Interrelation of the stagnant slab, Ontong Java Plateau, and intraplate volcanism as inferred from seismic tomography

We investigated the seismological structure beneath the equatorial Melanesian region, where is tectonically unique because an immense oceanic plateau, a volcanic chain and subduction zones meet. We conducted a multi-frequency P-wave tomography using data collected from an approximately 2-year-long seismic experiment around the Ontong Java Plateau (OJP). High-velocity anomalies were revealed beneath the center of the OJP at a depth of ~ 150 km, the middle-eastern edge of the OJP at depths of 200–300 km, and in the mantle transition zone beneath and around the OJP; low-velocity anomalies were observed along the Caroline volcanic island chain above 450 km depth. These anomalies are considered to be associated with the thick lithosphere of the OJP, remnant dipping Pacific slab, stagnant Pacific slab, and a sheet-like upwelling. The broad stagnant slab was formed due to rapid trench retreat from 48 to 25 Ma until when the OJP with thick lithosphere collided with a subduction boundary of the Pacific and Australian plates. This collision triggered slab breakoff beneath the arc where the dipping slab remained. The stagnant Pacific slab in the mantle transition zone restricted the plume upwelling from the lower mantle causing sheet-like deformed upwelling in the upper mantle.

Behaviors of Wet Plume Controlled by Olivine-Wadsleyite Phase Transition and Water Distribution

<p>Quaternary Intraplate volcanoes are sparsely distributed in Northeast Asia including Northeast China and Korean Peninsula and roles of the stagnant Pacific plate in the volcanoes have been studied. Recent geochemical studies suggest that the hydrated mantle in the mantle transition zone was incorporated in the wet plumes that were generated from the hydrated layer atop the stagnant slab, and the ascending wet plumes experienced partial melting in the shallow asthenosphere. To quantitatively evaluate the incorporation of the mantle in the transition zone into the wet plumes and their partial melting in the asthenosphere, we conducted a series of two-dimensional thermochemical numerical models by including the olivine-wadsleyite phase transition at the 410km discontinuity. The buoyancy is controlled by temperature, bound-water content and mineral phase. Viscosity reduction by the bound-water is added to the temperature-dependent viscosity. Particle tracers are used to track the incorporation of the mantle in the transition zone into the wet plumes. We vary the Clapeyron slope of the phase transition and water distributions in the mantle transition zone and hydrated layer of the stagnant slab to evaluate their effects on the behavior of the wet plumes. Results show that multiple wet plumes generated from atop the stagnant slab incorporate the hydrated mantle in the transition zone. Due to the endothermic phase transition at the 410 km discontinuity, the ascending wet plumes are retarded and laterally migrated beneath the 410 km discontinuity for several million years, and enter the overlying asthenosphere as merged large wet plumes. The ascending merged wet plumes laterally spread beneath the thermal lithosphere and experience partial melting, consistent with the interpretation based on the geochemical studies. The spacing of the merged wet plumes (~440 km) caused by the phase transition at the 410 km discontinuity is consistent with the sparse volcano distribution in Northeast China and Korean Peninsula.</p>

Stagnant slab front within the mantle transition zone controls the formation of Cenozoic intracontinental high-Mg andesites in northeast Asia

The geochemistry of Cenozoic intracontinental high-Mg andesites (HMAs) in northeast Asia, together with regional geophysical data, offers an opportunity to explore the genetic relationship between the formation of intracontinental HMAs and subduction of the Pacific plate. Compared with primary HMAs in arcs, Cenozoic intracontinental HMAs in northeast Asia have lower Mg# [100 × Mg/(Mg + Fe2+)] values (53–56) and CaO contents (5.8–6.6 wt%), higher alkali (Na2O + K2O) contents (5.15–6.45 wt%), and enriched Sr-Nd-Hf isotopic compositions (87Sr/86Sr = 0.7056–0.7059; εNd = −4.9 to −3.4; εHf = −4.7 to −2.6) as well as lower Pb isotope ratios (206Pb/204Pb = 16.76–19.19; 207Pb/204Pb = 15.42–15.45; 208Pb/204Pb = 36.71–37.11). These Cenozoic intracontinental HMAs are similar to Cenozoic potassic basalts in northeast China with respect to their Sr-Nd-Pb-Hf isotopic compositions but have higher SiO2 and Al2O3 contents and lower K2O, MgO, and light rare earth element contents. These features indicate that these Cenozoic intracontinental HMAs originated from the mantle, where recycled ancient sediments and water contributed to partial melting of peridotite. Combined with the presence of a large low-resistivity anomaly derived from the mantle transition zone (MTZ) near these intracontinental HMAs, and their occurrence above the stagnant slab front within the MTZ (at 600 km depth) in northeast Asia, we conclude that the stagnant slab front, with high contents of recycled ancient sediments and water, has controlled the formation of Cenozoic intracontinental HMAs in northeast Asia.

