We present results of marine MT acquisition in the Alboran sea that also incorporates previously acquired land MT from southern Spain into our analysis. The marine data show complex MT response ...functions with strong distortion due to seafloor topography and the coastline, but inclusion of high resolution topography and bathymetry and a seismically defined sediment unit into a 3‐D inversion model has allowed us to image the structure in the underlying mantle. The resulting resistivity model is broadly consistent with a geodynamic scenario that includes subduction of an eastward trending plate beneath Gibraltar, which plunges nearly vertically beneath the Alboran. Our model contains three primary features of interest: a resistive body beneath the central Alboran, which extends to a depth of ∼150 km. At this depth, the mantle resistivity decreases to values of ∼100 Ohm‐m, slightly higher than those seen in typical asthenosphere at the same depth. This transition suggests a change in slab properties with depth, perhaps reflecting a change in the nature of the seafloor subducted in the past. Two conductive features in our model suggest the presence of fluids released by the subducting slab or a small amount of partial melt in the upper mantle (or both). Of these, the one in the center of the Alboran basin, in the uppermost‐mantle (20–30 km depth) beneath Neogene volcanics and west of the termination of the Nekkor Fault, is consistent with geochemical models, which infer highly thinned lithosphere and shallow melting in order to explain the petrology of seafloor volcanics.
Key Points:
First marine MT survey in Alboran, an area of very complex topography and tectonic history
Two conductive anomalies are interpreted as fluids released from the Alboran subducting plate
We imaged the subducting slab as a resistor that changes its electrical properties at 200 km depth
A combined study of whole-rock major and trace elements, Sr–Nd isotopes, zircon U–Pb dating, and in situ zircon Hf–O isotopes has been carried out for late Mesozoic magmatic rocks in the Lower ...Yangtze River Belt. The results provide insights into the origin of mantle sources of magma above a subduction zone, and thus into the petrogenesis of high-K calc-alkaline rocks, shoshonites, and A-type granites on continental margins, and the associated tectonic transformation from a continental arc to a back-arc extensional setting. The late Mesozoic magmatism can be subdivided into three stages: high-K calc-alkaline intrusions (148–133Ma), shoshonitic rocks (133–127Ma), and A-type granitoids (127–123Ma). All the rocks have consistent arc-like trace element characteristics with positive anomalies of Rb, Th, U, Pb, and LREE, negative anomalies of Nb, Ta, and Ti, and enriched Sr–Nd–Hf isotopic signatures. The first-stage intrusions in the Tongling area usually host dark enclaves of diorite, have high Sr/Y ratios, and low Y contents, and contain zircons with relatively low εHf(t) values (−38.6 to −6.6) and high δ18O values (5.7‰ to 10.1‰). A few inherited zircons with Neoarchean to Paleoproterozoic ages and highly enriched Hf isotopic compositions were detected in both the host intrusive rocks and the enclaves. The second-stage Ningwu volcanics contain zircons with moderate εHf(t) values (−13.3 to −3.8) and elevated δ18O values (5.4‰ to 7.6‰). The third-stage intrusions can be divided into A1- and A2-type granitoids, and their zircons have relatively high δ18O values of 6.7‰ to 10.3‰ and high εHf(t) values of 0 to −7.9. Based on these geochemical data we drew the following conclusions. Before 148Ma, following metasomatism by slab-derived fluid/melts, partial melting of the lithospheric mantle produced basaltic magma in the context of a subducting paleo-Pacific plate. This basaltic magma mixed with magma derived from the Archean lower crust, and the underplated and thickened juvenile lower crust. From 148 to 133Ma, continuous production of these mixed magma resulted in their intrusions as basic rocks at shallow levels. Meanwhile, partial melting of the thickened juvenile crust formed the intermediate-acid rocks of the first stage. As subduction continued, and the dip angle of the subducting plate increased, the continental arc tectonic setting was transformed to one of back-arc extensions. Metasomatism and decompression induced intensive partial melting of the lithospheric mantle, and these magmas, together with a limited amount of assimilated crustal materials, formed the second-stage volcanics. Roll-back of the subducted slab resulted in extension, causing disassembling of the lower lithosphere and lithospheric thinning, and the upwelling of hot asthenosphere. The A2-type granites were the result of the reworking of the Mesoproterozoic juvenile crust in this tectonic setting of extension. On the other hand, the A1-type granitoids were formed from magmas that were derived both from the matasomatized mantle and from the A2-type granitic material that had its origins in the Mesoproterozoic juvenile crust. We suggest that an integrated and comparative study of the multiple stages of development of these magmatic rocks is the key to understanding the tectonic evolution and associated magmatic activities in this continental intraplate setting.
