain by a series of epidote amphibolite and greenschist facies lithologies in a highly deformed ductile shear zone. Peak metamorphic conditions of 770–900 °C and 11–15 kbar indicate metamorphism at depths far greater than can be accounted for by the preserved thickness of the ophiolite (∼15 km). In the mountains of northern Oman, the 1.2-km-thick Bani Hamid thrust sheet is composed of intensely folded granulite and amphibolite facies rocks within mantle sequence peridotites, exhumed by late-stage out-of-sequence thrusting along the Bani Hamid thrust. The Bani Hamid thrust slice includes two-pyroxene quartzites (± hornblende, cordierite, sapphirine), diopside + andradite garnet + wollastonite + scapolite marbles and calc-silicates and amphibolites (hornblende + plagioclase ± clinopyroxene ± biotite) with localized partial melting, intruded by hornblende pegmatites. The Bani Hamid granulites represent metamorphosed cherts and calcareous turbidites probably derived from the distal Haybi Complex and Oman Exotic limestones, which have an alkali basaltic substrate. Metamorphic modeling using the program THERMOCALC in the system NCKFMASHTO (Na 2 O-CaO-K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O-TiO 2 -O) gives peak pressure-temperature conditions of 850 ± 60 °C and 6.3 ± 0.5 kbar, a pressure that is much lower than that of the metamorphic sole, suggesting a different origin. The 206 Pb/ 238 U zircon dates indicate that the gabbroic crust of the ophiolite formed by ridge magmatism from before 96.1 to 95.5 Ma. The 206 Pb/ 238 U zircon dates from the metamorphic sole range from 95.7 to 94.5 Ma, and suggest that metamorphism and melting was either synchronous with or slightly postdated ridge magmatism. The Bani Hamid granulites are younger; zircon and titanite U-Pb dates span ca. 94.5–89.8 Ma. Peraluminous granitic dikes intruding the mantle sequence peridotites are as young as 91.4 Ma and likely reflect localized partial melting of crustal material during the late stage of the obduction process. A minimum of 130 km shortening is recorded by restoration of the major folds within the Bani Hamid thrust sheet, and more than 30 km offset has occurred along the west-directed breaching out-of-sequence Bani Hamid thrust. These rocks may be representative of deep-level duplexes imaged on recent seismic sections across the mountains of northern Oman–United Arab Emirates.