Variations In Sediment Thickness Research Articles

Empirical Equation between Predominant Frequency and Sediment Thickness in Plampang, West Nusa Tenggara

Soil characteristics mapping is one of the essential aspects in the development of a region. Soil characteristics such as the predominant frequency and thickness of the sediment layer are needed to ensure the capacity of soil against the load of an infrastructure construction. The existence of tectonic activity causes West Nusa Tenggara Province to be an area with high seismic activity. Along with the plan to accelerate the development of the Samota Area, seismic studies are one of the aspects that determine project feasibility. The predominant frequency value from the results of microtremor measurements is used in the development of an empirical formula to predict soil thickness in the Plampang area. The empirical formula from previous research is used as a conformity test of the empirical formula obtained. Empirical equations are then used to map variations in sediment thickness in the Plampang area, which are justified based on geological data. The results of the sediment thickness map based on the empirical formula for the Plampang area showed a good correlation with the predominant frequency and also showed a good correlation with the results of the formula that has been carried out by previous studies.

Basin-wide erosion and segmentation of the Plio-Pleistocene forearc basin in central Japan revealed by tephro- and biostratigraphy

The basement of the Tokyo metropolitan area consists of the Miocene–Pleistocene forearc basin fills that are well exposed around Tokyo Bay, especially on the Miura and Boso peninsulas. The forearc basin fills on these two peninsulas are called the Miura and Kazusa groups, and they were deposited during the late Miocene–Pliocene and Pliocene–middle Pleistocene, respectively. Because many biostratigraphic datum planes, paleomagnetic reversal events, and other chronostratigraphic tools are available for these deposits, they provide the “type stratigraphy” of other equivalent sedimentary sequences on the Japanese islands and in the northwest Pacific. However, the use of such stratigraphic markers has not been fully applied to understand the architecture of a basin-wide unconformity between the Miura and Kazusa groups called the Kurotaki unconformity. For our study, we made correlations among the Pliocene vitric tephra beds based on their stratigraphic levels, lithologic characteristics, the chemical compositions of glass shards, and calcareous nannofossil biostratigraphy. As a result, we were able to correlate tephra beds Ng-Ky25 just above the C3n.3n normal subchronozone (4.7 Ma), IkT16-An157.5 and IkT19-An158.5 near the top of the Mammoth reverse polarity subchronozone (3.21 Ma), and Ahn-Onr (2.6–2.7 Ma) across Tokyo Bay on the Miura and Boso peninsulas. We were able to recognize erosional surfaces and coeval mass-transport deposits immediately below the top of the Mammoth reverse polarity subchronozone, which suggests that submarine landslide(s) may have produced the lack of stratigraphic horizons (4.5–3.2 Ma) in the Miura and eastern Boso regions. Basal pebbly sandstone beds pervasively cover the erosional surfaces, and they show lateral variations into the thick (up to 60 m) mass-transport deposits and overlying turbidite sandstones. The lateral variations in sediment thickness of the post-failure deposits suggest that the basin-wide erosion was associated with the initial growth of a basin-bounding structural high that separates two distinct sub-basins in the forearc basin, which resulted in the subsequent onlapping deposition in the earliest stage of the Kazusa forearc basin. The basin-wide erosion marks the initiation of tectonic reconfigurations that led to segmentation of the forearc basin around the Tokyo Bay region.

Sediment Distribution and Treatment in the Inflow Water-Level-Fluctuating Zone of the Biliuhe Reservoir

Most of the pollutants carried by runoff accumulate in the form of sediment, impacting the capacity and water quality of reservoirs. To study the sediment distribution in the water-level-fluctuating zone (WLFZ) of a reservoir in North China with a long drying period and to explore its treatment, the Biliuhe Reservoir in Dalian, Liaoning Province, China, was selected as the study area. The sediment thicknesses along the thalwegs of the three tributaries were surveyed, including a detailed survey on the sediment thickness and particle size in Dapu, the inflow bay of the main river. According to our findings, the sediment distribution along the thalwegs is similar to the delta sedimentation. The inflow WLFZ, especially the inflow bay, is the main gathering area for sediment. Furthermore, the variation in sediment thickness in the top-set region has river siltation characteristics, which are mainly affected by the scouring and deposition of floods during the dry period. From the convex bank to the inside of the bay in Dapu, the hydrodynamic force of the sediment gradually weakens, the thickness gradually increases, and the bay is the main sedimentary area of the suspended load. A method of sediment reuse for vegetation buffer platform construction is proposed. This method can reduce the amount of sediment entering downstream and enhance the ability to remove the pollution along the bank of the reservoir.

