Geochronology and paleothermometry of Neogene sediments from the Vøring Plateau using Sr,C and O isotopes

The Neogene sediments from DSDP site 341 on the Vøring Plateau, Norwegian Sea, contain a thin glauconitic pellet-bearing subunit, which separates underlying pelagic clays from overlying glacial-marine sediments. Oxygen isotope measurements of benthic foraminifera show aδ¹⁸O shift of + 1%. during deposition of this subunit, probably a combined effect of a drop in bottom water temperature and a rise in seawaterδ¹⁸O. The chronology of this sedimentological and O isotope transition is, however, poorly constrained by fossil evidence.RbSr dating of glauconitic pellets indicates that the lower part of the glauconitic subunit was deposited 11.6 ± 0.2 Ma ago. Further geochronological evidence, derived from the Sr and C isotopic compositions of foraminifera compared with known seawater-time variations, indicates that the lower pelagic clays are early to middle Miocene, deposited at a mean rate of ∼ 15 m/Ma. The glauconitic subunit contains part of the middle Miocene and probably all of the late Miocene in a condensed sequence with a very low mean depositional rate (∼ 0.2 m/Ma). The overlying glacial marine sediments are probably Pliocene, with a high mean rate of deposition, ∼ 45 m/Ma.This is the first application of C, O and Sr isotopic stratigraphy combined with RbSr dating of glauconitic minerals, and it illustrates the applications of this integrated approach in geochronology.     

Thermal Structure and Rheology of the Upper Mantle Derived from Mantle Xenoliths from Gansu Province, Western China

Mantle xenoliths brought up by Cenozoic volcanic rocks onto the earth‘s surface may provide direct information about the upper mantle beneath the volcanic region. This paper presents the study on mantle xenoliths collected from Haoti village, Dangchang County, Gansu Province, western China. The main purpose of the study is to gain an insight into the thermal structure and rbeology of the upper mantle beneath the region. The results show that the upper mantle of the region is composed mainly of spinel lherzolite at shallower depth (52-75km), and garnet iberzolite at greater depth (greater than 75km), instead of harzburgite and dunite as proposed by some previous studies. The upper mantle geotherm derived from the equilibrium temperatures and pressures of xenoliths from the region is lower than that of North China, and is somewhat closer to the Oceanic geotherm. The crust-mantle boundary is determined from the geotherm to be at about 52km, and the Moho seems to be the transition zone of lower crust material with spinel iberzolite. If we take 1280℃ as the temperature of the top of asthenosphere, then the fithospbere-asthenosphere boundary should be at about 120km depth. The differential stress of the upper mantle is determined by using recrystallized grain size piezometry, while the strain rate and equivalent viscosity are determined by using the high temperature flow law of peridotite. The differential stress, strain rate and viscosity profiles constructed on the basis of the obtained values indicate that asthenospberic diapir occurred in this region during the Cenozoic time, resulting in the corresponding thinning of the lithosphere. However, the scale and intensity of the diapir was significantly less than that occurring in the North China region. Moreover, numerous small-scale shear zones with localized deformation might occur in the iithospberic mantle, as evidenced by the extensive occurrence of xenoliths with tabular equigranular texture.     

The Kverkfj?ll fissure swarm and the eastern boundary of the Northern Volcanic Rift Zone, Iceland

Rift zones at the divergent plate boundary in Iceland consist of central volcanoes with swarms of fractures and fissures extending away from them. Fissure swarms can display different characteristics, in accordance with their locations within the ∼50-km-wide rift zones. To better discern the characteristics of fissure swarms, we mapped tectonic fractures and volcanic fissures within the Kverkfj?ll volcanic system, which is located in the easternmost part of the Northern Volcanic Rift Zone (NVZ). To do this, we used aerial photographs and satellite images. We find that rifting structures such as tectonic fractures, Holocene volcanic fissures, and hyaloclastite ridges are unevenly distributed in the easternmost part of the NVZ. The Kverkfj?ll fissure swarm extends 60 km north of the Kverkfj?ll central volcano. Holocene volcanic fissures are only found within 20 km from the volcano. The Fjallgarear area, extending north of the Kverkfj?ll fissure swarm, is characterized by narrow hyaloclastite ridges indicating subglacial volcanism. We suggest that the lack of fractures and Holocene volcanic fissures there indicates decreasing activity towards the north in the easternmost part of the NVZ, due to increasing distance from the long-term spreading axis. We argue that arcuate hyaloclastite ridges at the eastern boundary of the Northern Volcanic Rift Zone are mainly formed during deglaciations, when three conditions may occur; firstly, eruption rate increases due to decompression of the mantle. Secondly, the high tensile stresses accumulated during glaciations due to lack of magma supply may be relieved as magma supply increases during deglaciations. Thirdly, faulting may occur during unloading due to differential movements between the thinner and younger Northern Volcanic Rift Zone crust and the thicker and older crust to the east of it.     

