Oceanic Slab Research Articles

Zircon xenocryst Hf-O isotopic compositions in the Qiyugou Au orefield: A record of Paleoproterozoic oceanic slab subduction in the Trans-North China Orogen

The Trans-North China Orogen (TNCO) is suggested to form under a transition from subduction to collision from 2500 Ma to ∼ 1850 Ma, and the Taihua TTG gneiss in the Xiong’ershan district is thought to have formed in an Andean-type continental arc setting or island arc setting during this geological event. However, to date, the geochemical record of the subducted oceanic slab is still lacking. This study focuses on Hf-O isotopic analyses of zircon xenocrysts from the Qi189 hornblende monzogranite, Qiyugou Au orefield, TNCO. The results reveal that zircon xenocrysts with ages from 2304 Ma to 1934 Ma generally have low-δ18O values from 2.66‰ to 3.83‰ and high-εHf(t) values from 2.7 to 7.8. These zircon grains were generated from remelting of high-temperature altered gabbroic rocks of the subducted oceanic slab during the transition from compression to extension. In this paper, we present new evidence for Paleoproterozoic oceanic slab subduction in the TNCO.

Generation of Nb-enriched mafic rocks and associated adakitic rocks from the southeastern Central Asian Orogenic Belt: Evidence of crust-mantle interaction

Melting of subducting oceanic lithosphere and associated melt-mantle interactions in convergent plate margins require specific geodynamic environment that allows the oceanic slab to be abnormally heated. Here we focus on the Early Mesozoic mafic rocks and granite porphyry, which provide insights into slab melting processes associated with final closure of the Paleo-Asian Ocean. The granite porphyry samples are calc-alkaline and distinguished by high Sr contents, strong depletion of heavy rare earth elements, resulting in high (La/Yb)N and Sr/Y ratios, and negligible Eu anomalies. Based on their high Na2O and MgO, low K2O contents, positive εHf(t) and εNd(t) and low (87Sr/86Sr)i values, we propose that the granite porphyry was likely derived from partial melting of subducting Paleo-Asian oceanic crust. The Nb-enriched mafic rocks are enriched in Rb, Th, U, Pb and K, and depleted in Nb, Ta, Ba, P and Ti, corroborating a subduction-related origin. Their heterogeneous Sr-Nd-Hf-O isotopic compositions and other geochemical features suggest that they were likely derived from partial melting of peridotitic mantle wedge interacted with oceanic slab-derived adakitic melts. Trace element and isotope modeling results and elevated zircon δ18O values suggest variable subducting sediments input into the mantle wedge, dominated by terrigenous sediments. Synthesizing the widely-developed bimodal rock associations, conjugated dikes, thermal metamorphism, tectonic characteristics, paleomagnetic constraints, and paleogeographical evidence along the Solonke-Changchun suture zone, we identify a slab window triggered by slab break-off, which accounts for slab melting and formation of the Nb-enriched mafic rocks and associated adakitic granite porphyry in southeastern Central Asian Orogenic Belt.

Late Jurassic Paleotethyan oceanic slab break-off revealed by Sr-Nd-Hf isotopes of Na-rich adakitic granites from northwestern Turkey

The Mesozoic geodynamic evolution of the Paleotethys Ocean is critical but a poorly known piece of the geological history of the Sakarya Zone, north Turkey. Here, we describe new integrated research of zircon U-Pb geochronology, bulk-rock major-trace element and Sr–Nd isotopes as well as Hf isotope of zircons from the granitic rocks of the Demirköy and Kızıldamlar plutons located in Bilecik area along the western Sakarya Zone, northwestern Turkey. Zircon U–Pb ages obtained by LA-ICP-MS reveal that the granitic magma intruded into the basement rocks during the Late Jurassic (ca. 162–157 Ma). The granites are tholeiitic and show slightly metaluminous to peraluminous geochemical affinities. The granite samples are adakitic, having relatively high Na2O (6.35–7.58 wt%) and Sr (172–580 ppm), and low K2O (0.39–1.13 wt%) and Y (3.2–5.3 ppm), and thus have high Sr/Y (70–250). The samples exhibit enrichment of light rare earth elements (REE) and large ion lithophile elements (LILE) and depletion in heavy REE and the high field strength elements (HFSE; e.g., Nb, Ti, Ta). The samples possess low 87Sr/86Sr(t) values of 0.70313 to 0.70372, and εNd(t) of −0.66 to +9.24. Zircon grains exhibit positive εHf (t) values of 5.0 to 12.8 and depleted mantle model ages of Hf (TDM1 = 0.24 to 0.68 Ga). These geochemical features and Sr–Nd–Hf radiogenic isotope systematics reveal that the adakitic rocks were likely formed by low degree melting (<10%) of the eclogitic part of an oceanic slab and subsequent melt fractionation and crustal contamination. An ongoing subduction scenario cannot explain the formation of the investigated adakitic rocks. We suggest that the adakitic plutons were generated through slab break-off after the final closure of the Paleotethys Ocean throughout the Late Jurassic (ca. 162–157 Ma). Considering the previous data, the continental back-arc basin system, leading to the opening of Neotethys Ocean at the southern margin of the continent, has been put forward to decipher the Late Jurassic tectonic evolution of the Paleotethys Ocean and the western Sakarya Zone.

