慢速、超慢速扩张洋中脊三维地震结构研究进展与展望

慢速与超慢速扩张洋脊是研究岩浆活动、构造运动、热液循环等相互作用的最佳场所;其复杂的三维空间的地震结构是构建构造动力学机制的基础.文章首先结合国际深海研究发展趋势,回顾了慢速扩张洋脊的三维地震结构研究进展,发现慢速扩张洋中脊与快速洋脊相似,也存在岩浆房或熔融体;然后,重点结合我国2010年1-3月首次在西南印度洋洋中脊开展的三维地震探测实验,提出了今后超慢速洋中脊的重要研究方向;此次地震数据初步处理结果表明,数据质量良好,为下一步三维层析成像研究打下坚实基础;相信此次研究将在超慢速扩张洋脊的形成演化机制上取得突破性进展,提升我国在国际大洋中脊研究中的地位.     

超慢速扩张洋脊:海洋地学研究新领域

超慢速扩张洋脊是近年发现的全球大洋中脊系统中一个新的类型,以低扩张速率、微弱的火山作用和缺乏转换断层为特征。阐述了超慢速扩张洋脊的特征及地理分布,简要介绍了美国、德国、丹麦、俄罗斯等国家对超慢速扩张洋脊的研究与探索以及国内研究现状。     

蛇纹岩化作用对超慢速洋中脊拆离断层发育的影响

在超慢速扩张的洋中脊区域,广泛发育的拆离断层将幔源橄榄岩大规模拆离到海底表面,上升出露的橄榄岩发生蛇纹岩化作用,极大地降低了岩石强度,促进了拆离断层的持续滑动,形成独特的光滑地形。本文利用快速拉格朗日连续介质分析法模拟研究蛇纹岩化作用对超慢速洋中脊拆离断层发育的影响。通过对比有蛇纹岩化作用和无蛇纹岩化作用下拆离断层发育的生命周期和力学差异,发现在蛇纹岩化作用下,岩石圈生成断层所需要应力降低了40%,导致断层下盘极易产生与原断层倾向相反的新断层;蛇纹岩化导致断层滑动摩擦力减小,有利于断层的持续滑动。理解蛇纹岩化作用和断层发育的关系对认识超慢速洋脊段末端的扩张机制具有重要意义。     

铀系不平衡与洋中脊下的岩浆形成及其迁移过程

综述了20多年来洋中脊新生玄武岩中铀系不平衡研究成果,分析了铀系不平衡的形成机制和主要影响因素,并探讨了铀系不平衡研究中的一些问题。结果表明:(1)玄武质岩浆中230Th/238U不平衡可能产生于石榴子石稳定的深部源区,也可能来自230Th的"内部增长"(in-growth),过剩230Th更可能形成于含铝单斜辉石;(2)226Ra/230Th不平衡可能来自岩浆的形成过程,也可能来自岩浆迁移过程;(3)铀系不平衡与岩浆滞留时间和上升速率紧密相关,226Ra的大量过剩显示,这个不平衡事件形成于8ka以内,如果不考虑玄武质岩浆迁移过程的事件,则意味着岩浆迁移时间不超过8ka。提出当前亟待解决的问题:①准确模拟地幔部分熔融条件并测定铀系子体在熔体/地幔残留相间的分配系数;②提出更合理的结晶分异模型;③探索流体对226Ra/230Th不平衡的影响。     

西南印度洋中脊超慢速扩张的构造和岩浆活动特征

西南印度洋中脊是当前研究超慢速扩张中脊岩石圈增生模式和热液硫化物矿产资源的热点区域.本研究从西南印度洋中脊的扩张速率变化与全球地质历史事件的关系、岩浆活动强度及其不同尺度上的运移规律、热点-中脊交互作用的地质现象和地球物理特征等方面,系统总结了有关西南印度洋中脊的最新认识.针对以上内容,提出了进一步工作建议,如调查航次的区域与方法、岩浆-构造活动与岩石圈状态的相互关系以及热点-中脊交互作用的动力学机制.     

