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1.
A number of examples are presented to substantiate that submarine landslides have occurred along most continental margins and along several volcano flanks. Their properties of importance for tsunami generation (i.e. physical dimensions, acceleration, maximum velocity, mass discharge, and travel distance) can all gain extreme values compared to their subaerial counterparts. Hence, landslide tsunamis may also be extreme and have regional impact. Landslide tsunami characteristics are discussed explaining how they may exceed tsunamis induced by megathrust earthquakes, hence representing a significant risk even though they occur more infrequently. In fact, submarine landslides may cause potentially extreme tsunami run-up heights, which may have consequences for the design of critical infrastructure often based on unjustifiably long return periods. Giant submarine landslides are rare and related to climate changes or glacial cycles, indicating that giant submarine landslide tsunami hazard is in most regions negligible compared to earthquake tsunami hazard. Large-scale debris flows surrounding active volcanoes or submarine landslides in river deltas may be more frequent. Giant volcano flank collapses at the Canary and Hawaii Islands developed in the early stages of the history of the volcanoes, and the tsunamigenic potential of these collapses is disputed. Estimations of recurrence intervals, hazard, and uncertainties with today’s methods are discussed. It is concluded that insufficient sampling and changing conditions for landslide release are major obstacles in transporting a Probabilistic Tsunami Hazard Assessment (PTHA) approach from earthquake to landslide tsunamis and that the more robust Scenario-Based Tsunami Hazard Assessment (SBTHA) approach will still be most efficient to use. Finally, the needs for data acquisition and analyses, laboratory experiments, and more sophisticated numerical modelling for improved understanding and hazard assessment of landslide tsunamis are elaborated.  相似文献   

2.
Most tsunamis are generated by earthquakes, with secondary, less frequent, mechanisms including subaerial and submarine landslides, volcanic eruptions and (extra‐terrestrial) bolide impacts. Different mechanisms generate tsunamis with different magnitudes, travel distances and impacts. Submarine landslides had been mapped and studied for decades but records suggested that only a few had generated tsunamis, and that these were minor. It was not until 1998, when a slump on the seabed offshore of northern Papua New Guinea caused a tsunami wave up to 15 m high that killed over 2200 people, was the significance of submarine landslides in tsunami generation realised. A combination of new (multibeam) seabed mapping technology and the development of improved numerical tsunami models for tsunami generation led to the recognition of the landslide tsunami mechanism of the PNG event. As a result the hazard from submarine landslides in tsunami generation is now recognized and better understood. Extensive mapping of ocean margins reveals that submarine landslides are common. Although many of these probably generated tsunamis, few have been identified, so their hazard remains uncertain. This article describes how the hazard from submarine landslide tsunamis was first recognized, how submarine landslides generate tsunamis, why they were previously discounted as a major hazard, and their potential hazards. An important aspect of the recognition of the tsunami hazard from submarine landslides has been the significance of geology, which has contributed to a subject previously dominated by seismologists.  相似文献   

3.
An examination of the deeply incised Ediacaran Wonoka canyons in the Adelaide Geosyncline (most recently interpreted as subaerial valleys) demonstrates their submarine origin, and confirms them as some of the best examples of ancient outcropping submarine canyons in the world. The entire canyon-fill succession is interpreted to be of deep-water (below wave base) origin, consisting of calcareous shale and siltstone together with a variety of mass-flow deposits including turbidites, grain flows and debris flows. The canyon fill lacks definitive shallow-water structures (e.g. mud cracks, fenestral fabrics or wave ripples) at all stratigraphic levels. Canyon-lining carbonate crusts that have previously been interpreted as non-marine calcretes or tufas (and used to suggest a non-marine origin for the canyons) are argued to be of deep-water, marine, microbial origin. Extremely negative carbon isotope values from the canyon-fill and canyon-lining crusts have a primary marine origin. Previously interpreted deepening upward trends in the canyon fill (used as evidence of a subaerial erosion episode followed by drowning) are suggested to be fining upward trends, caused by the transition from canyon cutting to canyon filling, with the majority of the fill being of deep-water slope origin. The basal conglomeratic canyon-fill sediments represent the last vestiges of the high-energy, deep-water, canyon-erosion environment in which the incisions formed. A deep-water origin for the canyons is consistent with all previous stratigraphic observations of the Wonoka canyons, including the conspicuous lack of regional unconformities in the lower Wonoka Formation, and their emanation from the deep-water facies of the Wonoka Formation. A submarine canyon origin also removes the need for extreme (~ 1 km) relative sea level fluctuation and associated problems (i.e. an enclosed basin with Messinian-style evaporative drawdown or thermal uplift above a migrating mantle plume) required by the subaerial valley hypotheses.  相似文献   

