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排序方式: 共有92条查询结果,搜索用时 171 毫秒
1.
Tiago M. Alves Vasilios Lykousis Dimitris Sakellariou Stamatina Alexandri Paraskevi Nomikou 《Geo-Marine Letters》2007,27(1):41-61
Bathymetric, 9.5-kHz long-range sidescan sonar (OKEAN), seismic reflection and sediment-core data are used in the analysis
of two tectonic troughs south of Crete, Eastern Mediterranean Sea. Here, up to 1.2 s two-way travel time (TWTT) of strata
have accumulated since the Middle Miocene in association with extension in the South Aegean region. The study area comprises
>100-km- long by >25-km-wide basins filled by sediments subdivided into two seismic units: (1) an upper Unit 1 deposited in
sub-basins which follow the present-day configuration of the southern Cretan margin; (2) a basal Unit 2, more than 500 ms
(TWTT) thick, accumulated in deeper half-graben/grabens distinct from the present-day depocentres. Both units overlap a locally
stratified Unit 3 comprising the pre-Neogene core complex of Crete and Gavdos. In this work, the interpreted seismic units
are correlated with the onshore stratigraphy, demonstrating that denudation processes occurring on Crete and Gavdos in response
to major tectonic events have been responsible for high sedimentation rates along the proximal southern Cretan margin. Consequently,
topographically confined sedimentary units have been deposited south of Crete in the last 12 Ma, including turbidites and
other mass-flow deposits fed by evolving transverse and axial channel systems. Surface processes controlling facies distribution
include the direct inflow of sediment from alluvial-fan systems and incising mountain rivers onto the Cretan slope, where
significant sediment instability processes occur at present. In this setting, seismic profiles reveal eight different types
of stratigraphic contacts on basin-margin highs, and basinal areas show evidence of halokinesis and/or fluid escape. The acquired
data also show that significant changes to the margin’s configuration occurred in association with the post-Alpine tectonic
and eustatic episodes affecting the Eastern Mediterranean. 相似文献
2.
The transport pathways of fine sediments (fraction <2 μm) along the Brazilian continental shelf from Ceará to the Amazon River
mouth were studied by means of clay mineral analyses. On the continental shelf southeast of the Amazon mouth, fluctuations
in clay mineral compositions reflect simple mixing between the suspended load of the North Brazil Current and sediment from
several smaller rivers. Previously, clay mineral variations west of the Amazon mouth have been explained by variable settling
velocities of different grain-size classes within the <2 μm fraction or by selective coagulation of individual clay mineral
groups. By contrast, our experiments with river bank samples show that selective coagulation does not occur in Amazon River
sediments. A more appropriate explanation for observed variations in clay mineral composition off the Amazon mouth seems to
be, similarly to that for the shelf between Ceará and the Amazon mouth, a mixing of Amazon sediments with suspended material
of the North Brazil Current. This interpretation is supported by data on clay mineral composition east and south of the Amazon
mouth, showing more affinity to sediments of the North Brazil Current than to the suspended load of the Amazon River. Additionally,
relatively low sedimentation rates and low concentrations of fine-grained sediments on the shelf suggest that high riverine
input by the Amazon River does not overprint the sediments of the North Brazil Current in this region. The strong North Brazil
Current shunts the Amazon suspended load in a north-westerly direction along the north-eastern coast of South America. Hence,
stronger sedimentation of Amazon sediments would occur only west of the river mouth. 相似文献
3.
4.
Bianca Carvalho Vieira Nelson Ferreira Fernandes Oswaldo Augusto Filho Tiago Damas Martins David R. Montgomery 《Environmental Earth Sciences》2018,77(6):260
The hillslopes of the Serra do Mar, a system of escarpments and mountains that extend more than 1500 km along the southern and southeastern Brazilian coast, are regularly affected by heavy rainfall that generates widespread mass movements, causing large numbers of casualties and economic losses. This paper evaluates the efficiency of susceptibility mapping for shallow translational landslides in one basin in the Serra do Mar, using the physically based landslide susceptibility models SHALSTAB and TRIGRS. Two groups of scenarios were simulated using different geotechnical and hydrological soil parameters, and for each group of scenarios (A and B), three subgroups were created using soil thickness values of 1, 2, and 3 m. Simulation results were compared to the locations of 356 landslide scars from the 1985 event. The susceptibility maps for scenarios A1, A2, and A3 were similar between the models regarding the spatial distribution of susceptibility classes. Changes in soil cohesion and specific weight parameters caused changes in the area of predicted instability in the B scenarios. Both models were effective in predicting areas susceptible to shallow landslides through comparison of areas predicted to be unstable and locations of mapped landslides. Such models can be used to reduce costs or to define potentially unstable areas in regions like the Serra do Mar where field data are costly and difficult to obtain. 相似文献
5.
