Subduction Processes Research Articles

Geochronological and geochemical study of the Late Carboniferous volcanic drilling cores from the Piedmont of the Hala'alate Mountain in West Junggar: Implications for stratigraphic division and tectonic evolution

The West Junggar tectono-magmatic processes are critical for reconstructing the amalgamation history of the Central Asian Orogenic Belt (CAOB). However, the lack of consensus regarding the geochronology, petrogenesis, and tectonic setting of the complex igneous rocks poses a challenge to comprehending the Late Paleozoic tectonic evolution of West Junggar. This study focuses on systematic geochronological and geochemical research of well-preserved drill cores from the Piedmont of the Hala'alate Mountain, including rhyolite, felsic tuff, basalt, and andesite. The ages of these rocks in Late Carboniferous (310–301 Ma) are constrained by zircon UPb dating, excluding them from the Jiamuhe Formation of the Early Permian. The rhyolites and felsic tuffs exhibit apparent affinity to the A-type granite series, characterized by high total alkali and FeOT/MgO values. Plagioclase controls the main residual facies and depleted isotopic compositions (εHf(t) = +10.2 to +13.4; εNd(t) = +7.6 to +8.1), indicating that the magma source originated from the partial melting of juvenile continental materials in an extensional background. The basalts and andesites display high LILEs/LREEs, LILEs/HFSEs ratios, along with low HFSEs, which correspond to arc-affinity geochemical compositions. The high zircon εHf(t) (+7.5 to +14.3) and whole-rock εNd(t) (+7.8 to +7.9) values indicate a depleted pyroxene-rich mantle source modified by metasomatic slab-derived fluids. This implies a large-scale upwelling of mantle materials driven by the overlying lithospheric crustal thinning. Combining the temporal variations of crustal thickness recorded by the Sr/Y and (La/Yb)N ratios in igneous rocks, the tectonic processes of West Junggar during the Carboniferous to Early Permian can be divided into three stages. The first stage involves sustained subduction processes from 350 Ma to 320 Ma, followed by the closure of the oceanic crust and collision orogenesis from 320 Ma to 310 Ma. Finally, and a post-collision extension setting occurred after 310 Ma.

Initiation of Mesoarchean nascent subduction zone in the North China Craton

Exploring the nature of Archean plate tectonics is crucial for understanding the geodynamic evolution of early Earth. While previous studies have focused on (ultra-)mafic metavolcanic and tonalite−trondhjemite−granodiorite (TTG) rocks, the crust-mantle interactions of Archean intermediate magmatism have received much less attention. Mesoarchean (ca. 2936−2855 Ma) dioritic and tonalitic-trondhjemitic magmatism occurred in the Jiaobei terrane, North China Craton. While low-silica (65.67−68.82 wt%) TTG magmatism persisted throughout the entire magmatic period, both the high-silica (69.79−74.54 wt%) TTG and biotite dioritic (MgO of 1.52−2.45 wt%) magmatism were transient (ca. 2922−2901 Ma). They were mainly sourced from metabasaltic crust, though the biotite dioritic and low-silica TTG rocks show variable mantle input. In comparison, the hornblende dioritic magmatism (MgO of 2.07−5.00 wt%) occurred later (ca. 2912−2865 Ma) and was derived from melt-metasomatized mantle sources. Spatially, the magmatism became younger from northeast to southwest. Combined with increasing crustal melting pressures after ca. 2920 Ma and mildly elevated zircon δ18O (up to +6.8‰) of TTG rocks, a Mesoarchean nascent subduction zone is proposed for the Jiaobei terrane that is featured by the recycling of supracrustal basaltic rocks to thickened lower crust (forming high-pressure and high-δ18O TTG) and subsequent melt-related mantle metasomatism (forming relatively high-Mg hornblende dioritic rocks). The recycled supracrustal materials may have lubricated the basal part of the thickened crust and then triggered the opening of a nascent mantle wedge. Further compilation of data from global Archean dioritic and TTG rocks reveals chemical discontinuities at ca. 3.6−3.5 Ga and after ca. 3.0 Ga; these periods are marked by the occurrence of both high-Mg (median MgO >3 wt%) dioritic magmatism and high-(La/Yb)N (median >30) and high-δ18O (median >5.9‰) TTG magmatism. These data, along with independent geological evidence, suggest that the nascent subduction processes may have initiated locally at ca. 3.6−3.5 Ga but globally after ca. 3.0 Ga.

