Subduction Events Research Articles

Where and when the Paleo-Asian Ocean was closed: new evidence from the Tuquan ophiolite

The Solonker suture marks the location of the final collision in southeastern Central Asian Orogenic Belt (CAOB). However, it is observed that the suture is obscured at the western margin of the Songliao Basin. Fortunately, the recently discovered Tuquan ophiolite is one of the important fragments within the Solonker suture. Testings of samples from the Tuquan ophiolite yielded LA-ICP-MS zircon U–Pb chronological data with weighted mean of 206 Pb/ 238 U ages from 265 ± 5 Ma to 279 ± 6 Ma which were interpreted to represent the formation age of the Tuquan ophiolite from the late Cisuralian to Guadalupian. The geochemical characteristics of the Tuquan ophiolite is related to supra-subduction zone features. We suggest that it represents an important bidirectional subduction event involving the oceanic crust of the Paleo-Asian Ocean beneath the Mongolia Block and the North China Craton during the early to late Permian. Integrating these findings with previous research indicates that the final subduction of the Paleo-Asian Ocean persisted until at least the late Permian. Our study provides further insights, indicating that the discovery of the Tuquan ophiolite could be regarded as the eastern extension of the Solonker suture. This finding will provide new constraints on the evolution of the southeastern CAOB.

LOMU Type Alkali Basalts in East Asia Sourced from Subduction Recycling of Multiple Ancient Crustal Components

Abstract The origin of alkali basalts with distinctive time-integrated low U/Pb (low μ, LOMU) from East Asia is controversial due to the complex geochemical and tectonic signatures reported from this region. We report new data on the petrology and geochemistry of the Higashi-Matsuura and Kita-Matsuura alkali basalts from Southwest Japan, which confirm the presence of a LOMU-type mantle component below the Japanese Islands, similar to East Asian intraplate volcanoes. We use whole rock geochemistry and mineral chemistry to determine mantle melting, magma evolution conditions and isotopic evolution of the LOMU mantle source. Petrological studies show that the Higashi-Matsuura alkali basalts (~3 Ma) were derived from a hydrous mantle source with ~880 μg/g H2O, at a pressure of 1.8 to 2.3 GPa at a mantle potential temperature of ~1300°C. Trace element modelling concludes that these basalts may have been generated through the hydrous upwelling and partial melting of the asthenospheric mantle containing ~15% dehydrated eclogitic component. Higashi-Matsuura alkali basalts show 206Pb/204Pb values of 17.72 to 18.04 which are among the lowest values from Southwest Japan. Relatively older (6–8 Ma) alkali basalts from the Kita-Matsuura area showing similar physicochemical characteristics, do not show LOMU-type isotopic trends. Trace element and Pb–Sr–Nd isotopic data indicate that the Higashi-Matsuura mantle component is similar to the extreme LOMU components reported from the Northeast China alkali basalts, as well as the recently discovered Petit Spot volcanoes on the Pacific Plate. We model the origin of the LOMU signature from the lowest reported Pb isotope ratios in East Asia, from intraplate alkali basalts in Northeast China. Our model suggests that at least two separate subduction events of marine sediments, at 1.8 and 2.2 Ga, are required to explain the observed Pb isotopic variation in the East Asian region. Other LOMU type basalts from East Asia, including Southwest Japan and Petit Spot, define a linear trend between the Xiaogulihe basalts and lithospheric mantle xenoliths, with 40% to 60% Pb in Higashi-Matsuura basalts being derived from the lithospheric mantle during upwelling. This suggests that the LOMU array in East Asia may have been formed by mixing between multiple ancient, subducted sediment components accumulated at the mantle transition zone for about 2 billion years, and its recent upwelling due to dehydration of the stagnant Pacific slab and related melting of the metasomatised asthenospheric mantle.

