Lateral Thickness Variations Research Articles

Axial EBIC oscillations at core/shell GaAs/Fe nanowire contacts

Electron beam induced current (EBIC) measurements were carried out in situ in the scanning electron microscope on free-standing GaAs/Fe core–shell nanowires (NWs), isolated from the GaAs substrate via a layer of aluminum oxide. The excess current as a function of the electron beam energy, position on the NW, and scan direction were collected, together with energy dispersive x-ray spectroscopy. A model that included the effects of beam energy and Fe thickness predicted an average collection efficiency of 60%. Small spatial oscillations in the EBIC current were observed, that correlated with the average Fe grain size (30 nm). These oscillations likely originated from lateral variations in the interfacial oxide thickness, affecting the resistance, barrier potentials, and density of minority carrier recombination traps.

Late Cretaceous to Miocene and Quaternary deformation history of the Chalk: Channels, slumps, faults, folds and glacitectonics

Late Cretaceous Chalk sedimentation history across the British Isles included (i) fault controlled uplift and subsidence in Northern Ireland and the Inner Hebrides and (ii) uplift along the lines of en echelon folds in Southern Britain and northern France. Synsedimentary slump folds and downslope displacement structures are compared with penecontemporaneous interbed slides and later tectonic folds and faults. Compressional strike-slip tectonic processes at Flamborough Head, Yorkshire, illustrate intra-Chalk slump beds in a half-graben setting. Progressive ‘growth’ of structures characterises early downslope slump folding, interbed sliding and some listric faulting. Sheet-flints replacing slide shear planes and early fractures provide evidence for early movements. Availability of open-slopes or the depth of burial under which the range of structures developed is reflected in the degree of disruption and fragmentation of chalk and flint. Fragmentation provides clues to the timing of events and origin of the Late Campanian Altachuile Breccia (Northern Ireland) and the Coniacian Hope Gap slides (Sussex). Fragmentation and formation of sheet flints together help distinguish intra-Chalk tectonics from Quaternary glacitectonic structures.The role of marl seams, high porosity chalk beds and hardgrounds on bed-sliding, décollement zones and disruption of chalk blocks from bedrock in glacitectonics is discussed. Chalk formations with marl seams develop a special style of fracturing related to early interbed sliding and pore-fluid escape structures. Marl-seams are shown to be primary sedimentary features and not the products of post depositional pressure-solution. More than any other formation the Late Santonian – Early Campanian Newhaven Chalk contains extensive sheet-flints and shows great lateral variation in thickness and lithology across the fold belts of southern England and northern France.

The influence of intrabasinal tectonics in the stratigraphic evolution of piggyback basin fills: Towards a model from the Tremp-Graus-Ainsa Basin (South-Pyrenean Zone, Spain)

Piggyback basins often display asymmetric sedimentary successions influenced by both regional and local syn-depositional tectonic structures. There are few examples where a complete exhumed piggyback basin fill can be analyzed, from proximal, close to source areas, to the distal deep-water sinks. The study of these basins is a key to extract the stratigraphic signal of intrabasinal tectonics, which often competes and can even overprint other controlling factors such as eustasy, climate, or autogenic processes. The purpose of this paper is to propose a more detailed model of the evolution of piggyback basin fills, which recognizes the influence of intrabasinal tectonic activity, specific to the piggyback context and the resulting stratigraphic architecture. We investigate the Tremp-Graus-Ainsa piggyback basin, located in the southern Pyrenees, during early Eocene, which was influenced by the development of the Montsec frontal thrust. Twenty four stratigraphic sections, totaling 14 km of logged thickness, are analyzed, covering 60 km of the basin. The stratigraphic surfaces have been correlated in order to define large-scale stratigraphic units and a new sequence stratigraphic framework. The influence of local tectonics in the basin record is identified by abrupt lateral sediment thickness variations, abrupt changes of depositional environments, local erosion surfaces, and changes in dominant sedimentary process regimes and paleocurrent directions through space and time. As a result, we propose a new tectono-stratigraphic model for the development of the Tremp-Graus-Ainsa Basin. Specifically, we identify (1) an underfilled stage with initial high flexural subsidence and a frontal subaqueous blind anticline thrust propagation that controls a dominantly carbonate environment, followed by a progressive increase in siliciclastic sediment supply, and (2) an overfilled stage with an increase in frontal thrust propagation causing new emergent sediment sources. This second stage evolves from an active emerged blind anticline thrust to a migrating thrust that moved along lateral ramps, and which caused the decoupling of the Tremp-Graus uplifted wedge-top sub-basin from the Ainsa subsiding foredeep domain. Finally, we provide a new model intrinsic to the piggyback basin development with four main stages, controlled by changes in intrabasinal tectonic activity, flexural subsidence and orogeny axial zone evolution, which ultimately modulate sediment supply and creation and distribution of accommodation space.