A Weighted Closure-Phase Statics Correction Method: Synthetic and Field Data Examples

Recorded seismograms are usually distorted by statics owing to complex geological conditions, such as lateral variations in sediment thickness or complex topographies. These distorted and discontinuous signals usually exist in either arrival times or amplitudes of waves, and they are most likely to be smeared as velocity perturbations along their associated raypaths. Therefore, statics may blur images of the target bodies or, even worse, introduce unexpected and false anomalies into subsurface structures. To partly resolve this problem, we develop a weighted statics correction method to estimate unwanted temporal shifts of traces using the closure-phase technique, which is utilized in astronomical imaging. In the proposed method, the source and receiver statics are regarded as independent quantities contributing to the waveform shifts based on their acquisition geometries. Numerical tests on both the synthetic and field cases show noticeable, although gradual, improvements in data quality compared to the conventional plus–minus (PM) method. In general, this method provides a straightforward strategy to reedit the travel times in seismic profiles without inverting for a near-surface velocity model. Moreover, it can be extended to any interferometrical methods in seismic data processing that satisfies the closure-phase conditions.

Comparison of along-track resolution of SARAL/AltiKa and ENVISAT-RA2 satellite altimeters over continental margins

ABSTRACT The objective of the present study is to systematically quantify the along-track resolution of Satellite for Argos and AltiKa (SARAL)/AltiKa and Environmental Satellite (ENVISAT)-RA2 altimeters over continental margins. Spectral coherence and power spectral density estimated from pairs of stacked (averaged) sea surface height (SSH) profiles from repeat tracks as a function of wave number are used to estimate along-track resolution and noise characteristics, respectively. The analyses were carried out along 96 SARAL/AltiKa and ENIVISAT-RA2 tracks covering continental margins of India, Gulf of Mexico, and South America. Our study clearly suggests that for all three sites the short-wavelength resolution of SARAL/AltiKa (15–18 km) is ~27–33% better than ENVISAT-RA2 (22–25 km) data. Variations in sediment thickness along the continental margins have significant influence in the along-track resolution of both altimeters. Strong geoid signals owing to thin sediments (0.1–0.3 km thick) along the South American margin provided higher resolution for both SARAL/AltiKa (15 km) and ENVISAT-RA2 (22 km) altimeters. Conversely, over the continental margins of India and Gulf of Mexico with huge sediment thickness (1–20 km), the resolution of SARAL/AltiKa and ENVISAT-RA2 drops to 18 and 25 km, respectively. We also observe that resolution of both altimeters is higher by 1–2 km for continental margins with narrow shelf. The present study has implications in the generation of high-resolution marine geoid and gravity anomalies using SARAL/AltiKa along with other altimeters.

Terrestrial heat flow of Jizhong depression, China, Western Bohai Bay basin and its influencing factors