Effects of episodic slow slip on seismicity and stress near a subduction-zone megathrust

Slow slip phenomena deep in subduction zones reveal cyclic processes downdip of locked megathrusts. Here we analyze seismicity within a subducting oceanic slab, spanning ~50 major deep slow slip with tremor episodes over 17 years. Changes in rate, b-values, and stress orientations of in-slab seismicity are temporally associated with the episodes. Furthermore, although stress orientations in the slab below these slow slips may rotate slightly, in-slab orientations 20–50 km updip from there rotate farther, suggesting that previously-unrecognized transient slow slip occurs on the plate interface updip. We infer that fluid pressure propagates from slab to interface, promoting episodes of slow slip, which break mineral seals, allowing the pressure to propagate tens of km further updip along the interface where it promotes transient slow slips. The proposed methodology, based primarily on in-slab seismicity, may help monitor plate boundary conditions and slow slip phenomena, which can signal the beginning stages of megathrust earthquakes.

First structural observation around the hinge of the Mongolian Orocline (Central Asia): Implications for the geodynamics of oroclinal bending and the evolution of the Mongol-Okhotsk Ocean

ABSTRACT To understand the origin of curved subduction zones has been one of the major challenges in plate tectonics. The Mongol-Okhotsk Orogen in Central Asia is characterized by the development of a U-shaped oroclinal structure that was accompanied by the continuous subduction of the Mongol-Okhotsk oceanic plate. Therefore, it provides a natural laboratory to understand why and how a subduction system became tightly curved. In this study, we provide the first structural observation around the hinge of the Mongolian Orocline (the Zag zone in Central Mongolia), with an aim to constrain the oroclinal geometry and to link hinge zone structures with the origin of the orocline. Our results show that rocks in the Zag zone are characterized by the occurrence of a penetrative foliation that is commonly subparallel to bedding. Both bedding and dominant fabric in the Zag zone are steeply dipping, and their strike orientations in a map view follow a simple curve around the hinge of the Mongolian Orocline, thus providing the first structural constraint for 3D geometry of the orocline. A secondary penetrative fabric parallel to the axial plane of the orocline was not observed, indicating a low degree of orogen-parallel shortening during oroclinal bending. Combining with available geological and geophysical data, we conclude that the Mongolian Orocline was developed in a period of Permian to Jurassic, and its origin was linked to the subduction of the Mongol-Okhotsk oceanic slab. We consider that the low-strain oroclinal bending likely resulted from the along-strike variation in trench retreat, which was either triggered by the negative buoyancy of the Mongol-Okhotsk oceanic slab, or driven by the relative rotation of the Siberian and North China cratons. Our results shed a light on 3D geometry and geodynamic mechanisms of large-scale oroclinal bending in an accretionary orogen.

How to quake a subducting dry slab at intermediate depths: Inferences from numerical modelling

The origin of intermediate-depth subduction seismicity is a topic of research since long time. While plate unbending is considered as one of the main stress loading mechanisms, the processes responsible for earthquake nucleation are still unclear and depend on the question of whether failure occurs in the wet dehydrating portion of the slab or in the predominantly dry portion. Recently, the seismogenic portions of subducting oceanic slabs have been proposed to consist of dominantly dry metaperidotite that deforms by seismic brittle failure in absence of fluid-mediated embrittlement.In this work, we quantify by numerical modelling the differential stress achievable during unbending of a subducting slab. We show that the presence of discrete hydrated domains in a dry, strong slab amplifies the differential stress to high seismogenic values (ca. 3-4 GPa in the 100-200 km depth range) at intermediate depths. We also consider the effects of low temperature plasticity in olivine that can hinder the build-up of high differential stress to the first 100 km of depth.

