Geomagnetic anomalies and regional geology in India along 18° N parallel

The mobile geomagnetic measurements made during 1970–73 by NGRI across the Indian Peninsula between Alibag and Kalingapatnam have been examined. It is noticed that the meanZ values after normal correction and the long periodZr/Hr ratio reveal different levels on the three different geological provincesviz., the Deccan traps, peninsular granites and the easternghat metamorphics.     

Structure of the lithosphere along the deep seismic sounding profile: Tien Shan—Pamirs—Karakorum—Himalayas

During 1973–1977, as part of the International Geodynamic Project, some seismic investigations of the Earth's crust have been carried out by geotraverses of the Tien Shan—Pamirs—Karakorum—Himalayas. The seismic data obtained together with other geophysical information, allow the construction and interpretation of the lithospheric section through the Pamirs-Himalayas structure. This section includes thick crust with complex layering, supra-asthenospheric and asthenospheric layers of the upper mantle. The thickness of the Earth's crust increases from 50–55 km in the north, in the Ferghana depression (Tien Shan), to 70–75 km in the south, near the Karakul Lake (Northern Pamir). It varies within 60–65 km for the Central and Southern Pamir, Karakorum and the Inner Himalayas. Its thickness is least (35–37 km) in the south, under the outer margin of the Himalayan foredeep. Extreme gravity minima and depressions on the geoid surface correspond to the regions with maximum thickness of the Earth's crust. The centers of the disturbing masses on the geoid surface are located in the vicinity of the asthenosphere's upper layer; this determines the effect of the whole lithospheric layer, including its asthenospheric layer, at intense changes of gravity anomalies. The asthenospheric upper layer is recorded at a depth of about 120 km, its base at a depth of 200 km, in the northern and southern regions, and 300 km in its central part (Southern Pamir, Karakorum). In the middle asthenospheric layer, wave velocities decrease to 7.5 km/sec, under the base they increase to 8.4 km/sec and reach 9.4 km/sec at a depth of about 400 km. In the supra-asthenospheric layer of the upper mantle, longitudinal and shear wave-velocities slightly increase (by less than 0.1 km/sec) towards its base.     

Geomagnetic induction responses of anisotropic conducting mantle

Phase change of dielectric magnesiowustite in the lower mantle may leave signatures in geomagnetic records of the globally distributed array of observatories. The related features appear in EM induction responses of lower mantle, which are studied theoretically. The surface EM field corresponding to a response of the earth with conductivity anisotropy in a mantle spherical layer is presented as the sum of the magnetic and electric modes. Equations for the fields of both modes and their relationship in a weakly anisotropic earth are obtained by the perturbation method. The two field modes are analyzed jointly and separately to characterize the conductivity tensor of the anisotropic lower mantle. The tensor elements corresponding to the tangential components of the field can be estimated from the magnetic mode alone recorded currently by the global network of geomagnetic observatories. For the tensor data to be complete, observatory data on lateral variations of the electric field are required in addition to three-component geomagnetic records.     

Anomalously high uplift rates along the Ventura—Santa Barbara Coast, California—tectonic implications

The NW—SE trending segments of the California coastline from Point Arena to Point Conception (500 km) and from Los Angeles to San Diego (200 km) generally parallel major right-lateral strike-slip fault systems. Minor vertical crustal movements associated with the dominant horizontal displacements along these fault systems are recorded in local sedimentary basins and slightly deformed marine terraces. Typical maximum uplift rates during Late Quaternary time are about 0.3 m/ka, based on U-series ages of corals and amino-acid age estimates of fossil mollusks from the lowest emergent terraces.In contrast, the E–W-trending segments of the California coastline between Point Conception and Los Angeles (200 km) parallel predominantly northward-dipping thrust and high-angle reverse faults of the western Transverse Ranges. Along this coast, marine terraces display significantly greater vertical deformation. Amino-acid age estimates of mollusks from elevated marine terraces along the Ventura—Santa Barbara coast imply anomalously high uplift rates of between 1 and 6 m/ka over the past 40 to 100 ka. The deduced rate of terrace uplift decreases from Ventura to Los Angeles, conforming with a similar trend observed by others in contemporary geodetic data.The more rapid rates of terrace uplift in the western Transverse Ranges reflect N—S crustal shortening that is probably a local accommodation of the dominant right-lateral shear strain along coastal California.     

