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A small tonalite – dacite body has been discovered in the Nakanogawa Group, Hidaka Belt, northern Japan, which is a Palaeogene subduction complex formed in the palaeo-Japan trench along the northeastern margin of Eurasia. The tonalites are characterized by extremely low K2O (0.2–0.3 wt.%) and relatively flat chondrite-normalized rare earth element patterns (La/Yb[N] ~2.3), similar to plagiogranites in ophiolites. The major-and trace-element characteristics are consistent with low-pressure partial melting of oceanic crust outside the garnet stability field. Zircon U – Pb dating of the tonalite yielded a weighted-mean 206Pb/238U age of 159.1 ± 1.6 Ma. Late Jurassic oceanic plateau-type basaltic rocks with a small amount of plagiogranitic rocks are distributed sporadically in subduction complexes along the palaeo-Japan arc – trench system. A similar zircon U – Pb age (151.6 ± 1.8 Ma) has been reported for greenstone in the Cretaceous subduction complex along the palaeo-Kuril arc – trench system. The tonalite – dacite block was probably derived from the subduction complex at the junction of the palaeo-Japan and -Kuril arc – trench systems. Thus, the greenstones and the tonalite body were likely part of a large oceanic plateau, which formed at the Izanagi – Pacific–Farallon ridge triple junction during the Late Jurassic – Early Cretaceous. [ABSTRACT FROM AUTHOR]

Valdivia Bank (VB) is a Late Cretaceous oceanic plateau formed by volcanism from the Tristan‐Gough hotspot at the Mid‐Atlantic Ridge (MAR). To better understand its origin and evolution, magnetic data were used to generate a magnetic anomaly grid, which was inverted to determine crustal magnetization. The magnetization model reveals quasi‐linear polarity zones crossing the plateau and following expected MAR paleo‐locations, implying formation by seafloor spreading over ∼4 Myr during the formation of anomalies C34n‐C33r. Paleomagnetism and biostratigraphy data from International Ocean Discovery Program Expedition 391 confirm the magnetic interpretation. Anomaly C33r is split into two negative bands, likely by a westward ridge jump. One of these negative anomalies coincides with deep rift valleys, indicating their age and mechanism of formation. These findings imply that VB originated by seafloor spreading‐type volcanism during a plate reorganization, not from a vertical stack of lava flows as expected for a large volcano. Plain Language Summary: Oceanic plateaus are large, elevated underwater features commonly formed from volcanic material from a hotspot. Valdivia Bank is a Late Cretaceous oceanic plateau in the southeast Atlantic Ocean formed by volcanism from the Tristan‐Gough hotspot near the Mid‐Atlantic Ridge. The origin and evolution of Valdivia Bank is poorly defined, but new magnetic data suggests the edifice originated through ridge‐centered volcanism, with lateral accretion of crust. This is unlike the evolution of a massive volcano, which would be expected to create a vertical stack of lava flows. Magnetic inversion modeling suggests the plateau was formed by seafloor spreading during the formation of anomalies C34n‐C33r, with the plateau becoming younger from east to west, rather than north‐south as predicted by some hotspot models. Results from International Ocean Discovery Program Expedition 391 paleomagnetism and biostratigraphy confirm the anomaly interpretation. Key Points: Valdivia Bank is characterized by quasi‐linear magnetic anomalies that are parallel to the inferred paleo‐Mid‐Atlantic RidgeMagnetic anomalies imply that the plateau becomes younger E‐W consistent with formation via seafloor spreading during anomalies C34n‐C33rRift valleys, division of C33r, and anomaly curvature imply complex ridge tectonics and a ridge jump [ABSTRACT FROM AUTHOR]

In this study, we for the first time applied a joint geodynamic‐geophysical inversion approach to oceanic plateau subduction models, and compared the subduction style and corresponding topography and Bouguer gravity of two representative subduction scenarios with passive or active collision. We showed that the case of passive collision of the Ontong Java Plateau (OJP) crust better explains the topography, gravity, and seismic data than the active collision scenario. This implies that the OJP did not control the regional dynamics during the collisional process. We conclude that previous studies may have overestimated the role of the OJP in triggering subduction initiation, subduction polarity reversal, and even Pacific Plate rotation. Plain Language Summary: The effect of the Ontong Java Plateau (OJP), the largest oceanic plateau on Earth, on subduction dynamics remains controversial. Proposed models for the evolution of the OJP range from dominantly "soft docking" that generates shallow subduction and little impact on nearby regions, to strong and active collision that results in deep plateau subduction, extensive accretion, subduction polarity reversal, and even major plate reorganization. Determining the subduction depth and accretion volume of OJP is the key to resolving this dispute. In the former, the oceanic plateau undergoes shallow subduction with its upper and middle crusts slightly accreted into the island arc, leading to relatively low topography and a flat Bouguer gravity profile, consistent with observation. The latter case leads to deep subduction of the plateau lithosphere, resulting in high topography, low Bouguer gravity, and crustal structures that all violate observation. Key Points: The joint geodynamic‐geophysical inversion approach can help distinguish ambiguous dynamic behaviors of oceanic plateau subductionThe Bouguer gravity and other observations of Ontong Java Plateau (OJP) favor the passive collision model over the active collision modelOJP should have played a passive role in the collision process, implying the overestimation in previous studies [ABSTRACT FROM AUTHOR]

The formation and geology of oceanic plateaus are incompletely understood, in part because geophysical data are sparse and of low density. This paper presents an improved high‐resolution bathymetry map for southern Shatsky Rise, by combining newly acquired multibeam bathymetry data from several cruises and by filling in gaps in multibeam coverage with the SRTM15+ global bathymetry model. This map provides important insights on the evolution of Shatsky Rise from plateau forming, to post‐plateau secondary volcanism, to sedimentation, erosion, and sedimentary mass wasting. Tamu Massif is segmented by several subdued bathymetric troughs that divide the massif flanks into smaller rises. The segmentation pattern suggests Tamu Massif was built by a series of eruptions along a jumping triple junction. The distribution of secondary volcanic cones is non‐uniform, with most occurring on the southern and eastern flanks. Most of these secondary cones occur as short chains that parallel nearby magnetic lineations, suggesting control by spreading ridge parallel faults. These results suggest the evolution of Shatsky Rise was largely controlled by spreading ridges during its active volcanic phase. Numerous escarpments were identified around the plateau, a majority of which parallel bathymetry contours, suggesting these features developed because of differential subsidence of the rise massifs. The map reveals widespread mass wasting and downslope sediment movement, shifting sediment distribution and changing the surface morphology. Improved topography improves our understanding of the evolution of Shatsky Rise and emphasizes the importance of bathymetry data in deciphering the geodynamic evolution of large igneous provinces. Plain Language Summary: Shatsky Rise is a large oceanic plateau located in northwest Pacific Ocean offshore Japan. It formed from hotspot volcanism that erupted along a moving triple junction, forming volcanic mountains along the way, of which Tamu Massif and Ori Massif are the largest. We present new high‐resolution bathymetry map of southern Shatsky Rise by merging newly available multibeam data with existing data and filling gaps in coverage with ba/thymetry estimated from satellite altimetry. Studies considered Tamu Massif as a large edifice formed from voluminous ridge‐centered eruptions. However, the map reveals Tamu Massif to be segmented by several shallow valleys that divide it into five smaller mountains, each forming at a jumping triple junction, suggesting that Tamu Massif was built by a series of ridge‐centered eruptions. The map also shows non‐uniform distribution of secondary volcanic cones over Tamu Massif, majority of them located over eastern and southeastern flanks due to preferential volcanism occurring near the Pacific‐Farallon ridge. When a volcano becomes dormant, it becomes covered with sediments, a process often thought to be simple accumulation. We discovered evidence of substantial sedimentary movement that has occurred and is likely still occurring, suggesting that volcano morphology can significantly vary long after it becomes volcanically inactive. Key Points: We present a high‐resolution bathymetry map of southern Shatsky Rise with greater multibeam coverage revealing small‐scale surface featuresTamu Massif is subtly segmented into 5 smaller rises consistent with formation by series of ridge eruptions along a jumping triple junctionSecondary volcanic cones are distributed mainly over the southern and eastern flanks, and many have trends implying spreading ridge control [ABSTRACT FROM AUTHOR]

The nature of the early Archean ocean floor remains a topic of important debate. There are relatively few well-preserved occurrences worldwide where such terrains may be studied in detail because of structural dismemberment, metamorphic overprinting and pervasive early stage hydrothermal alteration to recent weathering. The 3.41-Ga dominantly mafic formations of the Nondweni Greenstone Belt (NGB) covering 270 km2 in the south-eastern Kaapvaal Craton comprise submarine volcanics that exhibit a wide range of textural features, including pillows, chill zones and brecciated flow tops, and various spinifex textures, including the rare platy pyroxene type, cumulate layers, and tuffs. Channelized subaqueous lava lakes that underwent fractionation are capped by thick spinifex-textured units and pillows. Early stage seafloor alteration is regionally variable, ranging from intense to minimal, with preservation of original mineralogy in many areas. Mafic volcanic rocks of the NGB contrast with those of the Barberton Greenstone Belt both in the style of volcanism and in the associated compositional range of komatiitic basalt to basalt with a complete absence of high-Mg komatiites. Olivine-phyric rocks, or derivatives thereof, are largely absent and pyroxene is the main controlling phase with orthopyroxene in the most primitive komatiitic basalts and clinopyroxene in the evolved lava lake sequences. The abundance of orthopyroxene typifies the long-standing silica-enriched character of the Kaapvaal Craton. Three exceptionally well-preserved and well-exposed sequences were studied utilizing hand-drilled samples and deep coring providing unprecedented stratigraphic and textural detail and field controls for more than 400 samples. A unifying feature of the mafic volcanics of the NGB is the range of compositions and ratios of incompatible elements most clearly illustrated by a series of high- and low-Ti compositional lineages reflecting differing sources or degrees of mantle partial melting. Sharp boundaries between high- and low-Ti flow successions indicate sudden changes in the melting regimes or the interaction of flow sequences from different volcanic centres. Th/Nb ranges from 0.1 to 0.2 and reveals crustal contamination of primitive lavas. The primary magma that gave rise to the most primitive komatiitic basalts with 19.5% MgO was derived from partial melting of a mantle plume source in the garnet stability field. Trace element modeling shows that the sequences studied in detail have been modified by fractionation and crustal contamination with the most likely contaminant being the Ancient Gneiss Complex (3.43–3.66 Ga), which is extensively exposed in Eswatini and probably underlies the Paleoarchean terrains in the southern Kaapvaal Craton. The geotectonic setting was likely that of a submerged felsic crustal platform as enclaves within an oceanic plateau. [ABSTRACT FROM AUTHOR]

