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1.
Nisyros island is a volcano at the eastern edge of the Aegean volcanic arc within the Hellenic arc and trench system along the convergence zone of the Eurasian and African plates. Several fault zones have been mapped and analyzed on the island with fault displacements reaching 100?C150 m as deduced from the morphology and the offset of the stratigraphic formations of the volcano. Seismic activity during 1995?C1998 affected the island with damage along the western edge of the Mandraki town, related to the Mandraki fault. The geological, tectonic and morphological data on land show that the Mandraki fault throw is 80?C100 m and its length about 2 km. Its continuation northwards under the sea was studied within a systematic survey of the broader area of the Kos-Nisyros-Tilos islands; and the bathymetric and lithoseismic data showed the existence of some active tectonic structures. In the area of the Yali-Nisyros Channel the prolongation of the Mandraki fault has a 100 m high submarine scarp between the two sides of the fault. Morphological slopes along the fault are high between 20 and 50% in contrast to slopes of 1?C5% observed on top of the two adjacent tectonic blocks. The general structure both on land and offshore shows a westward tilt contemporaneous to the extension in the E-W direction observed in this area. Observations of the submarine fault during a dive with submersible Thetis showed spectacular landslides and loose rocks along the fault scarp and very abrupt linear topographic change along the strike of the fault. The synthesis of the onshore and offshore data on a digital topographic map shows that the Mandraki fault is a secondary structure of the major F3 fault zone of Nisyros which separates the neotectonic block/horst of Prophitis Ilias in the west from the Emborio/Nikia block in the east. The GPS data from the period 1997?C2001 show excellent agreement with the neotectonic block structure of Nisyros. The seismic hazard of the F3/Mandraki fault zone is discussed together with the volcanic hazard of Yali-Nisyros area with the general conclusion that the expected seismic magnitude of 6.1?C6.3 is significantly higher than that observed in 1995?C1998. The ascent of magma from a chamber 7.5?C8.5 km deep between the Yali and Nisyros islands may trigger tectonovolcanic activity similar to that observed at the end of the 19th century.  相似文献   

2.
Potential tsunami waves were modelled on the basis of the morphology and geological setting of a late glacial submarine landslide localized in the north-eastern sector of the Sea of Marmara, using a three-dimensional algorithm with the purpose of assessing the future risk of tsunamogenic landslides in the region. The landslide occurred off the Tuzla Peninsula on the north-eastern slope of the Ç?narc?k Basin, the easternmost of the three deep Marmara basins. The mass movement appears to be related to the Main Marmara Fault that passes below the toe of the failed mass. Observations from earlier manned submersible dives suggest that the initiation of the slide was facilitated by secondary faults associated with the Hercynian orogeny and involved Palaeozoic shales dipping southwards towards the deep basin. Radiocarbon dating of core material, together with the well-dated Marmara sapropel above the chaotically mixed landslide surface, reveal that the latest landslide event occurred about 17 14C ka b.p. The uppermost scar of the landslide is found at 250 m and its toe at about 1,200 m below the present sea level. At the time of the slide, the Marmara Sea Basin was lacustrine, with its water level at ?85 m. In plan view the landslide has a distinctively triangular shape and the lateral extent of its toe is about 10 km. Multibeam bathymetric data indicate that the sliding motion probably occurred in two phases: a slower phase affecting the eastern part, characterized by an undulating surface, and a more rapid phase affecting the western part that possibly created tsunami waves. In the seismic sections, older failed slide masses can be clearly identified; these were probably displaced during marine isotopic stage 6 (～127–160 ka b.p.). The front of this buried material is located more than 1.5 km further south of the fault. We used a three-dimensional, Green’s function-based potential theory approach, rather than shallow-water equations commonly used in conventional tsunami simulations. The solution algorithm is based on a source-sink formulation and an integral equation. The results indicate that the maximum height of the tsunami in the Ç?narc?k Basin could have reached about half the average thickness of the sliding mass over a lateral extent of 7 km. Assuming an average thickness of 30 m for the landslide, and considering that the water level at 17 ka b.p. was at about ?85 m, the modelling shows that the maximum wave height generated by the slide would have been about 15–17 m.  相似文献   

