K/Rb and (Sr/Sr)0 ratios of dredged submarine basalts

K/Rb and (⁸⁷Sr/⁸⁶Sr)₀ ratios were measured for 14 submarine basalts dredged from seamounts in the Pacific Ocean. The K/Rb ranges from 200 to 700, which is significantly lower than that of oceanic ridge tholeiites. Petrographic examination and the low value of K/Rb indicate that seamount basalts are alkaline.     

Crustal intrusion beneath the Louisville hotspot track

We report here the first detailed 2D tomographic image of the crust and upper mantle structure of a Cretaceous seamount that formed during the interaction of the Pacific plate and the Louisville hotspot. Results show that at ～ 1.5 km beneath the seamount summit, the core of the volcanic edifice appears to be dominantly intrusive, with velocities faster than 6.5 km/s. The edifice overlies both high lower crustal (> 7.2–7.6 km/s) and upper mantle (> 8.3 km/s) velocities, suggesting that ultramafic rocks have been intruded as sills rather than underplated beneath the crust. The results suggest that the ratio between the volume of intra-crustal magmatic intrusion and extrusive volcanism is as high as ～ 4.5. In addition, the inversion of Moho reflections shows that the Pacific oceanic crust has been flexed downward by up to ～ 2.5 km beneath the seamount. The flexure can be explained by an elastic plate model in which the seamount emplaced upon oceanic lithosphere that was ～ 10 Myr at the time of loading. Intra-crustal magmatic intrusion may be a feature of hotspot volcanism at young, hot, oceanic lithosphere, whereas, magmatic underplating below a pre-existing Moho may be more likely to occur where a hotspot interacts with oceanic lithosphere that is several tens of millions of years old.     

珍贝—黄岩海山链热-重力均衡动态调节机制

珍贝—黄岩海山链作为我国南海的残留扩张中心, 对其研究具有重要的科学意义. 本文运用均衡学方法, 通过重力异常数据反演了过珍贝—黄岩海山链剖面的地壳界面变化, 同时计算了岩石圈热结构状态, 在此基础上建立了珍贝—黄岩海山链的岩石圈地温结构模型. 通过均衡分析方法, 对剖面上测点的海底地形数据进行了热均衡和重力均衡分析, 得到了热均衡和重力均衡形变量. 结果表明, 在珍贝—黄岩海山链高热流区域, 热均衡作用可以产生最大约0.55 km的形变, 其重力均衡形变范围为0.77—1.89 km. 热均衡通过改变海底地形和地壳物质密度不断作用于重力均衡, 重力又反过来作用于热均衡, 形成了热均衡-重力均衡动态调节机制.      

Melting of enriched mantle beneath Pitcairn seamounts: Unusual U–Th–Ra systematics provide insights into melt extraction processes

U-series systematics as well as Sr isotopes were measured on young seamount lavas from the Pitcairn hotspot collected during the Polynaut cruise. The combined U-series and Sr isotope data reveal typical mixing relationships between two endmembers. One typical ‘plume’ endmember with radiogenic ⁸⁷Sr/⁸⁶Sr and relatively low ²³⁰Th/²³⁸U and a ‘lithosphere’ endmember with less radiogenic ⁸⁷Sr/⁸⁶Sr and relatively larger ²³⁰Th/²³⁸U. Remarkably, all the lavas, except for a few arguably older samples, are characterized by ²²⁶Ra deficits relative to ²³⁰Th. On the basis of water content and trace element systematics, we argue that this is due to melting in the presence of phlogopite, which is only stable at lithosphere temperatures. A melting model including the diffusive exchange of elements among phlogopite, garnet and melt is used to constrain melting conditions of the lithosphere. These unusual ²²⁶Ra–²³⁰Th signatures can be explained by relatively slow melting rates at low matrix porosity. Our model also demonstrates that the effective partitioning behavior is dependent on the melting rate. A simple thermal model for lithosphere heating and melting is in good agreement with predicted melting rates.     

