Slab Buoyancy Research Articles

The missing ridge Enigma: A new model for the Tuamotu Plateau conjugate and Peruvian flat slab

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

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

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

Buoyancy of slabs and plumes enhanced by curved post-garnet phase boundary

Buoyancy of slabs and plumes enhanced by curved post-garnet phase boundary

Venus' light slab hinders its development of planetary-scale subduction.

Terrestrial planet Venus has a similar size, mass, and bulk composition to Earth. Previous studies proposed that local plume-induced subduction existed on both early Earth and Venus, and this prototype subduction might initiate plate tectonics on Earth but not on Venus. In this study, we simulate the buoyancy of submerged slabs in a hypothesized 2-D thermo-metamorphic model. We analyze the thermal state of the slab, which is then used for calculating density in response to thermal and phase changes. The buoyancy of slab mantle lithosphere is primarily controlled by the temperatures and the buoyancy of slab crust is dominated by metamorphic phase changes. Difference in the eclogitization process contributes most to the slab buoyancy difference between Earth and Venus, which makes the subducted Venus' slab consistently less dense than Earth's. The greater chemical buoyancy on Venus, acting as a resistance to subduction, may have impeded the transition into self-sustained subduction and led to a different tectonic regime on Venus. This hypothesis may be further tested as more petrological data of Venus become available, which will further help to assess the impact of petro-tectonics on the planet's habitability.

The effects of plateau subduction on plate bending, stress and intraplate seismicity

AbstractThe influence of subducting plateaux on inter‐plate earthquakes has been extensively studied. However, its effects on plate bending, stress distribution and intraplate earthquakes remain unclear. Here we model the deflection of a subducting oceanic plate with an oceanic plateau regarded as a high flexure rigidity body near the trench using a thin plate model. Dozens of models were carried out to study the effects of the distance between plateau and trench on flexural bending and stress with variable flexural rigidity. We find that the influence of plateau depends on its location with respect to the trench axis and it begins to affect the plate bending when its distance from the trench axis is ~70–120 km. It changes the bending stress distribution and causes concentrated deformation to the trench‐ward of the plateau. After the plateau starts to subduct, pre‐existing bending faults may be reactivated by the differential forces caused by the lateral variation in slab buoyancy between the shallower seamounts/plateaus and the deeper slab.

The Role of Sediment Accretion and Buoyancy on Subduction Dynamics and Geometry

AbstractSubducted sediments are thought to lubricate the subduction interface and promote faster plate speeds. However, global observations are not clear‐cut on the relationship between the amount of sediments and plate motion. Sediments are also thought to influence slab dip, but variations in subduction geometry depend on multiple factors. Here we use 2D thermomechanical models to explore how sediments can influence subduction dynamics and geometry. We find that thick sediments can lead to slower subduction due to an increase of the megathrust shear stress as the accretionary wedge gets wider, and a decrease in slab pull as buoyant sediments are subducted. Our results also show that larger slab buoyancy and megathrust stress due to thick sediments increase the slab bending radius. This offers a new perspective on the role of sediments, suggesting that sediment buoyancy and wedge geometry also play an important role on large‐scale subduction dynamics.

Delayed Growth of Ferropericlase and Bridgmanite Controls Slab Residence at the 660‐km Discontinuity

AbstractCold subducted oceanic lithosphere may stall in the uppermost portions of the lower mantle (660–700 km depth) as a consequence of its slow thermal equilibration and the negative dP/dT of reactions forming ferropericlase and bridgmanite. Evaluations of slab buoyancy in the lower parts of the transition zone have mostly neglected the effects of dynamic P–T paths and the temperature‐dependent expansion or contraction of stalled slabs. Forward predictions from mineral equilibria and thermal modeling posit slab residence at the base of the mantle transition zone for c. 150–160 Myr, dependent on geotherm and the depth of stagnation (between 660–700 km). Slab components can attain density thresholds following heating and phase transformations over distinct periods: Basaltic crust will become positively buoyant after c. 4 Myr whereas ultramafic components will become negatively buoyant after c. 150–160 Myr. Slab components may thus separate and independently transit or escape the transition zone under circumstances of equilibrated mineral assemblages. The 660 km seismic discontinuity presents a filter for the partial recycling of the upper oceanic lithosphere into the upper mantle.

