共查询到20条相似文献，搜索用时 22 毫秒
1.
Abstract The deformation style of the Torlesse Terrane along the southern Kaikoura coast, South Island, New Zealand, records shallow level deformation processes within an accretionary prism during the Early Cretaceous. The beds exhibit complicated structural features resulting from multistage deformations in a lithological unit, that were intimately related with the dewatering and lithification of terrigenous sediments. The earliest phase of deformation throughout the transect studied was the development of pinch-and-swell structures and boudinage fabrics due to layer-parallel extension while the beds were poorly consolidated. This was followed locally by mesoscopic tight to close recumbent folding. The beds are cut locally by two phases of mudstone intrusions. The earlier phase was initiated by 'in situ' fluidization of mudstone layers, whereas the later phase represented intrusion of siliceous claystone probably derived from an overpressured decollement. Minor faults at high-angles to bedding by layer-normal shortening then disrupted the beds throughout the transect. The deformation was followed by formation of meso- and macroscopic scale open to gentle folds by layer-parallel shortening. Kilometer-scale differential stratal rotations were produced during the final main tectonic phase that occurred in association with post-accretion Neogene regional disturbance. 相似文献
2.
Dilute gravity current and rain-flushed ash deposits in the 1.8 ka Hatepe Plinian deposit,Taupo, New Zealand 总被引：1，自引：1，他引：0
Two groups of poorly sorted ash-rich beds, previously interpreted as rain-flushed ashes, occur in the ca. AD 180 Hatepe Plinian pumice fall deposit at Taupo volcano, New Zealand. Two ash beds with similar dispersal patterns and an aggregate thickness of up to 13 cm make up the lowermost group (A). Group A beds extend 45 km north-east of the vent and cover 290 km2. In the southern part of the group A distribution area, a coarse ash to lapilli-size Plinian pumice bed (deposit B) separates the two group A beds. The scarcity of lapilli (material seen elsewhere from the still-depositing pumice fall) in group A beds indicates that they were rapidly transported and deposited. However, this rapid transportation and deposition did not produce cross-bedding, nor did it erode the underlying deposits. It is proposed that thick (>600 m) but dilute gravity currents generated from the collapsing outer margin of the otherwise buoyant Hatepe Plinian eruption column deposited the group A beds. The upper ash beds (group C) consist of one to seven layers, attain an aggregate thickness of 35 cm, and vary considerably in thickness and number of beds with respect to distance from vent. Group C beds contain variable amounts of ash mixed with angular Plinian pumices and are genuine rain-flushed ashes. Several recent eruptions at other volcanoes (Ukinrek Maars, Vulcan, Rabaul, La Soufrère de Guadeloupe and Soufrière, St Vincent) have produced gravity currents similar in style, but much smaller than those envisaged for group A deposits. The overloaded margins of otherwise buoyant eruption plumes generated these gravity currents. Laboratory studies have produced experimental gravity current analogues. Hazards from dilute gravity currents are considerable but often overlooked, thus the recognition of gravity current deposits will contribute to more thorough volcanic hazard assessment of prehistoric eruption sequences. 相似文献
3.