Dynamics of intrathermocline vortices in a gyre flow over a seamount chain

The interaction of meddies with a complex distribution of seamounts is studied in a three-layer quasi-geostrophic model on the f-plane. This study aims at understanding if and how this seamount chain can represent a barrier to the propagation of these eddies and how it can be involved in their decay. The eddies are idealized as vortex patches in the middle layer, interacting with a regional cyclonic current and with ten idealized seamounts. The numerical code is based on the contour surgery technique. The initial position, radius, shape, number and polarity of the eddies are varied. The main results are the following: (1) Though they do not describe the unsteady flow, the streamlines of the regional and topographic flow provide a useful estimate of the vortex trajectories, in particular towards the major seamounts, where stronger velocity shears are expected. (2) The tallest and widest seamounts which have the largest vorticity reservoir are able to considerably erode the vortices, but also to draw anticyclones towards the seamount top. The ability of narrower seamounts to erode vortices is related to their multiplicity. (3) Only 1/3 of the anticyclones with about 30-km radius reach the southern boundary of the seamount chain, and their erosion is larger than 50 %. The other anticyclones are either completely eroded or trapped over a wide seamount top. Cyclones are less affected by seamounts because they oppose the topographic draft towards the seamount top and they drift along the side of the seamount. (4) Large vortices resist topographic erosion more efficiently. The rate of erosion grows from a few percent to about 35–50 % as the vortex radius decreases from about 60 to 30 km. Small cyclones are not eroded, contrary to small anticyclones (which completely decay), in relation with the different trajectories of these eddies in the vicinity of the seamounts. (5) The detailed vortex shape does not appear critical for their evolution, if they are close enough to the seamount chain initially. The interaction between a group of vortices initially north of the seamount chain can modify their trajectory to such an extent that they finally avoid collision with seamounts. (6) Finally, meddy trajectories across the Horseshoe Seamounts (data from the AMUSE experiment) show qualitative similarity with the vortex paths in the model. Several events of vortex decay also occur at comparable locations (in particular over the wide and tall seamounts) in the model and observations.     

A Paleogene magmatic overprint on Cretaceous seamounts of the western Pacific

The West Pacific Seamount Province (WPSP) represents a series of short-lived Cretaceous hotspot tracks. However, no intraplate volcanoes in advance of petit-spot volcanism erupted near a trench have been identified after the formation of the WPSP on the western Pacific Plate. This study reports new ages for Paleogene volcanic edifices within the northern WPSP, specifically the Ogasawara Plateau and related ridges, and Minamitorishima Island. These Paleogene ages are the first reported for basaltic rocks on western Pacific seamounts, in an area that has previously only yielded Cretaceous ages. The newly found Paleogene volcanisms overprint the Early–middle Cretaceous volcanic edifices, because the seamount or paleo-island material-covered reefal limestone caps on these edifices are uniformly older than the Paleogene volcanism identified in this study. This study outlines several possible causative factors for the Paleogene volcanism overprinting onto existing Cretaceous seamounts, including volcanism related to lithospheric stress, or a younger hotspot track within the northern part of the WPSP that records magmatism from ~60 Ma.     

Anomalous subsidence on the rifted volcanic margin of Pakistan: No influence from Deccan plume

The role of hotter than ambient plume mantle in the formation of a rifted volcanic margin in the northern Arabian Sea is investigated using subsidence analysis of a drill site located on the seismically defined Somnath volcanic ridge. The ridge has experienced > 4 km of subsidence since 65 Ma and lies within oceanic lithosphere. We estimate crustal thickness to be 9.5–11.5 km. Curiously < 400 m of the thermal subsidence occurred prior to 37 Ma, when subsidence rates would normally be at a maximum. We reject the hypothesis that this was caused by increasing plume dynamic support after continental break-up because the size of the thermal anomalies required are unrealistic (> 600 °C), especially considering the rapid northward drift of India relative to the Deccan-Réunion hotspot. We suggest that this reflects very slow lithospheric growth, possibly caused by vigorous asthenospheric convection lasting > 28 m.y., and induced by the steep continent–ocean boundary. Post-rift slow subsidence is also recognized on volcanic margins in the NE Atlantic and SE Newfoundland and cannot be used as a unique indicator of plume mantle involvement in continental break-up.     

