The structural state of the former lower continental crust in Calabria (S. Italy)

In the Northern Serre (S. Italy), a continuous section through a former lower continental crust is exposed (Schenk 1984, 1985). These granulite facies rocks have not been penetratively deformed during their uplift to higher crustal levels. Therefore, they allow to study the structural state of the lower continental crust. Most rockforming minerals, among them ortho- and clinopyroxene and cordierite, show recrystallization structures. Quarz basal subgrain boundaries are developed in lower levels of the lower crust and disappear toward higher levels. In addition to dimensional orientations of the rockforming minerals, the preferred crystallographic orientations of plagioclases, cordierite, amphibole, pyroxenes and, rarely, quartz reflect the strain orientation of the prograde high-T deformation. During this deformation, the stretching direction was constant NNE-SSW throughout the exposed lower crustal section. Shear indicators are rare. This points to the fact that deformation has been homogeneous and coaxial flattening. There is only a small variation in orientation and strength of textures throughout the exposed lower crust. Therefore, neighbouring layers of different composition rather than different deformation might serve as seismic reflectors in the former lower crust.

---

Zusammenfassung In der nördlichen Serre (Süditalien) ist ein zusammenhängender Bereich einer ehemaligen kontinentalen Unterkruste aufgeschlossen (Schenk 1984, 1985). Diese granulitfaziellen Gesteine sind bei ihrem Aufstieg in höhere Krustenbereiche nicht durchgreifend überprägt worden. Deshalb läßt sich an ihnen noch der strukturelle Zustand der ehemaligen Unterkruste studieren. Außer in Feldspäten, Quarz und Amphibol tritt auch in Ortho- und Klinopyroxen und in Cordierit Rekristallisation auf. In tieferen Krustenniveaus sind im Quarz basisparallele Subkorngrenzen entwickelt, die in höheren Krustenbereichen nicht auftreten. Sowohl die Gestaltregelungen der gesteinsbildenden Minerale als auch die kristallographischen Plagioklas-, Cordierit-, Amphibol-, Pyroxen- und (seltener) Quarzregelungen spiegeln die Strain-orientierung der prograden hochtemperierten Deformation wider. Während dieser Deformation blieb die Streckung im gesamten Unterkrustenbereich konstant NNE-SSW orientiert. Scherindikatoren sind selten. Beides deutet darauf hin, daß die Deformation homogen und als koachsiale Plättung abgelaufen ist. Im gesamten aufgeschlossenen Unterkrustenbereich variieren weder Intensität noch Orientierung der kristallographischen und gestaltlichen Regelungen der gesteinsbildenden Minerale wesentlich. Deshalb sollten in der ehemaligen Unterkruste Lagen unterschiedlicher Deformation weniger als seismische Reflektoren in Frage kommen als Lagen unterschiedlicher Zusammensetzung.

---

Résumé Le Massif de la Serre Septentrionale en Calabre (Sud de l'Italie) représente un tronçon d'une ancienne croûte continentale profonde (Schenk 1984, 1985). Les roches du faciès granulite qui le constituent n'ont pas subi de déformation penetrative lors de leur ascension dans les niveaux plus superficiels de la croûte. Leur étude permet donc de reconstruire les caractéristiques structurales de l'ancienne croûte profonde. Non seulement les feldspaths, le quartz et l'amphibole, mais encore l'orthopyroxène, le clinopyroxène et la cordiérite ont recristallisé. Dans les niveaux les plus profonds de la croûte on observe dans les cristaux de quartz des bordures de sous-grains parallèles à la base. Celles-ci n'apparaissent plus dans les niveaux plus superficiels. L'orientation des cristaux des minéraux constitutifs des roches, de même que l'orientation des réseaux cristallins du plagioclase, de la cordiérite, de l'amphibole, du pyroxène et (mais plus rarement) du quartz réflètent la direction de contrainte de la phase de déformation prograde de haute température. Pendant cette phase de déformation, la direction d'extension est demeurée NNE-SSW. Les témoins de cisaillement sont rares, ce qui montre que la déformation s'est effectuée de façon homogène, en relation avec un aplatissement coaxial. Les textures varient très peu en orientation comme en intensité dans toute la coupe de croûte profonde exposée. En conséquence, l'existence de réflecteurs sismiques devrait refléter des différences dans les compositions minéralogiques des roches plutôt que dans leurs caractéristiques structurales.

