Oxygen Isotope Evidence for Chemical Interaction of Ki lauea Historical Magmas with Basement Rocks

Klauea historical summit lavas have a wide range in matrix ¹⁸O_VSMOWvalues (4·9–5·6) with lower values in rockserupted following a major summit collapse or eruptive hiatus.In contrast, ¹⁸O values for olivines in most of these lavasare nearly constant (5·1 ± 0·1). The disequilibriumbetween matrix and olivine ¹⁸O values in many samples indicatesthat the lower matrix values were acquired by the magma afterolivine growth, probably just before or during eruption. BothMauna Loa and Klauea basement rocks are the likely sources ofthe contamination, based on O, Pb and Sr isotope data. However,the extent of crustal contamination of Klauea historical magmasis probably minor (< 12%, depending on the assumed contaminant)and it is superimposed on a longer-term, cyclic geochemicalvariation that reflects source heterogeneity. Klauea's heterogeneoussource, which is well represented by the historical summit lavas,probably has magma ¹⁸O values within the normal mid-ocean ridgebasalt mantle range (5·4–5·8) based on thenew olivine ¹⁸O values. KEY WORDS: Hawaii; Klauea; basalt; oxygen isotopes; crustal contamination     

东天山黄山-镜儿泉过铝花岗岩矿物学、地球化学及年代学研究

对东天山黄山-镜儿泉一带黄山南、镜儿泉、图拉尔根沟三个过铝花岗岩作了岩相学、矿物学、地球化学、sr-Nd同位素和锆石U-Pb年代学研究.锆石U-Pb LA-ICP-MS原位定年测得黄山南岩体结晶年龄为259.9±1.4Ma(MSWD=0.86),图拉尔根沟岩体结晶年龄为275.4±8.3Ma(MSWD=29),均侵位于二叠纪碰撞后伸展环境.三个过铝花岗岩均具有低锶同位素初始比值(Isr=0.6969～0.70396)、高εNd(t)值( 5.5～ 7.2)以及年轻的亏损地幔单阶段模式年龄(tDM=0.48～0.56Ga),表明其岩浆源区均为来源于亏损地幔的新生地壳岩石.这种新生地壳岩石可能为偏酸性的火山岩.三个岩体的矿物学和地球化学可分为两类:一类以黄山南白云母花岗岩为代表,为强过铝花岗岩(A/CNK>1.1),强烈亏损Ba、sr和Ti而富集Cs、Rb和K,具有高的Rb/sr(2.03～14.5)和Al2O3/TiO2(110～1592),低的Nb/Ta(3.24～6.76)比值,其稀土元素配分曲线呈"V"字形,显示强烈铕亏损(Eu/Eu*=0.04～0.55),表明这类岩体的直接源岩以泥质岩为主.另一类以图拉尔根沟二云母花岗岩为代表,为弱过铝(1   

台湾及其邻海的重力特征与构造、地震的关系

分析认为，在台湾及其邻海的重力场中，具有短波长特征的空间重力异常受地形与海深所制约．玉山的200×10     

Spatial and temporal variability of crustal magnetization of a slowly spreading ridge: Mid-Atlantic Ridge (20°–24° N)

The results of the two- and three-dimensional magnetic inversions performed on data located between 20°–24° N on the Mid-Atlantic Ridge indicate the crustal magnetization has decayed exponentially for the last 10 Ma, and that this decay has been fairly symmetric about the ridge axis. After removal of the mean temporal decay, the residual field is characterized by more positive magnetizations at the second-order discontinuities, regardless of initial magnetization direction. A model that involves the preferential emplacement of serpentinized lithologies near the discontinuities is proposed to explain this correlation. The temporal detrending method also indicates that several ridge-parallel depressions located on the flanks of the ridge axis are regions of more positive magnetizations. These bathymetric depressions may mark the locations of detachment faulting that occurred during amagmatic periods of extension. The general symmetry of the crustal magnetization about the ridge axis does not support the occurrence of continuous detachment faulting proposed to correspond to the inner and outer corners of ridge axis discontinuities.     

南海輓近海平面变化与构造升降初步研究

揭示南海輓近海平面变动和古岸线诸多遗迹,探讨晚更新世以来海平面变化一般规律,初步分析南海周缘中全新世和现代地壳构造升降速率,得出本区海平面上升率适中参考值为0.10—2.0mm/a,初步估算未来50年内海平面变化趋势在1.26—4.0mm/a之间;而构造升降受块断差异运动控制,一般规律是南海北部沿岸自东向西递减,由福建南部1.15mm/a至海南岛南部0.54mm/a。近代地壳形变资料亦证实这种变化规律。     

透空式水平板波浪上托力分布

通过系列模型试验，对波浪上托力沿板宽分布进行了详细分析，提出对于透空式平板结构，发生较大冲击压强或上托力的分布主要分为均布型和局部冲击型两类，由此得到压强分布的计算公式。大量试验资料表明，该公式与实验值有着较好的一致性。     

Estimation of Uplift Capacity of Rapidly Loaded Plate Anchors in Soft Clay

Plate anchors are extensively used in civil engineering constructions as they provide an economical alternative to gravity and other embedded anchors. The rate of loading is one of the important factors that affects the magnitude of soil resistance as well as soil suction force. This article outlines the effect of pullout rate on uplift behavior of plate anchors (70 mm diameter) buried in soft saturated clay by varying the pullout rate from 1.4 mm/min to 21.0 mm/min. The variation of breakout force and suction force with embedment depth and rate of pull are presented. A correlation between the rate of increase of undrained strength of clay and anchor capacity with rate of strain has been established. Finally an empirical equation has been proposed that includes the rate of pull in the estimation of breakout capacity of anchors.     