4.
Over the past 200 years of written records, the Hawaiian Islands have experienced tens of tsunamis generated by earthquakes in the subduction zones of the Pacific ‘Ring of Fire’ (for example, Alaska–Aleutian, Kuril–Kamchatka, Chile and Japan). Mapping and dating anomalous beds of sand and silt deposited by tsunamis in low-lying areas along Pacific coasts, even those distant from subduction zones, is critical for assessing tsunami hazard throughout the Pacific basin. This study searched for evidence of tsunami inundation using stratigraphic and sedimentological analyses of potential tsunami deposits beneath present and former Hawaiian wetlands, coastal lagoons, and river floodplains. Coastal wetland sites on the islands of Hawai΄i, Maui, O΄ahu and Kaua΄i were selected based on historical tsunami runup, numerical inundation modelling, proximity to sandy source sediments, degree of historical wetland disturbance, and breadth of prior geological and archaeological investigations. Sand beds containing marine calcareous sediment within peaty and/or muddy wetland deposits on the north and north-eastern shores of Kaua΄i, O΄ahu and Hawai΄i were interpreted as tsunami deposits. At some sites, deposits of the 1946 and 1957 Aleutian tsunamis are analogues for deeper, older probable tsunami deposits. Radiocarbon-based age models date sand beds from three sites to ca 700 to 500 cal yr bp , which overlaps ages for tsunami deposits in the eastern Aleutian Islands that record a local subduction zone earthquake. The overlapping modelled ages for tsunami deposits at the study sites support a plausible correlation with an eastern Aleutian earthquake source for a large prehistoric tsunami in the Hawaiian Islands.  相似文献   

5.
The Pacific coast, including the Kamchatka Peninsula, the Kuriles, the Sea of Japan, the Sea of Okhotsk, and the Bering Sea, is the main tsunami-prone area in Russia. The Far East tsunamis are much more frequent, extensive, and devastating than those in the Black, Caspian, Baltic, and White Sea coasts, as well as in major inland lakes of Baikal, Ladoga, etc. The tsunami catalog of the Russian Far East from 1737 to present lists 110 events with mainly near-field and few far-field sources (105 and 5 events, respectively). Most of the catalogued tsunamis (95 cases) were induced by earthquakes, and few events had volcanic (3), landsliding (2), meteorological (3), and unknown (2) triggers. Altogether there were eleven devastating tsunamis for the period of observations, with > 10 m heights, two of which were great events in 1737 and 1952, when the waves exceeded 20 m. The wave heights were in the range 2.5-10 m in fifteen hazardous tsunami events and within the tidal range (~ 1-2 m) in thirteen cases; the other events were small and detectable only instrumentally. Thus, the average recurrence times for tsunamis of different magnitudes in the Russian Pacific coast are 25 years for devastating events and 10-15 years for hazardous tsunamis; small tsunamis occur almost every year, according to statistics for the last sixty years collected at the regional network of tide stations. The topics discussed in the paper concern the completeness and reliability of the Far East catalog; distribution of tsunami events in space and time; correlation between the intensity of tsunami and the magnitude of the causative undersea earthquake; tsunami recurrence; tsunami warning; and long-term hazard assessment and mapping.  相似文献   

6.
While earthquakes generate about 90% of all tsunamis, volcanic activity, landslides, explosions, and other nonseismic phenomena can also result in tsunamis. There have been 53 000 reported deaths as a result of tsunamis generated by landslides and volcanoes. No death tolls are available for many events, but reports indicate that villages, islands, and even entire civilizations have disappeared. Some of the highest tsunami wave heights ever observed were produced by landslides. In the National Geophysical Data Center world-wide tsunami database, there are nearly 200 tsunami events in which nonseismic phenomena played a major role. In this paper, we briefly discuss a variety of nonseismic phenomena that can result in tsunamis. We discuss the magnitude of the disasters that have resulted from such events, and we discuss the potential for reducing such disasters by education and warning systems.  相似文献   