Assessing the Role of Water in Alaskan Flat‐Slab Subduction

AbstractLow‐angle subduction has been shown to have a profound impact on subduction processes. However, the mechanisms that initiate, drive, and sustain flat‐slab subduction are debated. Within all subduction zone systems, metamorphic dehydration reactions within the down‐going slab have been hypothesized to produce seismicity, and to produce water that fluxes melting of the asthenospheric wedge leading to arc magmatism. In this work, we examine the role hydration plays in influencing slab buoyancy and the geometry of the downgoing oceanic plate. When water is introduced to the oceanic lithosphere, it is incorporated into hydrous phases, which results in lowered rock densities. The net effect of this process is an increase in the buoyancy of the downgoing oceanic lithosphere. To better understand the role of water in low‐angle subduction settings, we model flat‐slab subduction in Alaska, where the thickened oceanic lithosphere of the Yakutat oceanic plateau is subducting beneath the continental lithosphere. In this work, we calculate the thermal conditions and stable mineral assemblages in the slab crust and mantle in order to assess the role that water plays in altering the density of the subducting slab. Our slab density results show that a moderate amount of hydration (1–1.5 wt% H2O) in the subducting crust and upper lithospheric mantle reduces slab density by 0.5%–0.8% relative to an anhydrous slab, and is sufficient to maintain slab buoyancy to 300–400 km from the trench. These models show that water is a viable factor in influencing the subduction geometry in Alaska, and is likely important globally.

Dynamic interactions between subduction zones

A common feature of tectonic reconstructions is the migration of subduction zones and their interactions, resulting in complex tectonic patterns. Reduced migration rates, uplift, strong rotations, and slab break-off are all tectonics features commonly associated with convergent margin interactions, showing that while subduction zones' motions fundamentally shape tectonics settings, their interactions hold the key to their complexities. Fundamental properties such as slab buoyancy, dip direction, and coupled mantle flow determine the motions of single hinge zones. When these zones approach, stress transfers between the slabs through the lithosphere and the mantle, thereby altering the subduction force balance, favoring, opposing, or diverting further motions. We illustrate the range of configurations reported in tectonic reconstructions and address their dynamics through the results of analog and numerical models of subduction dynamics in the upper mantle. The Cenozoic evolution of the Mediterranean provides relevant examples of such interactions, where the coeval activity of several subduction zones during the Africa-Europe convergence has strongly overprinted the current tectonics. A comparison of new analog models with the deep dynamics of the Mediterranean shows how the interactions among slabs and mantle may have shaped the distribution of basins and mountain belts, their uplift, and volcanism in this tectonic province. • Interaction between subduction zones modified surface tectonics and mantle dynamics. • Both deep and surface processes can be affected by stress propagation between slabs. • Slab-slab interactions have been relevant in multiple events in the Mediterranean.

Caribbean Slab Segmentation Beneath Northwest South America Revealed by 3‐D Finite Frequency Teleseismic P‐Wave Tomography

AbstractThe Caribbean plate subducts beneath northwest South America at a shallow angle due to a large igneous province that added up to 12 km of buoyant crust. The overriding plate lacks volcanism and exhibits Laramide‐style uplifts over 500 km from the trench. Here, we illuminate the subduction structures through finite frequency teleseismic P‐wave tomography and connect those structures to the Laramide‐style deformation on the overriding plate. We use a new data set collected from the Caribbean‐Mérida Andes seismic experiment comprised of 65 temporary broadband stations integrated with permanent stations from the Colombian and Venezuelan national networks. We identify three segments of subducting Caribbean plate with one segment completely detached from the surface. The timing of the detachment aligns with other regional events, including the uplift of the Mérida Andes, about 10 Ma. Slab buoyancy post‐detachment likely resulted in recoupling with the overriding plate, reactivation of Jurassic‐aged rift structures and subsequent uplift of the Mérida Andes. Mantle counterflow over the broken segment induced by rollback of the attached slab likely contributed to the uplift of the Mérida Andes. We conclude that the northern limit of subduction lies south of the Oca‐Ancón fault, though the fault itself may be the surface expression of the boundary. The southern limit of subduction lies south of our study area.

Subduction Dynamics and Mantle Pressure: 2. Towards a Global Understanding of Slab Dip and Upper Mantle Circulation

AbstractWe investigate the relationship between the global distribution of deep slab dips (at 250‐ to 300‐km depth) and pressure and circulation in the upper mantle. Using an analytic method to compute dynamic pressure in a 3‐D global upper mantle domain, and a force balance between slab dip, slab buoyancy, and pressure, we model dips for all major subduction zones. Overall, our models suggest that global‐scale mantle flow, as dictated by the shapes and velocities of Earth's plates and slabs, plays a fundamental role in creating the global pattern of slab dips. The dip trends of the South American and western Pacific subduction zones are controlled, in our models, by spatial variations in the dynamic pressure associated with flow. Our best fitting models produce global root mean square dip misfits of less than 10° for asthenospheric viscosities of 2.5–4.0 × 1020 Pas. This result is only obtained with a large flux of asthenosphere from upper to lower mantle at subduction boundaries, occurring on the overriding plate side of slabs, without which dips are significantly steeper than observed. This effect cannot be resolved by processes that affect only certain subduction systems and requires flux of asthenosphere into the lower mantle at subduction systems globally (or an alternative mechanism that produces more negative pressures on the overriding plate side of slabs). Upper mantle pressure fields that fit global slab dips yield negative dynamic pressure on the upper plate side of slabs, positive pressure on the subducting plate side, and an east‐to‐west pressure increase beneath the Pacific Plate.