Petrogenesis of Early Permian within-plate-type gabbros in the Western Qiangtang Terrane, Tibet: partial melting of lithospheric mantle metasomatized by subducted oceanic sediments on the northern margin of the Gondwana

ABSTRACT The extensively distributed Early Permian mafic rocks in the Qiangtang Terrane of the central Tibetan Plateau are the magmatic records of separation of this terrane from the Gondwana. Nevertheless, the origins and mantle source characteristics of these rocks continue to be subjects of debate. This study reports zircon U-Pb and Lu-Hf isotopic and whole-rock geochemical and Sr-Nd-Hf isotopic data on gabbros exposed in the Zougou Region, western Qiangtang Terrane. Our zircon U-Pb isotope age suggests that the Zougou gabbros (ZGs) were formed at ca. 290 Ma, representing newly discovered Early Permian mafic rocks within the Qiangtang Terrane. The ZGs display geochemical affinities with within-plate-basalts given that they are tholeiitic and have high Zr/Y and Ti/V ratios. Compared to previously examined Early Permian mafic rocks in the western Qiangtang Terrane, the ZGs are characterized by their distinct, nonradiogenic Nd (εNd(t) = -0.54 to −2.18) isotopic signatures, which cannot be attributed to crustal contamination or assimilation. This assertion is supported by their uniform whole-rock εNd(t), εHf(t), and zircon εHf(t) values, as well as by magma mixing modelling. In conjunction with regional geology, we propose that the ZGs were derived from an isotopically enriched lithospheric mantle metasomatized by previous subduction events. Furthermore, the positive decoupling of Hf-Nd isotopes (ΔεHf(t) = +2.21 to + 2.72), negative Nb-Ta anomalies, and variable Ba/Th ratios within the ZGs indicate that the metasomatic agents were fluids released from oceanic sediments. Our study provides insights into the compositions of the lithospheric mantle beneath the Qiangtang Terrane during the Permian, which is pivotal to discriminating the role of asthenospheric and lithospheric components in the formation of the contemporaneous mafic rocks in response to Gondwana break-up.

Determination of a normal orogenic palaeo-geothermal gradient with clay mineral and organic matter indices: a review

A collection of large data sets from different orogenic belts was compiled for a correlation between organic matter (OM) versus clay mineral (CM) indices calibrated with the vitrinite reflectance, (VR) vs Kübler-Indices (KI) method. Data selection was based on a normal geothermal gradient (25 to 35 °C/km) as determined in previous studies, e.g. by maturity modelling and clay mineral reaction progress calculations. In the Lower Austroalpine (Eastern Switzerland, European Alps) a 20 myr lasting metamorphic overprint caused an OM–CM thermal equilibrium among the indices used. The observed correlation enables to determine gradual changes in metamorphic factors such as pressure, temperature and time causing sensitive shifts of the gradient slope in the range of normal gradients. For New Caledonia, an identical correlation has been determined. Prior to re-equilibration of the VR/KI indices, sediments in New Caledonia of diagenetic to incipient metamorphic grade underwent a high-pressure subduction event. VR/KI indices are in or close to equilibrium, while slight differences in OM vs CM indices allow for a better understanding of polyphase conditions, especially with respect to pressure. Temperature estimations are identical despite of their poly-phase metamorphic history, which was mainly controlled by the last orogenic thermal event lasting > 5 to < 10 myr. In the eastern Helvetic Alps and Northern Calcareous Alps similar correlations were found with slightly different slopes. Comparison between different regions is possible when using KI standardization and same data discrimination. In both parts of the Alps a complex thermal history of short durations (< 5.0 myr for the Northern Calcareous Alps to 10 myr for the Helvetic Alps) caused similar VR/KI trends, but disequilibrium is suggested by weaker regression parameters. The following correlation is calculated for a moderate geotherm (55 to 74 mWm2, mean = 61 mWm2) and normal temperature gradient conditions (25 to 35 °Ckm−1): KI = 1.134e−0.305VR, (R2 = 0.880, n = 462) with VR given as %Rmax, KI as Δ°2θ (limited to values between 0.2 to 1.0 Δ°2θ). With increasing depth (z) a VR gradient of 1.4 ± 0.2%Rmaxkm−1 is determined and a KI gradient of 0.09 ± 0.002 Δ°2θ km−1 is observed. The study illustrates that a normal geotherm can be described by VR/KI correlation, even if different heating episodes may occur. For the detection of a poly-phase or plurifacial thermal history, several indices of clay minerals and organic matter with very different kinetics should be used, as e.g. demonstrated by strong differences in smectite content at equal VR/KI values versus structural depth. A specific interest is given to the correlation of vitrinite like solid bitumen reflectance as an alternative method to VR, the persistent preservation of liptinite macerals and the stability range of clay minerals and sub-greenschist facies critical minerals compared with VR/KI data. Until now, despite the Alps in this study, systematic liptinite maceral studies have not been published in other orogenic settings.