Polygonal mounds in the Barents Sea reveal sustained organic productivity towards the P–T boundary 下载免费PDF全文
Tiago M. Alves 《地学学报》2016,28(1):50-59
Three‐dimensional (3D) seismic‐reflection data from the Barents Sea show geometric similarities between Permian cool‐water mounds and older carbonate build‐ups. In detail, the Samson Dome area records the development of polygonal mounds in Upper Permian strata; at the same time, a gradual drowning event took place in the Barents Sea. The presence of these polygonal mounds is interpreted to reflect: (i) shallower conditions around the Samson Dome than in other parts of the Barents Sea; and (ii) earlier drowning of Upper Permian mounds towards the west and northwest into the Ottar Basin. Based on the recognition of mounds ~20 m below the Permian–Triassic stratigraphic boundary, this paper proposes for the first time that shallow areas of the Barents Sea, such as the Samson Dome, witnessed sustained organic productivity until the onset of the P–T extinction event. 相似文献
6.
Nutrient dynamics in Mediterranean temporary streams: A case study in Pardiela catchment (Degebe River, Portugal) 总被引:1,自引:0,他引:1
Ana Isabel Lilleb Manuela Morais Pedro Guilherme Rita Fonseca Antnio Serafim Ramiro Neves 《Limnologica》2007,37(4):337-348
Most of the streams in the Mediterranean region are temporary, following predictable seasonal of flooding and drying, with a transition from lotic conditions to shallow lentic conditions. The goal of our study was to assess the nitrogen and phosphorus dynamics in channel-bed processes of temporary streams between floods. Results show that, during winter, temperatures ranged between 9.5 and 11.2 °C and oxygen concentration ranged from 8.0 to 9.5 mg L−1, whereas, during summer, temperatures varied between 21.2 and 26.8 °C and oxygen between 1.2 and 5.3 mg L−1, with oxygen depletion in the pools during the night. The nitrate concentrations were far more abundant during winter (February), while ammonium concentration increased after stream fragmentation into pools (especially in July when oxygen depletion conditions favoured ammonification). Results on sediment profiles showed that the most active sediment layers for NH4-N are the top 2–3 cm, corresponding to the sediment depositional sites of the stream. Phosphate concentrations had larger variability, yet concentrations decreased from winter to spring and increased again in summer, when the shallow water pools were formed. Sediment profiles at the sediment depositional sites showed that PO4-P was more dynamic in the first 6 cm.
In Mediterranean temporary streams, nutrient dynamics vary seasonally, as the system transits from lotic conditions to shallow lentic conditions, evidencing the regeneration of nutrients from organic and inorganic matter during the flow cessation period. 相似文献
7.
Luísa D.V.Carvalho Tiago Jalowitzki Ricardo Scholz Guilherme O.Gonalves Marcelo Peres Rocha Rogério Silvestre Pereir Cristiano Lana Marco Paulo de Castro Gláucia Queiroga Reinhardt A.Fuck 《地学前缘(英文版)》2022,(1):182-203
We present major and trace element compositions of mineral concentrates comprising garnet xenocrysts,ilmenite,phlogopite,spinel,zircon,and uncommon minerals (ti... 相似文献
8.