The Jizhong depression in the western Bohai Bay Basin has a high thermal state. Heat flow is one of the most important parameters in geothermics, its measurement can help obtain a better understanding of the regional lithospheric thermal structure. In this study, continuous steady-state temperature logs were obtained from 14 deep boreholes. 136 core samples were measured for thermal conductivity and radiogenic heat production. The temperature data indicated that heat transfer within the sedimentary layer was dominated by heat conduction, with a geothermal gradient of approximately 31.5 ℃•km−1. The gradient value was almost 0 ℃•km−1 in the dolomite strata, due to their abnormally high thermal conductivity and permeability. Average thermal conductivity values was 2.46, 5.51 and 3.21 W•m−1•K−1, and the radiogenic heat production were 1.05, 0.26, and 0.74 μW•m−3 for the sandstone, dolomite and metamorphic rocks, respectively. Higher heat flow in the Jizhong depression is mainly distributed around the subsurface structure of sub-uplift region, which is near the fault between the sub-uplift region and the sub-depression region, and the value can reach 120.0 mW•m−2. With an increase in the dolomite buried depths or an increase in the overlying sandstone thickness, the surface heat flow value decreases. Finally, we discuss the factors influencing the shallow temperature: the lateral sediment thickness variations can markedly influence the temperature distribution. The influence of magma activity that happened long ago can be neglected.

The Caracas, Venezuela, Seismic Microzoning Project: Methodology, results, and implementation for seismic risk reduction

The city of Caracas has been struck by earthquakes several times in its history, and the most recent one, the M6.6 1967 event, evidenced strong site effects within the sedimentary valley. Therefore, Caracas has been the object of research related to earthquake hazard and damage since then, and a seismic microzoning project has been formally developed in the years 2005 to 2009. The principal results constitute response spectra at different subsoil conditions within the sedimentary valley and estimation of landslide hazard for hillside areas. The methodology used for the development of the soil response comprises probabilistic assessment of seismic hazard at rock sites; identification of soil and basin site effects; definition of microzones of similar seismic response. The seismic hazard in the area of Caracas was detailed for PGA and A1 (1-sec. period response), at four macrozones. We developed generic profiles for dynamic response using 1D equivalent linear analysis, considering variations in sediment thickness and VS,30. The analytical results were calibrated and approximate 2D-3D basin effects included. The distribution of microzones in the valley is based on geomorphologic, geological and geophysical modeling, with soil profiles from deep boreholes, and predominant periods from H/V. The earthquake triggered landslide hazard is evaluated using information regarding geology, geomorphology, geotechnical information, slope, weathering, and anthropic alterations, employing a modified Newmark method with Arias intensities. Thus, priority areas for intervention may be identified. Part of the study includes the evaluation of buildings regarding their typified structural reliability, calibrated with the 1967 Caracas earthquake damages. The derived priorities for retrofitting of existing buildings regarding their location within the different microzones are one of the key issues in the county ordinances that have been developed for the 5 counties in the Metropolitan Region. So far, one of them has been signed by the respective mayor, and the remaining 4 are in progress. Once all are signed, we expect seismic disaster risk plans to be developed to guarantee the use of the technical information for earthquake risk mitigation.

Sediment thickness variations of the Tangshan fault zone in North China from a dense seismic array and microtremor survey

Reliable knowledge of sediment thickness is beneficial for investigating buried active faults and improving seismic hazard assessments, especially in regions with recent strong earthquakes. In this study, we investigate the basement of sediments around the 1976 Ms 7.8 Tangshan earthquake region, North China using the microtremor horizontal-to-vertical spectral ratio (H/V) method. For the ambient vibration survey, we deploy a dense seismic array (comprising 152 seismographs with inter-station distances of approximately 1–4 km) and obtain the resonance frequency from each seismograph, which indicates the sediment thickness. The H/V method provides a potential tool for inferring the shallow subsurface structure. Based on the relations between resonance frequency and sediment thickness from permanent borehole seismic stations' records, we recover the sediment thickness around the Tangshan fault zone, which varies from less than 200 m in the northern region to more than 800 m in the southern region. Our results are generally consistent with those of previous drilling and geological studies. Variations in the thickness of sediments indicate that the buried fault system has experienced significant modifications over time, which is partially due to predominant tectonic control by the NE-SW trending Tangshan Fault. The obtained thickness of sediments provide an important basis for earthquake strong motion simulations, sediment crustal corrections for travel time tomography and active fault investigations in the Tangshan fault zone.

