Tectonic Regime Research Articles

Hard classification of polyphase fault-slip data: Improvement and application

The quickest approach to the separation of polyphase fault-slip data into single-phase subsets is perhaps the recognition in stress space of the hyperplanes, consisting of their corresponding datum vectors, whose normals are parallel to the solutions of the stress vector. However the uncertainty of the solution sign makes it impossible to distinguish the subsets with respect to the opposite real stress vectors. In order to overcome this drawback the existing hard-classification method was improved by incorporating the slip senses during the separation. A series of artificial two-phase fault-slip datasets with the measurement errors generated under the imposed stresses were taken to test the improved method. There is a high accuracy of the stress solution even when the measurement errors have a range of [-12°, 12°]. The new method is further applied to data of shear bands measured on the limbs of the NNE-trending syncline at the Shihtipin resort, east of Taiwan. Two stress estimates representing the strike-slip or compressional tectonic regimes were obtained on each limb. They indicate an anticlockwise rotation of the maximum principal stress axis from ESE–WNW or SE–NW to E–W during the folding, probably in response to the oblique collision between the Luzon volcanic arc and the South China continent.

The Western Gulf of Corinth (Greece) 2020–2021 Seismic Crisis and Cascading Events: First Results from the Corinth Rift Laboratory Network

AbstractWe investigate a seismic crisis that occurred in the western Gulf of Corinth (Greece) between December 2020 and February 2021. This area is the main focus of the Corinth Rift Laboratory (CRL) network, and has been closely monitored with local seismological and geodetic networks for 20 yr. The 2020–2021 seismic crisis evolved in three stages: It started with an Mw 4.6 event near the northern shore of the Gulf, opposite of Aigion, then migrated eastward toward Trizonia Island after an Mw 5.0 event, and eventually culminated with an Mw 5.3 event, ∼3 km northeast of the Psathopyrgos fault. Aftershocks gradually migrated westward, triggering another cluster near the junction with the Rion–Patras fault. Moment tensor inversion revealed mainly normal faulting; however, some strike-slip mechanisms also exist, composing a complex tectonic regime in this region dominated by east–west normal faults. We employ seismic and geodetic observations to constrain the geometry and kinematics of the structures that hosted the major events. We discuss possible triggering mechanisms of the second and third stages of the sequence, including fluids migration and aseismic creep, and propose potential implications of the Mw 5.3 mainshock for the seismic hazard of the region.

Tectonic Stress Distribution in the Song Tranh 2 Hydropower Reservoir: Implication for Induced Earthquake

The Song Tranh 2 hydropower reservoir was built in Tra My area, Quang Nam province, composing magmatic and high-grade metamorphic rocks of the northern part of the Kon Tum massif. Since the reservoir was put into operation, induced earthquakes have occurred in the Song Tranh 2 hydropower reservoir and its vicinity. Tectonically, the northwest-southeast to east-west striking faults developed strongly. Detailed analysis of slickensides and attitude of faults occurring in the studied area have shown that the northwest-southeast striking faults are reactivated as dextral ones during the Pliocene-Quaternary up to the present day. Based on the geometric distribution of the fault network, kinematic characteristics, and the youngest tectonic stress regime, we computed the distribution of tectonic stress in the studied area. Computation results show two positive anomalies of stress directly related to the northwest-southeast faults numbered 2, 10, 11a, 11b and sub-latitude striking fault numbered 1. These faults run in line with the local river channels and are likely to reactivate and generate induced earthquakes.

Melting of metasomatically enriched lithospheric mantle – Constraints from Pan-African monzonites (Damara Orogen, Namibia)