•Three stages of Late Mesozoic magmatisms in the LRYB were studied.•Three stages of magmatisms are characterized by different zircon Hf–O isotopes.•Petrogenesis of the three stage magmatisms reveals progressive deep processes.•These magmatisms were genetically related to Paleo-pacific plate subduction.•Tectonic setting transformed from continental arc to back arc at ca. 130±3Ma.
The propagation of slab break-off (slab tearing) is usually attributed to laterally variable plate convergence systems with a spatial transition between simultaneous oceanic subduction and ...continental collision. To study the process of slab tearing in a non-collisional geodynamic context, here we use a 3D thermo-mechanical numerical approach to model the oblique subduction of a homogeneous oceanic plate. We investigate the effects of the following parameters: (1) subduction obliquity angle, (2) age of oceanic slab, and (3) partitioning of boundary velocities (i.e., the ratio between the subduction component and the advance of the overriding plate in the total convergence). In our simulations, the retreat of the subduction zone leads to a thinning of the fore-arc and back-arc lithosphere, which are decoupled from the subducting slab by the rise of the hot asthenosphere from the underlying mantle wedge. As a consequence of the initial obliquity of the active plate margin, slab roll-back velocities are subject to progressive along-trench variations. Consistent with the gradual rotation of the trench, the front of the decoupling between the overriding and downgoing plates (together with predicted magmatic activity and topographic uplift) migrates in a horizontal direction. In the experiments with low angles of subduction obliquity (< 15°), relatively old subducting plates (> 50 Ma), and in the absence of the subduction component in the overall shortening, slab detachment either develops simultaneously along the entire length of the subduction zone or does not occur at all. In contrast, with higher subduction obliquity (≥ 15°), younger slabs (≤ 50 Ma) and in the presence of a boundary push on the oceanic side, the initial slab break-off is followed by the gradual growth of the “tear” window in the direction opposite to the migration path of the previously established plates decoupling. The sharp contrast in trench retreat rates between subduction zone segments affected and unaffected by slab detachment results in the arcuate shape of the trench. Furthermore, the direction of slab tearing may change from horizontal to vertical, eventually leading to the formation of a transform fault on the subducting plate. Our results show striking similarities with several features – such as trench curvature, subduction zone segmentation, magmatic production, lithospheric stress/deformation fields, and associated topographic changes – observed in many subduction zones (e.g., Marianas, New Hebrides, Mexico, Calabrian).
•The propagation of slab break-off is modelled in non-collisional settings.•Initial subduction obliquity leads to along-trench variations in slab retreat rates.•The young age of the subducting plate and high subduction obliquity favour slab tearing.•Originally horizontal slab tearing can turn into a vertical transform fault.•Slab tearing favours strong trench curvature as observed in many subduction zones.
At more than 500km in length, the mainly Jurassic–Early Eocene Gangdese batholith is one of the most important constituents of the southern Lhasa sub-block and provides an ideal site for study of ...Tibetan orogenesis. Recent studies on Gangdese intermediate-felsic intrusive rocks, mainly granites, demonstrate that remarkable crustal growth as well as an early Late Cretaceous (ca. 100–80Ma) magmatic “flare-up” event occurred in southern Tibet. However, the mechanism that drove this magmatic event and its relationship to crustal growth event are not yet clear. Here, we report detailed petrological, geochronological, geochemical and Sr–Nd–Hf isotopic data for recently identified norites and hypersthene-bearing hornblendites in the Milin area, southern Lhasa sub-block. These mafic rocks are dated at early Late Cretaceous (ca. 93Ma), and are characterized by relatively uniform Sr–Nd–Hf isotopic compositions ((87Sr/86Sr)i=0.7042 to 0.7047, εNd(t)=+2.9 to +3.6 and εHf(t)zircon=+10.9 to +17.0), suggesting that they evolved from similar parental magmas. The Milin norites and hornblendites are likely to be the products of mineral fractionation and accumulation from a common parental magma during the early and late stages of the magma evolution, respectively. Thermometric calculations indicate that pyroxenes from the Milin norites have high crystallization temperatures (1240–1349°C). The parental magmatic compositions calculated from pyroxene trace element compositions in the Milin norites show slightly flat to enriched light rare earth element (LREE) patterns (La/YbN=2.9–3.4; La/GdN=2.1–2.9) with variable negative Nb anomalies (Nb/LaN=0.18–0.81), indicating their dual or hybrid geochemical characteristics. We suggest that their parental magmas may have been generated by the interaction of upwelling asthenospheric and metasomatized lithospheric mantle. Taking into account the spatial and temporal distribution of the Mesozoic magmatic rocks and regional paleomagnetic data, we further suggest that the early-Late Cretaceous magmatic “flare-up” in the southern Lhasa sub-block was also triggered by the asthenospheric upwelling, which resulted from roll-back of subducted Tethyan oceanic slab.