Delineation of a quick clay zone at Smørgrav,Norway, with electromagnetic methods under geotechnical constraints

In many coastal areas of North America and Scandinavia, post-glacial clay sediments have emerged above sea level due to iso-static uplift. These clays are often destabilised by fresh water leaching and transformed to so-called quick clays as at the investigated area at Smørgrav, Norway. Slight mechanical disturbances of these materials may trigger landslides. Since the leaching increases the electrical resistivity of quick clay as compared to normal marine clay, the application of electromagnetic (EM) methods is of particular interest in the study of quick clay structures.For the first time, single and joint inversions of direct-current resistivity (DCR), radiomagnetotelluric (RMT) and controlled-source audiomagnetotelluric (CSAMT) data were applied to delineate a zone of quick clay. The resulting 2-D models of electrical resistivity correlate excellently with previously published data from a ground conductivity metre and resistivity logs from two resistivity cone penetration tests (RCPT) into marine clay and quick clay. The RCPT log into the central part of the quick clay identifies the electrical resistivity of the quick clay structure to lie between 10 and 80 Ω m. In combination with the 2-D inversion models, it becomes possible to delineate the vertical and horizontal extent of the quick clay zone. As compared to the inversions of single data sets, the joint inversion model exhibits sharper resistivity contrasts and its resistivity values are more characteristic of the expected geology. In our preferred joint inversion model, there is a clear demarcation between dry soil, marine clay, quick clay and bedrock, which consists of alum shale and limestone.     

Structure, stratigraphy, and eruption dynamics of a young tuff ring: Hanauma Bay, O’ahu, Hawai’i

The Hanauma Bay–Koko Head complex is one of several young volcanic landforms along the Koko fissure, in southeastern O’ahu. The Hanauma Bay region of the complex comprises two nested tuff rings, inner and outer Hanauma Bay, and multiple smaller vents. The internal structure of the inner tuff ring, well exposed due to subsequent breaching by the ocean and wave erosion, indicates that it formed during a minimum of five distinct phases of deposition that produced five mappable units. Significant inward collapses generated major unconformities that separate the units exposed in the inner wall. The planes of failure are cut by narrow steep-walled, locally overhung channels and gullies, suggesting that the collapse events were each followed by short time breaks during which the deposits were eroded by rainfall runoff. Within each pyroclastic unit, there are many local slump scars and unconformities, suggesting that minor instability of the inner wall was a near-constant feature. From bedding sags and surge bed forms, it is apparent that the vent shifted at least twice during tuff ring growth. Ballistic blocks in the youngest unit indicate that the eruption overlapped in time with a separate eruption to the north, most likely to be that of the Kahauloa tuff ring 880?m away.     

Gravity Data from the Teboursouk Area (“Diapirs Zone”, Northern Tunisia): Characterization of Deep Structures and Updated Tectonic Pattern

Located between eastern segments of the Atlas and Tell-Rif oro-genic belts, the “Dome zone” of northern Tunisia is characterized by the juxtaposition of various structures that mainly controlled the long geody-namic history of this part of the south-Tethyan Margin. To better understand the organization and deep extension of these structures, gravity data from the Teboursouk key area are proposed. These data include the plotting of Bouguer anomaly map and related parameters such as vertical and horizontal gradients, upward continuation and Euler solution. Compared to geological and structural maps available, they allow the identification of new deep structures and greater precision regarding the characteristics and organization of known ones; consequently, an updated structural pattern is proposed.     