3.
A Late Pleistocene submarine slide on the Bear Island Trough Mouth Fan   总被引：5，自引：2，他引：5  
A large submarine slide on the southern flank of the Bear Island Trough Mouth Fan, southwestern Barents Sea continental slope, has a run-out distance of about 400 km, a total volume of about 1100 km3, and is younger than 330 ka. Three seismic units, comprising mainly hemipelagic sediments has partly filled the slide scar. An increased sedimentation rate on the Bear Island Trough Mouth Fan from Late Pliocene time, probably in combination with abundant earthquakes, is the most likely cause of the slide. Based on these and previous studies, we suggest that large-scale slides were important sediment transport processes during Plio-Pleistocene.  相似文献   

4.
Reflection profiling in a region of anomalous topography and structure in the Bay of Bengal off Burma has revealed the presence of a large submarine slide (olistostrome) at the base of the continental slope off the Bassein River. The slide overlies a thick section of Bengal Deep-Sea Fan turbidites and has a complex internal structure consisting of two primary elements. The lower element is pervasively disturbed and is interpreted as a mudflow generated at the time of the slide which spread over a large area to as much as 35 km beyond the topographic toe. This mudflow poured into a distributary channel on the Bengal Fan and virtually filled it for 145 km along its length. The upper element comprises a series of relatively coherent blocks of stratified sediments (olistoliths) bounded by curved fault planes. The blocks have been transported as much as 55 km from the original Sunda Trench wall. Their dimensions, up to 360 m thick and 2.8 km between faults, are similar to olistoliths of the slide terrain in the Apennines. The blocks are blanketed by younger slope strata. The total area covered by the slide, including the mudflow, is almost 4,000 km2, and total volume of the slide is over 900 km3. Material of the slide consists of Bengal Fan turbidites offscraped above the Sunda Subduction zone and blanketed by rapidly deposited slope sediments from a western Irrawaddy River distributary (the Bassein River) during Late Quaternary glacial low sea level. This rapid loading, probably coupled with a large earthquake, triggered the slide.  相似文献   

5.
The Pacific deep western boundary current (DWBC) encounters an unstable continental margin where it flows across the New Zealand convergent plate boundary. Seismic profiles show the DWBC was intercepted by several submarine landslides, the latest (~38-100 ka) being the newly discovered Matakaoa debris flow. Occupying ~650 km3, the flow extends 200 km from Matakaoa re-entrant to Kermadec Ridge to form a 37-68 m high lobe in the current's path. This deposit appears to have (1) reduced the size of gaps in the western boundary, thereby reducing leakage of the DWBC, and (2) temporarily reduced the terrigenous supply into the flow by impeding the passage of turbidity currents from New Zealand.  相似文献   

6.
Abstract

As part of a National Oceanic and Atmospheric Administration (NOAA) program to understand bottom and nearbottom processes on the continental margin, the continental slope seaward of the coast of Delaware, just east of the Baltimore Canyon Trough, and northeast of Wilmington Canyon was studied in detail. With a suite of geophysical data, a 7.5 × 13.0‐km portion of the continental slope was surveyed and found to be composed of a large submarine slide, approximately 11 km 3 in volume. The slide varies from 50 to 300 m in thickness and is believed to be composed of Pleistocene Age sediments. The internal structure of the continental slope can be seen on the seismic reflection profiles, as well as the readily identifiable continuous slip surface. Pliocene to Cretaceous horizons comprise the continental margin with Pliocene to Eocene horizons truncated at the slip surface. Sediment failure occurred on the slope between the late Tertriary erosion surface, which shaped the continental margin, and the overlying Quaternary sediments. A mechanism suggested to have contributed to the sediment failure is a late Pleistocene lower stand of sea level. Creep of surficial sediments is believed to be active on the surface of the submarine slide, indicating present‐day instability.  相似文献   

