洋中脊动力学研究现状

对最近几年洋中脊动力学研究之进展给予综述.研究包括:(1)脊轴处中央U 形(?)谷的成因:(2)洋脊下地幔对流、熔融和熔体迁移之格局;(3)洋脊处上升流的流动结构与洋脊分节的关系;(4)岩石层对洋脊分节的影响:(5)洋中脊断裂带与热应力之关系等五个方面.对上述五问题之地质、地球物理或地球化学背景仅作简要说明.主要介绍洋中脊动力学之理论研究状况,即着重介绍几年来所提出的解析的、数值的以及物理实验的各种模型之基本思路及主要结论.     

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...     

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.     

特提斯演化的关键动力学过程与驱动力

特提斯系统经历了漫长的演化历史,包含了多期次的威尔逊旋回,是研究板块构造与地球动力学的理想对象.特提斯演化的典型特征是一系列大陆块体从南方的冈瓦纳大陆裂解,而后向北漂移,最终与北方的劳亚大陆碰撞拼合.该过程中,多期次特提斯洋盆(原特提斯洋、古特提斯洋、新特提斯洋)的张开和关闭是核心要素.本文以大洋板块的生命周期为主线,将特提斯系统演化分解为大陆裂解、俯冲起始、洋脊俯冲、大陆碰撞等四个关键动力学过程,并系统分析了每个关键过程的控制因素和驱动力.(1)特提斯系统窄条形大陆地体的裂解可能受控于板块俯冲,尤其是俯冲板片的远端牵引作用;而块状印度大陆的裂解及印度洋的张开可能是地幔柱与远端俯冲板块共同作用的结果.(2)特提斯系统周期性的大陆地体碰撞拼合产生多期次的俯冲跃迁,是地体碰撞产生的反推力、洋脊推力及板下地幔流牵引力共同作用的结果,并且岩石圈的弱化是关键因素.(3)洋脊俯冲往往伴随着板片断离,该构造体制的转换可能需要地幔流牵引力的辅助,从而实现板块俯冲的延续性;而洋脊俯冲对上盘和下盘都会产生一定的动力学效应,其特征地质记录可用于反演洋脊俯冲历史.(4)青藏高原的巨大重力势能意味着持续至今的印...     

西南印度洋脊中段Indomed-Gallieni洋中脊岩浆-构造动力模式

利用西南印度洋脊中段Indomed-Gallieni洋段49—51°E区段全覆盖高分辨率多波束水深地形资料,应用构造地貌学分析方法,结合区域地形及其他地球物理等资料,在分段分析49—51°E区段岩浆-构造动力学模式的基础上,进一步探讨了约10 Ma以来Indomed-Gallieni洋段的演化史.28、29洋段目前岩浆供应不足,在轴部不对称深断层的控制之下不对称扩张,属于超慢速扩张洋脊较常见的演化方式.轴部火山建造主要向北翼增生,发育与火山脊相关的火山地貌;南翼构造拉张作用强烈,地貌上可观察到大量断块,拆离断层可能大量存在.而27洋段水深浅、火山密集、轴部缺失裂谷,超慢速扩张下却具有较高的岩浆通量.Indomed-Gallieni洋段地形高地建造于一次岩浆增强事件,但应该不是因为Crozet热点的影响.27洋段为目前仍受该岩浆增强事件影响的唯一区段,但其强度和规模也在逐渐减小;包括28、29洋段在内的Indomed-Gallieni段其他部分,已重新恢复到岩浆供应不足的正常超慢速扩张洋脊演化模式.28、29洋段和27洋段岩浆供应均存在岩浆通量由多至少的周期,周期内岩浆供应较多时期轴部建脊,减少时期轴部火山建造裂离.但27洋段由于仍受岩浆增强事件的影响,与28、29洋段表现形式不同,主要表现为火山建造裂离方式、岩浆供应周期长短以及构造活动强烈程度的不同.     