Growth and linkage of a basin-bounding fault system: Insights from the Early Cretaceous evolution of the northern Polhem Subplatform, SW Barents Sea

Normal faults grow by either simultaneous increase in displacement and length (isolated model), or by rapid establishment of their final length before accumulating significant displacement (constant-length model), or a combination of these tow end-members. In this study, we integrate stratigraphic and structural observations with throw backstripping and time thickness maps to define the growth processes of a basin-bounding normal fault in the northern Polhem Subplatform, SW Barents Sea. During the initial 15 My of Early Cretaceous rifting, the fault system consisted of at least five en-echelon segments, which were ca. 5–10 km long. Throw backstripping indicates that fault segments were hard-linked after this initial stage to form a single 40 km long fault zone. Fault linkage resulted in displacement redistribution, with the locus of faulting being shifted from the center of each paleo-segment, to the center of the through going fault. Across fault incised valleys provide additional information on the topographic response to fault growth. Major valley incisions at the fault linkage zones outline the extent of the individual fault segments and support early isolated fault growth. Fault backstripping results suggest that valley incision and sedimentation rates kept up with fault slip rate, such that the incised valleys remained unaffected by the uplift of the latter fault linked stage. This study illustrates the importance of integrating stratigraphic and structural observations when reconstructing the evolution of basin-bounding normal faults. In particular, syn-rift erosional features, sediment thickness variations, sediment distribution, stratal geometries and onlap/truncation relationships are critical for estimating the growth of these structures.

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.

Automated delineation and characterization of drumlins using a localized contour tree approach

Drumlins are ubiquitous landforms in previously glaciated regions, formed through a series of complex subglacial processes operating underneath the paleo-ice sheets. Accurate delineation and characterization of drumlins are essential for understanding the formation mechanism of drumlins as well as the flow behaviors and basal conditions of paleo-ice sheets. Automated mapping of drumlins is particularly important for examining the distribution patterns of drumlins across large spatial scales. This paper presents an automated vector-based approach to mapping drumlins from high-resolution light detection and ranging (LiDAR) data. The rationale is to extract a set of concentric contours by building localized contour trees and establishing topological relationships. This automated method can overcome the shortcomings of previously manual and automated methods for mapping drumlins, for instance, the azimuthal biases during the generation of shaded relief images. A case study was carried out over a portion of the New York Drumlin Field. Overall 1181 drumlins were identified from the LiDAR-derived DEM across the study region, which had been underestimated in previous literature. The delineation results were visually and statistically compared to the manual digitization results. The morphology of drumlins was characterized by quantifying the length, width, elongation ratio, height, area, and volume. Statistical and spatial analyses were conducted to examine the distribution pattern and spatial variability of drumlin size and form. The drumlins and the morphologic characteristics exhibit significant spatial clustering rather than randomly distributed patterns. The form of drumlins varies from ovoid to spindle shapes towards the downstream direction of paleo ice flows, along with the decrease in width, area, and volume. This observation is in line with previous studies, which may be explained by the variations in sediment thickness and/or the velocity increases of ice flows towards ice front.

Effects of groundwater withdrawal on land subsidence in Kashan Plain, Iran

Kashan Plain is located in an arid area on the west side of central desert of Iran in north of Isfahan province. In recent decades, increasing population, industrial and agricultural developments in the region that require water withdrawal have resulted in the decline of groundwater and the occurrence of subsidence in this plain. Since 1991, earth fissures that have caused many damages to buildings, farmlands and roads have been observed in many parts of Kashan Plain. The lengths of some of these fissures are more than 2 km long. In this study, after determining the location and direction of fissures, the probable influential causes of earth fissuring were investigated. The results indicated that the main causes of this phenomenon are basically the decline of groundwater head, variation in sediment thickness due to bedrock anomalies and differential vertical compaction. In order to investigate the rate of land subsidence in the Kashan Plain, the Envisat ASAR data from 2003 to 2008 were used. The displacement maps were generated by InSAR time series analysis. The maximum rate of subsidence in this period was determined to be about 3.5 cm/year.