Evidence for crustal low shear-wave speed in western Saudi Arabia from multi-scale fundamental-mode Rayleigh-wave group-velocity tomography

We investigate the crustal and upper-mantle shear-velocity structure of Saudi Arabia by fundamental-mode Rayleigh-wave group-velocity tomography and shear-wave velocity inversion. The seismic dataset is compiled using ∼140 stations of the Saudi National Seismic Network (SNSN) operated by the Saudi Geological Survey (SGS). We measure Rayleigh-wave group-velocities at periods of 8–40 s from regional earthquakes. After obtaining 1-D shear-wave velocity models by inverting group-velocities at each grid node, we construct a 3-D shear-velocity model for Saudi Arabia and adjacent regions by interpolating the 1-D models. Our 3-D model indicates significant lateral variations in crustal and lithospheric thickness, as well as in the shear-wave velocity over the study region. In particular, we identify zones of reduced shear-wave speed at crustal levels beneath the Cenozoic volcanic fields in the Arabian Shield. The inferred reductions of 2–5% in shear-wave speed may be interpreted as possibly indicating the presence of partial melts. However, their precise origin we can only speculate about. Our study also reveals an upper-mantle low velocity zone (LVZ) below the Arabian Shield, supporting the model of lateral mantle flow from the Afar plume. Further geophysical experiments are needed to confirm (or refute) the hypothesis that partial melts may exist below the Cenozoic volcanism in western Saudi Arabia, and to build a comprehensive geodynamic–geological model for the evolution and present state of the lithosphere of the Arabian Plate and the Red Sea.

The impact of lateral variations in lithospheric thickness on glacial isostatic adjustment in West Antarctica

Differences in predictions of Glacial Isostatic Adjustment (GIA) for Antarctica persist due to uncertainties in deglacial history and Earth rheology. The Earth models adopted in many GIA studies are defined by parameters that vary in the radial direction only and represent a global average Earth structure (referred to as 1-D Earth models). Oversimplifying the actual Earth structure leads to bias in model predictions in regions where Earth parameters differ significantly from the global average, such as West Antarctica. We investigate the impact of lateral variations in lithospheric thickness on GIA in Antarctica by carrying out two experiments that use different rheological approaches to define 3-D Earth models that include spatial variations in lithospheric thickness. The first experiment defines an elastic lithosphere with spatial variations in thickness inferred from seismic studies.We compare the results from this 3-D model with results derived from a 1-D Earth model that has a uniform lithospheric thickness defined as the average of the 3-D lithospheric thickness. Irrespective of the deglacial history and sublithospheric mantle viscosity, we find higher gradients of present-day uplift rates (i.e. higher amplitude and shorter wavelength) in West Antarctica when using the 3-D models, due to the thinner-than-1-D-average lithosphere prevalent in this region. The second experiment uses seismically inferred temperature as an input to a power-law rheology, thereby allowing the lithosphere to have a viscosity structure. Modelling the lithosphere with a powerlaw rheology results in a behaviour that is equivalent to a thinner lithospheremodel, and it leads to higher amplitude and shorter wavelength deformation compared with the first experiment. We conclude that neglecting spatial variations in lithospheric thickness in GIA models will result in predictions of peak uplift and subsidence that are biased low in West Antarctica. This has important implications for ice-sheet modelling studies as the steeper gradients of uplift predicted from the more realistic 3-D model may promote stability in marine-grounded regions of West Antarctica. Including lateral variations in lithospheric thickness, at least to the level of considering West and East Antarctica separately, is important for capturing shortwavelength deformation and it has the potential to provide a better fit to Global Positioning System observations as well as an improved GIA correction for the Gravity Recovery and Climate Experiment data.