An embryonic intra-oceanic subduction in the Bangong−Nujiang Meso-Tethys: insights from Late Jurassic Nd[sbnd]Hf isotopic decoupling adakitic granodiorites in the Dongco ophiolite of central Tibet

Subduction of the Bangong−Nujiang Meso-Tethys not only bred world-class Cu − Au polymetallic deposits in central Tibet but also created the initial elevation of the Tibetan Plateau during the Late Cretaceous following its final closure. Consequently, the Meso-Tethys carry critical codes for the evolution of Tethyan realm and the development of the Tibetan Plateau but some fundamental aspects still remain poorly explored. Here we proposed an embryonic intra-oceanic subduction event in the Bangong−Nujiang Meso-Tethys during the Late Jurassic based on investigations of adakitic granodiorites intruded within the Dongco ophiolite suite, central Tibet. These granodiorites were dated at 150–156 Ma by zircon UPb isotopes and geochemically display an affinity with high-silica adakite (Sr/Y = 24–47; La/Yb = 6–16), specifically marked by high SiO2, Al2O3 and Na2O contents but low MgO, CaO + Na2O contents as well as extremely low Yb and Y contents. They have (87Sr/86Sr)i ratios varying from 0.7061 to 0.7069, εNd(t) values of −0.83 to 0.50, and εHf(t) values of 3.3 to 8.9 (with one exception at −5.0), showing a distinct NdHf isotopic decoupling (ΔεHf(t) = 2.1–7.6; ΔεHf(t) = εHf(t)–1.55 × εNd(t)–1.21). Geochemical modeling indicates that they were derived from partial melting of subducted Meso-Tethyan oceanic crust and moderate amounts of sediments. Residual plagioclases in the source region likely resulted in relatively lower Sr and La contents than typical high-silica adakite. Combined with previous studies, we suggest that the Dongco granodiorites formed in the early stage of an intra-oceanic subduction zone in the Bangong−Nujiang Meso-Tethys where subsidence of colder lithosphere inductively commenced along an active transform fault bounded by younger lithosphere during the Late Jurassic. This embryonic intra-oceanic subduction possibly occurred locally and was very short-lived.

Fossil metasomatized and newly-accreted fertile lithospheric mantle volumes beneath the Bakony-Balaton Highland Volcanic Field (central Carpathian-Pannonian region)

This study focuses on upper mantle xenoliths from Sabar-hill (Bakony-Balaton Highland Volcanic Field), a newly discovered locality of mantle xenoliths, situated in the central Carpathian-Pannonian region (CPR). The investigated seven peridotite and one pyroxenite xenoliths record two episodes in the geochemical evolution of the local subcontinental lithospheric mantle. The moderately deformed Group I xenoliths reveal modal contents (≥9 vol% orthopyroxene), major element characteristics (depletion in Al2O3 and CaO in orthopyroxenes) and trace element features (e.g., enrichment in U, Pb and Sr in clinopyroxenes) typical in supra-subduction zones (i.e., mantle wedge) xenoliths. All these suggest that the studied xenoliths were formed by silica-rich fluid/melt - peridotitic wall rock interactions. The tectonic background to the formation of Group I xenoliths is likely linked to the subduction of an oceanic slab during the Mesozoic-Paleogene. This may have happened far from the current position of Sabar-hill, to where the lithosphere, including the metasomatized mantle volume, was transferred via plate extrusion. The less deformed, dominantly protogranular and porphyroclastic Group II xenoliths are depleted in light rare earth elements (LREE). In contrast, they are enriched in clinopyroxene (≥11 vol%), incompatible major elements (e.g., Al and Na) and heavy rare earth elements (HREE), by a minimum of four times compared to primitive mantle. All these suggest that Group II xenoliths represent a geochemically fertile mantle partition and the degree of partial melting affected this mantle section was limited (≤7% based on trace elements). Group II xenoliths have higher equilibrium temperatures (1088–1168 °C) compared to Group I xenoliths (1018–1089 °C). The lack of petrographic and geochemical evidence pointing to metasomatism-related annealing in Group II xenoliths suggests that they are derived from a greater depth. It follows that Group II xenoliths could represent prior asthenosphere volumes which recently accreted to the lithosphere. The lithospherization of the upper part of the asthenosphere may have initiated after the tectonic inversion of the region, when compressional tectonic regime beneath the CPR gradually overtook the former extensional one in the last ∼8 Ma. The high bulk structural hydroxyl content in both Group I (22–1699 ppm) and II (55–320 ppm) xenoliths is likely linked to the subduction events that took place in the CPR from the Mesozoic onwards. These events transported significant amounts of fluid (H2O) back to the upper mantle, affecting both the mantle wedge and the asthenosphere.

