The seismicity of the Reykjanes Ridge

The distribution of earthquakes along the Reykjanes ridge is highly variable but follows a pattern which has remained essentially unchanged for the past fifty years. A number of possible interpretations are discussed.     

The Reykjanes Ridge between 63°10′N and Iceland

Along the Reykjanes Ridge from 63°10′N to 63°50′N we identify 10 en-echelon shallow closely spaced axial volcanic ridges (AVR). The AVRs are confined to the crest of the Reykjanes Ridge. This has been suggested to imply intense tectonic erosion of the ridges and subsequent sediment covering as they drift off axis. All AVRs studied are small and their length does not exceed that of individual eruptive fissures on land. Recent seismic activity is concentrated near Fuglasker seamount and the two AVRs in the Húllið graben. Tectonic strike changes drastically at the Skerjadjúp graben from N28°E to N40°E. Historic volcanic activity has been intense in this area with up to 14 eruptions, the last confirmed eruption occurring in 1926. New multibeam and backscatter data in the area of AVR 1 and the northernmost tip of AVR 2 show that the ridges were created by multiple eruptions, with clearly defined volcanic centres or cones, in contrast to monogenetic Pleistocene hyaloclastite ridges on land in Iceland with similar dimensions. In this area there was a series of eruptions during the years 1226–1238 in which the craters could have formed. The data also show normal faults in the Húllið graben. The western boundary faults have a drop of more than 25 m, while the eastern faults are smaller but denser.     

Seismic reflection survey of an oblique aseismic basement trend on the Reykjanes Ridge

A detailed (5 km track separation) seismic reflection survey of a portion of the upper flank of Reykjanes Ridge supports the existence of an oblique aseismic ridge, previously postulated from other data. The oblique basement ridge may have been formed by a magma center moving southwest under this portion of the Reykjanes Ridge at about 6 cm/yr between 7 and 5 mbyp. The oblique ridge is complex, being interrupted by saddles about every 30 km length. This spacing could reflect incipient, very weakly developed transverse fractures, or more probably the concentration of volcanic activity at particularly active vents, which shift southwestward every million years or so in response to the south-westward moving magma chambers entrained in the asthenosphere. Minor irregularities in the oblique ridge parallel crustal isochrons; such small features are probably elongate fissure eruptions restricted to a narrow spreading axis.     

A longitudinal seismic reflection profile of the Reykjanes Ridge: Part I — Evidence for west-flowing bottom water

A longitudinal seismic reflection profile along the east flank of Reykjanes Ridge, from Charlie fracture zone to the vicinity of Iceland, has important implications both for bottom water movement and for hypotheses of crustal generation at the axis of the mid-oceanic ridge. In this paper bottom water movement is considered. Between 52°N and 57°N Reykjanes Ridge is cut by about 12 fractures whose trend, inferred from other data, is approximately east-west. North of 57° there is little or no indication of east-west fracturing. Fracture valley bottoms are typically 1 km below the surrounding basement level; sediment fills are about 0.5 km; present bottoms are 2.1 to 2.8 km below sea level. Depositional asymmetry is apparent in 9 cases, 7 of which have the deepest and generally least reflective bottom at the northern edge. This suggests predominately west-flowing bottom currents, carrying Norwegian Sea overflow water through the fracture valleys, a result consistent with previously published data.     

A longitudinal seismic reflection profile of the Reykjanes Ridge: Part II — Implications for the mantle hot spot hypothesis