Mainland China has the most complex continental tectonics on Earth, making it one of the ideal laboratories for probing the tectonics and evolution of the Earth. However, no consensus has been reached regarding the basement tectonic architectures and evolution of mainland China. This paper focuses on the interpretation of aeromagnetic and seismic refraction data, closely incorporating available surface geological data, borehole data, and seismic data to trace the likely occurrences of oceanic plateau slices in the basement architecture of mainland China. We suggest that the highly positive aeromagnetic anomalies and thick high-velocity lower crustal layer indicate that the Junggar basin is floored by an oceanic plateau. The Tarim basin can be divided into a southeastern oceanic plateau block with strongly positive aeromagnetic anomalies and a northwestern passive continental margin block with weak aeromagnetic anomalies. Both blocks got amalgamated together during the late Paleoproterozoic, and the irregular, promiscuous aeromagnetic anomalies within the northwestern block are spatially consistent with and attributed genetically to the Tarim Large Igneous Province (LIP). The lower crust of the Qaidam basin is characterized by large P-wave velocity, suggesting that the basin is floored by an Early Paleozoic oceanic plateau, and the deep-seated subduction of the oceanic plateau could be responsible for the ultrahigh-pressure (UHP) metamorphism in the northern Qaidam margin during the Early Paleozoic. The western North China craton could be an assembly of multiple oceanic plateau fragments along irregular continental margins during the Paleoproterozoic. An intraplate suture (Miaowan suture) divides the Yangtze craton kernel to the east and the Shennong terrane to the west; both were amalgamated during the Neoproterozoic. The Yangtze craton kernel could be floored by an oceanic plateau basement characterized by large P-wave velocity and strongly positive aeromagnetic anomalies. All these oceanic plateau basements exhibit strong rigidity to resist later tectonic deformation and generally remain intact. • Junggar basin is floored by oceanic plateau of highly positive magnetic anomalies 3 and high velocity • Tarim basin is divided into northeastern oceanic plateau and southwestern passive 5 margin. • Qaidam basin is floored by oceanic plateau, whose subduction resulted in UHPM 7 in N. Qaidam margin • Western North China craton is an assembly of oceanic plateaus during the 9 Paleoproterozoic • Yangtze craton kernel is floored by oceanic plateau and divided by Miaowan suture 11 from Shennong terrane. [ABSTRACT FROM AUTHOR]

• For small-scale oceanic plateaus, the rifting manifests as a process from landward migration to seaward migration, accompanied by a transition from a symmetric to an asymmetric structure. • For large-scale oceanic plateaus, the rifting can form a symmetric structure during breakup. With larger initial lithospheric thinning, the crustal deformation distributes within a narrower region and forms the asymmetric structure. • The initial rifting migration dominates the Caroline Ridge deformation. A larger initial lithospheric thinning may exist beneath the small-scale section of Caroline Ridge during the Miocene rifting initiation. The dynamic rifting model of the Caroline Ridge, an oceanic plateau in the West Pacific, remains unclear. Previous studies have revealed that the crustal width of the Caroline Ridge clearly varies from the northwest to the southeast. Here, we investigate Caroline Ridge rifting using numerical simulation and reveal the relationships between different plateau sizes and rifting. For small-scale oceanic plateaus, the initial rift migration forms a symmetric structure. The strain transfers toward one side of the margin. The rifting manifests as a process from landward migration to seaward migration, accompanied by a transition from a symmetric to an asymmetric structure. With larger initial lithospheric thinning, larger ranges of crustal deformation appear with less basement tectonic subsidence. For large-scale oceanic plateaus, basement tectonic subsidence occurs within narrower ranges. The proximal half-graben structures rapidly abandon with relatively horizontal basement and small fault offsets on the rift flank, leading to a symmetric structure during breakup. With larger initial lithospheric thinning, crustal deformation occurs within a narrower region during the early stage. Landward rift migration dominates the deformation, resulting in an asymmetric structure. On the basis of these results, for the large-scale section of the Caroline Ridge, NW-trending normal faults originated from the landward rift migration. At present, seaward rift migration dominates the Caroline Ridge deformation, forming a relatively symmetric structure. For the small-scale section of the Caroline Ridge, the initial landward rift migration occurred. Larger initial lithospheric thinning may exist beneath the Caroline Ridge during the initiation of Miocene rifting. [ABSTRACT FROM AUTHOR]

An oceanic plateau, the Yakutat terrane, has entered the subduction system across southcentral Alaska. Its down‐dip fate and relationship to overlying volcanism is still debated. Broadband seismometers from the Wrangell Volcanism and Lithospheric Fate (WVLF) temporary experiment were deployed with <20 km spacing across southcentral Alaska to study this region. An array‐based deconvolution procedure is used to isolate the scattered P and S coda of teleseismic P waves for imaging discontinuity structure. This procedure is applied to WVLF and other dense seismic arrays across southcentral Alaska in a manner that accounts for near‐surface wavespeed variations. Two imaging techniques are employed: two‐dimensional migration and three‐dimensional common‐conversion‐point (CCP) stacking. Migrating the scattered phases along WVLF stations shows the ∼18 ± 4 km thick Yakutat crust subducting beneath the Wrangell Volcanic field to the NNE. It is offset from the Alaska‐Aleutian seismic zone laterally by 250 km to the southeast at 100 km depth, and dips more steeply (45°). At depths <45 km, CCP stacking reveals that the Yakutat crust is continuous for over 450 km along strike. This shallow continuity and deeper offset suggest a tear in the subducting Yakutat slab at depths >45 km, around 146°W. CCP stacking also reveals a continuous thin low‐velocity layer atop the underthrust Yakutat crust for >450 km along strike, at all depths <35 km. The uniform low‐velocity thrust zone indicates consistent properties through multiple rupture‐zone segments, showing that low‐velocity channels generally correspond with subduction megathrusts. Plain Language Summary: Typical oceanic crust that descends in subduction zones is 6–7 km thick. The subducting crust across southcentral Alaska is an >11 km thick oceanic plateau known as the Yakutat terrane. The western side of this study region has no subduction volcanism; the eastern side is home to the Wrangell Volcanic Field (WVF), one of the most voluminous continental volcanic fields in the world. Multiple seismometer arrays are used to image subsurface discontinuity structure associated with the subduction of the Yakutat terrane. We image the boundaries of the subducting Yakutat crust, and the base of the continental crust where it does not interfere with the subducting crust. The Yakutat terrane is a continuous feature for >450 km along strike at depths <45 km, but deeper it appears to be separated into two segments with >250 km of lateral offset, a 45° difference in orientation, and a 20° difference in dip angle. These differences provide strong evidence for a tear in the subducting plate that may be the cause of the extensive WVF. The results also show low seismic velocities atop the subducted Yakutat terrane at depths <35 km, which could be sediment being dragged down along the plate interface. Key Points: Yakutat oceanic plateau is subducting beneath Wrangell Volcanic Field (WVF) to at least 110‐km depthA thin low‐velocity layer delineates the megathrust atop the subducting Yakutat crust for over 450 km along‐strike, at depths <35 kmA tear in the subducting Yakutat terrane lies 100–200 km west/northwest of the WVF [ABSTRACT FROM AUTHOR]

The morphostructural characteristics of the main taxa of southern Indian Ocean, such as the Del Cano-Crozet, Conrad, and Kerguelen plateaus, are given based on structural-tectonic analysis. A depth map of the acoustic basement of the eastern part of the Afro-Antarctic megadepression is first built using the digitized NGDS database on the thickness of the sedimentary cover of the Indian Ocean, which is available on the Internet. A critical analysis of deep seismic sounding (DSS) data on the Kerguelen plateu hs been performed. A geochemical analysis of more than 100 samples of basalts, acidic rocks, and rocks of sedimentary origin, collected on the islands of the Kerguelen archipelago and raised by dredging and deep-sea drilling from the Kerguelen and Conrad plateaus, is carried out. The study has shown that the plateaus resemble the Madagascar Ridge in terms of morphostructure and a number of other features. It is concluded that these flat-topped horst massifs were the part of the "structural bridge" that connected Madagascar and the eastern part of mainland Antarctica no later than the Paleogene. A proposed alternative version of the geological interpretation of the deep seismic data states that the plateau has a relatively homogeneous structure along the strike, including a thick (at least 15-km) layer of continental crust. An analysis of the depth map of the acoustic basement made it possible to identify the depocenters of the relatively isolated Enderby, Shackleton and Labuan basins framing the Kerguelen Plateau on the southern side. The paragenetic connections of the Kerguelen Plateau with the basins indicate that the isolation of the Kerguelen Plateau from East Antarctica and the formation of the modern structure of the region began in the Late Cretaceous and ended with the last phase of tectonic activation in the Late Miocene–Pliocene. The results of geochemical analysis show that the geochemical and petrological anomalies of the plateau and islands are associated with the crust, and not with the mantle contamination, as well as with the continental nature of the Kerguelen Plateau. [ABSTRACT FROM AUTHOR]

Oceanic plateau crust is thicker and hotter than the surrounding "normal" oceanic crust, causing differential subsidence and subsequent strain between the two domains. Here, we show that recently active sedimentary faults that have been imaged adjacent to the Rio Grande Rise, an oceanic plateau in the western South Atlantic, have extensions that can penetrate at least ∼1.5 km into the upper oceanic crust. Our high‐resolution seismic velocity model indicates that these fault zones significantly reduce seismic velocities and increase inferred porosity and permeability. We also present physical property‐constrained fluid flow models that predict substantially increased fluxes along these pathways compared to crust not affected by faulting. As these phenomena are effectively enabled by the proximity of an oceanic plateau, we suggest that similar geologic settings globally may also exhibit elevated levels of tectonic and hydrothermal activity, with implications for the hydration of mature oceanic crust and the oceanic crustal biosphere. Plain Language Summary: Oceanic crust slowly sinks as it cools after its formation at midocean spreading centers, but it does so more slowly than usual at oceanic plateaus, where anomalous magmatism has left it relatively thick and buoyant. The different rates of sinking can induce faults that rupture the transitionary crust between the plateau and the surrounding oceanic crust. The Rio Grande Rise, located in the western South Atlantic Ocean, is an oceanic plateau at whose fringes recently active faults have been imaged in the sediments that cover the crust. We trace those faults at least 1.5 km deep into the crust and provide additional evidence for recent fault activity. We also show that fluids likely flow along those pathways, spilling into the ocean in much larger volumes than in normal oceanic crust. Faulting and associated fluid flow is likely also very active in crust bordering other oceanic plateaus globally, affecting the composition of the crust and the habitat of living organisms. We estimate the affected area to make up close to 10% of all oceanic crust on Earth. Key Points: Recently, active extensional faults adjacent to the Rio Grande Rise penetrate at least ∼1.5 km into the crustThe imaged faults significantly increase fluid circulation within the upper crust and fluid exchange with the oceanCrust adjacent to oceanic plateaus and other major bathymetric features may release an estimated 43 km3 of fluid into the oceans globally [ABSTRACT FROM AUTHOR]