东亚大陆伸展和裂谷作用与动力学

大陆岩石圈的伸展作用是块体分裂，大陆解体和漂移过程中必需的第一步，而伸展作用的模式则取决于驱动力系和大陆岩石圈的反响。所以不同类型裂谷的形成与地球深部地壳与地幔结构，深层过程及其力学机制密切相关。这是回答控制大陆生长与为什么会开裂的关键所在。本文在研究了系列裂谷构造的基点上，阐述了大陆裂谷的特点，地壳与地幔构造背景和地球物理场特征。论述了裂谷形成和其分类，讨论了我国攀西裂谷和莱茵地堑的深层过程。由     

郯庐古裂谷中段地壳结构的初步研究—二维动力学射线追踪方法的解释结果

横穿中国东部郯庐古裂谷中段的连云港—临沂—泗水地壳测深剖面资料解释工作随着解释方法的不同其研究程度也不断深入(见图1)。1983年作者曾进行了一维垂向非均匀地壳速度模型的解释,并结合重磁场等地球物理资料的综合分析,探讨了临沂8.5级地震的深部构造背景。本文是在上述工作基础之上利用动力学射线追踪方法,选用二维非均匀地壳速度模型,进一步研究郯庐古裂谷中段的地壳结构特征,初步探讨古裂谷的形成及其动力学演化过程。     

岩石层内与断裂后热释放有关的物理变化

我们根据岩石层的热状态来定义现代裂谷和古裂谷,现代裂谷是断裂后的岩石层尚未达到热平衡状态,而古裂谷是断裂后的岩石层已处于热稳定状态。将现代裂谷与古裂谷的物理特征进行对比,结果表明,它们的地貌、构造和岩浆特征方面虽有很多相同之处,但在地球物理特征方面却有一些明显的差异。在由现代裂谷转变为古裂谷时采用一个简单的岩石层热释放模型,我们可以看到,现代裂谷与古裂谷之间在岩石层的上地幔地震波速度和密度、表面热流、热结构与居里点深度、以及大地电磁性质方面的差异可以用宕石层冷却来解释。而单用冷却不能解释与断裂有关以及紧接断裂之后的复杂沉陷过程,也不能解释古裂谷内为什麽出现一些高速的壳层,以及地壳有时为什麽会加厚。假如考虑到岩石层的抗弯强度和与断裂有关的地壳的岩浆加厚,这些现象就可以得到解释。我们认为,许多现代裂谷下面的“上地幔异常”可能与断裂地壳岩浆加厚过程有关。     

西南印度洋超慢速扩张洋脊的岩浆一构造旋回：来自轴部火山洋脊地形变化和深海丘陵样式的证据

综合利用洋脊轴部的深拖侧扫声纳资料和轴外的水深数据,研究了超慢速扩张的西南印度洋洋脊处洋壳增生过程的瞬时变化。在洋脊各段的侧扫声纳图像中可以观察到轴部火山洋脊的长度与高度的差异,及这些火山建造不同的变形程度。这些差异是由于轴部火山洋脊处于其生命演化周期的不同发育阶段,包括火山建造期和构造裂解期。利用轴外侧的水深数据确定了每个洋脊段中许多大小均匀的深海丘陵。这些深海丘陵均显示不对称的形状,面向轴部为陡峭的断层崖,背向轴部为平缓倾斜的火山岩斜坡。这些深海丘陵是被运移到两翼的、已被裂解的早期轴部火山洋脊的残留,它们形成于连续的岩浆建造期和构造裂解期之中,即一个岩浆-构造旋回。在厚地壳的洋脊区段观察到大型深海丘陵,而在薄地壳的洋脊区段观察到小型深海丘陵。这说明岩浆供给量控制着深海丘陵的大小。在薄地壳的洋脊区段,深海丘陵有规律地等间隔排列,表明岩浆一构造循环的伪周期性过程持续约0．4ma,比厚地壳的洋脊区段的周期时间短4～6倍。我们认为,有规律的深海丘陵样式与长寿命洋脊段下部几乎恒定的岩浆持续供给有关。相比之下,在岩浆供给急剧减少并极不连续的情况下,不再存在有规律的深海丘陵样式。     

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.     

