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81.
Measurement and modeling of bed shear stress under solitary waves   总被引:1,自引:0,他引:1  
Direct measurements of bed shear stresses (using a shear cell apparatus) generated by non-breaking solitary waves are presented. The measurements were carried out over a smooth bed in laminar and transitional flow regimes (~ 104 < Re < ~ 105). Measurements were carried out where the wave height to water depth (h/d) ratio varied between 0.12 and 0.68; maximum near bed velocity varied between 0.16 m/s and 0.51 m/s and the maximum total shear stress (sum of skin shear stress and Froude–Krylov force) varied between 0.386 Pa and 2.06 Pa. The total stress is important in determining the stability of submarine sediment and in sheet flow regimes. Analytical modeling was carried out to predict total and skin shear stresses using convolution integration methods forced with the free stream velocity and incorporating a range of eddy viscosity models. Wave friction factors were estimated from skin shear stress at different instances over the wave (viz., time of maximum positive total shear stress, maximum skin shear stress and at the time of maximum velocity) using both the maximum velocity and the instantaneous velocity at that phase of the wave cycle. Similarly, force coefficients obtained from total stress were estimated at time of maximum positive and negative total stress and at maximum velocity. Maximum positive total shear stress was approximately 1.5 times larger than minimum negative total stress. Modeled and measured positive bed shear stresses are well correlated using the best convolution model, but the model underestimates the data by about 4%. Friction factors are dependent on the choice of normalizing using the maximum velocity, as is conventional, or the instantaneous velocity. These differ because the stress is not in phase with the velocity in general. Friction factors are consistent with previous data for monochromatic waves, and vary inversely with the square-root of the Reynolds number. The total shear stress leads the free stream fluid velocity by approximately 50°, whereas the skin friction shear stress leads by about 30°, which is similar to that reported by earlier researchers.  相似文献   
82.
Eastward migration of the Caribbean plate relative to the South American plate has caused lithospheric loading along the northern margin of South America, which is recorded by an 1100-km-long foreland basin which is oldest in the west (Maracaibo basin, 65-55 Ma) and youngest in the east (Columbus basin, eastern offshore Trinidad, 15-0 Ma). The Orinoco River has been the primary source of sediment for the basin since early Miocene. We have integrated approximately 775 km of deep-penetration 2D seismic lines acquired in the area of eastern offshore Trinidad as part of the 2004 “Broadband Ocean-Land Investigations of Venezuela and the Antilles arc Region” (BOLIVAR) project, 8000 km2 of shallow industry 3D seismic data, and published industry well data from offshore eastern Trinidad. Active mud diapirism in the Columbus basin is widespread and is related to overthrusting and tectono-sedimentary loading of upper Miocene-lower Pliocene age mud. Analysis of the shallow 3D seismic data reveals the presence of extensive gravity-flow depositional elements on the Columbus basin slope and the deepwater area. These stacked gravity-flow deposits are characterized by mass-transport deposits at the base, turbidite frontal-splay deposits, leveed-channel deposits, and capped by fine-grained condensed-section deposits. Exploration targets in the deepwater area are located towards the center of the Columbus basin, where northeast-trending fault-propagation folds are important Plio-Pleistocene trap-forming elements. Deep basin wells drilled in recent years have proven that turbidites were transported into the deepwater Columbus basin during the Plio-Pleistocene. Analysis of these well results suggests that a deeper oil charge is present within the deepwater Columbus basin area. The primary uncertainty for this variable hydrocarbon system is whether fault or diapiric pathways connect or divert the petroleum charge at depth with shallower reservoir rocks.  相似文献   
83.
