Gravity interpretation along seismic reflection profile DEKORP 9-N (northern Rhine Graben)

A 2-D gravity model, incorporating geophysical and geological data, is presented for a 110 km long transect across the northern Rhine Graben, coinciding with the 92 km long DEKORP 9-N seismic reflection profile. The Upper Rhine Graben is marked by a prominent NNE-striking negative anomaly of 30–40 mgal on Bouguer gravity maps of SW Germany. Surface geological contacts, borehole data and the seismic reflection profile provide boundary constraints during forward modelling.
Short-wavelength (5–10 km) gravity features can be correlated with geologic structures in the upper few km. At deeper levels, the model reflects the asymmetry visible in the seismic profile; a thicker, mostly transparent lower crust in the west and a thinner, reflective lower crust in the east. From west to east Moho depth changes from 31 to 26–28 km. The entire 40 mgal minimum can be accounted for by the 2–3 km of light sedimentary fdl in the graben, which masks the gravitational effects of the elevated Moho. The thickened lower crust in the west partly compensates for the mass deficit from the depressed Moho. A further compensating feature is a relatively low density contrast at the crust-mantle boundary of 0.25 g cm^-3. The Variscan must displays heterogeneity along the profile which cuts at an angle across the strike of Variscan structures. The asymmetry of the integrated crustal model, both at the surface and at depth suggests an asymmetric mechanism of rift development.     

VRANCEA99—the crustal structure beneath the southeastern Carpathians and the Moesian Platform from a seismic refraction profile in Romania

The VRANCEA99 seismic refraction experiment is part of an international and multidisciplinary project to study the intermediate depth earthquakes of the Eastern Carpathians in Romania. As part of the seismic experiment, a 300-km-long refraction profile was recorded between the cities of Bacau and Bucharest, traversing the Vrancea epicentral region in NNE–SSW direction.

The results deduced using forward and inverse ray trace modelling indicate a multi-layered crust. The sedimentary succession comprises two to four seismic layers of variable thickness and with velocities ranging from 2.0 to 5.8 km/s. The seismic basement coincides with a velocity step up to 5.9 km/s. Velocities in the upper crystalline crust are 5.9–6.2 km/s. An intra-crustal discontinuity at 18–31 km divides the crust into an upper and a lower layer. Velocities within the lower crust are 6.7–7.0 km/s. Strong wide-angle PmP reflections indicate the existence of a first-order Moho at a depth of 30 km near the southern end of the line and 41 km near the centre. Constraints on upper mantle seismic velocities (7.9 km/s) are provided by Pn arrival times from two shot points only. Within the upper mantle a low velocity zone is interpreted. Travel times of a PLP reflection define the bottom of this low velocity layer at a depth of 55 km. The velocity beneath this interface must be at least 8.5 km/s.

Geologic interpretation of the seismic data suggests that the Neogene tectonic convergence of the Eastern Carpathians resulted in thin-skinned shortening of the sedimentary cover and in thick-skinned shortening in the crystalline crust. On the autochthonous cover of the Moesian platform several blocks can be recognised which are characterised by different lithological compositions. This could indicate a pre-structuring of the platform at Mesozoic and/or Palaeozoic times with a probable active involvement of the Intramoesian and the Capidava–Ovidiu faults. Especially the Intramoesian fault is clearly recognisable on the refraction line. No clear indications of the important Trotus fault in the north of the profile could be found. In the central part of the seismic line a thinned lower crust and the low velocity zone in the uppermost mantle point to the possibility of crustal delamination and partial melting in the upper mantle.     

