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Maria V. Triantaphyllou Alexandra Gogou Margarita D. Dimiza Sofia Kostopoulou Constantine Parinos Grigoris Roussakis Maria Geraga Ioanna Bouloubassi Dominik Fleitmann Vassilis Zervakis Dimitris Velaoras Antonia Diamantopoulou Angeliki Sampatakaki Vassilis Lykousis 《Geo-Marine Letters》2016,36(1):51-66
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Sediment instabilities are common on the prodeltas of the seismically active continental margins of Western Greece. Sediment
failures on the low-angle (0.5°–2°) prodelta slopes manifest themselves as successions of peripheral rotational block slumps
restricted to the foresets of the late highstand systems tract (HST). The individual slump blocks are about 80–150 m long
and are bounded by growth faults acting as curved slip planes that extend to a mean depth of 10–15 m below seafloor. Shear
planes develop in the lower part of muddy and/or gas charged HST foresets. Deeper basal transparent muddy layers of the early
HST bottomset, together with the late Pleistocene transgressive systems tract sequences (TST), are mostly unaffected. On the
steeper (2°–6°) fan delta slopes of the western Gulf of Corinth debris flows and avalanches with a significant retrogressive component dominate
slope destabilisation. Sediment cores taken from landslide scarps and slide planes penetrated gas bubble releasing sediments
thereby indicating that failure planes are in the late HST foresets/upper part of the early HST bottomsets gas charged zone.
The foresets of the HST prodelta deposits display high water content (30–80%), low bulk density (1.4–1.9 g cm−3) and relatively low values of undrained shear strength (3–20 kPa). The water content of the HST distal muddy bottomsets is
relatively higher (50–110%) and bulk density relatively lower (1.3–1.7 g cm−3) with low values of shear strength (2–10 kPa). The shear strength of the gas releasing sediment layer displays lower values
(2–9 kPa) relative to the overlying, post failure, muddy sediments of the late 100–300 years. Slope stability was calculated
using the normalised soil parameter (NSP) method under undrained conditions for normally consolidated prodelta sediments.
This analysis indicates that instabilities could be induced by critical earthquake ground accelerations of 26.6–29.6% g for
the HST foresets and 12.4–14.1% g for the basal transparent layer belonging to the early HST bottomsets. Consequently the
early HST bottomsets has to be considered a potentially unstable layer since the regional peak ground accelerations (PGAs)
for the next 50 years are expected to range from 19 to 30% g. Moreover, our results show that new glide planes in the prodeltaic
sediment bodies of the seismically active continental margins of Western Greece will likely develop from the gas charged sediments
of the lower part of the HST foresets to the upper part of early HST bottomsets. 相似文献
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G.?Roussakis A.?P.?KarageorgisEmail author N.?Conispoliatis V.?Lykousis 《Geo-Marine Letters》2004,24(2):97-111
The vertical distribution patterns of grain-size parameters, carbonate and organic carbon contents, and clay mineral abundances were examined in ten sediment cores from basins of the northern Aegean Sea. Sedimentation rates for Holocene deposition were determined on the basis of 11 14C datings and indirectly from the age of the lower sapropel S1; they were estimated at 14.9, 18.9–21.8 and 34.7 cm 103 year–1 for the North Skyros, Athos, and North Limnos basins respectively. The sedimentation rates decrease gradually towards the southern basins, as a consequence of the greater distance from sediment supply sources. Also, sedimentation rates appear to decrease from the last glacial to Holocene units. The clay minerals illite, smectite, kaolinite and chlorite were identified in the cores. Generally, illite is the predominant mineral, showing a north–south-decreasing trend, followed by smectite; both minerals have a terrigenous origin, and smectite occurrence is higher in basins located near Limnos Island where volcanic formations prevail. Terrigenous illite is abundant in the sapropel S1, exhibiting the highest content of all sedimentological units. High illite content is unusual in eastern Mediterranean sapropels. This pattern is attributed to the proximity of the N. Aegean Sea basins to Balkan and Turkish land sources. In general, sediment mass gravity flow processes coupled with seasonal deposition from nepheloid layers are the predominant sedimentation mechanisms in the N. Aegean basins. The relative sea-level stand, the proximity to terrigenous sources (rivers), the morphology of the N. Aegean basins (small dimensions, isolated, steep slopes), as well as seismic activity and strong bottom currents are probably the major factors controlling sedimentation in the region. 相似文献
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Isidoros Livanos Paraskevi Nomikou Dimitris Papanikolaou Grigoris Rousakis 《Geo-Marine Letters》2013,33(6):419-431
A spectacular hummocky topography was discovered offshore of the south-eastern slope of the Nisyros island volcano in the eastern sector of the Aegean volcanic arc in 2000–2001, using multibeam bathymetric mapping and seismic profiling, and interpreted as part of a volcanic debris avalanche originating onland. During E/V Nautilus cruise NA011 in 2010, a detailed side-scan sonar and ROV exploration aimed at evaluating the surface morphology of this avalanche field. Combining the new data with selected older datasets reveals that the debris avalanche is characterized by numerous (at least 78) variously sized and shaped hummocks. Some of these are distinctly round, either scattered or aligned in groups, whereas others are elongated in the form of ridges. This is consistent with existing models accounting for variations in the longitudinal and lateral velocity ratio of landslides. Maximum dimensions reach 60 m in height above the sea bottom, 220 m in length and 230 m in width. The structures outline a large tongue-shaped, submarine hummock field of about 22.2 km2, approx. 4.8 km wide and 4.6 km long and with an estimated volume of 0.277 km3. Due to its characteristic shape, the collapsed volcanic flank is interpreted to represent a singular failing event, involving a rapid and virtually instantaneous downslope movement of the slide mass into the sea. Indeed, the H/L (height of 280 m vs. run-out of 7 km) ratio for the Nisyros slide is 0.04; plotted against volume, this falls within the theoretical bounds as well as measured values typical of submarine landslides. The timing of the event is probably related to the extrusion of Nikia lavas and their subsequent failure and formation of a main scarp observed at about 120 m depth on an 8-km-long seismic profile and a map of slope angle distribution, at the depth where the palaeo-coastline was located 40 ka ago. An inferred age of ca. 40 ka for the avalanche awaits confirmation based on dating of core material. 相似文献
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