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.     

Lithosphere thickness,subsidence of aseismic ridges,and volcano height

Two methods — collection of empirical subsidence data for aseismic ridges and observation of the increase in height of oceanic volcanoes with increasing lithospheric age — are used to try to discriminate between the thermal models for the oceanic lithosphere of McKenzie (1967), Parker and Oldenburg (1973), and Crough (1975). Although the volcano height data indicate that thickening of the lithosphere does occur, neither method is precise enough to allow preferential selection of one model over another.     

西南印度洋岩浆补给特征研究:来自洋壳厚度的证据

西南印度洋中脊为典型的超慢速扩张洋中脊,其岩浆补给具有不均匀分布的特征.洋壳厚度是洋中脊和热点岩浆补给的综合反映,因此反演洋壳厚度是研究大尺度洋中脊和洋盆岩浆补给过程的一种有效方法.本文通过对全球公开的自由空气重力异常、水深、沉积物厚度和洋壳年龄数据处理得到剩余地幔布格重力异常,并反演西南印度洋地区洋壳厚度,定量地分析了西南印度洋的洋壳厚度分布及其岩浆补给特征.研究发现,西南印度洋洋壳平均厚度7.5 km,但变化较大,标准差可达3.5 km,洋壳厚度的频率分布具有双峰式的混合偏态分布特征.通过分离双峰统计的结果,将西南印度洋洋壳厚度分为0~4.8 km的薄洋壳、4.8~9.8 km的正常洋壳和9.8~24 km的厚洋壳三种类型,洋中脊地区按洋壳厚度变化特征可划分为7个洋脊段.西南印度洋地区薄洋壳受转换断层控制明显,转换断层位移量越大,引起的洋壳减薄厚度越大,减薄范围与转换断层位移量不存在明显相关性.厚洋壳主要受控于该区众多的热点活动,其中布维热点、马里昂热点和克洛泽热点的影响范围分别约340 km,550 km和900 km.Andrew Bain转换断层北部外角形成厚的洋壳,具有与快速扩张洋中脊相似的转换断层厚洋壳特征.     

Evolution of the Taebaeksan Basin,Korea: I,early Paleozoic sedimentation in an epeiric sea and break‐up of the Sino‐Korean Craton from Gondwana

The Taebaeksan Basin is located in the mid‐eastern part of the southern Korean Peninsula and tectonically belonged to the Sino‐Korean Craton (SKC). It comprises largely the lower Paleozoic Joseon Supergroup and the upper Paleozoic Pyeongan Supergroup which are separated by a disconformity representing a 140 myr?long hiatus. This paper explores the early Paleozoic paleogeographical and tectonic evolution of the Taebaeksan Basin on the basis of updated stratigraphy, trilobite faunal assemblages, and detrital zircon U–Pb ages of the Joseon Supergroup. The Joseon Supergroup is a shallow marine siliciclastic‐carbonate succession ranging in age from the Cambrian Series 2 to Middle Ordovician. The Ongnyeobong Formation is the sole Upper Ordovician volcanic succession documented in the Taebaeksan Basin. It is suggested that in the early Paleozoic the Taebaeksan Basin was a part of an epeiric sea, the Joseon Sea, in east Gondwana. The Joseon Sea was the depositional site for lower Paleozoic successions of the SKC. Early Paleozoic sedimentation in the Joseon Sea commenced during the Cambrian Stage 3 (～ 520 Ma) and ceased by the end of the Darriwilian (～ 460 Ma). In the early Paleozoic, the SKC was located at the margin of east Gondwana and was separated from the South China Craton by an oceanic basin with incipient oceanic ridges, the Helan Trough. The spreading oceanic ridges and associated transform faults possibly promoted the uplift of the Joseon Sea, which resulted in cessation of sedimentation and break‐up of the SKC from core Gondwana by the end of the Ordovician.     

