Seismic properties of the upper mantle beneath Lanzarote (Canary Islands): Model predictions based on texture measurements by EBSD   总被引：1，自引：0，他引：1  

Pierre Vonlanthen  Karsten Kunze  Luigi Burlini  Bernard Grobety 《Tectonophysics》2006,428(1-4):65-85

We present a petrophysical analysis of upper mantle xenoliths, collected in the Quaternary alkali basalt fields (Series III and IV) from the island of Lanzarote. The samples consist of eight harzburgite and four dunite nodules, 5 to 15 cm in size, and exhibit a typical protogranular to porphyroclastic texture. An anomalous foliation resulting from strong recovery processes is observed in half of the specimens. The lattice preferred orientations (LPO) of olivine, orthopyroxene and clinopyroxene were measured using electron backscatter diffraction (EBSD). In most samples, olivine exhibits LPOs intermediate between the typical single crystal texture and the [100] fiber texture. Occasionally, the [010] fiber texture was also observed. Simultaneous occurrence of both types of fiber textures suggests the existence of more than one deformation regime, probably dominated by a simple shear component under low strain rate and moderate to high temperature. Orthopyroxene and clinopyroxene display a weaker but significant texture. The LPO data were used to calculate the seismic properties of the xenoliths at PT conditions obtained from geothermobarometry, and were compared to field geophysical data reported from the literature. The velocity of P-waves (7.9 km/s) obtained for a direction corresponding to the existing seismic transect is in good agreement with the most recent geophysical interpretation. Our results are consistent with a roughly W–E oriented fastest P-wave propagation direction in the uppermost mantle beneath the Canary Islands, and with the lithosphere structure proposed by previous authors involving a crust–mantle boundary at around 18 km in depth, overlaid by intermediate material between 11 and 18 km.  相似文献   

Post-deformational annealing of calcite rocks   总被引：3，自引：3，他引：3  

Auke Barnhoorn  Misha Bystricky  Luigi Burlini  Karsten Kunze 《Tectonophysics》2005,403(1-4):167-191

The evolution of microstructure and crystallographic preferred orientation (CPO) during post-deformational annealing was studied on three calcite rock types differing in purity and grain size: Carrara marble (98% calcite, mean grain size of 115 μm), Solnhofen limestone (96%, 5 μm) and synthetic calcite aggregates (99%, 7 μm). Samples were first deformed in torsion at 727 °C at a shear strain rate of 3 × 10^{− 4} s^{− 1} to a shear strain of 5 and subsequently heat-treated at 727 °C for various durations between 0 and 24 h. Microstructures and CPOs were analysed by optical microscopy, image analysis and electron backscatter diffraction (EBSD).All rock types deformed in the dislocation creep field at the same applied conditions, but their microstructures and CPOs after deformation and after annealing differed depending on starting grain size and material composition. In Carrara marble and in the synthetic calcite aggregate, a strong CPO developed during deformation accompanied by dynamic recrystallisation with significant changes in grain size. During annealing, widespread grain growth and subtle changes of CPO occurred, and equilibrated foam microstructures were approached after long annealing times. The CPO is the only feature in annealed samples indicating an earlier deformation phase, although it is not always identical to the CPO formed during deformation. In the more impure Solnhofen limestone, secondary phases on grain boundaries suppressed grain boundary mobility and prevented both the formation of a recrystallisation CPO during deformation and grain size modification during deformation and annealing.  相似文献   

Petrogenesis of ultramafic and mafic xenoliths from Mesozoic basanites in southern Sweden: constraints from mineral chemistry     

Tatjana Rehfeldt  Karsten Obst  Leif Johansson 《International Journal of Earth Sciences》2007,96(3):433-450

Jurassic basanite necks occurring at the junction of two major fault zones in Scania contain ultramafic (peridotites, pyroxenites) and mafic xenoliths, which together indicate a diversity of upper mantle and lower crustal assemblages beneath this region. The peridotites can be subdivided into lherzolites, dunites and harzburgites. Most lherzolites are porphyroclastic, containing orthopyroxene and olivine porphyroclasts. They consist of Mg-rich silicates (Mg# = Mg/(Mg + Fe_tot) × 100; 88–94) and vermicular spinel. Calculated equilibration temperatures are lower in porphyroclastic lherzolites (975–1,007°C) than in equigranular lherzolite (1,079°C), indicating an origin from different parts of the upper mantle. According to the spinel composition the lherzolites represent residues of 8–13% fractional melting. They are similar in texture, mineralogy and major element composition to mantle xenoliths from Cenozoic Central European volcanic fields. Dunitic and harzburgitic peridotites are equigranular and only slightly deformed. Silicate minerals have lower to similar Mg# (83–92) as lherzolites and lack primary spinel. Resorbed patches in dunite and harzburgite xenoliths might be the remnants of metasomatic processes that changed the upper mantle composition. Pyroxenites are coarse, undeformed and have silicate minerals with partly lower Mg# than peridotites (70–91). Pyroxenitic oxides are pleonaste spinels. According to two-pyroxene thermometry pyroxenites show a large range of equilibration temperatures (919–1,280°C). In contrast, mafic xenoliths, which are mostly layered gabbronorites with pyroxene- and plagioclase-rich layers, have a narrow range of equilibration temperatures (828–890°C). These temperature ranges, together with geochemical evidence, indicate that pyroxenites and gabbroic xenoliths represent mafic intrusions within the Fennoscandian crust.  相似文献   