Multiple ridge subduction processes in the southern Altaids: Implications from clinopyroxene chemistry and Sr–Nd–Hf isotopes of late carboniferous Nb-enriched, magnesian diorite-andesites in West Junggar, NW China

Magnesian (Mg# > 45) and/or Nb-rich (Nb > 6 ppm) dioritic-andesitic rocks have been identified in the Mayile area of southern West Junggar, Northwest China. A systematic petrological, geochronological, whole-rock geochemical, Sr–Nd–Hf isotope, and clinopyroxene chemistry study was conducted to constrain the petrogenesis, mantle source, and P–T conditions of magmas as well as the tectonomagmatic processes in the southern Paleo-Asian Ocean subduction zone. Mayile dioritic-andesitic rocks generated in the Late Carboniferous (306–312 Ma) have a high SiO2 content (52.3–63.2 wt%) and high Mg# (46–66) as well as sanukitic-like low LaN/YbN and Sr/Y ratios. They have high Nb contents (6.27–11.6 ppm) and Nb/U (9–18) ratios and are Nb-enriched sanukitic high-Mg andesites (HMAs). Clinopyroxene thermobarometry indicates that the parental magma of the Mayile HMAs experienced a high-temperature (1137–1199 °C) and shallow-level (0.8–0.17 GPa) partial melting in a mantle wedge. They have εNd(t) (+5.14 to +6.42), εHf(t) (+8.8 to +14.4), and zircon εHf(t) (+9.4 to +15.8) values and a slightly high (87Sr/86Sr)t ratio ranging from 0.703214 to 0.704140 relative to normal mid-ocean ridge basalt (N-MORB). The results suggest that West Junggar was dominated by continuous records of sanukitic rocks hinting at multiple ridge subduction processes in the southern Altaids. We reconstructed the subduction history of the Junggar Ocean in the Carboniferous-Permian from these particular observations in West Junggar, and gained insight into the gradual evolution of the tectonic setting, which transitioned from normal intraoceanic subduction to hot subduction of the young oceanic lithosphere from the Early to Late Carboniferous, with subsequent ridge subduction occurring mainly in the Late Carboniferous.

Kimberlite eruptions driven by slab flux and subduction angle

Kimberlites are sourced from thermochemical upwellings which can transport diamonds to the surface of the crust. The majority of kimberlites preserved at the Earth’s surface erupted between 250 and 50 million years ago, and have been attributed to changes in plate velocity or mantle plumes. However, these mechanisms fail to explain the presence of strong subduction signatures observed in some Cretaceous kimberlites. This raises the question whether there is a subduction process that unifies our understanding of the timing of kimberlite eruptions. We develop a novel formulation for calculating subduction angle based on trench migration, convergence rate, slab thickness and density to connect the influx of slab material into the mantle with the timing of kimberlite eruptions. We find that subduction angles combined with peaks in slab flux predict pulses of kimberlite eruptions. High rates of subducting slab material trigger mantle return flow that stimulates fertile reservoirs in the mantle. These convective instabilities transport slab-influenced melt to the surface at a distance inbound from the trench corresponding to the subduction angle. Our deep-time slab dip formulation has numerous potential applications including modelling the deep carbon and water cycles, and an improved understanding of subduction-related mineral deposits.

Carbonate-silicate interaction in subducting slabs recorded by Zn isotopes in western Alps metasediments

Carbonate-silicate interaction within subduction channels influences the chemical and physical properties of subducting carbon and is crucial to understanding the fate of carbonates in subduction zones. However, geochemical evidence for such interaction is still limited. Here we investigate zinc isotopes (expressed as δ66ZnJMC-Lyon) to decipher potential carbonate-silicate interaction in subducting slabs given the remarkable Zn isotopic difference between silicate and carbonate components. A suite of high- to ultrahigh-pressure metasediments containing variable amounts of carbonates from the Schistes Lustrés nappe (western Alps) record subduction processes to various depths (∼15–90 km) in a cool geothermal condition (∼8°C/km). Despite a broad range of metamorphic grades (0.8–2.9 GPa and 300–630°C) and carbonate contents (0–49 wt%), the bulk metasediments have nearly constant δ66Zn values (av. 0.21±0.05‰, 2sd, n=12), which are indistinguishable from their putative protoliths (0.25±0.07‰, 2sd, n=5). The lack of systematic variation of bulk sediment δ66Zn with metamorphic grades implies a limited transfer of Zn during prograde metamorphism. Therefore, sediments will preserve their original Zn isotopic compositions after subduction into the mantle. By contrast, the carbonate components in sediments obtained by leaching exhibit a progressive decline in δ66Zn from 0.82±0.01‰ to 0.20±0.04‰ and a modest increase in Zn concentration with pressure. These variations are best interpreted as a result of prolonged closed-system Zn isotopic exchange between isotopically heavy carbonates and light silicate components in the subducting slab. Our results demonstrate that a considerable amount of carbonates in subducting sediments can be retained at least down to the depths of the sub-arc mantle (>90 km). The strong pressure-dependent Zn isotopic fractionation (R2=0.87) during carbonate-silicate interaction makes Zn isotopes a novel proxy for the storage depth of subducting carbonates.