Sedimentary structures in the middle–late Miocene to early Pliocene Misaki Formation, Miura Group, Miura Peninsula, Central Japan, were studied, and paleocurrent data were interpreted as the result of deep-sea bottom-current flow. These current data were further compared with present-day bottom currents in the northwestern Pacific region. The Misaki Formation is thought to be a forearc deposit within the Izu oceanic arc, and is composed of thick volcaniclastic beds interbedded with siliceous biogenic clayey sediments. Sedimentary structures showing paleocurrent directions are involved in the upper part of the volcaniclastic beds, in the pumiceous beds just above the volcaniclastic beds, and in the pelagic sediments. Based on paleomagnetic data suggesting considerable rotation of the beds, all the current directions were reconstructed to their original orientation. The paleocurrents are summarized into the following three groups. The first group in the volcaniclastic beds indicates southeast-directed paleocurrent directions. The second group in the upper parts of volcaniclastic beds and in some pumiceous beds exhibits a southwest- and northeast-directed paleoflow. The third group usually observed in the pumiceous beds with parallel lamination displays a northwest- or southeast-directed paleocurrent. The origin of each group's paleoflow direction is attributed to turbidity current, internal tidal current, and contour current influences, respectively. Present-day observations of the deep-sea northwest Pacific suggest that most of the bottom-current indicators in the Misaki Formation are related to North Pacific Deep Water, possibly Antarctic Bottom Water as well as a combination of tidal and local effects. It is concluded that the beds of the Misaki Formation were deposited in the proto-Sagami basin ca 9 Ma and were formed under weak bottom currents in a wide and flat basin during colder climatic conditions, whereas the beds dated at ca 6 Ma were deposited under strong bottom-current flow, and were then accreted to the Honshu arc. 相似文献
4.
Turbidity currents may feature active sediment transport and rapid bed deformation, such as those responsible for the erosion of many submarine canyons. Yet previous mathematical models are built upon simplified governing equations and involve steady flow and weak sediment transport assumptions, which are not in complete accordance with rigorous conservation laws. It so far remains unknown if these could have considerable impacts on the evolution of turbidity currents. Here a fully coupled modeling study is presented to gain new insights into the evolution of turbidity currents. The recent analysis of the multiple time scales of subaerial sediment-laden flows over erodible bed [Cao Z, Li Y, Yue Z. Multiple time scales of alluvial rivers carrying suspended sediment and their implications for mathematical modeling. Adv Water Resour 2007;30(4):715–29] is extended to subaqueous turbidity currents to complement the fully coupled modeling. Results from numerical simulations show the ability of the present coupled model to reproduce self-accelerating turbidity currents. Comparison among the fully and partially coupled and decoupled models along with the analysis of the relative time scale of bed deformation explicitly demonstrate that fully coupled modeling is essential for refined resolution of those turbidity currents featuring active sediment transport and rapid bed deformation, and existing models based on simplified conservation laws need to be reformulated. 相似文献
5.
Guido Giordano Massimiliano Porreca Pietro Musacchio Massimo Mattei 《Bulletin of Volcanology》2008,70(10):1221-1236
The edifice of Stromboli volcano gravitationally collapsed several times during its volcanic history (>100 ka–present). The
largest Holocene event occurred during the final stage of the Neostromboli activity (∼13–5 ka), and was accompanied by the
emplacement of phreatomagmatic and lahar deposits, known as the Secche di Lazzaro succession. A stratigraphic and paleomagnetic
study of the Secche di Lazzaro deposits allows the interpretation of the emplacement and the eruptive processes. We identify
three main units within the succession that correspond to changing eruption conditions. The lower unit (UA) consists of accretionary
lapilli-rich, thinly bedded, parallel- to cross-stratified ash deposits, interpreted to indicate the early stages of the eruption
and emplacement of dilute pyroclastic density currents. Upward, the second unit (UB) of the deposit is more massive and the
beds thicker, indicating an increase in the sedimentation rate from pyroclastic density currents. The upper unit (UC) caps
the succession with thick, immediately post-eruptive lahars, which reworked ash deposited on the volcano’s slope. Flow directions
obtained by Anisotropy of Magnetic Susceptibility (AMS) analysis of the basal bed of UA at the type locality suggest a provenance
of pyroclastic currents from the sea. This is interpreted to be related to the initial base-surges associated with water–magma
interaction that occurred immediately after the lateral collapse, which wrapped around the shoulder of the sector collapse
scar. Upward in the stratigraphy (upper beds of UA and UB) paleoflow directions change and show a provenance from the summit
vent, probably related to the multiple collapses of a vertical, pulsatory eruptive column. 相似文献
6.
Previous quantitative studies of field-scale cyclic steps are mostly based on analysis of field data. Such studies have shed light on the erosion/deposition patterns over these morphological features as well as the magnitudes of the turbidity current parameters back estimated using the measured geometry data. However, it remains unclear to what extent such back estimated hydraulic features and erosion/deposition patterns can be numerically reproduced by process-based numerical models. Here, a tw... 相似文献
7.