Oceanic plateaus are characterized by overthickened crust and rheologically strong and chemically depleted mantle root with respect to normal oceanic lithosphere. The buoyant nature makes them have a great potential to resist subduction and initiate a new subduction zone behind the plateaus, yet the geodynamic process remains enigmatic. Here, we use 2D thermo‐mechanical modeling method to investigate the role of an oceanic plateau in subduction zone reorganization. We systematically examine the rheological strength and compositional density of oceanic plateau mantle and the size of oceanic plateau. Three distinct subduction styles are observed: (a) steep subduction without subduction zone reorganization, favored by a small‐sized (≤150 km) or lowly depleted oceanic plateau; (b) breakoff of subducting slab followed by flat‐slab subduction, which is conditional on a moderate‐sized (≥300 km) oceanic plateau with high degree depletion but relatively low strength; and (c) initiation of a new subduction zone behind the plateau (subduction zone jump), facilitated by a moderate‐sized oceanic plateau with a strongly depleted mantle root. The geodynamic results demonstrate that the overriding lithosphere undergoes a tectonic force switch from compression to tension in ∼5 Myr as the subduction zone is reorganized. This can be reflected in the crustal deformation of the overriding plate and used as an indicator to identify past subduction zone jumps. The spatial relationship predicted by the subduction zone jump models explains the Jurassic closure event of the accretion on southern Qiangtang at ∼175 Ma and the intraoceanic infant arc magmatism in the Meso‐Tethys at ∼165 Ma. Key Points: Different subduction styles controlled by the buoyancy and strength of the oceanic plateau, as well as its sizeSubduction zone jump occurs when the plateau has a strong rheological and highly depleted mantle rootThe continental tectonic force transformation systems can be used as an indicator to identify ancient subduction zone jumps [ABSTRACT FROM AUTHOR]

As one of the largest Phanerozoic orogens in the world, the Central Asian Orogenic Belt ( CAOB) is an excellent place for studying accretionary orogenic process and continental crust growth. It is widely accepted that the CAOB formed by long-lived evolution of the Paleo-Asian Ocean (PAO) from Neoproterozoic to Early Mesozoic, comprises various allochthonous fragments, such as micro continents, island arcs, seamounts/ oceanic plateaus, accretionary prisms, and ophiolitic melanges. The composition of ophiolitic melange is almost same in the western CAOB. Except the typical ophiolite components, the ophiolitic melanges contain pillow basalt, volcanic breccia, reef limestone, olistostrome, and terrestrial elastic rocks. Geochemically, the mafic rocks in ophiolitic melange can be divided into two groups of MORBand OIB-type. The MORB-type belongs to tholeiitic basalt series and shows subduction characteristics; the OIB-type belongs to alkaline basalt series, and is formed in seamount/ oceanic plateau affinity. The age, property and distribution of ophiolitic melanges and metamorphic rocks in the western CAOB indicate that the subduction initiation time of the PAO is no later than Early Neoproterozoic, and the locat ion should be on the southern margin of Siberia plate and gradually develop southward. In the early development of the PAO, the mechanism of subduction initiation should be mantle plume-induced dominant. However, in the later period, the seamount/ oceanic plateau may block the subduction channel, causing the subduction polarity reversal and transition, and the subduction initiation occurs. Initiation of new subduction zone is a difficult problem of Earth's plate tectonics regime. The reason is that the subduction initiation is an extremely short process, scarcity of geological record, and there are few cases of subduction initiation that are happening on Earth today. Therefore, it is necessary for geologists to work together to reveal the past and present lives of the PAO, and to solve the mystery of the subduction initiation of plate tectonics. [ABSTRACT FROM AUTHOR]

Shatsky Rise oceanic plateau was emplaced during a period of frequent geomagnetic polarity reversals, allowing reconstruction of its tectonic evolution using magnetic anomalies. Prior studies mainly focused on identifying magnetic isochrons and encountered difficulties in tracing magnetic lineations over high relief. We complied a large magnetic data set over Shatsky Rise and its environs, using 5.5 × 106 data points from 96 geophysical cruises spanning 54 years. The long‐time span and heterogeneity of component data sets made data merger a challenge. Contributions of internal and external fields, and spurious readings were removed during processing. A "backbone" method, using recent GPS‐navigated data as a foundation, was developed to improve the coherency of the data set. The singular characteristic of the new magnetic anomaly map is that linear magnetic anomalies are ubiquitous. In nearly every place where data are dense enough to delineate anomaly trends, the plateau and surrounding crust are characterized by linear anomalies. Discordant anomalies in some areas imply complex tectonics related to triple junction migration and ridge reorientation. Tamu Massif apparently formed along a segment of Pacific‐Farallon spreading ridge that rotated by 90° as a triple junction migrated through the edifice. Ori Massif appears to have formed on the Pacific‐Izanagi ridge between triple junctions. Shirshov Massif contains discordant lineations that may indicate a microplate. The pervasive occurrence of linear magnetic anomalies within Shatsky Rise implies that these volcanic edifices must have formed by spreading analogous to mid‐ocean ridges that formed anomalously thick crust. Key Points: A large and heterogeneous marine magnetic data set has been compiled to construct a new magnetic anomaly map for Shatsky RiseGPS‐navigated data were used as a "backbone" reference frame for older data to correct navigational errors and improve data agreementAll of Shatsky Rise is traversed by linear/curvilinear magnetic anomalies, indicating its formation by enhanced ridge volcanism [ABSTRACT FROM AUTHOR]

The syenitic rocks of South Rallier du Baty Intrusive Complex (SRBIC) represent intrusions into the oceanic plateau basalts of the south‐western Kerguelen Islands. The SRBIC was previously interpreted as a typical ring complex due to magma emplacement with cauldron subsidence. Our new structural and geochronological data reveal that it is a laccolith built between 11.6 and 7.9 Ma by successive injections of magma sheets around the crust–mantle boundary, with an average injection rate between 0.8 and 1.4 × 10−4 km3/year. These results establish strong similarities between the SRBIC, the only recorded example of a felsic laccolith in an oceanic intraplate setting, and many continental plutons emplaced in various geodynamic setting. The SRBIC thus has the characteristics of a continental plutonic complex emplaced in an oceanic plateau crust. We postulate the critical parameter relevant to causing such similarities and plutonic magmatism is crustal thickness. [ABSTRACT FROM AUTHOR]

Modern oceans contain large bathymetric highs (spreading oceanic ridges, aseismic ridges or oceanic plateaus and inactive arc ridges) that, in total, constitute more than 20–30% of the total area of the world's ocean floor. These bathymetric highs may be subducted, and such processes are commonly referred to as ridge subduction. Such ridge subduction events are not only very common and important geodynamic processes in modern oceanic plate tectonics, they also play an important role in the generation of arc magmatism, material recycling, the growth and evolution of continental crust, the deformation and modification of the overlying plates, and metallogenesis at convergent plate boundaries. Therefore, these events have attracted widespread attention. The perpendicular or high-angle subduction of mid-ocean spreading ridges is commonly characterized by the occurrence of a slab window, and the formation of a distinctive adakite-high-Mg andesite-Nb-enriched basalt-oceanic island basalt (OIB) or a mid-oceanic ridge basalt (MORB)-type rock suite, and is closely associated with Au mineralization. Aseismic ridges or oceanic plateaus are traditionally considered to be difficult to subduct, to typically collide with arcs or continents or to induce flat subduction (low angle of less than 10°) due to the thickness of their underlying normal oceanic crust (>6–7 km) and high topography. However, the subduction of aseismic ridges and oceanic plateaus occurred on both the western and eastern sides of the Pacific Ocean during the Cenozoic. On the eastern side of the Pacific Ocean, aseismic ridges or oceanic plateaus are being subducted flatly or at low angles beneath South and Central American continents, which may cause a magmatic gap. But slab melting can occur and adakites, or an adakite-high-Mg andesite-adakitic andesite-Nb-enriched basalt suite may be formed during the slab rollback or tearing. Cu-Au mineralization is commonly associated with such flat subduction events. On the western side of the Pacific Ocean, however, aseismic ridges and oceanic plateaus are subducted at relatively high angles (>30°). These subduction processes can generate large scale eruptions of basalts, basaltic andesites and andesites, which may be derived from fractional crystallization of magmas originating from the subduction zone fluid-metasomatized mantle wedge. In addition, some inactive arc ridges are subducted beneath Southwest Japan, and these subduction processes are commonly associated with the production of basalts, high-Mg andesites and adakites and Au mineralization. Besides magmatism and Cu-Au mineralization, ridge subduction may also trigger subduction erosion in subduction zones. Future frontiers of research will include characterizing the spatial and temporal patterns of ridge subduction events, clarifying the associated geodynamic mechanisms, quantifying subduction zone material recycling, establishing the associated deep crustal and mantle events that generate or influence magmatism and Cu-Au mineralization, establishing criteria to recognize pre-Cenozoic ridge subduction, the onset of modern-style plate tectonics and the growth mechanisms for Archean continental crust. [ABSTRACT FROM AUTHOR]

This study focuses on the mafic--ultramafic lavas of the Early Carboniferous outcrop in Mangxin, southwestern Yunnan, China. Picrites with 26-32 wt% MgO and a quenched texture are the most significant components of this rock association. This article divides the Mangxin picrites into two types. The mantle potential temperature (Tp) of these picrites is higher than the Tp range of mid-ocean ridges and reaches that of mantle plumes. According to the geochemical characteristics, the Type-1 picrites probably formed from the melting of the mantle plume head and were contaminated by ambient depleted mantle, whereas the Type-2 picrites formed from themelting of mantle plume tails. These plume-related mafic--ultramafic rocks in Mangxin and the ocean island basalt (OIB)-carbonate rock associations in many areas of the Changning--Menglian belt provide significant evidence for the improvement of previous models of the Palaeotethyan oceanic plateau. [ABSTRACT FROM AUTHOR]

Subduction of oceanic plateaux are events that have occurred infrequently in Earth's history. Although rare, these events are thought to considerably influence regional tectonics and global plate motions. In the mid‐Cretaceous the oceanic Hikurangi Plateau collided with the formerly active East Gondwana margin: An event which falls into the same of a sudden change from subduction to extensional processes, including graben formation, development rift basin development, and exhumation of metamorphic core complexes in the Zealandia continent as well as a global‐scale plate reorganization event. In this study, we use recently acquired seismic refraction and gravity data along one profile across the submarine Chatham Rise, which represent a former accretionary wedge of the East Gondwana subduction zone. We demonstrate that the southward extent of the subducted Hikurangi Plateau in the lower crust beneath the submarine Chatham Rise along our profile is only ~150. This is ~150 km less than the previously suggested extent. Furthermore, we interpret that a slice of the subducted Phoenix Plate remains attached to the southern edge of the Hikurangi Plateau. We suggest that cessation of subduction in response to the Hikurangi Plateau jamming as well as the rollback and detachment of the Phoenix Plate slab played an important role in the changing tectonic forces across Zealandia in mid‐Cretaceous. Moreover, we suggest that the subduction cessation along the Hikurangi Plateau segment led to the fragmentation of the Gondwana subduction zone and Phoenix Plate, which significantly influenced and potentially prolonged the global mid‐Cretaceous plate reorganization event. Key Points: P‐wave velocity and gravity models reveal the extent of the Hikurangi Plateau and Phoenix Plate beneath the submarine Chatham RiseFlattening, rollback, and detachment of the Phoenix Plate slab led to widespread extension in southern ZealandiaSubduction cessation and fragmentation of the Phoenix Plate triggered a global plate reorganization event at 105 Ma [ABSTRACT FROM AUTHOR]

