Comparison of gravimetric and seismic constraints on the structure of the Aegean lithosphere in the forearc of the Hellenic subduction zone in the area of Crete

The island of Crete is located in the forearc of the Hellenic subduction zone, where the African lithospheric plate is subducting beneath the Eurasian one. The depth of the plate contact as well as the internal structure of the Aegean plate in the area of Crete have been a matter of debate. In this study, seismic constrains obtained by wide-angle seismic, receiver function and surface wave studies are discussed and compared to a 3D density model of the region.The interface between the Aegean continental lithosphere and the African one is located at a depth of about 50 km below Crete. According to seismic studies, the Aegean lithosphere in the area of Crete is characterised by strong lateral, arc–parallel heterogeneity. An about 30 km thick Aegean crust is found in central Crete with a density of about 2850 kg/m³ for the lower Aegean continental crust and a density of about 3300 kg/m³ for the mantle wedge between the Aegean crust and the African lithosphere. For the deeper crust in the area of western Crete two alternative models have been proposed by seismic studies. One with an about 35 km thick crust and another one with crustal velocities down to the plate contact. A grid search is performed to test the consistency of these models with gravimetric constraints. For western Crete a model with a thick lower Aegean crust and a density of about 2950 kg/m³ is favoured. The inferred density of the lower Aegean crust in the area of Crete correlates well with S-wave velocities obtained by surface wave studies.Based on the 3D density model, the weight of the Aegean lithosphere is estimated along an E–W oriented profile in the area of Crete. Low weights are found for the region of western Crete.     

Seismicity at the convergent plate boundary offshore Crete,Greece, observed by an amphibian network

We investigate microseismic activity at the convergent plate boundary of the Hellenic subduction zone on- and offshore south-eastern Crete with unprecedented precision using recordings from an amphibian seismic network. The network configuration consisted of up to eight ocean bottom seismometers as well as five temporary short-period and six permanent broadband stations on Crete and surrounding islands. More than 2,500 local and regional events with magnitudes up to M _L = 4.5 were recorded during the time period July 2003–June 2004. The magnitude of completeness varies between 1.5 on Crete and adjacent areas and increases to 2.5 in the vicinity of the Strabo trench 100 km south of Crete. Tests with different localization schemes and velocity models showed that the best results were obtained from a probabilistic earthquake localization using a 1-D velocity model and corresponding station corrections obtained by simultaneous inversion. Most of the seismic activity is located offshore of central and eastern Crete and interpreted to be associated with the intracrustal graben system (Ptolemy and Pliny trenches). Furthermore, a significant portion of events represents interplate seismicity along the NNE-ward dipping plate interface. The concentration of seismicity along the Ptolemy and Pliny trenches extends from shallow depths down to the plate interface and indicates active movement. We propose that both trenches form transtensional structures within the Aegean plate. The Aegean continental crust between these two trenches is interpreted as a forearc sliver as it exhibits only low microseismic activity during the observation period and little or no internal deformation. Interplate seismicity between the Aegean and African plates forms a 100-km wide zone along dip from the Strabo trench in the south to the southern shore-line of Crete in the north. The seismicity at the plate contact is randomly distributed and no indications for locked zones were observed. The plate contact below and north of Crete shows no microseismic activity and seems to be decoupled. The crustal seismicity of the Aegean plate in this area is generally confined to the upper 20 km in agreement with the idea of a ductile deformation of the lower crust caused by a rapid return flow of metamorphic rocks that spread out below the forearc. In the region of the Messara half-graben at the south coast of central Crete, a southward dipping seismogenic structure is found that coalesces with the seismicity of the Ptolemy trench at a depth of about 20 km. The accretionary prism south of Crete indicated by the Mediterranean Ridge showed no seismic activity during the observation period and seems to be deforming aseismically.     

