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The subcontinental lithospheric mantle (SCLM) is now suggested by many workers to play a role in the formation of orogenic gold deposits in active continental margins, given that the gold and volatiles may be introduced into the SCLM during plate subduction. The giant Cretaceous Jiaodong gold province within the North China block occurs in a convergent margin setting where there is no terrane accretion, thus eliminating from consideration the commonly accepted metal and fluid source reservoir for orogenic gold--the metamorphosed upper crust of an active orogenic belt. Thus, the auriferous fluids were released below the high-grade Archean cratonic rocks from either enriched SCLM or directly from the subducted oceanic slab. Mercury (Hg) isotopes allow fingerprinting of isotopically distinct reservoirs, discriminating between the two possibilities. We present Hg isotope data for a set of pyrite grains, native gold, and lamprophyres from six main gold districts in the Jiaodong gold province. These samples dominantly yielded near-zero Δ199Hg (0‰ ± 0.1‰), within uncertainty of the mantle Δ199Hg value. The results provide novel evidence that the SCLM predominantly controlled the Hg budget of the Jiaodong gold deposits, implying that the SCLM acted as a buffer for the crustal mass-independent fractionation Hg signatures and a storage reservoir for gold and volatiles. Consequently, during slab subduction, gold and volatiles were recycled and stored within the SCLM and were later released from the mantle during uplift and relaxation of compressional tectonics. [ABSTRACT FROM AUTHOR]

The Bayingobi basin is located in the middle of Central Asia Orogenic Belt, at the intersection of Paleo‐Asian Ocean and Tethys Ocean, as well as the junction of multiple tectonic plates. This unique tectonic setting underpins the basin's intricate history of tectonic activity. To unravel the multifaceted tectono‐thermal evolution within the southwestern region of the basin and to elucidate the implications of sandstone‐hosted uranium mineralization, granitic and clastic rock samples were collected from the Zongnai Mts. uplift and Yingejing depression, and apatite fission track (AFT) dating and thermal history simulation analysis were performed. AFT dating findings reveal that the apparent ages of all samples fall within the range of 244 Ma to 112 Ma. In particular, the bedrock of the Zongnai Mts. and Jurassic detrital apatite fission tracks have undergone complete annealing, capturing the uplift‐cooling age. Meanwhile, the AFT ages of Cretaceous detrital rocks are either equivalent to or notably exceed the age of sedimentary strata, signifying the cooling age of the provenance. A comprehensive examination of AFT ages and palaeocurrent direction analyses suggests that the Cretaceous source in the Tamusu area predominantly originated from the central and southern sectors of the Zongnai Mts. uplift. However, at a certain juncture during the Late Early Cretaceous, the Cretaceous provenance expanded to include the northern part of the Zongnai Mts. uplift. Based on the results of thermal history simulations and previous studies, it is considered that the Tamusu area has undergone four distinct tectonic uplift events since the Late Paleozoic. The first is the Late Permian to Early Triassic (260–240 Ma), which is associated with the closure of the Paleo‐Asian Ocean and the accretionary orogeny within the Alxa region. The second uplift event took place in the Early Jurassic (190–175 Ma) and corresponded to intraplate orogeny following the closure of the Paleo‐Asian Ocean. The third uplift event is the Late Jurassic to Early Cretaceous (160–120 Ma), which is linked to the East Asia's position as the convergence center of multiple tectonic plates during this period. The fourth uplift event is linked to the Late Early Cretaceous (112–100 Ma), driven either by the westward subduction of the eastern Pacific plate or the mantle upwelling resulting from the Bangong–Nujiang oceanic lithosphere subduction and slab break‐off. The primary stress orientation for the first three tectonic uplift phases approximated a nearly SN direction, while the fourth stage featured a principal stress direction of NW. The fourth tectonic uplift event of the Late Early Cretaceous and basaltic eruption thermal event during this period likely exerted a significant influence on the formation of the Tamusu sandstone‐hosted uranium deposit. [ABSTRACT FROM AUTHOR]

This study integrates geophysical‐geochemical data to investigate the thermochemical structure of the lithosphere and sublithospheric mantle, along the Southern Tyrrhenian Basin, Apennines, Adriatic Sea, Dinarides, and Carpathians‐Balkanides. We present the lithospheric structure of the Adria microplate and the two opposing mantle slabs along its NE and SW margins. The modeling shows the presence of two asthenospheric mantle wedges aligning with the Apenninic and Dinaric continental mantle slab rollback, along with cold (−200°C) sublithospheric anomalies beneath Adria's NE and SW margins. In the northern Adria region, the lithosphere undergoes synchronous thinning in the Tyrrhenian domain and thickening toward the forefront of the northern Apennines. This is associated with the northeastward rollback of the SW Adriatic slab, leading to subsequent delamination of the continental mantle. In the southern Adria region, the complex deep structure results from the variably oriented lithospheric slabs, and nearly 90‐degree shift of the tectonic grain between the southern Apennines and the Calabrian Arc. At the SW Adria margin, beneath the northern Apennines, the thermal sublithospheric anomaly is attached to the shallower lithosphere, while a slab gap is modeled in the southern Apennines. One possibility is that the gap is due to a recent horizontal slab tear. Along the NE margin of Adria, the thermal anomaly penetrates to depths of about 200 km in the northern Dinarides and 280 km in the southern Dinarides, shallower than the SW Adria anomaly, which extends to at least 400 km depth. Plain Language Summary: We integrate geological and geophysical data (e.g., elevation, gravity, geoid, seismic tomography) to investigate the density and temperature distribution of the crust and lithospheric mantle down to 400 km, along a profile that extends from the Southern Tyrrhenian Basin and Apennines in Italy to the Southern Dinarides and Carpathians‐Balkanides in southern Europe. Taking advantage of our previous research in this region, we provide an integrated view of the lithospheric structure and the uppermost mantle of the Tyrrhenian Basin, Apennines, Adriatic Sea and Dinarides. Our results show notable variations in both the crust and the base of the lithosphere. These variations are observed to be shallower beneath the basins and deeper beneath the mountain belts, particularly in areas with elevated topography. Additionally, our observations highlight the presence of two cold and dense zones situated in the distal margins of the Adria microplate, specifically beneath the Apennines and Dinarides. The thermal anomalies indicate the presence of two oppositely subducting slabs with different geometries and depth of penetration. Their current location, depth and shape have largely controlled the geodynamic evolution of the study region in the last 30 My. Key Points: We unveil the thermochemical structure of the lithosphere and uppermost mantle of the Adria Microplate and its marginsResults reveal mantle wedges aligning with Apenninic and Dinaric slabs, and cold sublithospheric anomalies beneath NE and SW Adria marginsIn N Apennines the slab is attached while a gap is observed in S Apennines. At Adria's NE margin, the slab penetrates deeper southward [ABSTRACT FROM AUTHOR]

We present a new tectonic plate reconstruction that suggests substantial revisions to events associated with development of the Peruvian flat slab and resolves several long-standing issues regarding the subduction of bathymetric highs in the region. The Tuamotu Plateau is widely considered to be the product of Easter Plume magmatism, and plate reconstructions suggest it formed following initial plume ascent at ca. 55 Ma. The Nazca Ridge is also linked to the Easter Plume and is an obvious candidate to be the spreading ridge conjugate to the Tuamotu Plateau. Models for the paired evolution of the two ridges, however, generally stop at ca. 33 Ma because of the inability of plate reconstructions to associate the two ridges across a spreading center prior to this time. In addition, seafloor magnetic data demonstrate that the Tuamotu Plateau developed at a complexly shaped and evolving mid-oceanic ridge that precluded development of a simple mirror image conjugate of the type commonly employed in Nazca Ridge reconstructions. Seafloor isochrons also suggest that a ridge jump separated the Tuamotu Plateau from its conjugate at ca. 42 Ma. Global plate models offer an alternative approach to assessing conjugate development, by showing how a hypothetical conjugate to the Tuamotu Plateau is built up over time. Using such a model, we found that the conjugate that developed during the main stage of Tuamotu growth (55 Ma to 42 Ma) cannot be the Nazca Ridge, which appears to have initiated at ca. 42 Ma, when the Easter Plume diverted volcanism southward. We named the newly recognized conjugate the Enigma Ridge. Importantly, subduction of this ridge starting from ca. 17 Ma on the north Peruvian trench can account for the missing slab buoyancy previously attributed to the hypothesized, but controversial, Inca Plateau. The Enigma Ridge must still be providing far more buoyancy over a much greater area than the Nazca Ridge, which only began to subduct rather recently. [ABSTRACT FROM AUTHOR]

Differentiating between advancing and retreating subduction zones and delineating the timing and processes for switching between these end-member types are critical factors in understanding orogenic evolution. In this study, we used temporal composition variations and spatiotemporal distribution of igneous rocks to constrain the types and directions of the subduction zone beneath the Lhasa terrane in southern Tibet during the mid-Mesozoic. Comprehensive geochronological and geochemical data from mid- Mesozoic magmatic rocks in five areas of the central Lhasa terrane document two compositionally distinct magmatic suites: older (ca. 172-150 Ma) high-Mg diorite-granodiorite and younger (ca. 150-130 Ma) low-Mg granodiorite-syenogranite. The high-Mg rocks with enriched isotopes were likely generated by the mixing of Lhasa terrane basement- derived magmas with mantle-derived magmas that incorporated extensive contributions from subducted sediments. The low- Mg rocks with relatively depleted isotopes are interpreted as derived from a mixed lowercrust source (ancient basement and juvenile crust) with minor mantle contributions followed by crystal fractionation. Both high-Mg and low-Mg rocks show arc signatures and formed the 1200 km Shiquan-Zhegu-Menba arc. A combination of the Mesozoic geological and geochemical data suggests that both the Neo-Tethyan and Bangong Tethyan slabs were likely subducting beneath the Lhasa terrane, creating two opposing subduction zones. The change from high-Mg to low- Mg magmatism within the Shiquan-Zhegu- Menba arc likely records a tectonic switch from advancing to retreating subduction of the south-dipping Bangong Tethyan slab. Our results provide a good example for identification of subduction polarities and types of ancient orogenic belts through spatiotemporal distributions and changes in geochemical compositions of igneous rocks. [ABSTRACT FROM AUTHOR]

