Crustal architecture of central Victoria: results from the 2006 deep crustal reflection seismic survey

A ～400 km long deep crustal reflection seismic survey was acquired in central Victoria, Australia, in 2006. It has provided information on crustal architecture across the western Lachlan Orogen and has greatly added to the understanding of the tectonic evolution. The east-dipping Moyston Fault is confirmed as the suture between the Delamerian and western Lachlan Orogens, and is shown to extend down to the Moho. The Avoca Fault, the boundary between the Stawell and Bendigo Zones, is a west-dipping listric reverse fault that intersects the Moyston Fault at a depth of about 22 km, forming a V-shaped geometry. Both the Stawell and Bendigo Zones can be divided broadly into a lower crustal region of interlayered and imbricated metavolcanic and metasedimentary rocks and an upper crustal region of tightly folded metasedimentary rocks. The Stawell Zone was probably part of a Cambrian accretionary system along the eastern Gondwanaland margin, and mafic rocks may have been partly consumed by Cambrian subduction. Much of the Early Cambrian oceanic crust beneath the Bendigo Zone was not subducted, and is preserved as a crustal-scale imbricate thrust stack. The seismic data have shown that a thin-skinned structural model appears to be valid for much of the Melbourne Zone, whereas the Stawell and Bendigo Zones have a thick-skinned structural style. Internal faults in the Stawell and Bendigo Zones are mostly west-dipping listric faults, which extend from the surface to near the base of the crust. The Heathcote Fault Zone, the boundary between the Bendigo and Melbourne Zones, extends to at least 20 km, and possibly to the Moho. A striking feature in the seismic data is the markedly different seismic character of the mid to lower crust of the Melbourne Zone. The deep seismic reflection data for the Melbourne Zone have revealed a multilayered crustal structure that supports the Selwyn Block model.     

Crustal structure of the southern Dabie ultrahigh-pressure orogen and Yangtze foreland from deep seismic reflection profiling

A new 140‐km‐long seismic reflection profile provides a high‐resolution crustal‐scale image of the southern Dabieshan high‐pressure (HP) metamorphic belt and the Yangtze foreland fold‐and‐thrust belt. The seismic image of the stacked section shows that the southern Dabieshan metamorphic terrane and Yangtze foreland belt are separated by a large north‐dipping fault. In the foreland the upper crust is dominated by a series of folds and thrusts formed during the collisional stage in the mid‐Triassic; it was reworked by crustal extension resulting in the formation of a late Jurassic and Cretaceous red‐bed basin. The southern Dabieshan profile shows stacked crustal slabs developed along the margin of the collisional orogenic belt. The Moho reflectors at 10–11 s (～30–33 km) are seismically prominent and segmented by a number of south‐verging thrusts that were probably developed by foreland‐directed thrusting of the deeply subducted continental crust during exhumation. The seismic reflection profile suggests that structures related to the Triassic–Jurassic subduction and exhumation of the Yangtze plate are preserved despite the severe crustal extension superimposed during the late Mesozoic and Cenozoic.     

Crustal structure of mainland China from deep seismic sounding data

Since 1958, about ninety seismic refraction/wide angle reflection profiles, with a cumulative length of more than sixty thousand kilometers, have been completed in mainland China. We summarize the results in the form of (1) a new contour map of crustal thickness, (2) fourteen representative crustal seismic velocity–depth columns for various tectonic units, and, (3) a Pn velocity map. We found a north–south-trending belt with a strong lateral gradient in crustal thickness in central China. This belt divides China into an eastern region, with a crustal thickness of 30–45 km, and a western region, with a thickness of 45–75 km. The crust in these two regions has experienced different evolutionary processes, and currently lies within distinct tectonic stress fields. Our compilation finds that there is a high-velocity (7.1–7.4 km/s) layer in the lower crust of the stable Tarim basin and Ordos plateau. However, in young orogenic belts, including parts of eastern China, the Tianshan and the Tibetan plateau, this layer is often absent. One exception is southern Tibet, where the presence of a high-velocity layer is related to the northward injection of the cold Indian plate. This high-velocity layer is absent in northern Tibet. In orogenic belts, there usually is a low-velocity layer (LVL) in the crust, but in stable regions this layer seldom exists. The Pn velocities in eastern China generally range from 7.9 to 8.1 km/s and tend to be isotropic. Pn velocities in western China are more variable, ranging from 7.7 to 8.2 km/s, and may display azimuthal anisotropy.     