南海北部珠江口—琼东南盆地地壳速度结构与几何分层

基于南海北部大陆边缘珠江口—琼东南盆地深水区实施的14条近垂直深反射地震探测叠加速度谱,利用Dix公式将叠加速度剖面转换为地壳层速度剖面,并利用时深转换方法构建了深度域地壳层速度模型,综合各地壳速度剖面分析了南海北部大陆边缘珠江口与琼东南盆地不同深度层次的P波速度变化趋势以及地壳几何分层特征.结果表明,琼东南盆地区可分为4～8 km沉积层（V_P为1.7～4.7 km/s）、4～10 km厚的上地壳层（V_P为5.2～6.3 km/s）、5 km〗左右的下地壳层（V_P为6.4～7.0 km/s）以及2～6 km厚的高速下地壳底层（V_P＞7.0 km/s）.V_P＞7.0 km/s下地壳高速层的存在被认为是岩石圈伸展、下地壳底部底辟构造或者是残存的原始华夏下地壳基性层的地震学指示;综合研究区地球物理探测成果构建了跨越华南大陆与南海北部陆坡区剖面莫霍和岩石圈底界图像,揭示出岩石圈上地幔在华南大陆与南海北部大陆边缘的减薄特征.     

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.     

Deformation and stress regimes in the Hellenic subduction zone from focal Mechanisms

Fault plane solutions for earthquakes in the central Hellenic arc are analysed to determine the deformation and stress regimes in the Hellenic subduction zone in the vicinity of Crete. Fault mechanisms for earthquakes recorded by various networks or contained in global catalogues are collected. In addition, 34 fault plane solutions are determined for events recorded by our own local temporary network on central Crete in 2000–2001. The entire data set of 264 source mechanisms is examined for types of faulting and spatial clustering of mechanisms. Eight regions with significantly varying characteristic types of faulting are identified of which the upper (Aegean) plate includes four. Three regions contain interplate seismicity along the Hellenic arc from west to east and all events below are identified to occur within the subducting African lithosphere. We perform stress tensor inversion to each of the subsets in order to determine the stress field. Results indicate a uniform N-NNE direction of relative plate motion between the Ionian Sea and Rhodes resulting in orthogonal convergence in the western forearc and oblique (40–50^∘) subduction in the eastern forearc. There, the plate boundary migrates towards the SE resulting in left-lateral strike-slip faulting that extends to onshore Eastern Crete. N110^∘E trending normal faulting in the Aegean plate at this part is in accordance with this model. Along-arc extension is observed on Western Crete. Fault plane solutions for earthquakes within the dipping African lithosphere indicate that slab pull is the dominant force within the subduction process and responsible for the roll-back of the Hellenic subduction zone.     

New constraints on lithospheric thickness of the Iranian plateau using converted waves

The study of mantle lithosphere plays a key role to reveal predominant tectonic setting process of a region. The current geological and tectonic setting of Iran is due to the ongoing continental–continental collision of the Arabian and Eurasian plates. We applied a combined P and S receiver function analysis to the teleseismic data of nine permanent broadband seismic stations of the International Institute of Earthquake Engineering and Seismology located in different tectonic zones of Iranian plateau. More than 4 years of data were used to estimate the thickness of the crust and mantle lithosphere. According to our results, the crust is 50 km thick beneath the Zagros fold and thrust belt (ZFTB). We found the maximum Moho depth of approximately 70 km under the Sanandaj-Sirjan zone (SSZ) indicating the overthrusting of the crust of Central Iran onto the Zagros crust along the main Zagros thrust (MZT). Below the northeasternmost part of the Urumieh–Dokhtar Magmatic Arc (UDMA) and Central Iran, the Moho becomes shallower and lies at 40 km depth. Towards northeast, beneath the Alborz zone, the crust is 55 km thick. Based on S receiver functions, we provided new insights into the thickness of the Arabian and Eurasian lithospheres. The location of the boundary between these plates was estimated to be beneath the SSZ, which is slightly shifted northeastward relative to the surficial expression of the MZT. Furthermore, the Arabian plate is characterized by the relatively thick lithosphere of about 130 km beneath the ZFTB reaching 150 km beneath the SSZ, where the thickest crust was also observed. This may imply that the shortening across the Zagros is accommodated by lithospheric thickening. In contrast, UDMA and Central Iran are recognized by the thin lithosphere of about 80–85 km. This thin lithosphere may be associated with the asthenospheric upwelling caused by either lithospheric delamination or Neo-Tethys slab detachment beneath the Zagros collision zone.     

