Anomalous Layer Research Articles

Distribution and sedimentary characteristics of tsunami deposits along the Cascadia margin of western North America

Tsunami deposits have been found at more than 60 sites along the Cascadia margin of Western North America, and here we review and synthesize their distribution and sedimentary characteristics based on the published record. Cascadia tsunami deposits are best preserved, and most easily identified, in low-energy coastal environments such as tidal marshes, back-barrier marshes and coastal lakes where they occur as anomalous layers of sand within peat and mud. They extend up to a kilometer inland in open coastal settings and several kilometers up river valleys. They are distinguished from other sediments by a combination of sedimentary character and stratigraphic context. Recurrence intervals range from 300–1000 years with an average of 500–600 years. The tsunami deposits have been used to help evaluate and mitigate tsunami hazards in Cascadia. They show that the Cascadia subduction zone is prone to great earthquakes that generate large tsunamis. The inclusion of tsunami deposits on inundation maps, used in conjunction with results from inundation models, allows a more accurate assessment of areas subject to tsunami inundation. The application of sediment transport models can help estimate tsunami flow velocity and wave height, parameters which are necessary to help establish evacuation routes and plan development in tsunami prone areas.

Magma reservoir at Mt. Vesuvius: Size of the hot, partially molten, crust material detected deeper than 8 km

One- and two-dimensional VP models were obtained by TomoVes experiment, all characterized by low VP in the uppermost 500 m and a sharp discontinuity at about 2–3 km beneath the volcano. Large amplitude late arrivals were identified as P- to S-phases converted at the top, between 8 and 10 km deep, of a low-velocity layer with a dramatic drop of VS, from approximately 3.6 km/s to less than 1.0 km/s. Here, we synthesize the interpretation of Rayleigh wave dispersion measurements, made by several authors, to delineate the extent of such anomalous layer of hot, partially molten, crust material. Our non-linear inversion of broad-band dispersion measurements evidences a main feature of Somma-Vesuvius deep structure consisting of low VS layers at about 8–10 and 20 km of depth. The depth of the shallower low-velocity layer and the VS value above it are in agreement with TomoVes results, but the VS velocity reduction is of about 10%. If we assume VS equal to 1.0 km/s in our non-linear inversion, a thickness not greater than 0.35 km results. The volume occupied by this very low-velocity layer, sill-shaped, is compatible with the size of Mt. Vesuvius cone, but it develops above a much larger hot mass, which could be the parental source as the erupted products are only few percent of magma chamber.

Erratum: Anomalous layering at the liquid Sn surface [Phys. Rev. B70, 224206 (2004)

Received 23 November 2005DOI:https://doi.org/10.1103/PhysRevB.73.019901©2006 American Physical Society

Analogue and numerical modelling of accretionary prisms with a décollement in sediments

Abstract Active accretionary prisms at subduction margins generally include a horizontal detachment, décollement, within the sedimentary pile. The décollement, and its extension to undeformed regions (i.e., proto-décollement), corresponds to a layer of high fluid pressure. The deformation of the prisms, including such an anomalous layer, can be modelled and examined using analogue experiments and numerical simulations. Both these methods approximate the material under deformation as an assembly of particles (grains). The décollement layer is found to be best modelled by intercalating a layer with smaller internal frictional coefficient than the surrounding materials corresponding to the sediments. Our analogue experiments with dry sand and microglass beads reproduce structural geometry similar to that of interpreted seismic profiles at the toe of the prisms. Thrust faults originate from the horizontal beads layer and propagate upward with a constant angle of about 30°. Each of the fault bends produces a series of minor back thrusts. A particle image velocimetry (PIV) analysis revealed that the fault activity is characterized by intermittent reactivation and segmentation. The numerical simulations based on the distinct element method (DEM) were performed with similar kinematic settings and material properties as the analogue experiments. The numerical simulation results not only reproduce similar geometries as in the analogue experiments, but also show that the particle assembly experiences temporal variations in the deformation velocity and stress field as deformation propagates. This might be related to stick-slip motion of the frictional fault surfaces, which is a common feature of faulting during accretionary processes at subduction margins.