On Earth’s Mantle Constitution and Structure from Joint Analysis of Geophysical and Laboratory-Based Data: An Example

Determining Earth’s structure is a fundamental goal of Earth science, and geophysical methods play a prominent role in investigating Earth’s interior. Geochemical, cosmochemical, and petrological analyses of terrestrial samples and meteoritic material provide equally important insights. Complementary information comes from high-pressure mineral physics and chemistry, i.e., use of sophisticated experimental techniques and numerical methods that are capable of attaining or simulating physical properties at very high pressures and temperatures, thereby allowing recovered samples from Earth’s crust and mantle to be analyzed in the laboratory or simulated computationally at the conditions that prevail in Earth’s mantle and core. This is particularly important given that the vast bulk of Earth’s interior is geochemically unsampled. This paper describes a quantitative approach that combines data and results from mineral physics, petrological analyses of mantle minerals, and geophysical inverse calculations, in order to map geophysical data directly for mantle composition (major element chemistry and water content) and thermal state. We illustrate the methodology by inverting a set of long-period electromagnetic response functions beneath six geomagnetic stations that cover a range of geological settings for major element chemistry, water content, and thermal state of the mantle. The results indicate that interior structure and constitution of the mantle can be well-retrieved given a specific set of measurements describing (1) the conductivity of mantle minerals, (2) the partitioning behavior of water between major upper mantle and transition-zone minerals, and (3) the ability of nominally anhydrous minerals to store water in their crystal structures. Specifically, upper mantle water contents determined here bracket the ranges obtained from analyses of natural samples, whereas transition-zone water concentration is an order-of-magnitude greater than that of the upper mantle and appears to vary laterally underneath the investigated locations.     

The North Nuratau Fault Zone,Uzbekistan—Structure and evolution of a Palaeozoic Suture Zone

The Nuratau Fault Zone in eastern Uzbekistan forms part of the western prolongation of the Tien Shan, an extensive orogenic zone located along the margin of the Central Asian Orogenic Belt. The Nuratau region is geologically complex, forming part of the suture zone between the Kazakh-Kyrgyz continent and the Alai microcontinent. A model is proposed suggesting modified N-directed subduction model, where an extensive fold-and-thrust belt developed in the Nuratau region. This, coupled with significant transform activity would have resulted in major segmentation of the existing stratigraphy in the region, as well as the development of a foredeep basin to the north of the fold-and-thrust belt. Regional suturing and collision was variable. Indeed, the collisional history probably involved multi-phase subduction/accretion of various microcontinents, ancient island arcs, and fragments of oceanic islands. Final collision would have produced an eroding basinal high in the region of the Nuratau mountains, which issued sediment both northwards into the remnant basin of the Turkestan Ocean, but also to the south into the newly forming Amu Darya Basin.     

The State of the Art in Studying the Deep Structure of the Earth’s Crust and Upper Mantle beneath the Baikal Rift from Seismological Data

Izvestiya, Physics of the Solid Earth - Abstract—This review discusses the major milestone results yielded by the regional seismological studies of the deep structure of the Earth’s...     

The fragment of Paleo-Tethys ophiolite from central Qiangtang, Tibet:Geochemical evidence of meta-basites in Guoganjianian

N-MORB-type metabasites are discovered in the Guoganjianian area, central Qiangtang, Tibet, which are mainly metagabbro with cumulate structure and metabasalt. The rocks are distributed nearly from west to east unconformably underlying the Wanghuling Group of upper Triassic. On the basis of geo- chemical analysis, we find that the content of SiO2 is 43.03%―53.42%, and TiO2 1%―2.67%, Al2O3 16.75%―21.52%, CaO 7.03%―11.13%, K2O 0.05%―0.38%; the REE pattern is slight depletion or flat, and the trace spider diagram is like that of N-MORB, so we consider that the metabasite was formed under the setting of mid-ocean ridge or adult back-arc basin, and it is the fragment of Paleo-Tethys ophiolite.     

The formation of circular littoral cones from tube-fed pāhoehoe: Mauna Loa,Hawai'i

Pyroclastic cones along the southwest coast of Mauna Loa volcano, Hawai'i, have a common structure: (a) an early formed circular outer rim 200–400 m in diameter composed mostly of scoria and lapilli, and (b) one or more later-formed inner rims composed almost exclusively of dense spatter. The spatter activity locally fed short lava flows that ponded within the outer rims. Based on various lines of evidence, these cones are littoral in origin: relationships between the cones and associated flows; the degassed nature of the pyroclasts; and (although not unequivocal) the position of the cones relative to known eruptive vent locations on Mauna Loa. Additional support for the littoral interpretation comes from their similarity to (smaller) littoral cones that have been observed forming during the ongoing Kilauea eruption. The structure of these Mauna Loa cones, however, contrasts with that of standard Hawaiian littoral cones in that there is (or once was) a complete circle of pyroclastic deposits. Furthermore, they are large even though associated with tubefed phoehoe flows instead of 'a'. The following origin is proposed: An initial flow of tube-fed phoehoe into the ocean built a lava delta with a base of hyaloclastite. Collapse of an inland portion of the active tube into the underlying wet hyaloclastites or a water-filled void allowed sufficient mixing of water and liquid lava to generate strong explosions. These explosions broke through the top of the flow and built up the outer scoria/lapilli rims on the solid carapace of the lava delta. Eventually, the supply of water diminished, the explosions declined in intensity to spattering, and the initial rim was filled with spatter and lava.     

