Arc-type and intraplate-type ridge basalts formed at the trench-trench-ridge tripIe junction: Implication for the extensive sub-ridge mantle heterogeneity

Abstract ' In situ basalts' represent the ridge magmatism at and close to the ancient trench-trench-ridge triple junction. Such basalts in the Amami, Mugi, and Setogawa accretionary complexes, Southwest Japan, were described and analysed. The geochemical data show that the ' in situ basalts' include all the types of basalts, ranging from alkali basalts and high-alumina basalts to tholeiites, and the compositions tend towards intermediate and silicic rocks. The data also reveal that the ridge basalts are indistinguishable both from the island arc and intraplate basalts, no affinities with mid-ocean-ridge basalts. The sub-ridge mantle adjacent to the triple junction had a component of sub-arc mantle, and this mantle heterogeneity can be generated by the formation of a slab window.     

Resistivity and induced polarization response of a thin sheet

The relevant potential theory is given for a current point source in the presence of a conductive slab embedded in a homogeneous host region of infinite extent. The thin sheet representation is obtained from the exact integral formulation by a simple mathematical limit process. The same result is also deduced directly at the outset by applying a thin sheet boundary condition. The apparent resistivity for a two electrode array is then computed for the case where the bore hole intersects the thin sheet at right angles. The corresponding results for the dilution factor, relevant to the induced polarization response, are also obtained. It is shown that the apparent resistivity and the dilution factor are constant when the potential and the current electrode straddle the sheet but there is a characteristic decrease as the electrodes move away from the sheet.     

Thermal consequences of the formation of a slab window beneath the Mid-Cretaceous southwest Japan arc: A 2-D numerical analysis

Abstract   The development of voluminous granitic magmatism and widespread high-grade metamorphism in Mid-Cretaceous southwest Japan have been explained by the subduction of a spreading ridge (Kula–Pacific or Farallon–Izanagi plate boundaries) beneath the Eurasian continent and the formation of a slab window. In the present study, the thermal consequences of the formation of a slab window beneath a continental margin are evaluated through a 2-D numerical simulation. The model results are evaluated by comparison with the Mid-Cretaceous geology of southwest Japan. Of particular interest are the absence of an amphibolite- to granulite-facies metamorphic belt near the Wadati–Benioff plane, and significant melting of the lower crustal-mafic rocks sufficient to form a large amount of granitic magma. Because none of the model results simultaneously satisfied these two geological interpretations, it is suggested that subduction of plate boundaries in Mid-Cretaceous southwest Japan was not associated with the opening of a slab window. According to previous studies, and the results of the present study, two different tectonic scenarios could reasonably explain the geological interpretations for Mid-Cretaceous southwest Japan: (i) The spreading ridge did not subduct beneath the Eurasian continent, but was located off the continental margin, implying the continuous subduction of very young oceanic lithosphere; (ii) ridge subduction beneath the continental margin occurred after active spreading had ceased. Consequently, in both tectonic scenarios, the subduction of plate boundaries at the Mid-Cretaceous southwest Japan was not associated with a slab window, but very young (hot) oceanic lithosphere.     

柴北缘早古生代岛弧火山岩中埃达克质英安岩的发现及其地质意义

在柴达木北缘吉绿素滩间山群火山岩中发现埃达克质 (adakitic)英安岩 ,其主要地球化学特征如下 :SiO26 3.7%～ 6 4.3%,Al2 O3 15 .0 7%～ 15 .2 7%,MgO 3.47%～ 3.72 %,K2 O/Na2 O 0 .14～ 0 .2 5 ,Y和Yb含量较低 ,Y为17.82× 10 -6～ 18.10× 10 -6,平均 17.96× 10 -6,Yb为 1.47× 10 -6～ 1.5 5× 10 -6,Sr含量较高 ,为 5 2 7× 10 -6～ 5 37× 10 -6,亏损HREE ,具弱负Eu和Sr异常。除MgO高于正常的埃达克岩外 ,与世界上典型埃达克岩极为相似。据其地球化学研究结果并结合该区埃达克质英安岩 (5 14.2± 8.5Ma)和榴辉岩 (494.6± 6 .5Ma)的年龄数据 ,推测柴北缘板片在晚寒武世开始俯冲 ,俯冲板片在 75～ 85km深处开始部分熔融形成埃达克质岩浆 ,而熔融残留部分继续俯冲 ,在早奥陶世形成含石榴石残留相的榴辉岩。柴北缘埃达克质英安岩的发现进一步证实了早古生代柴北缘发生过洋陆俯冲作用。     

Post-collisional transition from calc-alkaline to alkaline volcanism during the Neogene in Oranie (Algeria): magmatic expression of a slab breakoff

During the Neogene, a magmatic change from calc-alkaline to alkaline types occurred in all the regions surrounding the western Mediterranean. This change has been studied in Oranie (western Algeria). In this area, potassic to shoshonitic calc-alkaline andesites (with La/Nb ratios in the range 4–6) were mainly erupted between 12 and 9 Ma. They were followed (between 10 and 7 Ma) by basalts displaying geochemical features which are transitional between calc-alkaline and alkaline lavas (La/Nb=1–1.7). After a ca. 3-Ma quiescence period, volcanic activity resumed, with the eruption of OIB-type alkaline basalts (La/Nb=0.5–0.6), from 4 to 0.8 Ma. A combined geochemical approach, using incompatible elements and Sr, Nd and O isotopes, allows us to conclude that the transitional basalts derived from the melting of a heterogeneous mantle source, at the boundary between lithosphere and asthenosphere. We propose that melting of a previously subduction-modified lithospheric mantle occurred between 12 and 10 Ma, in response to the upwelling of hot asthenosphere flowing up into an opening gap above a detached sinking slab. As a result, calc-alkaline magmas were formed. From 10 to 7 Ma, the transitional basalts were generated through melting of the boundary mantle zone between the lithosphere and the upwelling asthenosphere. During that stage, the contribution of the lithospheric source was still predominant. Then, as sinking of the oceanic slab progressed, the increasing uprise of the asthenosphere led to the formation and emplacement (from 4 to 0.8 Ma) of typical within-plate alkaline basalts derived from a plume-modified asthenospheric mantle.     

