岩浆与构造作用对洋壳厚度的影响——以西北印度洋为例

洋中脊及邻区洋盆的洋壳厚度能很好地反映区域岩浆补给特征,对于研究洋中脊内部及周缘岩浆活动和构造演化过程具有很好的指示意义.西北印度洋中脊作为典型的慢速扩张洋中脊,其扩张过程与周缘构造活动具有很强的时空关系.本文利用剩余地幔布格重力异常反演了西北印度洋洋壳厚度,由此分析区域内洋壳厚度分布和岩浆补给特征.研究发现,西北印度洋洋壳平均厚度为7.8 km,受区域构造背景影响厚度变化较大.根据洋壳厚度的统计学分布特征,将区域内洋壳分为三种类型:薄洋壳(小于4.5 km)、正常洋壳(4.5~6.5 km)和厚洋壳(大于6.5 km),根据西北印度洋中脊周缘(~40 Ma内)洋壳厚度变化特征可将洋中脊划分为5段,发现洋中脊洋壳厚度受区域构造活动和地幔温度所控制,其中薄洋壳主要受转换断层影响造成区域洋壳厚度减薄,而厚洋壳主要受地幔温度和地幔柱作用影响,并在S4洋中脊段显示出较强的热点与洋中脊相互作用,同时微陆块的裂解和漂移也可能是导致洋壳厚度差异的原因之一.     

西南印度洋中脊49.5°E离轴地壳结构

超慢速扩张西南印度洋中脊岩浆的集中供给在空间维度上表现为岩浆扩张段（NVR）与相邻的非转换断层不连续带（NTD）地壳结构的差异,而在时间维度上表现为离轴与沿轴地壳结构的差异.为了进一步揭示岩浆集中供给的时空分布特征,本文选取西南印度洋中脊热液区2010年海底地震仪深部探测中平行于洋中脊距轴部偏北约10 km的离轴测线d0d10,使用射线追踪正演和反演的方法,得到了NVR和NTD北侧离轴区域的地壳及上地幔P波速度结构,并与轴部速度结构进行了对比分析.研究结果表明：（1）NTD北侧离轴区域的地壳厚度约5.2 km,其厚度明显大于轴部NTD下方地壳厚度（~3.2 km）,由此推测洋脊轴部NTD区域形成的地壳在不断减薄;（2）NVR北侧离轴区域的地壳厚度约7.0 km,其厚度亦大于轴部NVR地壳厚度（~5.8 km）,表明在洋中脊演化过程中洋脊轴区域的岩浆供给在不断减少,其活动性在不断减弱.     

南海扩张期岩浆流体对洋壳厚度的影响：数值模拟

扩张期洋中脊热液循环系统的热排出与岩浆系统的热注入共同控制着洋壳厚度的生成,而岩浆流体是热液循环系统的流体成分之一,往往与下渗海水混合参与各圈层能量和物质传递,但其能量传递对洋壳厚度的影响机制目前还不清楚.利用有限元的数值模拟手段,对扩张期热液循环系统中岩浆流体与洋壳厚度的关系进行研究.结果表明：(1)相较于只有海水参与的对流循环,含有岩浆流体的热液循环造成的洋壳厚度的减薄量更大、热液喷口温度更高.(2)岩浆流体对洋壳厚度的二次减薄作用随其含量的增大而减弱,热液喷口温度随其含量的增大而升高.(3)南海岩浆水、地幔水含量和洋壳厚度的分布具有非均质性,东部次海盆的地幔水、岩浆水含量高于西南次海盆,前者的平均洋壳厚度也大于后者,并且在海盆残余扩张脊附近存在异常薄洋壳.结合模型结果分析认为,残余扩张脊附近的薄洋壳可能受到扩张期热液循环或后期岩浆流体的影响,而东部、西南次海盆的洋壳厚度差异可能是由于前者的岩浆流体含量高于后者而造成的.     

超慢速洋中脊岩浆作用的数值模拟

为了验证西南印度洋中脊50°E区域的残留熔融体与8~11 Ma前的岩浆供给活动的相关性,用有限元方法对洋壳模型进行热力学数值计算,以期解答超慢速洋中脊热液活动是由于古岩浆房长期持续供热,还是依赖周边热点提供持续的岩浆和热融熔问题.实验模拟了水平层状洋壳模型和地震试验得到的实际洋壳模型两种情况,对水平层状洋壳模型研究了上地幔有、无持续岩浆供给两种情况,对实际洋壳模型研究了一次岩浆供给的情况.结论如下:如果洋壳层底部没有持续热供应,岩浆房持续时间约为数千年或数万年;西南印度洋中脊中东段隆起区的热液活动和岩浆房最多持续存在0.8 Ma,现今热液活动的热源并不是8~11 Ma前的岩浆供给提供的.     