The relative impact of future storminess versus offshore dredging on suspended sediment concentration in a shallow coastal embayment: Rødsand lagoon,western Baltic Sea

Natural and human-induced disturbances challenge the resilience of coastal ecosystems. The relative impact of a projected increase in storm intensity versus a temporary sediment spill from the construction of the planned Fehmarnbelt Fixed Link on the suspended sediment concentrations was estimated in the shallow coastal embayment of Rødsand lagoon. The investigations were performed through a combination of field observations and numerical modeling. The numerical model was calibrated and validated with the field observations. The lagoonal response to a regional increase in wind speed showed temporarily amplified suspended particulate matter concentration (SPMC) levels (26–37%) and amplified exceedance duration above wash load (30–32%) in the entire lagoon, but especially in the shallow regions. The lagoonal response to a sediment spill outside Rødsand lagoon showed temporarily amplified SPMC levels (2–4%) and amplified exceedance duration above wash load (1–2%) in the entire lagoon, but especially in the regions in close proximity to the spill source. The regional response to increasing wind speed was thus 10-fold larger than the sediment spill where only advective sediment transport took place. This highlights the importance of the internal lagoonal processes as opposed to exterior disturbances. The study contributed to an improved understanding of the potential natural and human-induced stressors on the sediment dynamics in lagoonal ecosystems.

     

Edge Detection and Depth Estimation Using a Tilt Angle Map from Gravity Gradient Data of the Kozaklı-Central Anatolia Region,Turkey

In this paper the application of an edge detection technique to gravity data is described. The technique is based on the tilt angle map (TAM) obtained from the first vertical gradient of a gravity anomaly. The zero contours of the tilt angle correspond to the boundaries of geologic discontinuities and are used to detect the linear features in gravity data. I also present that the distance between zero and ±p\mathord

Application of surface-wave tomography to mineral exploration: a case study from Siilinjärvi,Finland

In order to assess the feasibility and validity of surface-wave tomography as a tool for mineral exploration, we present an active seismic three-dimensional case study from the Siilinjärvi mine in Eastern Finland. The aim of the survey is to identify the formation carrying the mineralization in an area south of the main pit, which will be mined in the future. Before acquiring the data, we performed an accurate survey design to maximize data coverage and minimize the time for deployment and recollection of the equipment. We extract path-averaged Rayleigh-wave phase-velocity dispersion curves by means of a two-station method. We invert them using a computationally efficient tomographic code which does not require the computation of phase-velocity maps and inverts directly for one-dimensional S-wave velocity models. The retrieved velocities are in good agreement with the data from a borehole in the vicinity, and the pseudo three–dimensional S-wave velocity volume allows us to identify the geological contact between the formation hosting most of the mineralization and the surrounding rock. We conclude that the proposed method is a valid tool, given the small amount of equipment used and the acceptable amount of time required to process the data.     

Fragmentation, transport and deposition of a low-grade ignimbrite: The Citlaltépetl Ignimbrite, Eastern México

 The Citlaltépetl Ignimbrite records one of the largest explosive events during the Holocene activity of Citlaltépetl Volcano (Pico de Orizaba). Multiple pyroclastic flow units, a fall deposit, and some lahar units were emplaced between 8500–9000 y B.P. as a result of repetitive but discrete explosive events. The whole ignimbrite resulted from discrete fluctuations in eruptive intensity that decreased with time. The initial pyroclastic flow pulse was by far the most violent and widespread event, and its deposits show conspicuous variations in structure and texture that could be associated with different mechanisms of transport and emplacement. Subpopulation Sequential Fragmentation Transport (SFT) analyses were carried out in order to determine the physical mechanisms that selectively concentrate or remove particles in the moving flows. We suggest that lateral and temporal changes in the flow rheology, in which fluidization, yield strength, entrainment of atmospheric air, and sedimentation played a dominant role in flow propagation and emplacement, may imprint a unique signature in the grain-size spectra. The lowermost unit of the Citlaltépetl Ignimbrite can be envisaged by a model in which progressive aggradation near the vent became replaced by en masse emplacement farther outward. Received: 24 March 1998 / Accepted: 9 October 1998