Broad depth range seismic imaging of the subducted Nazca Slab, North Chile

In this paper, we present a compilation of modern seismic and seismological methods applied to image the subduction process in North Chile, South America. We use data from active and passive seismic experiments that were acquired within the framework of the German Collaborative Research Center SFB267 ‘Deformation Processes in the Andes’. The investigation area is located between 20° and 25°S and extends from the trench down to 100 km depth. In the depth range between the sea bottom and 15 km, we process an offshore seismic reflection profile using a recently developed velocity-model-independent stacking procedure. We find that the upper part of the subducting oceanic lithosphere in this depth range is characterized by a horst-and-graben structure. This structure supports an approximately 3 km thick coupling zone between the plates. In the depth range between 15 and 45 km, we analyse the spatial distribution of aftershocks of the Antofagasta earthquake (1995). The aftershock hypocenters are concentrated in an approximately 3 km thick layer. Finally, in the depth range between 45 and 100 km, we apply Kirchhoff prestack depth migration to the onshore ANCORP profile. A double reflection zone is observed between 45 and 60 km depth, which may represent the upper and lower boundary of the subducted oceanic crust. Over the whole range down to more than 80–90 km depth, we obtain an image of the subducting slab. At that depth, the hypocenters of local earthquakes deviate significantly in the direction perpendicular to the slab face from the reflective parts of the slab. Consequently, our results yield a complete seismic image of the downgoing plate and the associated seismic coupling zone.     

Deep seismic refraction experiment in northeast Brazil: New constraints for Borborema province evolution

The Borborema Province of northeastern Brazil is a major Proterozoic crustal province that, until now, has never been explored using deep crustal seismic methods. Here are reported the first results obtained from a high-quality seismic refraction/wide-angle reflection profile that has defined the internal seismic velocity structure and thickness of the crust in this region. Almost 400 recording stations were deployed in the Deep Seismic Refraction (DSR) experiment through an NW–SE ca. 900 km linear array and 19 shots were exploded at every 50 km along the line. Data from the 10 southeastern most shots of the seismic profile were processed in this work. The main features and geological structures crossed by the studied portion of the profile belong to the so-called Central Sub-province of the Borborema tectonic province. The crustal model obtained is compatible with a typical structure of extended crust. The model was essentially divided into three layers: upper crust, lower crust, and a half-space represented by the shallower portion of the mantle. The Moho is an irregular interface with depth ranging between 31.7 and 34.5 km, and beneath the Central Sub-province it varies from 31.5 to 33 km depth, where its limits are related to major crustal discontinuities. The distribution of velocities within the crust is heterogeneous, varying vertically from 5.7 to 6.3 km/s in the upper crust and from 6.45 to 6.9 km/s in the lower crust. From the average crustal velocity distribution it is evident that the Central Sub-province has seismic characteristics different from neighboring domains. The crust is relatively thin and crustal thickness variations in the profile are subtle due to stretching that occurred in the Cretaceous, during the fragmentation of Pangaea, opening of the South Atlantic Ocean and separation of South America from Africa.     

A Geotransect in the Central Indian Shield,across the Narmada-Son Lineament and the Central Indian Suture

This paper reports on a geotransect in the central Indian shield along a 100 km wide NW-SE corridor between Hirapur and Rajnandgaon. This corridor has been selected based on two seismic profiles—a 235 km long seismic-refraction/wide-angle-reflection profile between Hirapur and Mandla and a 130 km long coincident deep-reflection/refraction profile between Seoni and Kalimati. Since the geologic, gravity, magnetic, and heat-flow data are available up to Rajnandgaon, the second part of the corridor has been extended by another 80 km in the absence of seismic data. From northwest to southeast, the transect corridor covers different tectonic units of the Late Archean to Mesoproterozoic Bundelkhand craton, the Paleoproterozoic to Mesoproterozoic Satpura mobile belt, the Middle Archean to Mesoproterozoic Kotri-Dongargarh mobile belt, and the Neoproterozoic Bastar craton.

The seismic results in the Bundelkhand craton show lower crustal velocity values at a very shallow depth; these data have now been interpreted as a lower-crustal intrusive body that is present throughout the Bundelkhand craton in the lower crust at depths of 23 to 25 km. Combined interpretation of seismic travel times with the gravity data indicate the presence of a local magmatic body at mid-crustal depth in the Satpura mobile belt. The crust-mantle boundary is at depths varying between 40 and 44 km.