The 560.8 ± 2.4 Ma old Okamutambo Pluton is the easternmost intrusion of the syn-collisional Otjimbingwe Alkaline Complex (OAC; Damara Belt, Namibia) and consists of mafic to intermediate alkaline rocks that belong to the monzodiorite-syenite series. The studied samples are potassic (K2O/Na2O > 1) and have moderately high magnesium (MgO: 5.7–2.6 wt%), nickel (Ni: 66–26 ppm), and chromium (Cr: 223–62 ppm) concentrations. LILE (Ba: 2425–1243 ppm; Sr: 1359–941 ppm) and HFSE (Zr: 447–202 ppm, Nb: 32.8–16.4 ppm, Hf: 4.7–9.4 ppm, Ta: 1.2–2.0 ppm) contents are also high. Strontium and Nd isotope data reveal the existence of two magmatic suites indicating a multi-source origin. Group 1 monzonites-quartz monzonites have moderately evolved Sr and Nd isotopic compositions (initial 87Sr/86Sr: 0.7066 to 0.7073; initial εNd: −3.5 to −5.0) and radiogenic Pb isotope ratios (206Pb/204Pb: 17.65–18.02; 207Pb/204Pb: 15.62–15.67; 208Pb/204Pb: 38.19–38.32). In contrast, group 2 monzodiorites-syenites display more evolved Sr and Nd isotopic compositions (initial 87Sr/86Sr: 0.7088 to 0.7090; initial εNd: −6.7 to −7.1) but similar Pb isotope ratios (206Pb/204Pb: 17.63–17.82; 207Pb/204Pb: 15.64–15.66; 208Pb/204Pb: 38.21–38.37). Differentiation involved AFC processes in group 1 monzonites-quartz monzonites whereas group 2 monzodiorites-syenites were modified by fractional crystallization. Although second-order processes were operative, high total alkali contents, incompatible trace element concentrations in excess of bulk crustal values, and evolved isotopic compositions of the most primitive samples are source-controlled and provide insight into the origin of the parental magmas. With reference to experimental data from the literature, it is inferred that the Okamutambo alkaline rocks represent evolved melts that were generated through melting of enriched lithospheric mantle (phlogopite-lherzolite). The observed negative Nb-Ta and Ti anomalies and positive Pb anomalies in primitive mantle-normalized trace element patterns are in line with a mantle source that contains a recycled crustal component. Isotopic compositions indicate that mantle enrichment is an ancient feature that might be linked to Proterozoic subduction. At 561 Ma, the geodynamic regime in the Damara Orogen was mainly characterized by compression during convergence and continental collision between the Congo and Kalahari cratons which is difficult to reconcile with the generation of the OAC as alkaline magmatism is commonly associated with extensional tectonic regimes. The OAC is, however, associated with a major suture zone that may have been involved in localized transtensional tectonics during oblique flat subduction and thus enabled the generation and ascent of mantle-derived alkaline melts.

Recycling of continental crust in the southern North China Craton: Constraints from the Sr–Nd–Pb–Hf–O isotopic compositions of Early Cretaceous Funiushan granites

This paper reports new zircon U–Pb ages, whole-rock geochemistry, and Sr–Nd–Pb–Hf–O isotopic compositions for the Funiushan (FNS) pluton of the southern North China Craton (NCC). These data and statistical analyses of previously reported compositions of late Mesozoic granites from the southern NCC are used to constrain late Mesozoic continental recycling and geodynamic processes. The FNS pluton includes coexisting early Early Cretaceous (140 Ma), late Early Cretaceous (130–116 Ma) and highly evolved granites (HEGs; 126–125 Ma). The early Early Cretaceous granites have high Sr contents (332–566 ppm), high Sr/Y (53–138) and (La/Yb)N (29–52) ratios, and low MgO contents (0.15–0.48 wt%). They have relatively low (87Sr/86Sr)i values (0.7051–0.7054), relatively high εNd(t) values (−4.11 to −3.96) and variable εHf(t) values (−22.9 to −0.53). They are also characterized by high initial 206Pb/204Pb values (17.893–17.975) and relatively low δ18O values (5.34‰–7.48‰) as well as Neoproterozoic (851–734 Ma) inherited zircons, suggesting that they have an adakitic affinity formed by recycling of subducted continental crust of the Yangtze Craton (YC) with input from a mantle component. In contrast, the late Early Cretaceous granites have relatively high (87Sr/86Sr)i values (0.7077–0.7085), low εNd(t) values (−14.2 to −12.0), relatively high initial 206Pb/204Pb values (17.398–17.497), εHf(t) values of −23.7 to −10.7, and δ18O values of 5.76‰–6.55‰. These data, and the Neoproterozoic zircon ages (716–702 Ma), indicate that the primary magmas formed from recycled YC crust with a minor contribution from the NCC basement. The late Early Cretaceous HEGs have relatively high εNd(t) values (−9.33 to −8.70), high initial 206Pb/204Pb values (17.656–17.885), intermediate εHf(t) values (−12.3 to −6.18), and relatively low δ18O values (5.22‰–6.72‰), indicating that they formed from subducted YC and mantle material. Previous work on granites of the southern NCC has established two phases of granite formation. Late Jurassic to early Early Cretaceous I-type granites (160–130 Ma) occur throughout the southern NCC. These are adakitic and formed from thickened lower continental crust (LCC). Late Early Cretaceous A-type granites (130–110 Ma) formed from thinned LCC. The differences between Late Jurassic to early Early Cretaceous granites and late Early Cretaceous granites reflect a transformation in tectonic regime from thickening to thinning, which is attributed to rollback of the subducting Paleo-Pacific Plate.