Imaging granitic plutons along the IBERSEIS profile

The parameters used for the acquisition of the IBERSEIS deep seismic reflection profile in the SW Iberian Peninsula provide seismic images of the deep crust as well as a high resolution section of the shallow subsurface. A very dense array of sources and receivers allowed high resolution tomographic studies in zones of special interest (granitic plutons). The three dimensional tomographic inversion produced velocity models along a 500 m wide and 1000 m deep strip along the IBERSEIS transect in the areas of La Bazana, La Dehesilla, Feria and Villafranca. In these high resolution velocity models (sampled by 50 × 50 × 50 m cells), high velocity anomalies indicate the geology and extension of the granitic plutons at depth. This directly correlates with the surface outcrops. Moreover, tomographic models provided valuable information for the geometry and characterization of fractured and fresh domains in a rock volume. Furthermore, a piggy back seismic acquisition experiment using additional seismic instrumentation from the University of Paris Sud (40-channel DMT) provided perpendicular, offline recordings of the Vibroseis sources. This additional recording system was deployed perpendicularly to the main IBERSEIS seismic reflection line and provided additional 3D control.     

Geomagnetic field anomalies over the Lombok Island region: an attempt to understand the local tectonic changes

During the last years, several investigations on the earthquakes and related tectonic structures along the Java Trench have been conducted. In this study, we focus on the Lombok Island—West Nusa Tenggara (Indonesia), which lies between the centres of the highest intensity of contiguous negative–positive geomagnetic anomalies in this region. The geological and tectonic structures, however, are not known in detail for this island, whereas a better knowledge of these conditions could lead to better estimate the potential earthquake risks and thus a possible improvement of the existing early warning system. We have performed a ground-based geomagnetic survey at 56 stations in the Lombok Island during October–November 2006 for a detailed mapping and interpretation of geomagnetic anomalies related to the geological and tectonic characteristics. The 2D and 3D magnetic maps show a general geomagnetic anomaly pattern in the Lombok Island which consists of repeated contiguous negative–positive anomalies. Two forward models have been proposed for a profile connecting a strongest apparent dipolar structure. The first model assumes a uniformly magnetized sphere as the source of the anomaly, and could be interpreted as a specific local structure composed by a quite large magnetic body. The second model considers several rocks with different susceptibilities and magnetizations, and could be interpreted as a discontinuity in the geological structures. This model agrees with the local geological surface conditions and the known large scale regional tectonic structure. Therefore, it is used to interpret our results in terms of tectonic characteristics, which suggests the potential existence of a new tectonic element (e.g. a local normal fault) in this region.     

Magnetotelluric investigations along the URSEIS profile: first results

An interpretation of an E–W profile of 30 magnetotelluric field stations along the URSEIS transect is presented. The magnetotelluric instruments used covered a period range from 0.2 to approximately 500 s and thus enabled a resolution of shallow and upper crustal structures. After decomposition of the impedance tensor a 2D interpretation was performed. The model obtained by a 2D inversion displays a conductive zone west of the Main Uralian Fault. It appears to be consistent with a tectonic division obtained by URSEIS seismic studies.     

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 velocity inhomogeneities along the Hirapur–Mandla profile, central India and its tectonic implications

Wide-angle seismic and gravity data across the Narmada-Son lineament (NSL) in central India are analyzed to determine crustal structure, velocity inhomogeneities and hence constrain the tectonics of the lineament. We present the 2-D crustal velocity structure from deep wide-angle reflection data by using a ray-trace inverse approach. The main result of the study is the delineation of fault-bounded horst raised to a subsurface depth (1.5 km) and the Moho upwarp beneath the NSL. The crust below the basement consists of three layers with velocities of 6.45–6.7, 6.2–6.5 and 6.7–6.95 km/s and interface depths of about 5.5–8.7, 14–17 and 18–23 km along the profile. The low-velocity (6.2–6.5 km/s) layer goes up to a depth of 5 km and becomes the thickest part (13 km), while the overlying high-velocity (6.45–6.7 km/s) layer becomes the thinnest (3 km) and upper boundary lies at a depth of 1.5 km beneath the NSL. The overall uncertainties of various velocity and boundary nodes are of the order of ±0.12 km/s and ±1.40 km, respectively. The up-lifted crustal block and the up-warping Moho beneath the NSL indicate that the north and south faults bounding the NSL are deeply penetrated through which mafic materials from upper mantle have been intruded into the upper crust. Gravity modeling was also undertaken to assess the seismically derived crustal features and to fill the seismic data gap. The lateral and vertical heterogeneous nature of the structure and velocity inhomogeneities in the crust cause instability to the crustal blocks and played an important role in reactivation of the Narmada south fault during the 1997 Jabalpur earthquake.     