A method of estimating the Moho density contrast with a tentative application of EGM08 and CRUST2.0

Based on Vening Meinez-Moritz global inverse isostatic problem, the Moho density contrast is formulated as that of finding a solution of a Fredholm integral equation of the first kind. We present solutions to this equation by combining global models of gravity (EGM08), topography (DTM2006) and seismic crust (CRUST2.0) to a resolution of 2°×2°. The test computations yielded Moho density contrasts ranging from 81.5 kg/m3 (in Pacific) to 988 kg/m³ (Tibet), with averages of 678 ± 78 and 334 ± 108 kg/m³ for continental and oceanic regions, respectively, and a global average of 448 ± 187 kg/m³. Estimated Moho depths range from 8 to 75 km with continental and oceanic averages of 36.6 ± 5.3 km and 12.9 ± 5.8 km, respectively, and a global average of 21 ± 12.5 km.     

Some consequences of the differences in seismic properties in old and young continental plates for isostatic compensation

We present a short review of seismological data and show that a difference of upper mantle structure is clearly resolved between old and young continental plates. An Archean plate is approximately 1 second faster than a Paleozoic plate for P waves. Such a change in plate structure implies that the density of an Archean lithosphere should be greater than the density of a Paleozoic lithosphere, if they have the same chemical composition. Applying isostasy, we would expect a Precambrian plate to sink and its surface to stand at a level 8 km beneath that of a Paleozoic plate. We first check that the weight of a crustal column is not smaller for a Precambrian crust than for a Paleozoic crust. Explosion seismology data show that isostatic equilibrium is nearly achieved in the crust. Therefore a compensating mechanism has to be acting in the subcrustal lithosphere. Two hypotheses are presented. Provided that the chemical composition of the lower lithosphere does not vary with time, a simple condition for a continental plate not to sink when aging, would be that the density of the lower lithosphere be ?0.1 g/cm³ smaller than the density of the asthenosphere. Another condition that would prevent plates to sink would be a lowering of the density of the lower lithosphere with age due for example to a variation of its chemical composition and to a reduction in the mean atomic weight. Both hypotheses imply that isostatic equilibrium of old continental plates is maintained by a geochemical mechanism. Although thermal evolution is a key factor for explaining the evolution of continents, it has to be complemented by a hypothesis on the change of the chemistry of the lower continental lithosphere with respect to the asthenosphere and/or with respect to age.     

Dalrymple Trough: An active oblique-slip ocean–continent boundary in the northwest Indian Ocean

The Dalrymple Trough marks part of the transform plate boundary between India and Arabia in the northern Arabian Sea. Oblique extension is presently active across this portion of the boundary at a rate of a few millimetres per year, and seismic reflection profiles across the trough confirm that it is an extensional structure. We present new swath bathymetric and wide-angle seismic data from the trough. The bathymetric data show that the trough is bounded by a single, steep, 3-km-high scarp to the southeast and a series of smaller, en-echelon scarps to the northwest. Wide-angle seismic data show that a typical oceanic crustal velocity structure is present to the northwest, with a crustal thickness of ~ 6 km. There is an abrupt change in crustal thickness and velocity structure at the northwestern edge of the trough, and the trough itself is underlain by 12-km-thick crust interpreted as thinned continental crust. Therefore we infer that Dalrymple Trough is an unusual obliquely extending plate boundary at which continental crust and oceanic crust are juxtaposed. The extensional deformation is focused on a single major fault in the continental lithosphere, but distributed over a region ~ 60 km wide in the oceanic lithosphere.     

Searching for Regional Crustal Velocity-Density Relations with the Use of 2-D Gravity Modelling – Central Europe Case