Modelling of convective turbulence with a two-equation k-ɛ turbulence closure scheme     

Adolf Stips  Hans Burchard  Karsten Bolding  Walter Eifler 《Ocean Dynamics》2002,52(4):153-168

 In this study we aim at comparing turbulence parameters from field observations and model simulations under convective conditions. The comparison is focused on the depth dependence and temporal dynamics of viscous and diffusive dissipation rates ɛ and χ. The near-surface observations were obtained by using a quasi-free-rising profiler which measured small-scale shear and temperature fluctuations to within the vicinity of the water surface. Convective conditions during the experiment are characterized by low wind speeds (between 0 and 4 m s⁻¹) and a typical heat loss of about 150 Wm⁻². We applied a state-of-the-art two-equation k-ɛ turbulence model with an algebraic second-moment closure scheme. The qualitative agreement of the turbulence quantities resulting from observations and simulations is rather good. The temporal dynamics of the temperature field is simulated correctly, whereas in the spatial dynamics some deficiencies of the model due to its local character can be seen. It is concluded that such models realistically reproduce convective turbulence and therefore represent a reasonable compromise between complexity and simplicity, so that they can be used with acceptable costs in large-scale models. Received: 31 October 2001 / Accepted: 20 September 2002 Acknowledgements The whole project was intitiated by Peter Schlittenhardt, who strongly supported the development of the observational technique in uprising mode and encouraged us to undertake the experiment in Lake Maggiore; for which we will thank him most. Performing the measurement campaign was only possible with the help of several colleagues from the Marine Environment Unit at the Joint Research Centre. Thanks to all of them, but especially to Dirk van d. Linde, Ulisse Devisioni, Bjarke Rasmussen and Hartmut Prandke. The Istituto Italiano di Idrobiologia, Verbania Pallanza, provided the boat Livia for the installation and removal of the measurement system; thanks to the master Stefano Maurizio for his engagement. Ute Tschesche provided the data evaluation and presentation software and Judith Challis helped to polish our English. Part of the study was␣supported by the PROVESS project (MAS3-CT97-0159); thanks to our sponsors at the European Commission. We are further grateful to two anonymous referees for their constructive comments. Responsible Editor: Charitha Pattiaratchi  相似文献   

Multiphase flow dynamics during CO2 disposal into saline aquifers   总被引：2，自引：0，他引：2  

Karsten Pruess  Julio García 《Environmental Geology》2002,42(2-3):282-295

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Texture development of polycrystalline anhydrite experimentally deformed in torsion     

Florian Heidelbach  Iona C. Stretton  Karsten Kunze 《International Journal of Earth Sciences》2001,90(1):118-126

A polycrystalline aggregate of anhydrite was deformed in torsion to a maximum shear strain of 8.1 at 700°C and a maximum shear strain rate of 5᎒^-3 s^-1. The crystallographic preferred orientation (CPO or texture) was investigated as a function of shear strain/shear strain rate in a radial profile from the centre to the edge of the sample. A deformation texture developed at shear strains of 1.5-2 (corresponding to shear strain rates of 1 to 1.3᎒^-3 s^-1) and reached a stable position relative to the kinematic frame at a shear strain of 3.7 (2.3᎒^-3 s^-1). Further shear strain only led to a small increase in texture strength but no change in the orientation relative to the kinematic frame. The CPO is very similar to naturally observed textures and can be explained by the activity of the {001}<010> and {012}<121> slip systems. Although independent mechanical data indicate that a change of mechanism from dislocation- to diffusion-controlled creep occurred at a shear strain of approximately 1.5, the texture does not weaken, but rather increases, in strength with higher shear strains.  相似文献   

Corrections for near surface effects: Decomposition of the magnetotelluric impedance tensor and scaling corrections for regional resistivities: A tutorial   总被引：1，自引：0，他引：1  

