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1.
We present GPS observations in Morocco and adjacent areas of Spain from 15 continuous (CGPS) and 31 survey-mode (SGPS) sites extending from the stable part of the Nubian plate to central Spain. We determine a robust velocity field for the W Mediterranean that we use to constrain models for the Iberia-Nubia plate boundary. South of the High Atlas Mountain system, GPS motions are consistent with Nubia plate motions from prior geodetic studies. We constrain shortening in the Atlas system to <1.5 mm/yr, 95% confidence level. North of the Atlas Mountains, the GPS velocities indicate Nubia motion with respect to Eurasia, but also a component of motion normal to the direction of Nubia-Eurasia motion, consisting of southward translation of the Rif Mountains in N Morocco at rates exceeding 5 mm/yr. This southward motion appears to be directly related to Miocene opening of the Alboran Sea. The Betic Mountain system north of the Alboran Sea is characterized by WNW motion with respect to Eurasia at ～1–2 mm/yr, paralleling Nubia-Eurasia relative motion. In addition, sites located in the Betics north of the southerly moving Rif Mountains also indicate a component of southerly motion with respect to Eurasia. We interpret this as indicating that deformation associated with Nubia-Eurasia plate motion extends into the southern Betics, but also that the Betic system may be affected by the same processes that are causing southward motion of the Rif Mountains south of the Alboran Sea. Kinematic modeling indicates that plate boundary geometries that include a boundary through the Straits of Gibraltar are most compatible with the component of motion in the direction of relative plate motion, but that two additional blocks (Alboran-Rif block, Betic Mountain block), independent of both Nubia and Eurasia are needed to account for the motions of the Rif and Betic Mountains normal to the direction of relative plate motion. We speculate that the southward motions of the Alboran-Rif and Betic blocks may be related to mantle flow, possibly induced by southward rollback of the subducted Nubian plate beneath the Alboran Sea and Rif Mountains.  相似文献   

2.
In September 1974, deep seismic sounding experiments were performed in the Alboran Sea. The crustal seismic profiles were carried out with shotpoints at sea along approximately the 36°N parallel and along 3°W and 5°W meridians with stations on land in Morocco and Spain following these three directions. The first interpretation of the data indicates a thinned continental crust with a Moho depth of 16 km on top of a slightly anomalous upper mantle (7.5<V p<7.9 km/s) beneath the center of the Alboran Sea. Towards Spain the transition to the continental margin is characterized by a very rapid thickening of the crust. Towards Morocco a rather abrupt thickening is observed only for the Rif region, while in the eastern part (north-south profile along 3°W) the dip of the Moho is very slight.  相似文献   

3.
Located at the Westernmost tip of the Mediterranean sea, the Gibraltar Arc is a very complex zone. The Betics in Spain and the Rif belt in Morocco surround the Alboran sea characterized by a thinned continental crust. The geodynamic evolution of this region results from the convergence of African and Iberian margins since the Late Cretaceous. It is controlled both by plate convergence and mantle dynamics, which significantly impact on morphology, sedimentary environments, tectonics, metamorphism and magmatism. We present here the contents of the special issue on the Gibraltar Arc and nearby regions, following the workshop organized at the University Abdelmalek Essaadi of Tetouan in Morocco from 27 to 28 October, 2011. The goal of this international workshop was to have an overview of the actual advance in research concerning the Rif and Betics chains, the Alboran basin, and their influence on the Iberian and African forelands.  相似文献   

4.
Within the Spain and Moroccan networks, a large volume of seismic data has been collected and used for investigating the lithosphere in the Betic–Rif Cordillera. The present study has two main goals: (1) Use the most actual seismological data from recent earthquakes in the Betic–Rif arc for investigating the lithosphere through the application of seismic local tomography techniques. (2) Define the possible structural blocks and explain the GPS velocities perturbation in this region. The resolution tests results indicate that the calculated images gave a close true structure for the studied regions from 5- to 60-km depth. The resulting tomographic image shows that the presence of two upper crust body (velocity 6.5 km/s) at 3- to 13-km depth between Iberian Betic and Moroccan Rif in the western and in the middle of Alboran Sea also shows the low velocity favoring the presence of melt in the base of these two bodies. The crustal bodies forms tectonic blocks in the Central Rif and in the Central Betic Cordillera.  相似文献   