In this paper we search for a reference relation between seismic P-wave velocity V and density ρ _ref for the continental crust. Based on the results of modern seismic experiments, we compiled 2-D seismic models into a network of four, each about 1100–1400 km long, continental-scale seismic transects cutting all main tectonic units in Central Europe. The Moho depth (about 52 km beneath the TESZ in SE Poland, to about 25 km beneath the Pannonian Basin) and the crustal structure of this area are characterised by a large variation. This structural variation provides an interesting basis for gravity studies and especially for analysing the difference of the density structure between two major tectonic provinces of distinctive age difference: Precambrian and Phanerozoic. The 2-D gravity modelling applied for crustal cross-sections representing the regional structure, based on a unified gravity anomaly map of the area, allows for a stable determination of some general features of the regional reference velocity-density relation for the continental crust. In general three major seismo-petrological types of rocks can be distinguished: sediments, crystalline crust and mantle. In compacted sediments the reference velocity-density relation is well described by the Gardner or Nafe-Drake model. Calculated gravity anomalies, using unified velocity-density relation for the whole crystalline crust, well describe observed anomalies, with an average difference of 14 mGal. However, calculated gravity anomalies, using separated velocity-density relations for the crystalline crust of Precambrian and Phanerozoic Europe, describe observed anomalies better than for the entire crust, with an average difference 12 mGal. The most important feature of these relations is the large differentiation of the derivative dρ _ref /dV in the crystalline crust, being about 0.3 g s/m⁴ for Precambrian, and about 0.1 g s/m⁴ for the Phanerozoic crystalline crust. The modelling suggests a very small density value in the uppermost mantle ρ = 3.11 g/cm³ below the younger area, while for the older area it is ρ = 3.3 g/cm³.     

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.     

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.     

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.     

Volcano spacings and lithospheric/crustal thickness in the Archaean

There is a linear relationship between the spacing of Pliocene-Pleistocene volcanoes and the thickness of the lithosphere and attenuated crust in the East African rift valley. Assuming that the physical-chemical properties of the Archaean and Cenozoic lithosphere and crust were broadly similar, we use the spacing of volcanic centres in the Abitibi greenstone belt of southern Canada to determine lithospheric and crustal thickness in the Archaean. The abitibi volcanoes have been deformed and so have elliptical cross-sections. In order to arrive at their original form we have removed the effects of tectonic strain by two alternative mechanisms of pure and simple shear which give comparable results. A mean original volcano spacing of 84–88 km suggests that the lithsophere was 80–90 km thick and that the crust was probably 35–45 km thick in this greenstone belt about 2700 m.y. ago. The crustal values are comparable with those determined by geochemical parameters and are consistent with the suggestion that greenstone belts formed in extensional marginal basins between crustal-thickened continental masses, deep sections of which are now seen in Archaean high-grade regions.     

3D Crustal and Lithospheric Structures in the Southeastern Mediterranean and Northeastern Egypt

Crustal and lithospheric thicknesses of the southeastern Mediterranean Basin region were determined using 3D Bouguer and elevation data analysis. The model is based on the assumption of local isostatic equilibrium. The calculated regional and residual Bouguer anomaly maps were employed for highlighting both deep and shallow structures. Generally, the regional field in the area under study is considered to be mainly influenced by the density contrast between the crust and upper mantle. Use of the gravity and topographic data with earthquake focal depths has improved both the geometry and the density distribution in the 3-D calculated profiles. The oceanic-continental boundary, the basement relief, Moho depth and lithosphere-asthenosphere boundary maps were estimated. The results point to the occurrence of thick continental crust areas with a thickness of approximately 32 km in northern Egypt. Below the coastal regions, the thickness of crust decreases abruptly (transition zone). An inverse correlation between sediment and crustal thicknesses shows up from the study. Furthermore, our density model reveals the existence of a continental crustal zone below the Eratosthenes Seamount block. Nevertheless, the crustal type beneath the Levantine basin is typically oceanic; this is covered by sedimentary sequences more than 14 km thick. The modeled Moho map shows a depth of 28–30 km below Cyprus and a depth of 26–28 km beneath the south Florence Rise in the northern west. However, the Moho lies at a constant shallow depth of 22–24 km below the Levantine Basin, which indicates thinning of the crust beneath this region. The Moho map reveals also a maximum depth of about 33–35 km beneath both the northern Egypt and northern Sinai, both of which are of the continental crust. The resulting mantle density anomalies suggest important variations of the lithosphere-asthenosphere boundary (LAB) topography, indicating prominent lithospheric mantle thinning beneath south Cyprus (LAB ~90 km depth), followed by thickening beneath the Eratosthenes seamount, Florence Rise, Levantine Basin and reaching to maximum thickness below Cyprian Arc (LAB ~115–120 km depth), and further followed by thinning in the north African margin plate and north Sinai subplate (LAB ~90–95 km depth). According to our density model profiles, we find that almost all earthquakes in the study area occurred along the western and central segments of the Cyprian arc while they almost disappear along the eastern segment. The active subduction zone in the Cyprian Arc is associated with large negative anomalies due to its low velocity upper mantle zone, which might be an indication of a serpentinized mantle. This means that collision between Cyprus and the Eratosthenes Seamount block is marked by seismic activity. Additionally, this block is in the process of dynamically subsiding, breaking-up and being underthrusted beneath Cyprus to the north and thrusted onto the Levantine Basin to the south.     