Structural anatomy of the newly recognized Paleozoic Dacaotan composite mélange belt: Implications for the subduction-accretion processes in the North Tianshan Ocean, southwestern Central Asian Orogenic belt

The North Tianshan Belt occupies a pivotal geological position within the southwestern Central Asian Orogenic Belt, making it an ideal location to unravel the long-term evolutionary history of the North Tianshan Ocean. However, how, and when the Tianshan oceanic basin was finally consumed and closed remains highly debated. To resolve these questions, here, we conducted a comprehensive investigation of the different litho-tectonic assemblages of the Dacaotan composite mélange belt in the North Tianshan Belt. Using detailed geological mapping, structural analyses, rock associations, and new geochemical and geochronological data, we divide the Dacaotan composite mélange belt into three litho-tectonic assemblages: (1) the southern sedimentary forearc (ca. 423–398 Ma) assemblage that is characterized by relatively coherent and well-bedded flysch sequences; (2) the central supra-subduction zone-type ophiolitic mélange assemblage (ca. 468–461 Ma) consisting of petrogenetically linked upper mantle and oceanic crustal components, which are exposed as tectonic slices or blocks within a serpentinite and sedimentary matrix; and (3) the northern accreted ocean plate stratigraphy mélange assemblage (ca.448–398 Ma) that is composed of accreted trench-fill turbidites, seamounts (OIB-like basic rocks) and ocean floor rocks (MORB-like basic rocks) dispersed in a sandy to pelitic matrix. Post-kinematic pluton and dyke reveal that the final accretionary event was completed prior to ca. 371 Ma. In conjunction with the spatial–temporal configurations of the different litho-tectonic assemblages, we conclude that the North Tianshan Ocean underwent bi-directional subduction and intra-oceanic subduction events during the Early Paleozoic. Our findings suggest the final consumption of the North Tianshan Ocean basin occurred before the Latest Devonian, resulting in the formation of the ultimate suture between the Central Tianshan Block and the North Tianshan Belt along the Kanggguertag-Dacantao composite mélange belt.

Zircon Petrochronology of Au-Rich Porphyry and Epithermal Deposits in the Golden Quadrilateral (Apuseni Mountains, Romania)

Abstract The Golden Quadrilateral of the Apuseni Mountains (Romania) represents the richest Au(-Cu-Te) porphyry and epithermal district of Europe and the Western Tethyan metallogenic belt. The Au(-Cu-Te) mineralization is associated with Neogene calc-alkaline magmatism along graben structures growing during the late stages of the Alpine-Carpathian orogeny. We use zircon petrochronology to study the time-space distribution, sources, composition, and timescales of the Au(-Cu-Te)-mineralizing magmatism and explore its link to regional tectonics. Our own and published U-Pb zircon ages document ore-forming magmatic activity between ~13.61 and 7.24 Ma. In combination with available paleomagnetic data, the new zircon ages corroborate the hypothesis that the magmatism in the Golden Quadrilateral evolved in a tectonic environment dominated by major (up to 70°) crustal block rotation. Hafnium isotope composition of Neogene zircon (εHf between –2 and 10) supports the predominant origin of the magmas from a heterogeneous lithospheric mantle, which may have been fertilized during an earlier Cretaceous subduction event and possibly by concurrent Miocene subduction. Xenocrystic zircon shows involvement of crustal sources resembling European continental basement. Fertility indicators, including Eu/Eu* and oxygen fugacity based on zircon composition, show no systematic correlation with the mineralizing events and/or age. High-precision (isotope dilution-thermal ionization mass spectrometry) U-Pb zircon geochronology demonstrates that the magmatic systems exposed at district scale evolved over less than ~100 k.y. and that durations of hydrothermal mineralization pulses were even shorter.

