Ridge Subduction Research Articles

Subduction of solidifying melt lens beneath ultraslow-spreading ridges revealed by (Grt-)amphibolites and dolerites in the Xiugugabu ophiolitic mélange, southwestern Tibet

Ridge subduction is an inevitable process during the evolution history of a vanished ocean. Due to its special characteristics, such as buoyant density, high heat fluxes and active magmatism, subduction of a ridge segment would leave specific fingerprints of high geothermal gradients and unique metamorphism. Therefore, investigations on ridge subduction could provide invaluable details of the evolution of oceanic lithosphere, and also help to understand the emplacement and preservation mechanism of worldwide ophiolites. This study mainly focuses on the mafic blocks of garnet-(Grt-)free and Grt-bearing amphibolites, foliated dolerites and dolerite dykes intruding into other blocks outcropped in the Xiugugabu ophiolitic mélanges of the west segment of the Yarlung-Tsangpo Suture Zone (YTSZ). On the basis of the detailed field, petrographical and geochemical investigations, combined with previous research on the other YTSZ ophiolites, this study highlights that the (grt-)amphibolites in the ophiolitic mélange are metamorphosed products under conditions of high geothermal gradients with protoliths to be subducted cumulate layered gabbros, which were likely to be newly crystallized in solidifying melt lenses beneath ridges. Hence, the generation of the XGGB ophiolite was contemporary with Neo-Tethyan ridge subduction. Intruding dolerite dykes show identical UPb ages to their intruded metamorphic soles, indicating that these amphibolites had gone through extremely rapid exhumation after peak conditions. Finally, scenarios of ridge destruction and subduction were proposed to complete the evolution history of the Neo-Tethyan Ocean and to provide an explanation for ophiolite emplacement and extremely rapid exhumation of metamorphic soles.

Magmatism during late Ordovician-early Silurian accretion of the Caledonides of Arctic Scandinavia: the Halti–Guolasjávri area revisited

In the Scandinavian Caledonides, evidence of syn-collisional magmatism related to extensional basin development immediately prior to late Silurian Baltica–Laurentia collision was considered restricted to exotic terranes until late Silurian ages were obtained from the Halti Igneous Complex (HIC), hosted by a thrust sheet (Corrovarre Nappe, CN) of continental affinity. Various orogenic models for the extension and magmatism, a. o. subduction flip, slab roll-back, and ridge subduction have been proposed. Crucial factors include the affinity (Baltican or exotic) of the CN, and the nature of the debated unconformity at the base of the overlying exotic Vaddas Nappe (Köli). This study reexamines a critical tectonostratigraphic section and reports U–Pb zircon ages (441–436 Ma) of palingenetic granitic dykes generated by the HIC. We reinterpret the CN as a slice of the continental margin and accreted with some other nappes of the Seve Nappe Complex (SNC) which decoupled from the continent-ocean transition (COT) at an early stage of subduction of the margin. The lower part of the bipartite Vaddas Nappe, composed of a very dense dolerite dyke swarm with screens of quartzite, marble and subordinate black schist, is reinterpreted as a continuation northwards of the uppermost nappe of the SNC in Indre Troms, derived from the Ediacaran outermost continental margin. The contact with the overlying telescoped succession of conglomerates, marbles, volcanics and turbidites of the Vaddas Nappe is tentatively interpreted as a tectonised unconformity. To a classical model of back-arc spreading outboard of Laurentia during closure of the Iapetus Ocean, we add the arrival of Baltica, with a prism of decoupled slices of the COT and exhumed/exhuming UHP nappes. The spreading centre of the basin provides a plausible setting for gabbros and pillow lavas of the Vaddas Nappe, and the shores of the basin for a Seve–Köli unconformity. Future PTt studies are required in order to be able to relate the proposed shallow, syn-exhumation HIC magmatism to the tectonometamorphic evolution of the HP–UHP nappes of the SNC.

Causes of Cretaceous subduction termination below South China and Borneo: Was the Proto-South China Sea underlain by an oceanic plateau?