Conundrums in loading-driven salt movement

Salt evacuation and rise can be driven by differential loading. Even less-dense (siliciclastic) overburden can subside with adequate differences in pressure head due to lateral variations in overburden density and/or thickness. Although the base of the thickest load sinks into salt, the top remains a topographic high so that further sedimentation in the same area is unlikely and subsidence should cease. Yet there are numerous examples where subsidence of less-dense sediment has continued in the absence of accepted mechanisms such as extension or contraction.Here I summarize the theoretical framework and relevant analog and numerical models before addressing several related conundrums in our understanding of loading-driven salt movement. First, how do minibasins less dense than salt sink enough to accumulate more sediment and thereby keep subsiding? Second, what happens when the overburden thins in the direction opposite to the top-salt dip, as on the flank of any minibasin? I then offer some thoughts on possible solutions and suggest that further research is needed to solve these problems.

Lateral Variations in Foreland Flexure of a Rifted Continental Margin: The Aquitaine Basin (SW France)

AbstractRift inheritance can play a key role in foreland basin geometry and behavior. If the foreland basin initiates soon after rifting, thermal cooling can also contribute significantly to subsidence. We investigate the effects of crustal inheritance (Aptian‐Cenomanian rifting) on the evolution of the Campanian to middle Miocene flexural Aquitaine foreland basin, northern Pyrenees, France. Surface and subsurface data define rifted crustal geometry and postrift thermal subsidence. Analysis of Bouguer gravity anomalies coupled with flexural modeling constrains the lateral variations of elastic thickness, plate flexure, and controlling loads. The Aquitaine foreland is divided along‐strike into three sectors. The relative role of surface and subsurface (i.e., buried) loading varies along‐strike, and the elastic thickness values decrease from the northeast (25 km) to the southwest (7 km) where the plate is the most stretched. The eastern foreland crust was not rifted and underwent a simple flexural subsidence in response to orogenesis. The central sector was affected by crustal stretching. Here the basin is modeled by combining topographic and buried loads, with postrift thermal subsidence. In the western sector, the foreland basin was created mainly by postrift thermal subsidence. The eastern and central sectors are separated by the Eastern Crustal Lineament, which is one of a series of inherited transverse faults that segment the orogen.

Seismic reflection and structuring characterization of deep aquifer system in the Dakhla syncline (Cap Bon, North-Eastern Tunisia)

The Dakhla syncline is located in the North-Eastern Tunisia. It is bounded by Abd El Rahmene anticline to the North-West, El Haouaria Graben to the North-East, Grombalia Graben to the South-West and the Mediterranean Sea to the East. The main aquifer reservoirs of Dakhla syncline are constituted by stacks of fluvial to deltaic Neogene sequences and carbonates. The interpretation of eight seismic reflection profiles, calibrated by wire line logging data of three oil wells, hydraulic wells and geologic field sections highlighted the impact of tectonics on the structuring geometry of aquifers and their distribution in elevated structures and subsurface depressions. Lithostratigraphic correlations and seismic profiles analysis through the syncline show that the principal aquifers are thickest within the central and northern part of the study area and thinnest to the southern part of the syncline.Seismic sections shows that the fracture/fault pattern in this syncline is mainly concentrated along corridors with a major direction of NW-SE and secondary directions of N-S, E-W and NE-SW with different release. This is proved by the complexity structure of Eastern Tunisia, resulted from the interaction between the African and Eurasiatic plates. Isochron maps of aquifers systems exhibited the structuring of this syncline in sub-surface characterized by important lateral and vertical geometric and thickness variations. Seismic sections L1, L2, L3, L4, L5 and petroleum wells showed an heterogeneous multilayer aquifers of Miocene formed by the arrangement of ten sandstone bodies, separated by impermeable clay packages. Oligo-Miocene deposits correspond to the most great potential aquifers, with respectively an average transmissivity estimated: Somaa aquifer 6.5 10−4 m2/s, Sandstone level aquifer 2.6 10−3 m2/s, Beglia aquifer 1.1 10−3 m2/s, Ain Ghrab aquifer 1.3 10−4 m2/s and Oligocene aquifer 2 10−3 m2/s. The interpretation of spatial variations of seismic units and the recognition of tectonic structures and their development may reveal some new insights for hydrogeological aspects.

