The elastic thickness of the lithosphere in the Pacific Ocean

In this study, we present determinations of the effective elastic thicknessT_e of the oceanic lithosphere along Pacific chains or archipelagoes.T_e is determined by computing the deflection of a continuous elastic plate under the load of volcanoes, and constrained by geoid heights provided by SEASAT. In the South Central Pacific, estimates of 14 km for the Marquesas and 6 km or less for the Pitcairn-Mururoa-Gloucester chain are in good agreement with a previous work in this region (Cook-Austral and Society chains). Around the Line Islands chain, SEASAT data reveal that the bathymetry is poorly known, preventing fine analysis. Meanwhile,T_e looks globally very low ( 6 km), except for three volcanoes but these results may be unreliable. The Easter chain features lowT_e values ( 6 km), with no noticeable variation along the chain. Higher values are found for a Samoan island, Manuae (24 km), and along the Hawaiian-Emperor seamounts chain (from 32 km at the eastern end of the chain to 21.5 km for the Hawaiian volcanoes, and from 25.5 to 15 km for the Emperor seamounts). The large number ofT_e estimates obtained in this study points out a noticeable difference between North and South Pacific results. Those from the North Pacific agree with the general trend (increase with the square root of age plate at loading time), while those from the South Central Pacific are much lower, according to their plate age. These lowT_e results from the South Pacific are only partly explained by taking account of thermal perturbations using the rejuvenation model. Therefore, these results then point out a regional difference in oceanic lithosphere.     

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.     

Denudational isostatic rebound of intraplate highlands: The lachlan river valley,Australia

A range of evidence from the Lachlan valley in the southeast Australian highlands is consistent with Neogene isostatic rebound in response to denudational unloading. This evidence is found along the inland edge of the highlands in the transition zone between the highlands proper and the Lachlan's inland alluviated valley and the intracratonic Murray Basin. The amounts and rates of uplift indicated by offsets of suballuvium bedrock profiles and the long profiles of Tertiary valley-filling basalts are consistent with modelling of denudational rebound using known rates of highland denudation and basinal sedimentation, and reasonable crustal properties. The modelling shows that weak to moderately strong strong lithosphere (effective elastic thickness, T_e = 1-25 km) and strong lithosphere (T_e = 100 km) are all consistent with the observed amounts of rebound. Strong lithosphere must be broken, however, to be consistent with the field data. Even in the Australian setting, which is characterized by very low rates of denudation, isostatic rebound in response to denudational unloading must be a significant factor in maintaining highland elevation and must be incorporated in models of long-term landscape evolution. It would be expected that denudational isostatic rebound would be an even more significant component of long-term landscape evolution in areas of higher denudation rates.     

Effective elastic thickness of the lithosphere from joint inversion in western China and its implications*

The western China lies in the convergence zone between Eurasian and Indian plates. It is an ideal place to study the lithosphere dynamics and tectonic evolutions on the continental Earth. The lithospheric strength is a key factor in controlling the lithosphere dynamics and deformations. The effective elastic thickness (T_e) of the lithosphere can be used to address the lithospheric strength. Previous researchers only used one of the admittance or coherence methods to investigate the T_e in the western China. Moreover, most of them ignored the internal loads of the lithosphere during the T_e calculation, which can produce large biases in the T_e estimations. To provide more reliable T_e estimations, we used a new joint inversion method that integrated both admittance and coherence techniques to compute the T_e in this study, with the WGM2012 gravity data, the ETOPO1 topographic data, and the Moho depths from the CRUST1.0 model. The internal loads are considered and investigated using the load ratio (F). Our results show that the joint inversion method can yield reliable T_e and F values. Based on the analysis of T_e and F distributions, we suggest (1) the northern Tibetan Plateau could be the front edge of the plate collision of Eurasian and Indian plates; (2) the southern and part of central Tibetan Plateau have a strong lithospheric mantle related to the rigid underthrusting Indian plate; (3) the southeastern Tibetan Plateau may be experiencing the delamination of lithosphere and upwelling of asthenosphere.     