Abundances,distribution and sizes of volcanoes in the Pacific Ocean and implications for the origin of non-hotspot volcanoes

In this paper I present data on the abundances, sizes and crustal age for all volcanoes (volcanic islands and seamounts) which appear on published bathymetric charts of the Pacific Ocean. These new data shed light on the origin of non-hotspot volcanoes and are important, in combination with data on the chemical compositions of seamounts and volcanic islands, for estimates of the bulk composition of ocean crust. These data also provide firm constraints on off-ridge oceanic volcanism models. Results of this study show that the size-frequency distribution of Pacific volcanoes is Poisson-like and that the smallest volcanoes are much more abundant than large ones. This study shows clearly that the most abundant volcanoes on the Earth are the submerged oceanic volcanoes which comprise 5–25% of the oceanic volcanic layer. On Pacific crust of Eocene age and younger, the abundance of volcanoes (number of volcanoes per unit area) increases monotonically with increasing age. Assuming steady state, the production rate of new off-ridge volcanoes (number of volcanoes per unit area per unit time) is inversely proportional to the square root of the lithosphere age [1]. On crust older than Eocene, the number of volcanoes per unit area of crust decreases monotonically with increasing age, however the total volume of lava represented by these edifices increases with increasing age. Size frequency distributions of volcanoes on swaths of successively older crust indicate that these abundance patterns are partly due to the effect of sediment burial of small edifices on old Pacific crust as well as the effect of increased lithosphere thickness on seamount size. These general patterns are not appreciably changed by omitting from consideration known hotspot volcanoes [2] and volcanoes built at fossil constructional plate margins [3].     

Geoid anomalies across fracture zones and the thickness of the lithosphere

Recent advances in the measurement and interpretation of geoid height anomalies provide a new way to estimate the thickness of the oceanic lithosphere as a function of crustal age. GEOS-III satellite altimetry measurements show abrupt changes in sea level across fracture zones which separate areas of lithosphere with different ages. These changes have the correct location, amplitude, and wavelength to be caused by the combined gravitational attraction of the relief across the fracture zone and the isostatic support of this relief. Eight profiles of geoid height and bathymetry across the Mendocino fracture zone are inverted to determine the depth of the isostatic compensation, assuming that the compensation occurs in a single layer. These depths are then interpreted with a thermal boundary layer model of lithospheric growth. To explain satisfactorily the geoid measurements, the thermal diffusivity of the upper mantle must be 3.3 × 10^?3 cm² s^?1 and the thickness of the lithosphere, defined as the depth at which the geotherm reaches 95% of its maximum value, must be9.1km m.y.^?1/2 × t^1/2, where t is lithospheric age.     

Origin and compensation of Chagos-Laccadive ridge, Indian ocean, from admittance analysis of gravity and bathymetry data

The Chagos-Laccadive ridge (CLR) is a prominent aseismic, volcanic ridge in the northern Indian ocean. The ridge, together with the Southern Mascarene plateau (SMP), to which it is genetically related, is considered as a volcanic trace of the Reunion hotspot. We have examined the isostatic compensation of the CLR through transfer function analysis of gravity and bathymetry data along seven profiles. The analysis suggests that the CLR is compensated locally, with an Airy crustal thickness (T_c) of 20 km. The rather low elastic plate thickness (T_e) of about 4 km implies that the volcanism of the ridge took place very near a spreading centre. The proximity of the Chagos fracture zone indicates that the emplacement was probably near a spreading centre-transform junction.     

Nonstationary dynamic control of seismic activity of platform regions by mid-ocean ridges

The plot of temporal variation in the seismic activity level in the central and eastern North American platform (NAP) is shown to be similar to that for the Mid-Atlantic Ridge (MAR). This fact was previously noted for Fennoscandia [Skordas et al., 1991]. The characteristic features of the MAR plot recur approximately every three years for Fennoscandia and every four to eight years for the NAP. These data indicate that the mid-ocean ridge largely controls the seismic activity of the adjacent platforms. The control is provided by the ridge push force. As a result of variations in this force due to the nonstationary process of dike intrusion in the axial zone of the ridge, disturbances of the stationary stress-strain state of the lithosphere migrate from the ridge. Using the Elsasser model, the observed time shift can be used for estimating the viscosity of the asthenosphere, amounting to 10¹⁷ Pa s with an accuracy of ±30% in the case considered. The disturbance amplitudes decaying away from the ridge are high enough to change the seismic activity of the adjacent platforms.     