Arc magmatism can provide crucial information about crustal recycling and growth in subduction zones. However, the crustal material transfer process and mechanism from subducting slab to overlying mantle wedge are controversial. Here, we present detailed petrological, geochronological, geochemical, and Sr-Nd-Hf isotopic data, and previously published data for two episodes of subduction-related magmatism in the Northern Yili Block, NW China, to study these issues in relation to the recycling of continental crust and the growth of two separate magmatic episodes at 400-350 Ma and 350-300 Ma, respectively. The Nd-Sr isotope modeling results, low Nd/Sr, and variable Hf/Nd ratios demonstrate that the arc magmatism of these two episodes could be derived from two distinct mélange sources that formed at the slab-mantle interface during subduction of the Junggar oceanic plate. In the first episode, magmatic rocks exhibit high Th/Yb and (La/Sm)N ratios and the decoupling of Nd-Hf isotopes, which leads to an interpretation that the primary magmas were derived from sources mainly mixed by sediments and mantle-wedge peridotites. In contrast, the magmas of the second episode exhibit high Ba/La and Ba/Th ratios and the coupling of Nd-Hf isotopes, which implies that these magmas were likely produced from mélange diapirs dominated by the mixing of midoceanic-ridge basalts and mantle peridotites. The episodes of Nd-Hf isotopic decoupling and coupling coincided with crustal material recycling and crustal growth in subduction zones, respectively. Considering that the Northern Yili Block at 400-350 Ma was characterized mainly by enriched Nd-Hf isotopes of magmatic rocks and no development of the forearc accretionary complex, while the Northern Yili Block at 350-300 Ma featured high εHf(t)-εNd(t) values of magmatic rocks, occurrences of extension-related magmatism, and distribution of the forearc accretionary complex and immature back-arc basin, we propose that these two different mélange sources probably developed in response to subduction transition. That is, one resulted from advancing and the other was relevant to retreating regime. The partial melting of these mélanges in the mantle wedge generated intermediate felsic magmas, which might have acted as a leading mechanism for crustal recycling and growth of the accretionary orogenic belt. [ABSTRACT FROM AUTHOR]

The Late Carboniferous to Early Permian is a critical period of the Chinese Tianshan, witnessing the tectonic transition from subduction to post-collisional extension during the final amalgamation of the Central Asian Orogenic Belt (CAOB). The late Carboniferous Mozbaysay mafic–ultramafic complex in the Qijiaojing–Balikun area, eastern North Tianshan, provides important clues for revealing the nature and timing of this tectonic transition. The Mozbaysay complex comprises mainly hornblende gabbros and lherzolites. LA-ICP-MS U-Pb zircon ages of hornblende gabbro yielded a weighted mean age of 306 ± 1.9 Ma for this complex. These mafic–ultramafic rocks have high contents of MgO (up to 30 wt.%), Cr (up to 2493 ppm), and Ni (up to 1041 ppm), but low contents of SiO2 (40.34–47.70 wt.%). They are enriched in LREE and show characteristics of enriched mid-ocean ridge basalts (E-MORB). The relatively high Th/Yb and Ba/Nb ratios imply the mantle sources could have been metasomatized by slab–mantle interaction with aqueous fluids from dehydration of the subducted slab. Thus, these mafic–ultramafic rocks were most likely produced by partial melting of the asthenospheric and lithospheric mantle with a slight influence of slab-derived fluids. Therefore, we suggest that the formation of these Late Carboniferous mafic–ultramafic rocks was triggered by the decompression-induced influx of asthenospheric heat and melting through a slab window during post-collisional slab breakoff. Combined with geological data, the petrogenetic links of the Late Carboniferous mafic–ultramafic rocks in eastern North Tianshan to slab breakoff suggest that the tectonic transition from convergence to post-collision most likely initiated in situ at ca. 306 Ma and lasted to ca. 300 Ma. [ABSTRACT FROM AUTHOR]

The West Junggar terrane (WJT) is known to tectonically correlate with the Central and East Kazakhstan terrane; however, the Middle Palaeozoic magmatism has rarely been reported in the southern WJT, leading to significant uncertainties on the tectonic evolution and correlation of the southwestern Central Asian Orogenic Belt (CAOB). Here, we present the adakitic quartz diorites with zircon U-Pb ages of 390–394 Ma in the Alashankou region to demonstrate the presence of Early-Middle Devonian magmatism in the southern WJT for the first time. They are characterized by low Yb (1.00–1.41 ppm) and Y (8.9–14.4 ppm) but high Sr (301.5–475.9 ppm), Sr/Y (23–41), and (La/Yb)N (8.1–11) values, typical of modern adakites. As suggested by the relatively high SiO2 (64.96–67.71 wt.%), moderate K2O (1.53–3.19 wt.%), but low MgO (1.56–2.31 wt.%), TiO2 (0.28–0.42 wt.%), Cr (≤13.7 ppm), Co (2.7–9.9 ppm), Ni (8.0–18.7 ppm), and Th (3.7–4.8 ppm) concentrations as well as the high positive ɛHf(t) values (+5.6 to +10.3) and low Nb/Ta ratios, the adakitic quartz diorites were probably generated by the partial melting of a slightly thickened lower crust, which consisted mainly of the Ediacaran-Middle Palaeozoic intra-oceanic arc rocks in the southern WJT. Combined with regional data, our new results support that the southern WJT probably evolved as the eastern extension of the Kazakhstan Early-Middle Devonian outer volcanic belt during the Middle Palaeozoic. The Early-Middle Devonian tectonic extension and resultant lower crust melting may thus be related to the slab retreat of the Junggar-Balkhash Ocean. [ABSTRACT FROM AUTHOR]

The process of slab tearing profoundly affects interchanges of material and energy between the lithosphere and asthenosphere and has been invoked as a trigger of magmatism in modern and ancient subduction‐collision systems. Horizontal slab tears may occur after continental collision and subsequent slab break‐off, whereas vertical slab tears may occur in response to along‐strike variations in rollback velocity, thereby ripping the subducting lithosphere. Previous studies have documented tearing of subducting slabs in modern convergent settings using geophysical methods, but in ancient collisional belts, where these methods cannot be applied, detection of slab tearing is more difficult, particularly if the oceanic plate was fully subducted and the geological evidence for subduction is limited. Here, we employ multiproxy element‐isotope imaging, as well as zircon petrochronology and whole‐rock geochemistry, to assess evidence of vertical tearing of the Mianlue slab in the Triassic Qinling Orogen in central China. Our results show evidence of anomalous, inferred tear‐related magmatism east of 107°E, at 225–210 Ma. Vertical tearing of the northward subducting Mianlue slab is recorded by geochemically anomalous igneous rocks, and the recognition of N–S to NE–SW trending boundaries defined by Sr‐Nd‐Hf isotopes, elemental ratios (Nb/Ta, and Ca/Al), Mg#, and reduced crustal thickness between 107° and 108°E. The inferred vertical tear may have been driven by slab rollback of the Mianlue slab at 225–210 Ma. The results show that multiproxy element‐isotope imaging (e.g., of εNd(t), εHf(t), (87Sr/86Sr)i, Nb/Ta, Mg#, and Ca/Al) can be used to reconstruct the evolution of fossil subduction systems. Plain Language Summary: Subduction zones are natural factories for the circulation of material in the Earth as they allow tectonic plates to descend into the mantle. Pieces of subducting plates, normally referred to as slabs, can be torn and segmented, but reconstructing the geometry of such tears in ancient convergent plate boundary settings is difficult. Based on the observation that slab tearing can trigger anomalous magmatism in convergent plate margins, we present a multiproxy geochemical and isotopic imaging method that enables us to understand the process of slab tearing in the Qinling Orogen (China) during the Triassic. We found that igneous rocks from the Qinling Orogen, east of 107°E, are geochemically anomalous (relative to typical arc rocks). These rocks are dated 225–210 Ma. By imaging various geochemical characteristics, our results demonstrate that the subducting Mianlue slab was likely torn at longitude 107°–108°E, thereby producing geochemically and isotopically anomalous mafic and felsic magmatism. Key Points: The geometry of slab tearing in an ancient orogen is constrained by a multiproxy geochemical approachAnomalous arc magmatism dated at 225–210 Ma is identified in the Qinling Orogen east of 107°EThe Mianlue slab underwent southward vertical slab tearing at 107°−108°E in the Late Triassic [ABSTRACT FROM AUTHOR]

Changes in magmatism and sedimentation along the late Neoproterozoic-early Paleozoic Ross orogenic belt in Antarctica have been linked to the cessation of convergence along the Mozambique belt during the assembly of East-West Gondwana. However, these interpretations are non-unique and are based, in part, on limited thermochronological data sets spread out along large sectors of the East Antarctic margin. We report new 40Ar/39Ar hornblende, muscovite, and biotite age data for plutonic (n = 13) and metasedimentary (n = 3) samples from the Shackleton–Liv Glacier sector of the Queen Maud Mountains in Antarctica. Cumulative 40Ar/39Ar age data show polymodal age peaks (510 Ma, 491 Ma, 475 Ma) that lag peaks in U-Pb igneous crystallization ages, suggesting igneous and metamorphic cooling following magmatism within the region. The 40Ar/39Ar ages are similar to ages in other sectors of the Ross orogen, but younger than detrital mineral 40Ar/39Ar cooling ages indicative of older magmatism and cooling of unexposed inboard areas along the margin. Detrital zircon trace element abundances suggest that the widespread onset of magmatism in outboard localities of the orogen correlates with a ~560–530 Ma decrease in crustal thickness. The timing of crustal thinning recorded by zircon in magmas overlaps with other evidence for the timing of crustal extension, suggesting that the regional onset of magmatism with subsequent igneous and metamorphic cooling probably reflects slab rollback that coincided with possible global plate motion changes induced during the final assembly of Gondwana. [ABSTRACT FROM AUTHOR]