The stratigraphic architecture, distribution and hydrocarbon potential of the organic-rich Kyalla and Velkerri shales of the Upper Roper Group (McArthur Basin)

Recent exploration studies on the stratigraphic framework, depositional environment and tectonic setting of the Mesoproterozoic Roper Group in the McArthur Basin, have led to an improved understanding of the highly prospective organic-rich shales of the Velkerri and Kyalla Formations. From a review of open-file drill core and well logs, several major regressive–transgressive (R–T) cycles have been identified along with four key facies associations within the Roper Group. The R–T cycles show sedimentary features typical of deposition within a clastic-dominated marine deltaic setting and show marked lateral variation in facies and thicknesses. The lateral thickness variations are particularly noticeable across regional 2D seismic lines. Seismic interpretation and well correlation confirm the extensive and relatively undeformed nature of the Velkerri and Kyalla Formations in the subsurface, they appear thickest in the south-east of the Beetaloo Sub-Basin.

Enceladus’s crust as a non-uniform thin shell: I tidal deformations

The geologic activity at Enceladus’s south pole remains unexplained, though tidal deformations are probably the ultimate cause. Recent gravity and libration data indicate that Enceladus’s icy crust floats on a global ocean, is rather thin, and has a strongly non-uniform thickness. Tidal effects are enhanced by crustal thinning at the south pole, so that realistic models of tidal tectonics and dissipation should take into account the lateral variations of shell structure. I construct here the theory of non-uniform viscoelastic thin shells, allowing for depth-dependent rheology and large lateral variations of shell thickness and rheology. Coupling to tides yields two 2D linear partial differential equations of the fourth order on the sphere which take into account self-gravity, density stratification below the shell, and core viscoelasticity. If the shell is laterally uniform, the solution agrees with analytical formulas for tidal Love numbers; errors on displacements and stresses are less than 5% and 15%, respectively, if the thickness is less than 10% of the radius. If the shell is non-uniform, the tidal thin shell equations are solved as a system of coupled linear equations in a spherical harmonic basis. Compared to finite element models, thin shell predictions are similar for the deformations due to Enceladus’s pressurized ocean, but differ for the tides of Ganymede. If Enceladus’s shell is conductive with isostatic thickness variations, surface stresses are approximately inversely proportional to the local shell thickness. The radial tide is only moderately enhanced at the south pole. The combination of crustal thinning and convection below the poles can amplify south polar stresses by a factor of 10, but it cannot explain the apparent time lag between the maximum plume brightness and the opening of tiger stripes. In a second paper, I will study the impact of a non-uniform crust on tidal dissipation.

Origin and mechanism of formation of the Oligo-Miocene ironstones of Umm Himar Formation, Turabah area, southwestern Arabian Shield, Saudi Arabia

Turabah area is located in the southwestern Arabian Shield of Saudi Arabia. The present study aims to shed light on the origin and mechanism of formation of the ironstones of the Tertiary sedimentary succession of the Umm Himar Formation in Turabah area. These ironstones are enclosed within mudstones and carbonates with less frequent sandstone. These lithologies show lateral and vertical facies changes and thickness variations and are stacked in cyclic nature within seven sedimentary units. The studied ironstones are present in two main horizons within the lower part of the succession as follows. (1) Lower massive and oolitic hematitic ironstones which are present as thin continuous and discontinuous bands and laminae within the varicolored tuffaceous mudstone unit of the lower part of the succession. The oolitic ironstones are present in successive small cycles. The lower parts of these cycles are composed of ferruginous friable mudstones. The middle parts are composed of hard ferruginous mudstones with abundant vertical and inclined burrows. The upper parts of these cycles are composed of indurated oolitic hematitic ironstones. The iron ooids of these ironstones were formed during successive and subsequent stages of diagenetic ferruginization of the precursor tuffaceous materials. (2) Upper bedded peloidal hematitic-magnetitic ironstones which are present in the hematitic-magnetitic ironstone unit no. 6 just overlying the bedded basalt unit no. 5. These ironstones were formed during subsequent stages of diagenetic ferruginization of precursor tuffaceous materials. These stages include the formation of green clays, devitrification, and finally ferruginization of the tuffaceous materials. This ironstone type is laterally changed into the penecontemporaneous bedded tuffaceous basalt.