The seismic-reflection data set identifies the presence of a suture at the Satpura mobile belt/ Kotri-Dongargarh mobile belt boundary. A well-defined Moho offset and a pattern of adjacent fabrics, each characterized by dips toward each other, mark tectonically imbricated crust on opposite sides of the suture.     

长江中下游庐江-枞阳火山岩矿集区深部结构与成矿作用

为探测长江中下游成矿带庐江-枞阳白垩纪火山岩盆地和铁、硫矿集区深部构造和地壳结构,探讨成矿深部控制条件,作者完成了穿越火山岩盆地的深反射地震剖面(150km,记录30s)和罗河铁矿区浅层高分辨反射地震剖面(20km),以及平行剖面的大地电磁、高精度重磁剖面,揭示了矿集区全地壳精细结构和电磁结构,同时开展区域构造测量和火山岩年代学研究,获得了新的认识。证实"耳状"的庐-枞火山岩盆地是一个沿北东向罗河断裂向东侧发育的非对称火山盆地,排除了另一半被断在西侧红层之下的判断;罗河断裂是一条切穿MOHO的深断裂,倾向南东,是引导地幔流体和岩浆上涌和喷发的通道,在中地壳形成岩浆房(反射亮斑);鉴别出多层界面,火山岩-侏罗系厚度约4~5km(其中火山岩厚度约3km),三叠系-震旦系变形层底界深度大致18~20km,变质基底组成中下地壳,MOHO平缓向北西倾,深度33~31km;追踪郯-庐断裂带的深部产状,陡立延伸到MOHO,宽约10km。从而揭示了早白垩系(132~127Ma)庐-枞火山岩矿集区深部过程与成矿、控矿作用。     

Silkeborg gravity high revisited: Horizontal extension of the source and its uniqueness

Silkeborg Gravity High is a dominant positive gravity anomaly in Denmark. It is associated with an igneous intrusion within the crust. A deep refraction seismic profile locates the top of the intrusion in depths between 11 km and 25 km. The present contribution should be read together with two other papers by the author (Strykowski, 1998; Strykowski, 1999) dealing with the modelling problems of the same area.Strykowski (1998) discusses an advanced method of geological stripping. The focus is on coupling various types of piecewise information (depth to the top/base of geological bodies/layers obtained from depth converted seismograms and interpolated to a horizontal grid, surface gravity data, and mass density information from boreholes). The objective is to model the surface gravity response of known sediments to a depth level of 10 km.Illustrated by the practical example (modelling of the source of Silkeborg Gravity High) Strykowski (1999) discusses methodological aspects of extracting information about the geometry of the source body (in 3D) from (geologically stripped) surface gravity data and from a cross-secting deep seismic profile. The average mass density contrast between the source body (the intrusion) and the surroundings is estimated. The used geometrical information from the seismogram is weak (only the depth interval). A remarkable result of this investigation is that the along profile cross section of the obtained (3D-)structure agrees with the geometrical information of the refraction seismic profile.The present paper is an attempt to extend this result to the rest of the sedimentary basin. Of particular interest is another positive gravity anomaly (another intrusion?) located to the north-west of the studied anomaly. A “final model” obtained here estimates the depth to the source body to 14 km.Nevertheless, the focus of the present paper is not on finding a particular “best model” of the subsurface, but on ambiguity considerations. Especially, on how the different assumptions alter the obtained model? The interesting aspect is whether the used assumptions are supported by the available information.     