The concentrations of the heavy metals in the four new Antarctic meteorites Yamato (a), (b), (c) and (d) and in Orgueil, Murray, Allende, Abee, Allegan, Mocs and Johnstown

All active midocean ridges show a uniform relationship between depth and age of the oceanic crust. Recently, it has been shown by numerical methods that convective flow in a Newtonian fluid will have a positive gravity anomaly and an upward surface deformation associated with an ascending limb. If there is thermal convection in the upper mantle, these calculations predict that there may be a correlation between free air gravity anomalies and differences from the uniform relationship between oceanic depth and age. To investigate such a correlation, we considered the crestal elevation and free air gravity anomaly over the crest of the midocean ridges. It has been suggested that the differences from the depth versus age relationship are related to spreading rate. Thus, we also considered a correlation between crestal elevation and changes in rate along the ridge axis.We found a positive correlation between free air gravity and differences in crestal depth of the midocean ridge system. We found no correlation between spreading rate and gravity and no uniform relationship which holds in all the oceans between spreading rate and observed crestal depths.The long wavelength gravity anomalies which are correlated with the differences in crestal depth cannot be supported by an 80 km thick lithosphere. Thus, they are considered evidence of flow within the aesthenosphere. Further, the correlation between gravity anomaly and differences in crestal depth has the same sign and gradient as predicted by the investigations of convection in a Newtonian fluid.     

Cyclic volcanic stratigraphy in a late ordovician marginal basin, West Norwegian Caledonides

The well-preserved extrusive sequence of the Solund-Stavfjord Ophiolite Complex (SSOC) in the West Norwegian Caledonides enables reconstruction of the uppermost oceanic crust that developed in a marginal basin. Basaltic sheet flows, pillow lavas and volcanic breccias are the main components of the extrusive sequence and show stratigraphic and structural evidence for a cyclic development. The first stage in a volcanic cycle is characterized by high extrusion rates yielding sheet flows, commonly with the thickest flow units at the base. Sequences of sheet flows can be correlated laterally for at least 6.5 km. Pillow lavas succeed the sheet flows later in a volcanic cycle with progressively smaller pillows forming at decreasing extrusion rates. Volcanic breccias occur towards the end of a volcanic cycle, but may also occur at lower stratigraphie levels. They are made generally of pillow breccias and hyaloclastites. The extrusive sequence of the SSOC oceanic crust was constructed through seven volcanic cycles that resulted in stratigraphic units with thicknesses ranging from 40 to 225 m. This architecture is comparable to sequences in in situ oceanic crust developed along slow- to intermediate-spreading ridges.     

Serpentinite protrusions in the oceanic crust

Serpentinite may be a significant component of the oceanic crust, not as a continuous layer, but as vertical tectonic protrusions and sills emplaced from the upper mantle into fault zones parallel to the axis of spreading ridges. The diapiric emplacement of serpentinite bodies occurs within 100–200 km of ridge axis, with a rate of ascent on the order of 1 mm/year. Serpentinite protrusions may cause small-scale linear magnetic anomalies parallel to ridge axis. Serpentinites are distributed in the oceanic crust according to an orthogonal pattern, with large serpentinite protrusions aligned along major fracture zones, and smaller serpentinite bodies emplaced in bands parallel to ridge axis.     

Impact of micro-organisms activity on the natural remanent magnetization of the young oceanic crust

Microbes have been widely reported in the deep subseafloor environment. Still it is difficult to detect a global chemical signature of bacterial activity in the oceanic crust. We carried out experiments up to 355 days exposing very young oceanic basalts to anaerobe sulfate reducing organisms in an in-vitro marine environment. The Natural Remanent Magnetization of samples was monitored during the whole duration of experiments and within this time frame the most magnetized sub-samples lost up to 30% of their original signal. Scanning electron microscope observations show cycling of iron from the titanomagnetites to iron sulfide phases. Our results suggest that microbes can have a major and fast impact on the magnetization of young oceanic basalts and could contribute to a global signal as the central anomaly magnetic high seen along ridges axis.     