Structural significance of an evaporite formation with lateral stratigraphic heterogeneities (Southeastern Pyrenean Basin, NE Spain)

We run a series of analogue models to study the effect of stratigraphic heterogeneities of an evaporite formation on thin-skinned deformation of the Southeastern Pyrenean Basin (SPB; NE Spain). This basin is characterized by the existence of evaporites, deposited during the Early-Middle Eocene with lateral variations in thickness and lithological composition. These evaporites are distributed in three lithostratigraphic units, known as Serrat Evaporites, Vallfogona and Beuda Gypsum formations and acted as décollement levels, during compressional deformation in the Lutetian. In addition to analogue modeling, we have used field data, detailed geological mapping and key cross-sections supported by seismic and well data to build a new structural interpretation for the SPB. In this interpretation, it is recognized that the basal and upper parts of the Serrat Evaporites acted as the main décollement levels of the so-called Cadí thrust sheet and Serrat unit. A balanced restoration of the basin indicates that thrust faults nucleated at the stratigraphic transition of the Serrat Evaporites (zone with lateral variations of thickness and lithological composition), characterized by a wedge of anhydrite and shale. The analogue models were setup based on information extracted from cross-sections, built in two sectors with different lithology and stratigraphy of the evaporites, and the restored section of the SPB. In these models, deformation preferentially concentrated in areas where thickness change, defined by wedges of the ductile materials, was inbuilt. Based on the structural interpretation and model results, a kinematic evolution of the SPB is proposed. The kinematic model is characterized by the generation of out-of-sequence structures developed due to lateral stratigraphic variations of the Serrat Evaporites. The present work shows a good example of the role of stratigraphic heterogeneities of an evaporite formation which acts as décollement level on structural deformation in a fold-thrust belt. The results of this work have implications for hydrocarbon exploration and are relevant for studying structural geometry and mechanics in shortened evaporite basins.

Role of Late Jurassic intra-oceanic structural inheritance in the Alpine tectonic evolution of the Monviso meta-ophiolite Complex (Western Alps)

AbstractThe eclogite-facies Monviso meta-ophiolite Complex in the Western Alps represents a well-preserved fragment of oceanic lithosphere and related Upper Jurassic – Lower Cretaceous sedimentary covers. This meta-ophiolite sequence records the evolution of an oceanic core complex formed by mantle exhumation along an intra-oceanic detachment fault (the Baracun Shear Zone), related to the opening of the Ligurian–Piedmont oceanic basin (Alpine Tethys). On the basis of detailed geological mapping, and structural, stratigraphic and petrological observations, we propose a new interpretation for the tectonostratigraphic architecture of the Monviso meta-ophiolite Complex, and discuss the role played by structural inheritance in its formation. We document that subduction- and exhumation-related Alpine tectonics were strongly influenced by the inherited Jurassic intra-oceanic tectonosedimentary physiography. The latter, although strongly deformed during a major Alpine stage of non-cylindrical W-verging folding and faulting along exhumation-related Alpine shear zones (i.e. the Granero–Casteldelfino and Villanova–Armoine shear zones), was not completely dismembered into different tectonic units or subduction-related mélanges as suggested in previous interpretations. The present-day architecture of the Monviso meta-ophiolite Complex results from nappe-scale folding with a significant component of shearing, and strain partitioning of the Alpine deformation, which were controlled by the inherited occurrence of (i) lateral and vertical variations of facies and thickness of sediments, (ii) an intra-oceanic fault-rock assemblage, which acted as weak horizons in concentrating deformation, and (iii) remnants of a volcanic ridge, which consists of massive metabasalt. Thus, the recognition of pre-collisional, intra-oceanic, tectonostratigraphic inheritance represents an important step in reconstructing the tectonic evolution of meta-ophiolite units in orogenic belts.

Comparison of crustal structures in the fossil spreading center of South China Sea basins and the tectonic significance