Ge Nanoclusters in GeO<sub>2</sub>: Synthesis and Optical Properties

Ge nanocrystals (NCs) in GeO2 films obtained with the use of two methods were studied. The first method is a film deposition from supersaturated GeO vapor with subsequent dissociation of metastable GeO on heterophase system Ge:GeO2. The second method is growth of anomalous thick native germanium oxide layers with chemical composition GeOx(H2O) during catalytically enhanced Ge oxidation. The obtained films were studied with the use of photoluminescence (PL), Raman scattering spectroscopy, high-resolution electron microscopy (HREM). Strong PL signals were detected in GeO2 films with Ge-NCs at room temperature. “Blue-shift” of PL maximum was observed with reducing of Ge excess in anomalous thick native germanium oxide films. Also a correlation between reducing of the NC sizes (estimated from position of Raman peaks) and PL “blue-shift” was observed. The Ge NCs presence was confirmed by HREM data. The optical gap in Ge-NCs was calculated with taking into account quantum size effects and compared with the position of the experimental PL peaks. It can be concluded that a Ge-NC in GeO2 matrix is a quantum dot of type I.

Microbial origin of excess methane in glacial ice and implications for life on Mars.

Methane trapped in the 3,053-m-deep Greenland Ice Sheet Project 2 ice core provides an important record of millennial-scale climate change over the last 110,000 yr. However, at several depths in the lowest 90 m of the ice core, the methane concentration is up to an order of magnitude higher than at other depths. At those depths we have discovered methanogenic archaea, the in situ metabolism of which accounts for the excess methane. The total concentration of all types of microbes we measured with direct counts of Syto-23-stained cells tracks the excess of methanogens that we identified by their F420 autofluorescence and provides independent evidence for anomalous layers. The metabolic rate we estimated for microbes at those depths is consistent with the Arrhenius relation for rates found earlier for microbes imprisoned in rock, sediment, and ice. It is roughly the same as the rate of spontaneous macromolecular damage inferred from laboratory data, suggesting that microbes imprisoned in ice expend metabolic energy mainly to repair damage to DNA and amino acids rather than to grow. Equating the loss rate of methane recently discovered in the Martian atmosphere to the production rate by possible methanogens, we estimate that a possible Martian habitat would be at a temperature of approximately 0 degrees C and that the concentration, if uniformly distributed in a 10-m-thick layer, would be approximately 1 cell per ml.

Study on Crustal Structures of Changbaishan-Jingpohu Volcanic Area using Receiver Functions

We have obtained 34 receiver functions from 71 teleseismic waveforms recorded by 34 stations that were installed in Changbaishan-Jingpohu volcanic area. Through inversing receiver function of 34 seismic stations, we obtain S wave velocity structures beneath the stations. The results revealed that the Moho depth is about 32-33km along Shenyang-Dunhua direction and becomes deeper toward its west side. In Changchun area, the Moho thickness reaches about 36km and under the crater of Changbaishan-Tianchi volcano, the depth of Moho reaches about 38km while beneath Jingpohu volcanic area, Moho depth is about 39km. Overall, the crustal thickness of studied area is thick in north and thin in south. We found an anomalous layer with obvious low Swave velocity beneath Changbaishan-Tianchi crater at about 10km depth. Beneath Jingpohu volcanic area there is also possibly a low S-wave velocity anomalous body at about 30km depth. The results show that the S-wave velocity gradients at Moho are obviously different between near crater and far apart from the crater. S-wave velocity gradient of Moho near crater is clearly smaller than that in area without volcano, which suggests that Moho structures beneath volcano are different from general Moho structures in other areas. The crust thickness on the two sides along Shenyang-Dunhua direction has a comparatively great changes and its location is almost the same as Dunhua-Mishan fault at near surface. Therefore, Dunhua-Mishan fault is a very important geological structure zone in the studied area.