5.
The goal of this paper is to study the velocity field and deformation parameters in Southern Spain and surrounding areas (Ibero-Maghrebian region) using GPS episodic measurements. Results are compared to those previously published as well as deformation parameters derived from seismic data. For this purpose, a geodetic GPS network of 12 stations was observed during eight field campaigns from 1998 to 2005 by the San Fernando Naval Observatory (ROA), Spain. Relative GPS velocities in the Gulf of Cadiz with respect to the stable part of Eurasia are ～4.1 mm/yr in a NW–SE to NNW–SSE direction. In the Betics, Alboran Sea and North of Morocco, velocities are ～4.4 mm/yr in a NW–SE direction, and they are ～2.3 mm/yr in a N–S direction in the eastern part of the Iberian Peninsula. These results are in agreement with the anticlockwise rotation of the African plate. GPS strain tensors are determined from the velocity model, to obtain a more realistic crustal deformation model. The Gulf of Cadiz is subjected to uniform horizontal compression in a NNW–SSE direction, with a rotation to N–S in the Alboran Sea and Northern Morocco. An extensional regime in a NW–SE direction, which rotates to W–E, is present in the Internal Betics area. In the Betic, Alboran Sea and North of Morocco regions we compare seismic deformation rates from shallow earthquakes with the determined GPS deformation rates. The comparison indicates a seismic coupling of 27%, while the remaining 73% might be generated in aseismic processes. Deformations measured in the Ibero-Maghrebian region with GPS could be interpreted in terms of either elastic loading or ductile deformation.  相似文献   

6.
Two-dimensional crustal velocity models are derived from passive seismic observations for the Archean Karelian bedrock of north-eastern Finland. In addition, an updated Moho depth map is constructed by integrating the results of this study with previous data sets. The structural models image a typical three-layer Archean crust, with thickness varying between 40 and 52 km. P wave velocities within the 12–20 km thick upper crust range from 6.1 to 6.4 km/s. The relatively high velocities are related to layered mafic intrusive and volcanic rocks. The middle crust is a fairly homogeneous layer associated with velocities of 6.5–6.8 km/s. The boundary between middle and lower crust is located at depths between 28 and 38 km. The thickness of the lower crust increases from 5–15 km in the Archean part to 15–22 km in the Archean–Proterozoic transition zone. In the lower crust and uppermost mantle, P wave velocities vary between 6.9–7.3 km/s and 7.9–8.2 km/s. The average Vp/Vs ratio increases from 1.71 in the upper crust to 1.76 in the lower crust.The crust attains its maximum thickness in the south-east, where the Archean crust is both over- and underthrust by the Proterozoic crust. A crustal depression bulging out from that zone to the N–NE towards Kuusamo is linked to a collision between major Archean blocks. Further north, crustal thickening under the Salla and Kittilä greenstone belts is tentatively associated with a NW–SE-oriented collision zone or major shear zone. Elevated Moho beneath the Pudasjärvi block is primarily explained with rift-related extension and crustal thinning at ∼2.4–2.1 Ga.The new crustal velocity models and synthetic waveform modelling are used to outline the thickness of the seismogenic layer beneath the temporary Kuusamo seismic network. Lack of seismic activity within the mafic high-velocity body in the uppermost 8 km of crust and relative abundance of mid-crustal, i.e., 14–30 km deep earthquakes are characteristic features of the Kuusamo seismicity. The upper limit of seismicity is attributed to the excess of strong mafic material in the uppermost crust. Comparison with the rheological profiles of the lithosphere, calculated at nearby locations, indicates that the base of the seismogenic layer correlates best with the onset of brittle to ductile transition at about 30 km depth.We found no evidence on microearthquake activity in the lower crust beneath the Archean Karelian craton. However, a data set of relatively well-constrained events extracted from the regional earthquake catalogue implies a deeper cut-off depth for earthquakes in the Norrbotten tectonic province of northern Sweden.  相似文献   