Michael H. Ort Giovanni Orsi Lucia Pappalardo Richard V. Fisher 《Bulletin of Volcanology》2003,65(1):55-72
The late Pleistocene trachytic Campanian Ignimbrite underlies much of the Campanian Plain near Naples, Italy, and occurs in valleys in the mountainous area surrounding the plain out to about 80 km from its source, the Campi Flegrei caldera. At sites within 15 km of the Campi Flegrei, anisotropy of magnetic susceptibility (AMS) principal directions indicate that, in the absence of significant topography, deposition came from a flow moving in a roughly radial direction. AMS studies of the more distal ignimbrite reveal downhill and/or downvalley flow directions prior to deposition, even where these directions are at high angles to a generally radial transport direction from the vent. On the flanks of Roccamonfina Volcano, flow was directly downhill, as if the source of the ignimbrite was the summit of the volcano. In most localities, the ignimbrite consists of a single massive deposit. In a few localities in the Apennine Mountains, however, the confluence of multiple drainage systems off mountains resulted in multiple local flow units that cannot be correlated between valleys. A detailed study of the ignimbrite in the flat Titerno River valley near Massa shows that the AMS fabrics are not due to late-stage creeping during deposition or compaction. Well-defined, but non-parallel AMS fabrics from vertical and lateral sections in the Massa area are best explained by the merging of gravity currents flowing down the valley and steep valley sides to form a single aggradational deposit. Clast compositions and AMS axes at Mondragone indicate that the pyroclastic flow encountered the Monte Massico massif and was partially blocked, so that flow during deposition was toward the Campi Flegrei. Similar AMS data from sites along the edge of the Campanian Plain indicate back-flow off the first ridge of the Apennine Mountains reached at least 5 km from their base. The Campanian Ignimbrite was deposited from a density-stratified pyroclastic flow. The depositional system consisted of the lower, denser portion of the current, and was controlled by topography. The grouping of the AMS axes is interpreted to indicate that deposition occurred under laminar flow conditions. 相似文献
8.
Upper Cretaceous oceanic red beds in southern Tibet:Lithofacies,environments and colour origin 总被引：6，自引：0，他引：6
Jansa Luba 《中国科学D辑(英文版)》2006,49(8):785-795
9.
磁组构通常指磁化率各向异性,即AMS(Anisotropy of Magnetic Susceptibility),是一种重要的岩石组构,是弱变形沉积岩地区灵敏的应变指示计.近年来,AMS在造山带及前陆地区的广泛应用为构造变形研究提供了极大的帮助,同时提升了该方法的理论认识.本文在研读最新相关文献与著作的基础上,结合笔者及研究团队在龙门山地区获得的磁组构研究成果,综述了磁组构在沉积岩地区构造变形研究中的应用进展,并基于现有的研究认识对关键问题进行讨论,提出以下几点认识:(1)磁性矿物分析是AMS研究的关键,应结合多种岩石磁学实验及光学与电子显微构造研究手段展开详细的磁性矿物学分析;(2)磁化率椭球与应变椭球的对应主轴在绝大多数情况下相互平行,但在不同期次、不同种类复杂的磁性矿物组成,或者多期次构造变形的影响下,AMS与应变的关系相对复杂,应比对高场和低温AMS及非磁滞剩磁各向异性(AARM)测试结果,获得不同矿物的优选定向特征,并对获得的组构进行分期;(3)AMS可以揭示造山带及其前陆地区的构造演化历史,并且是分析断层相关褶皱的有限应变特征和变形机制的重要方法,同时也是厘定断裂带变形性状和期次及运动学分析的有效手段;(4)磁组构形成于成岩作用早期或构造变形的最早阶段,能很好地记录褶皱和逆冲作用之前的平行层缩短变形,因此可以揭示同沉积阶段的古构造应力方向.后期足够强烈的构造变形能局部改造或彻底掩盖先存AMS记录,构造流体有关的同构造期结晶矿物或先存矿物的重结晶导致的再定向被认为是其根本原因;(5)斜交磁线理是一种特殊的磁组构类型,反映了区域构造叠加或多期构造变形作用或隐伏斜向逆冲等可能的构造过程,有必要结合多方面的地质证据对其成因作出合理解释. 相似文献
10.