A longitudinal seismic reflection profile of the Reykjanes Ridge, together with earthquake seismicity patterns, is interpreted in terms of the mantle plume hypothesis. Between 52°N and 57°N Reykjanes Ridge is cut by about 12 fractures whose trend, inferred from other data, is approximately east-west. North of 57° there is little or no indication of east-west fracturing.The 57°N transition from fractured to unfractured basement occurs about 900 km southwest of the postulated Iceland mantle plume. The fractured province exhibits higher seismicity and rougher basement, on transverse profiles, than does the unfractured province. A similar transition to rougher, more seismic ridge crest also occurs 900 km northeast of Iceland. We propose that flowage of hot, basalt-rich asthenosphere away from the Iceland hot spot keeps the axial lithosphere hot, thin, sparsely fractured, and relatively aseismic out to 900 km from the plume. Similar effects are evident in the vicinity of some other plumes located near spreading axes. Some plumes also exhibit a greater number of earthquakes at some distance from the spreading axis — possibly a reflection of non-axial igneous activity or fracturing due to local, plume-generated stresses.The regional basement slope along the longitudinal profile is about 8 × 10^?4. If this slope represents a balance between viscous and gravity forces in the flow, a viscosity of the order 10¹⁹ poises can be estimated from the Poiseuille equation.A peculiarly flat, opaque reflector was discovered near the Reykjanes axis, about 300 km southwest of Iceland. Several hypotheses are advanced to account for such reflectors by the exceptional volcanic activity associated with high plume discharge.     

Multifractal characterization of airborne geophysical data at the Oak Ridge facility

Scaling analyses on geophysical measurements of electrical conductivity, gamma radiation, and magnetic fields, at the Oak Ridge Reservation were conducted. The electrical conductivity and magnetic data exhibited multifractality in the north-south and east-west directions. The radiation data were observed to be non-scaling; a variogram with a sill was found to be more appropriate. The scaling of the EC and magnetic was generally within a range smaller than the maximum distance selected, as periodicity dominated at the larger distances. The electrical conductivity had anisotropy in the scaling of their variograms. But the magnetic data appear to have an isotropic scaling. The underlying statistics of the fields were near Gaussian for the electrical conductivity, but essentially Gaussian for the magnetic data. In environmental hydrogeology, knowledge of the spatial distribution of the intrinsic permeability, K, is very helpful in understanding the transport and spreading of contaminant plumes. Our previous studies have shown that the subsurface permeability, K, is multifractal. Detailed measurement of K is costly. Hence, large data sets of value collected both on a fine scale and over large distances are rare. In this study, we hypothesize that geophysical data could be used indirectly as a surrogate measurement for K, for obtaining statistical information on scale limited K data, and perhaps, directly at sites where K and electrical conductivity are correlated.     

Crustal structure of the Reykjanes Ridge near 62°N,on the basis of seismic refraction and gravity data

Explosion deep seismic sounding data sections of high quality had been obtained with RV Meteor in the Reykjanes Iceland Seismic Project (RRISP77 [Angenheister, G., Gebrande, H., Miller, H., Goldflam, P., Weigel, W., Jacoby, W.R., Pálmason, G., Björnsson, S., Einarsson, P., Pavlenkova, N.I., Zverev, S., Litvinenko, I.V., Loncarecic, B., Solomon, S., 1980. Reykjanes Ridge Iceland Seismic Experiment (RRISP 77). J. Geophys. 47, 228–238]) which close an information gap near 62°N. Preliminary results were presented by Weigel [Weigel, W., 1980. Aufbau des Reykjanes Rückens nach refraktionsseismischen Messungen. In: Weigel, W. (Ed.), Reykjanes Rücken, Island, Norwegischer Kontinentalrand. Abschlusskolloquium, Hamburg zur Meteor-Expedition, vol. 45. DFG, Bonn, pp. 53–61], and here we report on the data and results of interpretation. Clear refracted phases to 90 km distance permit crustal and uppermost mantle structure to be modelled by ray tracing. The apparent P-wave velocities are around 4.5, 6–6.5, 7–7.6 and 8.2–8.7 km/s, but no wide-angle reflections have been clearly seen. Accompanying sparker reflection data reveal thin sediment ponds in the axial zone and up to 400 m thick sediments at 10 Ma crustal age. Ray tracing reveals the following model below the sediments: (1) a distinct, 1–2 km thick upper crust (layer 2A) with Vp increasing with age (to 10 Ma) from <3.4 to 4.9 km/s and with a vertical gradient of 0.1–0.2 km/s/km, (2) a lower crust or layer 3 beginning at depths of 2 (axis) to 4 km (10 Ma age) below sea level with 6.1–6.8 km/s and similar vertical gradients as above, (3) the lower crust bottoms at 5.2–9.5 km depth below sea level (0–10 Ma) with a marked discontinuity, underneath which (4) Vp rises from about 7.5–7.8 km/s (0–10 Ma) with a positive vertical gradient of, again, 0.1–0.2 km/s/km such that 8 km/s would be reached at 12 km and deeper near the axis. Our preferred interpretation is that the mantle begins at the distinct discontinuity (“Moho”), but a deeper “Moho” of Vp ≈ 8 km/s cannot be excluded. From Iceland southward to 60°N several experiments show a decrease of crustal thickness from 14 to 8 km. Velocity trends with age across the ridge reflect cooling and filling of cracks, and thickness trends probably suggest volcanic productivity variations as previously suggested.Gravity inversion concentrates on a profile across the ridge with the above seismic a priori information; with 0.2–0.5 km depth uncertainty it leads to a good fit (±2.5 mGal where seismic data exist). Best fitting densities are (in kg/m³) for sediments, 2180; upper crust, 2450–2570; lower crust, 2850–2940; mantle lithosphere, 3215–3240 with a deficit for an asthenospheric wedge of no more than −100 kg/m³. The morphological ridges and troughs superimposed on the SE ridge flank are partly correlated, partly anti-correlated with the Bouguer anomaly and suggest that variable crustal density variations accompany the morphology variations.     