Genesis of carbonatite and associated U–Nb–REE mineralization at Huayangchuan, central China: Insights from mineral paragenesis, chemical and Sr-Nd-C-O isotopic compositons of calcite

The Huayangchuan deposit in the North Qinling alkaline province of Central China is a unique carbonatite-hosted giant U–Nb–REE polymetallic deposit. The mineralization is characterized by the presence of betafite, monazite, and allanite as the main ore minerals, but also exhibit relatively high budgets of heavy rare earth elements (HREE = Gd–Lu and Y). The origin of carbonatites has long been controversial, thus hindering our understanding of the genesis of the deposit. Here, we conducted an in-situ trace elemental, Sr–Nd isotopic, and bulk C–O isotopic analyses of multi-type calcites in the deposit. Two principal types (Cal-I and Cal-II), including three sub-types (Cal-I-1, Cal-I-2 and Cal-I-3) of calcites were identified based on crosscutting relationships and calcite textures. Texturally, Cal-I calcites in carbonatites display cumulates with the grain size decreasing from early coarse- (Cal-I-1) to medium- (Cal-I-2) and late fine-grained (Cal-I-3), whereas Cal-II calcites coexist with zeolite displaying zeolite–calcite veinlets. Geochemically, Cal-I calcites contain relatively high REE(Y) (151–2296 ppm), Sr (4947–9566 ppm) and Na (29–390 ppm) contents, characterized by right- to left-inclined flat distribution patterns [(La/Yb)N = 0.2–4.2] with enrichment of HREE(Y) (136–774 ppm), whereas Cal-II calcites display low REE, Sr and undetectable Na contents, characterized by a right-inclined distribution pattern [(La/Yb)N = 13.5, n = 16]. The U–Nb–REE mineralization, accompanied with intense and extensive fenitization and biotitization, is mainly associated with the Cal-I-3 calcites which show flat to relatively left-inclined flat REE distribution patterns [(La/Yb)N = 0.2–1.0]. Isotopic results show that Cal-I calcites with mantle signatures are primarily igneous in origin, whereas Cal-II are hydrothermal, postdating the U–Nb–REE mineralization. Cal-I calcites (Cal-I-1, Cal-I-2 and Cal-I-3) from mineralized and unmineralized carbonatites, displayed regular changes in REE, Na and Sr contents, but similar trace element distribution patterns and Sr–Nd-C-O isotopic signatures, indicating that these carbonatites originated from the same enriched mantle (EM1) source by low-degree partial melting of HREE-rich carbonated eclogites related to recycled marine sediments. The combination of trace elements and Sr-Nd isotopic composition of calcites further reveals that these carbonatites have undergone highly differentiated evolution. Such differentiation is conducive to the enrichment of ore-forming elements (U–Nb–REE) in the late magmatic–hydrothermal stages owing to extensive ore-forming fluids exsolved from carbonatitic melts. The massive precipitation of the U–Nb–REE minerals from ore-forming hydrothermal fluids may have been triggered by intense fluid–rock reactions indicated by extensive and intense fenitization and biotitization. Therefore, the Huayangchuan carbonatite-related U-Nb-REE deposit may have formed by a combination of processes involving recycled U–Nb–REE–rich marine sediments in the source, differentiation of the produced carbonatitic magmas, and subsequent exsolution of U–Nb–REE–rich fluids that precipitated ore minerals through reactions with wall rocks under the transitional tectonic regime from compression to extension at the end of Late Triassic.

Thermal state and evolving geodynamic regimes of the Meso- to Neoarchean North China Craton

Constraining thickness and geothermal gradient of Archean continental crust are crucial to understanding geodynamic regimes of the early Earth. Archean crust-sourced tonalitic–trondhjemitic–granodioritic gneisses are ideal lithologies for reconstructing the thermal state of early continental crust. Integrating experimental results with petrochemical data from the Eastern Block of the North China Craton allows us to establish temporal–spatial variations in thickness, geothermal gradient and basal heat flow across the block, which we relate to cooling mantle potential temperature and resultant changing geodynamic regimes from vertical tectonics in the late Mesoarchean (~2.9 Ga) to plate tectonics with hot subduction in the early to late Neoarchean (~2.7–2.5 Ga). Here, we show the transition to a plate tectonic regime plays an important role in the rapid cooling of the mantle, and thickening and strengthening of the lithosphere, which in turn prompted stabilization of the cratonic lithosphere at the end of the Archean.