7.
The ∼8.15 ka Storegga submarine slide was a large (∼3000 km3), tsunamigenic slide off the coast of Norway. The resulting tsunami had run-up heights of around 10–20 m on the Norwegian coast, over 12 m in Shetland, 3–6 m on the Scottish mainland coast and reached as far as Greenland. Accurate numerical simulations of Storegga require high spatial resolution near the coasts, particularly near tsunami run-up observations, and also in the slide region. However, as the computational domain must span the whole of the Norwegian-Greenland sea, employing uniformly high spatial resolution is computationally prohibitive. To overcome this problem, we present a multiscale numerical model of the Storegga slide-generated tsunami where spatial resolution varies from 500 m to 50 km across the entire Norwegian-Greenland sea domain to optimally resolve the slide region, important coastlines and bathymetric changes. We compare results from our multiscale model to previous results using constant-resolution models and show that accounting for changes in bathymetry since 8.15 ka, neglected in previous numerical studies of the Storegga slide-tsunami, improves the agreement between the model and inferred run-up heights in specific locations, especially in the Shetlands, where maximum run-up height increased from 8 m (modern bathymetry) to 13 m (palaeobathymetry). By tracking the Storegga tsunami as far south as the southern North sea, we also found that wave heights were high enough to inundate Doggerland, an island in the southern North Sea prior to sea level rise over the last 8 ka.  相似文献   

8.
SeaMARC II side-scan sonar data reveal that a large area of seafloor north and west of Easter Island has been disrupted by recent submarine volcanism. A large volcanic area begins approximately 60 km WNW of the island and extends for over 130 km to the west. The volcanic field is characterized by high backscatter intensity in the side-scan sonar records and is elevated 400–1000 m above the N-S seafloor fabric that surrounds it. This field, the Abu Volcanic Field, covers at least 2500 km2 and appears to consist of recent lava flows and small volcanoes. Backscatter intensity of the Abu Volcanic Field is similar to that of the adjacent ridge flank which is less than 0.4 Ma, suggesting a similar age for its formation. Two additional areas of high backscatter immediately north of Easter Island cover a combined area of over 300 km2. The sidescan sonar records show that these features are clearly of volcanic origin and are not debris flows from the nearby island. The flows are nearly 300 m thick and are morphologically similar to subaerial pahoehoe lava shields. Their high backscatter indicates that they are also the products of relatively recent submarine volcanic activity. The presence of these large areas of recent volcanism in the vicinity of Easter Island has important implications for the various models that have been proposed to explain the origin of the Easter Seamount Chain. In addition, the similar ages of Easter Island and the Easter Microplate suggest that the presence of a hotspot near or beneath this fast-spreading portion of the East Pacific Rise about 4.5 m.y. ago may have initiated the large-scale rift propagation that created the microplate.  相似文献   

9.
Landslide processes on the western slope of the Kuril Basin were studied using bathymetry and seismic data obtained under the international KOMEX and SSGH projects. Slope areas containing landslides, landslide blocks and mass-transport deposits were distinguished. Large-scale landslides occupying an area of more than 100 km2 are located in such areas of open continental margins as the slopes of the North Hokkaido Marginal Plateau and Terpeniya Ridge. Landslide blocks up to 2 km in size and mass-transport deposits are located in submarine canyons and fans in Terpeniya Bay. The age of landslides has been estimated as Middle Pleistocene–Holocene. Landslides are most likely triggered by seismic activity and gas saturation of sediments. Subsequent slope failure seems quite probable within the study area, and landslides capable of generating tsunamis may occur.  相似文献   