Geochronology, geochemistry, Sr–Nd–Hf isotope composition of the late Permian adakite in West Ujimqin, Inner Mongolia: petrogenesis and tectonic implications

We report herein on new zircon U–Pb ages, the major and trace elements of whole-rock, and the Sr–Nd–Hf isotope composition for adakitic intrusives collected from the West Ujimqin district in the Southeast region of the Central Asian Orogenic Belt (CAOB). These data provide important constraints on the petrogenetic evolution and geodynamic setting of late Permian magmatism in the Southeast CAOB. The U–Pb dating of zircon shows that the ages of Seerbeng pluton and Nuhetingshala pluton in West Ujimqin are 255.3 ± 0.71 and 254.4 ± 1.2 Ma, respectively, which signifies that these are products of magmatic activity in the late Permian. The adakitic intrusives are characterized by high levels of Sr (Sr ≥ 741 ppm), low Y, low Yb, high Sr:Y ratios, and strongly fractionated rare earth elements (10.3 &lt; LaN/YbN &lt; 22.5), which is similar to the features of the adakite. The magmatic zircons exhibit positive Hf values (+8.1 to +13.3), and young two-stage model ages vary from 430 to 760 Ma. The high εNd(t) and low (87Sr:86Sr)i indicate that the adakitic granite derived from the partial melting of subducted oceanic slab. The high level of Mg# [100 × Mg/(Mg + Fe) in atomic number] and abundant Cr–Ni indicate that magmatic melts interacted with olivine rocks in the mantle. Considering these results and the regional rock assemblies, we conclude that the Paleo-Asian Ocean had not yet completely closed in the late Permian, and northward subduction continued, with the subducted slab possibly breaking off.

A low-velocity layer atop the mantle transition zone beneath the western Central Asian Orogenic Belt: Upper mantle melting induced by ancient slab subduction

Increasing evidence shows that the low-velocity layer (LVL) atop the mantle transition zone (MTZ) may play a critical role in Earth's evolution. The existence and formation of the LVL in continental interiors remain ambiguous. This study investigates the P-wave velocity structure atop the MTZ by matching P- and sP-wave triplicated waveforms simultaneously in the western Central Asian Orogenic Belt far away from continental margins. Two groups of nearly orthogonal triplications are allocated using records of four earthquakes at stations from the China National Seismic Network and the Incorporated Research Institutions for Seismology. Our observations resolve an LVL with 21–23 km thickness and a P-wave velocity drop of approximately −5.7% to −5.8% atop the MTZ. The two nearly orthogonal directions show little variation in the velocity structure of the LVL. It is inferred that the LVL results from partial melting induced by water and/or other volatiles released from the subducted Paleo-Asian oceanic slab and, hence, has existed since the Paleozoic, with possible variations in melt fraction. The long-lived partially molten LVL may play an important role in the intraplate magmatism and mantle dynamics in the Central Asian Orogenic Belt. Our results provide strong seismic evidence for the LVL existing in the continental interior, indicating that the LVL may be pervasive in Eurasia because of vast subductions of ancient plates; this provides new insights into the material circulation in the ancient oceanic subduction zones.

Upper Triassic to Middle Jurassic magmatic evolution of northern Patagonia: Insights from the tectonic and crustal evolution of the Los Menucos area, North Patagonian Massif, Argentina