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.     

The Reykjanes Ridge: structure and tectonics of a hot-spot-influenced, slow-spreading ridge, from multibeam bathymetry, gravity and magnetic investigations

We report a comprehensive morphological, gravity and magnetic survey of the oblique- and slow-spreading Reykjanes Ridge near the Iceland mantle plume. The survey extends from 57.9°N to 62.1°N and from the spreading axis to between 30 km (3 Ma) and 100 km (10 Ma) off-axis; it includes 100 km of one arm of a diachronous ‘V-shaped' or ‘chevron' ridge. Observed isochrons are extremely linear and 28° oblique to the spreading normal with no significant offsets. Along-axis there are ubiquitous, en-echelon axial volcanic ridges (AVRs), sub-normal to the spreading direction, with average spacing of 14 km and overlap of about one third of their lengths. Relict AVRs occur off-axis, but are most obvious where there has been least axial faulting, suggesting that elsewhere they are rapidly eroded tectonically. AVRs maintain similar plan views but have reduced heights nearer Iceland. They are flanked by normal faults sub-parallel to the ridge axis, the innermost of which occur slightly closer to the axis towards Iceland, suggesting a gradual reduction of the effective lithospheric thickness there. Generally, the amplitude of faulting decreases towards Iceland. We interpret this pattern of AVRs and faults as the response of the lithosphere to oblique spreading, as suggested by theory and physical modelling. An axial, 10–15 km wide zone of high acoustic backscatter marks the most recent volcanic activity. The zone's width is independent of the presence of a median valley, so axial volcanism is not primarily delimited by median valley walls, but is probably controlled by the lateral distance that the oblique AVRs can propagate into off-axis lithosphere. The mantle Bouguer anomaly (MBA) exhibits little mid- to short-wavelength variation above a few milliGals, and along-axis variations are small compared with other parts of the Mid-Atlantic Ridge. Nevertheless, there are small axial deeps and MBA highs spaced some 130 km along-axis that may represent subdued third-order segment boundaries. They lack coherent off-axis traces and cannot be linked to Oligocene fracture zones on the ridge flanks. The surveyed chevron ridge is morphologically discontinuous, comprising several parallel bands of closely spaced, elevated blocks. These reflect the surrounding tectonic fabric but have higher fault scarps. There is no evidence for off-axis volcanism or greater abundance of seamounts on the chevron. Free-air gravity over it is greater than expected from the observed bathymetry, suggesting compensation via regional rather than pointwise isostasy. Most of the observed variation along the ridge can be ascribed to varying distance from the mantle plume, reflecting changes in mantle temperature and consequently in crustal thickness and lithospheric strength. However, a second-order variation is superimposed. In particular, between 59°30′N and 61°30′N there is a minimum of large-scale faulting and crustal magnetisation, maximum density of seamounts, and maximum axial free-air gravity high. To the north the scale of faulting increases slightly, seamounts are less common, and there is a relative axial free-air low. We interpret the 59°30′N to 61°30′N region as where the latest chevron ridge intersects the Reykjanes Ridge axis, and suggest that the morphological changes that culminate there reflect a local temperature high associated with a transient pulse of high plume output at its apex.     