10.
Three dimensional seismic data, offshore Brunei, provide evidence for a giant landslide with a volume of 1200 km3, an area of ∼ 5300 km2 and an average thickness of ∼ 240 m. It extends for over 120 km from the Baram Canyon in ∼ 200 m water depth to the deep basin floor of the North West Borneo Trough. The landslide is a unique example of a major submarine landslide located on a steep, tectonically active margin adjacent to a large river and canyon system. The landslide is mappable using 3D seismic data, which allow detailed imaging of internal flow structures, erosional headwall and the basal sliding surface. The landslide is a complex deposit, involving a chaotic debris flow matrix, with flow structures and blocks 500 to 1000 m wide and up to 250 m thick. Imaging of the basal sliding surface reveals large striations ∼ 30-120 km long, ∼ 100-600 m wide, and ∼ 10-30 m deep that show significant amounts of basal erosion. In the landslide source area we describe fluid escape structures, gas buildups and bottom simulating reflectors, which may provide a mechanism for weakening and triggering slope failure. We also report older landslides, buried several hundred meters beneath the basin floor that indicate giant landsliding is a recurrent process in the NW Borneo Trough.  相似文献   

11.
Regulations are being developed through the International Seabed Authority (ISBA) for the exploration and mining of cobalt-rich ferromanganese crusts. This paper lays out geologic and geomorphologic criteria that can be used to determine the size and number of exploration and mine-site blocks that will be the focus of much discussion within the ISBA Council deliberations. The surface areas of 155 volcanic edifices in the central equatorial Pacific were measured and used to develop a mine-site model. The mine-site model considers areas above 2,500 m water depth as permissive, and narrows the general area available for exploration and mining to 20% of that permissive area. It is calculated that about eighteen 100 km2 exploration blocks, each composed of five 20 km2 contiguous sub-blocks, would be adequate to identify a 260 km2 20-year-mine site; the mine site would be composed of thirteen of the 20 km2 sub-blocks. In this hypothetical example, the 260 km2 mine site would be spread over four volcanic edifices and comprise 3.7% of the permissive area of the four edifices and 0.01% of the total area of those four edifices. The eighteen 100 km2 exploration blocks would be selected from a limited geographic area. That confinement area is defined as having a long dimension of not more than 1,000 km and an area of not more than 300,000 km2.  相似文献   

12.
The Baiyun submarine slide complex (BSSC) along the Pearl River Canyon of the northern South China Sea has been imaged by multibeam bathymetry and 2D/3D seismic data. By means of maximum likelihood classification with slope aspect and gradient as inputs, the BSSC is subdivided into four domains, denoted as slide area I, II, III and IV. Slide area I is surrounded by cliffs on three sides and has been intensely reshaped by turbidity currents generated by other kinds of mass movement outside the area; slide area II incorporates a shield volcano with a diameter of approximately 10 km and unconfined slides possibly resulting from the toe collapse of inter-canyon ridges; slide area III is dominated by repeated slides that mainly originated from cliffs constituting the eastern boundary of the BSSC; slide area IV is distinguished by a conical seamount with a diameter of 6.5 km and a height of 375 m, and two slides probably having a common source that are separated from each other by a suite of residual strata. The BSSC is interpreted to be composed of numerous slide events, which occurred in the period from 10.5 to 5.5 Ma BP. Six specific factors may have contributed to the development of the BSSC, i.e., gas hydrate dissociation, gas-bearing sediments, submarine volcanic activity, seismicity, sedimentation rate and seafloor geomorphology. A 2D conceptual geological model combining these factors is proposed as a plausible mechanism explaining the formation of the BSSC. However, the BSSC may also have been affected by the Dongsha event (10 Ma BP) as an overriding factor.  相似文献   