The Upper Triassic magmatic and tectonic evolution of the central North Patagonian Massif is evaluated on the basis of the zircon U–Pb-Hf record and whole-rock geochemistry for volcanic rocks of the Los Menucos area. The sample analyzed in this study yields a 210 ± 3 Ma U–Pb zircon age, and the geochemical data for the Upper Triassic rocks are indicative of a high-K calk-alkaline arc-type magmatism, which also has adakitic signature related to melting of the subducted oceanic slab. The Hf isotope data for zircon from this sample, with ƐHf(T) values between −6.1 and −8.7, support a mixed source that may have involved melted slab and crustal reworking. A comprehensive evaluation of the Upper Triassic – Lower Jurassic magmatic and tectonic evolution of northern Patagonia is in agreement with the hypothesis of an Upper Triassic South Gondwanian flat-slab, which shifted the magmatic arc towards the continental interior at a time of crustal shortening and with arc-type adakitic magmatism. The Lower Jurassic magmatic record is consistent with slab detachment and later roll-back, which would have promoted crustal extension and intraplate magmatism, and also shifted the arc magmatism to the paleo-trench. As a consequence of this stage, the Subcordilleran and North Patagonian batholiths were emplaced. The progressively – with time - less negative Hf isotope signatures for magmatic zircon are consistent with less evolved magmas generated throughout the Jurassic. The Upper Triassic-Middle Jurassic magmatism in northern Patagonia was part of a complex tectonic evolution, which involved crustal shortening and extension during plate reorganization during the critical period of Gondwana supercontinent break-up.

Late Permian ultrapotassic rhyolites in SE Thailand: evidence for a Palaeotethyan continental rift basin

The volcanic rocks in the Chanthaburi zone are rarely reported, but are important in investigating the tectonic evolution of the Palaeotethyan Ocean in SE Thailand. Four rhyolitic samples from Ko Chang Island yield zircon ages of 258–254 Ma, confirming the presence of Late Permian volcanic rocks in SE Thailand. These rocks consist of Group 1 rhyolites and Group 2 rhyolitic ignimbrites and have high K 2 O contents of 4.92–7.10 wt% and A/CNK values of 1.10–1.69. They are enriched in the light rare earth elements, Rb, Th, U, Zr and Y and show negative anomalies of Ba, Sr, Nb, Ta and Ti, with clear Eu anomalies. Their whole-rock ε Nd ( t ) values range from −1.7 to −3.1. Zircon in situ ε Hf ( t ) and δ 18 O values range from 0.0 to +5.6 and from 8.2 to 9.6‰, respectively. They belong to the group of peraluminous, ultrapotassic A-type rhyolites and were derived from partial melting of a mixed source of Mesoproterozoic metasedimentary rocks with a component of juvenile mafic crust. These ultrapotassic rhyolites formed in a continental rift setting in response to the rollback of a subducted Palaeotethyan oceanic slab beneath the Indochina Block. Combining these results with previous geological observations, we propose that there were some sporadically distributed continental rift basins along the Eastern Palaeotethyan domain during the Permian. Supplementary material: Major oxides, trace element analytical and Nd isotopic data, LA–ICP–MS and SIMS zircon U–Pb ages and Zircon in-situ Hf–O isotopic compositions are available at https://doi.org/10.6084/m9.figshare.c.5635390

Early mesozoic arc–back-arc system in the leading edge of the Tibetan Plateau

The southern Lhasa subterrane is the leading edge of the Tibetan Plateau and preserves important records of the Neo-Tethyan oceanic subduction and India–Asia collision. However, the early tectonic evolutionary history of the Neo-Tethyan oceanic subduction remains unclear. Here we present results from a systematic study on the early Mesozoic (Middle Triassic–Late Jurassic) magmatic and sedimentary rocks in the southern Lhasa subterrane. These rocks are distributed zonally along the southern and northern parts of the subterrane, and have been classified into the north and south belts. The magmatic rocks in the north belt show a back-arc basin affinity, with accreted bimodal volcanic sequence and sediments from central and southern Lhasa subterranes, whereas the magmatic rocks in the south belt reveal typical magmatic arc characteristics. The sediments were derived from southern Lhasa subterrane, without the input of clastic components from the central Lhasa subterrane. Based on geochronological, geochemical, and sedimentary records from both belts, we conclude that the early Mesozoic magmatism was triggered by the northward subduction of the Neo-Tethyan oceanic slab, and its initial subduction likely occurred before the Middle Triassic (ca. 240 Ma). During the Early–Middle Jurassic (ca. 193–165 Ma), a short-lived episode of back-arc extension, we attribute to roll-back of the subducting Neo-Tethyan slab, resulted in the single magmatic arc that existed during the early stages (ca. 240–194 Ma) and evolved gradually into an arc–back-arc system. The north belt represents a back-arc basin setting associated with a continental arc, whereas an intra-oceanic and continental (island) arc simultaneously developed in the south belt, and both these belts were possibly generated under the same subduction system.