Tangential velocity profile for pseudoplastic power-law fluids in the hydrocyclone — A theoretical derivation

The tangential velocity profile for pseudoplastic power-law fluids in a hydrocyclone is determined with an analytical solution to a simplified momentum balance equation by the method of perturbation. A numerical solution of the non-linear two-point boundary-value problem by the finite-difference method provides a verification of the perturbation solution. The tangential velocity is found to be controlled by a modified Reynolds number involving the radial velocity at the outer radius of the hydrocyclone and the consistency and flow-behavior indices of the fluid. The radial position of the maximum tangential velocity decreases with an increase in the modified Reynolds number and/or the degree of pseudoplasticity of the fluid. This analysis suggests that improved performance of hydrocyclones may be possible for pseudoplastic fluid systems as the tangential velocity profiles are steeper and the free-vortex zone is extended inwards.     

Dipping basement reflectors along volcanic passive margins—new insight using data from the Kerguelen Plateau

Multichannel seismic reflection data from the Southern Kerguelen Plateau show many dipping basement reflectors associated with volcanic flows. These reflectors are quite similar in their shape to seaward-dipping basement reflectors observed along volcanic passive margins. On the Kerguelen Plateau the sources are updip of the basement reflectors, in the presently extinct and eroded volcanoes. We suggest that the same source/reflector geometry may also apply to the seaward-dipping basement reflectors observed along passive margins. We interpret these reflectors to be the result of volcanism on the passive margin which flowed in all directions into the newly created ocean basin at an early spreading stage.     

Tectonic implications of gravity anomalies associated with some continental rift valleys — a model study

An asthenospheric, diapir-type continental rift-valley model is developed. This model seems to satisfy the gravity anomalies observed over some of the rifts of East Africa without necessitating the assumption of excessively thick sections of low-density sediments or exceedingly low-density volcanics in the valleys. It is suggested that frequently observed large-amplitude Bouguer anomaly minima coinciding with some rift valleys are partially caused by hot creep of material from the lower crust into the mantle material in the rift zone.     

Middle and late Holocene sea-levels along the Israel Mediterranean coast — evidence from ancient water wells

During the course of a study of historical water wells along the Israeli shore, which has been in progress since 1984, an innovative method for investigating sea-level fluctuations was developed. Eighteen ancient water wells were re-dug, 14 by the author, and four by archaeologists. Most of the re-dug wells are found at archaeological sites located a very short distance inland from the present shoreline. Evidence of ancient ground-water levels found in the wells directly reflects on historic eustatic sea-level changes, and the rate at which the end of the post-glacial transgression advanced. A critical question concerns the durability and life span of these wells, as the true age is very important for the accurate reconstruction of a sea-level curve. The Pre-Pottery Neolithic well of Atlit-Yam, which is the oldest known well in the world (ca. 8000 yr old), enabled the most accurate sea-level reconstruction for early Holocene times. Sea-level rise during that period of the Atlit-Yam site was of the order of 20 mm yr⁻¹ at the beginning, slowing to 6–8 mm yr⁻¹ at the abandonment of this site at ca. 7.5 ka BP, when it was flooded by the advancing and rising sea. After reaching its present level, sea-level fluctuations for the past 2.5 millennia were not greater than 1.5 to 2 m. © 1997 John Wiley & Sons, Ltd.     

Flexural response of the Venetian foreland to the Southalpine tectonics along the TRANSALP profile

The Venetian Basin was affected by flexure related to the Southalpine shortening phase during the Middle Miocene – Early Pliocene. This downbending is quantified here using a two‐dimensional flexural model. A recently improved data set on basin geometry based on the bottom of the Serravallian–Tortonian clastic wedge, on palaeobathymetry and gravity anomalies is used to constrain the components of flexure and to test the importance of the initial bathymetry in evaluating the contribution of surface loads to deflection. A good fit is obtained assuming a northward broken plate configuration of the downbent Adriatic plate with an effective elastic thickness of 20 km. Results highlight that, in the studied region, flexure related to the Eastern Southern Alps is totally due to surface loads (topographic load partly replacing initial bathymetry) and that no hidden loads are required. Furthermore, the palaeobathymetry contributes up to 50% to the total flexure in the studied region.     