The South China, Indochina, and Borneo margins surrounding the South China Sea contain long-lived arcs that became inactive at approximately 85 Ma, even though an embayment of oceanic crust (the ‘Proto-South China Sea’) remained in the intervening region. This oceanic crust eventually subducted in the Cenozoic below Borneo and the Cagayan arc, while the modern South China Sea opened in its wake. To investigate the enigmatic cessation of Mesozoic subduction below South China and Borneo, we studied a fragment of oceanic crust and overlying trench-fill sediments that accreted to NW Borneo during the final stages of Paleo-Pacific subduction. Based on radiolarian biostratigraphy of cherts overlying the pillow basalts and detrital zircon geochronology of the trench-fill, we constrained the minimum age of the oceanic crust during accretion to 40 Ma. This shows that subduction cessation was not related to ridge subduction. Geochemical analysis of pillow basalts revealed an enriched mid-ocean ridge basalt signature comparable to oceanic plateaus. Using paleomagnetism, we show that this fragment of oceanic crust was not part of the Izanagi Plate but was part of a plate (the ‘Pontus’ Plate) separated from the Izanagi Plate by a subduction zone. Based on the minimum 40 Ma age of the oceanic crust and its geochemistry, we suggest that Mesozoic subduction below South China and Borneo stopped when an oceanic plateau entered the trench, while the eastern plate margin with the Izanagi Plate remained active. We show how our findings offer opportunities to restore plate configurations of the Panthalassa-Tethys junction region.

Ancient slabs beneath Arctic and surroundings: Izanagi, Farallon, and in-betweens

A detailed 3-D tomographic model of the whole mantle beneath the northern hemisphere (north of ~ 30°N latitude) is obtained by inverting a large amount of P-wave arrival time data (P, pP, and PP) to investigate transition of subducted slabs beneath Eurasia–Arctic–North America. We apply an updated global tomographic method that can investigate the whole mantle 3-D structure beneath a target area with high resolution comparable to that of regional tomography. The final tomographic model is obtained by performing independent calculations for 12 different target areas and stitching together the results. Our model clearly shows the subducted Izanagi and Farallon slabs penetrating into the lower mantle beneath Eurasia and North America, respectively. In the region from Canada to Greenland, a stagnant slab lying below the 660-km discontinuity is revealed. Because this slab has a texture that seems to be due to subducted oceanic ridges, the slab might be composed of the Farallon and Kula slabs that had subducted during ~60–50 Ma. During that period, a complex rift system represented by division between Canada and Greenland was developed. The oceanic ridge subduction and hot upwelling in the big mantle wedge above the stagnant slab caused a tensional stress field, which might have induced these complex tectonic events.

Timing of Izanagi-Pacific ridge subduction beneath Eurasia: Constraints from metamorphic soles in Hokkaido, Japan

The subduction of the Pacific Plate beneath Eurasia controls not only Cenozoic intraplate volcanism, but also deep-focus earthquakes along the continental margin of eastern Asia. However, the timing of subduction of the Paleo-Pacific (Izanagi)−Pacific ridge and the initial subduction of the Pacific Plate beneath Eurasia remain subjects of debate. Metamorphic soles provide key evidence for ridge subduction, and together with coeval igneous activity, they are widely used to constrain the timing of ridge subduction. Here, we present the results of a study of amphibolites from the Hidaka metamorphic belt, Hokkaido, northern Japan, which are interpreted to be metamorphic soles. Our integrated study involved secondary ion mass spectrometry (SIMS) U-Pb zircon dating, mineral chemistry, whole-rock geochemistry, and Sr-Nd-Pb-Hf isotopic analyses. SIMS U-Pb zircon dating indicates that the amphibolite-facies sole metamorphism occurred in the late Eocene. Phase equilibria modeling suggests that the peak P−T conditions of metamorphism were 8.0−9.5 kbar/700−730 °C. The amphibolites belong to the tholeiitic series, and they are relatively enriched in heavy rare earth elements relative to light rare earth elements, with depletions in Nb, Ta, P, and Ti. They have (87Sr/86Sr)i = 0.704207−0.704998, εNd(t) = +11.65 to +11.96, εHf(t) = +14.28 to +16.32, (206Pb/204Pb)i = 18.240−18.255, and (207Pb/204Pb)i = 15.522−15.525. The geochemistry of these amphibolites reveals that their protoliths had normal mid-oceanic-ridge basalt affinities, and their Pb isotopic signatures indicate Indian-type mantle rather than Pacific-type mantle. Considering the geochemistry of coeval igneous rocks, we conclude that the late Eocene metamorphism in the Hidaka metamorphic belt records the intraoceanic subduction that followed heat transfer from the incipient mantle wedge toward the top of the subducting plate, and it provides a key constraint on the timing of subduction of the Izanagi-Pacific ridge.