Revisiting Phanerozoic evolution of the Qinling Orogen (East Tethys) with perspectives of detrital zircon

This paper presents 5383 detrital zircon U–Pb and 2079 Lu–Hf isotopic data, shedding new light on the Phanerozoic tectonic evolution of the Qinling Orogen. The Silurian rocks are characterized by negative εHf(t) values of early Paleozoic and Neoproterozoic zircon, which in the Devonian to Carboniferous samples show both positive and negative εHf(t) values. Late Paleozoic zircon peak with negative εHf(t) values occurs in the upper Carboniferous to Cretaceous strata. The Permian to Cretaceous strata show peaks at 1.9 Ga and 2.5 Ga. Early Paleozoic and Neoproterozoic peaks are back in Upper Triassic to Cretaceous rocks. A Mesozoic peak with negative εHf(t) values occurs in Cretaceous rocks. In terms of the sources of detrital zircon, the South Qinling Belt and Yangtze Block are major contributors for Silurian strata; South Qinling and North Qinling belts are major sources for Lower Devonian to upper Carboniferous rocks; early Carboniferous detrital zircon related to subduction of Mianlue oceanic slab emerges in upper Carboniferous rocks; North China Block is an important source for Lower Permian to Upper Triassic rocks; Lower Cretaceous strata exhibit diverse provenance, including North Qinling Belt, North China Block, and South Qinling Belt. Provenance changes suggest that the initial spreading of the Mianlue Ocean occurred in the Early Silurian during which the South Qinling Terrane and Yangtze Block had not completely separated. The Mianlue Ocean evolved into a mature stage of spreading in the Early Devonian. Meanwhile, the closure of the Shangdan Ocean took place. The Mianlue Ocean started to subduct in the early Carboniferous. Its final closure occurred in the Late Triassic, leading to uplift and exhumation of Qinling Orogen and cutting off detritus transport from North China Block in the Jurassic. The Qinling Orogen underwent collapse in the late Early Cretaceous.

Caribbean Plate Boundaries Control on the Tectonic Duality in the Back‐Arc of the Lesser Antilles Subduction Zone During the Eocene

AbstractThe Eocene tectonic evolution of the easternmost Caribbean Plate (CP) boundary, that is the Lesser Antilles subduction zone (LASZ), is debated. Recents works shed light on a peculiar period of tectonic duality in the arc/back‐arc regions. A compressional‐to‐transpressional regime occurred in the north, while rifting and seafloor spreading occurred in Grenada basin to the south. The mechanism for this strong spatial variation and its evolution through time has yet to be established. Here, using 3‐D subduction mechanical models, we evaluate whether the change in the trench‐curvature radius at the northeast corner of the CP could have modulated the duality. We assume asymmetrical CP boundaries at the north (from east to west: oblique subduction to strike‐slip) and at the south (subduction‐transform edge propagator‐like behavior). Regardless of the imposed trench curvature, the southern half of our modeled CP undergoes a NW‐to‐W‐oriented extension due to the tendency of the southernmost part of the South‐America oceanic slab to rollback. In contrast, the tectonic regime in the northeast corner of the CP depends on the trench‐curvature radius. A low radius promotes transtension‐to‐transpression, with a NE‐oriented compressive component of the principal stress. A high radius largely reduces the compressive component and promotes an extensional regime similar to that in the south. We thus propose that an initially low‐curvature radius of the NE‐LASZ triggered the tectonic N‐S duality in the Eocene and led to an ephemeral period of transpression/compression at the north. However, an additional mechanism might have been required to locally enhance compression.