洋中脊扩张速率对洋壳速度结构的约束

洋中脊速度结构是揭示大洋岩石圈演化过程的重要约束.为探讨不同扩张速率下洋中脊的洋壳速度结构特征,挑选了全球152处快速(全扩张速率 90mm·a-1)、慢速(全扩张速率20～50mm·a-1)和超慢速(全扩张速率20mm·a-1)扩张洋中脊和非洋中脊的洋壳1-D地震波速度结构剖面,通过筛选统计、求取平均值等方法对分类的洋壳1-D速度结构进行对比研究,获得了不同扩张速率下洋中脊洋壳速度结构差异以及洋中脊与非洋中脊洋壳速度结构差异的新认识:(1)快速、慢速和超慢速扩张洋中脊的平均正常洋壳厚度分别为6.4km、7.2km和5.3km,其中洋壳层2的厚度基本相似,洋壳厚度差异主要源自洋壳层3;其洋壳厚度变化范围分别为4.9～8.1km、4.6～8.7km和4.2～10.2km,随着洋中脊扩张速率减小,洋壳厚度的变化范围逐渐增大;(2)快速扩张洋中脊的洋壳速度大于慢速和超慢速,可能与快速扩张脊洋壳生成过程中深部高密度岩浆上涌比较充足有关;(3)非洋中脊(10Ma)的洋壳比洋中脊(10Ma)的洋壳厚～0.3km,表明洋壳厚度与洋壳年龄有一定的正相关性.     

西南印度洋中脊地质构造特征及其地球动力学意义

西南印度洋中脊（SWIR）增生的洋壳面积仅占印度洋的15%左右,但其具有比东南印度洋中脊和西北印度洋中脊更悠久而复杂的演化历史.基于已有的地质、地球物理和地球化学等资料,系统总结了SWIR的地质构造特征,并讨论了SWIR的演化过程、洋脊地幔的不均一性、洋脊周边海底高原成因等核心问题.SWIR地形中段高、东西两段低,空间重力异常基本与地形变化一致.按转换断层一级边界可将SWIR划分为20个一级段.SWIR的磁异常条带呈现两端渐进式分布和中段带状分布特征,对应洋脊的三期演化历史.SWIR的地幔源区极不均一,尤其是中新元古代造山带根部集中拆离的中段.源区地幔的不均一性与大陆裂解和洋脊演化过程密切相关.SWIR的东端与西北印度洋中脊和东南印度洋中脊的邻近洋脊段具有地球化学亲缘性,西端与大西洋中脊和南美洲—南极洲洋中脊的邻近洋脊段具有地球化学亲缘性,这与SWIR的渐近式扩张有关.SWIR周边海底高原普遍具有较大的地壳厚度,其成因除了陆壳基底之外,可能与热点火山作用、热点-洋脊相互作用或热点-三联点相互作用有关,目前尚未形成统一的认识.SWIR的形成演化及其作用域内的熔融异常（如海底高原）是冈瓦纳大陆裂解、残留岩石圈地幔、软流圈地幔和深部地幔热柱物质共同作用的结果.了解SWIR的演化过程对揭示冈瓦纳大陆的裂解过程和印度洋的演化具有重要意义.     

西南印度洋洋中脊热液活动区综合地质地球物理特征

西南印度洋洋中脊(SWIR)是超慢速扩张洋脊的代表,是海洋地学研究热点.本文从SWIR多波束水深数据、重、磁数据和地震结构等几方面,阐述了SWIR热液活动区(49°39′E)的综合地质地球物理特征.SWIR热液活动不仅与扩张速率有关,构造作用更是一个重要控制因素;热液活动区位于Indomed和Gallieni转换断层之间,从水深地形上看,该区段洋脊是SWIR上水深最浅的区域之一,水深与MBA存在良好的镜像关系,MBA和RMBA低值意味着较厚的地壳厚度与较高的地幔温度,洋脊段27地壳厚度大于9km,可能是受到Crozet热点的影响;磁条带数据表明,此区段洋脊南北两翼呈不对称扩张,形成南翼的浅离轴域比北翼宽;在洋脊段28发现的活动热液喷口刚好位于热液蚀变形成的低磁强区内,具有良好的硫化物资源.这些认识必将为在该区首次实施的三维地震探测研究的地质地球物理解释及活动热液喷口的动力学机制研究打下坚实基础.     