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•Early Late Cretaceous (ca. 93Ma) norites in the Milin area, eastern Gangdese•Early Late Cretaceous (ca. 100–80Ma) magmatic “flare-up” event in southern Lhasa•The parental magmas of norite contain asthenospheric and lithospheric mantle components.•The pyroxenes from the norites have high crystallization temperatures (up to 1349°C).•Lithosphere–asthenosphere interaction occurred during slab roll-back.
Initial SE-dipping slow subduction of the Ligurian–Tethys lithosphere beneath Africa from Late Cretaceous to middle Oligocene twisting to a later faster E-dipping subduction of the subcrustal ...lithosphere is proposed as an efficient geodynamic mechanism to structure the arcuate Betic–Rif orogenic system. This new subduction-related geodynamic scenario is supported by a kinematic model constrained by well-dated plate reconstructions, tectonic, sedimentary and metamorphic data sets. The slow initial SE-dipping subduction of the Ligurian–Tethys realm beneath the Malaguide upper plate unit is sufficient to subduct Alpujarride and Nevado-Filabride rocks to few tens of kilometers of depth in middle Eocene times. The shift from SE- to E-dipping subduction during latest Oligocene–early Miocene was possibly caused by both the inherited geometry of the highly segmented Ligurian–Tethys domain and by the fast roll-back of the subducted lithospheric slab. The early Miocene rather synchronous multiple crustal and subcrustal processes comprising the collision along the Betic front, the exhumation of the HP/LT metamorphic complexes, the opening of the Alboran basin, its flooring by HP Alpujarride rocks and subsequent HT imprint, can be explained by the fast NW- and W-directed roll-back of the Ligurian–Tethys subcrustal lithospheric slab. The W retreat of the Ligurian–Tethys lithosphere in middle–late Miocene times could partly explain the initiation of its lateral tear and consequent subcrustal processes. From latest Miocene onward the Betic–Rif system evolved under both the northerly push of Africa resulting in tightening at crustal and subcrustal levels and by the distinct current dynamics of the steep lithospheric slab. The SW-directed scape of the Rif fold belt is one of the most striking evidences linked to the recent evolution of the squeezed Betic–Rif system between Africa and Iberia.
► Kinematic model for the Betic–Rif orogen with initial SE-dipping subduction ► Highly stretched Ligurian–Tethys lithosphere subducted beneath N-moving Africa ► Late Cretaceous to mid Oligocene slow subduction and HP metamorphic peak ► Subduction shifted to E-dipping in late Oligocene due to ocean segmentation. ► Coeval roll-back, Alboran back-arc extension and exhumation of HP rocks
The Mesozoic geology of SE China has been widely considered to relate to subduction of the Paleo-Pacific Plate beneath the Eurasia, accompanied with extensive felsic magmatism. However, the ...relationship between the origin of the voluminous crust-derived magmatism and the Paleo-Pacific slab subduction remains unclear. Here we present petrology, zircon U-Pb-Hf isotope data, and whole-rock geochemical and isotopic compositions of two episodes of Cretaceous silicic volcanic rocks (~114–113 Ma for Group 1 and ~ 100–93 Ma for Group 2) from southeastern Zhejiang Province in SE China, to provide new constraints on the geodynamic transition of the Paleo-Pacific slab subduction. Both volcanic groups show similarly unradiogenic whole-rock εNd(t) and zircon εHf(t) values (εNd(t) = −8.6 to −6.4 and εHf(t) = −8.5 to −0.7 for Group 1, and εNd(t) = −7.1 to −6.1 and εHf(t) = −7.8 to −2.2 for Group 2). The limited emplacement of coeval mafic magmas and absence of mafic microgranular enclave and intermediate lavas suggest an insignificant role of mantle-derived melt either as a parental magma for differentiation or a mafic component during magma mixing. Instead, the low MgO and high concentrations of SiO2 and incompatible elements and the evolved Nd and Hf isotopic compositions indicate derivation of these two groups of silicic magmas from similar hybrid crustal sources containing the metasedimentary rocks of Cathaysia basement and the newly accreted arc crust. Group 1 rocks contain a higher proportion of hydrous minerals (e.g., biotite) and Sr/Y and La/YbCN ratios but lower zircon saturation temperature than Group 2. Such variations in mineral and geochemical compositions could be mainly attributed to the change of melting conditions from a relatively deeper, colder and water-fluxed crust to a shallower, hotter and less hydrous state. By combining our new results and previous studies of the late Mesozoic volcanic rocks in SE China, we propose that a geodynamic transition from subduction to slab rollback of the Paleo-Pacific Plate was responsible for such P-T-H2O variations of the crustal source.
•1. Two episodes of Cretaceous felsic volcanic rocks were identified in SE China.•2. Both of them were formed by crustal melting at different P-T-H2O conditions.•3. Transition from subduction to slab rollback occurred during ca. 110–90 Ma.