伊豆-小笠原下方660km间断面的区域性差异

利用南、北加州地震台网、华盛顿州西北太平洋地震台网单分量短周期地震仪和德国、瑞士地震台网／台阵的三分量宽频带地震仪记录的1981～2000年伊豆——小笠原地区下方地震的波形资料，使用N次根倾斜叠加方法提取近源一侧来自间断面SdP转换震相，以研究660km间断面区域性差异．研究发现，自35Ｎ 到26Ｎ 的各剖面依次体现了和达 本尼奥夫带（Wadati-Benioff zone）倾角逐渐加大，且地震分布的最大深度也逐步增大，俯冲板块对660km间断面存在的影响也因此产生了差异性:没有俯冲板块影响的情况下，该间断面出现在CM）660km深度处，而受到俯冲板块明显作用的地区，则该间断面普遍出现下陷．转换点分布的一定程度分散性可能是间断面本身复杂结构、震相误判或一维球对称地球模型假定等因素造成的．这一转换点分布的分散性是一个亟需解决的问题．      

日本列岛下太平洋俯冲板块的精细结构

尽管许多学者对日本列岛下的太平洋俯冲板块做了大量的研究,但板块内部的结构(比如板块厚度,板块内地震波速度随深度的变化以及洋壳的俯冲情况等)仍然不太清楚。利用日本地区密集台网收集到的中深和深发地震到时数据来探讨上述问题。采用三维射线追踪正演模拟法,首先利用333个远震计算得到了日本地区太平洋板块的厚度为85km;然后利用3283个地震(震源深度大于40km)的130227条P波到时进一步研究板块内部的精细结构。结果显示,沿深度方向6个地层段(间隔100km)内的速度扰动值分别为5.5%,4.0%,3.5%,2.5%,2.0%和6.0%,在40～500km范围内速度扰动随深度的增加而减小,这与温度随深度的变化情况相一致。当深度大于500km时,速度扰动突然增大到6.0%,分析认为该异常可能由发生在东亚大陆边缘下方的深发地震无法精确定位导致的。最后利用40～500km深度范围内的近震测试得到日本东北和北海道地区下方洋壳俯冲的深度均为110km,平均厚度分别为7.5km和5km,相对于一维模型的速度扰动分别为1%和-3%。这说明洋壳在俯冲到110km以深时,由于受温度和压力的影响,逐渐脱水、变质,直至与板块融合。通过分析震源与洋壳的位置关系,本研究认为北海道地区比东北地区下方的俯冲洋壳可能含有更多的流体(比如水),导致两地区洋壳内的速度相差如此之大。此外,因为日本南部与洋壳对应的区域多为海洋,观测台站较少,所以本研究无法测试得到该区域内的洋壳俯冲情况。     

Physical conditions producing slab stagnation: Constraints of the Clapeyron slope, mantle viscosity, trench retreat, and dip angles

Recent seismic tomography has revealed various morphologies in the subducted lithosphere. In particular, significant flattening and stagnation of slabs around the 660-km boundary are seen in some areas beneath the northwestern Pacific subduction zones. We examined the cause of slab stagnation in terms of the Clapeyron slope of the phase transformation from ringwoodite to perovskite + magnesiowüstite, trench retreat velocity, dip angles, and high viscosity of the lower mantle based on two-dimensional (2-D) numerical simulations of thermal convection. In particular, we examined the conditions necessary for slab stagnation assuming a very small absolute value of the Clapeyron slope, which were proposed based on recent high-pressure, high-temperature (high P–T) experiments. Our calculations show that slabs tend to stagnate above the 660-km boundary with an increasing absolute value of the Clapeyron slope, viscosity jump at the boundary, and trench retreat velocity and a decreasing initial dip angle. Stagnant slabs could be obtained numerically for a realistic range of parameters obtained from high P–T experiments and other geophysical observations combining buoyancy, high lower-mantle viscosity, and trench retreat. We found that a low dip angle of a descending slab at the bottom of the upper mantle plays an important role in slab stagnation. Two main regimes underlie slab stagnation: buoyancy-dominated and viscosity-dominated regimes. In the viscosity-dominated regime, it is possible for slabs to stagnate above the 660-km boundary, even when the value of the Clapeyron slope is 0 MPa/K.     

Niobium-tantalum Fractionation During Slab Subduction:Implications for the Formation of Continental Crust

Compared with the oceanic crust, knowledge about the formation of the continental crust (CC) is relatively poor. Although melting of subducted slabs in the early history of the Earth has been considered as the major way that shaped the chemical characteristics of the CC by most geologists, as the CC shares many characteristics with modern adakites, some geologists argued that Archean TTG was formed in the same way as modern arcs rather than slab melting, whereas others proposed that melting at the bottom of the thickened oceanic crust was more important. Recently,the debate is mainly focused on the unique subchondritic Nb/Ta value of the CC, and particularly, how Nb and Ta fractionated from each other and consequently how, in detail, the CC was built.