Formation conditions of the young flat-slab in the wedge subduction zone

A wedge subduction can initiate after a ridge subduction and is characterized by the young and thin part of the plate subducting first. A series of 2D geodynamical models were built to investigate the factors in the development of a young flat-slab during the wedge subduction process after a ridge subduction. The factors include the convergence rate, the half spreading rate of the ridge, the initial subduction angle, and the thickness of the overriding plate. The results show that a flat-slab generally forms before the slab age reaches 40 Myr, except in special cases, for instance, when convergence rate≤1 cm/yr, the half spreading rate is low, or the initial subduction angle≥55°. The wedge subduction slab is prone to flatten because the young wedge subduction slab is naturally accompanied by buoyancy anomaly and has the potential to flatten at the relatively low convergence rate. There is no evidence as to whether a flat-slab exists south of Chile Triple Junction. According to the simulation results, the tomographic evidence, and the oceanic age distribution, it can be inferred that, from 46.5°S to ∼50°S, the wedge subduction is in the initiation stage and the formation of a flat-slap is impossible. Near 50–52°S, the slab possibly flattens when the initial subduction angle is 20–30°. From ∼52°S to ∼55°S, when the initial subduction angle is 20–40°, a flat subduction is likely to occur.

Clockwise rotation of SW Japan and timing of Izanagi–Pacific ridge subduction revealed by arc migration

Igneous rocks associated with the Cretaceous to Paleogene volcanic arc in SW Japan show ages that young from west to east in a direction parallel to the Median Tectonic Line suggesting corresponding translation of a heat source traditionally interpreted in terms of oblique subduction of a spreading ridge. However, recent oceanic plate reconstructions suggest ridge subduction may be younger than the main arc activity. Age compilations of 1227 points of felsic to intermediate Cretaceous and Cenozoic igneous rocks from the Japan arc show arc magmatism that can be separated into an early active period 130–60 Ma (stage 1), a subsequent period of quiescence 60–46 Ma (stage 2), which is followed by a resumption of igneous activity from 46 Ma onward (stage 3). In southwest Japan, the orientations of the magmatic arcs of stages 1 and 3 show and angular discordance of about 20°. The lack of active arc magmatism and the occurrence patterns of adakitic and high-Mg andesitic magmas indicate that ridge subduction occurred during stage 2. The arc age distribution pattern of stage 1 is explained by the slab shallowing related to a younging of the subducting slab as the ridge approaches. Furthermore, the obliquity of the arcs formed at stages 1 and 3 is explained by a 20° clockwise rotation of the inner zone of southwest Japan during the ridge-subduction phase. Oceanic plate reconstructions show counterclockwise rotation in the subduction direction after the ridge subduction phase, and coupling of the subducting oceanic plate with the upper plate would support microplate rotation in the inner zone. The new proposed tectonic reconstructions provide a framework to related Paleogene subduction of an active spreading ridge along the east Asia margin not only to the distribution of granitic bodies but also to rift-related basin formation on the eastern margin of the Eurasian continent and to rotation of crustal blocks indicated by paleomagnetic data of Cretaceous terranes.

Slab tearing and lithospheric structures in Luzon island, Philippines: constraints from P- and S-wave local earthquake tomography

Luzon Island is a complex setting of seismicity and magmatism caused by the subduction of the South China Sea lithosphere and the presence of a major strike-slip fault system, the Philippine Fault. Previous studies of the structure of this subduction zone have suggested that a ridge subduction system resulted in a slab tearing along the ridge. On the other hand, the Philippine Fault plays an important role in understanding how major strike-slip faults deform and displace at a continental scale. To constrain the lithospheric geological structure in the area and refine the slab tearing model, we performed a P- and S-wave seismic tomography travel time inversion using local earthquakes. The dataset has been combined from seismic phases reported by the International Seismological Centre and new pickings from six broadband seismic stations in northern Luzon. The three-dimensional P- and S-wave velocity models in Luzon Island were analyzed by applying the LOTOS package with a one-dimensional velocity model obtained from the VELEST program. Our tomographic images indicate contrasting velocity structures across the Philippine Fault to a depth of 60 km. Therefore, we suggest that the Philippine Fault might be a lithospheric structure that displaces both the crust and the upper mantle. The results also indicate regions of low-velocity slab windows from a depth of 40 km, which are interpreted as the sites of slab tearing. Compared with focal mechanisms and earthquake occurrence in this region, we propose that slab tearing extends from the fossil ridge and creates regional kinematic perturbations. The tearing produces shallow upwelling magma to stay in the chambers beneath the crust, which is in contrast to the magmatic system observed in other regions.