Earth’s Subduction May Have Been Triggered by the Same Event That Formed the Moon

The giant impact that formed the Moon may also have led to extrastrong mantle plumes that enabled the first subduction event, kick-starting Earth’s unique system of sliding plates.

980-970 Ma Sette-Daban event of the Siberian craton: new geochronological and geochemical data, relationship to LIP and potential connection with other LIPs

ABSTRACT The paper presents new geological, geochronological, geochemical and Nd-Sr isotopic data on the Meso- Neoproterozoic dolerites of the Siberian Craton. New U-Pb baddeleyite and apatite ages of a E-W-trending dyke and two sills from southeastern Siberia are 982 ± 11, 977 ± 7, 970 ± 31 and 972 ± 60 Ma, respectively, extending the area of distribution of the event by more than 100 km to the north. The Sette-Daban intrusions are subalkalic mostly low-Ti dolerites, although high-Ti dolerites have been locally documented as well. Trace element abundances in dolerites vary from typical to E-MORB to OIB with arc-like signatures represented by high Th/Yb and low TiO2/Yb ratios. All dolerite samples display moderately positive εNd(t) values varying from +3.3 to +7.7 and indicating the magmas were derived from a depleted mantle source. E-MORB and OIB intrusions are attributed to the different degree of interaction of magma between the depleted asthenospheric mantle and regions within the sub-continental lithospheric mantle (SCLM) that were metasomatically enriched during earlier subduction events. Available data on Sette-Daban event distribution, composition and duration satisfy the characteristics of a typical Large Igneous Province (LIP). Similar age 1000–950 Ma mafic magmatism is also recognized in the Baltic and Amazonian cratons and can be potentially correlated with the Sette-Daban event.

Onset of Plate Motion in the Presence of Chemical Heterogeneities in the Mantle and the Effect of Mantle Temperature

AbstractChemical heterogeneities of various origins have been observed in the Earth's mantle. Their assumed higher density acts on the style of mantle convection and therefore, as tectonic plates form the highly viscous upper boundary layer of mantle convection, the onset of surface motion is also affected by chemical heterogeneities. We perform 2D basally heated thermochemical mantle convection models which initially include layers of dense material at different mantle depths. In comparison to purely thermal convection models, the onset of first plate motion is delayed. This delay is even more pronounced, the deeper the location of dense material, the larger its volume and the higher its density. Additionally, we varied the starting temperature of our models. In an initially hot mantle, a strong temperature‐dependent viscosity contrast between the mantle and cold surface leads to a cold, highly viscous plate (stagnant lid). For an initially cold mantle, rising plumes first need to transport basal heat upwards to create a sufficient viscosity contrast. Consequently, the formation and subsequent breaking of the stagnant lid is delayed. Considering a hotter mantle after Earth's magma ocean phase, we find that plate motion can occur within approximately the first 0.5 Gyr of solid‐state convection if no chemical structures are present or for dense material situated at the surface. Deep chemical heterogeneities delay the onset by at least one order of magnitude. Furthermore, the early surface motion will have been more erratic than todays stable plate motion, probably with a few single subduction events.