Iris Thérèse Elise Verhagen Jaco Hugo Baas Ricardo Silva Jacinto William Dale McCaffrey Alan Geoffrey Davies 《Ocean Dynamics》2013,63(4):385-397
Many aquatic environments exhibit soft, muddy substrates, but this important property has largely been ignored in process-based models of Earth-surface flow. Novel laboratory experiments were carried out to shed light on the feedback processes that occur when particulate density currents (turbidity currents) move over a soft mud substrate. These experiments revealed multiple types of flow-bed interaction and large variations in bed deformation and bed erosion, which are interpreted to be related to the interplay between the shear forces of the current and the stabilising forces in the bed. Changes in this force balance were simulated by varying the clay concentrations in the flow and in the bed. Five different interaction types are described, and dimensional and non-dimensional phase diagrams for flow-bed interaction are presented. 相似文献
11.
Shun Nomura Giovanni De Cesare Mikito Furuichi Yasushi Takeda Hide Sakaguchi 《国际泥沙研究》2022,37(3):299-306
A turbidity current is a turbulent, particle-laden gravity current that is driven by density differences resulting from the presence of suspended sediment particles. The current travels downslope, bearing a large amount of sediment over a great distance, and forms fluvial and submarine bedforms. Knowledge of the spatio-temporal deposition profile of turbidity-deposited sediment is important for a better understanding of sediment transport by turbidity currents. In the current study, the depositi... 相似文献
12.
《国际泥沙研究》2016,(4):368-375
A wide range of methods are commonly used to measure deposited fine sediment and quantify substrate quality in rivers as part of bioassessment or monitoring programmes. In this laboratory-based experi-ment known amounts of three sediment types (sand, topsoil, peat) were added to mesocosms and four methods of measuring deposited fine sediment;turbidity, estimation of released sediment, Turner–Hillis deposited sediment sampler (DSS) and visual estimation of%surface cover were evaluated. The objective of the study was to evaluate which of these methods for estimating deposited sediment best dis-criminates between levels of deposited fine sediment added and assesses the effects of inter-observer variability between % surface cover estimations. While turbidity measurement and the resuspension method were strongly related to levels of added sediment, it proved difficult using the two methods to resolve differences between adjacent levels of added sediment e.g. 50 g and 100 g. Surface cover esti-mations were also strongly related to added sediment levels and were better able to distinguish between adjacent levels of added sediment. Furthermore, we found no significant differences between the %surface cover estimations between observers. Results from this laboratory experiment strongly endorse the use of visual estimation of surface cover in field studies. Further work evaluating the turbidity and re-suspension methods under field conditions would also be beneficial. 相似文献
13.
This study aims at gaining basic understanding about two specific phenomena that are observed in the highly turbid estuaries tidal Ouse, Yangtze and Ems, i.e. (1) the accumulation of suspended matter in the deeper parts of the estuaries and (2) the relatively high values of turbidity near the surface in the area of the turbidity maximum. A semi-analytical model is analysed to verify the hypothesis that these phenomena result from bottom slope-induced turbidity currents and from hindered settling, respectively. The model governs the dynamics of residual flow, driven by fresh water discharge, salinity gradients and turbidity gradients. It further uses the condition of morphodynamic equilibrium (no divergence of net sediment transport) to compute the residual sediment concentration. New aspects are that depth variations on flow and mixing processes, as well as flocculation and hindered settling of sediment, are explicitly accounted for. Tides act as a source of mixing and erosion of sediment only, thus processes like tidal pumping are not considered. Model results show that the estuarine turbidity maximum (ETM) shifts in the down-slope direction, compared to the case of a constant depth. Slope-induced turbidity currents, which are directed down-slope near the bottom and up-slope near the surface, are responsible for this shift, thereby confirming the first part of the hypothesis above. The down-slope shift of the ETM is reduced by currents resulting from gradients in depth-dependent mixing, which counteract turbidity currents, but which are always weaker. Including flocculation and hindered settling yields increased surface sediment concentrations in the area of the turbidity maximum, compared to the situation of a constant settling velocity, thereby supporting the second part of the hypothesis. Sensitivity experiments reveal that the conclusions are not sensitive to the values of the model parameters. 相似文献
14.