7.
《Engineering Geology》2001,59(1-2):115-132
Large landslides are common processes during the evolution of volcanoes and individual events can exceed several cubic kilometres in volume. Volcanic slope failures are a significant risk for the neighbouring population due to their huge volumes and great runout distances. Around the Canary archipelago, a total of seventeen deposits of large landslides have been found, and on Tenerife, seven large landslides have affected the subaerial and submarine morphology during the last ∼6 Ma. However, the causes of such mass movements are still poorly understood. This work analyses the events around the Canary Islands and focuses on the ones that occurred on Tenerife in order to obtain new insights into the mechanisms of large volcanic landslides. The study is divided into a first part that includes site investigations examining the general features favouring large-scale failures at volcanoes. The second part describes the laboratory tests used to analyse a residual soil that may be the potential slip surface of the slides on Tenerife. The site investigation revealed that regional tectonics and the climate have a significant influence on the spatial distribution of the landslides. Moreover, morphological and geological features such as deep fluvial canyons, a high coastal cliff and persistent dike intrusion may favour the initiation of slope failure. A typical residual soil sample from the lateral scarp of the La Orotava amphitheatre on Tenerife was studied by carrying out standard laboratory tests. The microstructure was analysed using environmental scanning electron microscopy and a particular bonding was found. This bonding was also detected by the geotechnical tests. Consolidation tests and direct shear tests revealed that the mechanical behaviour of the residual soil changes greatly if the bonding of the soil is broken. The bonded structure generally fails when the effective normal stress surpasses the yield strength of the bonding. In the case of large volcanic landslides with thicknesses up to several hundred meters, the high overburden easily exceeds this yield strength and generates a broken bonding. Therefore, volcanic residual soils, such as the one analysed in this study, are perfect candidates for the potential failure surfaces of large volcanic landslides. Referring to the La Orotava events, we assume that residual soil layers and morphological, geological and climatic features reduced the slope stability to critical conditions, whereas a strong earthquake associated with a caldera collapse episode may have finally triggered the landslide. The results obtained indicate that the residual soils play an important role in affecting the stability of volcano slopes and their destabilising influence significantly favours large-scale sliding. We suggest that the results obtained from this study can be applied to other locations since volcanic residual soils are common in volcanic areas.  相似文献   

8.
《Earth》2002,57(1-2):1-35
Landslides have been a key process in the evolution of the western Canary Islands. The younger and more volcanically active Canary Islands, El Hierro, La Palma and Tenerife, show the clearest evidence of recent landslide activity. The evidence includes landslide scars on the island flanks, debris deposits on the lower island slopes, and volcaniclastic turbidites on the floor of the adjacent ocean basins. At least 14 large landslides have occurred on the flanks of the El Hierro, La Palma and Tenerife, the majority of these in the last 1 million years, with the youngest, on the northwest flank of El Hierro, as recent as 15 thousand years in age. Older landslides undoubtedly occurred, but are difficult to quantify because the evidence is buried beneath younger volcanic rocks and sediments. Landslides on the Canary Island flanks can be categorised as debris avalanches, slumps or debris flows. Debris avalanches are long runout catastrophic failures which typically affect only the superficial part of the island volcanic sequence, up to a maximum thickness of 1 to 2 km. They are the commonest type of landslide mapped. In contrast, slumps move short distances and are deep-rooted landslides which may affect the entire thickness of the volcanic edifice. Debris flows are defined as landslides which primarily affect the sedimentary cover of the submarine island flanks. Some landslides are complex events involving more than one of the above end-member processes.Individual debris avalanches have volumes in the range of 50–500 km3, cover several thousand km2 of seafloor, and have runout distances of up to 130 km from source. Overall, debris avalanche deposits account for about 10% of the total volcanic edifices of the small, relatively young islands of El Hierro and La Palma. Some parameters, such as deposit volumes and landslide ages, are difficult to quantify. The key characteristics of debris avalanches include a relatively narrow headwall and chute above 3000 m water depth on the island flanks, broadening into a depositional lobe below 3000 m. Debris avalanche deposits have a typically blocky morphology, with individual blocks up to a kilometre or more in diameter. However, considerable variation exists between different avalanche deposits. At one extreme, the El Golfo debris avalanche on El Hierro has few large blocks scattered randomly across the avalanche surface. At the other, Icod on the north flank of Tenerife has much more numerous but smaller blocks over most of its surface, with a few very large blocks confined to the margins of the deposit. Icod also exhibits flow structures (longitudinal shears and pressure ridges) that are absent in El Golfo. The primary controls on the block structure and distribution are inferred to be related to the nature of the landslide material and to flow processes. Observations in experimental debris flows show that the differences between the El Golfo and Icod landslide deposits are probably controlled by the greater proportion of fine grained material in the Icod landslide. This, in turn, relates to the nature of the failed volcanic rocks, which are almost entirely basalt on El Hierro but include a much greater proportion of pyroclastic deposits on Tenerife.Landslide occurrence appears to be primarily controlled by the locations of volcanic rift zones on the islands, with landslides propagating perpendicular to the rift orientation. However, this does not explain the uneven distribution of landslides on some islands which seems to indicate that unstable flanks are a ‘weakness’ that can be carried forward during island development. This may occur because certain island flanks are steeper, extend to greater water depths or are less buttressed by the surrounding topography, and because volcanic production following a landslide my be concentrated in the landslide scar, thus focussing subsequent landslide potential in this area. Landslides are primarily a result of volcanic construction to a point where the mass of volcanic products fails under its own weight. Although the actual triggering factors are poorly understood, they may include or be influenced by dyke intrusion, pore pressure changes related to intrusion, seismicity or sealevel/climate changes. A possible relationship between caldera collapse and landsliding on Tenerife is not, in our interpretation, supported by the available evidence.  相似文献   