南海海盆三维重力约束反演莫霍面深度及其特征

利用南海海盆及周边最新的重力,经过海底地形、沉积层的重力效应改正,并采用岩石圈减薄模型的温度场公式,校正了从张裂边缘到扩张海盆的热扰动重力效应.通过研究区的地震剖面和少量声呐数据得到的莫霍面深度点作为约束,采用基于"起伏界面初始模型"的深度修正量反演迭代公式,反演、计算了研究区的莫霍面深度及地壳厚度.结果表明,海盆区莫霍面深度在8~14 km之间,地壳厚度在3~9 km之间;东部海盆和西南海盆残留扩张中心沿NNE向展布向西南延伸至112°E,莫霍面深度超过12 km,地壳厚度在6 km以上,而西北海盆没有明显的增厚扩张中心;在西南海盆北缘的中沙地块南侧,存在一个近EW向地壳减薄带,地壳厚度在9~10 km;莫霍面深度14 km的等深线和地壳厚度9 km的等值线可指示洋陆边界位置.     

南海西南海盆壳幔结构重力反演与热模拟分析

壳幔结构及扩张期后的岩浆活动是研究南海西南海盆形成演化的关键.本文针对NH973-1剖面开展壳幔密度结构重力反演,并依据重力反演的壳幔模型,定量模拟海底扩张期后的壳幔热结构与热演化过程.重力反演表明:西南海盆中央残余扩张脊之下存在一个较深的凹陷带,其下Moho面比两侧略深,呈现扩张期后的热沉降特点.热模拟发现:海盆扩张...     

90Ma以来热点与西南印度洋中脊的交互作用：海台与板内海山的形成

本文研究了西南印度洋底地幔热点．洋中脊交互作用与海台、海山形成的关系．先利用板块重构确定了西南印度洋区域中热点与洋中脊相对位置及海台、海山的形成年代,然后通过水深异常和艾里均衡模式计算了相应热点的岩浆熔融通量．计算结果显示,自90Ma以来,马里昂（Marion）热点的活动可分为三个阶段：与古罗德里格斯（Rodrigues）三联点相互作用阶段（90～73．6Ma）、与西南印度洋中脊相互作用阶段（73．6～42．7Ma）及板内火山活动阶段（42．7～0Ma）．这三个阶段分别对应于德尔卡诺隆起（Del Cano Rise）的东部、中部和西部区域洋底海台的形成．马里昂热点的活动强度和周期性明显受到了热点离洋中脊距离的影响．马里昂热点的活动周期约为25Ma,长过夏威夷和冰岛热点的活动周期（约15Ma）．     

Dynamics of the mid-ocean ridge plate boundary: Recent observations and theory

The global mid-ocean ridge system is one of the most active plate boundaries on the earth and understanding the dynamic processes at this plate boundary is one of the most important problems in geodynamics. In this paper I present recent results of several aspects of mid-ocean ridge studies concerning the dynamics of oceanic lithosphere at these diverging plate boundaries. I show that the observed rift valley to no-rift valley transition (globally due to the increase of spreading rate or locally due to the crustal thickness variations and/or thermal anomalies) can be explained by the strong temperature dependence of the power law rheology of the oceanic lithosphere, and most importantly, by the difference in the rheological behavior of the oceanic crust from the underlying mantle. The effect of this weaker lower crust on ridge dynamics is mainly influenced by spreading rate and crustal thickness variations. The accumulated strain pattern from a recently developed lens model, based on recent seismic observations, was proposed as an appealing mechanism for the observed gabbro layering sequence in the Oman Ophiolite. It is now known that the mid-ocean ridges at all spreading rates are offset into individual spreading segments by both transform and nontransform discontinuities. The tectonics of ridge segmentation are also spreading-rate dependent: the slow-spreading Mid-Atlantic Ridge is characterized by distinct bulls-eye shaped gravity lows, suggesting large along-axis variations in melt production and crustal thickness, whereas the fast-spreading East-Pacific Rise is associated with much smaller along-axis variations. These spreading-rate dependent changes have been attributed to a fundamental differences in ridge segmentation mechanisms and mantle upwelling at mid-ocean ridges: the mantle upwelling may be intrinsically plume-like (3-D) beneath a slow-spreading ridge but more sheet-like (2-D) beneath a fast-spreading ridge.     