The timeline of prolonged accretionary processes in eastern Central Asian Orogenic Belt: Insights from episodic Paleozoic intrusions in central Inner Mongolia, North China

Abstract Updating magmatic profile in crucial constituent terranes across the Central Asian Orogenic Belt presents a key to chronicling the timeline of prolonged accretionary processes and termination of the Paleo-Asian Ocean in the northern China–southern Mongolia tract. Here we performed a systematic geochronological and geochemical study on a spectrum of Paleozoic intrusions from the Erenhot region in central Inner Mongolia, North China, within the hinterland of the tract, with four distinct magmatic episodes unraveled. Combining these episodes with the previously documented events from contiguous regions defines two major tectono-magmatic cycles. The early Paleozoic cycle (500–450 Ma) evolved from initial fluid-fluxed tholeiitic and calc-alkaline granitoids to melt-fertilized mafic-intermediate magmatism. It appears to experience the initiation and maturation of a Western Pacific-type intra-oceanic arc system that culminated in ridge subduction. The late Paleozoic cycle expanded in magmatic expression from sporadic Late Devonian (373–365 Ma) calc-alkaline intermediate-felsic pulses through Early-Middle Carboniferous (356–320 Ma) medium to high-K calc-alkaline flare-up to a Late Carboniferous–Early Permian (310–277 Ma) province of diverse lithologies. These magmatic episodes seem to encompass a complete active continental arc–back-arc system that spanned from resuming oceanic plate subduction through slab rollback and backarc rifting to ridge-trench collision and backarc basin closure. Featuring a Rodinia-aged terrane affinity and a representative Paleozoic magmatic profile, the Erenhot region provides an optimal site for correlating the evolution of mosaic terranes in southern Mongolia and northern China, and for evaluating the coupled evolution of shifting tectonic regimes and plural crustal generation mechanisms within a retreating accretionary orogen.

Siderian mafic-intermediate magmatism in the SW Yangtze Block, South China: Implications for global ‘tectono-magmatic lull’ during the early Paleoproterozoic

It is debated whether global-scale tectono-magmatic quiescence prevails on the Earth during the early Paleoproterozoic. The early Paleoproterozoic meta-mafic and intermediate rocks in the southwestern Yangtze Block of South China could provide an opportunity to constrain not only the tectonic evolution of the Yangtze Block itself but also augment our understanding of the global tectono-magmatic lull during the early Paleoproterozoic. Zircon UPb geochronological study indicates that the meta-mafic rocks were emplaced at ca. 2.32 Ga, whereas the intermediate rocks were emplaced at ca. 2.35 Ga. These two lithologies are characterized by subduction-related whole-rock geochemical (such as depleted Nb, Ta and enriched Rb, K, Pb) and unradiogenic NdHf isotopic signatures (εHf(t) < 0, εNd(t) < 0), implying a derivation from a geochemically enriched lithospheric mantle modified by ancient subduction. Thermodynamic modeling further suggests that the intermediate rocks are products of the AFC process between mantle-derived basaltic magma and TTG-like basement rocks at shallow crustal levels. We interpret this as occurring within a post-orogenic extensional setting. Integrated with the published work, we further posit that the Yangtze Block experienced a tectonic regime transition from compressive orogenesis to post-orogenic extension during the early Paleoproterozoic, and the magmatic activities in several crustal blocks are still active until ~2.3 Ga and therefore places important constraints on the beginning of the tectono-magmatic lull.

Early Paleozoic tectono–magmatic evolution in the South Altun orogenic belt, northwest China: Insights from zircon U–Pb geochronology, Hf isotope and geochemistry of the granitoids

ABSTRACT Precise timing of granitoids and constraints of their magma sources are critical to understanding the subduction–collision tectonic evolution of the Altun orogenic belt (AOB), a vital part of the Qinghai-Tibet plateau in northwestern China. We provide in-situ zircon U–Pb age, Hf isotopic composition and whole-rock geochemistry of the Yusupualeke granitic pluton in the South Altun orogenic belt (SAOB), an integral unit of the AOB, to determine the tectono-magmatic evolution of the SAOB at early Palaeozoic. The Yusupualeke granitic pluton comprises the medium-coarse grained porphyritic monzogranite and medium-fine grained granodiorite. The monzogranite sample yielded a weighted mean 206Pb/238U age of 476.8 ± 3.6 Ma (MSWD = 0.59), while the granodiorite yielded a weighted mean 206Pb/238U age of 453.2 ± 4.7 Ma (MSWD = 0.013). Zircon U–Pb ages suggest that the subduction–collision process of the SAOB remained from the middle stage of the early Palaeozoic to the end. Both of the studied granitoids belong to metaluminous to weakly peraluminous series and show typical I-type granite characteristics with depletions in Ba, Nb, Sr, P, and Ti, and enrichments in light rare earth elements (LREEs), Rb, Th, K, and slightly negative Eu anomalies. Based on the geochronological data and regional geological background, we believe that the porphyritic monzogranites were formed during the northward subduction process of the South Altun ocean (SAO), while the granodiorites were formed during the tectonic regime transition stage. Besides, combing the previous research achievements with our newly obtained data, we put forward a new division and geodynamic model of the early Palaeozoic tectono-magmatic evolution for the South Altun orogenic belt.