Stratigraphic and thermochronologic data are used to study the processes that shaped the topography of the central Apennines of Italy. These are part of a major, active mountain belt in the center of the Mediterranean area, where several subduction zones control a complex topography. The Apennines were shaped by contraction at the front of the accretionary wedge overlying the subducting Adria microplate followed by extension at the wedge rear in response to eastward slab rollback. In the central Apennines, intermontane extensional basins on the western flank rise eastward toward the summit. We contribute with new data consisting of 28 (U-Th-Sm)/ He and 10 fission track ages on apatites to resolve a complex pattern of thermal histories in time and space, which we interpret as reflecting the transitional state of the orogen, undergoing a two-phase evolution related to initial slab retreat, followed by slab detachment. Along the Tyrrhenian coast, we document cooling from depths ≥3-4 km occurring between 8 and 5 Ma and related to the opening of marine extensional basins. Post--5 Ma, a broader region of the central Apennines exhibits cooling from variable depths, between <2 km in most areas and ≥3-4 km in the northeast, and with different onset times: at ca. 4 Ma in the west, at ca. 2.5 Ma in the center and northeast, and at ca. 1 Ma in the southeast. Between 5 and 2.5 Ma, exhumation is associated with modest topographic growth during the late stages of thrusting. Since 2.5 Ma, exhumation has concurred with the opening of intermontane basins in the west and in the east, with regional topographic growth and erosion, that we interpret to be associated with the locally detaching slab. [ABSTRACT FROM AUTHOR]

Compositional changes in successively erupted felsic rocks can be used to infer physical changes in lower crustal conditions and to enhance the understanding of the tectonic regime. This study presents geochronological, geochemical and isotopic data for two I-type granitic plutons in the Sonid Left Banner of the Central Asian Orogenic Belt. Our new data, together with compiled I-type granitoid data, reveal the presence of magma compositional transition at ∼305 Ma in the Baolidao arc-accretion belt. The early stage granitoids (330–305 Ma) are medium-K calc-alkaline with higher Sr/Y ratios. The late stage granitoids (305–270 Ma) are high-K calc-alkaline with lower Sr/Y ratios. The two-stage granitoids have roughly similar predominately positive Sr−Nd−Hf isotope values, but with a decreasing trend from the early to late stages. Geochemical data indicate that the early stage granitoids were generated by dehydration melting of juvenile mafic crust at amphibole-dominated depths. In contrast, the late stage granitoids were produced by dehydration melting of a mixed lithology containing juvenile K-rich mafic lower crust and supracrustal materials at the plagioclase-stable crustal level. We propose that the compositional transition of these granitoids can be linked with different slab behaviors of the northward subducting Paleo-Asian oceanic plate, and also with the back-arc tectonic settings. [ABSTRACT FROM AUTHOR]

Quantifying the rhythms and rates of magmatic-hydrothermal systems is critical for a better understanding of their controls on ore formation and the dynamics of magmatic reservoirs that feed them. We reconstructed the evolution of ore-forming fluids using hydrothermal quartz from the 17.4 Ma Zhibula skarn, Tibet. Ion probe analysis reveals sharp and dramatic changes in quartz δ18O values between 5‰ and -9.3‰, with fluid δ18O values varying between 2.8‰ and -18.2‰, which are best explained by transient meteoric water incursion into a hydrothermal system dominated by magmatic fluids. Two pulses of magmatic fluids and a meteoric water incursion event are inferred, which operated at the millennium scale (760-1510 yr) as constrained by the aluminum diffusion chronometer. Our results indicate that magmatic reservoirs are likely water unsaturated for most of their lifetime (>105 -106 yr), with transient and episodic fluid exsolutions (-10³ yr) being driven by magma replenishment or crystallization-induced water saturation. With focused and efficient metal deposition, multiple pulses of metallifer- ous fluids favor the formation of giant deposits with high grade. Meteoric water δ18O values (-25.4 ± 2.3‰) derived from Zhibula quartz further suggest a paleo-elevation of 5.9 ± 0.3 km; this transient early Miocene surface uplift plausibly was due to break-off of the oceanic slab attached to the Indian Plate. Our research highlights that ubiquitous hydrothermal quartz in orogenic belts can probe the dynamics of magmatic-hydrothermal systems and also quantify paleo-elevations, which has significant tectonic implications. [ABSTRACT FROM AUTHOR]

High-Mg andesites (HMAs) and their cognate intrusive rocks constitute volumetrically very small proportions of the total earth, and are mainly distributed along the edges of convergent plates. Petrogenetic studies can provide possible solutions for discrepancies in the geodynamics and subduction zone evolution. This paper presents the first ever reports of the newly discovered high-Mg diorite in Akechukesai area, the western part of the East Kunlun Orogenic Belt, and provides a reference for the evolutionary history and subduction mechanism of the Proto-Tethys Ocean. Akechukesai high-Mg diorites yielded a weighted mean zircon U-Pb dating age of 427.3 ± 2.3 Ma (Middle Silurian). Results of the geochemical analyses show that the high-Mg diorites were high-K calc-alkaline series with the SiO2 content ranging 50.40 to 55.41 wt%. They are characterized by high values of Mg# (67–77), high MgO (6.92–10.58 wt%), TiO2 (0.53–0.87 wt%), Cr (286–615 ppm), Ni (61–124 ppm), Ba (570–927 ppm) contents, and low FeOtotal/MgO ratios (0.54–0.89). Furthermore, they exhibit nearly flat right-declined rare-earth element (REE) patterns with slight LREE enrichment. The samples are enriched in large ion lithophile elements (e.g., Ba, Rb, and Th) and depleted in high field strength elements (e.g., Ta, Nb, and Ti). These geochemical features are analogous to the sanukitic high-Mg andesites. The mean value of the initial εHf(t) is −1.3, indicating that the source is enriched mantle. The values of Rb/Cs, Ba/La, and La/Sm ratios suggest that subducting sediments formed an important component of the magmatic source. The presence of water-bearing minerals such as amphibole and biotite indicate a water-rich and oxygen-rich primitive magma system. Petrogenetic analysis indicates that the Akechukesai high-Mg diorites probably formed by melts and aqueous fluids produced from partial melting of the subducting sediments interacting with mantle peridotites. We hypothesize that, after the closure of the Proto-Tethys Ocean Basin in the Middle Silurian, the deep subducted slab broke-off and formed a slab window, asthenospheric material upwelled heating the subducting sediments and causing them to melt. Thus, we suggest that the emplacement of the Akechukesai high-Mg diorites mark the commencement of post-collisional magmatism. [ABSTRACT FROM AUTHOR]

Migmatites can be produced in different stages of orogeny and thus have a great significance to constrain the evolution of an orogenic belt. Migmatites widely occur in the Tongbai orogen, but their formation ages and geological significance have not been well constrained. In this study, a combination of cathodoluminescence (CL) imaging, U-Pb-Hf-O isotopes and trace elements of zircons were carried for migmatites in the Tongbai orogen. Most inherited cores in a melanosome show variable U Pb ages and Hf isotope compositions with high δ18O values, suggesting they might be detrital zircons from its protolith. Whereas other relict cores in this sample have nebulously zoning or no zoning, flat HREE patterns with insignificant Eu anomalies and relatively low formation temperatures, indicating their formation under eclogite-facies conditions. They yield U Pb ages of 228 ± 3 Ma, providing direct evidence for the involvement of Triassic subducted continental materials. The overgrowth rims and some new grown grains in the migmatites show euhedral shapes, low CL luminescence, high Th/U ratios, clear negative Eu anomalies and steep HREE patterns, suggesting they are anatectic zircons formed during partial melting. They yield U Pb ages from 139 ± 2 to 132 ± 1 Ma. Combined with previous studies, we suggest that migmatization mainly occurred at 139–130 Ma in the Tongbai orogen. The anatectic zircons in the leucosomes show much lower δ18O values (5.79 ± 0.23‰ vs. 8.94 ± 0.16‰) and higher 176Hf/177Hf ratios (0.282302–0.282648 vs. 0.281728–0.282504) than the zircons in adjacent melanosomes, suggesting that the leucosomes might be externally sourced. According to a compilation of the geochronological information of migmatite and magmatism, the Tongbai-Dabie-Sulu orogens have a spatial and temporal variation of partial melting and magma activities in the post-collisional stage. These might result from the diachronical tectonic switch and collapse of the orogenic lithospheric keels caused by the subduction and rollback of the Paleo-Pacific slab. • The Tongbai migmatites formed at ca. 139–130 Ma. • There exist spatiotemporal variations of partial melting and magmatism in the Tongbai-Dabie-Sulu orogen. • A diachronous tectonic switch has been documented. [ABSTRACT FROM AUTHOR]

The geodynamic evolution of the Himalayan–Tibetan orogen during the transition from Neo-Tethyan oceanic to Indian continental subduction remains unclear. Therefore, we conducted a new geochronological, geochemical, and Sr–Nd–Pb–Hf isotopic study of post-collisional diabase dikes in the central segment of the southern Lhasa subterrane. The diabase dike samples yield zircon U–Pb ages of 51–49 Myr, consistent with the timing of the Gangdese magmatic flare-up (ca. 53–50 Myr). In addition, the diabase dikes exhibit a geochemical intraplate affinity and depleted Sr–Nd–Pb isotopic compositions. Their variable Nb contents allow, the dikes to be divided into low-Nb (<8 ppm) and high-Nb (>9 ppm) types, which represent the melting products of slab fluid-modified Gangdese lithospheric mantle and a mixed mantle source comprising Gangdese lithospheric and deep asthenospheric mantle, respectively. In addition, the wide variation in εHf(t) values (−10.0 to +12.3) of zircons from these diabase dikes is consistent with the Gangdese zircon Hf isotopic 'pull-down' at ca. 50 Ma. Integrating our new data with published results suggests that the zircon Hf isotopic 'pull-down' was a direct consequence of the involvement of subducted Indian continental components in the formation of the Gangdese continental lithosphere. The combined data further suggest that upwelling of the asthenosphere, triggered by breakoff of the Neo-Tethyan oceanic slab, resulted in extensive mantle and crustal melting in the Lhasa terrane and the Tethyan Himalaya in southern Tibet. • The diabase dikes in the Pengcuolin–Dazhuqu region were emplaced at ca. 50 Ma. • Indian continental components were involved in the Gangdese lithosphere at ca. 50 Ma. • The breakoff of the Neo-Tethyan slab is believed to have occurred at ca. 53–50 Ma. [ABSTRACT FROM AUTHOR]