This study focuses on the magmatic activities and seamounts in the fossil spreading center of three sub-basins in the South China Sea (SCS). The northwest sub-basin (NWSB), southwest sub-basin (SWSB), and east sub-basin (ESB) are of great importance for understanding the tectonic evolution and post-spreading magmatic activities of the SCS. Deep crustal structures beneath the spreading center of these basins have a significant bearing on the magmatic process. For this study, on the basis of a previous velocity model, we used two-dimensional ray-tracing methods and high-quality ocean-bottom seismometer (OBS) data collected in 2006 to image the NWSB deep P-wave velocity structures. We then performed travel-time analysis and estimated uncertainties of the final model parameters to prove that the velocity model was convincing. Finally, we compared the one-dimensional velocity structures among OBS profiles 2006-p1 in the NWSB, G8G0 in the ESB, and T1 in the SWSB. Our study of crustal structure yielded the following results: (1) the NWSB is confirmed to be oceanic crust compared with typical Atlantic oceanic crust. The crustal velocity is 5.7–7.0 km/s, whereas the crustal thickness is 6–8 km in the basin (90–130 km). There is no high-velocity layer. The Mohorovicic discontinuity (Moho) is distributed symmetrically along the spreading axis and gradually increases from the basin center to the edge. The depth of the Moho is 11–12 km in the basin (55–70 km) and changes dramatically to 16 km depth under the north margin (170–200 km) and the Zhongsha Islands (0–55 km). The depth of the Moho in all three sub-basins increases by 1 km in the spreading center. This suggests that crustal structure in the SCS is greatly influenced by magmatic activities. The Shuangfeng seamount, with a 6.5-km-thick crust, located in the center of the spreading ridge, is a post-spreading volcanic seamount. The 6.4-km/s velocity contour beneath the Shuangfeng seamount gradually rises toward the northwest part, and the velocity at the seamount bottom is lower than 7.4 km/s. Therefore, we speculate that the magma underplating does not exist. Consistent with multichannel seismic date, we conclude that the Shuangfeng seamount was mainly formed by an intrusive process: The magma intruded along the former extinct spreading axis. The Longnan seamount in the SWSB and the Jixiang seamount in the ESB were mainly formed by an eruption process, supposedly the result of the buoyant decompression melting mechanism. (2) The detailed velocity model of the NWSB shows that the oceanic crust exhibits obvious across-axis velocity variations. The lateral variations in crustal thickness and velocity are also found in the ESB and SWSB. This shows that the previously formed oceanic crust in all three SCS sub-basins are reformed by post-spreading magma. The difference in crustal velocity of the NWSB indicates that the magma supply varied in space during or even after the seafloor spreading. The OBS 2006-p1 velocity model indicates that the velocity contours gradually rise from the southeast to northwest (140–160 km) direction. The crust layer may have experienced asymmetric basalt magma flow, and the northwest part of the basin may have received more magma than the southeast part during the process of seafloor spreading. Low crustal velocities were found in the SWSB and ESB but not in the NWSB. This indicates an inadequate supply of magma in the NWSB. Compared with the other two sub-basins, the Cenozoic tectonic activities in the NWSB were less active. The residual magma might have been consumed after the NWSB spreading. Either there are no magma chambers in the NWSB spreading center, or the magma chambers are too small to detect.

Radioactive Occurrence in Sediments of Cenozoic Age near Bandha Village, Jaisalmer District, Rajasthan

ABSTRACT The radioactivity is dependent on the isotope and their concentration in the mineral such as potassium (K), uranium (U) and thorium (Th). In this paper the presence of a 3 to 5 m thick uranium and thorium bearing sediment of Cenozoic age is reported above Goru Formation of Jaisalmer basin. Gamma ray log response of Well-1 which falls under the study area has marked with very high GR (gamma ray) readings ranging from 350-1488 API coupled with high uranium and thorium content from spectral gamma ray log ranging from 92-178 ppm and 60-80 ppm respectively at the depth of 50 m from ground surface (-110m above MSL). Further studies are required to delineate the lateral thickness variation for mining purpose which may be a radioactive source.

An intrusive origin of some UG-1 chromitite layers in the Bushveld Igneous Complex, South Africa: Insights from field relationships