Seismic image of the deep crust at the eastern margin of the Alps (Austria): indications for crustal extension in a convergent orogen

A 39-km-long deep seismic reflection profile recorded during two field campaigns in 1996 and 2002 provides a first detailed image of the deep crust at the eastern margin of the Eastern Alps (Austria). The ESE–WNW-trending, low-fold seismic line crosses Austroalpine basement units and extends approximately from 20 km west of the Penninic window group of Rechnitz to 60 km SSE of the Alpine thrust front.The explosive-source seismic data reveals a transparent shallow crust down to 5 km depth, a complexly reflective upper crust and a highly reflective lowermost crust. The upper crust is dominated by three prominent west-dipping packages of high-amplitude subparallel reflections. The upper two of these prominent packages commence at the eastern end of the profile at about 5 and 10 km depth and are interpreted as low-angle normal shear zones related to the Miocene exhumation of the Rechnitz metamorphic core complex. In the western portion of the upper crust, east-dipping and less significant reflections prevail. The lowermost package of these reflections is suggested to represent the overall top of the European crystalline basement.Along the western portion of the line, the lower crust is characterised by a 6–8-km-thick band of high-amplitude reflection lamellae, typically observed in extensional provinces. The Moho can be clearly defined at the base of this band, at approximately 32.5 km depth. Due to insufficient signal penetration, outstanding reflections are missing in the central and eastern portion of the lower crust. We speculate that the result of accompanying gravity measurements and lower crustal sporadic reflections can be interpreted as an indication for a shallower Moho in the east, preferable at about 30.5 km depth.The high reflectivity of the lowermost part of the lower crust and prominent reflection packages in the upper crust, the latter interpreted to represent broad extensional mylonite zones, emphasises the latest extensional processes in accordance with eastward extrusion.     

Near-vertical seismic reflection image using a novel acquisition technique across the Vrancea Zone and Foscani Basin, south-eastern Carpathians (Romania)

The DACIA PLAN (Danube and Carpathian Integrated Action on Process in the Lithosphere and Neotectonics) deep seismic sounding survey was performed in August–September 2001 in south-eastern Romania, at the same time as the regional deep refraction seismic survey VRANCEA 2001. The main goal of the experiment was to obtain new information on the deep structure of the external Carpathians nappes and the architecture of Tertiary/Quaternary basins developed within and adjacent to the seismically-active Vrancea zone, including the Focsani Basin. The seismic reflection line had a WNW–ESE orientation, running from internal East Carpathians units, across the mountainous south-eastern Carpathians, and the foreland Focsani Basin towards the Danube Delta. There were 131 shot points along the profile, with about 1 km spacing, and data were recorded with stand-alone RefTek-125s (also known as “Texans”), supplied by the University Texas at El Paso and the PASSCAL Institute. The entire line was recorded in three deployments, using about 340 receivers in the first deployment and 640 receivers in each of the other two deployments. The resulting deep seismic reflection stacks, processed to 20 s along the entire profile and to 10 s in the eastern Focsani Basin, are presented here. The regional architecture of the latter, interpreted in the context of abundant independent constraint from exploration seismic and subsurface data, is well imaged. Image quality within and beneath the thrust belt is of much poorer quality. Nevertheless, there is good evidence to suggest that a thick (10 km) sedimentary basin having the structure of a graben and of indeterminate age underlies the westernmost part of the Focsani Basin, in the depth range 10–25 km. Most of the crustal depth seismicity observed in the Vrancea zone (as opposed to the more intense upper mantle seismicity) appears to be associated with this sedimentary basin. The sedimentary successions within this basin and other horizons visible further to the west, beneath the Carpathian nappes, suggest that the geometry of the Neogene and recent uplift observed in the Vrancea zone, likely coupled with contemporaneous rapid subsidence in the foreland, is detached from deeper levels of the crust at about 10 km depth. The Moho lies at a depth of about 40 km along the profile, its poor expression in the reflection stack being strengthened by independent estimates from the refraction data. Given the apparent thickness of the (meta)sedimentary supracrustal units, the crystalline crust beneath this area is quite thin (< 20 km) supporting the hypothesis that there may have been delamination of (lower) continental crust in this area involved in the evolution of the seismic Vrancea zone.     