On the interpretation of near-bottom water temperature anomalies

A positive water temperature anomaly of 0.11°C and an inverse gradient of potential temperature of 1.5 × 10^?2°C/m has been measured at the TAG hydrothermal field in the rift valley of the Mid-Atlantic Ridge at latitude 26°N by means of a thermistor array towed between 2 and 20 m above the seafloor. This anomaly appears to be associated with hydrothermal discharge from the oceanic crust. The temperature data are interpreted in terms of (1) a steady, turbulent thermal plume rising in a homogeneous, neutrally buoyant medium, and (2) turbulent diffusion in the ocean-bottom boundary layer. The calculations indicate that the thermal output of the TAG anomaly area is of the order of several megawatts, which is of the same order of magnitude as some continental geothermal systems. The thermal output from the TAG anomaly area represents a significant fraction of the total heat loss resulting from the generation of new lithosphere at the Mid-Atlantic Ridge at 26°N.     

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.     

A geophysical interpretation of the 1883 Krakatau eruption

From the present submarine topography in the vicinity of the Krakatau Islands it is concluded that the focus of the large explosions was situated to the northwest of the present Rakata Island. The channel between Krakatau and Sebesi Islands was completely blocked by banks of volcanic material immediately after the eruptions, and it is suggested that this material was mainly lithic fragments.The explosion sequences of Krakatau are deduced from the records of sea-waves and air-waves observed at Jakarta. The large tsunami was caused by the most violent explosion, simultaneously with the largest air-waves. It is inferred that the origin of the tsunami was a sudden upheaval of the seawater due to a violent explosion and that the height of the tsunami near its source was 30–40 m. Energy of the explosion is estimated from analyses of the air-waves as one order of magnitude greater than that of the 1956 Bezymianny eruption; thus, the 1883 Krakatau eruption may have caused explosive removal of more than 10 km³ of material.The subsurface structure of Krakatau Islands after the 1883 eruption is deduced from gravity anomalies. It is concluded that at the bottom of the caldera there are deposits of low density in the shape of an inverted cone 8 km in diameter and 1 km in depth. From the residual gravity anomaly observed over the caldera, one can estimate the mass deficiency there. This allows estimates to be made as to the amount of ejecta. Although large uncertainties remain, these data indicate that explosive removal of material was the main process responsible for the disappearance of the northern half of the former Rakata (Krakatau) Island in the 1883 eruption.     

地球物理联合反演研究综述

地球物理联合反演由于使反演问题的非唯一性得到有效限制而越来越受到人们的重视。本文概述了联合反演的发展现状及实现的方法，并讨论了其发展趋势及其局限性，指出地球物理联合反演是地球物理数据分析的理想工具，而非线性联合反演方法则是地球物理联合反演发展的方向。     

A deep geophysical study in the Baikal region

Experimental data show that in East Siberia resistivity curves, irrespective of their trends, are affected by galvanic (local) distortions. The preliminary step of the magnetotelluric data processing is to obtain a steady shape of resistivity curves reflecting a true deep section. For this purpose statistical averaging and different criteria of impedance rejecting were used. The available MTS curves were normalized by level to the global magnetovariation curves. Two-dimensional modelling was performed from several sublatitudinal profiles crossing the Baikal rift zone. Three-dimensional models based on two-dimensional modelling and on induction vector distribution have been computed via programs of M. N. Yudin. Following other researchers, two conductive layers are distinguished: i) the mid- and low crustal and ii) the mantle one, with the layer surface uplifted from 100–110 km depth in the southern Baikal rift zone to 60–70 km northeastwards along the eastern Baikal coast. The top of this layer seems to correspond to the asthenospheric roof. The asthenosphere deepening in southern BRZ is likely to be related to a decrease in the asthenospheric bulge width and an increase in the rate of lithospheric thickening with mantle degasing. The origin and evolution of the Baikal rift is considered, proceeding from the model of passive rifting with regard to a long-existing lithospheric inhomogeneity between the Siberian platform and the Sayan-Baikal folded area.     