中国大陆科学钻探井区深部构造地震波反射特征

中国大陆科学钻探（CCSD）井位于苏鲁超高压变质带南部的东海县境内。井区地震勘探是钻探综合研究项目之一,该项目采用世界先进的数字地震仪和三分量数字检波器,对line 1线剖面进行了测量。依据地震勘探成果和CCSD岩心资料,通过对该地震剖面深部构造反射特征的详细分析,发现该区深部反射构造具有明显的三单元层结构。第一单元层,约在11 km内,出现6个强反射带,产状基本一致,倾向SE,倾角28°。岩心资料和VSP勘探资料证实前4个反射带主要是榴辉岩带和韧性剪切带的反射,榴辉岩带与片麻岩成不规则层状相间分布。第二单元层,约在11~30 km,有4个比较清楚的反射带,反射弱,连续性差,带内分布有不规则的弱反射。其产状也基本一致,倾角很小,比较平缓,由浅到深,倾向逐渐变为NW,它与第一单元层为角度不整合接触关系。第三单元层,约在30 km以下深度,其顶部出现了比较强的反射带,连续性较好,产状较平缓,倾向NW;而在顶部反射带以下,则反射均匀,没有不规则反射带。据上述反射特征,结合该区地质背景,推测第一单元层为中朝与扬子板块俯冲带的折返地壳层,第二单元层为扬子克拉通层,主要为变质岩层,第三单元层为上地幔层,其顶部的反射为莫霍面的反射。     

Accompanying seismic refraction investigations along the profile DEKORP 2-North

Seismic refraction measurements were carried out along the DEKORP 2-N reflection line. Traveltime data have been inverted to velocity-depth distributions using x-t-inversion- und ray-tracing-methods. The velocity-model shows alternating layers of high and low velocity. High-velocity values range from 6.0 to 6.6 km/s in the upper crust and from 70 to 8.2 km/s in the lower crust. In low-velocity zones velocities do not exceed 6.25 km/s. The crust/mantle boundary lies in about 28–30 km depth. Correlations exist between the velocity-model of refraction seismics and the line drawing section of reflection seismics.

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Zusammenfassung Entlang dem tiefenseismischen Reflexionsprofil DEKORP 2-Nord wurden seismische Refraktionsmessungen durchgeführt. Die Laufzeitdaten wurden mit Hilfe von x-t-Inversions- und Ray-Tracing-Verfahren in Geschwindigkeits-Tiefen-Verteilungen transformiert. Es treten alternierende Hoch- und Niedriggeschwindigkeitszonen auf. In der oberen Kruste treten hohe Geschwindigkeitswerte im Bereich von 6,0 bis 6,6 km/s auf und in der unteren Kruste Werte von 7,0 bis 8,2 km/s. Die Geschwindigkeitswerte in Niedriggeschwindigkeitszonen überschreiten 6,25 km/s nicht. Die Grenze zwischen Erdkruste und Erdmantel liegt in einer Tiefe von etwa 28 bis 30 km. Es bestehen Übereinstimmungen zwischen dem Geschwindigkeitsmodell der Refraktionsseismik und der Struktur der reflexionsseismischen Sektion.

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Résumé Des mesures en sismique-réfraction ont été effectuées le long du profil de sismique réflexion DEKORP 2-N. On a réalisé une inversion des temps de propagation en distribution vitesse/profondeur par les méthodes de l'inversion x-t et du traçage de raie. Le modèle des vitesses fait apparaître des couches alternées à grandes et faibles vitesses. Les grandes vitesses sont de 6,0 à 6,6 km/sec dans la croûte supérieure et de 7,0 à 8,2 km/sec dans la croûte inférieure. Dans les zones à faible vitesse, celle-ci n'excède pas 6,25 km/sec. La profondeur du contact croûte/manteau est d'environ 28 à 30 km. Il existe une correspondance entre le modèle des vitesses de la sismique-réfraction et l'image fournie par la sismique-réflexion.