北冰洋Gakkel洋中脊的地幔熔融控制因素及非岩浆地壳增生

Spreading rate is a primary factor of mantle melting and tectonic behavior of the global mid-ocean ridges. The spreading rate of the Gakkel ridge decreases gradually from west to east. However, the Gakkel ridge can be divided into four thick-and-thin zones with varying crustal thicknesses along ridge axis. This phenomenon indicates that mantle melting of the Gakkel ridge is not a simple function of spreading rate. Mantle temperature, water content,mantle composition, and other factors are important in crustal accretion processes. Based on gravity-derived crustal thickness and wet melting model, we estimate that the mantle potential temperatures of the four zones are1 270, 1 220, 1 280, and 1 280°C(assuming that mantle water content equals to global average value), with corresponding mantle water contents of 210, 0, 340, and 280 mg/kg(assuming that mantle potential temperature is 1 260°C), respectivly. The western thinned crust zone is best modeled with low mantle temperature, whereas the other zones are mainly controlled by the enhanced conduction caused by the slower spreading rate. Along the Gakkel ridge, the crustal thickness is consistent with rock samples types. Predominated serpentinized peridotite and basalt are found in the area with crustal thickness 1.5 km and 2.5 km, respectively. The rock samples are including from basalt to peridotite in the area with crustal thickness between 1.5 and 2.5 km. Based on this consistency, the traditional magmatic accretion zone accounted for only 44% and amagmatic accretion accounted for 29% of the Gakkel ridge. The amagmatic accretion is a significant characteristic of the ultra-slow spreading ridge.     

北大西洋Mohns洋中脊扩张地壳构造特征的研究

北大西洋Mohns洋中脊是很重要的扩张带。中国第五次北极科学考察的重点之一就是获取Mohns洋中脊的磁力数据。通过获得的磁力和以往的重力与地震资料,研究洋中脊的扩张、地壳构造特征及厚度。这些研究有助于更好地理解地球上重要的地质过程、海底扩张和壳幔相互作用。     

慢速扩张洋中脊热液成矿的典型实例——青藏高原北部德尔尼铜矿地质对比研究

青海德尔尼铜矿是发育在青藏高原北部的块状硫化物矿床,其赋矿围岩为阿尼玛卿蛇绿岩套中的超基性岩,代表了古特提斯洋的残骸。通过对德尔尼铜矿的详细地质解剖,认为其保留了大量海底热液喷流的地质记录,包括:矿体上部普遍发育的薄层喷流岩;多孔状硫化物中保留下来的胶状结构、草莓结构和角砾结构;矿石的主要矿物组成;与黄铁矿碎屑同期形成的方解石、长英质胶结物;与大西洋Rainbow、TAG热液硫化物矿床相似的矿体分带性。根据洋脊玄武岩中TiO2全岩含量估算德尔尼蛇绿岩所代表的这一段古特提斯洋洋中脊为慢速-超慢速扩张洋中脊,半扩张速率为1.1~2.5cm/a。类比现今洋中脊热液硫化物成矿过程,认为德尔尼铜矿经历了海底喷流、冷却保存和俯冲侵位等3个阶段,其中海底喷流阶段可能与洋底核杂岩具有成因联系,俯冲侵位过程中的矿体和超基性岩、玄武岩受逆冲断层控制。与世界上其他陆地上保存的类似矿床相比,德尔尼铜矿时代年轻(石炭纪),矿床结构和构造及其围岩蚀变特征保存完整,是慢速-超慢速洋脊超基性围岩热液成矿作用的典型实例,可以称之为德尔尼型成矿作用,研究其成矿过程,对研究和理解现今洋中脊超基性岩系统热液成矿作用(尤其是深部成矿作用)具有重要的意义。     