In cordilleran-type orogens, subduction geometry exerts a fundamental control on the tectonic behavior of the overriding plate. An integrated low-temperature, large thermochronological data set is used in this study to investigate the burial and exhumation history of the overriding plate in northern Patagonia (40°-45°S). Thermal inverse modeling allowed us to establish that a ~2.5-4-km-thick section originally overlaid the Jurassic-Lower Cretaceous successions deposited in half-graben systems that are presently exposed in the foreland. Removal of the sedimentary cover started in the late Early Cretaceous. This was coeval with an increase of the convergence rate and a switch to a westward absolute motion of the South American Plate that was accompanied by shallowing of the subducting slab. Unroofing was probably further enhanced by Late Cretaceous to early Paleogene opening of a slab window beneath the overriding plate. Following a tectonically quiescent period, renewed exhumation occurred in the orogen during relatively fast Neogene plate convergence. However, even the highly sensitive apatite (U-Th)/He thermochronometer does not record any coeval cooling in the foreland. The comparison between Late Cretaceous and Neogene exhumation patterns provides clear evidence of the fundamental role played by inter-plate coupling associated with shallow slab configurations in controlling plate-scale deformation. Our results, besides highlighting for the first time how the whole northern Patagonia foreland was affected by an exhumation of several kilometers since the Late Cretaceous, provide unrivalled evidence of the link between deep geodynamic processes affecting the slab and the modes and timing of unroofing of different sectors of the overriding plate. [ABSTRACT FROM AUTHOR]

Extensive angular unconformities that occur across the Tianshan Orogen provide insight into the Paleozoic tectonic evolution of the southwestern Central Asian Orogenic Belt. This study presents new geochronological and geochemical data to unravel the age and origin of a pronounced angular unconformity in the Baluntai domain of the Chinese Western Tianshan. The angular unconformity separates a ductilely deformed metamorphic complex from overlying unmetamorphosed sequences. Detrital zircon age spectra suggest that the sedimentary samples received detritus from both the Precambrian basement and contemporaneous igneous rocks in the Central Tianshan and Yili blocks. The youngest detrital and metamorphic zircons in metasediments from the metamorphic complex reveal that their protoliths were deposited at ca. 419 Ma, and subsequently overprinted by metamorphism and ductile deformation at ca. 409 Ma. Along with ca. 532-407 Ma metavolcanics and foliated intrusions, the metamorphic complex constitutes a diachronous stratigraphic unit deposited during early Cambrian to late Silurian times, rather than a Precambrian basement as previously thought. The youngest detrital zircon age data from siliciclastic samples immediately above the angular unconformity constrain their deposition to ca. 407-397 Ma. In conjunction with a ca. 406 Ma rhyolitic tuff in the lower terrestrial siliciclastic unit and an early Carboniferous paleontological age of carbonates in the upper shallow-marine unit, it is suggested that the overlying unmetamorphosed sequences were accumulated during the early Devonian to the early Carboniferous. Thus, the angular unconformity in the Baluntai domain was formed between ca. 409 and 407 Ma. New and already published geochemical data reveal that the early Paleozoic (ca. 530-410 Ma) and late Paleozoic (ca. 410-320 Ma) igneous rocks are mainly medium- to high-K calc-alkaline I-type granitoids, which exhibit typical subduction-related geochemical signatures despite contrasting structural features. These geochronological and geochemical results are in accordance with those of similar early Devonian angular unconformities and Paleozoic igneous rocks in the southern Yili Block. The early Paleozoic igneous rocks display highly scattered zircon εHf(t) values, while the late Paleozoic rocks show a progressively increasing trend toward more positive values, attributed to slab rollback of the northward subducting South Tianshan Ocean under the Yili and Central Tianshan blocks. Thus, we propose that the formation of the angular unconformity was caused by tectonic conversion from contraction to extension due to slab rollback during the early Devonian. This study highlights the significance of geological and geochronological investigations of angular unconformities in the context of associated episodic magmatism and slab behavior in order to unravel distinct tectonic processes in the long-lived accretionary evolution of the Chinese Western Tianshan. [ABSTRACT FROM AUTHOR]

In this study, we present an estimate of the gravity signal of the slabs beneath the Alpine mountain belt. Estimates of the gravity effect of the subducting slabs are often omitted or simplified in crustal-scale models. The related signal is calculated here for alternative slab configurations at near-surface height and at a satellite altitude of 225 km. We apply three different modelling approaches in order to estimate the gravity signal from the subducting slab segments: (i) direct conversion of upper mantle seismic velocities to density distribution, which are then forward calculated to obtain the gravity signal; (ii) definition of slab geometries based on seismic crustal thickness and high-resolution upper mantle tomography for two competing slab configurations – the geometries are then forward calculated by assigning a constant density contrast and slab thickness; (iii) accounting for compositional and thermal variations with depth within the predefined slab geometry. Forward calculations predict a gravity signal of up to 40 mGal for the Alpine slab configuration. Significant differences in the gravity anomaly patterns are visible for different slab geometries in the near-surface gravity field. However, different contributing slab segments are not easily separated, especially at satellite altitude. Our results demonstrate that future studies addressing the lithospheric structure of the Alps should have to account for the subducting slabs in order to provide a meaningful representation of the geodynamic complex Alpine area. [ABSTRACT FROM AUTHOR]

Many convergent orogens, such as the eastern European Alps, display an asymmetric doubly vergent wedge geometry. In doubly vergent orogens, deepest exhumation occurs above the retro‐wedge. Deep‐seismic interpretations depict the European plate dipping beneath the Adriatic, suggesting the pro‐wedge location on the north side of the orogen. Our new thermochronometer data across the Eastern Alps confirm distinct shifts in the locus of exhumation associated with orogen‐scale structural reorganizations. Most importantly, we find a general Mid‐Miocene shift in exhumation (in the Tauern Window and the Southern Alps) and focus of modern seismicity across the Southern Alps. Taken together, these observations suggest a subduction polarity reversal at least since the Mid‐Miocene such that the present‐day pro‐wedge is located on the south side of the Alps. We propose a transient tectonic state of a slow‐and‐ongoing slab reversal coeval with motion along the Tauern Ramp, consistent with a present‐day northward migration of drainage divides. Plain Language Summary: When tectonic plates collide, they bend downwards and form two lithospheric wedges dipping in opposite directions, such as in the Eastern Alps. We present new crustal cooling data along a transect in the Eastern Alps confirming that surface rocks across the central Tauern Window originated from the deepest structural levels along the transect. South of the Tauern Window rocks were exhumed from higher depths compared to those north of it and were exhumed more recently, while seismic activity is also focused across the Southern Alps. These observations suggest a subduction polarity reversal because they are inconsistent with the original southern and northern locations of overriding and subducting plates, respectively, >15 million years ago. This interpretation is contrary to lithosphere‐scale tomography that shows no change in subduction polarity. Therefore, we propose a transient tectonic state, that is, a slow‐and‐ongoing subduction polarity reversal that initiated when Tauern Window rocks began their steep ascent to the surface along a deep‐seated fault known as the Tauern Ramp. This study bridges observations in the mantle, crust and on the surface over geologic time. Key Points: Thermochronologic data in the Eastern Alps is consistent with a transient tectonic state toward complete slab reversalThe pro‐wedge has switched from north to south of the Periadriatic Fault along TRANSALPMid‐Miocene motion along the Tauern Ramp is the consequence of slab‐reversal [ABSTRACT FROM AUTHOR]

We study the potential gravity effect of suggested slab configurations beneath the Alpine mountain belt. The opposing slab configurations are based on seismic crustal thickness estimates and high-resolution upper mantle tomographies. Direct conversion of upper mantle seismic velocities to densities results in a gravity response that reflects results in a gravity field that may be interpreted as related to the effect of subducting lithosphere, however the different contributing slab segments cannot be clearly identified. Therefore, we define the geometry of the upper slab interface by using the crustal thickness at 40 km depth as upper starting point. Based on seismic tomography, the slab interface is followed down to 200 km depth. We define two alternative models for the slab configuration in the Alpine region in line with recently proposed hypotheses. The gravity effect of these alternative models is calculated for (i) a simple constant density distribution in the slab and (ii) accounting for compositional and thermal variations with depth. The forward calculations predict a gravity effect of the slab up to 40 mGal and significant differences in the pattern of the anomalies. [ABSTRACT FROM AUTHOR]

After the onset of plate collision in the Alps, at 32–34 Ma, the deep structure of the orogen is inferred to have changed dramatically: European plate break-offs in various places of the Alpine arc, as well as a possible reversal of subduction polarity in the eastern Alps have been proposed. We review different high-resolution tomographic studies of the upper mantle and combine shear- and body-wave models to assess the most reliable geometries of the slabs. Several hypotheses for the tectonic evolution are presented and tested against the tomographic model interpretations and constraints from geologic and geodetic observations. We favor the interpretation of a recent European slab break-off under the western Alps. In the eastern Alps, we review three published scenarios for the subduction structure and propose a fourth one to reconcile the results from tomography and geology. We suggest that the fast slab anomalies are mainly due to European subduction; Adriatic subduction plays no or only a minor role along the Tauern window sections, possibly increasing towards the Dinarides. The apparent northward dip of the slab under the eastern Alps may be caused by imaging a combination of Adriatic slab, from the Dinaric subduction system, and a deeper lying European one, as well as by an overturned, retreating European slab. [ABSTRACT FROM AUTHOR]

The Carboniferous tectonic history of the northern West Junggar of NW China is of key importance for understanding the tectonics of the Altaids. This paper presents a systematic study of the emplacement of the E'min ophiolite, the ages of turbidites, and a stitching granite dike in order to constrain the late Carboniferous oceanic evolution of northern West Junggar. Our field investigation reveals that the Tarbagatay subduction–accretion complex formed by imbrication of turbidites and ophiolitic slices in the Western Tarbagatay Mountain, which is considered as the forearc of the Saur arc in northern West Junggar. Field relationships and zircon geochronology suggest that the emplacement of the E'min ophiolite was later than the minimum time of deposition of hanging wall turbidites, which have a zircon age of 324 Ma, and must be earlier than the crystallization time of a 311‐Ma stitching granite dike that intruded the younger turbidites. The turbidites become younger from north to south, with minimum deposition ages varying from 324 to 309 Ma. The 311‐Ma dike has an adakitic affinity suggesting that it may have formed by magma melted from a subduction slab wedge and from accreted material at ~310 Ma. Therefore, we propose an accretionary arc setting for the Saur arc, with southward migration of the trench and magma front. The 311‐Ma dike has a high Al enrichment and arc‐related geochemical signature suggesting that it was generated by melting of accreted trench sediments. This may indicate that subduction of the Saur arc may have lasted to at least the late Carboniferous. The development of such arc accretion suggests that the formation of the southern Altaids was fundamentally similar to that of accretionary orogens in the western Pacific. [ABSTRACT FROM AUTHOR]