Excessive subsidence of oceanic basins caused by recycled oceanic crust in the mantle source: A new perspective on the oceanic topography within Southeast Asia

Additional subsidence of oceanic basins compared to the half-space cooling prediction is thought to be a dynamic response to the underlying mantle convection induced by slab sinking. Here, we identified a significantly elevated proportion (10%−20%) of recycled oceanic crust (ROC) in the source of mid-ocean-ridge basalts (MORBs) from the South China Sea (SCS), illustrating the ROC accumulation within a confined upper-mantle range, possibly as a result of the long-term surrounding subduction events. The presence of this lithological heterogeneity in the ROC leads to an increase in mantle density by 0.45%−0.90% at most, thereby contributing to the observed excessive subsidence in the SCS basin when compared to other small basins affected by dynamic topography, while open oceans experience subsidence primarily due to thermal cooling. We propose that the ROC-induced density change in the upper mantle is crucial for generating the topographic anomalies in oceanic basins influenced by subducted slabs, alongside dynamic topography caused by mantle flow.

Eocene (48.7 Ma) eclogite at Chepelare, Central Rhodopes, Bulgaria: A key to the tectonics of the Rhodopes

AbstractThe presence of an Eocene subduction event, in addition to Jurassic and Cretaceous ones, in the Rhodopes of Greece and Bulgaria is controversial. We report the first dating of an eclogite from the Lower Allochthon of the Rhodopes. The eclogite is a mafic lens enclosed in orthogneiss of the Arda‐2 Unit at Chepelare, Central Rhodopes. It was metamorphosed under conditions of ca. 25 kbar, 700–750°C. Garnet grains have outward‐increasing Mg contents and omphacite inclusions with the highest jadeite contents in their rims, indicating garnet growth under increasing pressure. Lu–Hf geochronology yielded an age of 48.7 ± 1.5 Ma. The age is linked to a late stage of garnet growth close to peak pressure, i.e. to subduction.

Implications of input ground-motion selection techniques on site response analyses for different tectonic settings

This study investigates how current practices of input ground-motion selection influence site response analysis results and their variability, when considering different tectonic settings. Study sites in Seattle and Boston are chosen to represent tectonic settings with contributions to the seismic hazard from shallow crustal and subduction events, as well as stable continental regions, respectively. Selected input ground-motion suites for one-dimensional site response analysis represent variations in the target spectrum definition, spectral period of interest, seismic source, and ground-motion database. When directly incorporating different types of seismic sources (e.g. shallow crustal versus subduction) into target spectrum definitions and selecting ground motions from the corresponding databases (i.e. consistent with such seismic sources), differences on the estimated site response and its variability are observed. These effects are captured by spectral amplification factors and nonspectral intensity measures (significant duration and Arias intensity) and become particularly apparent for subduction zones. The variability in spectral amplification factors stemming from ground-motion selection techniques is found to be also a function of the characteristics of the site, becoming higher near the fundamental period of the site. Estimated responses at stiffer sites are more significantly influenced by ground-motion selection techniques, whereas the onset of nonlinear soil behavior at softer sites can reduce such variability.

Cretaceous to early Paleogene sediment provenance transition from continental to magmatic arc systems in the Northwestern Pacific Region

Unraveling the Paleo-Kuril Arc's origins is key to understanding northwest Pacific tectonics. The Paleo-Kuril Arc is viewed as an intraoceanic arc from trench subduction between the Izanagi and Pacific Plates. Alternatively, several studies suggest the Paleo-Kuril Arc as a continental magmatic arc, hypothesizing the existence of a mid-oceanic ridge and Paleogene subduction, placing the Paleo-Kuril Arc near the Okhotsk Block's southern edge. This study clarifies these hypotheses, previously clouded by limited geochronological data on deposits in the Paleo-Kuril Arc. We conducted U–Pb dating to examine the origins of detrital zircons from the Cretaceous–Paleogene Tokoro and Nemuro Belts of the Paleo-Kuril Arc. Cluster analysis, merging new and existing data, identified two unique detrital zircon age clusters. The abundance of Precambrian zircons in Cretaceous–Paleocene Paleo-Kuril Arc sandstones (Type 1 Cluster) suggests a continental magmatic origin, supporting the ridge subduction model. However, an early Eocene shift to a consistent local volcanic source (Type 2 Cluster) highlights a significant provenance change. This geochronological evidence, indicating a separation from continental sources, calls for further research to decode the simultaneous development of sediment sources in different geological belts, potentially tied to the ridge subduction event.