Exhuming an Accretionary Prism: A Case Study of the Kodiak Accretionary Complex, Alaska, USA

AbstractWe have carried a structural study across Kodiak accretionary complex in Alaska, USA, in order to describe its thermal structure and decipher the processes of exhumation. The accretionary complex consists of a stack of tectonic mélanges and coherent units. Mélanges are characterized by simple shear deformation with a pervasive network of top‐to‐the‐trench shear zones, whereas coherent units are affected principally by horizontal shortening, except for spatially limited outcrops in Kodiak Central Belt. Peak temperatures estimated using Raman spectroscopy of carbonaceous material range from 220 to 400°C through most of the complex. These temperatures coincide with estimates from metamorphic parageneses and are interpreted as temperatures of deformation achieved under a high gradient due to contemporaneous ridge subduction. The highest temperatures are recorded in the central part of the complex, pointing to a dome‐like structure. Lower temperatures (∼120°C) are recorded along the southeastern border of the complex, in slope sediments unconformably overlying accreted units. Based on the combination of structural and thermal data, we conclude that the rocks constituting the Kodiak complex experienced at least two stages of burial and then exhumation, with vertical motions reaching up to ∼13 km from the Paleocene to the present. Given the pervasive horizontal shortening within the wedge, exhumation resulted from prism thickening contemporaneous with surficial erosion. Recorded subsidence episodes may be local phenomena associated with thrusting or large‐scale processes associated with basal erosion. The rates of vertical motion range from 0.2 to 1.3 mm/yr, comparable with estimates in modern margins.

Hot subduction in the southern Paleo-Asian Ocean: Insights from clinopyroxene chemistry and Sr–Nd–Hf–Pb isotopes of Carboniferous volcanics in West Junggar

The chemical evolution and pressure–temperature conditions of subduction zone magmatism along ancient suture zones in orogenic belts can provide important information regarding plate convergence processes in paleo-oceans. Carboniferous magmatism in West Junggar is key to understanding the tectonothermal and subduction history of the Junggar Ocean, which was a branch of the Paleo-Asian Ocean, as well as the accretionary processes in the southwestern Central Asian Orogenic Belt (CAOB). We undertook a geochronological, mineralogical, geochemical, and Sr–Nd–Hf–Pb isotopic study of volcanic rocks from the Baikouquan area of West Junggar. We used these data to determine the petrogenesis, mantle source, and pressure–temperature conditions of these magmas, and further constrain the subduction and tectonic history of the Junggar Ocean. The studied volcanic rocks yielded zircon U–Pb ages of 342–337 Ma and are characterized by enrichments of large-ion lithophile elements (LILEs), and depletions in high-field-strength elements (HFSEs), indicative of an island arc affinity. The volcanic rocks have positive ƐNd(t) (5.83–7.04) and ƐHf(t) (13.47–15.74) values, 87Sr/86Sr(t) ratios of 0.704023–0.705658, and radiogenic 207Pb/204Pb(t) and 208Pb/204Pb(t) ratios at a given 206Pb/204Pb(t) ratio, indicative of a depleted mantle source contaminated by subduction-related materials. Geochemical modeling calculations indicate that ≤1% of a subduction component comprising fluid and sediment melt could have generated the source of the parental melts of the Baikouquan volcanic rocks. Clinopyroxene phenocrysts in the volcanic rocks are classified as high- and low-Ti clinopyroxene, and pressure–temperature calculations suggest the host rocks formed at high temperatures (∼1300 °C) and shallow to moderate depths (<2 GPa). The magma was probably generated by hot and hydrous melting in a mantle wedge in response to subduction of young, hot oceanic lithosphere. The present results, combined with published data, suggest that the Baikouquan volcanic rocks record a transition in tectonic setting from normal cold to anomalous hot subduction of young oceanic lithosphere close to a mid-ocean ridge. This indicates ridge subduction began shortly after 337 Ma. Our results provide new insights into the tectonomagmatic evolution during intra-oceanic subduction prior to ridge subduction.