Multiple sources and magmatic evolution of the Late Triassic Daocheng batholith in the Yidun Terrane: Implications for evolution of the Paleo-Tethys Ocean in the eastern Tibetan Plateau

Abstract Granitoids with diverse composition and tectonic settings provide important tools for exploring crustal evolution and regional geodynamic history. Here we present an integrated study using petrological, mineralogical, zircon U-Pb geochronological, whole-rock geochemical, and isotopic data on the Late Triassic Daocheng batholith in the Yidun Terrane with a view to understanding the petrogenesis of a compositionally diverse batholith and its implications for the evolution of the Paleo-Tethys Ocean in the eastern Tibetan Plateau. The different lithological units of the batholith, including granodiorite, monzogranite, and quartz diorite, with abundant mafic microgranular enclaves in the granodiorite (MME I) and monzogranite (MME II), show identical crystallization ages of 218–215 Ma. The mineral assemblage and chemical composition of the granodiorite are identical to those of tonalitic-granodioritic melts generated under water-unsaturated conditions. The insignificant Eu anomalies and low magmatic temperatures indicate hydrous melting in the source. The relatively narrow range of whole-rock chemical and Sr-Nd isotopes, as well as the zircon trace element and Hf isotopic compositions of the granodiorite, suggest a homogeneous crustal source for the magma. Our modeling suggests that the rock was produced by 20–50% of lower crustal melting. The Daocheng monzogranites display more evolved compositions and larger variations in Sr-Nd-Hf isotopes than the granodiorite, which are attributed to assimilation and the fractional crystallization process. This is evidenced by the presence of metasedimentary enclave and inherited zircon grains with Neoproterozoic and Paleozoic ages, a non-cotectic trend in composition, and the trend shown by the modeling of initial 87Sr/86Sr ratios and Sr. The quartz diorites and MMEs showing composition similar to that of andesitic primary magma have high zircon εHf(t) values and are characterized by enrichment in LILEs and depletion of HFSEs. They were derived from the partial melting of lithospheric mantle that had been metasomatized by slab melts and fluids. The MMEs in both rocks display typical igneous texture and higher rare earth element (REE) and incompatible element concentrations than their host granites. The presence of fine-grained margins, acicular apatite, and plagioclase megacrysts suggests a magma mingling process. The overgrowth of amphibole around the pyroxene, quartz ocelli rimmed by biotite, and oscillatory zones of plagioclase are all indicative of chemical diffusion. Their enriched Sr-Nd isotopes imply isotopic equilibrium with the host granites. Based on a comparison with the coeval subduction-related magmatism, we propose that subduction and subsequent rollback of the Paleo-Tethys (Garzê-Litang Ocean) oceanic slab was the possible mechanism that triggered the diverse Triassic magmatism within the eastern Tibetan Plateau.

Mantle wedge enrichment beneath southern Tibet during the late stage (100–45 Ma) of oceanic subduction: Geochemical constraints from mantle-derived intrusions

The Miocene K-rich mafic rocks in the Lhasa Terrane of the southern Tibetan Plateau are considered to be related to either the subduction of the Neo-Tethyan oceanic slab or Indian continental slab. In order to investigate how subduction of the Neo-Tethyan oceanic slab has geochemically affected the mantle beneath southern Tibet, we obtained geochronological and geochemical data for the late Early Cretaceous Cangguo gabbros (97 Ma) and early Eocene Langka diorites (51 Ma) in southern Tibet. Although both the Cangguo gabbros and Langka diorites are characterized by arc-like trace element patterns, with enrichment in light rare earth and large-ion lithophile elements (e.g., Ba, Pb, and U), depletion in high-field-strength elements (e.g., Nb and Ta), and depleted Sr–Nd–Hf isotopic compositions, there are some other major geochemical differences (e.g., Ba/La and Th/La ratios) between these two suites of rocks. These geochemical characteristics of the Cangguo gabbros and Langka diorites indicate they were produced by melting of the depleted mid-ocean ridge mantle (DMM) that was enriched by oceanic sediment-derived fluids and melts, respectively. Our data and previous studies suggest that dehydration and melting of subducted Neo-Tethyan oceanic sediments beneath the Lhasa Terrane occurred in the late stage (100–45 Ma) of Neo-Tethyan oceanic slab subduction, which may have enriched the mantle beneath the Lhasa Terrane. However, the mantle beneath the Lhasa Terrane that was enriched by the subduction of oceanic slab during 100–45 Ma is still isotopically more depleted than the source of Miocene post-collisional K-rich mafic rocks in the Lhasa Terrane. This indicates that the enriched component in the mantle source of the K-rich mafic rocks cannot be simply attributed to materials derived from the Neo-Tethyan oceanic slab, and other additional materials are required.