Non-axial breaching of a rift valley: Evidence from the Lord Howe Rise and the southeastern Australian margin

Present models of continental breakup envisage the formation of a rift valley which undergoes a protracted period of tectonism and eventual seafloor spreading in the axial part of the rift valley. This results in evidence of pre-breakup tectonism on most Atlantic-type margins in the form of normal blockfaults beneath the continental slope. The southeastern margin of the Australian continent has an unusually steep continental slope and shows little evidence of tectonism associated with the rift valley stage of development. The margin was formed by separation of the Lord Howe Rise and Australia during a phase of seafloor spreading in the Tasman Sea which lasted from about 80 to 60 m.y. B.P. Marine geophysical data over the central Lord Howe Rise indicate a contrast between the western and eastern part of of this structure. The western part shows faulted, rough basement topography, disturbed overlying sediments, and a relatively quiet magnetic field. The eastern part shows a smooth basement surface, undisturbed overlying sediments, and a high-amplitude, high-frequency magnetic field. It is suggested that the whole of the pre-breakup rift valley remained attached to the Lord Howe Rise. This explains the absence of rift valley structures within the eastern continental margin of Australia and implies non-axial breaching along the western boundary fault of a pre-Tasman Sea rift valley.     

南海北部区域构造和陆壳向洋壳的转化

1.海底扩张和陆壳大洋化,均能形成洋壳。陆壳转化为过渡壳是大洋化的必经阶段。 2.航磁测区内的中央海盆,具有类似于大洋中海底扩张形成的对称磁异常条带。 3.在西沙北裂谷、莺歌海裂谷型拗陷,以及属于断陷盆地性质的珠一、珠二拗陷等地,有沿着断裂上升的类似于洋壳成分的地幔物质喷溢或在地壳上层侵位,那里的地壳都有不同程度的减薄,属于过渡型地壳。西沙北裂谷的“莫霍面”,比相邻的南北两侧高出约10公里。 4.在南海北部发生多中心微型扩张和大洋化。     

Segmentation of the southern Mariana back-arc spreading center

SeaBeam multibeam bathymetry obtained during cruise SO-69 of research vessel (R/V) Sonne defines the segmentation and structure of ∼ 300 km of the Mariana back-arc spreading center south of the Pagan fracture zone at 17°33'N. Eight ridge segments, ranging from 14 to 64 km in length, are displaced as much as 2.7–14.5 km by both right- (predominantly) and left-lateral offsets and transform faults. An axial ridge commonly occupies the middle portion of the rift valley and rises from 200 to 700 m above the adjacent sea floor, in places shoaling to a water depth of 3200 m. An exception is the 60-km-long segment between 16°58' and 17°33'N where single peaks only a few tens of meters high punctuate the rift axis. Photographic evidence and rock samples reveal the presence of mostly pillow lavas outcropping on the axial ridges or peaks whereas the deeper parts of the rift valley floor (max. depth 4900 m) are heavily to totally sedimented. Abundant talus ramps along fault scarps testify to ongoing disruption of the crust. Lozenge-shaped collapse structures are covered by layers of sediment up to tens of centimeters thick on the rift valley floor. The presence of discrete volcanic ridges in the southern Mariana back-arc spreading region suggests that emplacement of oceanic crust at this slow spreading center occurs by `multi-site' injection of magma. Along-axis variations in length, crestal depth, and size of the axial ridges can be best explained by different stages in the cyclicity of magma supply along-axis.     

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.     

The mid-ocean ridge axial valley as a steady-state neck

In this paper the mid-ocean ridge axial valley is modelled as a steady-state lithospheric neck in which lithospheric stretching balances lithospheric accretion. Conversely, the axial high is a steady-state lithospheric bulge. The lithosphere is modelled as a thin plate with a Newtonian rheology. It is shown that an axial valley will occur if the rate of viscosity increase away from the ridge axis is faster than the rate at which accretion decreases. An axial high will occur if the opposite condition holds. This is consistent with the observation that axial valleys occur at low spreading rates and axial highs at high spreading rates. By fitting our model to profiles across the Mid-Atlantic Ridge and the East Pacific Rise and assuming the lithospheric thickness at the ridge axis to be 5 km, we find accretion widths of 6–8 km. We find the width over which there is a significant increase in lithospheric viscosity to be also 6–8 km.     