Anomalous layering at the liquid Sn surface

X-ray reflectivity measurements on the free surface of liquid Sn are presented. They exhibit the high-angle peak, indicative of surface-induced layering, also found for other pure liquid metals (Hg, Ga and In). However, a low-angle peak, not hitherto observed for any pure liquid metal, is also found, indicating the presence of a high-density surface layer. Fluorescence and resonant reflectivity measurements rule out the assignment of this layer to surface-segregation of impurities. The reflectivity is modelled well by a 10% contraction of the spacing between the first and second atomic surface layers, relative to that of subsequent layers. Possible reasons for this are discussed.

Flood-related, Organic-carbon Anomalies as Possible Temporal Markers in Reservoir Bottom Sediments

ABSTRACT Results of a study of sediment cores from four reservoirs in the upper Mississippi River Basin, USA, indicated that anomalous organic carbon concentrations associated with flood deposits may provide detectable temporal markers in reservoir bottom sediments. Temporal markers are needed for reservoir sediment studies to date sediment layers deposited between the 1963–64 cesium-137 peak and the present. For two of four reservoirs studied, anomalously low organic carbon concentrations were measured for a sample interval in the upper part of a sediment core. The anomalous interval was interpreted to have been deposited during the July 1993 flood that affected a large area of the upper Mississippi River Basin. Potentially, the July 1993 flood deposit may be used as a temporal marker in reservoir bottom sediments in parts of the basin affected by the flood. Several uncertainties remain regarding the viability of organic carbon as a temporal marker including the combination of flood, basin, and reservoir characteristics required to produce a recognizable organic carbon marker in the bottom sediment and the optimal sampling strategy needed to detect the marker in a sediment core. It is proposed that flood duration and basin size may be important factors as to whether or not an anomalous and detectable organic carbon layer is deposited in a reservoir.

One-dimensional resistivity modeling of NE Japan from stationary wide-band magnetotelluric data

Stationary magnetotelluric (MT) systems were set up at the Esashi station and the Mizusawa Geodetic Observatories of Geographical Survey Institute (GSI) in NE Japan. Since 1996, continuous 24 h resistivity and phase data have been acquired by the wide-band MT measurement devices. The data from the two sites were processed by the remote reference technique to minimize the effect of local noise. Data quality at the Mizusawa was sometimes poor, but use of the data as reference improved the MT data of the Esashi station. As a result, extremely stable apparent resistivity and phase curves were obtained from the Esashi station. The resistivity structure of NE Japan was estimated using two ways of 1-D modelling procedure by evaluating the stability of apparent resistivity and phase data. The modelling suggests the existence of anomalous layers which represent anisotropic or inhomogeneous crust and/or upper mantle structures of the NE Japan of fore-arc. These features may relate with Cretaceous tectonic events.

Biological, depositional, microspherule, and geochemical records of the Frasnian/Famennian boundary beds, South China

The investigation of shallow subtidal, inter-reef depression, and inter-reef platform sections from central Hunan of China shows that a transgression–regression cycle occurred in the Late Frasnian ( linguiformis Zone?). The lowest triangularis Zone to Middle triangularis Zone deposits are either very thin or variously missing in some shallow-water sections of Hunan. The uppermost Frasnian (upper linguiformis zone) black-shale interval (including black shale and carbonates) was not ubiquitously developed and probably formed in a regressive, shallower water and anoxic to dysoxic conditions suggested from higher Ce/La ratios, penecontemporaneous dolomitization of carbonates, and low faunal diversity. Two steps of the Frasnian/Famennian (F/F) mass extinction are postulated. The first is the extinction of the benthos near the end of the Frasnian which was probably caused by the sudden onset of anoxic environments. Shallow-water rugose corals and ostracods experienced sudden biomass loss right at the deposition of the black shale. The second is the extinction of pelagic fauna at the end of the Frasnian. Microspherules of probable impact origin are found in several layers in the Xikuangshan F/F boundary section, including two major peaks, respectively, at the Upper rhenana Zone and upper linguiformis zone, and two minor peaks, respectively, near the top of the linguiformis Zone and Lower triangularis Zone. The microtektites are white or brownish in color, characterized by high contents of Si, Al, and intermediate CaO, MnO, and FeO, and minor K 2O and Na 2O. Occurrences of these microtektites do not seem to be directly related to the F/F mass extinction. From the Lower rhenana Zone through the Upper crepida Zone there are four elemental anomalous layers. These elemental anomalies are interpreted to have resulted from the contemporaneous active rifting process and hydrothermal activities in South China.