The generation of erosive and accretionary margins has yet to be determined. Here, we explore the variations in the forearc structures of the New Guinea Trench (NGT) based on the latest high-quality multichannel seismic profiles perpendicular or subparallel to the New Guinea Trench axis. Our results reveal that for the domain of the oceanic crust subducted beneath the NGT (O-NGT), the thickness of the sedimentary strata decreases from the forearc sedimentary basin to the forearc slope. The basement is generally characterized by homogeneous intra-basement reflections, and few discontinuous, prominent layered reflections in the basement are identified. On the forearc slope, arcward-dipping fault-plane reflections have been identified within the basement, and an abrupt topographic decrease occurs in the eastern part of the forearc basement uplift. Widespread sedimentary sequences are identified as the present, transparent and horizontal seismic reflections with high continuities and low amplitudes. The sedimentary sequences show a large lateral variation of thickness and present folded features near the seaward-dipping faults due to the development of slump bodies. However, the sedimentary sequences in the forearc slope of the domain of the oceanic plateau subducted beneath the NGT (P-NGT) are obviously thicker than those of the O-NGT and present folded features. The stratigraphic thickness from the forearc sedimentary basin to the forearc slope clearly increases. Our initial results initially indicate that the NGT might constitute an internal reverse polarity subduction zone that changes from an erosive margin in the O-NGT to an accretionary margin in the P-NGT. • The domain of the oceanic crust subducted beneath the NGT (O-NGT) shows erosive margin features. • The domain of the oceanic plateau subducted beneath the NGT (P-NGT) shows accretionary margin features. • NGT constitutes an internal reverse polarity subduction zone that changes from an erosive margin to an accretionary margin. [ABSTRACT FROM AUTHOR]

This study focuses on the mafic-ultramafic lavas of the Early Carboniferous outcrop in Mangxin, southwestern Yunnan, China. Picrites with 26–32 wt% MgO and a quenched texture are the most significant components of this rock association. This article divides the Mangxin picrites into two types. The mantle potential temperature (T p) of these picrites is higher than the T p range of mid-ocean ridges and reaches that of mantle plumes. According to their geochemical characteristics, type-1 picrites probably formed from the melting of the mantle plume head and were contaminated by the ambient depleted mantle, whereas type-2 picrites formed from the melting of mantle plume tails. These plume-related mafic-ultramafic rocks in Mangxin and the ocean island basalt (OIB)-carbonate rock associations in many areas of the Changning–Menglian belt provide significant evidence for the improvement of previous models of the Palaeotethyan oceanic plateau. [ABSTRACT FROM AUTHOR]

Abstract Seismically detectable discontinuities at mid-depths of some cratonic lithospheric mantle define mid-lithosphere discontinuities (MLD), demonstrating that the lithospheric mantle is layered. The genesis and strength of the MLD are still in debate, most proposed models suggest the MLD is likely not weaker than the normal lithosphere, whereas other proposed models suggest that some metasomatised MLD rocks are weaker than the normal lithospheric mantle rocks. Thus, the weak MLD is likely a weakly-coupled layer at mid-depths in some cratonic lithosphere blocks, possibly influencing their stabilities. We assess the geodynamic significance of the MLD using geodynamic modeling. We propose that a weak MLD, with lower effective viscosity, can be connected to thinned cratonic margins during the evolution of some cratons and form continuously connected weak zones from cratonic margins to craton interiors, which can lead to lithospheric thinning or removal by extension, basal drag, delamination, thermochemical erosion, and other actions. Through analyzing different scenarios, we propose that some samples of weak MLDs can be found in a composite ophiolite profile formed on the Precambrian Karelian continental margin, with both continental and oceanic lithosphere, which is supported by chronological, petrological, and structural architectures of the profile. This creates new opportunities to directly study the properties of the MLD, which could help understand and settle the controversies on the origin of the MLD and its physical, chemical, and geophysical properties. Highlights • A weak MLD can provide opportunity for lithospheric destruction, when it is connected to the thinned cratonic margins. • A weak MLD can enhance the lithospheric destruction during extension, basal drag and delamination. • Buoyant cratonic roots can be left behind beneath oceanic basins along weak MLDs to form composite oceanic plateaus. • A composite ophiolite profile, with both continental and oceanic lithosphere, is the best place to sample the weak MLD rocks. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The Mikabu greenstones in SW Japan are fragments of accreted oceanic plateau. • Zircon U-Pb ages from the Mikabu greenstones show an igneous age of 155 Ma. • Zircon trace elements suggest depleted source mantle for the Mikabu protolith. • The Mikabu greenstones were parts of Jurassic oceanic plateau like Shatsky Rise. Abstract The Mikabu greenstones form accreted fragments of an ancient oceanic plateau, which are currently incorporated within the Cretaceous high P/T-type Sanbagawa metamorphic belt in SW Japan. Previous geological and petrological studies indicate that protoliths of the Mikabu greenstones were mafic/ultramafic volcano-plutonic rocks of oceanic plateau and/or islands. Despite their rock assemblage and metamorphic nature, their protolith age has been poorly constrained. By measuring U-Pb age of igneous zircon grains using LA-ICPMS, this study identified for the first time a Late Jurassic age for the protoliths of the Mikabu greenstones in the eastern Kii Peninsula (the Shurei complex). With an overlap within 2-sigma error, measured U-Pb ages of zircon grains have a weighted mean age of 154.6 ± 1.6 Ma (the Kimmeridgian, Late Jurassic), which is slightly younger than the radiolarian ages (Middle Jurassic) of deep-sea pelagic cherts associated with the Mikabu greenstones. This age difference indicates that mafic magmas intruded pelagic deep-sea sediments to form a volcanic edifice onto the pre-existing oceanic lithosphere. New measurements of trace element composition of the dated zircon grains further confirmed that the dated zircon grains are more similar to those in oceanic crusts rather than in continental crusts. In addition, the trace element composition suggests that the host mafic magma of the zircon grains was derived from depleted mantle similar to the mid-oceanic ridge-type source, rather than fertile mantle source like Hawaii and Iceland. On the basis of the common occurrence of similar mafic rocks from the Shatsky Rise in the modern NW Pacific Ocean, we conclude that the protoliths of the Mikabu greenstones had formed a part of an oceanic plateau/island, which was generated by the Late Jurassic intra-plate mafic magmatism in ancient mid-Pacific, probably together with the Shatzky Rise. [ABSTRACT FROM AUTHOR]

Subduction initiation is a crucial process for plate tectonics. Two scenarios of induced subduction initiation–subduction polarity reversal and subduction transference–are primarily attributed to the accretion of oceanic plateaus. However, the factors governing these two distinct types of subduction initiation remain elusive. Here, we employ 2D thermomechanical numerical models to investigate the dynamics of induced subduction initiation during oceanic plateau accretion. Specifically, we explore the impact of the thickness, density, and rheological strength of the oceanic plateau on subduction initiation. Our results reveal three geodynamic regimes: continuous subduction, subduction transference, and subduction polarity reversal. Thin oceanic plateaus tend to subduct along with slabs into the deep mantle, while thick oceanic plateaus often accrete to the margin, triggering subduction initiation. The rheological strength and density of an oceanic plateau play a significant role in determining whether subduction transference or polarity reversal occurs. Increasing the viscosity and decreasing the density of the oceanic plateau promotes strain concentration in front of the oceanic plateau, resulting in subduction polarity reversal. Our model results provide insights into the dynamics of subduction transference in the Caroline Sea and polarity reversal in Solomon in the western Pacific Ocean. • Numerical modeling of subduction transference and polarity reversal. • The styles of induced subduction initiation depend on the rheological strength and density of an oceanic plateau. • The formation of the lithospheric-scale shear zone is critical forthe type of induced subduction initiation. • The dynamics of two subduction initiations in the western Pacific Ocean are interpreted and analyzed. [ABSTRACT FROM AUTHOR]

The Caicedo Tuffs is a recently mapped Paleogene volcanic arc unit in the northern Andes, which provides insight into the magmatism of northwestern South America following its collision with the Caribbean-Colombian Oceanic Plateau (CCOP) in the latest Cretaceous. New regional geological mapping, igneous petrography, whole-rock and pyroxene geochemistry, and zircon U-Pb dating are provided for the northern sector of the eastern flank of the Western Cordillera of Colombia (Antioquia). These data reveal that the Caicedo Tuffs unit is predominantly composed of primary volcaniclastic deposits (tuff to breccia) that record mafic subaqueous volcanism and limited orogenic building shortly after collision of the CCOP. The geochemistry of these deposits includes major element characteristics and immobile trace element patterns that are typical of boninites. Zircons from these igneous rocks constrain a crystallization age of ca. 57 Ma, with meso-Proterozoic to Jurassic inherited zircons most likely reflecting continental recycling by turbidites in the subduction zone. The boninitic affinity of the Caicedo Tuffs and existing regional igneous and tectonic constraints can be explained by resuming of subduction and/or sinking of an older oceanic slab along northern South America following collision of the CCOP with South America. The boninitic, primary volcaniclastic deposits of the Caicedo Tuffs are crosscut by basaltic-gabbroic sills and dykes with oceanic plateau-like geochemical affinities, which lack evidence for hydrous melting and are compositionally similar to igneous rocks of the CCOP. These intrusions are interpreted to reflect decompression melting of plume-related mantle inherited during the collision of the CCOP with South America or slab detachment. The occurrence of oceanic plateau-like intrusions in a Paleocene volcanic arc casts doubt on the origin of other oceanic plateau sequences of the Western Cordillera, which are generally attributed to the Cretaceous CCOP. The Caicedo Tuffs is a unique boninite unit of which the detailed spatiotemporal extent and petrogenesis remain to be constrained, but that can provide valuable insight into tectono-magmatic processes during oceanic plateau-continent collision. • A new ca. 57 Ma-old submarine volcanic arc unit is documented in the NW Andes. • It records magmatism after collision of the Caribbean oceanic plateau with S America. • It includes Tonga-like boninites unlikely formed by typical subduction initiation. • Meso-Proterozoic to Paleocene zircons occur in the boninites. • The boninites are crosscut by oceanic plateau-like intrusions. [ABSTRACT FROM AUTHOR]