Permian A-type granites of the Western Carpathians and Transdanubian regions: products of the Pangea supercontinent breakup

Permian biotite leucogranites to granite porphyries and rhyolites form small intrusions in several Alpine tectonic units in the Western Carpathians and the Pannonian region (Slovakia and Hungary). Their A-type signature is inferred from main- and trace-element geochemistry, with high K, Rb, Y, REE, Zr, Th, Nb, Fe/Mg and Ga/Al, low Al, Mg, Ca, P, Sr, V and strong negative Eu-anomaly. This geochemical signature is further supported by the mineralogy comprising local hypersolvus alkali feldspars, annitic biotite and the presence and composition of HFSE accessory minerals. The δ18O values measured for zircon (mean value 8.3 ‰ ± 0.36) may be explained by the melting of igneous material of crustal origin and/or mantle basalts which interacted with low-temperature fluids. The in-situ SHRIMP U–Pb isotope dating of zircon from the granites highlights two different periods of magmatic crystallisation and pluton emplacement: the older 281 ± 3 Ma Cisuralian age in the southern part, Velence Hills in the Pannonian region (Transdanubian Unit) and younger Guadalupian ages in the northern part, the West-Carpathian area: 262 ± 4 Ma (Turčok, Gemeric Unit), 267 ± 2 Ma (Hrončok, Veporic Unit) and 264 ± 3 Ma (Upohlav, granitic pebbles in Cretaceous conglomerates of the Pieniny Klippen Belt). The ~ 280 to 260-Ma interval is simultaneous with post-orogenic or anorogenic, rift-related and mainly alkaline (A-type) magmatism on the broader European scale. Our study documents a close relationship between the Permian continental rifting and the Neotethyan Meliatic oceanic basin opening in the Middle Triassic. The A-type granites originated from the partial melting of the ancient lower crustal quartzo-feldspatic rocks with the possible contribution of meta-basic material from the mantle in an extensional tectonic regime consistent with disintegration of the Pangea supercontinent during the Permian–Triassic period.

Early and middle Miocene paleobathymetry of the Vienna Basin (Austria)

The Vienna Basin (VB) originated during the early Miocene and represents one of the largest onshore oil and gas field in Europe. The VB is composed of several horst and graben structures forming different subbasins, each with its own geodynamic evolution and deviating paleobathymetric developments during the Miocene. We present an analysis of water depth evolution along a NE-SW transect by applying a transfer function for benthic foraminifers. We document dramatic changes in the depth profile through time, which coincide with shifts of prevailing tectonic regimes. Bathyal conditions were established during the early Miocene piggy-back stage and the early middle Miocene extensional phase. A clear shallowing trend from upper bathyal to inner neritic conditions occurred during the middle Miocene extensional tectonic phase.All lowstand systems of relative sea level in the VB coincide with global Mi-events. The observed maxima of the relative sea level in the VB are vaguely in phase with the global record from the Ottnangian (late early Miocene) to the middle Badenian (middle Miocene) but exceed the range of global sea level rise by three to four times, suggesting a strong tectonic amplification. Fluctuations of the relative sea level during the late Badenian and Sarmatian (Serravallian, middle Miocene) range well within the global signal, documenting ceasing influence of tectonics.

Interactions between active tectonics and gravitational deformation along the Billecocha fault system (Northern Ecuador): Insights from morphological and paleoseismological investigations

The Billecocha plateau (4000 m a.s.l.) lies in the high elevation Ecuadorian Andes volcanic arc. It overhangs by 2000 m above the interandean valley. Both the plateau and surrounding volcanoes are heavily affected by active faulting characterized by straight, sharp and discontinuous scarps within a 6 km wide and 24 km long corridor. Contrasting interpretations have been proposed to explain the expression at surface of the so-called Billecocha fault system (BFS), from normal faulting related to postglacial elastic rebound or gravitational processes, to right-lateral faulting compatible with the North-Andean Sliver tectonic regime. The instrumental seismicity recorded around the BFS is low, however, a M~7 earthquake heavily struck the region in 1868.With the aim to discuss the kinematic and coseismic nature of the encountered deformations as well as the seismogenic character of the BFS, we performed (1) morphological analysis to map and quantify evidence of active faulting and (2) paleoseismological investigations across the longer segment of the fault system. In three trenches, we show that surface deformations are at least partly coseismic in origin during the Holocene with a minor lateral component, the last paleoseismic event being compatible in date with the 1868 earthquake. In addition, some of the enlightened paleoseismic events could have occurred in relationship with volcanic eruptions of the surrounding volcanoes.While field evidence of reverse and strike slip faulting suggests that regional tectonics could be involved, the geomorphological signature of the BFS at the mountain scale, as seen on the digital surface model, can partly be related to the development of deep seated gravitational deformations, hence suggesting an interaction between boundary (i.e. tectonic, volcanic) and body forces (i.e. gravity, post-glacial rebound). Further studies are however mandatory to better address the influence of each process at the BFS, in particular geodetic and seismological surveys.Given the available data, we suggest that the BFS could actually correspond to the distributed surface expression of the tectonic reactivation of the inherited Pujilí suture, enhanced by gravitational phenomenon. In this light, paleoearthquakes identified along the BFS may help evidencing the recurrence of major events in the region. However, it also imply that surface deformations along the BFS shall not be used without a careful and more detailed field work to derive fault slip rates for seismic hazard calculations.