Supplemental Material: Stagnant slab front within the mantle transition zone controls the formation of Cenozoic intracontinental high-Mg andesites in northeast Asia

Analytical methods, and supplemental figures and tables.<br>

Supplemental Material: Stagnant slab front within the mantle transition zone controls the formation of Cenozoic intracontinental high-Mg andesites in northeast Asia

Analytical methods, and supplemental figures and tables.<br>

P-wave velocity structures of the upper mantle and mantle transition zone beneath the northern South China Sea based on triplication fitting

&lt;p&gt;The South China Sea (hereafter as SCS) located in the southeastern Asia has been affected by the subduction of the western Pacific, Indo-Australian and Eurasian plates (Sun et al., 2018). Broadband P waveforms from the China Digital Seismograph Network (Zheng et al., 2010) for three intermediate-depth earthquakes that occurred closely in Mindoro, Philippine are used to detect velocity structures of the lowermost upper mantle and mantle transition zone (MTZ) beneath the northern SCS. The study area is divided into five profiles distributed from southwest to northeast azimuthally to reduce the computational costs and concern possible lateral variations (Li et al., 2018), and the corresponding 1-D best-fit velocity models are obtained from the observed and synthetic triplicated waveform fitting based on the iterative grid-search procedure. The searching grid can be described as below, three parameters for the low-velocity layer (LVL) atop the 410 km discontinuity (hereafter as the 410), five parameters for the high-velocity anomaly (HVA) atop the 660 km discontinuity (hereafter as the 660) and one parameter for the velocity perturbation below the 660. After the sensitivity tests of the synthetic waveforms with different parameters, the grid steps of the depth and velocity perturbation are set as 5 km and 0.5%, respectively.&lt;/p&gt;&lt;p&gt;Relative to the reference model IASP91 (Kennett and Engdahl, 1991), our results reveal that there are ubiquitous HVAs in five profiles at the bottom of the MTZ with a velocity increment of 1.5~3.5% and a thickness of 209~219 km, which show no apparent progressive velocity increment or decrement along the southwest-northeast direction. We prefer that the weak and abnormal thick HVAs are induced by the proto-SCS north slab remnants. We also observe an uplift 410 and depressed 660 with the depth change of 5 km and 5~15 km, respectively, which further support the low-temperature anomaly related to the stagnant slab. In addition, our results show there is an LVL atop the MTZ with a velocity decrement of 2.0~2.5% and a thickness of 60~75 km, and can be interpreted by the partial melting induced by upwelling materials from the MTZ, which are hydrated by water released from the stagnant slab. We infer that the LVL with little lateral variations may result from the percolation of the partial melts atop the MTZ under vertical pressure.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Kennett B L N, Engdahl E R. 1991. Traveltimes for global earthquake location and phase identification. Geophys. J. Int. 105(2): 429-465, doi:10.1111/j.1365-246X.1991.tb06724.x.&lt;/p&gt;&lt;p&gt;Li W, Wei R, Cui Q, et al. 2018. Velocity structure around the 410 km discontinuity beneath the East China Sea area based on the waveform fitting method. Chinese J. Geophys. 61(1): 150-160, doi:10.6038/cjg2018L0370.&lt;/p&gt;&lt;p&gt;Sun W, Lin C, Zhang L, et al. 2018. The formation of the South China Sea resulted from the closure of the Neo-Tethys: A perspective from regional geology. Acta Petrol. Sin. 34(12): 3467-3478, doi:1000-0569/2018/034(12)-3467-78.&lt;/p&gt;&lt;p&gt;Zheng X, Jiao W, Zhang C, et al. 2010. Short-Period Rayleigh-Wave Group Velocity Tomography through Ambient Noise Cross-Correlation in Xinjiang, Northwest China. Bull. Seismol. Soc. Am. 100(3): 1350-1355, doi:10.1785/0120090225.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;