Upper-plate conduits linked to plate boundary that hosts slow earthquakes

In shallow subduction zones, fluid behavior impacts various geodynamic processes capable of regulating slip behaviors and forming mud volcanoes. However, evidence of structures that control the fluid transfer within an overriding plate is limited and the physical properties at the source faults of slow earthquakes are poorly understood. Here we present high-resolution seismic velocity models and reflection images of the Hyuga-nada area, Japan, where the Kyushu-Palau ridge subducts. We image distinct kilometer-wide columns in the upper plate with reduced velocities that extend vertically from the seafloor down to 10–13 km depth. We interpret the low-velocity columns as damaged zones caused by seamount subduction and suggest that they serve as conduits, facilitating vertical fluid migration from the plate boundary. The lateral variation in upper-plate velocity and seismic reflectivity along the plate boundary correlates with the distribution of slow earthquakes, indicating that the upper-plate drainage system controls the complex pattern of seismic slip at subduction faults.

Ridge Subduction: Unraveling the Consequences Linked to a Slab Window Development Beneath South America at the Chile Triple Junction

AbstractThe subduction of an active spreading center generates a clear signature in the temporal evolution of subduction zones. It disrupts the typical arc‐type magmatism and intraplate seismicity, enhances the emplacement of backarc plateau lava and profoundly change the tectonics and topographic relief. These distinct observations are commonly linked to a slab window opening and mantle upwelling. The Chile Triple Junction provides the ideal setup to study the mid‐ocean ridge subduction process where both sides of the spreading center continue to subduct. Here, we use 2‐D numerical petrological‐thermomechanical modeling to focus on transient geodynamic processes caused by mid‐ocean ridge subduction. Model results show slab separation along the ridge axis with the opening of a slab window. During the opening, partial melts from the spreading center migrate toward the subcontinental mantle and high temperatures in the forearc are predicted. The temporal evolution of the modeled temperature is consistent with observed heat flow data, and with magmatism and high‐temperature metamorphism recorded in Chilean forearc rocks. Such migrated partial melts might explain the low viscosity inferred and low seismic velocity anomalies imaged in the slab window beneath South America, and the common geochemical signature of the Chile Ridge, the Taitao Ophiolite and the backarc magmatism. Following slab separation, our models suggest forearc uplift and changes in the stress regime, processes which are consistent with deformation records. Summarizing, our model of the geodynamic evolution of the Chile Ridge subduction provides a consistent framework that explains diverse records of magmatism, metamorphism, deformation and mantle physical properties.

Seamount and Ridge Subduction at the Java Margin, Indonesia: Effects on Structural Geology and Seismogenesis

AbstractThe Java ‐ Lesser Sunda margin, which features multi‐scale subducting oceanic basement relief, is classified as neutral (Lombok and Sumbawa) to erosional (Central Java to Bali) in comparison to its accretionary counterpart offshore Sumatra. However, a comprehensive analysis of how plate boundary and upper plate structure across the neutral to erosional transition are modulated by the subduction of oceanic basement relief is lacking to date. To shed light on the tectonic parameters that push the margin into the neutral or erosional domain, we combine multi‐channel reflection seismic images derived through a grid‐based P‐wave velocity inversion, and multibeam bathymetric maps. The data document how different scales of subducting topography modify seafloor morphology, upper plate structure, and décollement position. Large‐scale subducting features cause a landward shift of the deformation front, shortening of the accretionary wedge, and seafloor steepening at the relief's trailing edge. Small‐scale subducting ridges primarily impact the frontal prism resulting in over‐steepening at the trench and localized slope failure. Ahead of subducting relief, deformation of the accretionary wedge encompasses enhanced compression and a reduction in seafloor slope but appears independent of the size of the relief. Ridge and seamount subduction induce frontal erosion and basal erosion offshore Lombok and Bali, respectively. Our P‐wave velocity models indicate that the rigidity of the upper plate's base along the eastern Sunda margin is significantly lower than the worldwide trend. We conclude that this favors the genesis of tsunami earthquakes that have occurred on the Java margin.

Trace elements and organic geochemical fingerprinting of natural crude oils from the Monterey Formation, offshore Santa Maria Basin, California