Using electrical anisotropy for structural characterization of sediments: an experimental validation study

ABSTRACTImproved characterization of subsurface heterogeneity is important to better understand a range of key processes in the hydrologic cycle, such as overland flow, infiltration into the soil, evaporation to the atmosphere, and transpiration by plants. Recently, synthetic modelling studies have shown that information on subsurface heterogeneity can be obtained from the anisotropy in electrical resistivity. The objective of this paper is to experimentally validate the findings of this synthetic modelling study. In order to do so, we developed a new measurement procedure to determine the effective complex electrical resistivity from a set of current injections and voltage measurements on a heterogeneous sample. A synthetic modelling study showed that this new measurement procedure was able to reproduce the results of the previous study that showed how the electrical properties and the correlation length ratio of bimodal distributions of two materials can be obtained from the effective complex electrical resistivity measured in two perpendicular directions. After validation of the new measurement approach, we constructed two bimodal distributions in a 2D measurement cell and applied the newly developed measurement strategy. We were able to estimate the electrical properties, the volume fraction, and the correlation length ratio with good accuracy from the complex resistivity measurements in two directions. The remaining differences were attributed to variations in sediment thickness that occurred during sample preparation. We conclude that anisotropy should not be ignored but embraced when dealing with subsurface heterogeneity and that proper interpretation of anisotropy may actually be used to characterize subsurface heterogeneity.

Sedimentation rates test models of oceanic detachment faulting

AbstractLong‐lived detachment faults play an important role in the construction of new oceanic crust at slow‐spreading mid‐oceanic ridges. Although the corrugated surfaces of exposed low‐angle faults demonstrate past slip, it is difficult to determine whether a given fault is currently active. If inactive, it is unclear when slip ceased. This judgment is crucial for tectonic reconstructions where detachment faults are present, and for models of plate spreading. We quantify variation in sediment thickness over two corrugated surfaces near 16.5°N at the Mid‐Atlantic Ridge using near‐bottom Compressed High Intensity Radar Pulse (CHIRP) data. We show that the distribution of sediment and tectonic features at one detachment fault is consistent with slip occurring today. In contrast, another corrugated surface 20 km to the south shows a sediment distribution suggesting that slip ceased ~150,000 years ago. Data presented here provide new evidence for active detachment faulting, and suggest along‐axis variations in fault activity occur over tens of kilometers.

Directional flank spreading at Mount Cameroon volcano: Evidence from analogue modeling

AbstractMount Cameroon is characterized by an elongated summit plateau, steep flanks, and topographic terraces around its base. Although some of these features can be accounted for by intrusion‐induced deformation, we here focus on the contribution of edifice‐scale gravitational spreading in the structure of Mount Cameroon. We review the existing geological and geophysical data and morphostructural features of Mount Cameroon and surrounding sedimentary basins. Volcanic ridge gravitational spreading is then simulated by scaled analogue models on which fault formation is recorded using digital image correlation. Three sets of models are presented (i) models recorded in cross section (Type I), (ii) models recorded from above with a uniform (Type IIa), and (iii) nonuniform ductile layer (Type IIb). Type I models illustrate the formation of faults accommodating summit subsidence and lower flank spreading. Type IIa models favor displacement perpendicular to the long axis, with formation of a summit graben and basal folds, but fail to reproduce the steep flanks. Type IIb models investigate the effect of spatial variations in sediment thickness and/or properties consistent with geological evidence. Directional spreading of the volcano's central part perpendicular to the long axis is accounted for by a sediment layer with restricted lateral extent and increasing thickness away from the volcano axis. The later model closely reproduces key features observed at Mount Cameroon: steep upper flanks are accounted for by enhanced lateral spreading of the lower flanks relative to the summit. The relevance of these findings for understanding flank instabilities at large oceanic volcanoes is finally highlighted.