Crust and upper-mantle seismic anisotropy variations from the coast to inland in central and Southern Mexico (2): correlations with tectonic tremor

SUMMARY Seismic anisotropy in the flat slab region of Mexico is compared with tectonic tremor (TT) activity. The anisotropy is observed in three separate horizontal layers using a novel technique with receiver functions. Those layers are identified as the continental crust and the subducted flat oceanic slab and a thin (∼10 km thick) remnant mantle wedge between those two layers. The TT is located in two zones: (1) the Sweet Spot where most of the tremor is observed (∼160–180 km from the coast) and (2) the Transient Zone (∼80–110 km from the coast). Anisotropy within each layer is observed to be different within each of the tremor zones than just outside them. The changes are explained as due to hydration within those zones. Water releasing phase changes have previously been modelled to occur within those two zones in the subducted slab (Manea &amp; Manea). Water rising through each of the layers should generate the observed differences in anisotropy in those zones as the fast polarization direction and split times can differ between dry and hydrated material. This observation also correlates with the many observations of high pore fluid pressure associated with TT.

Petrogenesis of Early Cretaceous granitic rocks from the Haobugao area, southern Great Xing’an Range, northeast China: Geochronology, geochemistry and Sr-Nd-Hf-O isotope constraints

Early Cretaceous granitic rocks are widespread in the southern Great Xing’an Range (SGXR) in the eastern part of the Central Asia Orogenic Belt where they have been interpreted to have formed by remelting of juvenile materials derived from a depleted mantle source. We present new U-Pb ages, geochemistry and Sr-Nd-Hf-O isotopic data for ~140 Ma granitic rocks from the Haobugao area. The Xiaohanshan (XHS) quartz monzonite porphyry formed at 140 ± 2 Ma, the Wulanba (WLB) biotite granite formed at 142 ± 2 Ma, and the Wulanchulute (WLCLT) granite formed at 139 ± 2 Ma. They are A-type granites characterised by enrichment of light rare earth and high field strength elements. Based on the geochemistry and Sr-Nd-Hf-O isotope data, the quartz monzonite porphyry with elevated δ18O (6.6‰–7.4‰), high εNd(t) (5.8) and high εHf(t) (5.0–8.4) formed by the partial melting of juvenile material ultimately derived from depleted mantle which had been modified by melts derived from sediment on the downgoing slab. The biotite granite and granite with mantle-like δ18O (4.6‰–6.5‰, one value of 2.7‰), positive εNd(t) (1.4–1.8) and high εHf(t) (3.6–9.4) formed from partial melting of juvenile material derived from depleted mantle. The one low δ18O value from the biotite granite is probably the result of post magmatic alteration. The geochemical data suggest that the granitic rocks of the Haobugao area were not contaminated by older crust and formed in an extensional environment related to Paleo-Pacific oceanic slab rollback.

Terrane geodynamics: Evolution on the subduction conveyor from pre-collision to post-collision and implications on Tethyan orogeny

Terranes are passengers within drifting oceanic plate, and ride with the plate to subduction plate boundaries. Although oceanic lithosphere readily subducts into the mantle at the plate boundary, terranes may resist sinking or not, depending on a variety of controlling factors that are not very well understood. Further, the tectonic development of terranes prior to their arrival at a subduction zone is not well documented. We performed numerical experiments to explore these unknowns of terrane geodynamics during the whole pre- to post-collisional period. Our analyses reveal that terranes can undergo considerable extension prior to their arrival to subduction plate boundaries owing to the pull force exerted by sinking oceanic slabs. Increasing terrane crustal thickness, decreasing terrane width or imposed convergence velocity, and an intra-oceanic subduction setting aggrandizes the pre-collisional terrane extension. The numerical models identify increasing terrane crustal thickness and width, and decreasing imposed convergence velocity as factors that promote terrane accretion. Additionally, having a continental overriding plate at the subduction boundary increases the propensity for terrane accretion. Some intra-oceanic subduction experiments demonstrate ablative subduction and subduction polarity reversal events in connection with terrane collisions/subduction. We compare our models to the evolution of a controversial Tethyan terrane, the Nilufer oceanic plateau. Our models suggest that the Nilufer terrane may have undergone pre-collisional extension owing to the pull of the sinking Tethys Ocean plate. Further, uninterrupted subduction of the Tethys Ocean, despite the accretion of the Nilufer terrane, is illustrated by our results which have implications on other regions along the Tethyan orogenic belt.