Characterization of the magmatic response to scissor-like oblique continental collision and of the associated geodynamic mechanisms and crustal evolution is crucial to understanding the formation of collisional orogens. This paper presents geochronological, elemental, and isotopic data of three Triassic mafic–felsic intrusions formed during scissor-like continental collision in the Qinling orogenic belt (QOB), China. The biotite pyroxenite, gabbro, quartz monzonite, and syenite of the Jiuzigou (JZG) mafic–felsic intrusive complex were emplaced at 233–231 Ma and have extremely high Ba and Sr contents, fractionated rare-earth elements, insignificant Eu anomalies, and enriched Sr-Nd isotopes. A negative relationship between ε Nd (t) values and SiO 2 content suggests that the JZG complex was formed by assimilation and fractional crystallization (AFC) processes of enriched-mantle-derived melts. The Pangjiahe (PJH) and Laowangou (LWG) granite porphyries were emplaced at 224 Ma and 215 Ma, respectively, and are characterized by enriched large-ion lithophile elements, depleted high-field-strength elements, and enriched Sr and Nd isotopic compositions (0.70565–0.70648 and − 3.22 to −5.33 respectively), which indicate that these granite porphyries were sourced from mafic lower crust of the South Qinling terrane. The relatively high Mg numbers (45.8–49.4) might indicate mixing of crustal melt and minor mafic magma for the PJH granite porphyries. In contrast, the low Mg numbers (24.8–41.4) and obvious negative Eu anomalies of the LWG granite porphyries suggest an origin by fractional crystallization of crustal melts. The spatial–temporal distribution of the Triassic intrusions emplaced during the continental collision is proposed to have been produced by southeastward slab tearing in the QOB. Slab tearing may be a common geodynamic process during oblique collision generally. The origin and evolution of Triassic mafic and felsic intrusions in the QOB reveal that this oblique-collisional orogenic belt likely underwent a transition from net crustal growth to crustal reworking as collision progressed. • Mafic-felsic complex emplaced at 233–231 Ma was derived from an enriched mantle. • Granite porphyry emplaced at 224 Ma was formed by crustal melting with input of mantle. • Granite porphyry emplaced at 215 Ma was formed by fractionation of crustal melts. • Slab tearing is likely a regime inducing magmatism during oblique continental collision. • The oblique collision is characterized by a transition from crustal growth to reworking. [ABSTRACT FROM AUTHOR]

The high Sr/Y geochemical feature of granitoids can be attributed to various mechanisms, and elucidating genesis of high Sr/Y granitoids provides insights into the material recycling and magmatic processes at depth. In southeastern Central Asian Orogenic Belt (CAOB), many Middle Permian granitoids exhibit high Sr/Y ratios, but their origins remain unclear, inhibiting a comprehensive understanding of the magmatic response to the final closure of the Palaeo-Asian ocean. Here we present new zircon U-Pb ages, Lu-Hf isotopes and whole-rock geochemical data for the Middle Permian high Sr/Y monzogranites from central Inner Mongolia, southeastern CAOB. LA-ICP-MS zircon U-Pb data shows that these high Sr/Y rocks were emplaced during 273–261 Ma. They are calc-alkaline, sodium-rich and metaluminous to weakly peraluminous, with enriched large-ion lithophile elements (Rb, Th, K and Pb) and depleted high field strength elements (Nb, Ta, P and Ti), suggesting a mafic lower crustal source rather than evolved potassic crustal materials. Their relatively low (Gd/Yb)N (1.1–2.0), (Dy/Yb)N (1.0–1.3), Nb/Ta (7.9–10.9) ratios and flat heavy rare earth element patterns are characteristics of derivation from a relatively shallow depth with amphibolite as dominant residue. They also have highly variable εHf(t) values (−8.2 to +10.0) and TDMC (1814 to 649 Ma), similar to those of the Early Palaeozoic high Sr/Y intrusions along the Bainaimiao arc belt. Combined with data from literatures, we suggest that the high Sr/Y monzogranites in this study were probably generated by reworking of the newly underplated juvenile high Sr/Y lower crust of the Bainaimiao arc belt. Moreover, taking into account the regional investigations, the sublinear distributed Middle Permian magmatic rocks in the southeastern CAOB were likely associated with the incipient slab break-off of the Palaeo-Asian oceanic lithosphere following initial collision between the North China craton and the South Mongolia terranes. [ABSTRACT FROM AUTHOR]

In this paper, we present zircon U Pb dating, elemental geochemistry, and Sr–Nd–Hf isotopic compositions for Late Carboniferous intrusive rocks in the northern Great Xing'an Range, northeast China. We decipher the mechanisms which triggered lithospheric thinning under microblock-microblock collision and provide a new perspective on the Late Paleozoic evolution of the eastern Central Asian Orogenic Belt. Zircon U Pb dating indicates that Jifeng gabbroic diorites and Xinlin granites were formed during a short time interval at 322–321 Ma, whereas the Xinlin gabbros were crystallized at 300 ± 2 Ma. The Jifeng gabbroic diorites exhibit typical arc characteristics, depleted in HFSEs and enriched in Pb and have positive ε Nd (t) (+1.88 to +2.00) and ε Hf (t) (+6.6 to +10.7) values, suggesting they originated from the partial melting of depleted lithospheric mantle modified by slab fluids. The Xinlin gabbros belong to alkaline series and are characterized by the enrichment in HFSEs with positive Nb and Ta anomalies and depleted Nd Hf isotopic compositions (zircon ε Hf (t) = +12.5 to +18.9, whole rock ε Nd (t) = +8.0 to +8.1). High Nb concentrations and low La/Nb (0.81–0.83) and La/Ta (12.50–14.61) ratios of the Xinlin gabbros indicate an origin from a depleted asthenospheric mantle. The Xinlin granites have high-K calc-alkaline compositions, belonging to I-type granites. These granites have positive ε Hf (t) values (+0.2 to +15.1) and juvenile two-stage model ages (1314–506 Ma) with low Sr and high Yb contents, indicating they were formed by the partial melting of a juvenile lower crust under low-pressure conditions. These data, together with the temporal and spatial distribution and the geochemistry characteristics of the Carboniferous igneous rocks in the Great Xing'an Range, suggest that the amalgamation between the Erguna–Xing'an and Songnen blocks occurred before the Late Carboniferous. The occurrence of asthenospheric mantle-derived mafic rocks indicates localized thinning process of lithosphere. Late Carboniferous magmatism on the Erguna–Xing'an Block attributed to dripping or piece removal of the lithosphere due to asthenospheric fluctuation triggered by slab break-off. • Lithospheric and asthenospheric sources of mafic rocks represent the different stage of post-collisional magmatism. • The amalgamation of the Xing'an and Songnen blocks occurred before late Early Carboniferous. • Lithospheric dripping triggered by slab break-off generated the planar Late Carboniferous post-collisional magmatism. [ABSTRACT FROM AUTHOR]

To clarify the deep structure of the Taiwan Orogen formed by the collision between the Eurasian and the Philippine Sea plates, we determine a robust 3-D shear-wave velocity (Vs) model of the crust and upper mantle beneath the Taiwan Island and its surrounding regions by applying a novel joint inversion method to local and teleseismic P and S wave travel times, Rayleigh-wave phase velocities and S-wave receiver functions. A notable new feature revealed by our model is the morphologic variation of the subducting Eurasian slab along the orogen belt. Beneath South Taiwan, the subducting Eurasian slab is imaged as a continuous high-Vs zone down to ∼400 km depth with an obvious dip angle change at ∼70 km depth. Beneath Central Taiwan, the Eurasian slab is imaged as intermittent high-Vs anomalies in the upper mantle, whereas the Eurasian slab exhibits as a sheet-like high-Vs zone with a low dip angle down to only ∼100 km depth beneath North Taiwan. These results suggest that parts of the subducting Eurasian slab have been detached beneath the Philippine Sea east off the Taiwan Island, which may result in the remnant slab rebounding to underplate the upper plate and so promote mountain building on the Taiwan Island. • A 3-D Vs model of the Taiwan Orogen is derived from 4 kinds of seismic data. • Shape variations of the Eurasian slab are revealed along the orogen belt. • Parts of the Eurasian slab are detached beneath the Taiwan Orogen. [ABSTRACT FROM AUTHOR]

The metasomatism of mantle wedge peridotite by slab fluids is a common mechanism to generate mafic arc igneous rocks in oceanic subduction zones, which provides us with an excellent opportunity to study the crust-mantle interaction and temporal compositional evolution of slab fluids during oceanic subduction. Here, we identified three groups of mafic arc igneous rocks with zircon U Pb ages of 489 Ma for Group A, 461 Ma for Group B and 454 Ma for Group C in the Shangdan suture zone of the Qinling orogen, central China. From Group A to C, their SiO 2 , K 2 O, LILE and LREE contents as well as element ratios (e.g., Rb/Nb, Cs/Nb, Ba/Nb, (La/Yb) N and Th/Yb) and Sr-Nd-Hf-O isotopes show a progressively elevated or enriched trend, implying the temporal compositional change of slab fluids fluxing their mantle sources. According to the inverse calculation by program PRIMACALC 2 and mass balance calculation by program ABS5, we acquired the progressively elevated P-T conditions of slab dehydration/melting for Group A to Group C rocks, which are from 3.0 GPa and 735 °C to 4.7 GPa and 799 °C. Modelled results show that the proportion of sediment-derived melt in the slab fluids increased steadily from 5 wt% to 40 wt% as the subduction proceeded. Combining the geochemical with the modelled results, we conclude that the regular composition variations in the mafic arc igneous rocks from Group A to Group C reflect the compositional evolution of slab fluids with time during the oceanic subduction. Therefore, the studied mafic arc igneous rocks have recorded a continuous slab-mantle interaction and slab fluid evolution due to the subduction of Shangdan oceanic crust during the early Paleozoic. • Three groups of mafic arc igneous rocks are identified in the Qinling orogen. • They show temporal composition variation from depletion to enrichment. • This reflects slab fluid composition evolution with more sediment melts involved over time. [ABSTRACT FROM AUTHOR]