Structure of the crust and upper mantle beneath the central european rift system

The crustal structure of the central European rift system has been investigated by seismic methods with varying success. Only a few investigations deal with the upper-mantle structure. Beneath the Rhinegraben the Moho is elevated, with a minimum depth of 25 km. Below the flanks it is a first-order discontinuity, while within the graben it is replaced by a transition zone with the strongest velocity gradient at 20–22 km depth. An anomalously high velocity of up to 8.6 km/s seems to exist within the underlying upper mantle at 40–50 km depth. A similar structure is also found beneath the Limagnegraben and the young volcanic zones within the Massif Central of France, but the velocity within the upper mantle at 40–50 km depth seems to be slightly lower. Here, the total crustal thickness reaches only 25 km. The crystalline crust becomes extremely thin beneath the southern Rhônegraben, where the sediments reach a thickness of about 10 km while the Moho is found at 24 km depth. The pronounced crustal thinning does not continue along the entire graben system. North of the Rhinegraben in particular the typical graben structure is interrupted by the Rhenohercynian zone with a “normal” West-European crust of 30 km thickness evident beneath the north-trending Hessische Senke. A single-ended profile again indicates a graben-like crustal structure west of the Leinegraben north of the Rhenohercynian zone. No details are available for the North German Plain where the central European rift system disappears beneath a sedimentary sequence of more than 10 km thickness.     

Application of high-resolution seismic techniques in the evaluation of earthquake site response, Ottawa Valley, Canada

High-resolution seismic surveys, including P- and S-wave studies, have been conducted in an area of the Ottawa River valley located 80 km east of Ottawa (Canada). Based on dating of paleolandslides, the existence of paleoearthquake activity has been postulated in this area. The target zone for the seismic survey is characterized by surface disturbance and sediment deformation. P-wave seismic imaging was used to map the overburden–bedrock interface as well as to indicate reflecting boundaries within the overburden. The area of surface disturbance was found to overlie a buried bedrock basin, 8 km in diameter, infilled with a maximum thickness of 180 m of unconsolidated Quaternary sediments. Preliminary results of core logging show the presence of sand overlain by deformed fine sediments within the disturbed area. Shear-refraction studies reveal differences in the velocity–depth profiles between the disturbed area and the surrounding undisturbed areas. The shear-wave reflection method was used to produce a fundamental resonant period map for the area. Surface sediment disturbance was probably due to a combination of ground-motion amplification due to the basin (thick soft sediments) and the presence of water-saturated sand at depth.     

The structure of the crust and upper mantle to depths of 320 km beneath the Kaapvaal craton, from P wave arrivals generated by regional earthquakes and mining-induced tremors

Travel times from earthquakes recorded at two seismic networks were used to derive an average P wavespeed model for the crust and upper mantle to depths of 320 km below southern Africa. The simplest model (BPI1) has a Moho depth of 34 km, and an uppermost mantle wavespeed of 8.04 km/s, below which the seismic wavespeeds have low positive gradients. Wavespeed gradients decrease slightly around 150 km depth to give a ‘knee’ in the wavespeed-depth model, and the wavespeed reaches 8.72 km/s at a depth of 320 km. Between the Moho and depths of 270 km, the seismic wavespeeds lie above those of reference model IASP91 of Kennett [Research School of Earth Sciences, Australian National University, Canberra, Australia (1991)] and below the southern African model of Zhao et al. [Journal of Geophysical Research 104 (1999) 4783]. At depths near 300 km all three models have similar wavespeeds. The mantle P wavespeeds for southern Africa of Qiu et al. [Geophysical Journal International 127 (1996) 563] lie close to BPI1 at depths between 40 and 140 km, but become lower at greater depths. The seismic wavespeeds in the upper mantle of model BPI1 agree satisfactorily with those estimated from peridotite xenoliths in kimberlites from within the Kaapvaal craton.The crustal thickness of 34 km of model BPI1 is systematically lower than the average thickness of 41 km computed over the same region from receiver functions. This discrepancy can be partly explained by an alternative model (BPI2) in which there is a crust–mantle transition zone between depths of 35 and 47 km, below which seismic wavespeed increases to 8.23 km/s. A low-wavespeed layer is then required at depths between 65 and 125 km.     