7.
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.  相似文献   

8.
Major tectonic units of Spain have been investigated by deep seismic sounding experiments since 1974 to determine crustal structures and to delineate their differences. These areas are the central part of the Hercynian Meseta, and the Alpine chains: the Betic Cordillera in the south, including the Balearic promontory and the Alboran Sea, and the Pyrenees in the north.The main features of the crust and the upper mantle along a NNE-SSW cross-section from the Pyrenees to the Alboran Sea are described.The crust under the Meseta is typical of Hercynian areas found elsewhere in Europe, with an average thickness of 31 km, whereas the two Alpine regions are characterized by very large lateral inhomogeneities, such as rapid thickening of the crust to 50 and 40 km under the Pyrenees and the Betics, respectively. The deep-reaching E-W-trending North Pyrenean fault has a throw of 10–15 km at the base of the crust. A Pn velocity of 8.1 km s?1 is found under the entire Iberian Peninsula.In the Alboran Sea, strongly varying thicknesses of sediments, shallow variable depths to the Moho (～ 13 km under the Alboran ridge), and strong variations of Pn velocity between 7.5 and 8.2 km s?1 have been found.  相似文献   

9.
It has long been recognized that the Kii Peninsula in the southwest Japan arc is peculiar in a non-volcanic region, indicated by the presence of high temperature hot springs, high terrestrial heat flow and high 3He content in hot spring gases. Geophysical and geochemical studies were carried out to understand the geotectonic environment in the southern part of the Kii Peninsula. Most of the measured 3He / 4He ratios are similar or higher than air, indicating wide spread incorporation of mantle-derived helium into meteoric water. A region with rather high 3He / 4He ratios (> 4 RA) on the west side of the Omine Mountains coincides with the occurrence of high temperature hot springs. A deep crustal resistivity structure across the Omine Mountains was imaged by wide-band magnetotelluric soundings. A 2-D inversion with N–S strike using both TM and TE modes reveals two conductors, one in the upper (3–7 km depth) and the other in lower crust (25–35 km depth) to the west of the Omine Mountains. The distribution of microearthquakes and low-frequency tremors, and the existence of seismic reflectors indicate that the large conductor in the lower crust is related to aqueous fluids derived from the Philippine Sea plate. The upper-crustal conductive zone may also reflect the aqueous fluids trapped in the upper crust, which are presumably derived from the subducting slab. Considering the occurrence of seismic events in the subducting slab beneath the southern Kii Peninsula, the aqueous fluids generated by dehydration of the slab mantle could plausibly include MORB-type helium derived from the residual lithospheric mantle. Therefore, the high temperature hot springs and high 3He emanations in hot spring gases and other geotectonic events in the southern Kii Peninsula may be due to heat flux and mantle-derived helium discharged from aqueous fluid in the upper crust.  相似文献   

10.
This paper reports ten new surface heat-flow density (qs) values for central and southern Israel (central Sinai Microplate), whose crystalline crust and lithosphere formed as part of the Neoproterozoic Arabian-Nubian Shield. Heat flow was calculated in Mesozoic sediments using the classical approach of heat-flow determination by implementing in the analysis high-precision continuous temperature logs obtained in air- and/or water-filled boreholes. Thermal conductivity (TC) measured for a large suite of rock samples of lithotypes making up the sequence was assigned to temperature gradients in intervals for which the lithology was known. The heat-flow values obtained for different depth intervals in a borehole as well as the average values for the individual borehole locations cover a narrow range, attesting heat-conduction conditions. A steady-state thermal model along an E–W crustal cross section through the area shows that the observed systematic spatial distribution of the qs values, which range between 50 and 62 mW m−2, can primarily be explained by variations in the thickness of the upper crust and in the ratio between sedimentary and crystalline rocks therein. Given the time lapse of thermal heat transfer through the lithosphere, the qs data monitor the crustal thermal conditions prior to rift- and plume-related lithospheric thermal perturbations that have started in the larger area ca. 30 Ma ago. Observed and modeled qs display the best fit for a pre-Oligocene lithosphere–asthenosphere boundary (LAB) at ∼150 km, which would be at the upper end of LAB depths determined from stable areas of the Arabian Shield (150–120 km) not affected by the young, deep-seated thermal processes that have caused a further uprise of the LAB. Our data imply or predict that the surface heat flow of the Sinai Microplate generally tends to increase along N–S and W–E traverses, from ∼45–50 mW m−2 to ∼55–60 mW m−2. Surface heat flows on the order of 55–60 mW m−2 may be common in the northern Arabian Shield, where it exhibits typical lithosphere structure and composition and is unaffected by young heating processes, compared to values of ≤45 mW m−2 recently determined in the southern Arabian Plate for the Arabian Platform.  相似文献   