9.
The Yamansu skarn iron deposit is hosted in Early Carboniferous submarine lava flow and volcaniclastic rocks of the Yamansu Formation in Eastern Tianshan Mountains, NW China. The lava flows are predominantly basaltic, with minor andesites. Laser ablation inductively coupled plasma mass spectrometry (LAICP-MS) U–Pb zircon dating of the basalts and skarns yields almost coeval ages of 324.4 ± 0.94 and 323.47 ± 0.95 Ma, respectively. The basalts contain clinopyroxene and plagioclase phenocrysts with a considerable amount of Fe–Ti oxide minerals in the groundmass as interstitial phases, probably suggesting that olivine–, clinopyroxene- and plagioclase fractionated within the magma chamber. Geochemically, the basalts are characterized by slight variations in SiO2 (42.90–46.61 wt.%), P2O5 (0.08–0.12 wt.%), MnO (0.35–0.97 wt.%) and TiO2 (0.74–0.82 wt.%), and relatively large variations in CaO (6.93–15.13 wt.%), Al2O3 (14.71–19.93 wt.%), total Fe2O3 (8.14–12.66 wt.%) and MgO (4.96–8.52 wt.%). They possess flat to light rare earth element (REE)-depleted patterns and display variable degrees of depletions in high field-strength elements (HFSE), suggesting a transitional feature between MORB and arc volcanic rocks, and indicating a back-arc tectonic setting. Furthermore, the geochemical signature also suggests that the volcanic rocks of Yamansu Formation were produced by partial melting of the spinel-facies, asthenospheric mantle peridotite which had been metasomatized by slab-derived fluids. The broadly overlapping ages of the basalts and skarn mineralization suggests that the skarn formation in the Yamansu deposit is related to subaqueous volcanism. In combination with the available information including fluid inclusions and stable isotope data, we infer that the hydrothermal fluids that generated the skarns could be a mixture of evolved magma-derived fluids and convecting sea water driven by the heat from the shallow active magma chamber. The Yamansu basalts provided the source of iron for the skarn mineralization. We envisage the submarine volcanism, skarn alteration and iron mineralization in the Yamansu iron deposit as a continuous process, different from either conventional intrusion-related skarn type or submarine volcanic exhalation sedimentation type.  相似文献   

10.
The May 12, 2008 Wenchuan, China Earthquake which measured Mw = 8.3 according to Chinese Earthquake Administration – CEA (Mw = 7.9 according to the USGS) directly triggered many landslides, which caused about 20,000 deaths, a quarter of the total. Rock avalanches were among the most destructive landslides triggered by this seismic event, and have killed more people than any other type of landslide in this earthquake. The Donghekou rock avalanche, one example of a catastrophic avalanche triggered by the Wenchuan earthquake, occurred in Qingchuan and buried one primary school and 184 houses, resulting in more than 780 deaths, and in addition, caused the formation of two landslide dams, which formed barrier lakes.Combining aerial images (resolution of 0.5 m) with field investigations, this paper lists some parameters of 66 cases in one table, and details source characteristics of six typical cases. It has been found that most of the long runout rock avalanches have source areas with high relief and steep inclination, causing the debris in the travel courses to accelerate. There was also a large amount of saturated Holocene-age loose deposits formed by a river or gully that existed in the travel courses. Comparison studies indicate that saturated Holocene loose deposits in the travel courses could be the most important factor for the causes of the long runout characteristic of the rock avalanches especially when they traveled over gentle or even flat ground surfaces.Furthermore, the relationships among the relief slope gradient, runout and covered area are investigated, and a threshold line for predicting the maximum horizontal runout distance under certain change in elevation is presented.  相似文献   