The UG-1 chromitite in the Bushveld Complex is a 1m thick massive layer overlain by orthopyroxenite (4–9m thick) and underlain by anorthosite (1.5–2.0m thick), which is strongly interleaved with numerous thin layers of UG-1 chromitite (mm to 10s of cm thick). The chromitite-bearing anorthosite is further underlain by mottled anorthosite, which is devoid of chromitites. Sometimes, the massive UG-1 chromitite may be hosted entirely within orthopyroxenite; in this study, however, the orthopyroxenite occurs intermediate to the massive UG-1 layer and the footwall interleaved anorthosite unit, which has a 10cm thick chromitite layer at the contact. Though the UG-1 chromitites display complex features in the field outcrops (e.g., presence of numerous anorthosite autoliths, bifurcations, and dimpled contacts with anorthosite), the ones discussed in this study are distinctive and diagnostic of the emplacement mechanism for this chromitite layer. In this study of the UG-1 chromitite at the Impala Platinum mine, the chromitite is seen to form a small pothole (≈1.5m deep; 3.6m wide) that cuts down into the underlying orthopyroxenite (exposed only on the right side of the pothole) and anorthosite layers. A 15cm thick UG-1 chromitite layer (85 modal% chromites) is observed to protrude or branch out of the pothole margin and laterally extend into the anorthosite, with which it shows several crosscutting features. Examples include truncation of layering defined by interstitial pyroxene mottles within the footwall anorthosite against the protruding UG-1 layer, and truncation of several thinner UG-1 layers in the hanging wall anorthosite against erosive outlines of this chromitite layer. These observations strongly attest to the emplacement of the protruding UG-1 chromitite layer as a sill into the anorthosite. The presence of thin anorthosite lenses with high aspect ratios within this UG-1 layer, along with lateral variations in thickness of the layer, its irregular contact with the host anorthosite, and disturbances observed for the UG-1 layers in the footwall anorthosite adjacent to the erosive contact, implies that the superheated melt, parental to the chromitite, chemically dissolved the anorthosite and made space for it to flow as a sill within the latter. The probable mode of emplacement of the laterally protruding UG-1 layer would be through the opening of fractures related to the event of pothole formation where there is ponding of hot, thick and dense melt above the anorthosite. The melt subsequently filled the fractures and flowed laterally into the anorthosite thereby eroding it. An open magmatic system ensured continuous addition of melts in the pothole and their supply into the sill. This study thus emphasizes that some of the UG-1 chromitite layers within anorthosite can be emplaced as sills, and not all need to form from discrete cyclical pulses of chromite-saturated and plagioclase-saturated melts being brought into the chamber.

GaAs/AlAs triple-coupled cavity with InAs quantum dots for ultrafast wavelength conversion devices

We have investigated a GaAs/AlAs triple-coupled multilayer cavity structure with InAs quantum dots for an ultrafast wavelength conversion device. Three cavity modes with the resonance frequencies ω1, ω2, and ω3 were used for efficient wavelength conversion via a four-wave mixing (FWM) process. Identical frequency separation between two adjacent modes (ω1 − ω2 = ω2 − ω3) was successfully realized using a controlled lateral thickness variation across the wafer. Time-resolved FWM signals from the triple-coupled multilayer cavity were measured using 100 fs laser pulses. The incident laser pulses were divided into two pulses and each of them was spectrally shaped individually so that the input and control pulses only covered the ω1 and ω2 modes, respectively. The wavelength-converted FWM signal with a frequency of ω3 (= 2ω2 − ω1) was clearly observed when the sample was simultaneously irradiated with the input and control laser pulses.

The Don formation, Toronto, Canada: a record of the sangamonian interglacial and early wisconsinan (warm part of MIS 5e to a MIS 5 cold substage)

Regarded as a reference unit of the Sangamonian Interglacial in North America, the Don Formation was deposited however during a longer interval of time, including the interglacial climatic optimum of MIS 5e followed by an erosive and cool phase (intra-MIS 5e or MIS 5d), a boreal phase with a fir forest (late MIS 5e transition or MIS 5c), erosive episodes and the early part of a cold stadial (MIS 5d or MIS 5b). From the planar geometry of the beds exposed at the former Don Valley Brickyard in Toronto, the formation comprises five continuous allozones with lateral variations of thickness. Each allozone corresponds to dominant conditions of sedimentation in a shoreface of Lake Coleman within a limited accommodation space. Allozone D2 is correlated to other interglacial units of Eastern Canada, with ages between 128 and 115 ka. The interglacial climatic optimum is interrupted in the area by two cool events, the first one similar to the Tunturi event of Finland, the second one probably related to a Greenland Stadial. The heterogeneous upper Allozone D5 was deposited in shallow waters when a Northern Boreal Forest prevailed in the watershed. This zone is correlated to the Deschaillons Varves of the central St. Lawrence River Valley, deposited prior to the first regional Wisconsinan glacial advance in the central St. Lawrence River Valley and the resulting glacial Lake Scarborough Formation. From the 80 ka minimalU/Th age of the Deschaillons Varves and the 98 ± 8 ka TL age of the later marine La Perade Clay, Allozone D5 was deposited during the beginning of one of the cold episodes of MIS 5.