Petrography, geochemistry and geochronology of igneous rocks from the Jiangnan Orogen, South China: constraints on the Early Paleozoic tectonic evolution of the South China Block

The Early Paleozoic tectonic evolution of the South China Block (SCB) remains controversial related to intracontinental orogenic and oceanic subduction processes. We present whole-rock major and trace elemental data, LA-ICP-MS zircon U-Pb age and Lu-Hf isotopic data for the Early Paleozoic igneous rocks including granodiorites from the Yuechengling pluton and volcanic breccias from the Damingshan pluton from the western segment of the Jiangnan Orogen in the SCB. LA-ICP-MS zircon U-Pb dating yielded emplacement ages for the Yuechengling S-type granitoids of 438–436 Ma and the deposited age for the Damingshan volcaniclastic rocks to be later than 451 Ma. The Yuechengling granitoids have consistent SiO2, TiO2, Fe2O3T, MgO, and P2O5 contents, higher Al2O3 contents, and Na2O + K2O values, but lower Mg# values, compared with those of the Damingshan volcaniclastic rocks. All the studied samples exhibit enrichment in LREEs and moderate negative Eu anomalies (δEu = 0.63–0.75), with negative Ba, Sr, Nb, Ta, P, and Ti anomalies, and positive Rb, Th, U, Pb, and K anomalies. The granitoids have variable CaO/Na2O ratios of 0.22–1.11, negative εHf(t) values of −11.98 to −0.90, and corresponding TDM2 ages distributed from 2.37 to 1.55 Ga. The petrographic and geochemical characteristics of the Yuechengling granitoids indicate that their parental magma was derived from a crustal meta-greywacke and meta-pelite components in the Paleoproterozoic basement, and have undergone some degree of fractional crystallization. The volcaniclastic rocks have mostly negative εHf(t) values with partially positive (−27.54 to 8.73), and zircons with negative εHf(t) values (−27.54 to −0.14) show TDM2 ages of 3.79 to 1.63 Ga. Combined with petrographic and geochemical data, we suggest that the Damingshan volcaniclastic rocks were derived from Neoarchean-Neoproterozoic crustal materials and the felsic parental magma has undergone some degree of magma mixing with mantle material, and deposited soon after a Late Ordovician volcanic eruption (later than 451 Ma). Integrated with previous studies, our new data support the intracontinental orogenic model to account for the Early Paleozoic tectonic evolution. Thus, we suggest that the Early Paleozoic tectonic setting of the SCB was intracontinental orogeny rather than oceanic subduction-collision.

Boron Isotopic Systematics and Its Significance in Natural Processes: An Overview

In recent years, interest in boron has expanded from microscopic to macroscopic levels, and several studies have contributed to understanding the role of boron in earth and natural processes. The boron isotopic composition provides a unique perspective into the crystallization process in granites, pegmatites, and temperature variations. Boron isotopic studies have been used as a tracer to understand geothermal systems, rivers, rock processes, reconstruction of pH and pCO2, groundwater pollution, and further help in understanding the changes which have occurred in oceans through geological time. Furthermore, boron isotopes have also been utilized to understand the genesis of ores and understanding subduction processes and as a tracer in groundwater pollution. In plants, it acts as a micronutrient. However, its deficiency and the excessive amount may inhibit the growth of plants, bacteria, and fungi and may also affect the soil and aquatic microflora. Boron maintains and regulates several metabolic pathways, and its quantity above a certain level may prove detrimental to the environment. This overview explains boron isotope variations and their implications in earth sciences and natural processes.