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Crustal structure in the Barents-Kara region from detailed surveys by the method of deep seismic sounding. Paper 2

Seismogeologic sections for the Barents-Kara region along geotraverses 1-AR, 2-AR, and 3-AR with a total length of about 4000 km were obtained using the GODOGRAF software package developed at the Department of Seismometry and Geoacoustics of the Moscow State University. The data were travel times of refracted waves excited by approximately 100 sources along each traverse. This paper reports sections for the 3-AR traverse covering areas of the White Sea, Pechora Sea, and Kara Sea, and a geological interpretation of these. The sections cover depths down to 40–50 km and show basic crustal discontinuities, fold-thrust, rift, and paleospreading structural features, and paleosubduction zones. We characterize the possible character of the junction between the South Kara and North Kara basins. A geodynamic interpretation of the structures is provided for the Barents-Kara region.     

Recent vertical movements and their determination in the Rhenish Massif

The uplift of the Rhenish Massif started in late Tertiary time. The determination of the rates of recent vertical movements helps to interpret the whole, phenomenon. The data obtained by relevellings of high precision will be analysed by two different velocity models based on the concepts of Holdahl: the single-point model and the velocity surface model. Both models yield adjusted heights corresponding to a selected reference time and the velocities. The significance of the model parameters will be increased by introducing free adjustments with generalized inverses.     

Crustal structure of the ancestral northwestern California forearc region from seismic reflection imaging: implications for convergent margin tectonics

Analysis of a suite of 2-D seismic reflection profiles reveals that the northwestern Sacramento Valley and eastern Coast Range foothills, northern California, are underlain by a system of blind, west-dipping thrust faults. Homoclinally east-dipping and folded Mesozoic marine forearc strata exposed along the western valley margin define the forelimbs of northeast-vergent fault-propagation folds developed in the hanging walls of the thrusts. Exhumed coherent blueschists of the accretionary complex and attenuated remnants of the ophiolitic forearc basement presently exposed in the eastern Coast Ranges are in the hanging wall of the blind thrust system, and have been displaced from their roots in the footwall. Deep, east-dipping magnetic reflectors in the footwall of the thrust system may be fragments of sheared, serpentinized and attenuated ophiolitic basement. Restoration of slip on the thrusts suggests that the Coast Range fault, which is the exposed structural contact between the coherent blueschists and attenuated ophiolite, originally dipped east and is associated with the east-dipping magnetic reflectors in the footwall. This interpretation of the reflection data is consistent with previous inferences about the deep structure in this region, and supports a two-stage model for blueschist exposure in the eastern Coast Ranges: (1) blueschist exhumation relative to the forearc basin by attenuation of the ophiolitic basement along the east-dipping Coast Range fault system in late Cretaceous; (2) blueschists, attenuated ophiolite, and forearc strata all were subsequently uplifted and folded in the hanging wall of the blind thrust system beginning in latest Cretaceous–early Tertiary. The blind thrust system probably rooted in, and was antithetic to, the east-dipping subduction zone beneath the forearc region. Active transpressional plate motion in western California is locally accommodated, in part, by reactivation of blind thrust faults that originally developed during the convergent regime.     

深反射地震数据揭示的拉萨地体北部到羌塘地体南部地壳厚度的变化

由多个地体拼接而成的青藏高原,有着世界上最厚的地壳。在高原中部,从拉萨地体到羌塘地体,穿过班公湖—怒江缝合带(BNS)的地壳厚度变化长期存在争议。本文主要论述从拉萨地体北端横跨BNS到达羌塘地体的深地震反射剖面探测的结果。探测发现了清晰的Moho反射,揭示了拉萨地体—羌塘地体Moho深度和地壳厚度的变化。探测结果表明,在BNS下方Moho深度由南至北出现了6.2 km的急剧减小,并且与BNS向北28 km处的羌塘地体南部比较,地壳厚度变浅了12.5 km。否定了前人对BNS下方Moho存在20 km显著变化的认识。     

Orogenic structure of the Eastern Alps, Europe, from TRANSALP deep seismic reflection profiling