Crustal structure across the TESZ along POLONAISE''97 seismic profile P2 in NW Poland

The POLONAISE'97 (POlish Lithospheric ONset—An International Seismic Experiment, 1997) seismic experiment in Poland targeted the deep structure of the Trans-European Suture Zone (TESZ) and the complex series of upper crustal features around the Polish Basin. One of the seismic profiles was the 300-km-long profile P2 in northwestern Poland across the TESZ. Results of 2D modelling show that the crustal thickness varies considerably along the profile: 29 km below the Palaeozoic Platform; 35–47 km at the crustal keel at the Teisseyre–Tornquist Zone (TTZ), slightly displaced to the northeast of the geologic inversion zone; and 42 km below the Precambrian Craton. In the Polish Basin and further to the south, the depth down to the consolidated basement is 6–14 km, as characterised by a velocity of 5.8–5.9 km/s. The low basement velocities, less than 6.0 km/s, extend to a depth of 16–22 km. In the middle crust, with a thickness of ca. 4–14 km, the velocity changes from 6.2 km/s in the southwestern to 6.8 km/s in the northeastern parts of the profile. The lower crust also differs between the southwestern and northeastern parts of the profile: from 8 km thickness, with a velocity of 6.8–7.0 km/s at a depth of 22 km, to ca.12 km thickness with a velocity of 7.0–7.2 km/s at a depth of 30 km. In the lowermost crust, a body with a velocity of 7.20–7.25 km/s was found above Moho at a depth of 33–45 km in the central part of the profile. Sub-Moho velocities are 8.2–8.3 km/s beneath the Palaeozoic Platform and TTZ, and about 8.1 km/s beneath the Precambrian Platform. Seismic reflectors in the upper mantle were interpreted at 45-km depth beneath the Palaeozoic Platform and 55-km depth beneath the TTZ.

The Polish Basin is an up to 14-km-thick asymmetric graben feature. The basement beneath the Palaeozoic Platform in the southwest is similar to other areas that were subject to Caledonian deformation (Avalonia) such that the Variscan basement has only been imaged at a shallow depth along the profile. At northeastern end of the profile, the velocity structure is comparable to the crustal structure found in other portions of the East European Craton (EEC). The crustal keel may be related to the geologic inversion processes or to magmatic underplating during the Carboniferous–Permian extension and volcanic activity.     

共和盆地壳内部分熔融层存在的地球物理证据与干热岩资源区域性热源分析

新生代以来,共和盆地及其周缘造山带无火山、岩浆活动,印支期隐伏花岗岩体岩浆余热与放射性元素衰变生热等难以构成共和盆地干热岩资源的主要热源,而共和盆地又为一高温地热异常盆地.目前已基本探明了共和县恰卜恰与贵德县热水泉干热岩体2处,圈定出干热岩勘查目标靶区16处.区域重力和区域航磁调查、区域天然地震成像、盆地尺度天然地震背...     

Geophysical differences in the lithosphere between phanerozoic and precambrian Australia