13.
The northwest African margin has been affected by numerous large-scale landslides during the late Quaternary. This study focuses on a recent collapse of the Sahara Slide headwall and characterises the resulting flow deposit. Core and seismic data from the base of the upper headwall reveal the presence of blocky slide debris, comprising heavily deformed hemipelagic slope sediments. The blocky slide debris spilled over a lower headwall 60 km downslope and formed a thick transparent debris flow unit. Cores recovered 200–250 km farther downslope contain a surficial turbidite that is interpreted to be linked to the headwall collapse event based on timing and composition. One core located approximately 200 km from the headwall scar (C13) contains debrite encased in turbidite. The debrite comprises sheared and contorted hemipelagic mudstone clasts similar as those seen in the vicinity of the Sahara Slide headwall, and lacks matrix. This debrite pinches out laterally within 25 km of C13, whereas the accompanying turbidite can be correlated across 700 km of the northwest African margin. The linked turbidite–debrite bed is interpreted to have formed through recent failure of the steep Sahara Slide headwall that either 1) generated both a debris flow and a turbidity current almost simultaneously, or 2) generated a debris flow which with entrainment of water and progressive dilution led to formation of an accompanying turbidity current.  相似文献   

14.
Geophysical observations demonstrate that the archipelagic apron surrounding the Marquesan hot-spot volcanoes is derived almost entirely from mass wasting processes. Seismic reflection and refraction data constrain the volume of the apron sediments to approximately 200,000 km3, with thicknesses reaching over 2 km in the deep portions of the moat near the edge of the volcanic edifice. Seismic velocities average 4 to 5 km s–1 in the sediments, and 6 km s–1 at the top of the underlying basement. Single channel seismic profiles show acoustically chaotic cores in the sediments of the apron, which are interpreted as debris flows from mass wasting events. We deduce that the apron is formed by catastrophic collapses that may involve volumes over 100 km3 tens to hundreds of times during the lifetime of a volcano. Comparison with similar data from the Hawaiian Islands yields the result that the total volume of volcanics and their derived sediments along the strike of the chains is only slightly smaller for the Marquesas, implying comparable eruption rates. However, the ratio of sediment to surface volcanic load is much larger for the latter, leading to an overfilled moat in the Marquesas and an underfilled moat at Hawaii. The much larger size of the Hawaiian islands can be explained as the combined effects of a higher thermal swell, loading a stiffer elastic plate, and proportionately less mass wasting.  相似文献   

15.
The Cengio sandstone member of the Tertiary Piedmont Basin in northwestern Italy has a conservatively estimated volume of 2.5 to 3 km3 (length: 6.4 km; width: 4.8 km; thickness: 170 m). It is interpreted as a sandstone-rich submarine fan deposit. The Cengio member consists of eight tabular depositional sandstone lobes that are 5- to 25-m thick. These lobes filled a submarine structural depression and onlap and/or pinch-out against bounding slope mudstones. The stacking of the lobe units was related to synsedimentary tectonism.  相似文献   

16.
Aeromagnetic data collected over the Offshore Mahanadi Basin along the Eastern margin of India display high amplitude magnetic anomalies. The presence of a Cretaceous volcanic sequence masks the seismic response from the underlying basement and results in poor quality seismic data. In this study spectral analysis of the aeromagnetic data collected over this part of the Offshore Mahanadi Basin was carried out. Results of this analysis indicate the presence of a high density, highvelocity (6.45 km/s) mafic layer within the crystalline basement varying from 4–6 km depth. This intra-basement layer seems to have been affected by a number of lineaments, which have played a role in the evolution of the Mahanadi Offshore Basin. The western part of the offshore basin is affected by the volcanism related to the 85°E Ridge, whereas the intense anomaly band (900 nT) offshore Puri, Konark and Paradip is interpreted as a combined effect of crystalline Precambrian basement overlain (i) by Cretaceous volcanic rocks of variable thickness (25–860 m) and (ii) by a mafic layer within the basement.  相似文献   

17.
The Cengio sandstone member of the Tertiary Piedmont Basin in northwestern Italy has a conservatively estimated volume of 2.5 to 3 km3 (length: 6.4 km; width: 4.8 km; thickness: 170 m). It is interpreted as a sandstone-rich submarine fan deposit. The Cengio member consists of eight tabular depositional sandstone lobes that are 5- to 25-m thick. These lobes filled a submarine structural depression and onlap and/or pinch-out against bounding slope mudstones. The stacking of the lobe units was related to synsedimentary tectonism. Margin setting represents fan and/or source area  相似文献   