Mantle flow and melting beneath oceanic ridge–ridge–ridge triple junctions

Plate boundary geometry likely has an important influence on crustal production at mid-ocean ridges. Many studies have explored the effects of geometrical features such as transform offsets and oblique ridge segments on mantle flow and melting. This study investigates how triple junction (TJ) geometry may influence mantle dynamics. An earlier study [Georgen, J.E., Lin, J., 2002. Three-dimensional passive flow and temperature structure beneath oceanic ridge-ridge-ridge triple junctions. Earth Planet. Sci. Lett. 204, 115–132.] suggested that the effects of a ridge–ridge–ridge configuration are most pronounced under the branch with the slowest spreading rate. Thus, we create a three-dimensional, finite element, variable viscosity model that focuses on the slowest-diverging ridge of a triple junction with geometry similar to the Rodrigues TJ. This spreading axis may be considered to be analogous to the Southwest Indian Ridge. Within 100 km of the TJ, temperatures at depths within the partial melting zone and crustal thickness are predicted to increase by ~ 40 °C and 1 km, respectively. We also investigate the effects of differential motion of the TJ with respect to the underlying mantle, by imposing bottom model boundary conditions replicating (a) absolute plate motion and (b) a three-dimensional solution for plate-driven and density-driven asthenospheric flow in the African region. Neither of these basal boundary conditions significantly affects the model solutions, suggesting that the system is dominated by the divergence of the surface places. Finally, we explore how varying spreading rate magnitudes affects TJ geodynamics. When ridge divergence rates are all relatively slow (i.e., with plate kinematics similar to the Azores TJ), significant along-axis increases in mantle temperature and crustal thickness are calculated. At depths within the partial melting zone, temperatures are predicted to increase by ~ 150 °C, similar to the excess temperatures associated with mantle plumes. Likewise, crustal thickness is calculated to increase by approximately 6 km over the 200 km of ridge closest to the TJ. These results could imply that some component of the excess volcanism observed in geologic settings such as the Terceira Rift may be attributed to the effects of TJ geometry, although the important influence of features like nearby hotspots (e.g., the Azores hotspot) cannot be evaluated without additional numerical modeling.     

洋中脊构造及地震调查现状

介绍了洋中脊的全球分布和构造特征,对全球主要的、不同扩张速率的洋中脊进行了分类和列表描述;对洋中脊的构造特征,如地形特征、地壳厚度与扩张速率的关系及扩张轴下的岩浆房的特征、洋中脊与地幔柱的相互作用进行了阐述。回顾了海底地震仪在洋中脊构造调查中的应用及取得的主要成果。简要介绍了我国将用海底地震仪开展洋中脊构造调查的技术路线。     

Magmatism and tectonic processes in Area A hydrothermal vent on the Southwest Indian Ridge

The hydrothermal vent in Area A(37.78°S,49.65°E)is the first active hydrothermal vent discovered on the Southwest Indian Ridge(SWIR).Heat source and adequate bulk permeability are two necessary factors for the formation of a hydrothermal vent.Along the SWIR 49.3°E to 51.2°E,the gravity-derived crustal thickness is up to 9.0 km,much thicker than the average thickness of the global oceanic crust.This characteristic indicates that the magma supply in this area is robust,which is possibly affected by a hotspot.The large-scale residual mantle Bouguer anomalies(RMBA)reveal prominent negative-gravity anomalies between the first-order ridge segment(from Indomed to Gallieni,46.0°E to 52.0°E)and the Marion-Del Cano-Crozet region.These anomalies indicate the channel of the hotspot-ridge interaction.The tomography data corrected with theoretical thermal model indicate that the low-velocity anomalies corresponding to this channel can reach the base of the lithosphere.Near the hydrothermal vent area,the topography and crustal thickness at the off-axis area are extremely asymmetrical.South of the SWIR,the high topography corresponds to the thinning crustal thickness.The residual isostatic topography anomalies indicate that Area A is a deviation from the local isostatic equilibrium,similar to the characteristics of the transform fault inside corner.The forward profiles of the magnetic data indicate that the thinning magnetic layer at the south side of Area A corresponds to the shallow,high-velocity area revealed by the OBS,which is the result of tectonic extension of a detachment fault.The active tectonic processes in Area A can provide sufficient crustal permeability to the hydrothermal circulation and may form massive sulfide deposits.     