Upper mantle roots of Siberian craton basement structures along the Rift DSS profile

We investigate the upper mantle velocity structure through processing first arrival data from peaceful nuclear explosions. The reported 2D model has been obtained by ray tracing for a spherical Earth, unlike the classical plane-approximation approach with subsequent spherical symmetry corrections, which is not always applicable to a laterally heterogeneous subsurface. The upper mantle velocity highs and lows imaged to 200–220 km depths show obvious correlation with major structures of the craton basement. Namely, low-velocity zones are observed beneath basins, the largest (to 8.0–8.1 km/s) under the Vendian–Early Cambrian Sayan–Yenisei syneclise. A discontinuous high-velocity layer (8.6–8.7 km/s) at depths between 150 and 240 km is underlain by a zone of lower velocity (8.50–8.55 km/s) down to the 410 km discontinuity, where the velocity at the top of the transition zone is 9.4–9.5 km/s.     

Crustal structure of the Eastern Alps along the TRANSALP profile from wide-angle seismic tomography

The objective of the TRANSALP project is an investigation of the Eastern Alps with regard to their deep structure and dynamic evolution. The core of the project is a 340-km-long seismic profile at 12°E between Munich and Venice. This paper deals with the P-wave velocity distribution as derived from active source travel time tomography. Our database consists of Vibroseis and explosion seismic travel times recorded at up to 100 seismological stations distributed in a 30-km-wide corridor along the profile. In order to derive a velocity and reflector model, we simultaneously inverted refractions and reflections using a derivative of a damped least squares approach for local earthquake tomography. 8000 travel time picks from dense Vibroseis recordings provide the basis for high resolution in the upper crust. Explosion seismic wide-angle reflection travel times constrain both deeper crustal velocities and structure of the crust–mantle boundary with low resolution. In the resulting model, the Adriatic crust shows significantly higher P-wave velocities than the European crust. The European Moho is dipping south at an angle of 7°. The Adriatic Moho dips north with a gentle inclination at shallower depths. This geometry suggests S-directed subduction. Azimuthal variations of the first-break velocities as well as observations of shear wave splitting reveal strong anisotropy in the Tauern Window. We explain this finding by foliations and laminations generated by lateral extrusion. Based on the P-wave model we also localized almost 100 local earthquakes recorded during the 2-month acquisition campaign in 1999. Seismicity patterns in the North seem related to the Inn valley shear zone, and to thrusting of Austroalpine units over European basement. The alignment of deep seismicity in the Trento-Vicenza region with the top of the Adriatic lower crust corroborates the suggestion of a deep thrust fault in the Southern Alps.     

Preliminary interpretation of the magnetic anomalies in the area of the seismic reflection profile DEKORP 2-N

The magnetic anomalies in the surroundings of the DEKORP 2-N profile — within a rectangle of 230 km N-S and 170 km E-W — are interpreted by a two-layer model where the shape of an interface between an upper nonmagnetic and a lower magnetized layer can be determined. The depth and the amplitudes of the vertical undulations of the interface may vary within certain intervals. Along the DEKORP profile in its Rhenohercynian portion this interface is adjusted to the seismic results by fitting the interface to a weakly reflecting band observed in the seismic section. Thereby the depth of the corresponding zone within the Earth's crust can be extrapolated from the profile line into the surrounding area.     

青藏铁路沿线多年冻土区天然气水合物的地质、地球化学异常

通过对青藏铁路沿线多年冻土区重点地段野外冻胀、坍塌、冷泉冒气现象的观测及对低空大气、冷泉气体、表层冻土沉积物、地下冰等不同介质中气体的地球化学分析,发现部分冻土区如托纠山、昆仑山口、雁石坪等地区存在着明显的地质地球化学异常。这些气体地球化学异常可能主要是由青藏高原多年冻土区不同类型的断裂作用把深部气体带至浅部冻土沉积物中形成的,另一部分可能与浅部有机质转化成烃的作用相关,还有一部分可能与冰川发生位移造成的烃类气体聚集体重新分配有关;地质异常现象可能与该区构造热活动或天然气水合物形成的演化有关。对比前人的模拟计算结果,本次地质地球化学异常区的冻土厚度基本能满足天然气水合物形成的温压条件,这些异常可能与天然气水合物有关。