11.
Sinadinovsk  C.  Somerville  M.  McCue  K. F.  Talai  B. 《Natural Hazards》2001,24(3):285-294
Tsunamis originating in Indonesia and Papua New Guinea can be locally devastating and also occasionally threaten coastal areas of northern Australia. Tsunamis may be caused by earthquakes, volcanoes or submarine landslides. In Rabaul a unique set of records exists of tsunamis and seiches generated by tectonic earthquakes, volcanic earthquakes and a volcanic eruption. Spectral analysis of the digitised waveforms has been undertaken to compare the effects of the different sources and to estimate the characteristic natural periods of Rabaul Harbour. Seiches in Rabaul Harbourhave dominant periods of 20 to 30 min, a period band that is also typical for coastal recordings of tsunamis.  相似文献   

12.
The Neoproterozoic Wadi Ranga metavolcanic rocks, South Eastern Desert of Egypt, constitute a slightly metamorphosed bimodal sequence of low-K submarine tholeiitic mafic and felsic volcanic rocks. The mafic volcanic rocks are represented by massive and pillow flows and agglomerates, composed of porphyritic and aphyric basalts and basaltic andesites that are mostly amygdaloidal. The felsic volcanic rocks embrace porphyritic dacites and rhyolites and tuffs, which overlie the mafic volcanic rocks. The geochemical characteristics of Wadi Ranga volcanic rocks, especially a strong Nb depletion, indicate that they were formed from subduction-related melts. The clinopyroxene phenocrysts of basalts are more akin to those crystallizing from island-arc tholeiitic magmas. The tholeiitic nature of the Wadi Ranga volcanics as well as their LREE-depleted or nearly flat REE patterns and their low K2O contents suggest that they were developed in an immature island arc setting. The subchondritic Nb/Ta ratios (with the lowest ratio reported for any arc rocks) and low Nb/Yb ratios indicate that the mantle source of the Wadi Ranga mafic volcanic rocks was more depleted than N-MORB-source mantle. Subduction signature was dominated by aqueous fluids derived from slab dehydration, whereas the role of subducted sediments in mantle-wedge metasomatization was subordinate, implying that the subduction system was sediment-starved and far from continental clastic input. The amount of slab-derived fluids was enough to produce hydrous magmas that follow the tholeiitic but not the calc-alkaline differentiation trend. With Mg# > 64, few samples of Wadi Ranga mafic volcanic rocks are similar to primitive arc magmas, whereas the other samples have clearly experienced considerable fractional crystallization.The low abundances of trace elements, together with low K2O contents of the felsic metavolcanic rocks indicate that they were erupted in a primitive island arc setting. The felsic volcanic rocks are characterized by lower K/Rb ratios compared to the mafic volcanic rocks, higher trace element abundances (~ 2 to ~ 9 times basalt) on primitive arc basalt-normalized pattern and nearly flat chondrite-normalized REE patterns, which display a negative Eu anomaly. These features are largely consistent with fractional crystallization model for the origin of the felsic volcanic rocks. Moreover, SiO2-REE variations for the Wadi Ranga volcanic rocks display steadily increasing LREE over the entire mafic to felsic range and enriched La abundances in the felsic lavas relative to the most mafic lavas, features which are consistent with production of the felsic volcanic rocks through fractional crystallization of basaltic melts. The relatively large volume of Wadi Ranga silicic volcanic rocks implies that significant volume of silicic magmas can be generated in immature island arcs by fractional crystallization and indicates the significant role of intra-oceanic arcs in the production of Neoproterozoic continental crust. We emphasize that the geochemical characteristics of these rocks such as their low LILE and nearly flat REE patterns can successfully discriminate them from other Egyptian Neoproterozoic felsic volcanic rocks, which have higher LILE, Zr and Nb and fractionated REE patterns.  相似文献   