11.
《Journal of Geodynamics》2007,43(4-5):95-114
A three-dimensional model for the central Fennoscandian Shield was constructed for analysing the thermal, the rheological and the structural conditions in the lithosphere. The mesh covers a rectangular area in the southern Finland with horizontal dimensions of 500 km × 400 km and a depth extent of 100 km. Structural boundaries are derived from the several deep seismic soundings carried out in the area. Constructed model is first used in the calculation of the thermal and the rheological models and secondly in analysing the stress and the deformational conditions with the obtained rheology. Thermal and structural models are solved with the finite element method. The calculated surface HFD is between 40 and 48 mW m−2 in the Proterozoic southern part and below 40 mW m−2 in the older and northern Archaean part of the model. The calculated rheological strength shows a layered structure with two individual rheologically weak layers in the crust and strong layer in the upper part of the lower crust. The minimum brittle–ductile transition (BDT) depth is around 10 km in the southern part of the model while in the north and north-eastern parts the BDT depth is around 45–50 km. Comparison with the focal depth data shows that as most of the earthquakes occur no deeper than the depth of 10 km are they located in the brittle regime. Resulting stress conditions and possible regions of deformation after the model is subjected to pressure of 50 MPa reveals that the stress field is quite uniformly distributed in different crustal layers and that the elastic parameters control more the state of the stress than the applied rheological structure. In the upper crust, the stress intensity has values between 42 and 45 MPa whereas in the middle crust the values are around 50 MPa. Comparison of the 3-D model with earlier 2-D models shows that some differences in the results are to be expected.  相似文献   

12.
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.  相似文献   

13.
《Journal of Geodynamics》2008,45(3-5):149-159
Locations of the Eger Rift, Cheb Basin, Quaternary volcanoes, crustal earthquake swarms and exhalation centers of CO2 and 3He of mantle origin correlate with the tectonic fabric of the mantle lithosphere modelled from seismic anisotropy. We suggest that positions of the seismic and volcanic phenomena, as well as of the Cenozoic sedimentary basins, correlate with a “triple junction” of three mantle lithospheres distinguished by different orientations of their tectonic fabric consistent within each unit. The three mantle domains most probably belong to the originally separated microcontinents – the Saxothuringian, Teplá-Barrandian and Moldanubian – assembled during the Variscan orogeny. Cenozoic extension reactivated the junction and locally thinned the crust and mantle lithosphere. The rigid part of the crust, characterized by the presence of earthquake foci, decoupled near the junction from the mantle probably during the Variscan. The boundaries (transitions) of three mantle domains provided open pathways for Quaternary volcanism and the ascent of 3He- and CO2-rich fluids released from the asthenosphere. The deepest earthquakes, interpreted as an upper limit of the brittle–ductile transition in the crust, are shallower above the junction of the mantle blocks (at about 12 km) than above the more stable Saxothuringian mantle lithosphere (at about 20 km), probably due to a higher heat flow and presence of fluids.  相似文献   

14.
We provide new petrological evidence for the strong influence of water on the formation of the oceanic lithospheric mantle, the subcontinental mantle above, and the continental lithosphere. Our analysis throws new light on the hypothesis that new continental lithosphere was formed by the passage of silicate-rich aqueous fluid through the sub-continental mantle. In order to investigate this hypothesis, we analyzed a representative collection of lherzolite and harzburgite xenoliths from the sample volcano known as “The Thumb”, located in the center of the Colorado Plateau, western United States. The studied sample collection exhibits multi-stage water enrichment processes along point, line and planar defect structures in nominally anhydrous minerals and the subsequent formation of the serpentine polymorph antigorite along grain boundaries and in totally embedded annealed cracks. Planar defect structures act like monomineralic and interphase grain boundaries in the oceanic lithosphere and the subcontinental mantle beneath the North American plate, which was hydrated by the ancient oceanic Farallon plate during the Cenozoic and Mesozoic eras. We used microspectroscopical, petrological, and seismological techniques to confirm multi-stage hydration from a depth of ∼150 km to just below the Moho depth. High-resolution mapping of the water distribution over homogeneous areas and fully embedded point, line and planar defects in olivine crystals of lherzolitic and harzburgitic origin by synchrotron infrared microspectroscopy enabled us to resolve local wet spots and thus reconstruct the hydration process occurring at a depth of ∼150 km (T ≈ 1225 °C). These lherzolites originated from the middle part of the Farallon mantle slab; they were released during the break up of the Farallon mantle slab, caused by the instability of the dipping slab. The background hydration levels in homogeneous olivines reached ∼138 ppm wt H2O, and the water concentration at the planar defects could reach up to ∼1000 ppm wt H2O. However, the formation of antigorite in grain boundaries was found to be the primary hydration mechanism for harzburgitic samples originating from the subcontinental mantle (for hydration, T ≈ 600 °C). Additionally, the formation of antigorite in lherzolites could be found in annealed cracks. From these observations, we conclude that hydration induces multi-stage water enrichment of the mantle wedge by a process that is dominated by the growth and movement of ubiquitous cracks, which acts as planar defects. Cracks in the mantle seem to be the an important feature in both the water cycle of the subduction zone and the formation of the continental lithosphere.  相似文献   