Spatial variations of flexure parameters over the Tibet–Quinghai plateau

We investigate the Tibet–Quinghai plateau and the Tarim basin in terms of spatial variations of the elastic thickness (T_e) in the frame of the thin plate flexure model. The method of investigation makes use of a convolutive method, which allows high spatial resolution of the flexure properties and overcomes some of the problems tied to the spectral admittance/coherence methodologies. We study the relation between the topographic and subsurface loads and the observed crust–mantle interface (CMI) undulations, the latter having been obtained from gravity inversion. The gravity data used for the inversion are a unique set of high quality data available over the Chinese part of the plateau, and constitute the highest resolution grid today available in this impervious area. The gravity inversion is constrained by results from the study of the propagation of seismic waves. The two extensive sedimentary basins, the Tarim and the Qaidam basins, are modeled by forward gravity modeling. The oscillations of the CMI obtained from the gravity inversion agree well with those expected by loading the thin plate model of spatially variably elastic thickness with the surface and subsurface loads. It is found that the modeling of the sedimentary basins is essential in the flexure analysis. The spatial variations of elastic thickness correlate with the extensions of the different terrains that constitute the plateau. Most of the Tibet plateau has low T_e, varying in the bounds 10–30 km, with lower values in the Qiangtang terrain, where the T_e reaches 8 km. The Tarim and the Qaidam basins, Precambrian platforms overlain by sediments, are rigid and have a T_e of up to 110 km and 70 km, respectively. The flexural analysis distinctly discerns the Tibet plateau, with thick crust, part of which is molten, from the cratonic areas, the Tarim and Qaidam basins, which though of thinner crust, act as undeformable rigid blocks.     

喜马拉雅地区与天山地区地壳强地震的孕震因素探讨

利用哈佛大学GCMT数据中心和前人积累的历史地震资料(1962~2016年M W>4.0地震)以及Crust2.0地壳结构统计分析了喜马拉雅地区、天山地区的地壳区域构造与地震活动间的相关性。此外,利用GFZ地学研究中心提供的静态卫星重力模型GGM03S/EGM2008和地形模型Topo计算了2个地区的各类重力异常场,同时还模拟了不同地壳弹性参数下的重力异常场,结果表明喜马拉雅地区重力异常场在水平、垂直方向的梯度特征远大于天山地区的异常特征,且喜马拉雅地区的有效弹性板厚度Te(6~15km)小于天山地区的有效弹性板厚度Te(20~30km)。最后,利用喜马拉雅地区与天山地区的GPS震间三维形变场约束了断层运动模型,结果显示两者主前缘断裂的断层闭锁深度及应力积累状态存在较大的差异。因此认为,造成青藏高原及邻区的边界地壳区域地震活动性差异的动力学因素,与地壳有效弹性板厚度、孕震断层参数及区域应力积累状态等密切相关。     

Isostasy of the Eastern Anatolia (Turkey) and Discontinuities of its Crust

This paper presents a probable isostatic model of the East Anatolian Region, which lies in a belt of significant plate movements. Probable locations of the horizontal and vertical discontinuities in the crust structure were determined using the normalized full gradient (NFG) method. For the purpose of explaining the mechanism that supports topography corresponding to the crust thickness in the region, calculations of effective elastic thickness (T _e) were carried out initially by utilizing admittance and misfit functions. According to these results, the effective elastic thickness value obtained was less than the crust thickness, even though the isostatic model does not conform with the Airy model. Consequently, it was assumed that there could be problems beneath the crust. Hence, the NFG method was applied on the Bouguer gravity data of the region in order to investigate probable discontinuities in the crust structure. According to the NFG results, vertical structural transitions were observed at a depth ranging between 10 and 30 km, which begin immediately north of the Bitlis Zagros Suture Zone (BZSZ) and continue in a northerly direction. The relationship between the effective elastic thickness (T _e; 13 km in average as determined in the last stage), and the seismogenic zone in the region was investigated. If the T _e value happens to be less then the crustal thickness, then one can say that there are problems in the crustal structure of the region similar to Eastern Anatolia. Indeed, when NFG results of the study area are examined, numerous vertical and horizontal discontinuities in the crust can be observed. These discontinuities, which correspond to low Bouguer gravity anomalies and shallow focal depth-earthquakes, are probably the source of the factors which rule the tectonic mechanism of the region.     

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.     