Multichannel seismic evidence for tectonic migration in the Mariana subduction system

AbstractThe Mariana subduction system is an ideal place for ocean–ocean convergence boundary research. As a typical example of an extensional subduction model, the subduction process, subduction mechanism and subduction effect of this system have long been popular research topics. The seismic reflection information of oceanic sedimentary layers shown on multichannel seismic profiles is an intuitive record of small‐ to medium‐scale structural activity and sedimentary sequence evolution and is an important source for studying plate‐coupling processes and tectonic dynamic action. This paper is based on the raw multichannel seismic data of MGL1204 in the Mariana arc forearc region; these data were obtained by using linear interference filter technique noise attenuation technology and multiple velocity iterations to accurately image the oceanic sedimentary layer, and the imagery is used to study the Cenozoic fracture activity and residual strata thickness distribution characteristics of the middle part of the Mariana forearc region and to explore the tectonic migration characteristics of the Cenozoic sedimentary basins. We believe that due to the influence of regional stress field changes caused by the movement directions of the Pacific Plate and Philippine Sea Plate towards the Eurasian Plate, the development of secondary structures in the western Pacific subduction zone shows a characteristic west‐to‐east migration, and the structural characteristics of the Mariana forearc Cenozoic sedimentary basin are consistent with the regional tectonic features. The faults, stratigraphy and depocentres of the oceanic sedimentary layer show a migration pattern from west to east and from north to south overall; 25 and 13 Ma are identified as two key tectonic migration periods, and the tectonic migration characteristics and time records of the basin also provide direct evidence for the plate reconstruction model of trench retreat and Philippine Sea plate rotation.

On unusual conditions for the exhumation of subducted oceanic crustal rocks: How to make rocks hotter than models

The thermal structure of subduction zones controls many important processes such as metamorphic devolatilization, arc magmatism, and seismicity. However, the thermal state of subducted oceanic slab predicted by subduction zone thermal models down to ∼80 km depth is systematically cooler than inferred from exhumed metamorphic slab rocks, raising questions about the current understanding of subduction dynamics portrayed by these models. Upon synthesizing previous petrological studies and estimating slab ages of ancient subduction zones from metamorphic terranes, we think that exhumation of subducted oceanic metamorphic rocks is extremely rare and likely requires unusual processes that are not an integral component of normal subduction dynamics. We construct simple numerical scenarios to illustrate how the thermal regime under some unusual conditions at the beginning and ending stages of a subduction margin may deviate from the normal subduction process. At the beginning stage of subduction, a shallow maximum decoupling depth (MDD) between the slab and mantle wedge enables viscous mantle wedge flow to reach a shallow depth, bringing heat to make slab rocks warmer than in a mature subduction zone. At the ending stage of subduction, if subduction cessation is in the form of slab stalling and if the stalled slab is not immediately exhumed, the slab rocks can be warmed up by the heat from the warm mantle wedge and underlying asthenosphere. In either situation, the slab rocks can be much warmer than normal before they are exhumed by other dynamic processes. Observed exhumed rocks are not expected to represent the normal subduction process and should not be directly compared with thermal models that are designed for the normal process. To extract important information about general geodynamic processes from these rocks, we must first understand the processes these rocks went through.

Granitoids indicating moderate to high heat production associated with oceanic subduction system in the NE Tibetan Plateau

Regional heat flow provides a direct surface indication of the thermal state and energy balance of the lithosphere. Heat flux in the Tibetan Plateau remains poorly studied due to the lack of adequate information on heat flow. In this study, we investigated granitoids from the Gonghe-Guide district located in the NE Tibetan Plateau, which have important significance as hot and dry rocks with potential for geothermal resources. Samples from granitoid rocks as well as felsic dikes were collected from this area, and their geochemical data were combined with regional geophysical and drilling core data to evaluate a high-heat-flow anomaly. Our data show that granitoid rocks, including granodiorite, monzogranite, and syenogranite, crystallized at ca. 253−240 Ma, while felsic dikes formed ca. 230 Ma. The former were emplaced during the oceanic subduction process, whereas the latter formed in the syncollisional stage associated with the closure of the paleo-Zongwulong Ocean. Geochemically, these granitoid rocks are metaluminous to slightly peraluminous, with an average differentiation index (DI) value of 80, and they are classified as weakly to moderately fractionated I-type granites. The felsic dikes are peraluminous with high DI values (average of 95), typical of highly fractionated I-type granites. In terms of their high K and especially U and Th abundances, the calculated radioactive heat generation produced by the granodiorite, monzogranite, syenogranite, and felsic dikes is 1.85, 2.91, 6.85, and 3.02 μW/m3, respectively. Their moderate to high heat production generates a local heat-flow anomaly of 14.8−22.2 mW/m2 above the regional value of subduction zones, accounting for 11%−18% of the total regional heat flow. In combination with regional magnetotelluric data, the high-heat-flow anomaly may be attributed to an additional heat-flow contribution from a partial melt layer beneath this region. Furthermore, there is a significant positive correlation between the radioactive heat generation and magmatic fractionation, indicating that the enrichments of heat-producing elements are controlled by fractional crystallization and subsequently source composition. The granitoid rocks with high heat generation correspond to the large-scale intermediate-felsic magmatism during the subduction stage. We propose a model wherein a continuous subduction process during the closure of the paleo-Zongwulong Ocean and protracted cooling and crystallization processes resulted in the enrichment in heat-producing elements.