Longevity of small-scale (‘baby’) plumes and their role in lithospheric break-up

SUMMARYControversy between advocates of ‘active’ (plume-activated) versus ‘passive’ (driven by external tectonic stresses) modes of continental rifting and break-up has persisted for decades. To a large extent, inconsistencies between observations and models are rooted in the conceptual model of plumes as voluminous upwellings of hot material sourced from the deep mantle. Such large-scale plumes are expected to induce intensive magmatism and topographic uplift, thereby triggering rifting. In this case of an ‘active’ rifting-to-break-up system, emplacement of plume-related magmatism should precede the onset of rifting that is not observed in many rifted continental margins, thus providing a primary argument in favour of an antiplume origin for continental break-up and supercontinent fragmentation. However, mantle plumes are not restricted to whole-mantle (‘primary’) plumes emanating from the mantle-core boundary but also include ‘secondary’ plumes originating from the upper mantle transition zone or shallower. Over the last decades a number of such ‘secondary’ plumes with horizontal diameters of only ∼100–200 km (therefore, sometimes also called ‘baby’ plumes) have been imaged in the upper mantle below Europe and China. The longevity of such small-scale plumes and their impact on geodynamics of continental break-up have so far not been explored. We present results of a systematic parametrical analysis of relatively small thermal anomalies seeded at the base of the lithosphere. In particular, we explore the effects of variations in initial plume temperature (T = 1500–1700 °C) and size (diameter of 80–116 km), characteristics of the overlying lithosphere (e.g. ‘Cratonic’, ‘Variscan’, ‘Mesozoic’ and oceanic) and intraplate tectonic regimes (neutral or far-field extension of 2–10 mm yr–1). In tectonically neutral regimes, the expected decay time of a seismically detectable ‘baby’-plume varies from ∼20 to &amp;gt;200 Myr and is mainly controlled by its initial size and temperature, whereas the effect of variations in the thermotectonic age of the overlying lithosphere is modest. These small but enduring plumes are able to trigger localized rifting and subsequent continental break-up occurring from ∼10 to &amp;gt;300 Myr after the onset of far-field extension. Regardless of the thermomechanical structure of the lithosphere, relatively rapid (tens of Myr) break-up (observed in models with a hot plume and fast extension) favours partial melting of plume material. In contrast, in the case of a long-lasting (a few hundreds of Myr) pre-break-up phase (relatively cold plume, low extension rate), rifting is accompanied by modest decompressional melting of only ‘normal’ sublithospheric mantle. On the basis of the models presented, we distinguish two additional modes of continental rifting and break-up: (1) ‘semi-active’ when syn-break-up magmatism is carrying geochemical signatures of the deep mantle with deformation localized above the plume head not anymore connected by its tail to the original source of hot material and (2) ‘semi-passive’ when the site of final lithospheric rupture is controlled by a thermal anomaly of plume origin but without invoking its syn-break-up melting. These intermediate mechanisms are applicable to several segments of the passive continental margins formed during Pangea fragmentation.

Geochronology and Sr-Nd-Pb-Hf-O isotope geochemistry of Miocene intrusive rocks from Tsushima Islands, Japan: Constraints on petrogenesis and tectonic setting