Crustal reworking and growth during India–Asia continental collision: Insights from early Cenozoic granitoids in the central Lhasa Terrane, Tibet

Studying granitoids can provide important knowledge on the composition of continental crust and the geodynamic processes of continental growth. Nevertheless, the mechanisms of granitoids formation during continental collision remain uncertain. In this paper, we present new zircon U–Pb–Hf isotope and whole‐rock Sr–Nd isotope compositions, and major‐ and trace‐element data for granitoids from the Chacangka area of the central Lhasa Terrane, southern Tibet. The Chacangka quartz monzonites crystallized at ca. 61 Ma, whereas the monzonites and granites were emplaced at ca. 51 Ma. The quartz monzonites are peraluminous and have high 87Sr/86Sri ratios (0.7129–0.7131), and their low εNd(t) (−7.65 to −7.10) and zircon εHf(t) (−9.3 to −7.5) values are more depleted than those of ancient crustal basement beneath the central Lhasa Terrane, which suggests that they originated from a mixed source of juvenile and ancient lower crust. Compared with the quartz monzonites, the monzonites have much higher contents of MgO (2.34–3.62 wt%) and values of Mg# (42.3–46.3) and zircon εHf(t) (−6.9 to 1.9) and show more depleted Sr–Nd isotopes, indicating that more juvenile materials were involved in their source. The granites have Sr–Nd isotope compositions that are similar to those of the coeval monzonites but have higher SiO2 contents, lower CaO and MgO contents, and lower values of Mg#, suggesting that they were differentiates of the monzonites. The elemental and isotopic compositions of the Chacangka granitoids exhibit a marked transformation from 61 to 51 Ma, probably indicating breakoff of the Neo‐Tethyan oceanic slab. The Neo‐Tethyan slab breakoff not only induced the upwelling of deep material and eruption of magma but also caused re‐melting and destruction of ancient crustal basement of the central Lhasa Terrane.

A review of Devonian–Carboniferous magmatism in the central region of Argentina, pre-Andean margin of SW Gondwana

We review a widespread project database for Devonian–Carboniferous magmatism in the Sierras Pampeanas and Frontal Cordillera between 27° and 35°S, including petrological, geochemical, geochronological, and isotope data, with compiled data from the literature and some new results. We distinguish four main magmatic domains: 1) Devonian Arc, 2) Devonian Foreland, 3) Carboniferous Arc, and 4) Carboniferous Retro-Arc. Devonian segmented subduction led to two oceanic slab configurations: 1) flat-slab subduction in the outboard region and resubduction >800 km inland from the trench, including lithosphere delamination of the upper plate and break-off of the subducted oceanic slab, along with asthenospheric mantle upwelling (31°–33° 30′S), and 2) normal subduction (34° - 35°S). In the first configuration the arc magmatism was absent, but voluminous foreland magmatism was developed, including small-scale high silica adakite. The second geodynamic setting took place during the latest Devonian and Carboniferous when the oceanic slab roll-back occurred. Arc and retro-arc magmatism were developed coetaneously between 27° and 31°S, with northwest migration that could be explained by movement relative to hot asthenospheric mantle. The arc is represented by calc-alkaline granitoids, whereas retro-arc magmatism consisted of (a) metaluminous to weakly peraluminous A-type granites, and (b) strongly peraluminous A-type granites, these latter with sometimes incomplete isotopic homogenization of the parental magma. Devonian–Carboniferous magmatic evolution here is explained by segmented tectonic subduction and a switch-off and switch-on geodynamic model. Magmatic activity was mainly continuous from ca. 390 to 320 Ma, although compositional variations occurred through time and space (foreland, arc, and retro-arc). Major involvement of mantle sources in the genesis of the Carboniferous arc granites at 28°–30°S contrast with a dominant continental signature in the granites of the Devonian arc at 34°–35°S. These differences are explained by two different configurations in the subduction system related to advance (28°–30°S) or retreat (34°–35°S) of the subducted ocean slab. The main conclusion of this work is that the complex interaction of oceanic and continental plates can produce different types of magmatism (or its absence): subduction processes do not consist only of an oceanic plate sinking under a continental plate.