Aruba, Bonaire, and Curaçao are islands aligned along the crest of a 200-km-long segment of the east-west-trending Leeward Antilles ridge within the broad Caribbean-South America plate boundary zone presently characterized by east-west, right-lateral strike-slip motion. The crust of the Leeward Antilles ridge represents the western segment of the Cretaceous-early Cenozoic Great Arc of the Caribbean, which obliquely collided, with the continental margin of northern South America in early Cenozoic time. Following the collision, the ridge was affected by folding and was segmented by oblique, northwest-striking normal faults that have produced steep-sided, northwest-trending, elongate islands and narrow shelves separated by deepwater, sediment-filled and fault-controlled basins. In this paper, we present the first fault slip observations on the Neogene carbonate rocks that cover large areas of all three islands. Our main objective is to quantify the timing and nature of Neogene to Quaternary phases of faulting and folding that have affected the structure and topography of this area including offshore sedimentary basins that are being explored for their petroleum potential. These data constrain three fault phases that have affected Aruba, Bonaire, and Curaçao and likely the adjacent offshore areas: 1) NW-SE-directed late Paleogene compression; 2) middle Miocene syndepositional NNW-SSE to NNE-SSW extension that produced deep rift basins transverse to the east-west-trending Leeward Antilles ridge; and 3) Pliocene-Quaternary NNE-trending compression that produced NW-SE-trending anticlines present on Aruba, Curaçao and Bonaire islands. Our new observations - that include detailed relationships between striated fault planes, paleostress tensors, and bedding planes - show that prominent bedding dips of Neogene limestone on Aruba, Bonaire and Curaçao were produced by regional tectonic shortening across the entire Leeward Antilles ridge rather than by localized, syndepositional effects as proposed by previous workers. We interpret Pliocene-Quaternary NNE-directed shortening effects on the Leeward Antilles ridge as the result of northeastward extrusion or “tectonic escape” of continental areas of western Venezuela combined with southeastward shallow subduction of the Caribbean plate beneath the ridge.  相似文献   
84.
A large-scale contourite drift complex has been recognised on multi-channel 2D reflection seismic data acquired in the south-eastern Davis Strait and adjacent Labrador Sea slope offshore West Greenland between 63°?C66°N. Based on well-tie data, the drift complex developed from the Middle Miocene to the Recent. It has been mapped in a wide variety of water depths ranging from about 700?m, at a NNW-ESE-elongated crest located above structural highs in the Davis Strait, to more than 2,000?m beyond the slope to the Labrador Sea. The overall drift geometry has been described by subdivision into two first-order seismic units, enabling the generation of time-isochore maps. The reflection patterns demonstrating current-related deposition are illustrated by seismic examples. The time-isochores of the two first-order seismic units show lateral changes in their depocentres: the lower unit is absent in a zone slightly displaced south-westwards of the present-day crest, indicating changes in the prevailing deepwater current system during the Early Pliocene. The observations can be explained by two alternative palaeoceanographic scenarios: (1) either the present-day oceanographic setting with Arctic?CAtlantic water exchange across the Davis Strait was largely established by the mid-Miocene, with only minor adjustments during the Early Pliocene caused by tectonic movements, or (2) it became established during the Early Pliocene as a consequence of enhanced northward flow across the Davis Strait due to lowering of the sill depth.  相似文献   
85.
Listwanite from the Luobusa ophiolite,Tibet,forms a narrow,discontinuous band along the eastern part of the southern boundary fault. We undertook a detailed petrographic and geochemical study to understand the mineral transformation processes and the behaviour of major and trace elements during listwanite formation. Three alteration zones characterized by distinct mineral components and texture are recognized and,in order of increasing degree of alteration,these are: zoneIII is rich in serpentine minerals; zoneII is rich in talc and carbonates; and zoneI is mainly composed of carbonates and quartz. Geochemical data for the three alteration zones show significant modification of some major and trace elements in the protolith,although some oxides show linear correlations with Mg O. Gold mineralization is recognized in the Luobusa listwanite and may signify an important target for future mineral exploration. Gold enrichment occurs in both zoneI and zoneIIand is up to 0.91 g/t in one sample from zoneI. We show that CO2-rich hydrothermal fluids can modify both the occurrence and composition of chromite grains,indicating some degree of chromite mobility. Low-Cr anhedral grains are more easily altered than high-Cr varieties. The compositions of chromite and olivine grains in the listwanite suggest a dunite protolith.  相似文献   
86.
<正>Ophiolites are fragments of ancient ocean lithosphere emplaced on continental margins,in island arcs or in accretionary prisms,and have long been studied to better understand the evolution of ocean basins and collision of tectonic plates,the processes of mountain building and the occurrence of valuable ore bodies,such as podiform  相似文献   
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