We present geochemical and Sr-Nd-Pb-Hf-O isotopic data and zircon U-Pb ages for Miocene intrusive rocks (granitoids, mafic microgranular enclaves (MMEs), and diabases) from Tsushima Islands between Kyushu (Japan) and southeastern Korea. A granite yielded an age of 16.23 ± 0.16 Ma. The granitoids are mostly I-type, but are transitional between metaluminous and peraluminous compositions. All samples exhibit enrichment in LREE and LILE (e.g., K, Rb, and Pb), and depletion in HFSE (e.g., Nb, Ta, and Ti), implying formation in a subduction-related tectonic setting. However, on tectonic discrimination diagrams, data for all samples plot between the orogenic and anorogenic fields, indicating that Tsushima Islands were in a transitional tectonic regime from subduction to continental extension at ca. 16 Ma. The δ18Oclinopyroxene values of a diabase range from 5.65‰ to 5.84‰. The Sr-Nd-Pb isotopic compositions of the diabases (87Sr/86Sr = 0.70423–0.70481; εNd = −0.8 to +3.6; 206Pb/204Pb = 18.27–18.40) are within the range of Miocene Eoil basalts from the Cenozoic Yangnam Basin in southeastern Korea, indicating the mantle source was a metasomatized mantle wedge. The granitoids are characterized by more enriched isotopic compositions than the diabases (87Sr/86Sr = 0.70611–0.70756; εNd = −6.0 to −3.8; 206Pb/204Pb = 18.43–18.55), and the MMEs have an intermediate isotopic composition. δ18Oquartz values of the granitoids vary from 8.68‰ to 9.25‰, and zircons from a granite have εHf(t) values of −1.7 ± 1.2. The MMEs may have formed by partial melting of amphibolitic lower crust, whereas the granitoids possibly had a hybrid source consisting of amphibolite and graywacke.

Up-to-date PSHA along the Gulf of Aqaba-Dead Sea transform fault

The Gulf of Aqaba Dead Sea Transform fault (DSTF) is a plate boundary. Its southern section separates Sinai Peninsula, Egypt and Saudi Arabia. Also, it passes through several countries. A development plan with a huge budget is drawn to develop the Gulf of Aqaba region. This plan includes the construction of King Salman Bridge. Therefore, the assessment of the seismic hazard (SH) at this seismically active region is inevitable for the sustainability of these mega projects. This paper presents a probabilistic seismic hazard assessment (PSHA) study along the DSTF. An earthquake catalogue including the paleo-seismological studies, historical records and instrumental seismicity is compiled. It is processed by converting various magnitude types to the moment magnitude (Mw) using recently derived mathematical equations. Three de-clustering techniques have been used to de-cluster the catalogue. Then, a completeness test is performed. The obtained earthquake catalogue is implemented to plot two different smoothed seismicity maps (SSMs). These maps and two areal source models are utilized to represent the seismicity along the DSTF. Five ground-motion prediction equations (GMPEs) are used to predict the ground-motion, three out of them are for the seismic sources located at shallow active crust tectonic regime and two for the intermediate depth earthquakes generating at the subduction zone south of Cyprus. The previously mentioned SH inputs are implemented through the logic-tree framework. The produced maps illustrate that the PGA values in the DSTF region range from 0.02g to 0.4g and 0.06 to 0.48 for 475 and 2475 years return period (RP), respectively. Also, seismic hazard maps using the non-de-clustered catalogue are produced.

A non-arc tectonic setting for the evolution of Archean gabbro anorthosite Complexes: Evidence from the Singhbhum Craton, eastern India

Understanding the petrogenetic evolution of Archean gabbroic cumulates provides a window to the mantle geodynamics operated in the early Earth. Gabbroic cumulates are associated with various tectonic settings including convergent plate boundaries, mantle plumes, mid-ocean ridges, anorogenic, post-orogenic, continental rift settings and oceanic plateaus. The exposures of preserved gabbroic suites in the Archean are limited. The Mesoarchean Mayurbhanj Gabbro Anorthosite Complex (GAC) is a well-preserved mafic body situated along the eastern margin of the craton along with concomitant anorogenic Mayurbhanj granite. These rocks are devoid of deformational structures and/or mineral recrystallization. Based on their mineralogical characteristics, GAC rocks are mainly grouped into gabbronorite, leucogabbronorite, leucogabbro and gabbro. The minimal abundance of magmatic amphibole and the variation in the anorthite content (An36-An84) rules out the role of a hydrous mantle source in the evolution of these rocks. The studied samples display tholeiitic trend transitioning to calc-alkaline field, major and trace element data indicates the fractional crystallization of olivine and pyroxene. The trace element systematics of the Mayurbhanj GAC show LILE enrichment over HFSE depletion, with negative Nb-Ta and mixed Zr-Hf and Ti anomalies. The geochemical signatures such as elevated Th/Yb, Nb/Yb, variable Dy/Yb and Dy/Dy* and varying Gd/YbCN ratios suggest that parental magmas of the Mayurbhanj GAC originated from a heterogeneous depleted mantle source. The studied rocks' Sr-Nd isotopic systematics reveal that their parental magma has undergone assimilation by a Paleoarchean felsic crust. We argue that the ‘arc-like’ geochemical signatures displayed by these rocks resulted from crustal contamination and may not be due to subduction origin. Consolidating the field, geochemical and isotopic evidence suggests that the Mesoarchean Mayurbhanj GAC possibly evolved from an anhydrous heterogeneous mantle source in a non-compressive tectonic regime.