---

Serre (Schenk, 1984, 1985). . . , , . , , . , , , , () . NNE-SSW. . , . , , . , , .

     

Mineral inclusions in diamonds from the Kelsey Lake Mine, Colorado, USA: Depleted Archean mantle beneath the Proterozoic Yavapai province

Thirty-four silicate and oxide inclusions large enough for in situ WDS electron microprobe analysis were exposed by grinding/polishing of 19 diamonds from the Kelsey Lake Mine in the Colorado-Wyoming State Line Kimberlite district. Eighteen olivines, seven Cr-pyropes, four Mg-chromites, and one orthopyroxene in 15 stones belong to the peridotite (P) suite and three garnets and one omphacite in three stones belong to the eclogite (E) suite. The fact that this suite is dominated by the peridotite population is in stark contrast to the other diamond suites studied in the State Line district (Sloan, George Creek), which are overwhelmingly eclogitic. Kelsey Lake olivine inclusions are magnesian (17 of 18 grains in 9 stones are in the range Fo 92.7-93.1), typical of harzburgitic P-suite stones worldwide, but unlike the more Fe-rich (lherzolitic) Sloan olivine suite. Mg-chromites (wt% MgO = 12.8-13.8; wt% Cr₂O₃ = 61.4-66.6) are in the lower MgO range of diamond inclusion chromites worldwide. Seven harzburgitic Cr-pyropes in five stones have moderately low calcium contents (wt% CaO = 3.3-4.3) but are very Cr-rich (wt% Cr₂O₃ = 9.7-16.7). A few stones have been analyzed by SIMS for carbon isotope composition and nitrogen abundance. One peridotitic stone is apparently homogeneous in carbon isotope composition (δ¹³C_PDB = −6.2‰) but with variable nitrogen abundance (1296-2550 ppm). Carbon isotopes in eclogitic stones range from “normal” for the upper mantle (δ¹³C_PDB = −5.5‰) to somewhat low (δ¹³C_PDB = −10.2‰), with little internal variation in individual stones (maximum difference is 3.6‰). Nitrogen contents (2-779 ppm) are lower than in the peridotitic stone, and are lower in cores than in rims. As, worldwide, harzburgite-suite diamonds have been shown to have formed in Archean time, we suggest that the Kelsey Lake diamond population was derived from a block of Archean lithosphere that, at the time of kimberlite eruption, existed beneath the Proterozoic Yavapai province. The mixed diamond inclusion populations from the State Line kimberlites appear to support models in which volumes of Wyoming Craton Archean mantle survive buried beneath Proterozoic continental crust. Such material may be mixed with eclogitic/lherzolitic regimes emplaced beneath or intermingled with the Archean rocks by Proterozoic subduction.     

Layered mantle structure beneath the western Guyana Shield, Venezuela: Evidence from diamonds and xenocrysts in Guaniamo kimberlites

Mantle xenoliths and xenocrysts from Guaniamo, Venezuela kimberlites record equilibration conditions corresponding to a limited range of sampling in the lithosphere (100-150 km). Within this small range, however, compositions vary considerably, but regularly, defining a strongly layered mantle sequence. Major and trace element compositions suggest the following lithologic sequence: highly depleted lherzolite from 100 to 115 km, mixed ultra-depleted harzburgite and lherzolite from 115 to 120 km, relatively fertile lherzolite from 120 to 135 km, and mixed depleted harzburgite and relatively fertile lherzolite from 135 to 150 km. Based on comparison with well-documented mantle peridotites and xenocrysts from elsewhere, we conclude that the Meso-proterozoic Cuchivero Province (host to the Guaniamo kimberlites) is underlain by depleted and ultra-depleted shallow Archean mantle that was underplated, and uplifted, by Proterozoic subduction, perhaps more than once. These Proterozoic subduction events introduced less-depleted oceanic lithosphere beneath the Archean section, which remains there and is the source of the abundant Guaniamo eclogite-suite diamonds that have ocean-floor geochemical signatures. Although diamond-indicative low-Ca Cr-pyrope garnets are abundant, they are derived primarily from the shallow depleted layer within the field of graphite stability, and the rare peridotite-suite diamonds are either metastably preserved at these shallow depths, or were derived from the small amount of depleted lithosphere sampled by these kimberlites that remains within the diamond stability field (the mixture of Archean and Proterozoic mantle in the depth range 135-150 km).     