Some remarks on isostasy and the long-term behavior of the continental lithosphere

The interpretation of linear transfer function (admittance) studies of the relationship between gravity anomalies and topography depends on what assumptions are made in limiting the acceptable set of solutions. However, whatever assumptions are made, the value of the flexural rigidity determined from admittance studies of Australia and the continental United States is less than about 10²⁸ dyne-cm (5-km plate thickness). This is much less than values determined from other studies of the loading of continental lithosphere which consistently give values in the range of 4 × 10³⁰ to 2 × 10³¹ dyne-cm (elastic plate thickness of 35–60 km). It is argued that the latter values represent the long-term rigidity of the continental lithosphere and that the rigidity does not change greatly with age of loading.     

The effective elastic thickness of the lithosphere in the collision zone between Arabia and Eurasia in Iran

The effective elastic thickness, T_e, has been calculated in the collision zone between Arabia and Eurasia in Iran from the wavelet coherence. The wavelet coherence is calculated from Bouguer anomalies and topography data using the isotropic fan wavelet method, and gives T_e values between 14.2 and 62.2 km. The lower value is found in the Central Iranian Blocks and the East Iranian Belt which are bounded by several large strike-slip faults with lithospheric origin. The higher value occurs in the east of the South Caspian Sea Basin. The resulting T_e map shows positive and negative correlation with shear wave velocity and surface heat flow, respectively. A comparison between the seismogenic thickness (T_s) and T_e in Iran suggests that T_e > T_s. Results of the load ratio in Iran indicate that in most of the study area surface loads are much more prevalent than subsurface loads, except in the Central Iranian Blocks and NW of Iran. Intermediate to low T_e values in Iran were inherited from multiple rifting and orogenic activities from Late Precambrian (∼650 Ma) to present day which are not only reflected in thin and warm lithosphere but also an increasing seismicity rate.     

Elastic thickness structure of South America estimated using wavelets and satellite-derived gravity data

We used a wavelet formulation of the classical spectral isostatic analysis to invert satellite-derived gravity and topography/bathymetry for elastic thickness (T_e) over South America and its surrounding plates. To provide a homogeneous representation of the gravity field for this vast region, we corrected free-air anomalies derived from a combination of terrestrial/marine gravity data with data from the GRACE and CHAMP satellite missions (model EIGEN-CG03C) by a simple Bouguer slab using a smoothed representation of surface relief (wavelengths > 125 km). The resulting Bouguer anomaly compares well with terrestrial data acquired in the Central Andes and allows T_e to be confidently estimated for values greater than 10 km. The T_e map resolves regional-scale features that are well-correlated with known surface structures and shows maximum values of 100 ± 15 km over the Archean–Neoproterozoic core of the continent, decreasing to less than 30 km around continental margins. Several regions of the oceanic plates and continental margins have an elastic thickness less than 10 km. We performed a quantitative analysis by comparing the elastic thickness with the thermal structure predicted from the age of oceanic crust and igneous–metamorphic rocks. This demonstrates that oceanic plates have been weakened by thermal interaction with hotspots and locally by fracturing and hydration near the trench. We observe that only the nucleus of the continent has resisted the thermomechanical weakening induced by the rifting of Africa and South America along the passive margin and the Andean orogeny along the active margin. This latter region shows along-strike variations in T_e that correlate with the geotectonic segmentation of the margin and with the pattern of crustal seismicity. Our results reveal that the rigidity structure follows the segmentation of the seismogenic zone along the subduction fault, suggesting a causal relationship that should be investigated in order to improve the understanding and predictability of great earthquakes and tsunamis.     

Effective elastic thickness of island arc lithosphere under Japan

Abstract Using topography and observed gravity anomalies, we have estimated the effective elastic thickness as a measure of strength of Japanese island arc lithosphere. The thickness is found to range from about 3 km to >20 km. The thickness seems to be controlled primarily by the thermal state of the lithosphere. The higher the heat flow, the thinner is the elastic plate. However, several areas show significant deviations. The smaller effective elastic thickness in the northern Ryukyu arc than that inferred from heat flow may be attributed to the stress regime. In Japan, extensional tectonics are going on only in the Ryukyu arc region. Shallow subducting slab under the south-western Japan frontal arc probably increases the effective thickness by several kilometers. The determined effective elastic thickness suggests that when we consider vertical movements in the volcanic arc, we should take account of topographic and subsurface loading over a few hundred kilometers. However, if the dip of the slab is shallow, the flexural responses of the underlying slab, not only that of the island arc lithosphere, should be taken into account for the compensation, as is the case of the south-western Japan frontal arc.     