The TRANSALP Group, comprising of partner institutions from Italy, Austria and Germany, acquired data on a 340 km long deep seismic reflection line crossing the Eastern Alps between Munich and Venice. Although the field work was split into four campaigns, between fall 1998 and summer 2001, the project gathered for the first time a continuous profile across the Alps using consistent field acquisition and data processing parameters. These sections span the orogen itself, at its broadest width, as well as the editor Fred Davey and the two adjacent basins. Vibroseis and explosion data, complementary in their depth penetration and resolution characteristics, were obtained along with wide-angle and teleseismic data. The profile shows a bi-vergent asymmetric structure of the crust beneath the Alpine axis which reaches a maximum thickness of 55 km, and 80–100 km long transcrustal ramps, the southward dipping ‘Sub-Tauern-Ramp’ and the northward-dipping ‘Sub-Dolomites-Ramp’. Strongly reflective patterns of these ramps can be traced towards the north to the Inn Valley and towards the south to the Valsugana thrust belt, both of which show enhanced seismicity in the brittle upper crust. The seismic sections do not reveal any direct evidence for the presence of the Periadriatic Fault system, the presumed equivalent to the Insubric Line in the Western Alps. According to our new evolutionary model, the Sub-Tauern-Ramp is linked at depth with remnants of the subducted Penninic Ocean. The ‘crocodile’-type model describes an upper/lower crustal decoupling and wedging of both the European and the Adriatic–African continents.     

Crustal structure of the Barents Sea from seismic data

In 1976, the Institute of Physics of the Earth and the Institute of Oceanology, the U.S.S.R. Academy of Sciences, carried out deep seismic soundings in the Barents Sea along a profile 700 km long northeast of Murmansk. A system of reversed and overlapping traveltime curves from 200 to 400 km long has been obtained. The wave correlation was effected by several independent approaches, which identified on the records the refracted and reflected waves from boundaries in the Earth's crust and the upper mantle. Different methods were applied for the solution of the inverse problem: the isochrone method, the intercept-time method, and the iteration method.The use of these different methods gives an indication of the general applicability of the interpretation and of the most reliable elements in the seismic model.All the interpretations and representations of the section positively establish an essentially horizontal inhomogeneity of the Earth's crust in the Barents Sea. On the whole the structure is similar to that of deep sedimentary basins of the East European platform. The thickness of the sedimentary layer varies from 8 to 17 km, the average crustal thickness is about 35–40 km; the velocities in the upper part of the consolidated crust are 5.8–6.4 km/s; in the lower crust they are 6.8–7.0 km/s and higher.     

NIZUSE: a deep seismic reflection line in north-eastern Germany

Out of a dense network of seismic reflection lines for hydrocarbon exploration in the North-east German Basin, several lines were recorded to 12 s TWT to obtain information about the structure of the crust and the crust-mantle transition. One of these profiles is presented here. This stretches for 110 km in a NNE direction between Neustrelitz and the island of Usedom. It reaches from the External Variscides in the south across the North German Massif into the Rügen-Pomorze Terrane in the Baltic Sea. Below Cenozoic-Mesozoic-Paleozoic cover with clear reflections down to base Zechstein, the reflectivity varies considerably with depth and also laterally. The Paleozoic and Precambrian sediments and basement are generally void of reflections, but the lower crust and the Moho show strong reflections. To the north the reflectivity decreases, and the Moho depth increases to beyond the bottom of the record section at 12 s. There are no direct indications for deep-reaching faults such as the Trans-European Fault in the north. The North German Massif acted as a ramp towards the Variscan Orogeny, similar to the London-Brabant Massif further west.     