Cretaceous large-scale metal accumulation triggered by post-subductional large-scale extension, East Asia

Effective exploration for ore deposits usually seeks distinct conditions within fundamentally prospective geological settings that might lead to enhanced ore-forming processes. Large-scale metal accumulation to form ore deposits is usually associated with specific tectonic events. Many studies have found that significant mineralization is genetically related to plate subduction and accretion/collision tectonic regimes. Here we describe how an active continental margin extensional setting can also define an important tectonic environment for the development of a world-class regional metallogenic province subsequent to subduction and compressional stresses. The extensive and diverse group of mineral deposits formed from 145 to 70 Ma along the eastern Asian continental margin (EACM), from northern Vietnam, through eastern China, Korea, Japan, and to the Sikhote-Alin region of Far East Russia, represents a good example of a regional extension-related metallogenic province. The varied mineral deposits with distinct fluid and metal sources occur in a series of NE- or NNE-trending pull-apart basins and metamorphic core complexes, as well as along associated transtensional faults, reflecting the controls of an extensional regime. A new tectono-metallogenic model for the Cretaceous ore-forming events of eastern Asia is proposed to highlight the exploration potential in such tectonic settings throughout the world.

Petrogenesis and tectonic significance of the middle Neoproterozoic highly fractionated A-type granite in the South Qinling block

AbstractThe South Qinling block, a segment of the Yangtze craton involved in the Qinling–Dabie orogen, is critical for understanding the tectonic evolution of eastern China. However, the tectonic setting of the South Qinling block and the northern margin of the Yangtze block during middle Neoproterozoic time has long been the subject of debate, with two distinctly different models (continental rift or volcanic arc) proposed. Here, a comprehensive study of zircon U–Pb geochronology and geochemistry has been carried out on the Chengwan granitic pluton from the Suizao terrane in the South Qinling block. The granites are monzogranite and syenogranite in lithology, and are mainly composed of potash feldspar, quartz, plagioclase and biotite. This suite has long been regarded as a Palaeozoic magmatic pluton, but zircon U–Pb ages of 809 ± 9 Ma and 816 ± 4 Ma are obtained in this study. The granites are metaluminous to strongly peraluminous with high alkali contents, and exhibit highly fractionated features, including high SiO2, low Zr/Hf ratios, rare earth element tetrad effects and enrichment of K and Rb. They show Hf–Nd isotopic decoupling, which may be genetically related to their petrogenetic process. Based on the geochemical features and the positive εHf(t) values of the zircons, it is indicated that the granites may have been derived from partial melting of juvenile tonalitic rocks by biotite breakdown under fluid-absent conditions. The Chengwan granite geochemically belongs to the A2-subtype granites, suggesting that it might have formed in a post-orogenic tectonic setting. The highly fractionated A-type granite in this study may represent extensional collapse shortly after the collisional events in the South Qinling block, and thus indicate a tectonic regime switch, from compression to extension, as early as middle Neoproterozoic time. Integrating our new data with documented magmatic, metamorphic and sedimentary events during middle Neoproterozoic time in the region may support a continental rift model, and argues against arc models.

Timor Collision Front Segmentation Reveals Potential for Great Earthquakes in the Western Outer Banda Arc, Eastern Indonesia

In Eastern Indonesia, the western Outer Banda arc accommodates a part of the oblique Australian margin collision with Eurasia along the Timor Trough. Yet, unlike the Wetar and Alor thrusts of the Inner Banda arc in the north and the adjacent Java subduction zone in the west, both recent and historical seismicity along the Timor Trough are extremely low. This long-term seismic quiescence questions whether the Banda Arc collision front along the Timor Trough is actually fully locked or simply aseismic and raises major concerns on the possible occurrence of large magnitude and tsunamigenic earthquakes in this vulnerable and densely populated region. Here, we jointly analyze multibeam bathymetry and 2D seismic reflection data acquired along the Timor Trough to characterize the location, nature, and geometry of active faults. Discontinuous narrow folds forming a young accretionary prism at the base of the Timor wedge and spatially correlated outcropping normal faults on the bending northwest Australian shelf reveal two concurrent contrasting styles of deformation: underthrusting and frontal accretion. We find that those tectonic regimes and their associated seismic behaviors depend on 1) the thickness of the incoming and underthrusting Cenozoic sedimentary sequence, 2) the vergence of inherited normal faults developed within the continental shelf, and 3) the depth of the décollement beneath the Timor wedge. Based on the along-strike, interchanging distinct deformation style, we identify the mechanical and seismic segmentation along the Banda arc collision front and discuss the implications for earthquake and tsunami hazards along the western Outer Banda arc region.