Upper plate deformation and trench retreat modulated by subduction-driven shallow asthenospheric flows

Upper plate deformation within a subduction zone depends on the complex relationship between surface plate motions, trench motion, slab pull and asthenospheric flow. Previous modelling studies suggest that trench motion rates should be related to slab buoyancy, but this relationship is neither clear nor verified by observations of natural subduction systems. Trench motion is also thought to induce upper plate deformation; however, no clear correlation has been identified between the direction of trench motion and the mode of upper plate deformation. In this study, we construct 2-D thermo-mechanical models to explore the relationship between slab pull, trench retreat and upper plate deformation, focusing on subduction systems with retreating trenches. We start with quasi-steady-state subduction and introduce a positive density anomaly into the slab to transiently increase slab pull. We vary both the value of the density anomaly and the properties of the upper plate to isolate key controls on trench retreat and upper plate deformation. Our models indicate that asthenospheric flow responds to changes in slab pull and influences both trench retreat and upper plate deformation. We propose that trench retreat depends on the competition between shallow and opposite asthenospheric flows below the subducting and upper plates, and that a fast sub-slab flow can hamper trench retreat even when slab buoyancy is high. After a transient slab pull increase, the mode of upper plate deformation partly depends on the upper plate's ability to translate horizontally: an upper plate with a ridge at its trailing edge deforms by shortening, while a fixed upper plate may deform by extension. Finally, in some cases, upper plate deformation seems to allow trench retreat if the upper plate is weak enough to be deformed by the basal shear from underlying asthenospheric flow. Our results provide insights into retreating subduction systems with contrasting upper plate deformation modes, such as the compressive Andes and the extensional Aegean.

The role of arc migration in the development of the Lesser Antilles: A new tectonic model for the Cenozoic evolution of the eastern Caribbean

Abstract Continental arc systems often show evidence of large-scale migration both toward and away from the incoming plate. In oceanic arc systems, however, while slab roll-back and the associated processes of backarc spreading and arc migration toward the incoming plate are commonplace, arc migration away from the incoming plate is rarely observed. We present a new compilation of marine magnetic anomaly and seismic data in order to propose a new tectonic model for the eastern Caribbean region that includes arc migration in both directions. We synthesized new evidence to show two phases of backarc spreading and eastward arc migration toward the incoming Atlantic. A third and final phase of arc migration to the west subdivided the earlier backarc basin on either side of the present-day Lesser Antilles arc. This is the first example of regional multidirectional arc migration in an intra-oceanic setting, and it has implications for along-arc structural and geochemical variations. The back and forth arc migrations were probably due to the constraints imposed by the neighboring American plates on this isolated subduction system, rather than variations in subducting slab buoyancy.

The relationship between mantle potential temperature and oceanic lithosphere buoyancy

The Earth's mantle potential temperature (TP) is thought to have cooled by ∼250∘C since the Archean, causing a progressive change in both the structure and composition of oceanic lithosphere. These variables affect the negative buoyancy of subducting slabs, which is known to be an important force in driving plate motions. However, the relationship between TP and slab buoyancy remains unclear. Here, we model the formation and subduction of oceanic lithosphere as a function of TP, to investigate how TP influences the buoyancy of subducting slabs, and by extension how buoyancy forces may have changed through time. First, we simulate isentropic melting of peridotite at mid-ocean ridges over a range of TP (1300–1550∘C) to calculate oceanic lithosphere structure and composition. Second, we model the thermal evolution of oceanic plates undergoing subduction for a variety of scenarios (by varying lithospheric thickness, slab length and subduction velocity). Finally, we integrate the structural, compositional and thermal constraints to forward model subduction metamorphism of oceanic plates to determine down-going slab density structures. When compared with ambient mantle, these models allow us to calculate buoyancy forces acting on subducting slabs. Our results indicate that oceanic lithosphere derived from hotter mantle has a greater negative buoyancy, and therefore subduction potential, than lithosphere derived from cooler mantle for a wide range of subduction scenarios. With respect to the early Earth, this conclusion supports the viability of subduction, and models of subduction zone initiation that invoke the concept of oceanic lithosphere being primed to subduct. However, we also show that decreases to lithosphere thickness and slab length, and reduced crustal hydration, progressively reduce slab negative buoyancy. These results highlight the need for robust estimates of early Earth lithospheric properties when considering whether subduction was operative at this time. Nevertheless, our findings suggest that subduction processes on the early Earth may have been uniformitarian.