Crustal structure of the northern part of the Proterozoic Cuddapah basin of India from deep seismic soundings and gravity data

A crustal depth section was obtained from Deep Seismic Soundings (DSS) along the Alampur-Koniki-Ganapeshwaram profile, cutting across the northern part of the Proterozoic Cuddapah basin, India, running just south of latitude 16° N and between longitude 78° E and 81°E. The existence of a low-angle thrust fault at the eastern margin of the Cuddapah basin (Kaila et al., 1979) was confirmed along a second profile. Another low-angle thrust, along which charnockites with the granitic basement are upthrust against the Dharwars was delineated further east. The contact of the khondalites (lower Precambrian) with quaternary sediments near the east coast of India seems to be a fault boundary, which may be responsible for the thick sedimentary accumulation in the adjoining offshore region.The basement in the western part of the Cuddapah basin is very shallow and is gently downdipping eastward, to a depth of 1.7 km about 20 km west of Atmakur. It attains a depth of about 4.5 km in the deepest part of the Kurnool sub-basin, around Atmakur. Under the Nallamalai ranges its depth varies between 3.5 and 6.5 km, with an easterly dip. In the region north of the Iswarkuppam dome, the basement is at a depth of about 5.0 km, to about 6.8 km in the eastern part of the Cuddapah basin. Outside the eastern margin of the basin, the depth of the basement is about 1.8 km and further eastwards it is exposed. A fault at the contact of the khondalites with quaternary sediments near the east coast brings the basement down to a depth of approximately 1.3 km.In the Kurnool sub-basin the depth to the Moho discontinuity varies from 35 km under Atmakur to 39 km under the Nallamalai hills. In the region of the Iswarkuppam dome it is at a depth of about 36 km, deepening to about 39 km before rising to 37 km towards the east. Two-dimensional velocity modelling using ray-tracing techniques tends to confirm these results.Gravity modelling of the crustal structure, utilizing a four-layer crustal model in most parts along this profile, conforms to the observed gravity values. A weak zone in the eastern part of the profile where high-density material (density 3.05 g/cm³) has been found seems to be responsible for the gravity high in that part.     

Crustal stucture of the Barents-Kara Region based on detailed deep seismic sounding

In 1995–1998 and 2003–2005, detailed deep seismic soundings were undertaken in the Barents-Kara Region along geotraverses 1-AR, 2-AR, 3-AR with a total length of over 3000 km. Seismic cross-sections, up to 50 km deep as an average, were obtained using the software package GODOGRAPH designed at the Department of Seismometry and Geoacoustics of the Lomonosov Moscow State University. The study was based on refraction traveltime curves with approximately 100 curves per profile. The sections obtained along the 1-AR and 2-AR traverses were geologically interpreted. The main crustal boundaries, fold-thrust structural features of the lower crust and a suture zone between the North Barents Basin and the Caledonian Orogenic Belt were distinguished. Based on our data, the structure of the suture can be interpreted as an ancient subduction zone. The possible pattern of tectonic movements of the Barents Plate is characterized.     

Magnetotelluric study of the earth's crust along the deep seismic reflection profile DEKORP 2-N

In 1986/1987, 53 magnetotelluric soundings were carried out along the deep seismic reflection line DEKORP 2-N crossing the Münsterland basin and the Rhenish Massif. Examination of the data suggests one-dimensional interpretation to be appropriate for the Münsterland sites and still reasonable as an approximation for the major part of the Rhenish Massif sites. In some cases, the data are disturbed by man-made noise, in particular at the northern border of the Rhenish Massif, or affected by static shift distortion effects evidencing small scale, near surface electrical conductivity inhomogeneities. One-dimensional modeling for undistorted stations reveals a good conductor at a depth of 6–8 km in the Münsterland basin and at a depth of 14–16 km in the Rhenish Massif, shallowing to the north. Thus far, earlier MT results for these regions are largely corroborated. Comparison with reflection seismic results shows the conductor of the Rhenish Massif to coincide fairly well with a poorly reflective zone in the middle crust associated with strong reflectors at its upper and lower boundaries.The discussion of possibly responsible electrical conduction mechanisms considers electronic conduction to be most favorable for explanation of the Münsterland good conductor. Black shales with high organic content are regarded as being primarily capable of producing the observed high conductance because of adequate pre-graphitization of organic matter which may form a conducting network at conditions of very low grade metamorphism. In accordance with the crustal structure discernible from seismic reflections, an extension of this black shale horizon to the south into the Rhenish Massif is discussed. At the base of overthrusted rock masses it could have served as a gliding horizon during the Variscan folding era. Mobilization of its organic content and redeposition in shear zones would explain the good conductor of the Rhenish Massif by means of electronic conduction. Therefore, concurrence between the good conductor and strong seismic reflecting elements appears conceivable provided the latter represent shear zones as well. Electrolytic conduction, often supposed to be causative to high conductivity layers at midcrustal depths, would require considerable connected pore space. If the latter interpretation is correct the good conductor observed will indicate a recent thermal or extensional process.