The lithosphere in the central-eastern Mediterranean area

The lithosphere beneath the central-eastern Mediterranean area has been investigated by the inversion of the regional dispersion relations derived from analysis of surface waves. It is possible to distinguish several types of crust with average S-wave velocities in the range 3.0–3.8 km/sec, and thicknesses varying from a minimum of about 30 km, which corresponds to the Apennines, Crete and Otranto Channel regions, to a maximum of about 51 km beneath the Ionian Sea, which can be considered as a submerged continent. Associated with these crustal features, large lateral variations have been detected in the lithosphere thickness, which varies from a minimum of about 30 km corresponding to the Tyrrhenian Sea and south of Crete to a maximum of about 130 km corresponding to south-eastern Alps and north-central Greece, while the sub-Moho S-wave velocity varies in the range 4.2–4.8 km/sec. The constraint furnished by our results to the geological-tectonic setting of the investigated area, characterized by the continent continent collision between Africa and Europe, is pointed out.Publication No. 405, P. F. Geodinamica, CNR, Roma, Italy.     

喜马拉雅造山带西部拉昂错蛇绿岩带区域地壳深部结构及其可能构造归属研究

前人研究表明喜马拉雅造山带西部出露的拉昂错蛇绿混杂岩为新特提斯洋壳岩石圈的一部分,代表了新特提斯洋的关闭及其随后大洋岩石圈物质的仰冲.鉴于拉昂错蛇绿岩的构造演化历史尚不明确,前期对于拉昂错蛇绿岩带构造归属的研究主要基于岩石学研究和地表地质调查等,缺少精细的深部地壳结构进行运动学指示,因此证明拉昂错蛇绿混杂岩体的构造归属并非易事.本次研究中,我们对前期获得的一条南北向延伸穿过雅鲁藏布江缝合带和喜马拉雅造山带西部拉昂错蛇绿岩体的112 km长的深反射地震剖面进行了构造解释.高分辨率的深反射地震剖面清晰地显示了喜马拉雅山脉西部造山带内发育良好的地壳双冲构造几何结构,该地壳尺度双冲构造将印度俯冲地壳物质从底部运移到上部.同时,地震剖面还显示拉昂错蛇绿岩体和雅鲁藏布江蛇绿岩体在上地壳深处呈倾向相反但底部相通的结构构造.结合前人的岩石学/地球化学/地表地质研究成果,我们认为拉昂错蛇绿岩体为雅鲁藏布江缝合带蛇绿岩体的一部分.印度俯冲前缘的双冲构造折返将深部物质带到地表过程的同时,还将部分雅鲁藏布江蛇绿混杂岩携带至南侧距主缝合带位置大约20 km的拉昂错蛇绿岩区域.     

Insights into the Crustal Structure and Geodynamic Evolution of the Southern Granulite Terrain,India, from Isostatic Considerations