Pressure–Temperature History of the >3 Ga Tartoq Greenstone Belt in Southwest Greenland and Its Implications for Archaean Tectonics

The Tartoq greenstone belt of southwest Greenland represents a well-preserved section through >3 Ga old oceanic crust and has the potential to provide important constraints on the composition and geodynamics of the Archaean crust. Based on a detailed structural examination, it has been proposed that the belt records an early style of horizontal convergent plate tectonics where elevated temperatures, compared to the modern-day, led to repeated aborted subduction and tonalite–trondhjemite–granodiorite (TTG) type melt formation. This interpretation hinges on pressure–temperature (P–T) constraints for the belt, for which only preliminary estimates are currently available. Here, we present a detailed study of the pressure–temperature conditions and metamorphic histories for rocks from all fragments of the Tartoq belt using pseudosection modelling and geothermobarometry. We show that peak conditions are predominantly amphibolite facies, but range from 450 to 800 °C at up to 7.5 kbar; reaching anatexis with formation of TTG-type partial melts in the Bikuben segment. Emplacement of the Tartoq segments into the host TTG gneisses took place at approximately 3 Ga at 450–500 °C and 4 kbar as constrained from actinolite–chlorite–epidote–titanite–quartz parageneses, and was followed by extensive hydrothermal retrogression related to formation of shear zone-hosted gold mineralisation. Tourmaline thermometry and retrograde assemblages in mafic and ultramafic lithologies constrain this event to 380 ± 50 °C at a pressure below 1 kbar. Our results show that the convergent tectonics recorded by the Tartoq belt took place at a P–T gradient markedly shallower than that of modern-day subduction, resulting in a hot, weak and buoyant slab unable to generate and transfer ‘slab pull’, nor sustain a single continuous downgoing slab. The Tartoq belt suggests that convergence was instead accomplished by under-stacking of slabs from repeated aborted subduction. The shallow P–T path combined with thermal relaxation following subduction stalling subsequently resulted in partial melting and formation of TTG melts.

Subduction Orogeny and the Late Cenozoic Evolution of the Mediterranean Arcs

The Late Cenozoic tectonic evolution of the Mediterranean region, which is sandwiched between the converging African and European continents, is dominated by the process of subduction orogeny. Subduction orogeny occurs where localized subduction, driven by negative slab buoyancy, is more rapid than the convergence rate of the bounding plates; it is commonly developed in zones of early or incomplete continental collision. Subduction orogens can be distinguished from collisional orogens on the basis of driving mechanism, tectonic setting, and geologic expression. Three distinct Late Cenozoic subduction orogens can be identified in the Mediterranean region, making up the Western Mediterranean (Apennine, external Betic, Maghebride, Rif), Central Mediterranean (Carpathian), and Eastern Mediterranean (southern Dinaride, external Hellenide, external Tauride) Arcs. The Late Cenozoic evolution of these orogens, described in this article, is best understood in light of the processes that govern subduction orogeny and depends strongly on the buoyancy of the locally subducting lithosphere; it is thus strongly related to paleogeography. Because the slow (4–10 mm/yr) convergence rate between Africa and Eurasia has preserved the early collisional environment, and associated tectonism, for tens of millions of years, the Mediterranean region provides an excellent opportunity to elucidate the dynamic and kinematic processes of subduction orogeny and to better understand how these processes operate in other orogenic systems.

Numerical modeling of stress and topography coupling during subduction: Inferences on global vs. regional observables interpretation

The mechanical strength of the oceanic lithospheres exerts a fundamental control on the dynamics and tectonics of convergent margins. Here, we use numerical models of coupled subducting-upper plates and mantle to investigate the impact of the subducting lithosphere properties on the parameters commonly measured at subduction zones. The models predict a correlation among parameters such as subducting and convergence velocities, slab dip and curvature, and stress coupling with the upper plates, such signature emerges in a dynamic system through the slab buoyancy. Similar trends are found when assessing the impact of laboratory-constrained non-linear rheologies uncertainties on the mechanical strength and mantle flow, although some features are more affected. When the role of upper plate thickness is tested, we find a positive correlation of upper plate subsidence and fore-arc topography with the buoyancy of the down-going plate. However, plasticity in the crust limits the development of topography, becoming increasingly relevant with thinner upper plates, so that no correlation can be established. Similar control is found on the width of the long-wavelength dynamic subsidence, due to flexure, which increases non-linearly with the upper plate thickness. The models constrain relations among parameters within the dynamic system driven that allow inferences on the observable parameters. The comparisons supports the idea that the subduction dynamics emerges as a global phenomenon in plate motions, bending and dynamic subsidence, yet Andean-type margin tectonics is best addressed in a regional context.