The Raspas Metamorphic Complex is a high-pressure unit that includes eclogites, blueschists, and metapelites, with minor garnet amphibolites and greenschists. Although its geochemistry and ages are relatively well known, metamorphic constraints and the origin of its protolith remain unclear. In this study, we present new geochemical, metamorphic, and cooling age data to understand the protolith, metamorphic conditions, and the tectonic implications of this feature for the Northern Andes. Geochemical analyses reveals rocks with N- to E -MORB signatures and trace element ratios typical of a heterogeneous mantle source, like oceanic plateau lavas. Blueschists originated from a deep enriched source, similar to primitive mantle, while eclogites are similar to MORB. Iterative thermodynamic modeling of an eclogite indicates a low T/P gradient of ∼406 °C/GPa during the formation of the assemblage garnet (core), quartz, katophorite, and omphacite cores at ∼598 °C and 1.43 GPa, followed by the formation of atoll garnets and zoisite at 661 °C and 1.63 GPa, around 133 Ma. Peak metamorphic conditions were attained after isobaric heating at 709 °C and 1.61 GPa, resulting in the assemblage of garnet (rim), zoisite (rim), omphacite (rim), and katophorite. 40Ar/39Ar ages around 129 Ma indicate fast exhumation rates of 1.25 to 0.5 cm/yr. We propose that the Raspas Metamorphic Complex represents the interaction of the South American plate with an oceanic plateau during the Early Cretaceous. During the early stages of subduction, it created a flat slab geometry around 130 Ma. As the plateau transformed to eclogite, the decrease in buoyancy caused an increase in subduction dip during a regional roll-back at 129–123 Ma, which triggered the exhumation along the high-pressure belt. • Eclogite register the subduction and exhumation of an oceanic plateau. • The Northern Andes interacted with another oceanic plateau in the Early Cretaceous. • Fast exhumation rates of 1.25 to 0.5 cm/y after isobaric heating caused by a rollback. [ABSTRACT FROM AUTHOR]

Highlights • Ori Massif ocean plateau is characterized by linear magnetic anomalies. • Modeling indicates the source magnetization is alternating polarity blocks. • Ori Massif must have formed by spreading ridge volcanism. • The linear anomalies imply that eruptions were restricted to the ridge axis. Abstract Many oceanic plateaus have been emplaced at or adjacent to mid-ocean ridges. To explain plateau volume and thickened crust compared to normal oceanic crust, hotspot–ridge interaction is commonly assumed, but the manner of interaction remains unclear. The Shatsky Rise oceanic plateau is a large volcanic mountain that formed at a triple junction during Late Jurassic and Early Cretaceous time. Recent drilling and seismic investigations suggest that the intermediate edifice in the rise, Ori Massif, is a central volcano. Paradoxically, magnetic lineations were traced across parts of Ori Massif, implying formation at a spreading ridge. In this study, we examined magnetic anomalies over and around Ori Massif to obtain insights about the formation of this volcanic edifice. Magnetic data from 21 cruises were corrected, combined, and gridded to construct a magnetic anomaly map. Forward and inverse magnetic modeling was done to investigate the magnetic structure of Ori Massif. The results imply that this large volcanic edifice is predominantly characterized by linear magnetic anomalies resulting from alternating normal and reversed polarity magnetization blocks, analogous to magnetic anomalies recorded by spreading-ridges. This magnetic structure is not expected for a central volcano that was built by long runout lava flows, implying that Ori Massif eruptions must have been constrained near the ridge axis. Magnetic bights on the north and south boundaries of Ori Massif imply that it was bracketed by triple junctions, indicating complex ridge tectonics during the formation of Shatsky Rise. The surprising finding that Ori Massif is traversed by coherent linear magnetic anomalies indicates that oceanic plateaus can record seafloor spreading magnetic anomalies despite large crustal thickness. Other oceanic plateaus also record linear magnetic anomalies, implying a link between divergent plate boundaries and oceanic plateau volcanism. [ABSTRACT FROM AUTHOR]

The Aberdeen Lake supracrustal belt is one of several Neoarchean greenstone belts preserved within the Rae domain of the Western Churchill Province. We divide the belt into: (1) a Lower Sequence of ca. 2750 Ma komatiite, TTG (tonalite-trondhjemite-granodiorite) intrusions, and TTG-contaminated Fe-tholeiitic mafic gneiss; (2) a Middle Sequence of ca. <2687 Ma psammopelite gneiss, iron formation and ca. 2680 Ma intermediate to felsic intrusive rock; and (3) a ca. <2650 Ma Upper Sequence of pelite to psammopelite gneiss with minor iron formation and arkosic gneiss. The common association of komatiite with quartzite units and bimodal metavolcanic rocks, has previously been interpreted to indicate a continental rift setting for the formation of Neoarchean greenstone belts across the Rae domain. However, the lack of clastic metasedimentary rocks and felsic metavolcanic rocks in the Lower Sequence of the Aberdeen Lake supracrustal belt, the presence of uncontaminated komatiite, and the general absence of inherited zircons older than ca. 2750 Ma in the TTG and mafic gneiss units, suggest that the Lower Sequence formed as an oceanic plateau or oceanic crust between older, Paleoarchean to Mesoarchean, proto-Rae microcratons. Erosion of the cratons and the emergence of the oceanic plateaux due to their accretion/subcretion to these cratons shed ca. 2687–2870 Ma detritus that were deposited as the Middle Sequence into adjacent oceanic basins. Renewed uplift of the Lower and Middle sequences led to the emplacement of ca. 2680 Ma felsic intrusions, and the later deposition of the ca. <2650 Ma Upper Sequence. The involvement of oceanic plateaux is a new tectonic interpretation for the formation of Neoarchean greenstone belts in the Rae domain. [ABSTRACT FROM AUTHOR]

Abstract: Recent GPS observations show that slow slip events in south central Alaska are segmented along strike. Here we review several mechanisms that might contribute to this segmentation and focus on two: along‐strike variation of slab geometry and effective normal stress. We then test them by running numerical simulations in the framework of rate‐and‐state friction with a nonplanar fault geometry. Results show that the segmentation is most likely related to the along‐strike variation of the effective normal stress on the fault plane caused by the Yakutat Plateau. The Yakutat Plateau could affect the effective normal stress by either lowering the pore pressure in Upper Cook Inlet due to less fluids release or increasing the normal stress due to the extra buoyancy caused by the subducted Yakutat Plateau. We prefer the latter explanation because it is consistent with the relative amplitudes of the effective normal stress in Upper and Lower Cook Inlet and there is very little along‐strike variation in Vp/Vs ratio in the fault zone from receiver function analysis. However, we cannot exclude the possibility that the difference in effective normal stress results from along‐strike variation of pore pressure due to the uncertainties in the Vp/Vs estimates. Our work implies that a structural anomaly can have a long‐lived effect on the subduction zone slip behavior and might be a driving factor on along‐strike segmentation of slow slip events. [ABSTRACT FROM AUTHOR]

The documented occurrence of ancient continental cratonic roots beneath several oceanic basins remains poorly explained by the plate tectonic paradigm. These roots are found beneath some ocean-continent boundaries, on the trailing sides of some continents, extending for hundreds of kilometers or farther into oceanic basins. We postulate that these cratonic roots were left behind during plate motion, by differential shearing along the seismically imaged mid-lithosphere discontinuity (MLD), and then emplaced beneath the ocean-continent boundary. Here we use numerical models of cratons with realistic crustal rheologies drifting at observed plate velocities to support the idea that the mid-lithosphere weak layer fostered the decoupling and offset of the African continent's buoyant cratonic root, which was left behind during Meso-Cenozoic continental drift and emplaced beneath the Atlantic Ocean. We show that in some cratonic areas, the MLD plays a similar role as the lithosphere-asthenosphere boundary for accommodating lateral plate tectonic displacements. [ABSTRACT FROM AUTHOR]

Birimian volcanic rocks of the Mana District are located in the an important gold-mineralized segment of the Paleoproterozoic Houndé greenstone belt, western Burkina Faso, which contains cumulative resources of ∼11 Moz. Five orogenic gold deposits (∼8 Moz) are hosted in or close to basaltic rocks. Theses rocks were studied to investigate their possible role as a gold source in younger orogenic gold deposits. They are Fe-rich tholeiitic basalts with flat REE patterns, with (La/Yb) N = 0.96–1.3 and without negative Eu anomaly (Eu/Eu* = 0.92–1.26). The basalts also have low initial Sr isotopic ratios (0.693612–0.702190) and positive εNd values (+2.25 to +3.14). Using a Ce/Nb vs. Th/Nb diagram and various plume-related basalts worldwide for comparison, the Mana basalts are shown to be plume-related. In addition, using Zr/Nb vs. Nb/Th and Nb/Y vs. Zr/Y binary diagrams and reference fields, the Mana basaltic rocks appear to have formed directly above the plume head. Because plume-related basalts tend to be enriched in gold relative to MORB, we propose that the gold endowment of the Mana district is mostly related to the occurrence of plume-related basaltic rocks, which may have served as an important metal stock during subsequent remobilization for forming the orogenic gold deposits. We also propose that for gold exploration, two simple geochemical diagrams involving Zr, Y, Nb and Th could be used at an early stage to test the origin of the basaltic rocks and hence indirectly establish the fertility of a specific belt for hosting orogenic gold deposits. [ABSTRACT FROM AUTHOR]

The Siletz terrane, a predominantly mafic accreted oceanic terrane, is located in the Cascadia forearc region of Oregon, Washington and Vancouver Island. The terrane represents a late Palaeocene–Eocene large igneous province that consists of pillow lavas, massive flows and intrusive sheets. Previously it has been proposed that the Siletz terrane represents either an accreted oceanic plateau, hotspot island chain, backarc basin, island arc, or a sequence of slab window volcanics. A province-wide geochemical reassessment of the terrane, including new high precision Sr-Pb-Nd-Hf isotope data, has been used to assess the validity of the proposed tectonomagmatic models for the Siletz terrane. The trace element data show little evidence of crustal contamination, or an arc signature, and the samples have rare earth element (REE) patterns that are flat to light REE enriched. These features are similar to other oceanic plateaus such as the Ontong Java and the Caribbean. Initial isotope ratios range from 206 Pb/ 204 Pb: 18.751 to 19.668, 207 Pb/ 204 Pb: 15.507 to 15.661, 208 Pb/ 204 Pb: 38.294 to 39.2128, 176 Hf/ 177 Hf: 0.28300 to 0.28316 (ε Hf : 9.0 to 14.5), 143 Nd/ 144 Nd: 0.51282 to 0.51299 (ε Nd : 5.0 to 8.1) and 87 Sr/ 86 Sr: 0.70302 to 0.70380. These data are consistent with a mantle source of the Siletz terrane that appears to have been heterogeneous and slightly enriched. The enriched signature has characteristics of both EM2 and HIMU components and this, combined with a calculated mantle potential temperature well above ambient mantle, indicates derivation of the Siletz magmatism from a mantle plume, possibly the Yellowstone Hotspot. We therefore conclude that the Siletz terrane represents an accreted oceanic plateau. [ABSTRACT FROM AUTHOR]

Newly collected, high resolution multi-beam sonar data are combined with previous bathymetry data to produce an improved bathymetric map of Shatsky Rise oceanic plateau. Bathymetry data show that two massifs within Shatsky Rise are immense central volcanoes with gentle flank slopes declining from a central summit. Tamu Massif is a slightly elongated, dome-like volcanic edifice; Ori Massif is square shaped and smaller in area. Several down-to-basin normal faults are observed on the western flank of the massifs but they do not parallel the magnetic lineations, indicating that these faults are probably not related to spreading ridge faulting. Moreover, the faults are observed only on one side of the massifs, which is contrary to expectations from a mechanism of differential subsidence around the massif center. Multi-beam data show many small secondary cones with different shapes and sizes that are widely-distributed on Shatsky Rise massifs, which imply small late-stage magma sources scattered across the surface of the volcanoes in the form of lava flows or explosive volcanism. Erosional channels occur on the flanks of Shatsky Rise volcanoes due to mass wasting and display evidence of down-slope sediment movement. These channels are likely formed by sediments spalling off the edges of summit sediment cap. [ABSTRACT FROM AUTHOR]