Metasomatized mantle under the Eastern Mexican Alkaline Province: evidence from the Oligocene Rincón Murillo Gabbro, Sierra de San Carlos-Cruillas, NE México

The Cenozoic Sierra de San Carlos-Cruillas (SSCC) magmatic complex belongs to the Eastern Mexican Alkaline Province (EMAP). The SSCC mainly consists of plutonic and subvolcanic bodies displaying diverse lithological compositions. These rocks exhibit both OIB- and arc-like geochemical characteristics. In this contribution, we present new petrographic, geochemical, isotopic, and geochronological data of the Rincón Murillo Gabbro (RMG), situated in the southern part of the SSCC. The RMG is nepheline normative and displays an OIB-like intraplate character. Laser ablation ICP-MS U-Pb analyses on titanites yielded Oligocene (Rupelian) crystallization ages of 31.8 ± 1.8 and 31.0 ± 1.7 Ma. The 87Sr/86Sr(i) (0.703400–0.704159) and 143Nd/144Nd(i) (0.512732–0.512815) ratios indicate a mantle origin. Trace element data indicate the participation of different enriched mantle components in the source. One component is related to previous subduction processes, generating the HFSE-depleted arc-like magmas of San José Gabbro in the northern part of SSCC. The second component is related to deep mantle melts with a slightly carbonatitic signal and HFSE-enriched fluids derived from the subducted Hess conjugate plateau generating different enriched sources in the upper mantle under the SSCC. The passage, eclogitization, detachment and foundering of the Hess plateau gave rise to an asthenospheric upwelling that triggered the low to moderate partial melting degrees (3–15%) of an amphibole-bearing garnet peridotite, generating the OIB-like parental magmas of RMG, located in the southern part of SSCC. Two-component mixing, and AFC models indicate that variable degrees of crystal fractionation and minor crustal assimilation are involved in the evolution of Rincón Murillo Gabbro.

Teleseismic traveltime tomography of the Andaman subduction zone

The Andaman subduction zone is characterized by great earthquakes and strong tectonic activity, which are closely related to the subduction of the Indian plate. To gain insight into the subduction processes, the geometry of the downgoing Indian plate must be investigated. At present, the geometry of the subducting Indian plate remains unclear owing to the historical lack of a seismic network deployed in the Andaman Sea and the surrounding area. In this study, we employed a teleseismic traveltime tomography method to image the deep structures beneath the Andaman Sea. The principle of the tomography method is that regional earthquakes are regarded as seismic stations, and seismic stations at teleseismic distances are treated as teleseismic events based on the reciprocity theorem. With this type of teleseismic traveltime tomography, we selected a total of 179 regional earthquakes that were recorded by 4515 stations, and we manually selected 40,369 high-quality P-wave first arrivals. From the inversion of these seismic traveltime data, we obtain P-wave images at depths from 300 km to 700 km. Our tomographic images reveal three distinct features. First, we observe high-velocity anomalies beneath the Andaman Sea at depths of 300–700 km. The orientation of the high-velocity anomalies is nearly consistent with that of the Andaman trench and volcanic arc. The high-velocity anomalies in the upper mantle are laterally discontinuous; a gap at approximately 9°N divides the high-velocity anomalies into two parts. Second, high-velocity anomalies are found in the mantle transition zone (MTZ) below the Andaman trench. Third, strong low-velocity anomalies are detected below the volcanic group in the Mergui Basin. Based on our tomographic results, we infer the currently subducting Indian plate beneath the Andaman Sea, an ancient slab segment in the MTZ, and upwelling hot materials below the Mergui Basin.

Late Cryogenian–Ediacaran magmatism in southern Borborema Province, NE Brazil: Ages, sources, petrogenesis, and tectonic setting