Subduction of thick oceanic plateau and high‐angle normal‐fault earthquakes intersecting the slab

AbstractThe role of seamounts on interplate earthquakes has been debated. However, its impact on intraslab deformation is poorly understood. Here we present unexpected evidence for large normal‐fault earthquakes intersecting the slab just ahead of a subducting seamount. In 1995, a series of earthquakes with maximum magnitude of 7.1 occurred in northern Ryukyu where oceanic plateaus are subducting. The aftershock distribution shows that conjugate faults with an unusually high dip angle of 70–80° ruptured the entire subducting crust. Seismic reflection images reveal that the plate interface is displaced over 1 km along one of the fault planes of the 1995 events. These results suggest that a lateral variation in slab buoyancy can produce sufficient differential stress leading to near‐vertical normal‐fault earthquakes within the slab. On the contrary, the upper surface of the seamount (plate interface) may correspond to a weakly coupled region, reflecting the dual effects of seamounts/plateaus on subduction earthquakes.

The dynamical control of subduction parameters on surface topography

AbstractThe long‐wavelength surface deflection of Earth's outermost rocky shell is mainly controlled by large‐scale dynamic processes like isostasy or mantle flow. The largest topographic amplitudes are therefore observed at plate boundaries due to the presence of large thermal heterogeneities and strong tectonic forces. Distinct vertical surface deflections are particularly apparent at convergent plate boundaries mostly due to the convergence and asymmetric sinking of the plates. Having a mantle convection model with a free surface that is able to reproduce both realistic single‐sided subduction and long‐wavelength surface topography self‐consistently, we are now able to better investigate this interaction. We separate the topographic signal into distinct features and quantify the individual topographic contribution of several controlling subduction parameters. Results are diagnosed by splitting the topographic signal into isostatic and residual components, and by considering various physical aspects like viscous dissipation during plate bending. Performing several systematic suites of experiments, we are then able to quantify the topographic impact of the buoyancy, rheology, and geometry of the subduction‐zone system to each and every topographic feature at a subduction zone and to provide corresponding scaling laws. We identify slab dip and, slightly less importantly, slab buoyancy as the major agents controlling surface topography at subduction zones on Earth. Only the island‐arc high and the back‐arc depression extent are mainly controlled by plate strength. Overall, his modeling study sets the basis to better constrain deep‐seated mantle structures and their physical properties via the observed surface topography on present‐day Earth and back through time.

Contribution of slab-derived fluid and sedimentary melt in the incipient arc magmas with development of the paleo-arc in the Oman Ophiolite

Major-element and trace-element geochemistry of fresh volcanic glass, and the whole-rock Hf and Nd isotopic compositions of volcanic rocks from the Oman Ophiolite were used to understand the extent to which slab-derived fluids and melts were involved in magma generation during incipient arc development. The spreading stage (V1) samples have trace-element characteristics similar to those of mid-ocean-ridge basalts, with Hf and Nd isotopic compositions of the Indian mantle domain. The incipient arc stage (V2) volcanic glasses have lower abundances of high field strength elements (HFSEs) and middle and heavy rare earth elements (MREEs and HREEs), and higher abundances of large ion lithophile elements (LILEs) than the V1 glasses. Progressive increases in B, Pb, and LILEs in the V2 stage glasses with age indicate an increasing contribution of slab-derived fluids from earlier arc tholeiite (LV2) to later boninite (UV2). The LV2 was generated by the contribution of amphibolite-derived fluid. The UV2 is subdivided into low-Si UV2 and high-Si UV2 magmas, with the former having lower SiO2, and LILE, and higher Na2O, HFSE and REE concentrations than the latter. The low-Si UV2 magma was generated with the involvement of high-temperature slab-derived fluid. In contrast, the high-Si UV2 shows a spoon-shaped trace-element pattern and Hf and Nd isotope chemistry, indicating the involvement of sediment melt in its magma genesis. Decreasing HFSEs and HREEs in the magmas with age indicate progressive depletion of the source mantle during the V2 arc magmatism, suggesting an absence of convection in the mantle wedge that resulted from subduction of a hot, buoyant slab beneath young, hot lithosphere.