Intraplate strain generally focuses in discrete zones, but despite the profound impact of this partitioning on global tectonics, geodynamics, and seismic hazard, the processes by which deformation becomes localized are not well understood. Such heterogeneous intraplate strain is exemplified in central Asia, where the Indo-Eurasian collision has caused widespread deformation while the Tarim block has experienced minimal Cenozoic shortening. The apparent stability of Tarim may arise either because strain is dominantly accommodated by pre-existing faults in the continental suture zones that bound it—essentially discretizing Eurasia into microplates—or because the lithospheric-scale strength (i.e., viscosity) of the Tarim block is greater than its surroundings. Here, we jointly analyze seismic velocity, gravity, topography, and temperature to develop a 3-D density model of the crust and upper mantle in this region. The Tarim crust is characterized by high density, v s , v p , and v p / v s , consistent with a dominantly mafic composition and with the presence of an oceanic plateau beneath Tarim. Low-density but high-velocity mantle lithosphere beneath southern (southwestern) Tarim underlies a suite of Permian plume-related mafic intrusions and A-type granites sourced in previously depleted mantle lithosphere; we posit that this region was further depleted, dehydrated, and strengthened by Permian plume magmatism. The actively deforming western and southern margins of Tarim—the Tien Shan, Kunlun Shan, and Altyn Tagh fault—are underlain by buoyant upper mantle with low velocity; we hypothesize that this material has been hydrated by mantle-derived fluids that have preferentially migrated along Paleozoic continental sutures. Such hydrous material should be weak, and herein strain focuses there because of lithospheric-scale variations in rheology rather than the pre-existence of faults in the brittle crust. Thus this world-class example of strain partitioning arises not simply from the pre-existence of brittle faults but from the thermo-chemical and therefore rheological variations inherited from prior tectonism. [ABSTRACT FROM AUTHOR]

The study presents major and trace element compositions of whole-rocks and minerals of 24 spinel harzburgite xenoliths from the Lac Michèle locality in the northern part of the Kerguelen Archipelago (South Indian Ocean). The samples are modally homogeneous and large enough to provide representative whole-rock samples. Their Mg# are high (0.91 to 0.93) and they have 16–29 wt.% orthopyroxene (opx) and low clinopyroxene contents (0.1–2.8 wt.%). They display a wide range of serpentinisation, which result in LOI contents ranging from 0 to 3.5 wt.%. The spinel-bearing harzburgites from Lac Michèle are the most refractory peridotites identified so far among peridotite xenoliths available on the Kerguelen Archipelago and within the oceanic lithosphere. By contrast with most of the peridotite xenoliths from Kerguelen, they have been more preserved from post-formation processes such as metasomatic processes. The major and trace element compositions of the least serpentinised spinel harzburgites indicate an origin by ~ 30% of polybaric decompression fractional melting between 5GPa and ≤ 1 GPa. Thus, the spinel harzburgites from Lac Michèle, situated at the top of the Kerguelen lithospheric mantle, are residues of melting that took place over a broad range of depth and mostly in the garnet stability field. Our results, in comparison with published data on mantle xenoliths worldwide, show that spinel harzburgites from Lac Michèle have major and modal compositions that fall in the range of cratonic peridotites rather than abyssal peridotites, oceanic island peridotites, subduction zones peridotites and off-cratonic peridotites. This indicates (i) they formed in similar condition as for the ancient continental lithospheric mantle or (ii) they are fragment of ancient continental lithospheric mantle incorporated in the Kerguelen plateau. [ABSTRACT FROM AUTHOR]

Abstract Recent geophysical research programs survey the Tamu Massif within the Shatsky Rise oceanic plateau in the northwest Pacific Ocean to understand the formation of this immense volcano and to test the formation hypotheses of large igneous province volcanism. Massive sheet basalt flows are cored from the Tamu Massif, implying voluminous eruptions with high effusion rates. Seismic reflection data show that the Tamu Massif is the largest single volcano on Earth, characterized by a central volcanic shield with low-gradient flank slopes, implying lava flows emanating from its center and spreading massive area on the seafloor. Velocity model calculated from seismic refraction data shows that crustal thickness has a negative correlation with average velocity, implying a chemically anomalous origin of the Tamu Massif. Seismic refraction and reflection data reveal a complete crustal structure across the entire volcano, featured by a deep crust root with a maximum thickness of ∼30 km, and Moho geometry is consistent with the Airy Isostasy. These recent findings provide evidence for the two end-member formation models: the mantle plume and the plate boundary. Both are supported by some results, but both are not fit with some either. Consequently, plume–ridge interaction could be a resolution that awaits future investigations. [ABSTRACT FROM AUTHOR]

The oceanic plateau is proposed to be produced by the plume head. Seeking the missing plume head associated with seamount chains is an important task to evaluate the plume theory. While the subduction initiation and process of the oceanic plateau is debatable, recent seismic studies suggest that the Hawaiian-Emperor oceanic plateau has been subducted into the lower mantle. The sinking rate is higher than previously estimated and even higher than that of the normal slab. It is intriguing, as the phase transitions would cause a density barrier for the basaltic materials from ∼660-km to ∼720-km depths, and slow down the subduction of the slabs, especially for the oceanic plateau with a thickened basaltic crust. Here, we perform geodynamic numerical experiments to investigate the foundering process of flat stagnant slabs from the transition zone into the lower mantle. We compare models of both the normal slab and slabs bearing oceanic plateau and suggest that a basaltic underplate layer beneath the peridotite mantle lithosphere in the oceanic plateau can lead to a higher foundering rate. The underplated oceanic plateau can reach ∼1000-km depth in ∼50 Myrs. These models provide a new perspective in understanding the fate of subducted oceanic plateau, suggesting that missing plume heads might be found in the lower mantle. • Provides a new perspective in seeking subducted plume head in the mantle. • Presents various foundering processes of flat stagnant slabs from the transition zone into the lower mantle. • Underplated oceanic plateau can reach ∼1000-km depth in ∼50 Myrs, agreeing with recent seismic imaging studies. [ABSTRACT FROM AUTHOR]

• TTG suites from the Katoro area yield ages at ca. 2.7 Ga, which fall within the peak crustal growth period in the northern part of the Tanzania Craton. • The Katoro TTG gneisses are trondhjemitic series rocks, and they are most likely to originate from partial melting of rutile-free, garnet- and amphibole-bearing, juvenile thickened lower mafic crust. • The Katoro TTG rocks formed during the assembly of the Tanzania Craton. • The tectonic evolution model in this part of the Tanzania Craton needs to invoke a scenario that crustal growth caused by subduction-related modification of an oceanic plateau. The northern part of the Tanzania Craton is mainly comprised of several low-grade metamorphic granite-greenstone belts that are exposed on the east and south shores of Lake Victoria. Due to their gold-endowment, these greenstone belts have been the main focus of research and were interpreted to form in various tectonic settings involving back-arc basin, island arc or continental arc, and oceanic plateau. However, the TTG gneiss, as the dominant component of Archean crust, has received far less attention from the scrutiny of modern analytical methods than its counterpart, the mafic volcanics of the greenstone belts. Here, we present a comprehensive geochemical dataset including zircon U-Pb age, zircon Lu-Hf isotope results, as well as bulk-rock geochemical compositions and Sm-Nd isotopic analyses for TTG suites from the Katoro area in northwestern part of Tanzania. Zircon U-Pb dating for three TTG samples yielded 207Pb/206Pb ages of 2688 ± 19 Ma, 2717 ± 19 Ma and 2695 ± 15 Ma, respectively, and the results fall within the peak crustal growth period in northern part of the Tanzania Craton. The Katoro TTGs have relatively high contents of SiO 2 (70.77–74.69 %) and Na 2 O (4.74–5.16 %), high ratios of Sr/Y (59.49–167.25) and (La/Yb) N (17.27–50.72), but low MgO (0.24–0.70 %) and K 2 O (1.71–3.26 %) contents, low Nb/Ta ratios (8.50–16.60), and most of them belong to the trondhjemitic series rocks. They are characterized by strong enrichment of LILEs and LREEs with slightly positive Eu anomalies (Eu/Eu*=1.05–1.38), but depleted in Nb-Ta and Ti, as well as heavy REEs and Y. They have positive ε Hf (t) values (average ε Hf (t) = +1.90 ± 0.50 or + 2.07 ± 0.33) and ε Nd (t) values (+1.19 to + 2.40) but lower than the contemporaneous depleted mantle, which is inconsistent with a subducting slab signature, and it is proposed that the Katoro TTGs most likely originated from partial melting of rutile-free, garnet- and amphibole-bearing lower mafic crust of an oceanic plateau. The results presented here suggest that the tectonic evolution in this part of Tanzania Craton needs to invoke a scenario in which crustal growth caused by subduction-related modification of an oceanic plateau. [ABSTRACT FROM AUTHOR]

Recent research from the Shatsky Rise in the western Pacific Ocean provides new insights on the formation and evolution of this oceanic plateau as well as tests of mantle models to explain anomalous large igneous province (LIP) volcanism. Recent Shatsky Rise studies cored the igneous pile (Integrated Ocean Drilling Program Expedition 324), imaged the interior with seismic refraction and multichannel seismic reflection data, and mapped magnetic anomalies adjacent to the plateau to provide new constraints on its tectonic history. Coring data show that Tamu Massif, the largest edifice within Shatsky Rise, is characterized by massive sheet flows, similar to flows caused by voluminous eruptions in continental flood basalts. Core data also indicate that the massive eruptions waned as the plateau evolved and smaller edifices were built. Seismic data show intrabasement reflectors within Tamu Massif that indicate volcanism from its center, indicating that this is an enormous shield volcano with abnormally low flank slopes and thick crust (~ 30 km). Paleomagnetic data record minimal geomagnetic field variations, consistent with the inference of massive, rapid volcanism. Altogether, the physical picture indicates that Shatsky Rise was built by massive, rapid eruptions that formed enormous volcanoes. Geochronologic data support the previously inferred age progression, with the volcanic massifs formed along the trace of a triple junction starting from Tamu Massif and becoming progressively younger to the northeast. These data weaken support for rapid emplacement because they show that the last eruptions atop Tamu Massif encompassed several million years between the final massive flows as well as a long hiatus of ~ 15 Myr until late stage eruptions that formed a summit ridge. They may also indicate that the last eruptions on Tamu and Ori massifs occurred while the triple junction was hundreds of kilometers distant. Furthermore, magnetic anomaly data indicate that the plate boundary reorganization associated with Shatsky Rise formation occurred several million years prior to the first Tamu Massif eruptions, suggesting plate boundary control of Shatsky Rise initiation. Geochemical and isotopic data show that Shatsky Rise rocks are variably enriched, with the majority of lavas being similar to mid-ocean ridge basalts (MORB). However, the data indicate deeper (> 30 km) and higher partial degree of melting (15–23%) as compared with normal MORB. Melting models indicate that the magma experienced a mantle temperature anomaly, albeit only a small one (~ 50 °C). Some lava compositions suggest the involvement of recycled subducted slab material. Recent investigations of Shatsky Rise initially envisaged a competition between two end-member models: the thermal plume head and the fertile mantle melting beneath plate extension (aka, plate model). Both hypotheses find support from new data and interpretations, but both do not fit some data. As a result, neither model can be supported without reservation. Noting that most basaltic oceanic plateaus have formed at triple junctions or divergent plate boundaries, we suggest that this dichotomy is artificial. Oceanic plateau volcanism is anomalous and focused at spreading ridges for reasons that are still poorly understood, mainly owing to uncertainties about mantle convection and geochemical reservoirs. Shatsky Rise investigations have vastly improved our understanding of the formation of this oceanic plateau, but highlight that important work remains to understand the underlying nature of this volcanism. [ABSTRACT FROM AUTHOR]