The Sergipano Orogenic System (SOS) in southern Borborema Province (NE Brazil) hosts voluminous Neoproterozoic plutonism related to the Brasiliano/Pan-African Orogeny. This paper presents comprehensive whole-rock geochemical data, titanite U-Pb ages, and the first combined zircon U-Pb and Lu-Hf isotope results for plutonic rocks from the Macururé Domain to constrain their sources, petrogenesis, and tectonic setting. Three magmatic episodes are recognized and record the evolutionary stages of the orogen. (i) Early-collisional magmatism (643–628 Ma) comprises gabbros and diorites with minor tonalites characterized by well-developed tectonic foliation and evidence of solid-state deformation. These rocks are magnesian, high-K calc-alkaline, LILE- and LREE-enriched and provide subchondritic εHf(t) values (−6.5 to −4.7) and Orosirian Hf-TDMC model ages (1.83–1.94 Ga). Such features indicate derivation from a lithospheric mantle source metasomatized by incorporating crustal components through subduction processes prior to magma generation, possibly related to the Rhyacian Orogeny (2.20–1.96 Ga). Extensive mixing/mingling between basaltic and crust-derived magmas took place at lower crustal depths, producing coeval hybrid diorites and quartz-diorites. (ii) Syn-collisional magmatism (630–624 Ma) encompasses biotite- and muscovite-bearing granodiorites and monzogranites, preserving their structures parallel to the schistosity of the country rocks. These rocks are leucocratic, weakly metaluminous to peraluminous, and contain abundant surmicaceous enclaves. Petrographic features and geochemical composition suggest an origin related to the partial melting of graywacke protoliths with a subordinate igneous component. (iii) Late-collisional magmatism (625–600 Ma) includes undeformed and isotropic monzonites and granodiorites, which truncate the regional foliation. These rocks are consistently metaluminous and magnesian, showing affinities with the high-K calc-alkaline to shoshonite series. Trace element modeling with subchondritic εHf(t) values (−8.3 to −4.1) and Paleoproterozoic Hf-TDMC model ages (1.77–2.03 Ga) demonstrate that reworking of ancient lower mafic crust played an important role at this time. The integration of our data with previously published results leads us to conclude that the geodynamic evolution of the SOS along the western Gondwana margin is better explained by large-scale lithospheric extension followed by basin inversion and continental collision.

Low‐Temperature Thermochronological Perspective on Geodynamic Evolution of the Cathaysia Block Since Early Mesozoic

AbstractMesozoic intrusive rocks are extensively outcropped in the Cathaysia Block (CB), indicating that they underwent significant exhumation after being formed. However, tectonothermal evolution of the CB during Mesozoic–Cenozoic times is still poorly constrained and associated geodynamic mechanisms driving the regional exhumation remain elusive. Toward this end, we present first zircon and apatite (U‐Th)/He data of eight Mesozoic granitic plutons distributed across the intracontinental CB. Our new dating results are integrated with a compilation of regional low‐temperature thermochronological data to determine the CB evolution in a tectonic and topographic evolution framework. Zircon and apatite (U‐Th)/He central ages of the eight granitoids range from 146 to 30 and 82 to 31 Ma, respectively, implying a long‐lasting exhumation of the intracontinental CB. Inverse thermal modeling of the thermochronological data for the eight plutons indicates that the intracontinental CB underwent three exhumation phases at 150–110, 110–85, and 66–38 Ma, of which the former two exhumation phases were prolonged and significant. A compilation of regional thermochronological data reveals a propagating locus of fast exhumation phase from the intracontinental CB to the seaward epicontinental CB over time. Combined with other geological evidence, we infer that primary exhumation events of the CB resulted from changing subduction processes of the Paleo‐Pacific Plate, which include slab break‐off and foundering in the Late Jurassic, progressive slab retreat in the Early Cretaceous, and normal subduction in the Late Cretaceous, with minor exhumation events presumably triggered by the Paleogene opening of the South China Sea Basin.

Index active tectonic relative of Karangsambung geopark area, Central Java, Indonesia: assessment for developing geological disaster-based areas

Karangsambung as a “subduction fossil” of the Cretaceous-Paleocene on the island of Java, has a complex geological structure. The current subduction process in the south of Java with a north-south compression force has contributed to the reactivation of the geological structure in Karangsambung. The index active tectonic relative (IAT) of the Karangsambung area reflects the current level of tectonic activity. IAT is obtained from quantitative calculations of geomorphic indices, including streams length - gradients index (Sl), the ratio of valley floor width to valley height (Vf), basin asymmetry (Af), basin shape (Bs). mountain front sinuosity (Smf), the hypsometric integral (HI), and drainage density (Dd). The index active tectonic relative (IAT) calculation with an average IAT value of 2.37 indicates that tectonic activity is in the medium/moderate category. In addition, an attempt was made to compare the regional neotectonic activity with the relative tectonic activity of the study area. Based on this geomorphic analysis, the study area may have a lower seismic risk but needs to be evaluated more carefully for regional seismic hazards. The IAT value can be used as an initial reference for geological disaster assessment, especially earthquakes hazard in the development of the Karangsambung National Geopark Area.