Ferrobasalts from the Spiess Ridge segment of the Southwest Indian Ridge

Highly vesicular, microporphyritic basaltic rocks have been dredged from the slow-spreading Spiess Ridge segment of the Southwest Indian Ridge. All the samples recovered are hyalocrystalline with plagioclase, clinopyroxene and olivine as phenocryst and microphenocryst phases. Titanomagnetite occurs as euhedral microphenocrysts in some of the more evolved samples. In terms of bulk rock and quench glass chemistry the lavas are characterised by highly evolved compositions(e.g. FeO*=10.3−14.2%;TiO₂=2.0−3.4%;K₂O=0.50−1.1%;MgO=6.0−3.5%;Zr=160−274ppm;Nb=14−32ppm) and can be classified as ferrobasalts. Isotopic and incompatible element ratios of the lavas(e.g.⁸⁷Sr/⁸⁶Sr=0.70325−0.70333;Zr/Nb=8.4−11.3;Y/Nb=2.3−1.4) indicate their strongly “enriched” nature (see also Dickey et al. [6]).

Quantitative major and trace element modelling indicates that most of the compositional variations observed can be attributed to low-pressure fractional crystallisation of plagioclase, clinopyroxene and minor olivine and titanomagnetite. The range in composition can be accounted for by up to 65% fractional crystallisation.

We suggest that the extreme differentiation of the Spiess Ridge lavas is related not to spreading rate, but to rate of magma supply. The basaltic melts appear to have evolved in a newly established zone of magmatic activity, associated with the most recent northward jump of the Bouvet triple junction, where they were effectively isolated from significant admixture of primitive magmas.     

发展技术,增强基础-地球物理仪器国际化与地球物理技术在工程上的应用研讨会综述

2004年,中国地球物理学会仪器专业委员会在北京召开了“地球物理仪器国际化问题”(4月)和“地球物理技术在工程上的应用”(8月)两个研讨会。会议发表了大量论文,展示了目前我国先进的地球物理仪器和地球物理技术在工程方面的应用成就．承《地球物理学进展》编辑部支持,决定在2004年第四期和2005年第一期,连续刊载经专家审查和推荐的地球物理仪器和技术方面的部分优秀论文,     

A Bayesian trans-dimensional approach for the fusion of multiple geophysical datasets

We propose a Bayesian fusion approach to integrate multiple geophysical datasets with different coverage and sensitivity. The fusion strategy is based on the capability of various geophysical methods to provide enough resolution to identify either subsurface material parameters or subsurface structure, or both. We focus on electrical resistivity as the target material parameter and electrical resistivity tomography (ERT), electromagnetic induction (EMI), and ground penetrating radar (GPR) as the set of geophysical methods. However, extending the approach to different sets of geophysical parameters and methods is straightforward. Different geophysical datasets are entered into a trans-dimensional Markov chain Monte Carlo (McMC) search-based joint inversion algorithm. The trans-dimensional property of the McMC algorithm allows dynamic parameterisation of the model space, which in turn helps to avoid bias of the post-inversion results towards a particular model. Given that we are attempting to develop an approach that has practical potential, we discretize the subsurface into an array of one-dimensional earth-models. Accordingly, the ERT data that are collected by using two-dimensional acquisition geometry are re-casted to a set of equivalent vertical electric soundings. Different data are inverted either individually or jointly to estimate one-dimensional subsurface models at discrete locations. We use Shannon's information measure to quantify the information obtained from the inversion of different combinations of geophysical datasets. Information from multiple methods is brought together via introducing joint likelihood function and/or constraining the prior information. A Bayesian maximum entropy approach is used for spatial fusion of spatially dispersed estimated one-dimensional models and mapping of the target parameter. We illustrate the approach with a synthetic dataset and then apply it to a field dataset. We show that the proposed fusion strategy is successful not only in enhancing the subsurface information but also as a survey design tool to identify the appropriate combination of the geophysical tools and show whether application of an individual method for further investigation of a specific site is beneficial.