A study of teleseismic P-wave tomography was carried out along the Central Orogenic Belt of China (COBC) consisting of the Qinling, Tongbai, Hong'an, Dabie and Sulu orogens that were created by northward subduction of the South China Block beneath the North China Block to subarc depths in the Paleozoic to Mesozoic. The results show the presence of high velocity anomaly underneath the COBC and its adjacent cratonic regions but the absence of high velocity anomaly in the lower lithosphere beneath the COBC. The high velocity anomaly is deep underneath the COBC, possibly representing the relic of the subducted Paleo-Tethyan oceanic slab predating the continental deep subduction of the South China Block. The high velocity anomaly is also deep underneath the southeastern part of the North China Block, possibly being a relic of the subducted Paleo-Pacific slab. However, no relic of the deeply subducted continental slab is seismically detected in the lower lithosphere beneath the Dabie and Sulu orogens, though the occurrence of ultrahigh-pressure metamorphic rocks in the two orogens has clearly demonstrated continental deep subduction to subarc depths in the Triassic. Instead there is only the relatively flat and uniform Moho boundary beneath the COBC region, indicating that the subduction-thickened continental lithosphere along the COBC has lost its root. It is inferred that the orogenic root beneath the COBC region has been foundered into the asthenospheric mantle at the postcollisional stage. The foundering would have taken place in the Early Cretaceous as indicated by the extensive occurrence of magmatic and migmatitic rocks along the COBC. As a consequence, the seismic tomography cannot detect the deeply subducted continental slab remnant from the Triassic period. It is possible that the tomographic visibility of continental subduction in the lower lithosphere is limited to the past 100 Myr. [ABSTRACT FROM AUTHOR]

The Central Asian Orogenic Belt (CAOB), characterized by its significant Phanerozoic accretion, develops tremendous post-Carboniferous granites in its southern part and their origins are still unclear, hindering our understanding on the CAOB late-staged evolution, especially its crust growth history. Herein, we presented zircon U-Pb-Hf-O and whole-rock geochemical and Sr Nd isotopic data for the granitic rocks of early Permian (minor late Carboniferous) from the Khan-Bogd area in southern Mongolia. These early Permian granitic rocks formed at ∼280–290 Ma, together with the coeval previously-studied A-type granites there, are mainly high temperatures (∼800–960 °C) granitic rocks and have contributions of deep substances such as relicts of oceanic slab, lithospheric or asthenospheric mantles, as suggested by their zircon Hf O, and whole-rock Sr Nd isotopes. Similar cases occurred commonly in other areas of southern CAOB and we infer that this is related to a significant shift of the early Permian non-subduction-related magmatism from the earlier arc magmatism. Combined with the accompanying significant contraction of the Paleo- Asian Ocean (PAO), we concur the occurrence of the closure of multiple secondary oceans of the PAO and later accompanying slab-breakoff occurred commonly after the Carboniferous in the southern CAOB. Thus, we propose that the relicts of oceanic slabs, as an important magma origin of these post-Carboniferous granites, were reasonably caused by the commonly-occurred slab breakoff and therefore, it is of great importance for the vertical accretion of the post-Carboniferous evolution of the CAOB, rather than mantle plume and other models. [Display omitted] • The studied early Permian granites have contributions of relicts of oceanic slab. • The relicts of oceanic slab probably resulted from oceanic slab breakoff. • The slab breakoff is an important vertical accretion mechanism there. [ABSTRACT FROM AUTHOR]

The Andean orogenic belt, located in the Central Andes “Pampean flat-slab” segment in northern Chile (27–29°S), is composed of two major tectonic regions: the Coastal Cordillera and the Frontal Cordillera. To understand their internal tectonic styles, history of growth and the shortening absorbed by the upper crustal structure of this segment, we combined regional geological mapping data, new ages obtained from radiometric U–Pb dating, and a semibalanced and restored cross-section 225.18 km in length. The results as shown in the previous Mesozoic extensional fault systems, established in northern Chile by the Gondwana breakup, have played a fundamental role in the orogenic buildup. The central structure is characterized by an asymmetric basin (Upper Cretaceous–Paleocene) confined by a doubly vergent fault system composed of inverted faults related to the edges of the Mesozoic Chañarcillo and Lautaro Basins. The U–Pb geochronological data obtained from synorogenic volcano-sedimentary deposits and the angular unconformities recorded between the Cenozoic geological units have revealed that the compressive deformation in this segment started at around ~ 80 Ma by tectonic inversion in the eastern Coastal Cordillera and western Frontal Cordillera, however, the presence of Paleocene and Miocene synorogenic successions at the footwall of the basement reverse faults of the Frontal Cordillera suggests a migration of Andean deformation from the west to the east during the Paleocene–Miocene by propagation of ramps involving inherited basement highs. The pre-compression restoration makes it possible to estimate 40.94 km of minimum shortening, concentrated by inversion anticlines and fault-controlled basement highs across the Frontal Cordillera. [ABSTRACT FROM AUTHOR]

Widespread rhyolitic and mafic volcanism in the northern and southern sectors respectively of Eastern Sierras Pampeanas, central western Argentina, are associated with an important phase of extension and uplift linked to slab break-off on latest stages of the Pampean Orogeny. The main orogenic deformation took place between 540 and 535 Ma based on new ages available for this region. New U–Pb ages in zircons from the Oncán Rhyolite and new and old recalculated zircons from Los Burros Rhyodacite in the northern sector of Sierras de Córdoba, together with new U–Pb ages of the southern sector, indicate that volcanic and subvolcanic rocks of both sectors are partially coeval and unconformably overlying and/or intruding the basement rocks during a period of exhumation and subsequent cooling at 530-520 Ma. These data are in agreement with previous estimates for the final uplift of the Sierras de Córdoba based on the K–Ar cooling ages. The southern sector of Eastern Sierras Pampeanas may represent deeper structural levels within the crust and is characterized by the occurrence of small mafic bodies with OIB-like signature. New and reinterpreted U–Pb SHRIMP and TIMS ages on zircons and monazites in the metamorphic associated rocks date this episode, which is related to a rapid slab break-off event at 519-515 Ma. This episode is also associated with the emplacement of peraluminous granitoids, and with the extension, cooling and final uplift of the rocks affected by the Pampean Orogeny all along the Eastern Sierras Pampeanas. [ABSTRACT FROM AUTHOR]

The Argentine Andes between 34° and 36°S comprise a region that lies immediately south of the transition from the Chilean-Pampean flat-slab to a normal subduction segment. Several key changes take part through this area, such as a westward shift of the magmatic arc front, the end of Paleozoic outcrops of the Frontal Cordillera denoting a decline in exhumation, the change in strike of the orogenic belt, and a ∼2 km decrease in maximum and mean topography. We assess the associated segmentation of the deformation by studying the local seismicity. By analyzing one year of seismological data obtained from a local network, we found a significant shift in the low magnitude (M < 3) crustal seismicity pattern at ∼34.8°S. To the north, most events occur along the topographic front. In contrast, to the south, no events are observed in the frontal region but are instead located closer to the Andean axis. In addition, we used gravity data to determine elastic thickness in the region to compare crustal seismic activity with the pattern of crustal rigidity. We discuss this and other geological, tectonic, and climatic factors as possible controls on seismicity segmentation. We do not find a controlling role by neither elastic thickness nor precipitation patterns. Instead, we propose a thermal contrast due to the volcanic arc's geometry, and differing stress transfer from the subducting slab owing to mantle wedge heterogeneities, as the main active controls on the seismicity distribution; modulated by a passive control by pre-Andean rheological features of the crust. • Crustal seismicity in the Southern Central Andes is studied with a local network. • A shift in the spatial pattern is found across the Atuel river, Mendoza, Argentina. • To the north, there is seismic activity in the Andean front. • Southward, seismicity is absent in the front but only present near the inner region. • Thermal contrast and variable stress transfer from the slab are the main controls. [ABSTRACT FROM AUTHOR]

The petrogenesis of adakite and adakitic rocks and their tectonic settings are debated. Adakites may be derived by many processes which include: partial melting of subducted oceanic slab or lower crustal material; fractional crystallization of basaltic magma; magma mixing processes associated with arc-related and non-arc tectonic settings. It may be inaccurate to assume that adakitic rocks are solely derived from subducted oceanic slabs. The development of accretionary orogens, as it relates to adakite formation, is a topic of significant interest. Geochemical data coupled with field observations are vitally important to address the interplay between tectonics and magma genesis. In this comment, we discuss the petrogenetic processes and possible tectonic setting of the "adakite" in the southern Beishan region of the Central Asian Orogenic Belt (CAOB), to provide insight on the use of geochemical data to discriminate the tectonic setting of their formation. Based on our field observations, we disagree with the model presented by Zheng et al. (2020) as it pertains to the existence of an "ophiolitic mélange" and the evidence of a Permian arc system but rather, advocate that the adakitic rocks at Beishan are generated at an extensional setting. [ABSTRACT FROM AUTHOR]

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. • P-wave velocity at depth 410 km by modelling P and sP triplicated waveforms. • A low-velocity layer atop mantle transition zone in Central Asian Orogenic Belt. • Pervasive upper mantle melting in Eurasia induced by vast ancient subductions. [ABSTRACT FROM AUTHOR]

• 24904 RFs from 1182 events were collected by 307 stations to study MTZ. • Effect of velocity of upper mantle was studied via IASP91 and a 3D velocity model. • MTZ thickening was seen under the Western Alps due to uplift of discontinuities. • Localized arc-shaped thinning of MTZ was located along the alpine orogenic belt. • Temperature rise by mantle upwelling promoted topographic uplift of orogenic belt. To better constrain the deep structure and dynamics of the Western Alps, we studied the mantle transition zone (MTZ) structure using P-wave receiver functions (RFs). We obtained a total of 24904 RFs from 1182 events collected by 307 stations in the Western Alps. To illustrate the influence of the heterogeneity on the upper mantle velocity, we used both IASP91 and three-dimensional (3-D) velocity models to perform RF time-to-depth migration. We documented an MTZ thickening of about 40 km under the Western Alps and most of the Po Plain due to the uplift associated with the 410-km discontinuity and the depression associated with the 660-km discontinuity. Based upon the close spatial connection between the thickened MTZ and the location of the subducted slabs, we proposed that the thick MTZ was due to the subduction of the Alpine slab through the upper MTZ and the presence of remnants of subducted oceanic lithosphere in the MTZ. The uplift associated with the 410-km discontinuity provided independent evidence of the subduction depth of the Western Alps slab. In the Alpine foreland in eastern France, we observed localized arc-shaped thinning of the MTZ caused by a 12 km depression of the 410-km discontinuity, which has not been previously reported. This depression indicated a temperature increase of 120 K in the upper MTZ, and we proposed that it was caused by a small-scale mantle upwelling. Hardly any uplift of the 660 km discontinuity was observed, suggesting that the thermal anomaly was unlikely to be the result of a mantle plume. We observed that the thinning area of the MTZ corresponded to the area with the highest uplift rate in the Western Alps, which may have indicated that the temperature increase caused by the mantle upwelling contributed to the topographic uplift. [ABSTRACT FROM AUTHOR]