15.
We present fundamental-mode Rayleigh-wave azimuthally anisotropic phase velocity maps obtained for the Great Basin region at periods between 16 s and 102 s. These maps offer the first depth constraints on the origin of the semi-circular shear-wave splitting pattern observed in central Nevada, around a weak azimuthal anisotropy zone. A variety of explanations have been proposed to explain this signal, including an upwelling, toroidal mantle flow around a slab, lithospheric drip, and a megadetachment, but no consensus has been reached. Our phase velocity study helps constrain the three-dimensional anisotropic structure of the upper mantle in this region and contributes to a better understanding of the deformation mechanisms taking place beneath the western United States. The dispersion measurements were made using data from the USArray Transportable Array. At periods of 16 s and 18 s, which mostly sample the crust, we find a region of low anisotropy in central Nevada coinciding with locally reduced phase velocities, and surrounded by a semi-circular pattern of fast seismic directions. Away from central Nevada the fast directions are ～ N–S in the eastern Great Basin, NW–SE in the Walker Lane region, and they transition from E–W to N–S in the northwestern Great Basin. Our short-period phase velocity maps, combined with recent crustal receiver function results, are consistent with the presence of a semi-circular anisotropy signal in the lithosphere in the vicinity of a locally thick crust. At longer periods (28–102 s), which sample the uppermost mantle, isotropic phase velocities are significantly reduced across the study region, and fast directions are more uniform with an ～ E–W fast axis. The transition in phase velocities and anisotropy can be attributed to the lithosphere–asthenosphere boundary at depths of ～ 60 km. We interpret the fast seismic directions observed at longer periods in terms of present-day asthenospheric flow-driven deformation, possibly related to a combination of Juan de Fuca slab rollback and eastward-driven mantle flow from the Pacific asthenosphere. Our results also provide context to regional SKS splitting observations. We find that our short-period phase velocity anisotropy can only explain ～ 30% of the SKS splitting times, despite similar patterns in fast directions. This implies that the origin of the regional shear-wave splitting signal is complex and must also have a significant sublithospheric component.  相似文献   

16.
We investigate the mantle dynamics beneath the North China Craton (NCC) and surrounding regions based on a synthesis of recent P-wave mantle tomographic data down to depths of 600–800 km and their correlation with the surface geological features, with particular reference to the Paleoproterozoic tectonic events associated with the incorporation of the NCC within the Columbia supercontinent amalgam. From the tomographic images, we identify a hot corridor in the mantle transition zone beneath the central region of the Western Block of the NCC sandwiched between two cold corridors. This scenario is similar to the donut-shaped high-velocity anomaly surrounding a region of low-velocity anomaly in the lowermost mantle under the Pacific and suggests that the cold regions might represent slab graveyards which provide the fuel for the plumes rising from the center. A tomographic transect along the collisional suture of the NCC with the Columbia supercontinent, covering the Yinshan-Ordos Blocks in the Western Block through the Central Orogenic Belt and into the Eastern Block of the NCC reveals a ca. 250 km thick lithospheric keel below the Ordos Block defined by a prominent high-velocity anomaly. We identify slab break-off and asthenospheric upwelling in this region and suggest that this process probably initiated the thermal and material erosion of the tectosphere beneath the Eastern Block from the Paleoproterozoic, which was further intensified during the Mesozoic when a substantial part of the sub-continental mantle lithosphere was lost. We visualize heat input from asthenosphere and interaction between asthenosphere and overlying carbonated tectosphere releasing CO2-rich fluids for the preservation of ultra-high temperature (ca. 1000 °C) metamorphic rocks enriched in CO2 as well as high-pressure mafic granulites as a paired suite in this region. We also identify a hot swell of the asthenosphere rooted to more than 200 km depth and reaching up to the shallow mantle in the tomographic section along 35°N latitude at a depth of 800 km. This zone represents a cross-section through the southern part of the NCC. The surface distribution of Paleoproterozoic Xiong’er lavas and mafic dykes in this region would indicate that this region might have evidenced similar upwellings in the past. Our study has important implications in understanding the evolution of the NCC and suggests that the extensive modification of the mantle architecture and lithospheric structure beneath one of the fundamental Precambrian nuclei of Asia had a prolonged history probably dating from the Paleoproterozoic suturing of the NCC within the Columbia supercontinent amalgam.  相似文献   