Based on our recent analytical method development using microwave digestion and triple-quad ICP-MS, we have analyzed up to 57 elements for 20 natural crude oil samples from the offshore Santa Maria Basin of California, a prolific producing basin thought to be sourced dominantly from the Monterey Formation. Our goal is mainly to see if there were oil-oil correlations between wells (fractured reservoir mixing) or no correlation (locally sourced from an unconventional restricted depth reservoir or compartment), and which part (upper and/or lower) of the Monterey Formation acted as the primary source of oil. The results show distinct fingerprints based on V/Mo and Ni/Mo ratios derived from each of the wells where the oils were sampled during stem tests, indicating that they might be partially correlated with restricted intervals of the stratigraphy (or lithofacies). The V/Mo and Ni/Mo ratios of the oils also show that the lower Phosphatic-carbonate lithofacies of Monterey Formation may be a primary source of the oils. Vanadium and Ni contents, V/(Ni + V) ratios, and sulfur content of the oils indicate that the Monterey Formation was deposited under reducing marine conditions. The linear correlation between Mg and Ca contents and non-linear correlation between P and Ca contents in the oil samples may indicate that dolomite-rich rather than apatite-rich portions of Monterey Formation may be a critical source rock component. The similarity between REY distribution patterns of the oils and authigenic carbonate implies that carbonates might have been correlated with formation of the oils. Positive Eu anomalies in most oils reveal possible hydrothermal influences associated with magmatic and slab window effects as a consequence of the Farallon-Pacific Ridge subduction, higher heat flow, and later transtensional and transpressional deformation during deposition of Monterey Formation. The comparable REY distribution patterns for the oils and outcrop shales of Lions Head reveal potential oil-source correlations.

A newly defined latest Carboniferous-Permian ridge subduction in the southern Altaids: Insights from adakitic, S-type, and I-type granitoids in the northern East Junggar (NW China)

ABSTRACT Late Palaeozoic evolution of the Chinese Altai-East Junggar orogenic collage is vital for a better understanding of the accretionary evolution in the southern Altaids. This paper reports new geochronological, geochemical, and isotopic data for the magmatic rocks collected from the northern part of the Dulate arc. The ~304.6 Ma tonalite samples present typical adakitic features of high Sr (396–714 ppm), low Y (1.42–2.60 ppm), Yb (0.11–0.24 ppm), and high Sr/Y (177.25–440.23) ratios. Combining high Mg# (55.35–57.21) and depleted isotopic composition (εHf(t): +9.89 to +13.37 and εNd(t): +5.36 to +6.06), we suggest that they were derived from the partial melting of a subducted oceanic slab. The ~299.4 Ma monzogranite samples have high A/CNK values (1.08 to 1.25), CIPW normative corundum contents (1.28–3.25 wt.%), and positive εHf(t) (+5.14 to +8.63) and εNd(t) (+1.08) values, which are similar to S-type granite that may be generated by the melting of greywackes. The monzogranite (~286.9 Ma) and rhyolitic porphyry (~283.9 Ma) are high-K calc-alkaline and similar to the highly fractionated I-type granites with depleted isotopic features (εHf(t): +11.68 to +14.99 and +0.75 to +7.18, εNd(t): +5.91 and +6.18). The monzogranite and rhyolitic porphyry could have been derived from a depleted mantle source, but the rhyolitic porphyry probably suffered partial assimilation of the overlying crust materials. Combining regional rock associations of mafic-ultramafic complexes, adakitic rocks, high-Mg diorites, and A-type granites with close spatial-temporal relationship, we conclude that a mid-oceanic ridge of the Ob-Zaisan Ocean subducted southward beneath the Dulate arc during the latest Carboniferous to Permian. With the opening of the slab window, the upwelling asthenosphere provided high heat flux and triggered various magmatism in the upper plate. We propose that ridge subduction is one of the most effective mechanisms of continental growth in the southern Altaids.

Cretaceous Nb-enriched basalts in the western Gan-Hang belt, South China: A response to slab window opening during the Paleo-Pacific ridge subduction

The role of recycled slab-mantle interaction in the formation of mantle-derived rocks during the Paleo-Pacific subduction remains to be tested. In this study Nb-enriched basalts (NEBs) are identified in Shanggao, western part of the Gan-Hang belt, South China. These volcanic rocks have 40Ar/39Ar ages of ∼90 Ma. They are tholeiitic to calc-alkaline in composition and have low SiO2 (50.1–53.4 wt%), high MgO (5.86–7.10 wt%), and Nb (10.7–14.3 ppm) contents. These NEBs have high Nb/La (0.91–0.99) and Nb/U (14.8–21.0) ratios and chondrite-normalized patterns with slightly enriched light rare earth elements (LREEs) ((La/Yb)N = 5.32–6.52). Their primitive mantle-normalized patterns are similar to E-MORB-like characteristics. These features indicate that these NEBs were derived from a mixed source between the asthenospheric mantle and slab-derived adakitic components in the garnet stability field. Their Sr-Nd-Pb-Hf isotopic compositions also reflect the contribution of the subducted oceanic crust including minor sediments in the source. These NEBs are spatially associated with adakitic rocks, A-type granites, and related mineral deposits in the Gan-Hang belt, clearly supporting a tectonic model in which the Cretaceous ridge subduction and slab window opening occurred during the enhanced slab rollback of the Paleo-Pacific plate. The melting of residual eclogites caused by episodic upwelling of the asthenospheric mantle can account for the formation of the NEBs in a back­arc extensional setting.