[Display omitted] • Zircon xenocrysts have low-δ18O and high-εHf(t) values. • Zircon xenocrysts were generated from high temperature altered gabbroic rocks of subducted oceanic slab. • This paper provides new evidence for 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. [ABSTRACT FROM AUTHOR]

The Neoproterozoic Jiangnan accretionary orogenic belt recorded the accretion and collision of the Yangtze and Cathaysia blocks to form a stablized South China Block, but related geometry and kinematics is poorly constrained, leading to largely varied tectonic models. Here, we present detailed field investigation and kinematic analysis of the plutonic-metamorphic complexes in the Yuanbaoshan and Sanfang areas of the west Jiangnan orogenic belt, which enables identification of extensional granite-cored domes. In the dome margins, down-dipping lineations display a radial pattern and dome dominated foliations are extensively developed. The shearing structures within the plutonic-metamorphic complexes display extensional shearing surrounding the Yuanbaoshan and Sanfang granitic dome cores. Gneissic granites and massive ones from both the Yuanbaoshan and Sanfang plutons yield comparable crystallization ages of ca. 835–823 Ma that are within age errors of each other, as are the sheared recrystallized asymmetric quartz veins and mylonites dated at 831 Ma. Overall ages of the deformed Sibao Group and the undeformed overlying Danzhou Group, along with those of the granite plutons and mylonites, suggest formation of the granite-cored domes at ca. 835–823 Ma, coeval to the timing of emplacement of the granitic plutons. Locally, top-to-the-E thrusting structures are also observed in the west Yuanbaoshan and Sanfang areas and are inferred as at ca. 860–835 Ma, coinciding well with E- or SE- directed structures developed elsewhere in the Jiangnan orogenic belt, but in contrast with doming extensional shearing structures. Therefore, overall geometry and kinematics in the west Jiangnan belt indicate development of granitic dome related extensional ductile shearing deformation dated at ca. 835–823 Ma and a possible top-to-the-E compressional ductile thrusting deformation within 860–835 Ma. Given the previously inferred regional geology observations, along with age and chemical data across the Jiangnan orogenic belt, the dominant extensional shearing deformation in the region argue for a slab roll-back event within an accretionary belt, typical of domes-and-basins structures formed in accretionary convergent continental margin. The top-to-the-E thrusting is here interpreted as corresponding to compressional regime generated by the west directed subduction of oceanic crust beneath the northern Guangxi continental margin arc in the west Jiangnan orogenic belt. • Two stages of Neoproterozoic ductile deformation identified in the west Jiangnan accretionary orogenic belt. • The ca. 835–823 Ma extensional domes-and-basins were generated by a slab roll-back event in an accretionary belt. • The ca. 860–835 Ma top-to-the-E thrusting indicates west directed subduction beneath the western Jiangnan belt. [ABSTRACT FROM AUTHOR]

Abstract: A growing set of data indicates a stark contrast between the evolution of two types of ultrahigh-pressure (UHP) terranes: large terranes that evolved slowly (over 10–30Myr), and small terranes that formed and were exhumed on timescales of <10Myr. Here we compare the characteristics – area, thickness, formation rate, exhumation rate, age, and tectonic setting – of these two endmember types of UHP terrane worldwide. We suggest that the two UHP terrane types may form during different orogenic stages because of variations in the buoyancy and traction forces due to different proportions of subducting crust and mantle lithosphere or to different rates of subduction. The initial stages of continent collision involve the subduction of thin continental crust or microcontinents, and thus tectonic forces are dominated by the density of the oceanic slab; subduction rates are rapid and subduction angles are initially steep. However, as collision matures, thicker and larger pieces of continental material are subducted, and the positive buoyancy of the down-going slab becomes more prominent; subduction angles become gentle and convergence slows. Assessing the validity of this hypothesis is critical to understanding the physical and chemical evolution of Earth''s crust and mantle. [Copyright &y& Elsevier]

Abstract: The Baishiquan and Pobei Early Permian mafic–ultramafic intrusions were emplaced into Proterozoic metamorphic rocks in the Central Tianshan and the Beishan Fold Belt, northern Xinjiang, NW China. The Baishiquan intrusion comprises mainly gabbro, and mela-gabbro sills occurring within and along the margins of the gabbro body. In the Pobei intrusion, two distinct gabbroic packages, a lower gabbro and the main gabbro, are intruded and overlain by small cumulate wehrlite bodies. Both intrusions are characterized by enrichments of large ion lithophile elements and Th and U relative to the high field strength elements, and show strong negative Nb and Ta anomalies and positive K and Pb anomalies, leading to higher Th/Yb and Nb/Yb than in mid-ocean ridge basalt and ocean island basalt. These features are comparable with subduction-related mafic rocks and post-collisional magmas. Geological and geochemical considerations indicate that the parental magmas of the two intrusions were derived from decompression melting of ascending asthenosphere and reacted with overlying subduction-modified lithospheric mantle. We believe that these parental magmas were generated by post-collisional extension along the Chinese Tianshan, perhaps triggered by slab break-off or delamination of thickened lithosphere. Relatively lower (143Nd/144Nd)i and higher (87Sr/86Sr)i than other Permian mafic–ultramafic intrusions in the eastern Chinese Tianshan indicate that the parental magmas of these two intrusions experienced significant contamination by old crustal rocks. [Copyright &y& Elsevier]

The well-preserved magmatic arc in the eastern Pontides orogenic belt of northeastern Turkey offers critical clues on convergent margin tectonics associated with the late Mesozoic–early Cenozoic geodynamic evolution of the eastern Mediterranean region. Here we investigate the petrology, geochemistry, and U-Pb zircon chronology of adakitic intrusions from the Pulur region in the southern zone of the eastern Pontides belt. The intermediate to felsic intrusives in the Pulur region are characterized by the presence of abundant plagioclase, amphibole, and biotite phenocrysts. The rocks display extreme light rare earth element enrichment (La Yb p13.69–51.08); high Al2O3 (15.46–17.47 N N wt%), Na2O (3.57–8.43 wt%), Sr (324.8–1468 ppm), and La (13.9–55.8 ppm) concentrations; a high Sr/Y ratio (35– 473); and low Y (3–12.6 ppm) and heavy rare earth element concentrations, closely comparing with the typical features of adakites. Zircons extracted from the Pulur adakites show euhedral crystal shapes, oscillatory zoning patterns, and high Th/U ratios (up to 3.33) consistent with their magmatic origin. Their 206Pb/238U ages show a range of 55.21– 53.07 Ma, correlating the magmatism to the early Eocene. We propose a tectonic model involving the initiation of a slab rollback and simultaneous opening of a slab window during ridge subduction in a south-dipping subduction zone for the origin of the Tertiary adakitic and nonadakitic magmatism in the eastern Pontides orogenic belt. We also identify a close relation between slab rollback processes and backarc extension, which led to the opening of the Tertiary basins in the southern part of the eastern Pontide orogenic belt. The eastern Pontide orogenic belt is one of the world’s largest metallogenic provinces, and it hosts various economic mineralizations such as volcanogenic massive sulfide, epithermal gold-silver, porphyry copper-molybdenum, skarn-type, and chromite. However, the adakitic intrusions exposed in the southern part of the eastern Pontide orogenic belt are devoid of ore deposits, which leads us to propose that the Lutetian or later nonadakitic granitic intrusions exposed at the north of the Gumushane-Bayburt-Ispir line were probably responsible for the epithermal gold mineralization in this belt. [ABSTRACT FROM AUTHOR]

Accretionary orogens are the primary sites for continental growth, but the rate and amount of crust generation throughout its evolution are poorly constrained. In this contribution, our new results about the genesis of two intermediate-felsic plutons are combined with a compiled granitoid dataset to evaluate the amount and rate of crustal growth throughout the evolution of the Western Kunlun orogen, which is a typical accretionary orogen associated with the consumption of the Proto-Tethys during the early Paleozoic. The ca.446 Ma Sanshili pluton was formed through interactions between metasomatized mantle wedge-derived oxidized magmas and the lower arc crust, as indicated by high whole-rock Mg#, high Ce4+/Ce3+ ratios (308–861) of ca.446 Ma zircons, and the existence of inherited zircons with ages of 546–472 Ma. The Yirba dioritic to granodioritic pluton was emplaced at 474 ± 3 Ma. Samples from the Yirba pluton are characterized by high K 2 O content, higher Mg# (40–49) than pure crustal melts, slightly higher Y + Nb concentrations, and high Th/Nb ratios, and slightly enriched to depleted Hf–Nd isotopes. Combined with the presence of the 502–531 Ma inherited zircons, the Yirba pluton is suggested to from through differentiation of the metasomatized lithospheric mantle derived-magmas beneath the juvenile intra-oceanic arc in combination with crustal reworking during regional extensions. The Yirba pluton, together with contemporary A 1 -type granites, thus marks an extension event at ca. 475 Ma in the Western Kunlun orogen. The compiled dataset reveals three magmatic flare-ups at 530–500 Ma, 480–470 Ma, and 445–430 Ma, corresponding to two slab rollback events and the slab break-off after the final closure of the Proto-Tethys. The three episodes of more intensive magmatism are associated with more radiogenic Hf–Nd isotopes and increased Nb/La and Nb/Y ratios, indicating more contributions from intraplate-like sources during lithospheric extensions. Meanwhile, element ratios (La/Yb, Sm/Yb, and Sr/Y) that are sensitive to crustal thickness are also elevated. These phenomena are consistent with rapid juvenile crust generation during extensional stages of accretionary orogens. Our study has also shown that the rate of new crust production is quite uniform for different extensional events. The crustal generation rate during slab break-off is much higher than that during slab rollback, although the proportions of juvenile inputs in granitoids formed during slab rollback are relatively higher. This may reflect extra inputs from partial melting of oceanic slabs and subducting sediments like those in continental collision zones and/or rapid asthenospheric upwelling coupled with enhanced crust reworking during slab break-off. • The Yirba pluton was formed at ca.475 Ma during regional extensions. • There are three magmatic flare-ups in the Western Kunlun orogen. • The crust generating rate is uniform for different extensional events. [ABSTRACT FROM AUTHOR]