Crustal structure and origin of the Cape Verde Rise

The Cape Verde Islands are located on a mid-plate topographic swell and are thought to have formed above a deep mantle plume. Wide-angle seismic data have been used to determine the crustal and uppermost mantle structure along a ~ 440 km long transect of the archipelago. Modelling shows that ‘normal’ oceanic crust, ~ 7 km in thickness, exists between the islands and is gently flexed due to volcano loading. There is no direct evidence for high density bodies in the lower crust or for an anomalously low density upper mantle. The observed flexure and free-air gravity anomaly can be explained by volcano loading of a plate with an effective elastic thickness of 30 km and a load and infill density of 2600 kg m^− 3. The origin of the Cape Verde swell is poorly understood. An elastic thickness of 30 km is expected for the ~ 125 Ma old oceanic lithosphere beneath the islands, suggesting that the observed height of the swell and the elevated heat flow cannot be attributed to thermal reheating of the lithosphere. The lack of evidence for high densities and velocities in the lower crust and low densities and velocities in the upper mantle, suggests that neither a crustal underplate or a depleted swell root are the cause of the shallower than expected bathymetry and that, instead, the swell is supported by dynamic uplift associated with the underlying plume.     

Three-dimensional admittance analysis of lithospheric elastic thickness over the Louisville Ridge

Using bathymetry and altimetric gravity anomalies, a 1° 9 1° lithospheric effective elastic thickness(Te) model over the Louisville Ridge and its adjacent regions is calculated using the moving window admittance technique. For comparison, three bathymetry models are used: general bathymetric charts of the oceans, SIO V15.1,and BAT_VGG. The results show that BAT_VGG is more suitable for calculating T e than the other two models. T e along the Louisville Ridge was re-evaluated. The southeast of the ridge has a medium Te of 10–20 km, while Te increases dramatically seaward of the Tonga-Kermadec trench as a result of the collision of the Pacific and IndoAustralian plates.     

Active deformation in the Vrancea region,Rumania

Recent and historical seismicity as well as reliable fault plane solutions are used to study the active deformation caused by the occurrence of intermediate depth (60–170 km) earthquakes of the Vrancea region, Rumania. In this area, located in the southeastern part of the Carpathian arc, the westward subduction of the Carpathian trench has terminated, leaving continental lithosphere, at present, at the arc. The principalT axis of the intermediate depth events trends N159°E and has a plunge of 74°, which is the same as the dip of the subducted plate. TheP axis has a trend of 314° and a shallow plunge of 15°. The analysis of the moment tensor of six focal mechanisms showed that the dominant mode of deformation of the subducted lithosphere is a down-dip extension at a rate of about 2 cm/yr, based on seismicity data.     

Isostatic Consequences of Giant Landslides on the Hawaiian Ridge

—Giant landslides, like melting glaciers, lead to a redistribution of mass which will have isostatic consequences. Three-dimensional numerical modeling experiments were devised to examine how this mass redistribution affects the isostatic flexural curve. A debris avalanche of 10–40% of pre-slide Oahu is required to account for the 1200–5000 km³ Nuuanu deposit, while only ～ 1% of pre-slide Hawaii Island is necessary to generate the 200–800 km³ Alika I and II avalanche deposits. Trials were run using 25, 30, and 40 km elastic plate thicknesses (T _e ). The island uplift resulting from the Nuuanu slide was calculated to be 23 m and 109 m for 10% and 40% volume slides, respectively, both using T _e = 25 km. A rebound of 10 m and 49 m was calculated for the same volumes, respectively, using T _e = 40 km. A greater amount of uplift is expressed direct lyover the failed flank, causing the edifice to tilt away from the calved-off portion. The landslide deposit depresses the plate several meters beneath the debris field itself. Smaller slides (e.g., Alika I and II) do not produce as much flexural response, with 17 m and 7 m uplift for T _e = 25 and 40 km, respectively. The effects of slow moving, intact slumps where the failed blocks remain relatively close to the island pedestal were examined for the case of the Hilina slump, making up approximately 10% of the Hawaii Island edifice. Perhaps more significant than the uplift for the Hilina slump, comparable to that for the 10% Nuuanu debris avalanche, is the 114 m and 56 m of downwarp beneath its massive slumped foot (T _e = 25 and 40 km, respectively). The landslide rebound process, in the case of a relatively large landslide, should be considered as an added component to the evolutionary course of oceanic islands.     