A three-dimensional(3D) numerical model of unstable turbidity currents is developed based on the mechanism of sediment transport and turbulence theory.In this model,numerical simulation of turbidity currents without subsequent supply of muddy water was conducted using the same parameters as were used in the flume experiments.The evolution process of turbidity currents of completely losing supplies observed in the experiment was simulated by the model;validation of the numerical model and the algorithm was conducted.If momentarily interrupted process is regarded as a special case of the gradually interrupted,based on the preceding numerical simulation validity,it is feasible to simulate the motion law of turbidity currents under losing gradually supplies.By this method in this article,the characteristic of sediment-laden flow of losing gradually supplies was obtained,as well as its relationship between front velocity and sediment concentration. 相似文献
15.
鹅掌河泥石流对四川邛海影响的初步研究 总被引：5，自引：0，他引：5
沉积作用在湖泊的演化和消亡过程中至关重要,洪水泥石流的淤积作用对我国西南地区广泛分布的构造断陷湖泊是一个普遍的环境问题．通过调查鹅掌河泥石流发育背景,人类的活动对泥沙进入邛海的影响和对比1988年与2003 年邛海水下地形图得出:在近30年的时间内,鹅掌河洪水和泥石流改沿固定河堤流入邛海,在湖底形成浊流,将更大量泥沙带到湖中．浊流在鹅掌河河口的水下扇陡坡上形成一水下冲沟,在湖底沉积区形成一条长2 km的水下堤．计算发现, 鹅掌河泥石流带入邛海的泥沙远大于一般土壤侵蚀产沙量,其中以1996,1997和1998三年泥石流输入的泥沙量最多．鹅掌河洪水和泥石流对邛海的影响以及水下地形、湖泊基本特征和湖泊环境的变化,应引起人们对邛海保护的高度关注． 相似文献
16.
J. Trofimovs C. Foster R. S. J. Sparks S. Loughlin A. Le Friant C. Deplus L. Porritt T. Christopher R. Luckett P. J. Talling M. R. Palmer T. Le Bas 《Bulletin of Volcanology》2012,74(2):391-405
The 20th May 2006 lava dome collapse of the Soufrière Hills Volcano, Montserrat, had a total non-dense rock equivalent (non-DRE) collapse volume of approximately 115?×?106?m3. The majority of this volume was deposited into the ocean. The collapse was rapid, 85% of the mobilized volume being removed in just 35?min, giving peak pyroclastic flow flux of 66?×?103?m3?s?1. Channel and levee facies on the submarine flanks of the volcano and formation of a thick, steep-sided ridge, suggest that the largest and most dense blocks were transported proximally as a high concentration granular flow. Of the submerged volume, 30% was deposited from the base of this granular flow, forming a linear, high-relief ridge that extends 7?km from shore. The remaining 70% of the submerged volume comprises the finer grain sizes, which were transported at least 40?km by turbidity currents on gradients of <2°. At several localities, the May 2006 distal turbidity currents ran up 200?m of topography and eroded up to 20?cm of underlying substrate. Multiple turbidites are preserved, representing current reflection from the graben margins and deflection around topography. The high energy of the May 2006 collapse resulted in longer submarine run out than the larger (210?×?106?m3) Soufrière Hills dome collapse in July 2003. 相似文献
17.