Na and Cr contents in clinopyroxenes from peridotites: A possible discriminant between “sub-continental” and “sub-oceanic” mantle

The compilation of data available in the literature and new analyses show that clinopyroxenes are significantly richer in Na and poorer in Cr in peridotites associated with high-grade metamorphic rocks than in ultramafites from oceanic environments, considered as “sub-continental” or “sub-oceanic” mantle, respectively. Two distinctive fields can be drawn in the Na-Cr plot. This fact is related to the large amount of basic magma provided by the oceanic mantle along the mid-oceanic ridges.Application of this Na-Cr diagram to clinopyroxenes from peridotites in orogenic belts and appearing as xenoliths in volcanic rocks and kimberlites (“nodules”) allows us to specify their origin, taking into consideration that the clinopyroxene composition is controlled by several factors each of which gives rise to a particular trend:P-T. conditions, mineral facies, partial melting and crystal fractionation, metasomatism. It appears that oceanic-type mantle may be found under continents in extensional areas having evolved towards rift systems, and in ophiolites. The latter exhibit different degree of depletion related to their formation in two main geotectonic situations: mid-oceanic ridges and active margin systems.     

Geotectonic bipolarity, evidence from the pattern of active oceanic ridges bordering the Pacific and African plates

Active oceanic ridges are part of the global system of diverging plate boundaries encircling the Earth. They represent weak zones of the lithosphere. They are isostatically equilibrated. The system as a whole is considered to be well adapted to the present field of plate driving forces. The search for regularities in the pattern of active oceanic ridges may, therefore, provide valuable information as to the large-scale characteristics of structures and processes in the Earth’s mantle. Two large belts of active oceanic ridges are envisaged: (1) The semi-circular belt bordering the Pacific plate which extends from South of Tasmania to Northwest of Vancouver Island over a length of 20,000 km. It appears to encircle a center P1 in the central Pacific region. (2) The circum-African belt bordering the African plate which extends from the Azores to the Gulf of Aden over a length of 24,000 km. It appears to encircle a center A1 in central Africa. The attempt is made to determine the position of these centers. Extent and position of the ridge systems are described by 34 fixed points. Points R01–R20 mark the circum-African ridge system, points R21-R34 the Pacific ridge system. A least-squares adjustment is used to determine the optimum position of the centers P1 and A1. Center P1 of the Pacific ridge system is located at 169.8°W/2.6°S. Center A1 of the circum-African ridge system is located at 11.6°E/2.4°N. The location error of the centers is less than 2.8°. In view of the great extent of the ridge systems, and considering the fact that the location of P1 and A1 is based on independent data sets, the nearly antipodal and equatorial position of the centers is remarkable. The newly defined centers P1 and A1 are located close to the Pacific pole P, at 170°W/0°N, and the African pole A, at 10°E/0°N. Within the limits of error the center P1 coincides with pole P, the center A1 with pole A. Originally, these poles were introduced in order to describe a fundamental hemispherical symmetry which is apparent in the evolution of the Earth’s lithosphere during the last 180 Ma. The new results confirm the unique position of poles P and A in the global tectonic framework.     

Global tectonics and the geoid

A digitised tectonic model, initially built up for regionalization of Rayleigh waves, is applied to the geoid in order to define the mean geoid heights of the following regions: 3 oceanic regions, namely young oceans (0–30 Ma) middle-aged oceans (30–80 Ma) and old oceans (> 80 Ma); trenches and subduction zones; mountains; and shields. The relative importance of the deep sources is damped or enhanced by progressively removing or adding the lower or higher degrees of the geoid. A statistical approach allows us to quantify the success of the correlation between tectonics and these filtered geoids.Significant variations are observed in these correlations for oceanic regions (including subduction zones) with a cut-off between degree-2 and higher degrees. For degrees ? 3, a well-known trend is observed: high values correspond to young oceans (ridges) and low values to old oceans, high values are also obtained for subduction zones. On the contrary, and unexpectedly, for the degree-2 alone a trend reversal is observed: geoid lows are observed over ridges and geoid highs over old oceans; trenches give the same geoid amplitude than old oceans. Clearly this denotes a degree-2 convection pattern connected to plate tectonics. In addition it is shown that the minimum and maximum inertia axes of the surface distribution of young oceans, and independently of old oceans and trenches, coincide with the Earth's equatorial inertia axes (74°E and 164°E), i.e., with the equatorial extremes of the degree-2 geoid.Plate tectonics is uncorrelated with the polar anomaly of the degree-2 geoid, namely the flattening which is not accounted for by Earth rotation. A north-south axisymmetric convection with a degree-2 pattern is proposed to explain this extra flattening; this model is supported by the latitude dependence of the depth of oceanic ridges.     