13.
In order to identify the mud diapirs and mud volcanoes off SW Taiwan, we have examined ∼1500 km long MCS profiles and related marine geophysical data. Our results show ten quasi-linear mud diapirs, oriented NNE–SSW to N–S directions. Thirteen mud volcanoes are identified from the multibeam bathymetric data. These mud volcanoes generally occur on tops of the diapiric structures. Moreover, the active mud flow tracks out of mud volcanoes MV1, MV3 and MV6 are observed through the high backscatter intensity stripes on the sidescan sonar images. The heights of the cone-shaped mud volcanoes range from 65 m to 345 m, and the diameters at base from 680 m to 4100 m. These mud volcanoes have abrupt slopes between 5.3° and 13.6°, implying the mudflow is active and highly viscous. In contrast, the flat crests of mud volcanoes are due to relative lower-viscosity flows. The larger cone-shaped mud volcanoes located at deeper water depths could be related to a longer eruption history. The formation of mud diapirs and volcanoes in the study area are ascribed to the overpressure in sedimentary layers, compressional tectonic forces and gas-bearing fluids. Especially, the gas-bearing fluid plays an important role in enhancing the intrusion after the diapirism as a large amount of gas expulsions is observed. The morphology of the upper Kaoping Slope is mainly controlled by mud diapiric intrusions.  相似文献   

14.
The Pb and Sr isotope ratios of Plio-Pleistocene volcanic rocks from the Aleutian volcanic arc are used as tracers of the lithospheric subduction process at the converging Pacific and Bering plates. Aleutian arc lavas do not have the same Pb isotopic compositions as volcanic rocks of the subducted Pacific ocean crust or the nearby Pribilof Islands, but appear to contain an ‘old continental crustal component’ with high 207Pb/204Pb ratio, as has been found in some other volcanic arcs.87Sr/86Sr ratios in the Aleutian volcanic arc rocks average 0.70322, slightly higher than fresh volcanic rocks from normal ridge segments, but within the range of values from ‘Icelandic’ ridge segments, oceanic islands and the Pribolof Islands. The Pb and Sr isotopic compositions of Aleutian lavas show a positive correlation and the range of values does not change for volcanoes distributed along strike in the arc, even though the crustal type in the hanging wall of the Benioff zone changes from oceanic in the west to continental in the east. Since the basement of the continental arc segment is older than the basement of the oceanic segment, and probably has a different isotopic character, the constancy of isotopic ratios along the arc argues against contamination by wall rocks of the type exposed in the arc.A sufficient explanation for the isotopic data is the mixture of several per cent of continent-derived sediment with melt derived from the underthrust oceanic crust and overlying mantle. This small amount of contaminant is difficult to document by geophysical observations. Such a model implies extensive recycling of Ba, Pb, K and Rb through volcanism at convergent plate margins like the Aleutians.  相似文献   

15.
New field work, in addition to zircon geochronology, Nd isotopes and reconnaissance geochemical data allow the recognition of Paleoproterozoic volcanic and metavolcanic sequences in the São Luís Craton of northern Brazil. These sequences record at least five volcanic pulses occurring probably in three distinct epochs and in different tectonic settings. (1) The Pirocaua Formation of the Aurizona Group comprises early arc-related calc-alkaline metapyroclastic rocks of 2240 ± 5 Ma formed from juvenile protoliths in addition to minor older crustal components. (2) The Matará Formation of the Aurizona Group holds mafic tholeiitic and ultramafic metavolcanic rocks of back arc and/or island arc setting, which are likely coeval to the Pirocaua Formation. (3) The Serra do Jacaré volcanic unit is composed of tholeiitic basalts and predominantly metaluminous, normal- to high-K calc-alkaline andesites of 2164 ± 3 Ma formed in mature arc or active continental margin from juvenile protoliths along with subordinate older (Paleoproterozoic) materials and associated to the main calc-alkaline orogenic stage. (4) The Rio Diamante Formation consists of late-orogenic metaluminous, medium-K, calc-alkaline rhyolite to dacite and tuffs of 2160 ± 8 Ma formed in continental margin setting from reworked Paleoproterozoic crust (island arc) with incipient Archean contribution. (5) The Rosilha volcanic unit is composed of weakly peraluminous, medium-K, calc-alkaline dacite and tuff formed probably at about 2068 Ma from reworked crustal protoliths. As a whole the volcanic and metavolcanic rocks record and characterized better the previously proposed orogenic evolution of the São Luís Craton.  相似文献   