Exploration and discovery in Yellowstone Lake: results from high-resolution sonar imaging, seismic reflection profiling, and submersible studies

‘No portion of the American continent is perhaps so rich in wonders as the Yellow Stone’ (F.V. Hayden, September 2, 1874)Discoveries from multi-beam sonar mapping and seismic reflection surveys of the northern, central, and West Thumb basins of Yellowstone Lake provide new insight into the extent of post-collapse volcanism and active hydrothermal processes occurring in a large lake environment above a large magma chamber. Yellowstone Lake has an irregular bottom covered with dozens of features directly related to hydrothermal, tectonic, volcanic, and sedimentary processes. Detailed bathymetric, seismic reflection, and magnetic evidence reveals that rhyolitic lava flows underlie much of Yellowstone Lake and exert fundamental control on lake bathymetry and localization of hydrothermal activity. Many previously unknown features have been identified and include over 250 hydrothermal vents, several very large (>500 m diameter) hydrothermal explosion craters, many small hydrothermal vent craters (1–200 m diameter), domed lacustrine sediments related to hydrothermal activity, elongate fissures cutting post-glacial sediments, siliceous hydrothermal spire structures, sublacustrine landslide deposits, submerged former shorelines, and a recently active graben. Sampling and observations with a submersible remotely operated vehicle confirm and extend our understanding of the identified features. Faults, fissures, hydrothermally inflated domal structures, hydrothermal explosion craters, and sublacustrine landslides constitute potentially significant geologic hazards. Toxic elements derived from hydrothermal processes also may significantly affect the Yellowstone ecosystem.     

Reconstruction of atmospheric CO2 from ice-core data and the deep-sea record of ontong Java plateau: the Milankovitch chron

We provide a reconstruction of atmospheric CO₂ from deep-sea sediments, for the past 625000 years (Milankovitch chron). Our database consists of a Milankovitch template of sea-level variation in combination with a unique data set for the deep-sea record for Ontong Java plateau in the western equatorial Pacific. We redate the Vostok ice-core data of Barnola et al. (1987). To make the reconstructions we employ multiple regression between deep-sea data, on one hand, and ice-core CO₂ data in Antarctica, on the other. The patterns of correlation suggest that the main factors controlling atmospheric CO₂ can be described as a combination of sea-level state and sea-level change. For best results squared values of state and change are used. The square-of-sea-level rule agrees with the concept that shelf processes are important modulators of atmospheric CO₂ (e.g., budgets of shelf organic carbon and shelf carbonate, nitrate reduction). The square-of-change rule implies that, on short timescales, any major disturbance of the system results in a temporary rise in atmospheric CO₂.     

Gravity reduction with three-dimensional atmospheric pressure data for precise ground gravity measurements

The redistribution of air masses induces gravity variations (atmospheric pressure effect) up to about 20 μgal. These variations are disturbing signals in gravity records and they must be removed very carefully for detecting weak gravity signals. In the past, different methods have been developed for modelling of the atmospheric pressure effect. These methods use local or two-dimensional (2D) surface atmospheric pressure data and a standard height-dependent air density distribution. The atmospheric pressure effect is consisting of the elastic deformation and attraction term. The deformation term can be well modelled with 2D surface atmospheric pressure data, for instance with the Green's function method. For modelling of the attraction term, three-dimensional (3D) data are required. Results with 2D data are insufficient.From European Centre for Medium-Range Weather Forecasts (ECMWF) 3D atmospheric pressure data are now available. The ECMWF data used here are characterised by a spacing of Δ and Δλ = 0.5°, 60 pressure levels up to a height of 60 km and an interval of 6 h. These data are used for modelling of the atmospheric attraction term. Two attraction models have been developed based on the point mass attraction of air segments and the gravity potential of the air masses. The modelling shows a surface pressure-independent part of gravity variations induced by mass redistribution of the atmosphere in the order of some μgal. This part can only be determined by using 3D atmospheric pressure data. It has been calculated for the Vienna Superconducting Gravimeter site.From this follows that the gravity reduction can be improved by applying the 3D atmospheric attraction model for analysing long-periodic tidal waves including the polar tide. The same improvement is expected for reduction of long-term absolute gravity measurements or comparison of gravity measurements at different seasonal times. By using 3D atmospheric pressure data, the gravity correction can be improved up to some μgal.     