Reply to “Comments on ‘Poles of rotation’”

Analysis of published data on sea floor spreading for the different oceans demonstrates a close correlation between interruptions of spreading at sea and compressive periods on land and between periods of spreading activity and periods of “no compression” of the orogenic regions. The evolution of both orogenic and oceanic areas appears to be rhythmic. The model is generalized to a dynamic model for the Earth's crust in which periods of global compression and extension follow alternately. Such a model fits better the geological evidences from orogenic regions than the present model for sea floor spreading which postulates an expansion in the mid ocean ridges and a compression along the continental margins underthrusted by oceanic crust.     

Oceanic ridges and transform faults: Their intersection angles and resistance to plate motion

The persistent near-orthogonal pattern formed by oceanic ridges and transform faults defies explanation in terms of rigid plates because it probably depends on the energy associated with deformation. For passive spreading, it is likely that the ridges and transforms adjust to a configuration offering minimum resistance to plate separation. This leads to a simple geometric model which yields conditions for the occurrence of transform faults and an aid to interpretation of structural patterns in the sea floor. Under reasonable assumptions, it is much more difficult for diverging plates to spread a kilometer of ridge than to slip a kilometer of transform fault, and the patterns observed at spreading centers might extend to lithospheric depths. Under these conditions, the resisting force at spreading centers could play a significant role in the dynamics of plate-tectonic systems.     

Evidence for the protracted construction of slow-spread oceanic crust by small magmatic injections

Gabbroic cumulates drilled south of the Kane Transform Fault on the slow-spread Mid-Atlantic Ridge preserve up to three discrete magnetization components. Here we use absolute age constraints derived from the paleomagnetic data to develop a model for the magmatic construction of this section of the lower oceanic crust. By comparing the paleomagnetic data with mineral compositions, and based on thermal models of local reheating, we infer that magmas that began crystallizing in the upper mantle intruded into the lower oceanic crust and formed meter-scale sills. Some of these magmas were crystal-laden and the subsequent expulsion of interstitial liquid from them produced ‘cumulus’ sills. These small-scale magmatic injections took place over at least 210?000 years and at distances of ∼3 km from the ridge axis and may have formed much of the lower crust. This model explains many of the complexities described in this area and can be used to help understand the general formation of oceanic crust at slow-spread ridges.     

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.     

海洋热流与地幔上升流

从洋中脊上升的地慢物质带上来的大量热量,使在洋中脊裂谷处的海底热流及温度最高,向海沟逐渐减小．由于热胀冷缩,海底地势在洋中脊处最高．作者根据板块模型由一维运动物体的热传导方程推出裂谷处上升物体的温度分布公式．用此分布公式作为垂直边的边界条件,严格地求解了运动板块的二维运动物体热传导方程．用此方程研究了各参数与观测量之间的关系．通过改变公式中的参数值,使计算的理论曲线与实测海底热流一年龄及海底深度一年龄曲线拟合,从而求出了板块的厚度Ｌ＝９７ｋｍ,地幔上升速度ｕ＝３．ｇｍｍ／ａ,热膨胀系数ａ＝３．３７×１０－５／℃及温度分布．该方法克服了在洋中脊处理论热流值趋于无限大的问题,并可以计算出地幔上升流的速度及宽度．     

Lithospheric model with thick oceanic crust at the continental boundary: A mechanism for shallow spreading ridges in young oceans

In a general lithospheric model of a simple divergent ocean and continental margin that satisfies the constraints of isostasy and gravity anomalies, the free-air gravity anomaly at the margin is modelled by an oceanic crust that thickens exponentially toward the margin from its common value of 6.4 km about 600 km from the margin to 17.7 km at the margin; this postulated thickening is supported empirically by seismic refraction measurements made near continental margins. The thickness of the oceanic crust matches that of the continental lithosphere at breakup, as observed today in Afar and East Africa, and is interpreted as the initial oceanic surface layer chilled against the continental lithosphere. With continued plate accretion, the chilled oceanic crust thins exponentially to a steadystate thickness, which is achieved about 40 m.y. after breakup. These findings contrast with the generally held view that the oceanic crust has a uniform thickness.During the first 40 m.y. of spreading, the thicker oceanic crust, of density 2.86 g/cm³, displaces the denser (3.32 g/cm³) subjacent material; by isostasy, the spreading ridge and the rest of the seafloor thus stand higher in younger( <40m.y.) oceans than they do in older(>40m.y.) oceans. This is postulated to be the cause of the empirical relationship between the crestal depth of spreading ridges and the age (or half-width) of ocean basins.     