Isostatic response of the Laccadive Ridge from admittance analysis of gravity and bathymetry data

The Laccadive Ridge (L-R), trending roughly parallel to the west coast of India, is an intriguing segment of the northernmost Chagos-Laccadive Ridge (C-L-R) system. Although crustal nature and isostatic response of the southern C-L-R is well known, there are no similar studies on the L-R. In the present study, the isostatic response of the lithosphere beneath the L-R is estimated so as to characterize its crustal nature, total crustal as well as effective elastic plate thickness and mode of compensation. Twelve gravity and bathymetry profiles across the ridge were analyzed using linear transfer function and forward model techniques. The observed admittance function within the diagnostic waveband of 250 < λ > 80 km (0.025 < k > 0.080 km⁻¹) fits well with (i) the Airy model whose average crustal thickness (T_c) and density are 17 ± 2 km and 2.7 × 10³ kg m⁻³, respectively, and (ii) the thin plate flexure model of isostasy with an effective elastic plate thickness (T_e) of 2–3 km. The estimated average crustal thickness and density are in good agreement with published seismic refraction results over the ridge. The results of the present study support an Airy model of isostasy for the L-R. The low T_e value, in view of other published results in the study area, suggests stretched and loaded continental lithosphere of the L-R during the evolution of the western continental margin of India.     

Thermal cooling of the oceanic lithosphere: new constraints from geoid height data

Up to now, tests of thermal models of the oceanic lithosphere as it cools and moves away from the ridge crest have been based mainly on topography and heat flow data. However, large areas of the ocean floor deviate from the normal subsidence due to thermal contraction and heat flow data are not very sensitive to the form of the model.

Cooling of the lithosphere causes a short-wavelength step in the geoid across fracture zones that can also be used to constrain thermal models. We have analyzed geoid data at fracture zones from the SEASAT altimeter measurements in the entire Pacific Ocean and redetermined parameters of the cooling models. We find that the data reveal two distinct regimes of cooling; one for seafloor ages in the range 0–30 Ma, the other beyond 30 Ma; this does not appear to be correlated with particular fracture zones but rather it is representative of the whole area studied, i.e., the entire south Pacific and northeast Pacific Ocean. These two trends may be interpreted in terms of two different (asymptotic) thermal thicknesses of the plate model. The smaller thermal thickness ( 65 km) found for ages <30 Ma—compared to 90 km in the age range 30–50 Ma—calls for some kind of thermal perturbation in the vicinity of the ridge crest.

From the results obtained in this study, we conclude that the half-space cooling model is unable to explain the data, that beyond 30 Ma, a simple plate model gives a satisfactory fit to the data but in the younger plate portion (ages < 30 Ma) the cooling history of the oceanic lithosphere is much more complex than predicted by the usual cooling models. Furthermore, the depth-age relationship obtained from the geoid-derived thermal parameters departs significantly beyond 30 Ma from the widely used Parsons and Sclater's depth-age curve, predicting a lesser subsidence.     

Fundamentals of ridge crest topography

A linear relationship between the sea floor depth and the square root of age has been found for ocean lithosphere spreading from mid-ocean ridges. The asymptotic solution of depth as a function of age for the thermally contracting lithosphere predicts a linear dependence of depth ontwith a proportionality involving the initial lithosphere temperature, the thermal diffusivity, and the isostatic expansion coefficient averaged to include any temperature dependent phase changes. Empirical depth observations, when plotted as a function of the square root of age, bear out this prediction well, but there is a variation in the gradient,ht, along the ridge on a fine scale (up to 20% over 200 km). This implies a fundamental variation of the contraction parameter over the same scale, most probably of compositional origin. Details of a more complete cooling model near the ridge crest, including a crust of different thermal parameters than those of the mantle, predict a crestal height about 0.2 km below that of the simplified model. Individual profiles from the southeast Pacific show no such crestal deviation, and it is concluded that by quickly cooling the new crust, hydrothermal circulation may remove any effects of the crust which would be seen in the topography of a lithosphere cooled totally by conduction. The straightness of depth versust for older ocean data (to 80 m.y.) precludes any basal isothermal boundary shallower than 100 km.     