The Southern Granulite Terrain of India, formed through an ancient continental collision and uplift of the earth’s surface, was accompanied by thickening of the crust. Once the active tectonism ceased, the buoyancy of these deep crustal roots must have supported the Nilgiri and Palani-Cardamom hills. Here, the gravity field has been utilized to provide new constraints on how the force of buoyancy maintains the state of isostasy in the Southern Granulite Terrain. Isostatic calculations show that the seismically derived crustal thickness of 43–44 km in the Southern Granulite Terrain is on average 7–8 km more than that required to isostatically balance the present-day topography. This difference cannot be solely explained applying a constant shift in the mean sea level crustal thickness of 32 km. The isostatic analysis thus indicates that the current topography of the Southern Granulite Terrain is overcompensated, and about 1.0 km of the topographic load must have been eroded from this region without any isostatic readjustment. The observed gravity anomaly, an order of magnitude lower than that expected (−125 mGal), however, shows that there is no such overcompensation. Thermal perturbations up to Pan-African, present-day high mantle heat flow and low Te together negate the possible resistance of the lithosphere to rebound in response to erosional unloading. To isostatically compensate the crustal root, compatible to seismic Moho, a band of high density (2,930 kg m⁻³) in the lower crust and low density (3,210 kg m⁻³) in the lithospheric mantle below the Southern Granulite Terrain is needed. A relatively denser crust due to two distinct episodes of metamorphic phase transitions at 2.5 Ga and 550 Ma and highly mobilized upper mantle during Pan-African thermal perturbation reduced significantly the root buoyancy that kept the crust pulled downward in response to the eroded topography.     

The Lithosphere-Asthenosphere System in the Calabrian Arc and Surrounding Seas – Southern Italy

A fairly detailed structural model of the lithosphere-asthenosphere system (thickness, S- and P-wave velocities of the crust and of the uppermost mantle layers) has been defined in the Calabrian Arc region (Southern Tyrrhenian Sea, Calabria and the northwestern part of the Ionian Sea) in Southern Italy using seismic data from literature as a priori constraints of the nonlinear inversion of surface-wave data. The main features identified by this study are: (1) A very shallow (less then 10 km deep) crust-mantle transition in the Southern Tyrrhenian Sea and a very low v_s just below a very thin lid, in correspondence of the submarine volcanic bodies Magnaghi, Marsili and Vavilov, while the v_s in the lid is quite high in the area that separates Marsili from Magnaghi-Vavilov; (2) a shallow and very low v_s layer in the uppermost mantle in the areas of the Aeolian Islands, Vesuvius, Phlegraean Fields and Ischia, which represents their shallow-mantle magma source; (3) a thickened continental crust and lithospheric doubling in Calabria; (4) a crust about 25-km thick and a mantle velocity profile versus depth consistent with the presence of a continental rifted lithosphere, now thermally relaxed, in the investigated part of the Ionian Sea; (5) the subduction towards northwest of the Ionian lithosphere below the Southern Tyrrhenian Sea; (6) the subduction of the Adriatic/Ionian lithosphere underneath the Vesuvius and Phlegraean Fields.     

鄂尔多斯地块深部岩石圈电性结构研究

近年来新的研究成果反应出鄂尔多斯地块岩石圈并不是一个具有深根的完整的刚性块体,尤其在鄂尔多斯北部以及河套地堑发现有大范围的下地壳-上地幔低速低阻物质,如果这一情况属实,那么人们对鄂尔多斯地块的认识将发生大的变化.为此,我们在华北克拉通西部布设了一条穿过鄂尔多斯地块、河套地堑和阴山造山带的南北向大地电磁剖面,试图通过深部电性结构的探测提供更多信息.该剖面全长约850 km,共布设54个宽频测点和17个长周期测点.二维和三维反演结果均表明:鄂尔多斯地块内部以38°N为界,南部和北部电性结构存在明显差异.鄂尔多斯地块南部地壳至上地幔150 km深度范围内整体表现为高阻,具有刚性克拉通的特征;鄂尔多斯地块北部到河套地堑之间下地壳出现低阻层,特别是鄂尔多斯北端与河套地堑接壤地段,深部存在一个规模较大的下地壳-上地幔低阻异常体,该异常体从河套地堑开始,横向上向南延伸到鄂尔多斯地块内部约200 km,纵向上从下地壳向下延伸到上地幔(约100 km深度).根据该异常体的空间特征,参考该区地震波低速异常体的分布,我们认为鄂尔多斯北部及河套地堑中下地壳到上地幔存在热物质,其原因与深部的构造活动有关(软流圈热物质上涌、侧向流动等),这一情况可能反映出鄂尔多斯地块北部岩石圈深部正处于被改造(或者破坏)阶段,这对进一步认识青藏高原东北缘与华北克拉通之间的深部关系具有一定的启示作用.