17.
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.  相似文献   

18.
A temporary seismological network of broadband three-component stations has been deployed N–S to investigate the crust and upper mantle structure across the Ordos Block and the Yinshan Mountains. P wave receiver functions reveal the Moho depth to be about 41 km beneath the central Ordos Block and down to 45 km beneath the northern Ordos Block, a slight uplifting to 42–43 km beneath the Hetao Graben, increasing to 47–48 km beneath the Yinshan Mountains and then decreasing to 44 km beneath the northern Yinshan Mountains along the profile. In the Ordos Block, the crustal Vp/Vs ratio (about 1.80) south to the Hetao Graben differs from that (about 1.75) beneath the center Ordos Block. The crustal Vp/Vs ratio is significantly lower (about 1.65–1.70) beneath the Yinshan Mountains. The P wave receiver function migration imaging suggests relatively flat discontinuities at 410 and 660 km, indicating the lack of a strong thermal anomaly beneath this profile at these depths, and a low S wave velocity anomaly in the upper mantle beneath the Hetao Graben. We suggest that the low S wave velocity anomaly may be attributable to heat and that the thermal softening advances the evolution of the Hetao Graben, while the lower-crustal ductile flows transfer from the Hetao Graben to the northern Ordos Block, resulting in crustal thickening.  相似文献   

19.
Inversion of local earthquake travel times and joint inversion of receiver functions and Rayleigh wave group velocity measurements were used to derive a simple model for the velocity crustal structure beneath the southern edge of the Central Alborz (Iran), including the seismically active area around the megacity of Tehran. The P and S travel times from 115 well-located earthquakes recorded by a dense local seismic network, operated from June to November 2006, were inverted to determine a 1D velocity model of the upper crust. The limited range of earthquake depths (between 2 km and 26 km) prevents us determining any velocity interfaces deeper than 25 km. The velocity of the lower crust and the depth of the Moho were found by joint inversion of receiver functions and Rayleigh wave group velocity data. The resulting P-wave velocity model comprises an upper crust with 3 km and 4 km thick sedimentary layers with P wave velocities (Vp) of ～5.4 and ～5.8 km s?1, respectively, above 9 km and 8 km thick layers of upper crystalline crust (Vp ～6.1 and ～6.25 km s?1 respectively). The lower crystalline crust is ～34 km thick (Vp ～ 6.40 km s?1). The total crustal thickness beneath this part of the Central Alborz is 58 ± 2 km.  相似文献   

20.
We have studied the dependency between incoming plate structure, bending-related faulting, lithospheric hydration, and outer rise seismic activity offshore Maule, Chile. We derived a 2D Poisson's ratio distribution from P- and S-wave seismic wide angle data collected in the trench-outer rise. High values of Poisson's ratio in the uppermost mantle suggest that the oceanic lithosphere is highly hydrated due to the water infiltration through bending-related normal faults outcropping at the seafloor. This process is presumably facilitated by the presence of a seamount in the area. We conclude that water infiltrates deep into the lithosphere, when it approaches the Chile trench, producing a reduction of crustal and upper mantle velocities, supporting serpentinization of the upper mantle. Further, we observed a mantle Vp anisotropy of 8%, with the fast velocity axis running normal to the abyssal hill fabric and hence in spreading direction, indicating that outer rise processes have yet not affected anisotropy.The first weeks following the megatrust Mw = 8.8 Maule earthquake in 2010 were characterized by a sudden increase of the outer rise seismic activity, located between 34° S and 35°30′ S. We concluded that this phenomenon is a result of an intensification of the water infiltration process in the outer rise, presumably triggered by the main shock, whose epicenter was located some 100 km to the south east of the cluster.  相似文献   

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