We present laboratory experiments to study the consequence of the rupture of a subducting slab on the deep geometry of the subduction zone. In our experiments, slab break-off occurs after the entrance of buoyant material at trench causing a slowing down of the subduction and producing an increase of the shortening rate and a transient episode of subduction dip reversal. We discuss the potential application of these processes to the recent collisional evolution of the Alps proposing that the rupture of the slab produced a reorganization of the belt with a transition from one sided to a doubly vergent orogen. [ABSTRACT FROM AUTHOR]

We propose a flat-slab subduction model for Mesozoic South China based on both new sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon data and a synthesis of existing structural, geochronological, and sedimentary facies results. This model not only explains the development of a broad (~1300-km-wide) intracontinental orogen that migrated from the coastal region into the continental interior between Ca. 250 Ma and 190 Ma, but can also account for the puzzling chain of events that followed: the formation of a shallow-marine basin in the wake of the migrating foreland fold-and-thrust belt, and the development of one of the world's largest Basin and Range-style magmatic provinces after the orogeny. The South China record may serve as an example of the multiple effects of flat-slab subduction, including migrating orogenesis and foreland flexure, synorogenic sagging behind the active orogen, postdelamination lithospheric rebound, and the development of a Basin and Range-style broad magmatic province. [ABSTRACT FROM AUTHOR]

Integrated lithospheric modelling, based on the combined interpretation of gravity, geoid and topography data sets, highlights a previously undocumented lithospheric thinning beneath the Zagros collisional belt (Iran), which we propose to relate to recent slab break-off at the continent–ocean transitional lithosphere. Recent published data on the distribution of seismicity at depth support this interpretation. In agreement with other published models for the Zagros Mountains, the overlying crust exhibits, by contrast, a noticeable thickening, reaching a maximum of 52 km. The consequent thermal uplift expected from slab break-off is suggested to have modified the Zagros wedge taper and triggered the recently documented switch from thin-skinned to thick-skinned deformation in the Zagros Fold–Thrust Belt.Terra Nova, 17, 1–6, 2005 [ABSTRACT FROM AUTHOR]

Distinct types of mafic rocks, including gabbro-norite, gabbro-diorite, and monzogabbro-diorite, are exposed in the southern Chinese Altai. Zircon U-Pb dating of these intrusions reveals that they were emplaced at ca. 283–275 Ma. Geochemical data show that these intrusions are characterized by low Ce/Pb (3.1–10.3), low Nb/U (5.5–38.9), and high Th/Yb (0.4–2.4) ratios. They are slightly LREE enriched, with selective enrichments of LILE and LREE over HFSE and significant Nb-Ta and Zr-Hf depletions. These rocks have depleted Nd isotopic compositions (εNd(t) = 1.8–5.0). In addition, the monzogabbro-diorite samples contain abundant primary amphibole and biotite and show high Rb/Y ratios. The above geochemical characteristics and mineral associations indicate that their parental magmas were hydrous and likely resulted from partial melting of subduction-metasomatized sub-continental lithospheric mantle (SCLM). In contrast, the orthopyroxene-bearing gabbro-norite and gabbro-diorite intrusions show anhydrous and high-T mineral assemblages, implying that the hot asthenosphere must have been also involved in their magma genesis. Given the absence of oceanic island basalt (OIB)-like signals in our samples, the contribution of the coeval Tarim mantle plume can be precluded. A slab break-off scenario is proposed for the Chinese Altai after its collision with the East Junggar in Permian, with products of mafic magmas generated by the interaction between partial melts of SCLM and upwelling depleted asthenospheric mantle. Apart from the Altai Range, the coeval mafic-ultramafic magmatism in the Beishan and East Tianshan orogens share comparable petrogenesis, indicating a general tectonic regime of the western CAOB during this transitional period. • Permian mafic intrusions in the Chinese Altai were formed in an extensional setting. • Partial melting of the SCLM and asthenosphere gave rise to the mafic magmatism. • Upwelling of the asthenosphere was triggered by slab break-off. • A post-collisional tectonic regime prevailed in the western CAOB during the Permian. [ABSTRACT FROM AUTHOR]

A series of ancient Tethys ocean basins have been consumed beneath the Indian Ocean and are hard to observe due to their subduction below the southern Eurasian margin. The presence of the delaminated Gondwana fragments beneath the Indian Ocean also has long been suspected. These microplate tectonics lead to a fairly complex shallow crust and deep mantle configuration, which makes it difficult to reconstruct East Gondwana and discuss the geodynamics of continental rift-drift-collision in the Tethyan Realm. Lithospheric or crustal thinning due to the dehydration of the underlying stagnant slab is a good tracer of the subducted plate. Using the gravity-derived oceanic crust and Euler rotation method, we obtained an asymmetric crustal thickness across the Indian Ocean ridges. Areas of anomalous intra-plate crustal thinning were revealed and applied to track the possible subducted slabs. Then, combined with other geophysical data, two Meso-Tethyan slabs, one Neo-Tethyan slab, and two Indian slabs were delimited. From north to south, the slabs beneath India and the Northeast Indian Ocean constitute a south-dipping and step-shaped architecture. It requires a double subduction zone or a two-stage India-Eurasia collision model to be driven by the slab pull, which is still open for debate. [ABSTRACT FROM AUTHOR]

The slab dynamic processes, such as roll-back during oceanic subduction and break-off during collision orogenesis, would affect the movement of mantle material and the interaction between crust and mantle, accompanied by significant amounts of magmatism. Providing accurate depictions for these processes are extremely difficult, especially in the case of ancient orogens. Here, we present an integrated study of petrology, geochronology, geochemistry and Sr–Nd–Hf isotopes for Early Paleozoic granitoids from the Duojielaka, Ledu and Binglingsi plutons of the southeastern Qilian orogen. Our LA-ICP-MS U Pb zircon data show that these granitoids were emplaced at 476–432 Ma. The petrological and geochemical data suggest that the ~476 Ma Duojielaka porphyritic syenogranites formed by feldspar-dominated fractionation from relatively K-rich basaltic rock-derived adakitic parental magma. The ~454 Ma Duojielaka biotite monzogranites are highly fractionated I-type granites and were derived from basement orthogneisses mixed with minor mafic rocks. The Ledu fine- and medium-grained quartz diorites (~446 and ~452 Ma) are high-K calc-alkaline to shoshonitic and were produced by partial melting of relatively old K-rich mafic crust followed by minor mixing with mantle-derived magmas. The ~432 Ma Binglingsi biotite monzogranites with adakitic characteristics most likely originated by partial melting of thickened mafic lower crust. Based on the new data, in combination with previously published data, we suggest that the roll-back of the South Qilian oceanic slab occurred during the Middle Cambrian to early Middle Ordovician. The Duojielaka porphyritic syenogranites represented crustal anatexis in an arc front associated with slab roll-back. The Late Ordovician–early Middle Silurian intrusive rocks probably resulted from diachronous slab break-off in the post-collisional stage. The rocks record the opening of the break-off window from east to west and the gradual heat propagation from the central part to the southern and northern margins of the eastern Central Qilian due to the widening of break-off window and asthenospheric upwelling. We propose that complex geodynamic processes and their thermal effects can be traced by a comprehensive consideration of the petrogenetic, temporal and spatial evolution of related magmatism. • Genesis of Early Paleozoic granitoids (476–432 Ma) in the eastern Qilian orogen. • Magmas were predominantly derived from diverse crustal sources. • Magma generations were related to slab roll-back and diachronous break-off. • Orogenic geodynamic processes and their thermal effects can be traced by magmatism. [ABSTRACT FROM AUTHOR]

Understanding how long-term subduction dynamics relates to the short-term seismicity and crustal tec tonics is a challenging but crucial topic in seismotectonics. We attempt to address this issue by linking long-term geodynamic evolution with short-term seismogenic deformation in the Northern Apennines. This retreating subduction orogen displays tectonic and seismogenic behaviors on various spatiotemporal scales that also characterize other subduction zones in the Mediterranean area. We use visco-elasto-plastic seismo-thermo-mechanical (STM) modeling with a realistic 2D setup based on available geological and geophysical data. The subduction dynamics and seismicity are coupled in the numerical modeling, and driven only by buoyancy forces, i.e., slab pull. Our results suggest that lower crustal rheology and lithospheric mantle temperature modulate the crustal tectonics of the Northern Apennines, as inferred by previous studies. The observed spatial distribution of upper crustal tectonic regimes and surface displacements requires buoyant, highly ductile material in the subduction channel beneath the internal part of the orogen. This allows protrusion of the asthenosphere in the lower crust and lithospheric delamination associated with slab retreat. The resulting surface velocities and principal stress axes generally agree with present-day observations, suggesting that slab delamination and retreat can explain the dynamics of the orogen. Our simulations successfully reproduce the type and overall distribution of seismicity with thrust faulting events in the external part of the orogen and normal faulting in its internal part. Slab temperatures and lithospheric mantle stiffness affect the cumulative seismic moment release and spatial distribution of upper crustal earthquakes. The properties of deep, sub-crustal material are thus shown to influence upper crustal seismicity in an orogen driven by slab retreat, even though the upper crust is largely decoupled from the lithospheric mantle. Our simulations therefore highlight the effect of deep lower crustal rheologies, self-driven subduction dynamics and mantle properties in controlling shallow deformation and seismicity. • Simulation links crustal tectonics, seismicity with long-term geodynamics driven solely by slab pull. • Lithospheric delamination allowed by a ductile lower crust is needed for a realistic stress pattern. • Deep material properties (e.g., slab temperature, stiffness) significantly affect crustal seismicity. [ABSTRACT FROM AUTHOR]