西南印度洋中脊岩石地球化学特征及其岩浆作用研究

作为超慢速扩张脊的代表,西南印度洋中脊(SWIR)因其独一无二的地形地貌特征、洋壳结构、洋壳增生机制、岩浆和热液活动以及深部动力学过程,近30年来成为国内外研究的热点区域。基于近年来对SWIR玄武岩、辉长岩及橄榄岩的岩石学和地球化学研究成果总结,重点探讨了沿SWIR轴向（大尺度）以及单个洋脊分段（小尺度）的岩石地球化学变化特征及其影响因素,阐述了SWIR的岩浆供应及洋壳增生模式。其中,在9°～16°E斜向扩张脊,以构造作用为主的洋脊扩张模式导致了更宽的洋壳增生带和显著的地球化学异常;而在50°～51°E脊段,发育了强烈的火山活动,其成因机制包括克洛泽热点与洋中脊相互作用、微热点、古老熔融事件的残留地幔再熔融等几种观点。此外,西南印度洋中脊龙旂热液区（～49.7°E）的最新研究表明,其热液循环路径与拆离断层的发育密不可分,热液流体循环最深可达莫霍面以下6 km。因此,在今后的一段时间,应进一步加强SWIR不同空间尺度地幔源区性质、洋中脊构造与岩浆作用过程、热点-洋中脊相互作用和岩浆-热液活动与成矿等主要科学问题的研究。     

洋中脊分段性及其拓展和叠接机制

各种扩张速率下的洋中脊被转换断层和非转换断层分成许多段(长10到100km不等),而且这种岩浆活动和构造的分段特性在大西洋中脊表现特别明显。洋脊分段特征可以划分为4级。其中,转换断层是1级间断,其错断洋脊距离大于30km,其长度可达1000km左右,存在寿命可达10Ma。相邻转换断层之间的距离间隔也与扩张速率有关：扩张速率最慢处的距离间隔小于200km,中速一快速扩张脊为600～1000km,扩张速率大于140mm／a的脊段上未发现转换断层,总体上转换断层间距随扩张速率而增加。叠接拓展中心、斜向剪切带、火山间隔和横向断错等分别为2～4级间断,出现在两条转换断层之间,使洋中脊错断距离逐渐减少。2～4级区段的洋中脊长度也越来越小,存在的寿命也越来越短,直至4级区段的洋中脊长度一般小于10km,存在寿命为10^2～10^4a。这种分段性无论在超快速、快速、中速、慢速扩张脊,还是超慢速扩张脊都存在,其分段机制都与洋中脊拓展、叠接、跃迁(或废弃)、死亡过程密切相关,而拓展、叠接过程又受多种动力要素控制。正是洋中脊分段的动力机制控制了中央裂谷的存在与否。     

Accretion,structure and hydrology of intermediate spreading-rate oceanic crust from drillhole experiments and seafloor observations

Downhole measurements recorded in the context of the Ocean Drilling Program in Hole 504B, the deepest hole drilled yet into the oceanic crust, are analyzed in terms of accretion processes of the upper oceanic crust at intermediate spreading-rate. The upper part of the crust is found to support the non steady-state models of crustal accretion developed from seafloor observations (Kappel and Ryan, 1986; Gente, 1987). The continuous and vertical nature of borehole measurements provides stratigraphic and structural data that cannot be obtained solely from seafloor studies and, in turn, these models define a framework to analyze the structural, hydrological, and mineralogical observations made in the hole over the past decade.Due to the observed zonation with depth of alteration processes, and its relation to lava morphologies, the 650-m-thick effusive section penetrated in Hole 504B is postulated to be emplaced as the result of two main volcanic sequences. Massive lava flows are interpreted as corresponding to the onset of these sequences emplaced on the floor of the axial graben. The underlying lava made of structures with large porosity values and numerous cm-scale fractures is thus necessarily accreted at the end of the previous volcanic episode. On top of such high heterogeneous and porous intervals, the thick lava flows constitute crustal permeability barriers, thereby constraining the circulation of hydrothermal fluids.Accreted in the near vicinity of the magma chamber, the lower section is that exposed to the most intense hydrothermal circulation (such as black smokers activity). Once capped by a massive flow at the onset of the second volcanic phase, the lower interval is hydrologically separated from ocean-waters. A reducing environment develops then below it resulting, for example, in the precipitation of sulfides. Today, whereas the interval corresponding to the first volcanic episode is sealed by alteration minerals, the second-one is still open to fluid circulation in its upper section. Thus, upper part of the volcanic edifice is potentially never exposed to fluids reaching deep into the crust, while the lower one is near the ridge axis.Considering that most of the extrusives are emplaced within a narrow volcanic zone, the first unit extruded for a given vertical cross-section is necessarily emplaced at the ridge-axis. In Hole 504B, the 250-m-thickTransition Zone from dikes to extrusives is interpreted as the relict massive unit flooding the axial graben at the onset of the first volcanic sequence, and later ruptured by numerous dikes. Further from the axis, the same massive unit constitutes a potential permeability cap for vertical crustal sections accreted earlier. Also, the upper 50 meters of the basement might be considered as the far-end expression of massive outpours extruded near the ridge-axis.     