Cannikin atomic bomb recordings indicate that there are differences in travel-times from the Aleutian Islands test site to Phanerozoic and Precambrian provinces in Australia of up to 1.1 s. Explosion seismic studies in central and southeastern Australia enable travel-time corrections for crustal and upper mantle structure to be made to recordings of such teleseismic events. Structure in the upper 60 km can account for, at most, about 0.2 s of the residual difference, but attempts to constrain the remaining residual time to the region above the Lehmann discontinuity at about 200 km depth are difficult to reconcile with explosion seismic models. Regional differences in seismic velocity structure between Phanerozoic and Precambrian Australia therefore appear to exist at depths greater than 200 km.Electrical conductivities within the mantle have been investigated using two methods. Long-period electromagnetic depth sounding using magnetometer arrays demonstrates that conductivities increase at about 200 km under Phanerozoic Australia but not until about 500 km depth under Precambrian Australia. Shorter period magnetotelluric measurements can only resolve shallower structures; these too indicate a similar trend but with sub-crustal conductivities increasing at less than 100 km under Phanerozoic Australia. Magma at these depths and shallower may be the source for Cainozoic volcanism in eastern Australia. Under Precambrian central and northern Australia magnetotelluric investigations indicate that pronounced conductivity increases do not occur until depths of 150–200 km are reached.Oceanic magnetic observations indicate that the Australian lithospheric plate as a whole is separating from Antarctica at a rate of about 7 cm/yr. The seismic and conductivity structures under the continental region of this plate indicate that lateral inhomogeneities possibly extend to depths as great as 500 km and are probably caused by the passage of eastern Australia over a hot spot. Hawaiian studies indicate that hot spots are not local features but result from large scale disturbances in the mantle. Conductivity increases commencing in the depth range 100–250 km may give an indication of uppermost zones within which the Palaeozoic lithospherc has been substantially modified resulting in elevated surface heat flow, volcanism and seismic travel-time anomalies.     

Seismic model of the lithosphere of the East European Platform beneath the Baltic Sea-Black Sea profile

This paper presents the results of seismic measurements along the Baltic Sea-Black Sea profile. The basic wave groups recorded up to distances of 900 km are characterized. The main elements of a lithospheric model of the southwestern part of the Precambrian East European Platform are given. The thickness of the Earth's crust is about 45 km and the mean velocity of the crust is about 6.3 km/s. At a depth of 65 km, the velocity increases from 8.2 to 8.5 km/s. In the depth interval 110 to 135 km, there is a series of layers with low and high velocities. The lower boundary of the lithosphere is probably defined by the boundary at a depth of 110 km.     

Seismic velocities from explosions off the central coast of New South Wales

Travel times from explosions fired on the continental shelf off the central coast of New South Wales were observed at permanent stations and spreads of seismic exploration instruments, and combined with existing results to give a seismic crustal profile across part of southeastern Australia. An intermediate layer, dipping to the southwest, underlies the surface rocks and has a P velocity of about 6–52 km./sec. Beneath Sydney, its top may either be in contact with the basin sediments at a depth of about 5 km., or separated from them by a wedge of a few kilometres of 6 km./sec. material. The Mohorovi?i? discontinuity (M) is at a depth of 25 km., dips to the southwest at about 4 degrees, and the velocity under it is about 7.86 km./sec. The depth to the top of the intermediate layer under the Snowy Mountains is about 20 km., and the revised depth to M is about 42 km. M dips at about 2° to the southwest in this region, and the velocity at the top of the mantle is 8.1 km./sec.     

An introduction to the European Geotraverse Project: First results and present plans