Origin of Deccan Trap lavas: evidence from combined trace element and Sr-, Nd- and Pb-isotope studies

Geochemical and isotope results are presented from a new study of the most southern basalts in the Deccan Trap, India. Three chemical formations are recognised, two of which can be correlated with the established stratigraphy in Mahabaleshwar and imply a regional southerly dip of 0.06° over a distance of 250 km. In detail Sr-isotope variations within the Ambenali and Mahabaleshwar Formations can be shown to reflect three distinct end-members which provide new constraints for petrogenetic models. Pb-isotope data for selected basalts exhibit a wide range with²⁰⁶Pb/²⁰⁴Pb= 16.87–22.45, and a linear correlation on a Pb—Pb diagram. The least contaminated Ambenali basalts plot within the Pb-array, and interaction with mantle lithosphere involves a shift to less radiogenic Pb whereas contamination with crust is characterised by more radiogenic Pb. Unlike the Karoo and Parana continental flood basalt provinces only four flow units within the southern Deccan appear to contain a significant contribution from mantle lithosphere. The Mahabaleshwar and Ambenali Formation basalts exhibit a striking negative Pb—Sr isotope trend which is presently regarded as one of the features of interaction with shallow level lithospheric mantle. It further suggests that basalts from the Walvis Ridge, Kerguelen and Ninetyeast ridge all remobilised such shallow level material, and that the Deccan basalts which were not affected by crustal contamination reflect interaction between asthenospheric material similar to T-type MORB, but related to the Reunion hotspot, and continental mantle lithosphere of the Indian plate.     

Tectonic modelling in the Bjørnøya West Basin of the Western Barents Sea1

The Bjøirnøya West Basin lies between latitudes 73° and 74°, longitudes 16°E and 18°E, contains at least 8 km of sediments deposited from the Late Jurassic, and is of considerable interest for hydrocarbon exploration. The Cenozoic extensional tectonics in the basin can be clearly seen from seismic data with normal faulting and from subsidence curves with rapid subsidence. The extension occurred during the Late Palaeocene with active extension lasting about 6 million years (m.y.) followed by thermal cooling. The tectonic subsidence within the study area shows a three-phase development: phase 1, synrift (58–52 Ma (million years before the present day)), is characterized by rapid subsidence; phase 2, postrift (52–5 Ma), by slow subsidence with occasional uplift; and phase 3 (5–0 Ma), by rapid subsidence. An adaptive finite-element model, with consideration of the radiogenic heat production in the lithosphere, has been used to model the subsidence and heat flow. The modelling of subsidence shows the β-factor distribution varying from 1.9 to 3.5 with an average of 2.4 for the uniform lithospheric extension. The heat-flow modelling predicts a rapid increase of heat flow during the Early Palaeocene. The maximum heat flow at about 52 Ma, which could be as much as 3.0 hfu (10^?6 cal/cm²/s), was followed by a decrease in heat flow. A plate-weakening model has been proposed to explain the rapid subsidence for the last 5 m.y. by flexure of the elastic lithosphere which is weakened by a decrease in elastic thickness caused by an increase of the temperature gradient in the lithosphere. The plate-weakening model predicts a heat-flow increase at 5 Ma of up to 2.0 hfu. Our study, using quantitative modelling of the tectonic subsidence, provides a partial (if not a full) understanding of the tectonic development and thermal evolution of the Bjønøya West Basin.     

The effect of flow at Maud Rise on the sea-ice cover – numerical experiments

 The role of seamounts in the formation and evolution of sea ice is investigated in a series of numerical experiments with a coupled sea ice–ocean model. Bottom topography, stratification and forcing are configured for the Maud Rise region in the Weddell Sea. The specific flow regime that develops at the seamount as the combined response to steady and tidal forcing consists of free and trapped waves and a vortex cap, which is caused by mean flow and tidal flow rectification. The enhanced variability through tidal motion in particular modifies the mixed layer above the seamount enough to delay and reduce sea-ice formation throughout the winter. The induced sea-ice anomaly spreads and moves westward and affects an area of several 100 000 km². Process studies reveal the complex interaction between wind, steady and periodic ocean currents: all three are required in the process of generation of the sea ice and mixed layer anomalies (mainly through tidal flow), their detachment from the topography (caused by steady oceanic flow) and the westward translation of the sea-ice anomaly (driven by the time-mean wind).     