16.
Rock avalanches are common in the Mont Blanc massif, which is bordered by valleys with large resident and tourist populations and important highways. This paper combines historical data with detailed geomorphological mapping, stratigraphic observation, and absolute and relative dating, to interpret several deposits resulting from rock avalanching onto glaciers.Nineteen rock falls and rock avalanches are described, ranging in volume from 10,000 m3 to 10 × 106 m3. They occurred between 2500 BP and AD 2007 at six sites. The events at three sites (Miage and Drus Glaciers, and Tour des Grandes Jorasses) are characterised by short travel distances; those at Brenva, Triolet, and Frébouge Glaciers exhibit excessive travel distances.Interactions between rock avalanches and glaciers are of four types: (i) rock-avalanche triggering, where glacial and paraglacial controls include debuttressing of rockwalls due to glacier thinning and retreat, oversteepening of rock slopes by glacial erosion, and effects of glaciers on permafrost; (ii) rock-avalanche mobility, in which mobility and travel distance are modified by channelling of rock-avalanche debris by moraines and valleys, incorporation of ice and snow (often >50% for large events), and irregularities on the glacier surface; (iii) deposit sedimentology, where melting of incorporated ice transforms the final deposit by reducing its thickness typically to <5 m, and debris of variable thicknesses is juxtaposed in a hummocky deposit with chaotic piles of angular rock debris; and (iv) glacier dynamics where insulating debris deposited upon a glacier produces a debris-covered glacier of different dynamics, and high elevated scars can favour the formation of small glaciers.  相似文献   

17.
《Gondwana Research》2014,25(1):338-357
Four isolated metamorphic complexes located within post-collisional granitoids occupying up to 70% of the total area, were distinguished in Sinai (Egypt) and Elat area (southern Israel), the northernmost part of the Arabian–Nubian Shield. The metamorphic rocks include metasediments, felsic and mafic metavolcanic rocks intruded by granitic, dioritic, and gabbroic plutons, all subjected to penetrative deformation.We present new SIMS U–Pb dating of zircons from 13 rock units comprising metasediments, volcanic rocks, gneisses and plutons from three metamorphic complexes (Sa'al, Feiran–Solaf, and Kid). In addition we present a SIMS U–Pb titanite age of a granitic gneiss previously dated using zircon. On the basis of the new and published U–Pb data, three successive Meso- to Neoproterozoic island arcs formed during a period of ca. 500 My are recognized. The Sa'al arc (represented by the oldest arc rocks in the ANS) evolved from 1.03 to 0.93 Ga (100 My); the Feiran–Elat arc developed from ca. 870 to 740 Ma (130 My), and the Kid arc formed from ca. 640 to 620 Ma (20 My). Evidence for an older, ca. 1.1 Ga, pre-Sa'al island arc was established from the zircon xenocryst population, though no exposures of rocks of this age were found. In the Sa'al and Kid arcs both volcanic and sedimentary rocks are preserved, whereas in the Elat–Feiran arc volcanic rocks are missing. We suggest that at ~ 700 Ma the Elat−Feiran arc was subjected to rifting that resulted in separating of the Qenaia block and its movement to the SE.  相似文献   