Edgardo Can-Tapia George P. L. Walker Emilio Herrero-Bervera 《Journal of Volcanology and Geothermal Research》1997,76(1-2)
We studied the anisotropy of magnetic susceptibility (AMS) of 22 basaltic flow units, including S-type pahoehoe, P-type pahoehoe, toothpaste lava and 'a'
emplaced over different slopes in two Hawaiian islands. Systematic differences occur in several aspects of AMS (mean susceptibility, degree of anisotropy, magnetic fabric and orientation of the principal susceptibilities) among the morphological types that can be related to different modes of lava emplacement. AMS also detects systematic changes in the rate of shear with position in a unit, allowing us to infer local flow direction and some other aspects of the velocity field of each unit. 'A'
flows are subject to stronger deformation than pahoehoe, and also their internal parts behave more like a unit. According to AMS, the central part of pahoehoe commonly reveals a different deformation history than the upper and lower extremes, probably resulting from endogenous growth. 相似文献
18.
Toshihiro Ike Gregory F. Moore Shin'ichi Kuramoto Jin-Oh Park Yoshiyuki Kaneda Asahiko Taira 《Island Arc》2008,17(3):358-375
Abstract When seamounts and other topographic highs on an oceanic plate are subducted, they cause significant deformation of the overriding plate and may act as asperities deeper in the seismogenic zone. Kashinosaki Knoll (KK) is an isolated basement high of volcanic origin on the subducting Philippine Sea Plate that will soon be subducted at the eastern Nankai Trough. Seismic reflection imaging reveals a thick accumulation of sediments (∼1200 m) over and around the knoll. The lower portion of the sedimentary section has a package of high-amplitude, continuous reflections, interpreted as turbidites, that lap onto steep basement slopes but are parallel to the gentler basement slopes. Total sediment thickness on the western and northern slopes is approximately 40–50% more than on the summit and southeastern slopes of KK. These characteristics imply that the basal sedimentary section northwest of KK was deposited by infrequent high-energy turbidity currents, whereas the area southeast of KK was dominated by hemipelagic sedimentation over asymmetric basement relief. From the sediment structure and magnetic anomalies, we estimate that the knoll likely formed near the spreading center of the Shikoku Basin in the early Miocene. Its origin differs from that of nearby Zenisu Ridge, which is a piece of the Shikoku Basin crust uplifted along a thrust fault related to the collision of the Izu–Bonin arc and Honshu. KK has been carried into the margin of the Nankai Trough, and its high topography is deflecting Quaternary trench turbidites to the south. When KK collides with the accretionary prism in about 1 My, the associated variations in sediment type and thickness around the knoll will likely result in complex local variations in prism deformation. 相似文献
19.
20.
In the southern part of Rhodes, Greece, rhyolitic subaqueous pyroclastic deposits are interbedded with Tertiary, deep water, marine sediments. The lowermost and best exposed of these deposits — the Dali Ash — is described here. The deposit has been previously described as a deep water welded subaqueous ignimbrite. This paper shows that there is no evidence of welding, and texture previously reported were misidentified. The Dali Ash consists of a lower massive unit (5 m thick), overlain by a sequence of ash-turbidites (2.5 m thick). The lower unit was deposited by a high concentration turbidity current and the ash-turbidites by dilute turbidity currents. Foraminifera are dispersed throughout the deposit and indicate that all the sedimentary gravity flows were cold water/particulate systems. A palaeomagnetic study also suggests they were deposited cold. The Dali Ash can be interpreted as the lateral equivalent of a subaerial pumiceous pyroclastic flow deposit (ignimbrite). The ash-turbidites then may be redeposited slumps off the submarine slope of the lower massive unit, or, may represent later, smaller pyroclastic flows in the eruption. Other alternatives for the origin of the Dali Ash are fully discussed to show the problems in interpreting submarine volcanigenic sediments. It is possible that the deposits are not even a primary eruptive product and are remobilized pyroclastic debris, slumped, for example, off the sides of a shallow marine rhyolitic tuff ring. 相似文献