Late-stage foreland growth of China’s largest orogens (Qinling, Tibet): Evidence from the Hannan-Micang crystalline massifs and the northern Sichuan Basin, central China

This paper addresses the timing of final foreland growth of China’s largest orogens: the Mesozoic Qin Mountains (Qinling) and the Cenozoic Tibetan Plateau. In particular, we ask when the front of the Qinling orogen fold-thrust belt was emplaced, and when the northern Sichuan Basin was affected by the eastward growth of the Tibetan Plateau. We employ zircon and apatite fission-track and (U-Th)/He dating in the Proterozoic crystalline rocks of the Hannan-Micang massifs and the sedimentary rocks of the northern Sichuan Basin. The Hannan-Micang rocks remained in the zircon fission-track partial annealing zone (240 ± 30 °C) throughout the Paleozoic–Middle Triassic (481–246 Ma). From the late Middle Jurassic (ca. 165 Ma) to the early Late Cretaceous (ca. 95 Ma), enhanced cooling and exhumation, with rates of 1.2–2.5 °C/m.y. and 0.04–0.10 mm/yr, respectively, record propagation of the Qinling orogen into its leading foreland; the timing of foreland growth is supported by sedimentologic evidence, i.e., regional variation in sediment thickness and depocenter migration. Negligible cooling and exhumation since the Late Cretaceous (ca. 95 Ma) likely mark the end of the foreland fold-thrust belt formation and the subsequent persistence of a low-relief landscape that occupied extensive parts of central China; cooling and exhumation rates of 0.38–0.70 °C/m.y. and <0.02 mm/yr characterize this tectonic stagnation period. Accelerated cooling (4–5 °C/m.y.) since the Late Miocene (13–8 Ma), derived from apatite fission-track temperature-time path models, signifies involvement of the Hannan-Micang massifs and the northern Sichuan Basin into the eastward-growing Tibetan Plateau. Their inclusion into the plateau growth initiated faulting and stripped off 1.4–2.0 km of rock from the Hannan-Micang massifs and northern Sichuan Basin.

Nonlinear sediment thickness increase on the western East Pacific Rise flank, 45°S

Sediment thickness was evaluated on the western flank of the East Pacific Rise (EPR) at 45°S, based on high-resolution seismic data gathered during cruise 213/2 of R/V Sonne in 2011. Two zones with distinctly different sediment thickness were identified, separated by a transitional zone bordering a pseudo-fault. Sediment in the more distal zone 2 is almost twice as thick (~120 m) as in zone 1 close to the EPR. This is in contrast to the expected progressive sedimentary column thickening with seafloor age and distance from the spreading axis. The younger of two seismic units detected within the sedimentary column (EPR-2) occurs mainly in the distal zone on crust older than 9 Ma, whereas on younger crust it is present only in small isolated bodies. Both sedimentary units drape the basement. The drape is interpreted to represent particle settling from suspension and a generally low regional primary productivity. The spatial variation in sediment thickness cannot be explained by existing models, and other processes considered in the present case are (1) higher productivity in the western sector of the survey area, where thicker sediments were observed (zone 2), (2) the formation of sediment drifts near basement highs (‘seamount effect’), due to flow of Lower Circumpolar Deep Water affecting sediment deposition, and (3) erosion and/or non-deposition of the younger EPR-2 unit, due to elevated bed shear stresses associated with eddies transferring kinetic energy to the seafloor

Abrupt climatic events during Oxygen Isotope Stage 3 recorded in terrestrial sediments in the Netherlands: a multi-proxy approach