18.
As part of the ‘National Hydrographic and Oceanographic Research Plan for the Spanish Exclusive Economic Zone’, multibeam bathymetry and seismic reflection profiles were obtained in the Canary Islands aboard the R/V Hespérides. The submarine flanks of the Anaga offshore extension of Tenerife Island are here studied to analyze its geomorphology. In the north sector of the Anaga submarine massif, the extension of the Anaga Debris Avalanche has been mapped for the first time, and a volume of 36 km3 was calculated. The relationship between the Anaga and Orotava Debris Avalanches is also described. Faulting has been recognized as a key process for the occurrence of debris avalanches and the growth of volcanic lineaments. Moreover, faulting affects previous structures and the channelling of debris flows. Structural analysis shows the typical radial pattern of an oceanic island. In addition, a NE-SW dominant direction of faulting was obtained, consistent with the Tenerife Island structural trend seen in the Anaga Massif and Cordillera Dorsal. NW-SE and E-W are two other main trends seen in the area. Special interest is manifest in two long faults: ‘Santa Cruz Fault’ bounds the southern edge of Anaga offshore Massif with a length of 50 km and a direction that changes from NE-SW to almost E-W. The Güimar Debris Avalanche was probably channeled by this fault. The ‘Guayotá Fault’ was recognized in several seismic profiles with a N-S direction that changes towards NW-SE at its southern end. This fault affects the more recent sediments with a vertical offset of 25–30 m, along 60~km. It has been interpreted as a transpressive strike-slip fault.ZEE Working Group**F. Carrilloa, C. Matéa, M. Ballesterosb, M. Vaquerob, J. Martín-Dávillac and J.A. MaríncaInstituto Hidrográfico de la Marina. CádizbInstituto Español de Oceanografía. Madrid.cReal Observatorio de la Armada. S. Fernando, Cádiz.  相似文献   

19.
The western Svalbard continental margin contains thick sediment sequences with areas known to contain gas hydrates. Together with a dynamic tectonic environment, this makes the region prone to submarine slides. This paper presents results from geophysical mapping of the deepest part of the high Arctic environment, the Molloy Hole. The mapping includes multibeam bathymetry, acoustic backscatter and sub-bottom profiling. The geophysical data reveal seabed features indicative of sediment transport and larger-scale mass wasting. The large slide scar is here referred to as the Molloy Slide. It is located adjacent to the prominent Molloy Hole and Ridge system. The slide is estimated to have transported >65 km3 of sediments over the deep axial valley of the Molloy Ridge, and further into the Molloy Hole. A unique feature of this slide is that, although its run-out distance is relatively short (<5 km), it extends over an enormous vertical depth (>2,000 m) as a result of its position in a complex bathymetric setting. The slide was most likely triggered by seismic activity caused by seafloor spreading processes along the adjacent Molloy Ridge. However, gas-hydrate destabilization may also have played a role in the ensuing slide event.  相似文献   

20.
As a supplementary study, we used passive seismic data recorded by one ocean bottom seismometer (OBS) station (49°41.8′E) close to a hydrothermal vent (49°39′E) at the Southwest Indian Ridge to invert the crustal structure and mantle transition zone (MTZ) thickness by P-to-S receiver functions to investigate previous active seismic tomographic crustal models and determine the influence of the deep mantle thermal anomaly on seafloor hydrothermal venting at an ultra-slow spreading ridge. The new passive seismic S-wave model shows that the crust has a low velocity layer (2.6 km/s) from 4.0 to 6.0 km below the sea floor, which is interpreted as partial melting. We suggest that the Moho discontinuity at ~9.0 km is the bottom of a layer (2–3 km thick); the Moho (at depth of ~6–7 km), defined by active seismic P-wave models, is interpreted as a serpentinized front. The velocity spectrum stacking plot made from passive seismic data shows that the 410 discontinuity is depressed by ~15 km, the 660 discontinuity is elevated by ~18 km, and a positive thermal anomaly between 182 and 237 K is inferred.  相似文献   

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