Anomalous layer expansion in thin niobium films during hydrogen absorption

Hydrogen absorption in Nb layers of Fe/Nb and W/Nb films was explored in situ by small-angle neutron/ x-ray reflectometry, high-angle x-ray diffraction, and EXAFS spectroscopy. In addition to the hydrogen-induced Nb lattice expansion, the experiments reveal an anomalously large thickness expansion of the Nb layers. This expansion is interpreted to be caused by the rearrangement of Nb atoms, and the creation of additional lattice planes in response to the hydrogen-induced lateral lattice strain. This effect is likely the origin of the extraordinarily large hydrogen concentrations found in these films.

The influence of treatment time on plasma nitrocarburised compound layers for pure iron

Plasma nitrocarburising was carried out in a CO gas atmosphere with 3 mbar at 570°C for various times to investigate the effect of treatment time on the structure of the compound layer. The compound layers produced were composed of mainly the γ' phase for a slow cooled specimen and mixed γ'/e phases for a fast cooled specimen during the initial stage. With the increasing of the treatment time up to 10 hours, the proportion of the e phase relative to the γ' phase in the compound layer and the compound layer thickness were increased. When the treatment time was further increased to 30 hours, an anomalous phenomenon was observed: the e phase was not stabilised and the compound layer thickness was decreased. It was found that the anomalous compound layer growth is associated with the removal of surface material by sputtering or the formation of oxynitride.

Universality and scaling in the disordering of a smectic liquid crystal.

We present experimental and theoretical studies of the effects of quenched disorder on one-dimensional crystal ordering in three dimensions. This fragile smectic liquid crystal layering, the material with the simplest positional order, is also the most easily deformed periodic structure and is, therefore, profoundly affected by disorder, introduced here by confinement in silica aerogel. Theory and experiment combine to characterize this system to an extraordinary degree, their close accord producing a coherent picture: crystal ordering is lost, giving way to extended short-range correlations that exhibit universal structure and scaling, anomalous layer elasticity, and glassy dynamics.

Transition to plastic motion as a critical phenomenon and anomalous interface layer of a 2D driven vortex lattice

The dynamic transition between the ordered flow and the plastic flow is studied for a two-dimensional driven vortex lattice, in the presence of sharp and dense pinning centers, from numerical simulations. For this system, which does not show smectic ordering, the lattice exhibits a first order transition from a crystal to a liquid, shortly followed by the dynamical transition to the plastic flow. The resistivity provides a critical order parameter for the latter, and critical exponents are determined in analogy with a percolation transition. At the boundary between a pinned region and an unpinned one, an anomalous layer is observed, where the vortices are more strongly pinned than in the bulk.

Fabrication of Optically Designed Multidomain Layer Structure using Azo-Dye-Doped Ferroelectric Liquid Crystal Having N*‒C* Phase Sequence

We proposed an optical fabrication method of designed multidomain structure in an azo-dye-doped ferroelectric liquid crystal with chiral nematic (N*) – chiral smectic C (SmC*) phase sequence (NC-FLC). This layer alignment control is based on the N*−SmC* phase transition induced by the photoisomerization of doped azo-dye. This photo-assisted layer switching can be applied to the fabrication of the designed multidomain structure in the electrooptic elements such as display, optical memory and optical grating. Anomalous layer switching upon the electric field has also been observed.