Fine-grained sandy–silty channel–belt and floodplain deposits of the Berettyó–Körös Rivers, a main eastern transverse tributary system of the modern Tisza River in the eastern part of the Pannonian Basin, were deposited during the Late Pleistocene under net subsiding conditions. The palaeo-drainage network pattern of a 2500 km2 large part of the alluvial plain was reconstructed based on interpretation of airborne photographs and analysis of 18th century topographic maps, which show the natural river patterns that predate the introduction of river regulation schemes. The investigation showed that a large meandering river system, with two main channel belts surrounding a floodbasin, entered the alluvial plain from the northeast, and a braided river entered the alluvial plain from the southeast. Detailed sedimentary logs of seven continuous corings and several sand and clay-pit sections were used to characterize different alluvial units. Optical luminescence dating (OSL) of 25 quartz samples and four 14C datings showed that the sediments are of Late Pleniglacial to Late Glacial age. Transport directions inferred from heavy mineral analyses combined with the OSL ages strongly suggest that the large meandering system represents the palaeo-Tisza River, which supposedly flowed along the northeast–southwest striking Érmellék depression during the Late Pleniglacial. The braided river can be regarded as a precursor to the Fekete and Fehér–Körös Rivers, which entered the alluvial plain from the southeast during the Late Glacial. The interpretation of seismic profiles, field measurements of neotectonic activity, and variations in thickness of sediments along the studied profile revealed that river development was largely controlled by subsidence along the Érmellék depression until 14 to 16 ky, and by uplift of the southeastern part of the catchment area. The studied fluvial successions also document the response of the palaeo-Tisza and Körös system to the climate changes of the Weichselian Late Pleniglacial–Late Glacial period. Much of the sand from the meandering zones was deposited during the Bølling–Allerød and Ságvár–Lascaux interstadials, whereas some dated sand units from the braided zone represent the Older and Younger Dryas. The error ranges of OSL dates, which often exceed the duration of Weichselian substages and subdivisions, prevented an unambiguous correlation of the studied sections with the millennial-scale climate changes of the last 25 ky. Meandering and braided river activity coexisted under different climate conditions, whereas locations of the main channel belts are related to subsidence anomalies. The results of our study thus clearly indicate that tectonics was the primary control on river development.

Lithospheric density variations and Moho structure of the Irish Atlantic continental margin from constrained 3-D gravity inversion

A 3-D density anomaly model of the Irish Atlantic continental margin was generated from a regional inversion of the free air gravity data constrained by bathymetric and sediment thickness information. The model results are compared against a recent regional Moho structure compilation from velocity models from crustal-scale wide-angle reflection/refraction surveys. Using the inverted model, a regional map of Moho structure across the margin agrees well with the regional seismic compilation and also provides information in areas lacking deep seismic coverage. The density anomaly structure of the crust across the margin is investigated along model slices and using volume rendering of crustal layers. These views reveal extreme thinning,of the upper crust and a dominance of lower crustal densities in regions where exhumation of,serpentinized mantle has previously been interpreted. Using the regional view afforded by, the,inversion, the areal extent of these zones is tracked into regions lacking seismic constraints. Using the regional density anomaly model, variations in sediment thickness and crustal thickness are investigated to compute stretching factors across the margin and to identify zones,which deviate from local isostatic compensation. These zones generally correlate with faults,and rifting trends providing insights that could be useful for future palaeoreconstructions of North Atlantic rifting.

Lithospheric structure of the Chinese mainland determined from joint inversion of regional and teleseismic Rayleigh‐wave group velocities

We processed a large number of vertical‐component seismograms recorded by broadband seismic stations in and around China and ultimately retrieved ∼33 140 regional and ∼10 360 teleseismic fundamental‐mode group‐velocity measurements at 40 s periods, with fewer measurements for shorter and longer periods. We directly inverted the processed group‐velocity measurements for a three‐dimensional lithospheric S‐velocity model of the Chinese mainland. Synthetic test results and data fit or misfit analysis demonstrated the reliability of our surface‐wave tomographic inversion. The imaged upper‐crustal low velocities are consistent with variations in sediment thickness; for example, the Tarim Basin, which contains a great thickness of sediments, is characterized by a strong, shallow, low‐velocity anomaly. High lithospheric velocities are observed to varying depths in the North China Craton, Yangtze Craton, and Tarim Craton, indicating varying thicknesses of the lithosphere beneath these cratonic areas. Low asthenospheric velocities are widely imaged in eastern and southern China, around the Tarim Basin, and along a roughly north‐south belt in central China. The low asthenospheric velocities in eastern and southern China are attributed to partial melting of a subducted slab associated with the westward subduction of an oceanic plate, while those in central China and around the Tarim Basin are attributed to Indian‐Eurasian collision.