Crust forming processes on the early Earth have long been debated and relatively few rock and mineral samples exist with which to evaluate many hotly contested themes. The Acasta Gneiss Complex contains rock units with crystallization ages exceeding 4.0 Ga, making them the oldest known evolved rock units in the world. However, the AGC has experienced a long and complex history with multiple periods of igneous intrusion, deformation and metamorphism. Indeed, previous workers have demonstrated that orthogneisses within the AGC have igneous ages ranging from ∼4.03 to ∼3.4 Ga. This large range in crystallization ages gives us the opportunity to investigate the evolution of Earth’s earliest known continental crust through a period of greater than 1 billion years. Here we present an updated geologic map of key areas within the Acasta Gneiss Complex in which we delineate units based upon age as well as composition. Whole-rock geochemistry, zircon LA-ICPMS U–Pb geochronology and SIMS O-isotope analyses from a large suite of samples indicate a significant change in mode of crust formation over 400 million years. These data document a gradual change from shallow crustal processes generating basaltic to andesitic compositions at 4.02 Ga to deep-seated partial melting of hydrated basalt, represented by voluminous Archean TTG-like intrusions at 3.6 Ga. We find no evidence that classic Archean TTG-like rock units are present within the AGC prior to 3.6 Ga, suggesting a significantly different tectonic process at work prior to this time. We invoke an oceanic plateau-like model to describe the evolving nature of crust formation within the AGC, which forms a buoyant, evolved nucleus. This nucleus then initiates deep-seated partial melting of mafic crust forming voluminous TTG-like units at ∼3.6 Ga. This ultimately serves to stabilize the crust and forms a nucleus for later formation of the Slave craton. [ABSTRACT FROM AUTHOR]

Shatsky Rise in the Northwest Pacific is the best example so far of an oceanic plateau with two potential hotspot tracks emanating from it: the linear Papanin volcanic ridge and the seamounts comprising Ojin Rise. Arguably, these hotspot tracks also project toward the direction of Hess Rise, located ∼1200 km away, leading to speculations that the two plateaus are connected. Dredging was conducted on the massifs and seamounts around Shatsky Rise in an effort to understand the relationship between these plateaus and associated seamounts. Here, we present new 40 Ar/ 39 Ar ages and trace element and Nd, Pb, and Hf isotopic data for the recovered dredged rocks and new trace elements and isotopic data for a few drill core samples from Hess Rise. Chemically, the samples can be subdivided into plateau basalt-like tholeiites and trachytic to alkalic ocean-island basalt compositions, indicating at least two types of volcanic activity. Tholeiites from the northern Hess Rise (DSDP Site 464) and the trachytes from Toronto Ridge on Shatsky’s TAMU massif have isotopic compositions that overlap with those of the drilled Shatsky Rise plateau basalts, suggesting that both Rises formed from the same mantle source. In contrast, trachytes from the southern Hess Rise (DSDP Site 465A) have more radiogenic Pb isotopic ratios that are shifted toward a high time-integrated U/Pb (HIMU-type mantle) composition. The compositions of the dredged seamount samples show two trends relative to Shatsky Rise data: one toward lower 143 Nd/ 144 Nd but similar 206 Pb/ 204 Pb ratios, the other toward similar 143 Nd/ 144 Nd but more radiogenic 206 Pb/ 204 Pb ratios. These trends can be attributed to lower degrees of melting either from lower mantle material during hotspot-related transition to plume tail or from less refractory shallow mantle components tapped during intermittent deformation-related volcanism induced by local tectonic extension between and after the main volcanic-edifice building episodes on Shatsky Rise. The ocean-island-basalt-like chemistry and isotopic composition of the Shatsky and Hess rise seamounts contrast with those formed by purely deformation-related shallow mantle-derived volcanism, favoring the role of a long-lived mantle anomaly in their origin. Finally, new 40 Ar/ 39 Ar evidence indicates that Shatsky Rise edifices may have been formed in multiple-stages and over a longer duration than previously believed. [ABSTRACT FROM AUTHOR]

Oceanic plateaus are large igneous provinces formed by extraordinary eruptions that create thick oceanic crust, whose structure is poorly known owing to the lack of deep-penetration seismic data. Multichannel seismic (MCS) reflection and wide-angle refraction data allow us to show Moho structure beneath a large part of the Shatsky Rise oceanic plateau in the northwest Pacific Ocean. Moho reflectors in the two data sets can be connected to trace the interface from the adjacent abyssal plain across much of the interior. The reflectors display varied character in continuity, shape, and amplitude, similar to characteristics reported in other locations. Beneath normal crust, the Moho is observed at ∼13 km depth (∼7 km below the seafloor) in MCS data and disappears at ∼20 km depth (∼17 km below the seafloor) beneath the high plateau. Moho at the distal flanks dips downward towards the center with slopes of ∼0.5°–1°, increasing to 3°–5° at the middle flanks. Seismic Moho topography is consistent with Airy isostasy, confirming this widely-applied assumption. Data from this study show that crustal thickness between the massifs in the interior of the plateau is nearly twice normal crustal thickness, despite the fact that this crust records apparently normal seafloor spreading magnetic lineations. The Moho model allows improved estimates of plateau area ( 5.33 × 10 5 km 2 ) and volume ( 6.90 × 10 6 km 3 ), the latter assuming that the entire crust was formed by Shatsky Rise volcanism because the massifs formed at spreading ridges. This study is unique in showing Moho depth and structure over an extraordinarily large area beneath an oceanic plateau, giving insight to plateau structure and formation. [ABSTRACT FROM AUTHOR]

The formation mechanisms of flat slabs in South America remain unclear. To quantitatively evaluate the earlier proposed mechanisms, we simulate the post-100 Ma subduction history below South America using 4-D geodynamic models by progressively incorporating plate kinematics, seafloor ages and key tectonic features including the buoyant oceanic crust, continental cratons, oceanic plateaus (i.e. the inferred Inca plateau, subducting Nazca Ridge and Juan Fernandez Ridge), as well as deformable trench profiles according to recent geological reconstructions. We find that, in the absence of an overriding plate and subducting buoyancy features, the seafloor age affects slab dip angle by controlling the slab's mechanical strength (i.e., the resistance to bending) and negative buoyancy (integrated positive density anomaly that enhances bending). Our models show that slab strength dominates its buoyancy at age >30 Ma and the opposite for younger ages. The existence of a thick overriding plate reduces the slab dip by increasing dynamic suction, and individual cratonic roots further lead to along-trench variations of dip angle reduction. While dynamic suction from the overriding plate generates a permanent reduction of the long-wavelength slab dip angle, it is the final addition of subducting oceanic plateau and aseismic ridges that produces the transient and localized flat-slabs as observed. These results suggest that all mechanisms except the buoyancy features affect the slab dip only at large spatial scales. Our best-fit model with all the above tectonic features included provides a good match to both the upper mantle Benioff zones and the temporal evolution of volcanic arcs since the mid-Miocene. The imperfect match of the Peruvian flat-slab is likely associated with the uncertain 3-D configuration of the Amazonian craton. [ABSTRACT FROM AUTHOR]

The oceanic lithosphere subduction has been attested to exert the first-order control on the formation of the continental crust, but the role of oceanic plateaus on the crustal growth is elusive. It is difficult to identify the component of the oceanic plateaus in the continental crust because of their disappearance after subduction into deep mantle or obduction to be a part of the ophiolite mélanges in the ancient orogenic belts. Amdo microcontinent in the central Tibetan Plateau is an ideal place to assess the role of oceanic plateaus on crustal growth as it preserves the well-exposed arc magmatic rocks and fragments of the Meso-Tethyan oceanic plateaus to its south. Here, we present an integrated investigation of zircon U Pb ages and Hf isotope, as well as whole-rock elements and Sr-Nd-Hf isotopes of a suit of dacites in the Amdo microcontinent. Zircon U Pb dating indicates that the Amdo dacites formed at ca. 117 Ma, postdated the major formation period of the Meso-Tethyan oceanic plateaus. The Amdo dacites have relatively high SiO 2 (63.45–66.82 wt%) contents and low Mg# values (20.0–36.1), belonging to calc-alkaline series. The zircon thermometer and oxybarometer give estimated temperature and oxygen fugacity of crystallization as ca. 700–720 °C and ΔFMQ = + 0.98, respectively. The Amdo dacites have low Y (5.51–6.61 ppm) and Yb (0.44–0.53 ppm) contents and high La/Yb ratios (28.4–30.3). Despite their low Sr/Y ratios (6.86–13.5) caused by a small degree of fractionation of plagioclase, the Amdo dacites have an adakitic geochemical affinity, which was most likely derived from the thickened continental lower crust. These dacites have more depleted Sr-Nd-Hf isotope compositions (whole-rock 87Sr/86Sr i = 0.709796–0.710563, ε Nd (t) = −3.27 to −2.82, ε Hf (t) = 4.20–4.83 and zircon ε Hf (t) = 4.2–8.8) than the Neoproterozoic and Cambrian orthogneisses, as well as the Jurassic magmatic rocks. Simple binary mixing modeling reveals that the Amdo dacites could represent the hybrid melts of 50–60% of the subducted Meso-Tethyan oceanic plateau materials with 50–40% of the Amdo ancient continental crustal materials. The decoupled whole-rock Nd Hf isotopes (Δε Hf (t) = 7.55–8.43) of the Amdo dacites could be explained by the addition of zircon-free sediments overlie on the oceanic plateaus. These results indicate that subduction of the Meso-Tethyan oceanic plateaus with/without the overlying sediments have made a substantial contribution to continental crustal growth. In conjunction with previous studies, we conclude that the subduction of the Meso-Tethyan oceanic plateaus may have intermittently occurred for around 37 million years, thus highlighting the significance of subduction of the Meso-Tethyan oceanic plateaus during the secular evolution of the central Tibetan Plateau. • ∼117 Ma Amdo dacites first reported here formed under arc magmas-like oxidized environment. • Amdo dacites were most likely derived from the thickened continental lower crust. • Amdo dacites represent a hybrid melt of the ancient crustal basement and oceanic plateau materials. • Subduction of the oceanic plateau has an enormous potential for crust growth. [ABSTRACT FROM AUTHOR]