Remobilization of carbon in the lithospheric mantle during decratonization

The sub-continental lithosphere mantle (SCLM) contains considerable amounts of carbon and is responsible for significant CO2 emissions via magmatism during major heating and rifting events. However, the role of SCLM in Earth's carbon cycle during other geological processes, such as decratonization, a process of major removal and replacement of cratonic SCLM, remains poorly understood. The North China Craton is an archetype of Archean craton having been decratonized during the Mesozoic with extensive magmatism developed. Here we show that the primary melt of Early Cretaceous Yixian basalts contained abundant CO2 (0.6 wt%), through investigating melt inclusions trapped in olivine crystals. The melt inclusions are alkalic and chemically distinct from their sub-alkalic host basalts, which could be explained by that the primary alkalic melt was reactively modified during magma ascent in the SCLM. The primary melt has geochemical characteristics consistent with partial melting of a veined carbon-rich amphibole clinopyroxenite source in the SCLM formed metasomatically during circum-craton subduction events. The findings highlight that the carbon introduced into the cratonic SCLM during subduction could be subsequently reactivated and transferred to the crust and atmosphere through decratonization-related magmatism, which might represent a major process regarding the SCLM in regulating the terrestrial carbon cycle.

The layered gabbroic intrusion in the Southern Eastern Desert of Egypt: Implications for Fe-Ti-rich magmatism at the end of the East African Orogeny

The Rahaba layered intrusion in the Southern Eastern Desert of Egypt is mainly composed of pyroxene hornblende-, and hornblende gabbros. It hosts thick FeTi oxide ore layers that are composed of titanomagnetite with subordinate ilmenite and magnetite. To understand the petrogenesis of the FeTi rich gabbroic rocks, the nature of their parent high-Fe primitive mantle melts and condition of FeTi oxide layer formation, we obtained new mineral chemistry and whole-rock major and trace element compositions. The geochemical characteristics of the gabbroic rocks resemble those of picro-basalts, suggesting generation from picritic basaltic-like melts of tholeiitic affinity. Their high-Fe-Ti-Mg nature (TiO2, ∼1.91 wt%; MgO, ∼11.0 wt%; Fe2O3, ∼14 wt%) probably indicates elevated mantle potential temperatures due to upwelling of hot asthenospheric mantle. Rare earth elements (REEs) of clinopyroxene show flat patterns with depletion in LREEs, similar to those of the host gabbros and picritic basaltic lava, suggesting a mantle origin for their parental melts. Moreover, clinopyroxene compositions show differentiation trends similar to low-pressure igneous intrusions, and reveal a rift-related origin. Chemistry of plagioclase (An45–62), clinopyroxene (Wo38.4–45.3, En39.9–43, Fs14–19.3; Mg#: 0.71–0.92) and olivine (Fo56–80) shows wide compositional ranges due to variable degrees of crystal fractionation. The calculated equilibrium temperature for the Rahaba Fe-Ti-rich gabbros is mainly 820–1000 °C, at pressure below 5 kbar, reflecting depth of rock solidification of <15 Km. These rocks show both subduction-like and rift-related geochemical signatures comparable to other post-collisional layered intrusions in the Arabian-Nubian Shield. The low average Ce/Pb (8.3) and high Ba/Nb (391) ratios together with the enriched large-ion lithophile elements (Cs, Ba, K and Sr) and depleted high-field strength elements (Th, Nb, Ta and Zr) reflect the contribution from a preceding subduction event. The lineament map, structural lineament and lineament density map reveal main WNW-ESE and NW-SE structural trends of the Najd System in the Rahaba area. The Rahaba FeTi rich gabbroic intrusion was likely formed by initial impact of a mantle plume head with Fe-rich streaks at the base of metasomatized lithospheric mantle during an extensional phase associated with activity of the Najd-related strike-slip faults, at the end of the East African Orogeny. The resultant mantle-derived melt compositions and Najd-related structures possibly controlled the distribution of FeTi ore deposits during this Orogeny. The Rahaba gabbroic intrusion was emplaced during post-collisional extension after slab break-off and lithospheric delamination following collision of accreted island-arcs with the Saharan Metacraton.