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Zusammenfassung In den Jahren 1986 und 1987 wurden 53 magnetotellurische Sondierungen entlang des tiefenreflexionsseismischen Profils DEKORP 2-N in der Münsterländer Bucht und im Rheinischen Schiefergebirge durchgeführt. Die Auswertung der Daten läßt erkennen, daß eine eindimensionale Interpretation für die Meßpunkte in der Münsterländer Bucht angemessen ist und eine zulässige Näherung für den größten Teil der Meßpunkte im Rheinischen Schiefergebirge darstellt. In einigen Fällen sind die Daten durch künstliche elektromagnetische Signale gestört, besonders am nördlichen Rand des Rheinischen Schiefergebirges, oder durch »Static Shift«-Effekte verzerrt. Letztere weisen auf kleinräumige, oberflächennahe Leitfähigkeitsinhomogenitäten hin. Eindimensionale Modellrechnungen an ungestörten Stationen zeigen einen guten Leiter in 6 bis 8 km Tiefe in der Münsterländer Bucht und einen weiteren in 14 bis 16 km Tiefe im Rheinischen Schiefergebirge, der nach Norden hin ansteigt. Frühere MT-Ergebnisse für diese Gebiete werden damit weitgehend bestätigt. Ein Vergleich mit den reflexionsseismischen Ergebnissen macht deutlich, daß die Tiefenlage des guten Leiters im Rheinischen Schiefergebirge sehr gut mit einer reflexionsarmen Zone in der mittleren Kruste zusammentrifft, die an ihrer oberen und unteren Grenze von kräftigen Reflektorenbündeln eingeschlossen ist.Eine genaue Betrachtung der in Frage kommenden elektrischen Leitungsmechanismen ergibt, daß Elektronenleitung als wahrscheinlichste Erklärung für den guten Leiter in der Münsterländer Bucht anzusehen ist. Schwarzschiefer mit einem hohen Gehalt an organischem Material können die beobachteten großen Leitfähigkeiten erzeugen, wenn das organische Material durch eine niedriggradige Metamorphose prägraphitisiert ist und ein Netzwerk elektrischer Leiterbahnen ausbildet. In Anlehnung an die aus den seismischen Reflexionen ableitbare Krustenstruktur wird eine Ausdehnung dieses Schwarzschiefer-Horizontes nach Süden in das Rheinische Schiefergebirge diskutiert. An der Basis der überschobenen Gesteinspakete könnte er als Gleithorizont während der variszischen Faltung gedient haben. Eine mögliche Mobilisation seines organischen Gehaltes und Wiederablagerung in Scherzonen könnte den guten Leiter des Rheinischen Schiefergebirges somit ebenfalls durch Elektronenleitung erklären. Es liegt deshalb nahe anzunehmen, daß der gute Leiter unmittelbar identisch ist mit den Bündeln von Reflektoren, sofern diese ebenfalls als Überschiebungsbahnen anzusprechen sind. Elektrolytische Leitung, die oft als Ursache für Schichten mit hoher Leitfähigkeit in der mittleren und unteren Kruste vermutet wird, erfordert einen beträchtlichen konnektierten Porenraum. In diesem Fall wäre der gute Leiter als Hinweis auf rezente Wärmezufuhr oder Dehnungsprozesse zu deuten.