Seismic and electromagnetic evidence of dehydration as a free water source in the reactivated crust

According to recent estimates, the continental mid-crust contains 35–40 per cent amphibolites. Heating of the crust by an underlying mantle plume, for example beneath continental rifts, high plateaus, and areas of intraplate volcanic activity, releases water. Dehydration of amphibole-bearing rocks at depths of 20–40 km occurs mainly in the temperature range 650–700 °C, and this releases about 0.4 wt per cent of water. Seismic tomography studies of the crust in the Kirgyz Tien Shan Range, where the age of the tectonic activity is less than 30 Ma, revealed a low-velocity zone in the mid-crust. The velocity of P waves was 0.4 km s−1 lower than in normal crust. MT sounding data in the region show the existence of a low-resistivity layer with an average resistivity of about 25 Ω m at the depth of the low-velocity layer. The spatial correlation of the observed anomalous layers and calculated effect of fluid phase on seismic and electric parameters of rocks suggests the presence of aqueous fluids released by the heating of the mid-crust.

The deep crustal accretion beneath the Laxmi Ridge in the northeastern Arabian Sea: the plume model again

The Laxmi Ridge is the most intriguing structural feature of the northeastern Arabian sea. It is char- acterized by unusual crustal structure and anomalous gravity signature. Though the earlier geophysical examinations provide some vital information about its crustal configuration, its origin and evolution have remained unsolved. Using the available seismic information, the present 2-D together with 3-D gravity modelings of the Laxmi Ridge crust:mantle system brought out a transitional layer between the depth of 11-22 km. This anomalous layer is not confined beneath the ridge axis but found to be present in the entire eastern basin and interpreted as a massive mafic intrusion beneath the region. Thickness of this layer at the base of the crust beneath the Laxmi Ridge decreases gradually towards the north-west. However, its extension towards the southeast and ultimate connection with the Chagos-Laccadive Ridge makes the western bound- ary of the magmatic crustal accretion along the west coast of India. It is suggested that the Deccan plume head mushrooming beneath the region has modified the crust with a huge magmatic intrusion. The then spreading centre coupled with the Deccan volcanic eruption is held responsible for the present day con- figuration of the Laxmi Ridge.

Crustal accretion beneath the Koyna coastal region (India) and Late Cretaceous geodynamics

In 1967 a major earthquake in the Koyna region attracted attention to the hitherto considered stable Indian shield. The region is covered by a thick pile of Deccan lava flows and characterized by several hidden tectonic features and complex geophysical signatures. Although deep seismic sounding studies have provided vital information regarding the crustal structure of the Koyna region, much remains unknown. The two available DSS profiles in the region have been combined along the trend of Bouguer gravity anomalies. Unified 2-D density modelling of the Koyna crust/mantle suggests a ca. 3 km thick and 40 km wide high velocity/high density anomalous layer at the base of the crust along the coastline. The thickness of this anomalous layer decreases gradually towards the east and ahead of the Koyna gravity low the layer ceases to be visible. Based on the seismic and gravity data interpretation in the geodynamical/rheological boundary conditions the anomalous layer is attributed to igneous crustal accretion at the base of the crust. It is suggested that the underplated layer is the imprint of the magmatism caused by the deep mantle plume when the northward migrating Indian plate passed over the Reunion hotspot.

The rectification of wind-driven flow due to its instabilities

Within a quasi-geostrophic two-layer model of the wind-driven ocean circulation, an idealized symmetric double gyre flow is considered. When lateral friction is large enough, this flow is steady and characterized by a motionless lower layer. As friction is decreased, the flow undergoes a transition to time-dependence through a Hopf bifurcation, associated with a mixed barotropic/baroclinic instability. It is shown that the nonlinear self-interaction of this oscillatory unstable mode induces a nonzero time mean response in the lower layer. The origin of this deep flow is clarified through a weakly nonlinear analysis near critical conditions. It is explained how the perturbation is associated with both fluxes of anomalous layer thickness and Reynolds' stresses, due to vertical and horizontal phase lags, respectively. Furthermore, it is explained how the patterns of vorticity input, integrated over a cycle of the perturbation, induce a forcing of the second layer. Results deduced from computed trajectories at supercritical conditions support the conclusions from the weakly nonlinear analysis. This time-dependent view of the origin of the deep circulation is complementary to the classical (steady) picture based on the closure of geostrophic contours and forcing by interfacial friction, and may help to interpret results of eddy-resolving numerical models.