Reviewing the geodynamic impact of aseismic ridges subduction on the tectonic-magmatic evolution of the Southern Puna plateau

The Southern Puna (∼24° S to 27° S) is an outstanding segment of the Central Andes where the subduction of aseismic ridges, multi-scale lithospheric foundering, and the transition from normal Andean subduction to the Chilean-Pampean flat slab occur. The exponential increase in the volcanological and geophysical knowledge of the Southern Puna provides a unique opportunity to reappraise its geodynamic evolution. This paper presents an updated review of the spatial-temporal distribution of the volcanism along with a reassessment of the main geophysical and morphostructural features of this region. Contrasting our observations with new kinematic reconstructions of the Taltal and Copiapó ridges, we infer that their subduction under the Southern Puna plateau has played a fundamental role in its Neogene-Quaternary geodynamic evolution (e.g shallow subduction, lithospheric delamination). Specifically, we propose a new alternative hypothesis where the Transversal Volcanic Chains (TVCs), characterizing the Southern Puna, are related to slab tearing. Slab tearing is a valid explanation for the occurrence of local anomalous peaks of melt productivity in a relatively cold retreating mantle wedge during slab shallowing. In our model, slab tearing under the Southern Puna was influenced by the flexural deformation related to slab dip changes as a result of the anomalous slab buoyancy during the subduction of Copiapó and Taltal ridges.

Tearing on the southward subducting Kelameili oceanic lithosphere in the early Devonian: Evidence from the magmatism in the Harlik arc, southern Altaids

The coexistence of adakites, high-Mg andesites (HMAs) and Nb-enriched basalts (NEBs) records special subduction geodynamics, for instance, subduction initiation, subduction of oceanic ridges or slab tearing, which are widespread in the circum-Pacific subduction zone. The Altaids in Central Asia shared similar tectonic evolution to the western Pacific. However, slab tearing has not been widely reported, which hinders a better comparison between the Altaids and the western Pacific. In this study, we report the contemporaneous early Devonian granodiorites and mafic microgranular enclaves (MMEs) in the Sidingheishan area of the Harlik arc, southern Altaids. The granodiorites have high Sr (534–683 ppm) but low Y (7.79–10.2 ppm) and Yb (0.71–1.68 ppm) contents, with corresponding high Sr/Y values (29.78–76.93). In combination with their juvenile isotopes (initial 87Sr/86Sr = 0.70333–0.70356, εNd(t) = +7.4 − +8.01) and high Mg# numbers (50.9–53), they indicate that their primitive magmas were derived from slab melting. The MMEs show similarly high Sr contents (507–696 ppm), and identically juvenile isotopic signatures (initial 87Sr/86Sr = 0.70346–0.70356, εNd(t) = +7.14 − +7.5) to their host granodiorites. However, their relatively low silica contents (55–60 wt%), evolved Mg# numbers (44.8–55.2) and high Nb contents (12.7–24.7 ppm) demonstrate that they are the fractionation products of NEBs, which were produced by the partial melting of the mantle that was previously metasomatized by these adakitic magmas. Considering all the available evidence, we propose a slab tearing model in which the southward subducting Kelameili oceanic lithosphere underneath the Harlik arc underwent rollback in the early Devonian and its differential rollback rates caused slab tearing. Subsequent asthenosphere upwelling stimulated the Kelameili oceanic lithosphere to melt, producing the adakitic magmas, some of which intruded the crust after limited interaction with the mantle peridotites and underwent crystallization of dominant amphibole in the crust. The rest reacted completely with the mantle peridotites, which underwent further melting to produce the Nb-enriched basaltic magmas. They intruded into the adakitic magma chamber after fractional crystallization of olivine, clinopyroxene and garnet in the deep, forming the MMEs and promoting the remobilization of those hornblendites. This study reveals the development of slab tearing that are closely associated with slab rollback in the Altaids, providing further evidence that argues for the similarity between the Altaids and the western Pacific.