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Résumé En 1986 et 1987, 53 sondages magnéto-telluriques ont été effectués le long du profil de sismique-reflexion DEKORP 2-N mené à travers le bassin du Münsterland et le massif schisteux rhénan. L'examen des résultats montre qu'une interprétation uni-dimensionnelle convient à la région du Münsterland et est acceptable en première approximation pour la majeure partie du massif rhénan. Dans quelques cas, les mesures sont perturbées par des signaux électro-magnétiques d'origine artificielle, particulièrement au bord nord du massif rhénan, ou encore sont affectées par des effets de «static shift» traduisant des hétérogénéités de la conductivité électrique près de la surface. La modélisation uni-dimensionnelle, établie pour les stations non perturbées, met en évidence un bon conducteur situé à une profondeur de 6 à 8 km dans le Münsterland et un autre à 14 à 16 km dans le massif rhénan, ce dernier s'élevant vers le nord. Les résultats de mesures magnéto-telluriques antérieures sont ainsi confirmés. Par comparaison avec le profil de sismique-reflexion, le conducteur du massif rhénan correspond à une zône peu reflective de la croûte moyenne limitée à son toit et à son mur par des réflecteurs marqués.La discussion des mécanismes susceptibles d'être responsables de la conduction électrique fait apparaître la conduction électronique comme la plus probable dans le cas du conducteur du Münsterland. Des schistes noirs riches en matière organique peuvent justifier la haute conductivité en raison de la pré-graphitisation qui, dans des conditions de métamorphisme de très faible degré, peut engendrer un réseau conducteur. En liaison avec les structures crustales déduites de la sismique-réflexion, l'extension latérale éventuelle de cet horizon pourrait avoir joué le rôle de lubrifiant à la base de masses charriées au cours du plissement varisque. La mobilisation de son contenu organique reprécipité dans des shear-zones expliquerait la présence du bon conducteur sur le massif rhénan, par un processus de conduction électronique. Ainsi pourrait s'expliquer la coïncidence du bon conducteur et des bons réflecteurs sismiques, puisque ces derniers s'interprètent comme des surfaces de charriage. Un processus de conduction électronique, souvent considéré comme responsable de couches à haute conductivité dans la croûte moyenne, exige la présence d'un volume important de pores interconnectés. Si une telle interprétation est correcte, le bon conducteur observé serait l'indice d'un échauffement ou d'un processus extensif récent.

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DEKORP 2Nord , N-S 230 , E-W 170 , . . , , . DEKORP , , . , .

     

基于深反射地震剖面的沂沭断裂带岩石圈精细结构

为揭示沂沭断裂带深部结构及发生—发展过程,查清断裂切割深度及对岩石圈地幔的破坏,探讨沂沭断裂带的构造组合样式、运动方式、地壳稳定性及其对资源环境的约束作用,研究团队于2019年在沂沭断裂带南段沂南—莒县附近布设了一条长约60 km的深反射地震剖面,系统采集了沂沭断裂带和两侧地块的地震数据,对沂沭断裂带深部岩石圈精细结构进行了解剖.结果显示,该区岩石圈结构在横向上表现为以沂沭断裂带为界的块状结构特征,地壳厚度约30.8～39.5 km;莫霍面总体呈西浅东深态势,并被西倾的沂水—汤头断裂(F2)和昌邑—大店断裂(F4)错断,垂直落差达10.5 km.与浅部"两堑夹一垒"的构造组合样式不同的是,沂沭断裂带在深部剖面上表现为由沂水—汤头断裂(F2)和昌邑—大店断裂(F4)向上延伸与分叉散开的多条断裂组成"双枝状"构造组合样式.断裂带内被断层切割的界面反射波多呈向上的拱弧形,其构造形迹具有伸展、挤压和走滑并存的特征,推断这些界面为层间滑脱构造,它们指示了沂沭断裂带"多层滑移"构造运动方式.该断裂带不仅切穿了近地表、壳内地质界面,F2、F4断裂还向下切割莫霍面,深入岩石圈地幔,是深达地幔的深大断裂构造带,为地幔热物质的上涌提供了通道,对中生代的岩浆活动和内生成矿具有控制作用.地震剖面西端的铜井金矿成矿与沿F2断裂上侵的铜井杂岩体关系密切;剖面东端的火山机构保存完整,没有明显构造破坏痕迹,据此认为沂沭断裂带左行走滑主要发生在早白垩世青山期以前,其后水平滑移量应不大.从区域地质分布及地震反演结果看,昌邑—大店断裂(F4)明显将山东省分割为鲁西和鲁东两个地质构造单元,因此将其作为区域地质构造分界线是合理的.本项研究结果进一步加深了沂沭断裂带深、浅部结构的认识,为分析研究沂沭断裂带的深部过程和浅部构造响应及对资源环境的影响提供了资料约束.     