Basin boundary seismic effects in Mexico City southern region

AbstractRapid changes in geotechnical and geological ground conditions lead to significant ground motion variability. This condition mainly occurs at the so-called basin edges, where there is an abrupt transition between soft highly compressible soils and stiffer materials. This problem becomes more relevant in areas where ground subsidence drastically changes the dynamic response of high plasticity clay deposits, such as those found in Mexico City, due to fundamental site period evolution with time. This paper presents site response analyses at an abrupt transition area in the southeast Mexico City region, along the edges of the Xochimilco-Chalco lakes. Considerable damage associated with three-dimensional wave propagation effects was observed in this zone during the September 2017 Puebla-Mexico earthquake. A series of three-dimensional finite difference numerical models of the basin edge were developed to evaluate ground motion variability, considering topographic effects and soil non-linearities. Good agreement between the computed response and the observed damage during the 2017 Puebla-Mexico earthquake reconnaissance was found. In addition, several normal and subduction events with a return period of 250 years were considered to evaluate the effect that frequency content, and strong ground motion duration have on the soil response variability. From the results gathered here, it was established the relevance of accounting for three-dimensional wave propagation fields to assess site effects at basin-edge zones properly and to be able to implement proper risk mitigation measurements at these zones.

Statistical characterization of full-margin rupture recurrence for Cascadia subduction zone using event time resampling and Gaussian mixture model

AbstractEarthquake occurrence modeling of large subduction events involves significant uncertainty, stemming from the scarcity of geological data and inaccuracy of dating techniques. The previous research on statistical modeling of full-margin ruptures of the Cascadia subduction zone attempted to address these issues. However, the adopted resampling method to account for the uncertain marine turbidite age data from the Cascadia subduction zone was not sufficient in the sample size. This study presents a statistical approach based on the Gaussian mixture model applied to significantly larger resampled Cascadia age data. The results suggest that the 3-component Gaussian mixture model outperforms the 2-component Gaussian mixture model and the 1-component renewal models by capturing the long gap and short-term clustering. The developed Gaussian mixture model is well suited to apply to probabilistic seismic and tsunami hazard analysis and the calculation of long-term probability of the future full-margin Cascadia events by considering the elapsed time since the last event.

Munabulake ophiolitic mélange: Implication for the evolution history of the north branch of the Proto-Tethys ocean

One of the ophiolites that record the Proto-Tethys Ocean’s episodic closure is the Munabulake ophiolitic mélange, which is situated in the middle of the Kunlun–Qaidam and Altun–Qilian blocks. Detailed field mapping revealed that the Munabulake ophiolitic mélange comprises local (ultramafic rocks, basalts, andesites, gabbros, diorites, and plagiogranites) and exotic (marble, gneiss, schist, and amphibolite) blocks, many of which are in the schist matrix (Qimantage Group). Based on geochronological, geochemical, and petrological observations, the mafic rocks in the Munabulake ophiolitic mélange can be categorized into three categories: 498-Ma OIB-like gabbros, 468-Ma Hawaiian alkaline basalt-like dolerite and pillow basaltic slices, and 428 Ma massive SSZ-like ultramafic rocks. The 501–452 Ma I-type granites exhibit arc affinities due to the oceanic crust subduction. These findings, along with spatial relationships, suggest that the Early Paleozoic ophiolite complex, island arc rocks, and accretionary complex generated as an intra-oceanic arc system as a result of obduction of the south Altun Ocean’s onto the Central Altun block within a north-directed subduction event. A dextral strike–slip was evident throughout the Early Paleozoic oceanic crust subduction based on the whole set of planar and linear structural data, and the subduction polarity was likely to the north. According to the ophiolitic mélange’s youngest rocks and the existence of 413 Ma granite dykes that are widely exposed in the Munabulake ophiolitic mélange, the Munabulake ophiolitic mélange was most likely emplaced during the Middle Silurian. This Munabulake ophiolitic mélange is similar in age and petrochemical characteristics to the other ophiolites in the South Altun subduction–collision belt, indicating that the Manabulak ophiolite mélange is a westward extension of the Apa–Mangya subduction–collision belt, which formed during the northward subduction of the South Altun Ocean slab during the Early Paleozoic. Thus, the final closing time of the South Altun Ocean is between 413 and 428 Ma.