Thermal and rheological state of the lithosphere and specific features of structuring in the rift zone of the Reykjanes Ridge (from the results of numerical and experimental modeling)

Specific features of the bottom topography structure and the character of morphostructural segmentation of the rift zone of the Reykjanes Ridge change substantially along the ridge strike with increasing distance from Iceland’s hotspot. A clearly pronounced regularity of changes is observed in the rift zone’s morphology from the axial uplift (in the northern part of the ridge) to the rift valleys (in the southern part of the ridge) through an intermediate or transitional type of morphology. The results of numerical modeling showed that changes in the rift zone’s morphology along the Reykjanes Ridge strike are largely caused by changes in the degree of mantle heating and depend on the intensity of magma supply. It is shown that under conditions of ultraslow spreading, it is these parameters that control the presence or absence of crustal magma chambers, as well as the thickness of the effectively-elastic layer of the axial lithosphere. The experimental modeling of topography-forming deformations and structuring on the Reykjanes Ridge showed that under oblique extension, specific features of the formation of axial fractures and the character of their segmentation mainly depend on the thickness of the axial lithosphere, its heating zone width, and the kinematics of spreading. The experiments also showed that the tendency of fractures to develop obliquely to the extension axis is caused by the action of the inclined zone of the location of the deformation, and shear deformations play a substantial role in the lithosphere’s destruction as the inclination angle increases.     

长江断裂带东延问题及裂谷特性的讨论

长江断裂带东延至镇江后并未终止,而是于镇江、扬中之间南移了约三十公里,展布于江苏江阴、沙洲、靖江、南通境内,于吕泗以北进入黄海。同时讨论了长江断裂带的属性,指出长江断裂带应是一条自晚中生代以来的现代大陆裂谷带     

西藏中部构造特征及印度板块仰冲问题

以负磁场为背景的航磁特征,揭示了基底是由浅变质的副变质岩组成的,属于柔性基底。 藏北断裂在磁场图上不明显。 雅鲁藏布江断裂是举世闻名的缝合线。反映雅鲁藏布江断裂的航磁异常带,具有强度大、连续性好和长度大的特点,为世界上罕见的大陆线性异常带。地面地质和各种地球物理资料查明断面是南倾的。 晚三迭世开始,印度板块在特提斯洋壳上向北仰冲,并导致特提斯在始新世的关闭。两大陆碰撞后,仰冲运动结束,代之以印度板块对欧亚板块南缘的推挤。     

Evolution of rifted ocean ridges

A major question in seafloor tectonics has been, how does the 2-km-deep rift valley characteristic of slow-spreading ridges evolve into the relatively horizontal undulating relief of the rift mountains? Deep-tow studies of the Mid-Atlantic Ridge suggest that the primary mechanism for transformation of the rift valley topography is normal faulting along fault planes which dip away from the valley axis. The faulting occurs in a narrow zone just beyond the outer walls of the rift valley. This model allows for a steady-state evolution of the rift valley into the rift mountains in which the state of stress in the oceanic lithosphere continues to be in horizontal deviatoric tension throughout the entire process. Alternate mechanisms involving reverse faulting or regional tilt may be active but are found to be of less importance. Implications for various dynamic models of the rift valley are discussed.     

The gravity signatures of isostatic,thermally-expanded ridge crests

The gravity anomaly has been computed above isostatic, thermally-balanced speading centers that cool by conduction through their top surfaces. Isothermal, and therefore isodense, surfaces were treated as topographic boundaries between layers of different density, and Fourier transforms of power series of the topographic height were used to find the gravity. Convergence requires that the anomaly tend to zero with increasing distance from the ridge crest, and when this is obtained, a crestal positive anomaly is flanked by compensating negatives. Both the magnitude and the spatial width of the anomalies decrease with increasing spreading rate.The ～5 mgal gravity anomalies observed over fast-spreading ridges are matched well by the calculations, but slow-spreading ridges usually have a central rift valley in place of the smooth crest of the idealized isostatic thermal model. The mass deficiency of the valley cancels out the ～40 mgal positive peak that would otherwise occur. The short-wavelength anomaly amplitudes of such ridges are less than 25 mgal and follow the observed local rift valley and flanking ridge topography closely. Excess positive gravity and topography of the flanking ridges suggests compensation of the mass deficiency in the rift valley. However, long-wavelength gravity anomalies such as those observed in the northern Mid-Atlantic cannot be due to topography that is isostatically compensated at a shallow depth. These must be caused either by dynamic forces or by large-scale density differences compensated at much greater depths.