18.
The composite Zhaheba ophiolite complex, exposed in Eastern Junggar in the Southern Altaids, records an unusually long record of oceanic crust and magmatic arc evolution. The Zhaheba ophiolite complex consists of ultramafic rocks, gabbro, diorite, basalt and chert intruded by diabase dikes and diorite porphyry. These rocks are overlain by a several-km-thick section of tuffaceous rocks, volcaniclastic sedimentary rocks, and intermediate volcanic rocks. The igneous rocks of the ophiolite complex show negative Nb and Ta anomalies and LREE enrichment relative to HREE, suggesting the influence of fluids derived from a subducting oceanic slab. The LA-ICPMS U–Pb age of zircons from gabbro is 495.1 ± 3.5 Ma. Zircon ages from diorite and basalt are 458.3 ± 7.2 Ma and 446.6 ± 6.0 Ma, respectively. The basalt is locally overlain by bedded chert. Diabase dikes and diorite porphyry yield the U–Pb ages of 421.5 ± 4.1 Ma and 423.7 ± 6.5 Ma, respectively. The age of stratigraphically lower part of the overlying volcanic–volcaniclastic section is constrained to be about 410 Ma, the maximum depositional age of the tuffaceous sandstone from U–Pb detrital zircon ages. Late rhyolite at the top of the stratigraphic section yielded a U–Pb zircon age of 280.3 ± 3.7 Ma. The age and stratigraphic relationships for the Zhaheba ophiolite complex and related rocks suggest that the period of ~ 70 Ma of initial supra-subduction magmatism was followed by construction of a mature island arc that spanned an additional 140 Ma. Many other ophiolites in the southern Altaids appear to record similar relationships, and are represented as substrates of oceanic island arcs covered by island arc volcanism in supra-subduction zone. The occurrence of the Zhaheba ophiolite complex with tuffaceous and intermediate to felsic volcanic rocks is different from the rock association of classic Tethyan SSZ ophiolites but similar to some ophiolites in North America. Although the Zhaheba ophiolite belt is flanked by the Dulate arc in the north and Yemaquan arc in the south, it cannot stand a suture between two arcs. It is suggested that Devonian–Carboniferous Dulate arc was built on the late Cambrian–middle Ordovician Zhaheba supra-subduction oceanic crust. The late Carboniferous rocks and early Permian rocks in Dulate arc are interpreted to form in the extensional process within Zhaheba–Dulate arc composite system.  相似文献   

19.
Small Islands south off Hahajima, the southernmost of the Ogasawara Archipelago, consist of primitive basalts (<12 wt.% MgO) to dacite erupted during the transitional stage immediately following boninite volcanism on the incipient arc to sustained typical oceanic arc. Strombolian to Hawaiian fissure eruptions occurring on independent volcanic centers for the individual islands under a shallow sea produced magnesian basalt to dacite fall-out tephras, hyaloclastite and a small volume of pillow lava, which were intruded by NE-trending dikes. These volcanic strata are correlated to the upper part (<40 Ma) of the Hahajima main island. Volcanic rock samples have slightly lower FeO*/MgO ratios than the present volcanic front lavas, and are divided into three types with high, medium and low La/Yb ratios. Basalt to dacite of high- and medium-La/Yb types show both tholeiitic (TH) and calc-alkaline (CA) differentiation trends. Low-La/Yb type belongs only to TH basalt. The multiple magma types are coexistence on the each island. TH basalts have phenocrysts of olivine, clinopyroxene and plagioclase, while CA basalts are free from plagioclase phenocrysts.  相似文献   

20.
Seismic, sidescan sonar, bathymetric multibeam and ODP (Ocean Drilling Program) data obtained in the submarine channel between the volcanic islands of Gran Canaria and Tenerife allow to identify constructive features and destructive events during the evolution of both islands. The most prominent constructive features are the submarine island flanks being the acoustic basement of the seismic images. The build-up of Tenerife started following the submarine stage of Gran Canaria because the submarine island flank of Tenerife onlaps the steeper flank of Gran Canaria. The overlying sediments in the channel between Gran Canaria and Tenerife are chaotic, consisting of slumps, debris flow deposits, syn-ignimbrite turbidites, ash layers, and other volcaniclastic rocks generated by eruptions, erosion, and flank collapse of the volcanoes. Volcanic cones on the submarine island flanks reflect ongoing submarine volcanic activity. The construction of the islands is interrupted by large destructive events, especially by flank collapses resulting in giant landslides. Several Miocene flank collapses (e.g., the formation of the Horgazales basin) were identified by combining seismic and drilling data whereas young giant landslides (e.g., the Güimar debris avalanche) are documented by sidescan, bathymetric and drilling data. Sediments are also transported through numerous submarine canyons from the islands into the volcaniclastic apron. Seismic profiles across the channel do not show a major offset of reflectors. The existence of a repeatedly postulated major NE-SW-trending fault zone between Gran Canaria and Tenerife is thus in doubt. The sporadic earthquake activity in this area may be related to the regional stress field or the submarine volcanic activity in this area. Seismic reflectors cannot be correlated through the channel between the sedimentary basins north and south of Gran Canaria because the channel acts as sediment barrier. The sedimentary basins to the north and south evolved differently following the submarine growth of Gran Canaria and Tenerife in the Miocene.  相似文献   

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