Tiago A. Coimbra Jorge H. Faccipieri Dany S. Rueda Martin Tygel 《Studia Geophysica et Geodaetica》2016,60(3):500-530
Since the early days of seismic processing, time migration has proven to be a valuable tool for a number of imaging purposes. Main motivations for its widespread use include robustness with respect to velocity errors, as well as fast turnaround and low computation costs. In areas of complex geology, in which it has well-known limitations, time migration can still be of value by providing first images and also attributes, which can be of much help in further, more comprehensive depth migration. Time migration is a very close process to common-midpoint (CMP) stacking and, more recently, to zero-offset commonreflection- surface (CRS) stacking. In fact, Kirchhoff time migration operators can be readily formulated in terms of CRS parameters. In the nineties, several studies have shown advantages in the use of common-reflection-point (CRP) traveltimes to replace conventional CMP traveltimes for a number of stacking and migration purposes. In this paper, we follow that trend and introduce a Kirchhoff-type prestack time migration and velocity analysis algorithm, referred to as CRP time migration. The algorithm is based on a CRP operator together with optimal apertures, both computed with the help of CRS parameters. A field-data example indicates the potential of the proposed technique. 相似文献
9.
Tiago Carrilho Biló Ilson Carlos Almeida da Silveira Wellington Ceccopieri Belo Belmiro Mendes de Castro Alberto Ricardo Piola 《Ocean Dynamics》2014,64(10):1431-1446
The Brazil Current (BC) is likely the least observed and investigated subtropical western boundary current in the world. This study proposes a simple and systematic methodology to estimate quasi-synoptic cross-sectional speeds of the BC within the Santos Basin (23° S–26° S) based on the dynamic method using several combinations of data: Conductivity, temperature, and depth (CTD), temperature profiles, CTD and vessel-mounted Acoustic Doppler Current Profiler (VMADCP), and temperature profiles and VMADCP. All of the geostrophic estimates agree well with lowered Acoustic Doppler Current Profiler (LADCP) velocity observations and yield volume transports of -5.56 ±1.31 and 2.50 ±1.01 Sv for the BC and the Intermediate Western Boundary Current (IWBC), respectively. The LADCP data revealed that the BC flows southwestward and is ~100 km wide, 500 m deep, and has a volume transport of approximately -5.75 ±1.53 Sv and a maximum speed of 0.59 m s?1. Underneath the BC, the IWBC flows northeastward and has a vertical extent of approximately 1,300 m, a width of ~60 km, a maximum velocity of ~0.22 m s?1, and a volume transport of 4.11 ± 2.01 Sv. Our analysis indicates that in the absence of the observed velocities, the isopycnal (σ 0) of 26.82 kg m?3 (~500 dbar) is an adequate level of no motion for use in geostrophic calculations. Additionally, a simple linear relationship between the temperature and the specific volume anomaly can be used for a reliable first estimate of the BC-IWBC system in temperature-only transects. 相似文献
10.
Classification of washover dynamics in barrier islands 总被引:1,自引:0,他引:1
Ana Matias
scar Ferreira Ana Vila-Concejo Tiago Garcia Joo Alveirinho Dias 《Geomorphology》2008,97(3-4):655-674
This study systematically classifies washover dynamics with reference to coastal changes along the Ria Formosa barrier islands (Southern Portugal). Identification of washovers using a sequence of 11 sets of aerial photographs dated between 1947 and 2001 allowed a classification to be developed based on: (1) overwash evolution (increasing, decreasing, or constant overwash processes); (2) the mechanisms promoting washover formation (exceptional to infrequent oceanographic conditions, washout processes, structural erosion, inlet dynamics, and human interventions); and (3) the mechanisms promoting washover cessation (berm development, structural erosion, dune development, inlet dynamics, and human interventions). A total of 369 different washovers were observed along the Ria Formosa barriers during the study period, with 209 washovers being formed in various types of dune morphology and 303 being obliterated. The number of washovers was relatively stable from 1947 to 1972, and increased dramatically between 1972 and 1976 probably as a result of the development of immature inlet margins and downdrift starvation. From 1976 to 2001, washover occurrences declined and their spatial dimensions decreased, leading to a decrease in overwash activity over this time. Overall, the dominant formation mechanisms of washovers in the Ria Formosa were inlet dynamics (accounting for 57% of washovers formed) and structural erosion (20%), with human intervention mechanisms accounting for 12%. The cessation of washovers was dominated by dune development (33% of the washovers obliterated) followed by inlet dynamics (24%) and structural erosion (19%), while human intervention mechanisms accounted for 13%. The classification should be of use for the coastal management of barrier systems including the definition of overwash-prone areas and the determination of the relative importance of the mechanisms contributing to washover formation and cessation. 相似文献