The European Geotraverse (EGT) is an international, multidisciplinary project focused on a north-south orientated lithospheric profile, 4000 km long and of varying width, extending from northernmost Scandinavia to North Africa. This profile consists of three interlinking Segments (Northern, Central, and Southern) comprising a continuous succession of tectonic provinces ranging from the oldest Precambrian areas of the Baltic Shield to the currently active area of the Western Mediterranean. The broad aim of the EGT Project is to obtain a better three-dimensional picture of the structure, state, and composition of the continental lithosphere to use as a basis for an understanding of its evolution and dynamics. All of the 12 major projects that constitute the EGT “Joint Programme” have now been initiated, and several of these projects are nearing completion.Along the Northern Segment, data from “The Fennoscandian Long-Range Project” (FENNOLORA), a 2000 km long seismic profile across the Precambrian Baltic Shield, show that except beneath southern Sweden, the Shield is characterized by a high-velocity, 40–50 km thick crust—including a 5–10 km thick crust-mantle transition zone. An alternating series of 4–6 high- and low-velocity zones is present in the subcrustal lithosphere, the base of which increases in depth from ca. 110 km to ca. 230 km from south to north beneath the Shield. The top of the mantle transition zone lies at a depth of about 450 km. The second major project along this Segment, the EUGENO-S (European Geotraverse Northern Segment—Southern Part) project, is a multidisciplinary study of the Fennoscandian Border Zone, and was largely completed in 1984 with the realization of a large-scale seismic experiment. Preliminary interpretation of the excellent data obtained indicate the presence of strong lateral variations in internal crustal structure beneath the Danish Basin. Field work for a third major project, a multidisciplinary transect of the Archaean and Early Proterozoic terrains in the northernmost part of the Shield (the “Polar Profile”), was carried out in 1985.A series of deep seismic reflection lines has so far been realized in the area of the Central Segment in the context of German national programmes. First interpretations of the seismic data from a 260 km long profile across the two main intra-Variscan (Hercynian) lineaments have shown the presence of numerous horizons making up a highly reflective zone in the lowermost 10 km of the crustal section studied, and distinct changes in reflectivity between the main Variscan tectonic zones. In 1986, the entire Segment will be investigated in detail in an ambitious international programme of integrated geological and geophysical studies.A series of seismic experiments (termed EGT-S) have been carried out across the Southern Segment (in 1982, 1983, and 1985). Interpretation of data from the 1982 and 1983 experiments have led to several interesting results, including:

• 1.(1) the suggestion that two “crust-mantle”-like interfaces exist beneath the Po Basin (at depths of about 35 and 50 km) and adjacent tectonic units, these interfaces marking a deep contact zone between the Adriatic and European plates,
• 2.(2) in the area between Genoa and Corsica, the Ligurian Sea is underlain by a greatly thinned, distinctly layered section of continental crust, and
• 3.(3) Corsica and Sardinia are underlain by bowl-shaped, “typically” Variscan continental crusts.

The 1985 phase of seismic surveys focused on crustal structure beneath Tunisia and the adjacent seas.In addition, two off-traverse projects are being realized. First, a wide-aperture network of autonomously recording seismic stations (“NARS”), installed along the line Gothenburg-Málaga between 1982 and 1984, is already yielding high-quality data on the upper 600–700 km of the mantle. Second, an investigation of lithospheric seismic anisotropy in the area of the Iberian Peninsula is being organized for 1987–1988.Finally, of great importance are the systematic compilation of existing data and, where needed in critical regions, collection of new geophysical and geological data presently being carried out for the entire area encompassed by the EGT. It is expected that these compilations will be completed by 1987, at about the same time that full results from the main large-scale seismic experiments become available, enabling the construction of an integrated lithospheric cross-section along the EGT, requiring a final phase of intensive multidisciplinary collaboration.     

地震正演技术在深反射地震剖面探测中的应用

地震波正演模拟技术广泛应用于浅层勘探,此方法可以将地质模型和地震模型有机结合起来,验证和指导地震资料的采集、处理和解释。基于石油反射地震技术发展起来的深反射地震剖面探测技术,经过几十年的发展及应用,已经非常成熟,但到目前为止,地震波正演技术在深反射地震剖面探测中的应用却很少。本文利用跨越四川盆地深反射地震剖面来开展正演研究,通过对比拟合正演模拟数据和实际地震数据的层位到时,不断修正速度、层位等参数,建立最终深度域地质模型,为构造剖面提供较为准确的地壳厚度、莫霍面深度等地层信息。通过深度域地质模型,揭示出扬子板块西北缘新元古代古俯冲的角度约30°,俯冲的深度达到60 km。     

A seismic section of the upper mantle along the FENNOLORA profile (Baltic Shield) on the basis of the 2D inhomogeneous model of a medium

The data from the deep seismic sounding along the 2100 km long FENNOLORA profile are considered. Interpretation is made by the method of homogeneous functions with the Earth??s curvature taken into account. A seismic section of up to 200 km depth is the result. The image of a thick northward-dipping lithospheric slab, which is located beneath the lithosphere in the mantle, is obtained. The data we obtained are compared with the results of other authors.