张裂大陆边缘形成演化的数值模拟

本文在等黏态角落流模型的基础上,建立了上升离散地幔流场,将该流场作用于大陆岩石圈底部,能解释岩石圈减薄、裂解并最终形成海底扩张和张裂大陆边缘等一系列过程.数值模拟的结果表明,岩石圈在上升离散地幔流的作用下发生依赖于深度的伸展减薄,表现为不同深度的拉张因子不同,地表热流显著升高,热扰动引起的均衡调整造成地表沉降,同时热扰动造成岩石圈流变强度尤其是在变形中心处显著减小,脆性变形临界深度变浅,而韧性变形范围扩大.在上升离散地幔流的持续作用下最终导致大陆裂解,岩石圈地幔出露,形成海底扩张和张裂大陆边缘.     

Ancient Os isotope signatures from the Ontong Java Plateau lithosphere: Tracing lithospheric accretion history

In order to better understand the nature and formation of oceanic lithosphere beneath the Early Cretaceous Ontong Java Plateau, Re–Os isotopes have been analysed in a suite of peridotite xenoliths from Malaita, Solomon Islands. Geological, thermobarometric and petrological evidence from previous studies reveal that the xenoliths represent virtually the entire thickness of the southern part of subplateau lithospheric mantle (< 120 km). This study demonstrates that vertical Os isotopic variations correlate with compositional variations in a stratified lithosphere. The shallowest plateau lithosphere (< 85 km) is dominated by fertile lherzolites showing a restricted range of ¹⁸⁷Os/¹⁸⁸Os (0.1222 to 0.1288), consistent with an origin from ~ 160 Ma Pacific lithosphere. In contrast, the basal section of subplateau lithospheric mantle (~ 95–120 km) is enriched in refractory harzburgites with highly unradiogenic ¹⁸⁷Os/¹⁸⁸Os ratios ranging from 0.1152 to 0.1196, which yield Proterozoic model ages of 0.9–1.7 Ga. Although the whole range of Os isotope compositions of Malaita peridotites is within the variations seen in modern abyssal peridotites, the contrasting isotopic compositions of shallow and deep plateau lithosphere suggest their derivation from different mantle reservoirs. We propose that the subplateau lithosphere forms a genetically unrelated two-layered structure, comprising shallower, typical oceanic lithosphere underpinned by deeper impinged material, which included a component of recycled Proterozoic lithosphere. The impingement of residual but chemically heterogeneous mantle, mechanically coupled to the recently formed, thin lithosphere, may have a bearing on the anomalous initial uplift and late subsidence history of the seismically anomalous plateau root.     

Why the Sacramento Delta area differs from other parts of the great valley: Numerical modeling of thermal structure and thermal subsidence of forearc basins

Data on present-day heat flow, subsidence history, and paleotemperature for the Sacramento Delta region, California, have been employed to constrain a numerical model of tectonic subsidence and thermal evolution of forearc basins. The model assumes an oceanic basement with an initial thermal profile dependent on its age subjected to refrigeration caused by a subducting slab. Subsidence in the Sacramento Delta region appears to be close to that expected for a forearc basin underlain by normal oceanic lithosphere of age 150 Ma, demonstrating that effects from both the initial thermal profile and the subduction process are necessary and sufficient. Subsidence at the eastern and northern borders of the Sacramento Valley is considerably less, approximating subsidence expected from the dynamics of the subduction zone alone. These results, together with other geophysical data, show that Sacramento Delta lithosphere, being thinner and having undergone deeper subsidence, must differ from lithosphere of the transitional type under other parts of the Sacramento Valley. Thermal modeling allows evaluation of the rheological properties of the lithosphere. Strength diagrams based on our thermal model show that, even under relatively slow deformation (10⁻¹⁷ s⁻¹), the upper part of the delta crystalline crust (down to 20–22 km) can fail in brittle fashion, which is in agreement with deeper earthquake occurrence. Hypocentral depths of earthquakes under the Sacramento Delta region extend to nearly 20 km, whereas, in the Coast Ranges to the west, depths are typically less than 12–15 km. The greater width of the seismogenic zone in this area raises the possibility that, for fault segments of comparable length, earthquakes of somewhat greater magnitude might occur than in the Coast Ranges to the west. The text was submitted by the authors in English.