Crustal Thinning of the Northern Continental Margin of the South China Sea

Magnetic data suggest that the distribution of the oceanic crust in the northern South China Sea (SCS) may extend to about 21 °N and 118.5 °E. To examine the crustal features of the corresponding continent–ocean transition zone, we have studied the crustal structures of the northern continental margin of the SCS. We have also performed gravity modeling by using a simple four-layer crustal model to understand the geometry of the Moho surface and the crustal thicknesses beneath this transition zone. In general, we can distinguish the crustal structures of the study area into the continental crust, the thinned continental crust, and the oceanic crust. However, some volcanic intrusions or extrusions exist. Our results indicate the existence of oceanic crust in the northernmost SCS as observed by magnetic data. Accordingly, we have moved the continent–ocean boundary (COB) in the northeastern SCS from about 19 °N and 119.5 °E to 21 °N and 118.5 °E. Morphologically, the new COB is located along the base of the continental slope. The southeastward thinning of the continental crust in the study area is prominent. The average value of crustal thinning factor of the thinned continental crust zone is about 1.3–1.5. In the study region, the Moho depths generally vary from ca. 28 km to ca. 12 km and the crustal thicknesses vary from ca. 24 km to ca. 6 km; a regional maximum exists around the Dongsha Island. Our gravity modeling has shown that the oceanic crust in the northern SCS is slightly thicker than normal oceanic crust. This situation could be ascribed to the post-spreading volcanism or underplating in this region.     

Discussion: A review on the behaviour of helical piles as a potential offshore foundation system

Abstract

Helical piles have emerged as an attractive foundation system for offshore applications with renewed interest from the offshore community. Significant research gap currently exists in transferring this technology offshore and this paper discusses how existing and emerging knowledge can be successfully used to bridge some of the gaps. We focus on the Coupled Eulerian Lagrangian (CEL) large deformation finite element (LDFE) modelling technique that is commercially available and can be used to model the three-dimensional installation process with consideration of strain rate and softening effects in soft offshore clays. A helical pile of L?=?7.5?m long is modelled with one or two large-diameter helices (D?=?2?m) attached to a central shaft of d?=?0.5?m in diameter.The net effect of strain rate and softening is to increase the installation torque. The measured torque is within the range of 200–400?kN.m for the offshore clay and the pile geometry studied. Additional helices increase the uplift force but to a lesser degree than that of the measured torque. Remoulding induced strength reduction is found to be within the range of 25–33% of the intact clay strength. Issues of extracting and reusing offshore helical pile foundations are discussed.     

Three-dimensional inversion of marine magnetic anomalies: Implications for crustal accretion along the Mid-Atlantic Ridge (28°–31°30′ N)

We present magnetic field data collected over the Mid-Atlantic Ridge in the vicinity of the Atlantis Fracture Zone and extending out to 10 Ma-old lithosphere. We calculated a magnetization distribution which accounts for the observed magnetic field by performing a three-dimensional inversion in the presence of bathymetry. Our results show the well-developed pattern of magnetic reversals over our study area. We observe a sharp decay in magnetization from the axis out to older lithosphere and we attribute this decay to progressive low temperature oxidation of basalt. In crust which is 10 Ma, we observe an abrupt increase in magnetic field intensity which could be due to an increase in the intensity of magnetization or thickness of the magnetic source layer. We demonstrate that because the reversal epoch was of unusually long duration, a two-layer model comprised of a shallow extrusive layer and a deeper intrusive layer with sloping polarity boundaries can account for the increase in the amplitude of anomaly 5. South of the Atlantis Fracture Zone, high magnetization is correlated with bathymethic troughts at segment end points and lower magnetization is associated with bathymetric highs at segment midpoints. This pattern can be explained by a relative thinning of the magnetic source layer toward the midpoint of the segment. Thickening of the source layer at segment endpoints due to alteration of lower oceanic crust could also cause this pattern. Because we do not observe this pattern north of the fracture zone, we suggest it is a result of the nature of crustal formation process where mantle upwelling is focused. South of the fracture zone, reversals along discontinuity traces only continue to crust 2 Ma old. In crust >2 Ma, we observe bands of high, positive magnetization along discontinuity traces. We suggest that within the discontinuity traces, a high, induced component of magnetization is produced by serpentinized lower crust/upper mantle and this masks the contribution of basalts to the magnetic anomaly signal.