Late Carboniferous igneous rocks are widespread in the western Tianshan, but the tectonic settings for these rocks remain controversial. We report a plagioclase 40Ar/39Ar age, and geochemical, Sr–Nd ...isotope and LA–ICPMS clinopyroxene trace element data for gabbros in the Luotuogou region. The tholeiitic Luotuogou gabbros give a Late Carboniferous (312±1Ma) 40Ar/39Ar age and are characterized by high and variable εNd(t) values ranging from +3.7 to +7.8. They have geochemical features of both intra-plate and island arc magmatic rocks, i.e., relatively high TiO2 (0.6–2.2wt.%), Nb (4.2–24ppm) and Zr (51.4–283ppm) contents combined with variable and slightly high Nb/La ratios (0.24–1.8, mostly>0.7), and negative to positive Nb anomalies. The gabbros contain zoned clinopyroxenes, with Mg- and Cr-rich cores. Their parental magmas, as calculated using trace element data from Cr-rich (>3000ppm) clinopyroxene cores and clinopyroxene/basaltic liquid partition coefficients, show enrichments in incompatible elements, and prominent negative to slightly positive Nb anomalies, indicative of the influence of subduction-related compositions in their mantle source. These features indicate that the Luotuogou gabbros were most likely formed by interactions between asthenospheric and metasomatized lithospheric mantle. They were most plausibly formed by mixing between the asthenospheric mantle-derived and metasomatized lithosphere mantle-derived melts. Mixing was the result of asthenosphere upwelling triggered by roll-back of the subducted Paleo-Junggar Oceanic Plate rather than mantle plume-related rifting or post-collisional break-off during the Late Carboniferous.
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► ~312Ma Luotuogou gabbros in Central Tianshan. ► They have geochemical features of both intra-plate and island arc magmatic rocks. ► Their parental magmas originated from subduction-related mantle source. ► Interactions between asthenospheric and metasomatized lithospheric mantle. ► Asthenosphere upwelling triggered by roll-back of the subducted oceanic plate.
The Gibraltar Arc includes the Betic and Rif Cordilleras surrounding the Alboran Sea; it is formed at the northwest-southeast Eurasia-Nubia convergent plate boundary in the westernmost Mediterranean. ...Since 2006, the Campo de Dalias GNSS network has monitored active tectonic deformation of the most seismically active area on the north coast of the Alboran Sea. Our results show that the residual deformation rates with respect to Eurasia range from 1.7 to 3.0 mm/year; roughly homogenous west-southwestward displacements of the northern sites occur, while the southern sites evidence irregular displacements towards the west and northwest. This deformation pattern supports simultaneous east-northeast-west-southwest extension, accommodated by normal and oblique faults, and north-northwest-south-southeast shortening that develops east-northeast-west-southwest folds. Moreover, the GNSS results point to dextral creep of the main northwest-southeast Balanegra Fault. These GNNS results thus reveal, for the first time, present-day interaction of the roll-back tectonics of the Rif-Gibraltar-Betic slab in the western part of the Gibraltar Arc with the indentation tectonics affecting the eastern and southern areas, providing new insights for improving tectonic models of arcuate orogens.
•Early Cretaceous granodiorites were recognized in northern Lhasa terrane.•The granodiorites are divided into high-K and low-K types.•The high-K type is product of slab roll-back.•The low-K type is ...product of the subsequent slab break-off.
The late Mesozoic geodynamic evolution of the Lhasa terrane (Tibet) remains controversial due to a lack of systematic geochemical and chronological data. Here we present the results of geochemical and zircon U–Pb geochronological studies of two granodiorite plutons in the Dachagou area of the northern Lhasa terrane. The zircon U–Pb dating yielded magmatic crystallization ages of 104 and 117Ma, indicating pluton emplacement in the Early Cretaceous. On the basis of their K2O contents, the granodiorites can be divided into low-K calc-alkaline granodiorites (LKG, 104Ma) and high-K calc-alkaline granodiorites (HKG, 117Ma), and both types are characterized by the adakitic signatures of high Na2O and Sr contents, low Yb and Y abundances, and high Sr/Y ratios. The fact that the LKG and HKG formed at different times and have different geochemical characteristics suggests different petrogenetic mechanisms. Specifically, the LKG were generated by the interaction of an oceanic-slab-derived melt with enriched mantle, while the HKG were derived via the partial melting of the lower crust followed by hybridization with a significant amount of sediment and a minor amount of mantle. Based on these data and the regional tectonic setting, we propose that the LKG and HKG resulted from different geodynamic mechanisms: the HKG were the product of slab roll-back, while the LKG resulted from the subsequent slab break-off during the southwards subduction of the Bangong–Nujiang Ocean seafloor.
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