The Ordos Basin in China has about 10 km of Neoarchean to Quaternary sediments covering an enigmatic basement of uncertain origin. The basement is tectonically stable, has a thick mantle root, low heat flow, few earthquakes, and has been slowly subsiding for billions of years. The basement has geophysical signatures that indicate it is dominantly intermediate to mafic in composition, and is similar to some other cratons world-wide, and also to several major oceanic plateaus. It was accreted to the amalgamated Eastern Block and Central Orogenic belt of the North China Craton (NCC) in the Paleoproterozoic, then involved in several Proterozoic tectonic events including being over-thrust by an accretionary orogen, and intruded by Andean arc-related magmas, and then involved in a continent-continent collision during amalgamation with the Columbia Supercontinent. Thus, the basement rocks are deformed, metamorphosed to granulite facies, and determining their initial origin is difficult. We suggest that the data is consistent with an origin as an oceanic plateau that accreted to the NCC and, later experienced different episodes of differentiation associated with later subduction and collisions. Formation of cratonic lithosphere by accretion of oceanic plateaus may be one mechanism to create stable cratons. Other cratons that apparently formed by partial melting of underplated and imbricated oceanic slabs are stable in some cases, but also re-activated and “de-cratonized” in some cases in Asia, where they have been affected by younger subduction, hydration, slab roll-back, and melt-peridotite reactions. This suggests that the initial mode of craton formation may be a factor in the preservation of stable cratons, and de-cratonization is not only influenced by younger tectonic activity. [ABSTRACT FROM AUTHOR]

The Ontong Java Plateau (OJP) represents the result of a significant event in the Earth's geologic history. Limited geophysical and geochemical data, as well as the plateau's relative isolation in the Pacific ocean, have made interpretation of the modern day geologic structure and its 120 Ma formation history difficult. Here we present the highest resolution image to date of the wave speed structure of the OJP region. We use a data set that combines Rayleigh waves extracted from both ambient noise and earthquake waveforms and an iterative finite-frequency tomography methodology. The combination of datasets allow us to best exploit the limited station distribution in the Pacific and image wave speed structures between 35 km and 300 km into the Earth. We image a region of fast shear wave speeds, greater than 4.75 km/s, that extends to greater than 100 km beneath the plateau. The wave speeds are similar to as observed in cratonic environments and are consistent with a compositional anomaly that resulted from the residuum of eclogite entrainment during the plateau's formation. The combination of our imaged wave speed structure and previous geochemical work suggest that a surfacing plume head entrained eclogite from the deep mantle and accounts for the anomalous buoyancy characteristics of the plateau and observed fast wave speeds. [ABSTRACT FROM AUTHOR]

Carbonatites in island arcs are crucial for understanding the processes of magmatism and carbon recycling in subduction zones. Here we report carbonatitic pockets in Ordovician intra-oceanic island-arc volcanic rocks and in Ocean island basalts (OIB) of Cambrian ophiolites in the South Qilian Accretionary Belt, NW China. All these carbonatitic pockets are composed of relatively pure calcites, some of which have accessory minerals of apatite, sphene, hematite, magnetite and allanite. The 87Sr/86Sr (i) ratios of calcite range from 0.703833 to 0.706732, concentrated at about 0.705, which is close to the composition of Enriched Mantle 1 (EMI) affected by fluids from the subduction zone. The C-O isotopic compositions show trends that evolve away from primary carbonatite (δ13C PDB = −9.0 ~ −1.0 ‰, 18O SMOW = 10.1– 16.5 ‰), probably caused by calcite fractional crystallization. High Sr, Th, U, Pb and low Nb, Zr, Hf, Ti contents and 87Sr/86Sr (i) ratios indicate that these carbonatitic pockets derived from carbonatite magma, and may have been altered by subduction-zone fluids. Carbonatitic pockets in ankaramites represent primary mantle-derived carbonate melts/liquids with low REE (rare earth elements) abundances (<10 ppm) and δ13C PDB. Elevated REE contents and LREE/HREE (light / heavy rare earth elements) ratios suggest that carbonatitic pockets in high-Al andesite may have experienced fractionation crystallization. The EMI-type carbonatitic pockets occurrence in the two different rock assemblages may reflect an interaction between an oceanic arc and an oceanic plateau, deriving from the recycling of Proterozoic marine carbonates through the lower mantle. [ABSTRACT FROM AUTHOR]

We disagree the transitional supra-subduction zone model of Feng et al. (Int J Earth Sci (Geol Rundsch) 105:491-505, 2016) for the tectonic setting of Jifeng ophiolite suite in NE China. Existence of the komatiite in the Jifeng ophiolite indicates an oceanic plateau environment for this ophiolite suite within the so-called Xinlin-Xiguitu ocean. [ABSTRACT FROM AUTHOR]

We report the occurrences of the remnants of a Meso-Tethyan oceanic plateau, encompassing an area of ~ 2 × 10 5 km 2 in central Tibet. The plateau remnants include large volumes of pillow basalt formed largely by emergent to subaerial eruption, minor ultramafic intrusives and cumulates, exotic blocks of limestone, radiolarian chert, graywacke, and shale. Isotopic and paleontological dating suggest two major plateau eruptive events at 193–173 Ma and at 128–104 Ma, respectively. The basalts are characterized by enrichment of incompatible elements and a wide range of Sr–Nd isotope composition (initial ε Nd from –3.71 to + 7.9, initial 87 Sr/ 86 Sr from 0.703927 to 0.707618). The trace element and Sr–Nd isotopic data suggest that these basalts are of affinity with those from the Kerguelen and Tethyan plumes, indicative of a plume mantle upwelling origin with involvement of continental material. The wholesale obduction of the Meso-Tethyan oceanic plateau, along with the dismembered normal oceanic crustal fragments, over the Tibetan continental crust could have given rise to perhaps 2 km elevation of central Tibet during the Late Cretaceous. [ABSTRACT FROM AUTHOR]

This study presents laser step-heating Ar/Ar age determinations of basaltic lava samples from Tamu Massif, the oldest and largest edifice of the submarine Shatsky Rise in the northwest Pacific and Earth's proposed largest volcano. The rocks were recovered during Integrated Ocean Drilling Program Expedition 324, which cored 160 m into the igneous basement near the summit of Tamu Massif. The analyzed lavas cover all three major stratigraphic groups penetrated at this site and confirm a Late Jurassic/Early Cretaceous age for the onset of Shatsky Rise volcanism. Lavas analyzed from the lower and middle section of the hole yield plateau ages between 144.4 ± 1.0 and 143.1 ± 3.3 Ma with overlapping analytical errors (2σ), whereas a sample from the uppermost lava group produced a significantly younger age of 133.9 ± 2.3 Ma suggesting a late or rejuvenated phase of volcanism. The new geochronological data infer minimum (average) melt production rates of 0.63-0.84 km/a over a time interval of 3-4 million years consistent with the presence of a mantle plume. [ABSTRACT FROM AUTHOR]

Active continental margins represent both sites of crustal loss by tectonic erosion, and gain by juvenile magmatism and oceanic arc-plateau accretion. Estimating the gain component depends on distinguishing between growth of margins by either accretion of oceanic crust, or juvenile magmatic addition during extension. Distinguishing between these allochthonous and autochthonous models is frequently ambiguous due to difficulties in interpreting the tectonic origin of mafic rocks. We present a detailed geochronological and isotopic study constraining the evolution of the continental margin of northwest South America during 190-113 Ma. Semi-continuous slab rollback resulted in crustal thinning, juvenile magmatism, and ultimately, the removal of continental slivers; negating previous hypotheses that the margin evolved by the accretion of an island arc. However, basic crustal growth estimates, along with comparisons between rocks exposed in Northern Peru and the Northern Andes imply that uneven losses of continental crust occurred along northwest South America for Mesozoic exposures, and that the accretion of the Caribbean Large Igneous Province (CLIP) onto NW South America probably played a crucial role in maintaining and preserving Mesozoic continental crust north of the Huancabamba deflection. [ABSTRACT FROM AUTHOR]

Abstract: Until recently it was assumed that the Archaean continental crust (made of TTGs: tonalites, trondhjemites, and granodiorites) was generated through partial melting of MORB-like basalts in hot subduction environments, where the subducted oceanic crust melted at high pressure, leaving a garnet-bearing amphibolitic or eclogitic residue. However, recent geochemical models as well as basalt melting experiments have precluded MORB as a plausible source for TTGs. Rather, geochemical and experimental evidences indicate that formation of TTG required a LILE-enriched source, similar to oceanic plateau basalts. Moreover, subduction is a continuous process, while continental growth is episodic. Several “super-growth events” have been identified at ~4.2, ~3.8, ~3.2, ~2.7, ~1.8, ~1.1, and ~0.5Ga, which is inconsistent with the regular pattern that would be expected from a subduction-driven process. In order to account for this periodicity, it has been proposed that, as subduction proceeds, descending residual slabs accumulate at the 660-km seismic discontinuity. When stored oceanic crust exceeds a certain mass threshold, it rapidly sinks into the mantle as a cold avalanche, which induces the ascent of mantle plumes that in turn produce large amounts of magmas resulting in oceanic plateaus. However, melting at the base of thick oceanic plateaus does not appear to be a realistic process that can account for TTG genesis. Modern oceanic plateaus contain only small volumes (≤5%) of felsic magmas generally formed by high degrees of fractional crystallization of basaltic magmas. The composition of these felsic magmas drastically differs from that of TTGs. In Iceland, the interaction between a mantle plume and the mid-Atlantic ridge gives rise to an anomalously (Archaean-like) high geothermal gradient resulting in thick basaltic crust able to melt at shallow depth. Even in this favorable context though, the characteristic Archaean TTG trace element signature is not being produced. Consequently, internal recycling of oceanic plateaus does not appear to be a suitable process for the genesis of Archaean continental crust. A possible alternative to this scenario is the subduction of oceanic plateaus. This hypothesis is supported by a present-day analog. In Ecuador, the Carnegie ridge, which is an oceanic plateau resulting from the Galapagos hot spot activity, is being subducted beneath the South American plate. Not only are the resulting magmas adakitic (TTG-like) in composition, but the volcanic productivity is several times greater than in other parts of the Andean volcanic arc. Above the location where the plateau is subducted, the arc is wide and the quaternary volcanoes numerous (about 80 active edifices). The volcanic productivity of each individual volcano also is more intense than away from the subduction focal point with an average output rate of about 0.4–0.5km3·ka−1 compared with only about 0.05–0.2km3·ka−1 for production rates at volcanoes erupting in the rest of the arc. Consequently, we infer that occasional subduction of oceanic plateaus throughout Earth's history can account for the episodic nature of crustal growth. Additionally, the generation by this mechanism of huge volumes of TTG-like magmas would readily dominate the crustal growth record. [Copyright &y& Elsevier]