Upper-plate response to ridge subduction and oceanic plateau accretion, Washington Cascades and surrounding region: Implications for plate tectonic evolution of the Pacific Northwest (USA and southwestern Canada) in the Paleogene

Abstract The interaction between subduction zones and oceanic spreading centers is a common tectonic process, and yet our understanding of how it is manifested in the geologic record is limited to a few well-constrained modern and ancient examples. In the Paleogene, at least one oceanic spreading center interacted with the northwestern margin of North America. Several lines of evidence place this triple junction near Washington (USA) and southern British Columbia (Canada) in the early to middle Eocene, and we summarize a variety of new data sets that permit us to track the plate tectonic setting and geologic evolution of this region from 65 to 40 Ma. The North Cascades segment of the voluminous Coast Mountains continental magmatic arc experienced a magmatic lull between ca. 60 and 50 Ma interpreted to reflect low-angle subduction. During this period of time, the Swauk Basin began to subside inboard of the paleo-trench in Washington, and the Siletzia oceanic plateau began to develop along the Farallon plate–Kula plate or Farallon plate–Resurrection plate spreading center. Farther east, peraluminous magmatism occurred in the Omineca belt and Idaho batholith. Accretion of Siletzia and ridge-trench interaction occurred between ca. 53 and 49 Ma, as indicated by: (1) near-trench magmatism from central Vancouver Island to northwestern Washington, (2) disruption and inversion of the Swauk Basin during a short-lived contractional event, (3) voluminous magmatism in the Kamloops-Challis belt accompanied by major E-W extension east of the North Cascades in metamorphic core complexes and supra-detachment basins and grabens, and (4) southwestward migration of magmatism across northeastern Washington. These events suggest that flat-slab subduction from ca. 60 to 52 Ma was followed by slab rollback and breakoff during accretion of Siletzia. A dramatic magmatic flare-up was associated with rollback and breakoff between ca. 49.4 and 45 Ma and included bimodal volcanism near the eastern edge of Siletzia, intrusion of granodioritic to granitic plutons in the crystalline core of the North Cascades, and extensive dike swarms in the North Cascades. Transtension during and shortly before the flare-up led to &amp;gt;300 km of total offset on dextral strike-slip faults, formation of the Chumstick strike-slip basin, and subhorizontal ductile stretching and rapid exhumation of rocks metamorphosed to 8–10 kbar in the North Cascades crystalline core. By ca. 45 Ma, the Farallon–Kula (or Resurrection)–North American triple junction was likely located in Oregon (USA), subduction of the Kula or Resurrection plate was established outboard of Siletzia, and strike-slip faulting was localized on the north-striking Straight Creek–Fraser River fault. Motion of this structure terminated by 35 Ma. These events culminated in the establishment of the modern Cascadia convergent margin.

Patagonian landscape modeling during Miocene to Present-day slab window formation

The subduction of seismic oceanic ridges often results in the formation of slab windows, which can affect not only the heat flow and retroarc volcanism, but also the exhumation and topographic evolution of the upper plate. An active and world-class example of a slab window is southern Patagonia, in southernmost South America, which is related to the subduction of the seismic oceanic South Chile Ridge between the middle–late Miocene and the Present day. How the subduction of the ridge and formation of the slab window have influenced the evolution of the Patagonian landscape, exhumation and topography is still under debate. Some works have proposed orogenic deformation mostly affecting the Pacific margin and hinterland areas, or an inherited early Miocene tectonic relief generated before the slab window formation. Others have preferred epeirogenesis hypotheses, such as dynamic uplift or isostatic rebound as a result of lithospheric thinning associated with asthenospheric or lithospheric mantle changes. In this work, we analyze the landscape evolution at medium (orogen-scale) and long wavelengths (embracing the whole of southern Patagonia, from coast to coast) using FastScape a landscape numerical model. This program was coupled with an optimization scheme (the Neighborhood Algorithm) suitable for nonlinear inverse problems. The “goodness” (fit to data) of our landscape evolution models was evaluated using: i) cooling ages, and ii) maximum elevations, in order to provide constraints on the uplift rates, erosion efficiency and effective elastic thickness. We then used the best values to compare two forward models representing medium- versus long-wavelength processes. Our results indicate that long-wavelength uplift geometry (including dynamic uplift and/or lithospheric rebound from thinning) involving areas from the Andes to the Atlantic coast was required from 12 Myr to the Present day in order to reproduce not only the youngest cooling ages but also Present-day topography.