A rolling mill effect in asthenosphere beneath oceanic spreading centers

From structural studies in seventeen ophiolite massifs, information has been drawn about the activity of the asthenosphere beneath oceanic spreading centers. This information, together with geophysical data pertaining to oceanic ridges, has been integrated into a numerical model. It is inferred that for a fast-spreading ridge (< 5 cm/yr) a local diapiric uprise of melted peridodite is superimposed to the main circulation driven by the lithosphere drifting. As a result, the upward flow splits, at a depth of 35 km, into a 10–20 km wide axial jet and a horizontal flow diverging away from the ridge. The small diapir channels upward most of the partially molten peridotites. Magmatic activity in the crust is thus constrained to an area of similar horizontal extension, in agreement with ophiolitic and oceanic data. Also in agreement with findings in ophiolites, the meeting at a few tens of kilometers away from the ridge of the two diverging mantle circulations, creates a change in shear sense of the flow. From there on, the horizontal diverging flow is compatible with lithosphere drift.     

Implications of melting for stabilisation of the lithosphere and heat loss in the Archaean

The increased depth and volume of melting induced in a higher temperature Archaean mantle controls the stability of the lithosphere, heat loss rates and the thickness of the oceanic crust. The relationship between density distributions in oceanic lithosphere and the depth of melting at spreading centres is investigated by calculating the mineral proportions and densities of residual mantle depleted by extraction of melt fractions. The density changes related to compositional gradients are comparable to those produced by thermal effects for lithosphere formed from a mantle which is 200°C or more hotter than modern upper mantle. If Archaean continental crust formed initially above oceanic lithosphere, the compositional density gradients may be sufficient to preserve a thick Archaean continental lithosphere within which the Archaean age diamonds are preserved. The amount of heat advected by melts at mid-ocean ridges today is small but heat advected by melting becomes proportionally more important as higher mantle temperatures lead to a greater volume of melt and as the rate of production of oceanic plates increases. Archaean tectonics could have been dominated by spreading rates 2–3 times greater than now and with mantle temperatures between ca. 1600°C and 1800°C at the depth of the solidus. Mid-ocean ridge melting would produce a relatively thick but light refractory lithosphere on which continents could form, protected from copious volcanism and high mantle temperatures.     

Geoid anomalies over Gorringe Ridge,North Atlantic Ocean

Gorringe Ridge is a strong uplifted block of oceanic crust and upper mantle lying at the eastern end of the Azores-Gibraltar plate boundary. The geoid over this structure derived from Seasat altimeter data exhibits a 9-m height anomaly with a north-south lateral extension smaller than 200 km. An attempt is made to interpret this geoid together with the gravity anomalies and with the seismicity, which has been compiled as a function of depth.It is first shown that the flexure of the oceanic lithosphere due to the ridge loading does not provide a good fit of the geoid anomalies and probably should be discarded, as it assumes a continuous unfractured elastic plate.Models involving local heterogeneities are then tested. The comparison of the observed geoid anomalies with the anomalies due to the uncompensated relief indicates that the topographic high has no shallow compensation.Uncompensated models, previously proposed to explain the gravity anomalies, are tested using the geoid. One model (Purdy and Bonnin, in Bonnin [11]), which involves an uplift of upper mantle material at depth, generates too strong geoid anomalies and must be discarded. Another model, which represents a nascent subduction zone (Le Pichon et al. [25]), fits both the gravity and geoid anomalies, but leads to difficulties in explaining the deep seismicity north of Gorringe Ridge.A model in isostatic equilibrium is also able to fit both gravity and geoid anomalies. This model involves a deep root of density 3.0 g cm^?3, as has been previously proposed for many oceanic ridges and plateaus. This model is compatible with the deep seismicity, but the origin of this low-density material at great depth is up to now an unresolved question.More likely, dynamical models taking into account the forces induced by the convection related to the slow plate convergence in this area will have to be considered.