Impact of Solid Hydrocarbon on the Composition of Fluid Phase at the Subduction (Experimental Simulation)

Experiments conducted in the olivine–serpentine–anthracene–metal (FeNi) system have shown that the recrystallization of olivines occurs under substantially reduced conditions with active participation of hydrocarbons, especially paraffins; moreover, their amount increases with increasing pressure and temperature. During the decomposition of serpentine, a large amount of water is released; therefore, the fluid at relatively low P-T parameters (2 GPa, 1100 °C) has mainly water–hydrocarbon composition. With an increase in pressure up to 3–4.5 GPa and temperature up to 1300–1400 °C, the composition of the fluid changes greatly towards an increase in the relative amount of hydrocarbons, while the main share is occupied by light (C1–C4) aliphatic hydrocarbons. Therefore, a biogenic material with a carbon–hydrogen composition can make a certain contribution to the carbon budget in subduction processes when falling into the subduction zones and may affect the oxygen fugitivity in the subducted slab.

Precambrian history of the Pacific Mantle Domain: New constraints from Woodsreef and Port Macquarie serpentinized spinel harzburgites of the New England Orogen, Australia

Abstract The present-day mantle is divided into the African and Pacific domains by the circum-Pacific subduction girdle. Very little is known about the mantle composition of the Pacific Domain before 120 Ma due to the scarcity of the oceanic record, having mostly been destroyed by subduction processes. Accreted oceanic lithosphere (ophiolites) in orogens along the Paleo-Pacific margins provide rare opportunities to partially fill this knowledge gap. The early Cambrian (530–505 Ma) Weraerai ophiolite mélange in the New England Orogen in Eastern Australia represents fragments of the now-consumed Paleo-Pacific oceanic plate, predecessor of the Panthalassa and Pacific oceanic plates, accreted to east Gondwana during ca. 410–277 Ma. Early work revealed the presence of an accreted volcanic island(s) of possible mantle plume origin. However, due to their heavy alteration and weathering, the geochemical signature of the mafic rocks in the Weraerai ophiolite cannot be used to certify their plume origin with confidence. Therefore, mantle rocks found in the ophiolitic belt offer an alternative way to decipher the origin of the oceanic lithospheric fragments in the Weraerai ophiolite mélange. Here we report the petrographic, major, and trace elements composition, including highly siderophile elements (HSE), and Re-Os isotope composition for 16 serpentinized spinel harzburgite samples from Woodsreef and Port Macquarie. The observed spinel-orthopyroxene symplectite intergrowth textures are interpreted as garnet-breakdown textures due to cooling from an initial high temperature of >1200°C. Silicon and Al contents and Mg# of serpentinized spinel harzburgite, as well as heavy rare earth element modelling results, suggest a high-degree of melt extraction of 20–30% in the garnet stability field and in an anhydrous environment, probably in ocean island or oceanic plateau environments. The samples in this study have HSE concentrations interpreted to have resulted from post-melting processes rather than a melt extraction feature, indicating that their Re-Os model ages need to be interpreted with caution. Nevertheless, the melt depletion ages obtained from the Woodsreef and Port Macquaries samples range between 1.4 and 1.1 Ga, consistent with previous studies on other Pacific-rim ophiolitic rocks. We argue that these ages might be related to a major depletion event during the transition between supercontinents Nuna and Rodinia. Such depletion events affected a large proportion of this section of the mantle before the incorporation of the peridotites into the oceanic lithosphere in the Paleo-Pacific associated with rapid ascent of mantle plumes. This interpretation is consistent with the occurrence of accreted volcanic islands in the Werearai ophiolite mélange, as shown by the OIB chemical signatures of some of the mafic rocks, and their association with shallow water limestones that formed in near-equatorial latitudes. The proposed ca. 530–510 Ma Paleo-Pacific Ocean mantle plume event coincides with a global peak of oceanic mantle plume events that may record the legacy of a circum-Rodinia subduction girdle driving antipodal mantle superplume episodes.

Characterizing the porosity structure and gas hydrate distribution at the southern Hikurangi Margin, New Zealand from offshore electromagnetic data.