Ross W. Groom  Karsten Bahr 《Surveys in Geophysics》1992,13(4-5):341-379

This paper primarily examines the effects of small-scale or near-surface conductivity inhomogeneities on the magnetotelluric (MT) impedance tensor. These effects cause three different types of distortion results. (1) The well-knownstatic shifts of sounding curves. (2) When the underlying regional setting is two-dimensional then the two regional impedances are mixed in an arbitrary coordinate system. Thus the level and shape of each sounding curve is distorted as are the phases. (3) At sufficiently high frequencies these effects generate anomalous magnetic fields that in turn alter the background phases.This tutorial first explores the usefulness of various MT tensor analysis techniques to overcome the problem of phase mixing and to recover regional information in the presence of local geological noise. Synthetic and experimental data are considered. A sequence ofa priori models of increasing complexity are described. The use of appropriate decompositions of the MT tensor each with an increasing number of parameters is emphasised. In a second part, phase mixing and static shifts are examined from a synoptic view. Some static shift removal techniques that can be used in conjunction with the decomposition are discussed. This paper is not a review but rather an investigation of a few methods that the authors have found useful with field data.  相似文献   

The North-Chilean Coast Range — an example for the development of an active continental margin     

Dr. Karsten Berg  Dr. Christoph Breitkreuz  Dr. Klaus -Werner Damm  Siegfried Pichowiak  Prof. Dr. Werner Zeil 《International Journal of Earth Sciences》1983,72(2):715-731

Analysis of extended plutons in the Coast Range of North Chile between 25°30′ and 26°35′ led to the recognition of a complex magmatic and structural evolution from the Upper Paleozoic to the Tertiary. The ascension of the intrusive bodies is dictated by deep-seated block tectonics. Generally the chemistry changes from S-type magmas in the Paleozoic to I-type magmas in the Mesozoic and Cainozoic. This is accompanied by a change in the structural geology of the continental margin which we present in six hypothetic phases (Devonian-Tertiary). We are only at the beginning of an encompassing synthesis of the genesis of the Andean orogen.  相似文献   

Collisional tectonics of the Baltic Shield in the northern Gulf of Bothnia from seismic data of the BABEL project     

Karsten Gohl  Laust B. Pedersen 《Geophysical Journal International》1995,120(1):209-226

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Climatic and limnological changes at Lake Karakul (Tajikistan) during the last ~29 cal ka     

Liv Heinecke  Steffen Mischke  Karsten Adler  Anja Barth  Boris K. Biskaborn  Birgit Plessen  Ingmar Nitze  Gerhard Kuhn  Ilhomjon Rajabov  Ulrike Herzschuh 《Journal of Paleolimnology》2017,58(3):317-334

We present results of analyses on a sediment core from Lake Karakul, located in the eastern Pamir Mountains, Tajikistan. The core spans the last ~29 cal ka. We investigated and assessed processes internal and external to the lake to infer changes in past moisture availability. Among the variables used to infer lake-external processes, high values of grain-size end-member (EM) 3 (wide grain-size distribution that reflects fluvial input) and high Sr/Rb and Zr/Rb ratios (coinciding with coarse grain sizes), are indicative of moister conditions. High values in EM1, EM2 (peaks of small grain sizes that reflect long-distance dust transport or fine, glacially derived clastic input) and TiO₂ (terrigenous input) are thought to reflect greater influence of dry air masses, most likely of Westerly origin. High input of dust from distant sources, beginning before the Last Glacial Maximum (LGM) and continuing to the late glacial, reflects the influence of dry Westerlies, whereas peaks in fluvial input suggest increased moisture availability. The early to early-middle Holocene is characterised by coarse mean grain sizes, indicating constant, high fluvial input and moister conditions in the region. A steady increase in terrigenous dust and a decrease in fluvial input from 6.6 cal ka BP onwards points to the Westerlies as the predominant atmospheric circulation through to present, and marks a return to drier and even arid conditions in the area. Proxies for productivity (TOC, TOC/TN, TOC _Br ), redox potential (Fe/Mn) and changes in the endogenic carbonate precipitation (TIC, δ¹⁸O _Carb ) indicate changes within the lake. Low productivity characterised the lake from the late Pleistocene until 6.6 cal ka BP, and increased rapidly afterwards. Lake level remained low until the LGM, but water depth increased to a maximum during the late glacial and remained high into the early Holocene. Subsequently, the water level decreased to its present stage. Today the lake system is mainly climatically controlled, but the depositional regime is also driven by internal limnogeological processes.  相似文献