Spatial variations in the effective elastic thickness of the lithosphere in Southeast Asia

As a proxy for long-term lithospheric strength, detailed information on lateral effective elastic thickness (Te) variations can aid in understanding the distribution pattern of surface deformation and its response to long-term forces. Here we present high-resolution maps of spatial variations of Te for the complex SE Asian region by analyzing the coherence of topography and Bouguer gravity anomaly data. We find that after considering the gravity deficit of less dense sediment, the recovered Te maps are more representative of the geology, particularly in elongated rift basins. The results show that the Te variation pattern in SE Asia, in general, agrees well with its tectonic provinces and major tectonic boundaries. The oceanic basins, the Indosinian suture zones between the Indochina and Sibumasu blocks, and the Makassar Strait are characterized by low Te, while moderate and high Te values are recovered in the Khorat plateau, West Burma, the Singapore Ridge, the Con Song Swell, Borneo, the northern Australian margin and the Molucca Sea. The Te pattern in the south Indonesian margin is complicated by the approach and collision of oceanic plateaus and seamounts with the fore-arc region. The heterogeneous strength features are consistent with the complex assemblage of different tectonic units, and significant deformation during Cenozoic tectonic events. In the Indochina Peninsula, the extruded displacement during the India-Eurasia collision might have been partitioned and absorbed by the combined mechanism of the extrusion and viscous tectonic models. As a result, the offshore displacements of the major strike-slip faults in the South China Sea are much smaller than originally assumed, thus having less effect on the development of the South China Sea than other mechanisms such as the slab pull of the proto-South China Sea. Since the displacement driven by the boundary tectonic forces has been greatly absorbed and decreased by subduction and deformation in the active margins and adjacent weak regions, the motion velocity of the interior regions is greatly lower than the boundary active margins, and they are largely free of seismicity and volcanism. Our results suggest that East Borneo might share a similar crustal basement, and represent a broad tectonic zone of the destroyed Meso-Tethys Ocean extending from West-Middle Java, through East Borneo to northern Borneo of the Sarawak and Sabah. The Indosinian zones between the Indochina and Sibumasu blocks might extend further southeastward across Billiton Island to offshore of southern Borneo, and the Singapore platform and SW Borneo might belong to the same block. The results also show that the internal load fraction F is high in the coastal area of South China, the northern margin of the South China Sea, and the coastal area of Indochina, which, in general, agrees with the distribution of a high-velocity lower crustal layer and Late Cenozoic basaltic rocks.

New stratigraphic interpretation of the Soultz-sous-Forêts 30-year-old geothermal wells calibrated on the recent one from Rittershoffen (Upper Rhine Graben, France)

The Soultz-sous-Forets site (Alsace, France) is the first pilot geothermal project in the world that furnished the proof of concept of enhanced geothermal systems by producing energy from a deep-seated granite. The Alsace region, with its hundreds of previous drillings and seismic history related to both oil and potash exploitation, has been shown to have high subterranean temperatures. In the Soultz-sous-Forets geothermal project, most attention has focused on the basement and the sedimentary cover/basement interface. Thus, the sedimentary portion of the well (approximately 1.4 km thick) has never been fully investigated. The only available data on the Soultz-sous-Forets sedimentary cover are from an old masterlog (GPK-1) with interpretations of the tops and bases of the main geological formations, and from some other less well-documented well logs (EPS-1 and GPK-2). The main challenge of this work is therefore to re-interpret the old well data to provide precise and detailed chrono-lithostratigraphic logs for GPK-1 and GPK-2, especially in their sedimentary portions. These new investigations of the GPK wells have been possible due to the new data collected in the recent geothermal wells at Rittershoffen (GRT-1 and GRT-2, located 6.5 km from Soultz-sous-Forets), which are characterized by a quite complete stratigraphic succession. Both sites have been explored by deep drilling operations aiming to exploit the heat extracted from a deep granitic basement (Palaeozoic) covered by a stack of 1.4- and 2.2-km-thick sedimentary rocks (Mesozoic to Cenozoic) at Soultz-sous-Forets and Rittershoffen, respectively. Thus, the Rittershoffen chrono-lithostratigraphic logs have been used as a baseline to interpret the sedimentary succession in GPK-1 and GPK-2. In conclusion, all the well logs are compared for stratigraphic comparisons. With approximately 1400 m of sedimentary cover at Soultz-sous-Forets instead of 2200 m at Rittershoffen, the correlation between the two sites showed many differences exist in the sedimentary columns: layers missing due to erosion, lateral thickness variations in formations, and—above all—the occurrence of at least four major fracture zones affecting the units. In addition, a structural analysis was made to more precisely define the limits of the geological formations observed and to present more arguments for the presence of fracture zones or faults.