The dynamics of accretionary prisms and the processes that take place along subduction interfaces are controlled, in part, by the porosity and fluid overpressure of both the forearc wedge and the sediments transported to the system by the subducting plate. The Hikurangi Margin, located offshore the North Island of New Zealand, is a particularly relevant area to investigate the interplay between the consolidation state of incoming plate sediments, dewatering and fluid flow in the accretionary wedge and observed geodetic coupling and megathrust slip behaviour along the plate interface. In its short geographic extent, the margin hosts a diversity of properties that impact subduction processes and that transition from north to south. Its southernmost limit is characterized by frontal accretion, thick sediment subduction, the absence of seafloor roughness, strong interseismic coupling and deep slow slip events. Here we use seafloor magnetotelluric (MT) and controlled-source electromagnetic (CSEM) data collected along a profile through the southern Hikurangi Margin to image the electrical resistivity of the forearc and incoming plate. Resistive anomalies in the shallow forearc likely indicate the presence of gas hydrates, and we relate deeper forerarc resistors to thrust faulting imaged in colocated seismic reflection data. Because MT and CSEM data are highly sensitive to fluid phases in the pore spaces of seafloor sediments and oceanic crust, we convert resistivity to porosity to obtain a representation of fluid distribution along the profile. We show that porosity predicted by the resistivity data can be well fit by an exponential sediment compaction model. By removing this compaction trend from the porosity model, we are able to evaluate the second-order, lateral changes in porosity, an approach that can be applied to EM data sets from other sedimentary basins. Using this porosity anomaly model, we examine the consolidation state of the incoming plate and accretionary wedge sediments. A decrease in porosity observed in the sediments approaching the trench suggests that a protothrust zone is developing ∼25km seaward of the frontal thrust. Our data also imply that sediments deeper in the accretionary wedge are slightly underconsolidated, which may indicate incomplete drainage and elevated fluid overpressures of the deep wedge.

Provenance and Tectonic Setting Discrimination and Their Effects on the Pore-Scale Fluid Flow of Black Shale: Take the Micangshan Tectonic Belt, Central China, as an Example

The Lower Paleozoic reservoir in the Micangshan tectonic belt is a new shale gas exploration area with excellent potential. However, the black shale’s sedimentary environment and tectonic setting and their effects on pore-scale fluid flow have not been thoroughly elucidated and urgently require further research. A large amount of trace and rare earth element data of black shale in this area was tested to analyze the sediment’s provenance, tectonic setting, and coupling process, and nuclear magnetic resonance was applied for fluid flow ability determination. The characteristics of trace elements and rare earth elements in shale samples were drilled from the Upper Sinian Doushantuo Formation, the Lower Cambrian Niutitang Formation, and the Lower Silurian Longmaxi Formation in the Micangshan tectonic belt. The results reveal that the provenance of the shales was dominated by island arc volcanic rocks and some ancient fine-grained sediments. The provenances of shale in the Micangshan, Huijunba, and Zhenba areas may have a strong genetic relationship with the igneous rocks from the nearest Hannan massif. The provenance of the Niutitang shale in the Ningqiang area may comprise sediments derived from the Longmenshan continental rift system. The provenance of the shale tends to transition from the island arc to the continental margin from the depositional stage of the Doushantuo Formation to the Longmaxi Formation. The Micangshan tectonic belt inherited the tectonic evolution of the South China block, North China block, and Qinling, and those blocks provided the igneous provenance of the shale by island arcs, which were formed due to weathering caused by the process of subduction and collision orogeny. The movable fluid distribution of the Longmaxi Formation and Niutitang Formation is relatively uniform, and excessive pursuit of large amount of movable fluid will be counterproductive to the shale reservoir. The proportion of the cracks was generally less than 3% except for one sample from the Doushantuo Formation with 12.06%. Based on the centrifugation force in this research, the low fluid flow limits were less than 1.47, and there are strong positive relationships between movable fluid saturation and saturated porosity. In this study, the coupling relationship between the material source and pore scale flow is discussed for the first time, and the results can provide theoretical references for local provenance analysis and tectonic movement.

Coupling between Cenozoic extensional exhumation in North China and the subduction of the Pacific Plate

Both the timing and mechanism for the formation of the topographic stepping in East Asia along the North-South Gravity Lineament (NSGL) are critical to understand the coupled influence of tectonics, climate, and surface processes on landscape evolution. The Yanshan Mountains lie in the central portion of the NSGL and offer a chance to reveal the link between surficial tectonic deformation and the underlying subduction to understand the continental dynamics and paleoclimate of NSGL. In this study, we report 22 new apatite (UTh)/He (AHe) data from the Yanshan Mountains. Age-elevation profiles indicate a period of rapid exhumation initiated at ∼27–26 Ma, while the AHe ages generally increase with the distance from the fault coincides with northwestward tilting of the fault footwall. Combining a series of shallow extensional events in East Asia with deep process of the Pacific plate subduction, we suggest that the widely-distributed late Oligocene rapid exhumation represents a phase of Cenozoic re-destruction of the North China Craton (NCC). The Cenozoic re-destruction of the NCC and topographic stepping along the NSGL may be related to the retreat, dehydration, and decarbonization of the underlying stagnant Pacific slab.