Water level observations from unmanned aerial vehicles for improving estimates of surface water–groundwater interaction

Integrated hydrological models are usually calibrated against observations of river discharge and piezometric head in groundwater aquifers. Calibration of such models against spatially distributed observations of river water level can potentially improve their reliability and predictive skill. However, traditional river gauging stations are normally spaced too far apart to capture spatial patterns in the water surface, whereas spaceborne observations have limited spatial and temporal resolution. Unmanned aerial vehicles can retrieve river water level measurements, providing (a) high spatial resolution; (b) spatially continuous profiles along or across the water body, and (c) flexible timing of sampling. A semisynthetic study was conducted to analyse the value of the new unmanned aerial vehicle‐borne datatype for improving hydrological models, in particular estimates of groundwater–surface water (GW–SW) interaction. Mølleåen River (Denmark) and its catchment were simulated using an integrated hydrological model (MIKE 11–MIKE SHE). Calibration against distributed surface water levels using the Differential Evolution Adaptive Metropolis algorithm demonstrated a significant improvement in estimating spatial patterns and time series of GW–SW interaction. After water level calibration, the sharpness of the estimates of GW–SW time series improves by ~50% and root mean square error decreases by ~75% compared with those of a model calibrated against discharge only.     

Synchronous pattern of moisture availability on the southern Tibetan Plateau since 17.5 cal. ka BP – the Tangra Yumco lake sediment record

A possible asynchronicity of the spatial and temporal moisture availability on the Tibetan Plateau has been a controversial subject of discussion in recent years. Here we present the first attempt to systematically investigate possible spatial and temporal variations in moisture availability by examining two lakes, Tangra Yumco and Nam Co, on an east–west transect on the southern Tibetan Plateau using identical proxies for palaeoenvironmental reconstruction. In this study, an independent record from Tangra Yumco was analysed applying a multi‐proxy approach to reconstruct variations in moisture availability since the Lateglacial. Results were subsequently compared with previously published records from Nam Co and additional records from Tso Moriri (northwestern Himalaya) and Naleng Co (southeastern Tibetan Plateau). Our results show that Tangra Yumco was at least partially ice covered prior to 17.1 cal. ka BP. A temperature rise after 17.1 cal. ka BP probably resulted in thawing of the permafrost. At 16.0 cal. ka BP moisture availability increased, representing an initial monsoonal intensification. Warmer conditions between 13.0 and 12.4 cal. ka BP and cooler conditions between 12.4 cal. ka BP and the onset of the Holocene reflect the Bølling‐Allerød and Younger Dryas. At the onset of the Holocene moisture availability rapidly increased, with moisture highest prior to 8.5 cal. ka BP when temperatures were also highest. After 8.5 cal. ka BP the moisture availability gradually decreased and showed only minor amplitude variations. These findings are consistent with the records from large lakes like Nam Co, Tso Moriri, and Naleng Co, revealing a synchronous pattern of moisture availability on the southern Tibetan Plateau.     

An experimental database for the development,calibration and verification of constitutive models for sand with focus to cyclic loading: part I—tests with monotonic loading and stress cycles

For numerical studies of geotechnical structures under earthquake loading, aiming to examine a possible failure due to liquefaction, using a sophisticated constitutive model for the soil is indispensable. Such a model must adequately describe the material response to a cyclic loading under constant volume (undrained) conditions, amongst others the relaxation of effective stress (pore pressure accumulation) or the effective stress loops repeatedly passed through after a sufficiently large number of cycles (cyclic mobility, stress attractors). The soil behaviour under undrained cyclic loading is manifold, depending on the initial conditions (e.g. density, fabric, effective mean pressure, stress ratio) and the load characteristics (e.g. amplitude of the cycles, application of stress or strain cycles). In order to develop, calibrate and verify a constitutive model with focus to undrained cyclic loading, the data from high-quality laboratory tests comprising a variety of initial conditions and load characteristics are necessary. The purpose of these two companion papers was to provide such database collected for a fine sand. The database consists of numerous undrained cyclic triaxial tests with stress or strain cycles applied to samples consolidated isotropically or anisotropically. Monotonic triaxial tests with drained or undrained conditions have also been performed. Furthermore, drained triaxial, oedometric or isotropic compression tests with several un- and reloading cycles are presented. Part I concentrates on the triaxial tests with monotonic loading or stress cycles. All test data presented herein will be available from the homepage of the first author. As an example of the examination of an existing constitutive model, the experimental data are compared to element test simulations using hypoplasticity with intergranular strain.