Dynamical geochemistry of the Hawaiian plume

This paper presents a simple dynamical model for melting and trace element distribution in the Hawaiian mantle plume. I model the plume as a partially molten stagnation point flow against the oceanic lithosphere, and obtain solutions for the temperature, melt migration rate, and trace element concentration within it. Trace element concentrations in the melt exceed simple batch melting predictions by up to 70%. The magnitude of this effect depends strongly on the solid-melt partition coefficientK. Trace elements with differentK therefore experience a “dynamical fractionation” within the plume, and incompatible trace element ratios such asLa/Ce always exceed the batch melting predictions. I suggest a simple model for plume-lithosphere interaction in which melts from these two sources mix in proportions determined by thermodynamic constraints. The model can explain the composition of basalts from Haleakala if the degree of melting of the lithosphereF₁ decreases with time from roughly 10% for tholeiites to 2% for alkalic basalts. These values are considerably higher than previous estimates ofF₁ < 1%, and imply correspondingly smaller and more realistic values ( 10 km) for the thickness of the melted part of the lithosphere. Partial melting of additional depleted sources such as the asthenosphere is therefore not required by the Haleakala data. Estimates ofF₁ are highly sensitive to the values chosen for the partition coefficients, however, and should therefore be interpreted with caution.     

Crustal and upper mantle structure of stable continental regions in North America and northern Europe

From an analysis of many seismic profiles across the stable continental regions of North America and northern Europe, the crustal and upper mantle velocity structure is determined. Analysis procedures include ray theory calculations and synthetic seismograms computed using reflectivity techniques. TheP wave velocity structure beneath the Canadian Shield is virtually identical to that beneath the Baltic Shield to a depth of at least 800 km. Two major layers with a total thickness of about 42 km characterize the crust of these shield regions. Features of the upper mantle of these region include velocity discontinuities at depths of about 74 km, 330 km, 430 km and 700 km. A 13 km thickP wave low velocity channel beginning at a depth of about 94 km is also present.A number of problems associated with record section interpretation are identified and a generalized approach to seismic profile analysis using many record sections is described. TheS wave velocity structure beneath the Canadian Shield is derived from constrained surface wave data. The thickness of the lithosphere beneath the Canadian and Baltic Shields is determined to be 95–100 km. The continental plate thickness may be the same as the lithospheric thickness, although available data do not exclude the possibility of the continental plate being thicker than the lithosphere.     

Geothermal profile and crust-mantle transition beneath east-central Queensland: Volcanology, xenolith petrology and seismic data

Geothermobarometry of garnet granulite and garnet websterite xenoliths in basalts from numerous localities in east-central Queensland gives P-T points that fall along the geotherm previously defined for southeastern Australia. This elevated geotherm is ascribed to the advective transport of heat by Tertiary-Recent magmas ponded at the crust-mantle boundary. The lower crust in this region consists dominantly of mafic granulites, representing frozen basaltic melts and cumulates. Spinel lherzolite becomes a dominant rock type at depths of ca. 30 km, and persists, interlayered with pyroxenites, to depths of ca. 55 km. Seismic reflection profiles show a “layered lower crust” between depths of 20 and 36 km depth. The lithologically defined crust-mantle boundary lies within this zone, at least 6 km above the seismically defined Moho. This interpretation is consistent with the observed velocity (V_p) gradient downward through the layered zone. The constructed geotherm implies that the bottom of the lithosphere beneath eastern Australia is shallower than ca. 100 km. This makes it unlikely that the diamonds of eastern Australia are derived from local intrusions, unless these are > 200 Ma old.