Tectonic structure and petrology of the Antarctic plate boundary near the Bouvet triple junction

An en echelon suite of four fracture zones, trending approximately N40°E, has been discovered during a survey of the Southwest Indian Ocean Ridge between Bouvet Island and 14°E. The largest of these fracture zones, the Islas Orcadas and Shaka, are less than 30 km wide, have more than 3 km of vertical relief, and are respectively 100 and 200 km in length. The morphology of these and the Bouvet and Prince Edward fracture zones have been used to compute a pole for the relative motion between Africa and Antarctica. This pole, at 4°S and 32°W, is within the range of previously computed pole positions.Ridge basalts were dredged at three separate locations: at the Conrad fracture zone near 55°40′S and 3°51′W, at the Islas Orcadas fracture zone near 54°5′S and 6°4′E, and at the ridge crest near 11°E. In addition, samples from a probable upper mantle intrusion were recovered from one wall of the Islas Orcadas fracture zone. The opposite wall was very different consisting entirely of normal mid-ocean ridge basalt.     

A hafnium isotope and trace element perspective on melting of the depleted mantle

New Hf isotope and trace element data on mid-ocean ridge basalts (MORB) from the Pacific Ocean basin are remarkably uniform (¹⁷⁶Hf/¹⁷⁷Hf≈0.28313–0.28326) and comparable to previously published data [Salters, Earth Planet. Sci. Lett. 141 (1996) 109–123; Patchett, Lithos 16 (1983) 47–51]. Atlantic MORB have ¹⁷⁶Hf/¹⁷⁷Hf ranging from 0.28302 to 0.28335 confirming the wide range originally identified by Patchett and Tatsumoto [Geophys. Res. Lett. 7 (1980) 1077–1080]. Indian MORB define an even wider range, from 0.28277 to 0.28337, but three exotic samples have very unradiogenic Hf isotope compositions. Their very low ¹⁷⁶Hf/¹⁷⁷Hf ratios, together with their trace element characteristics, require the presence of unusual plume-type material beneath the Indian ridge. All other Indian MORB have uniform Hf isotope compositions at about 0.2832, and define a small field displaced to the right of other MORB in Hf–Nd isotope space. The distinct nature of Indian MORB is best explained by the presence in Indian depleted mantle of old recycled oceanic crust and pelagic sediments. Sm/Hf ratios calculated from new high-precision rare earth element and Hf trace element data do not vary in MORB in the same way as in ocean island basalts (OIB): ratios are constant in OIB, but decrease with increasing Sm contents in MORB. The constancy of Sm/Hf in OIB is probably due to an overwhelming influence of residual garnet during melting. By contrast, the decrease of Sm/Hf in MORB is due to the effect of clinopyroxene in the residue of melting beneath ridges, an interpretation confirmed by quantitative modeling of melting. The relationship between Sm/Nd and Lu/Hf ratios in MORB does not require the presence of garnet in the residual mineralogy. The decoupling of Lu/Hf ratios and Hf isotope compositions – the so-called Hf paradox [Salters and Hart, EOS Trans. Am. Geophys. Union 70 (1989) 510] – can be explained by melting dominantly in the spinel field at shallow depths beneath mid-ocean ridges.     

Electrical Structure in Marine Tectonic Settings

This review paper presents recent research on electrical conductivity structure in various marine tectonic settings. In at least three areas, marine electromagnetic studies for structural exploration have increasingly progressed: (1) data accumulations, (2) technical advances both for hardware and software, and (3) interpretations based on multidisciplinary approaches. The mid-ocean ridge system is the best-studied tectonic setting. Recent works have revealed evidence of conductive zones of hydrothermal circulation and axial magma chambers in the crust and partial melt zones of the mid-ocean ridge basalt source in the mantle. The role of water or dissolved hydrogen and its redistribution at mid-ocean ridges is emphasized for the conductivity pattern of the oceanic lithosphere and asthenosphere. Regions of mantle upwelling (hotspot or plume) and downwelling (subducting slab) are attracting attention. Evidence of heterogeneity exists not only in the crust and the upper mantle, but also in the mantle transition zone. Electrical conductive zones frequently overlap seismic low-velocity zones, but discrepancies are also apparent. Some studies have compared conductivity models with the results of seismic and other studies to investigate the physical properties or processes. A new laboratory-based conductivity model for matured oceanic lithosphere and asthenosphere is proposed. It takes account of both the water distribution in the mantle as well as the thermal structure. It explains observed conductivity patterns in the depth range of 60–200 km.