Analysis of simrad EM12 multibeam bathymetry and acoustic backscatter data for seafloor mapping,exemplified at the Mid-Atlantic Ridge at 45° N

The EM12 multibeam echosounder can record acoustic backscatter information as well as high resolution bathymetry. The dataset presented, from the axis of the Mid-Atlantic Ridge at 45° N, was the first EM12 survey of a mid-ocean ridge. This paper presents methods for utilising the backscatter information. Data processing enables the production of a mosaic of acoustic backscatter, and visualisation techniques are investigated to provide initial qualitative views of the combined backscatter and bathymetry datasets. The co-registration of the backscatter and bathymetry data enables quantitative analysis of their relationships. Various sites of different geological type have been selected and their angular acoustic backscattering relationships estimated, including the effect on backscatter of incidence angle, its regional variability with bottom type and the influence of bottom slope. Incidence angles and bottom type are shown to affect backscatter to a similar degree, while slopes appear to contribute little. The geometry of hull-mounted systems, such as the EM12, is significantly different from that of conventional sidescan sonars, such as GLORIA, and the backscatter images from the two types differ in various respects. Because of the wide variations in incidence angle that are common with hull-mounted systems, and the importance of incidence angle in determining backscatter strength, it is vital to consider the effect of incidence angle during interpretation.     

Numerical simulation of the thermal maturation, oil generation and migration in the Songliao Basin, Northeastern China

The discovery of the giant Daqing oil field in the Songliao Basin led to the realisation of the significant petroleum potential of non-marine basins. In order to reconstruct the basin evolution and oil formation, an integrated organic geochemical-basin modelling study along a regional transect across the Songliao Basin was conducted. It provided a regional heat flow evolution model, and revealed post-orogenic or late syn-orogenic maturation in the Central Depression and pre-orogenic maturation in the Southeast Uplift Zone. Kinetic parameters of petroleum generation for the lacustrine source formations are the basis for the simulation of oil generation and migration in the Songliao Basin. Using the principle activation energy peaking at 54 kcal/mol and a pre-exponential factor of about 4.2·10²⁷ Ma⁻¹, the simulation obtained a relatively good match with the measured transformation ratios. The Qingshankou Formation in the West and East Central Depressions constituted the major source in the basin. Major oil generation, migration and accumulation occurred during the Early Tertiary. In the West Central Depression, the generated oils migrated upwards into the Yaojia Formation followed by the updip migration into the Daqing Anticline and towards the local structural high along the West Slope. In contrast, the oil migration in the East Central Depression was dominated by the downward movement from the lower member of the Qingshankou Formation followed by the updip migration towards the Caoyang Anticline. The simulated oil accumulations are in good agreement with discovered oil fields, implying a potential application of the model for prediction and evaluation of new exploration targets in the basin.