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。     

Crustal structure and tectonics of the Hidaka Collision Zone, Hokkaido (Japan), revealed by vibroseis seismic reflection and gravity surveys

This study is the first integrated geological and geophysical investigation of the Hidaka Collision Zone in southern Central Hokkaido, Japan, which shows complex collision tectonics with a westward vergence. The Hidaka Collision Zone consists of the Idon'nappu Belt (IB), the Poroshiri Ophiolite Belt (POB) and the Hidaka Metamorphic Belt (HMB) with the Hidaka Belt from west to east. The POB (metamorphosed ophiolites) is overthrust by the HMB (steeply eastward-dipping palaeo-arc crust) along the Hidaka Main Thrust (HMT), and in turn, thrusts over the Idon'nappu Belt (melanges) along the Hidaka Western Thrust (HWT). Seismic reflection and gravity surveys along a 20-km-long traverse across the southern Hidaka Mountains revealed hitherto unknown crustal structures of the collision zone such as listric thrusts, back thrusts, frontal thrust-and-fold structures, and duplex structures. The main findings are as follows. (1) The HMT, which dips steeply at the surface, is a listric fault dipping gently at a depth of 7 km beneath the eastern end of the HMB, and cutting across the lithological boundaries and schistosity of the Hidaka metamorphic rocks. (2) A second reflector is detected 1 km below the HMT reflector. The intervening part between these two reflectors is inferred to be the POB, which is only little exposed at the surface. This inference is supported by the high positive Bouguer anomalies along the Hidaka Mountains. (3) The shallow portion of the IB at the front of the collision zone has a number of NNE-dipping reflectors, indicative of imbricated fold-and-thrust structures. (4) Subhorizontal reflectors at a depth of 14 km are recognized intermittently at both sides of the seismic profile. These reflectors may correspond to the velocity boundary (5.9–6.6 km/s) previously obtained from seismic refraction profiling in the northern Hidaka Mountains. (5) These crustal structures as well as the back thrust found in the eastern end of the traverse represent characteristics of collisional tectonics resulting from the two collisional events since the Early Tertiary.     

深地震反射剖面揭示嫩江断裂带中段的深部几何形态与地壳形变

嫩江断裂带对探讨松辽盆地的演化、 与大兴安岭的盆山耦合关系以及兴安地块与松嫩地块的归属问题具有重要研究意义。本文利用深地震反射剖面, 揭示了嫩江断裂带中段的深部几何形态, 表明它目前呈花状构造样式; 它也并非松辽盆地内部的拆离断层, 而是一条超壳断裂, 控制了大兴安岭的物质东移和松辽盆地的中生代沉积; 现今的嫩江断裂带同晚古生代兴安地块与松嫩地块拼合的嫩江构造拼贴带并没有继承性关系; 莫霍面可能在后期的演化过程中扮演了滑脱层的作用, 致使向西俯冲的地幔体向东位移。     

Crustal studies of the Koyna region using explosion data from deep seismic soundings

This study is based on the seismic data collected as a result of explosions carried out during the 1976 and 1978 Deep Seismic Sounding (DSS) field operations in the Koyna region. These shots were exploded from twelve shot points by the National Geophysical Research Institute along the Guhagar-Chorochi and Kelsi-Loni profiles.Refraction studies of the records reveal a two-layered crust. The top layer consists 17 km of granite and the second layer 19 km of basalt, giving the average depth of the Moho as 36 km in the region. The velocities of the phases P_g, P* and P_n have been computed as 5.82 ± 0.01, 6.61 ± 0.05 and 8.23 ± 0.05 km/sec respectively and those of S_g, S* and S_n as 3.41 ± 0.00, 4.09 ± 0.07 and 4.60 ± 0.08 km/sec respectively. The shear wave velocity in the basement rock has been found to be lower in comparison with other regions of the peninsular India.In some cases reflections were recorded both from the Moho as well as from the intermediate layer. These reveal a crustal thickness of 39 km with 19 km of granitic and 20 km of basaltic layers.Coda signal durations from DSS explosions recorded by microearthquake seismographs indicate a lateral heterogeneity in the crust on either side of Karad in an east-west direction.