Offshore geological hazards in the Hellenic arc

Abstract

The Hellenic Arc is located within one of world's most seismically active areas and has experienced extreme tectonism through Tertiary and Quaternary times. This activity controls the rates of uplift and subsidence and determines the sediments supply and depot centers. This paper discusses the various geological hazards detected in selected parts of the Hellenic Arc and examines the causative factors. The areas surveyed were the North Aegean Trough, the Kythera Ridge in the outer island arc, the Corfu/Kefalinia/Zante shelf/slope, and the Thermaicos, Corinth, Patras, Amvrakia, and Malliacos gulfs, as well as the Trichonis Lake. The potential geological hazards observed are (1) active faulting, (2) sediment instabilities, (3) gas‐charged sediments, (4) salt doming, (5) erosion, transport, and deposition of sediments, and (6) seismicity, volcanism, and tsunamis. The major types of sediment instabilities that have been documented on seabed include (1) surficial sediment creep in slopes ranging from 1 to 2° resulting infolding and faulting of the surficial sediments, (2) translational and rotational slides in slopes ranging from 2 to 40°, (3) debris flow, and (4) turbidity currents. Factors that contribute to slope instability in the Hellenic Arc are (1) sloping bottom, (2) thick accumulations of Plio‐Quatemary sediments, (3) present day high rates of sedimentation, (4) closely spaced active faults, (5) earthquakes, and (6) active diapirism. The contribution of long period waves to slope failure in these areas appears to be of minor importance, since the slope failures occur in depths of more than 150 m. Gas‐charged sediments and pockmarks have been observed in areas associated with deltaic, fjord‐like, and open sea environments. This gas is presumably formed by the decomposition of biogenic material. Numerous disasters that took place during historical times and greatly affected the coastal zone were caused by the above‐mentioned hazards. Damage to offshore installations are limited to cable failure.     

Sedimentation processes and basin-filling depositional architecture in an active asymmetric graben: Strava graben, Gulf of Corinth, Greece

This paper presents data on the sedimentation processes and basin-fill architecture in an incipient submarine intrabasinal graben, the Strava graben. The Strava graben is a relatively small intrabasinal structure about 15 km long and 3 km wide formed some time during the late Pleistocene. It connects the Alkyonidhes basin to the Corinth basin, in the Aegean back arc, which is characterized by fast rates of extension and intensive seismicity. Analysis and interpretation of high-resolution 3.5-kHz and sparker profiles together with sonar imagery have shown that gravity-driven sediment transport, triggered by earthquakes, is the dominant sedimentation process and that this sediment forms the vast bulk of the basin-fill. The sediment deposited in the Strava graben is derived from the uplifted footwall blocks bounding the graben and is transported to the basin initially as liquefied flows, some of which may progressively evolve to turbidity flows. The deposits of the liquefied flows have accumulated in the graben floor as aggradational stacks, consisting of sheet-like, low-relief lobes, forming base of slope aprons that are fed by multiple sediment sources along active faults. In addition to the lateral (footwall-derived) sediment transport there is also a gravity-controlled axial transport. The axial transport has formed a depositional system in the down-dip termination of the Strava graben, where it enters the Corinth basin. The axial depositional system grows outwards and upwards and consists of liquefied flow depositional lobes which are separated by turbidites. The sedimentation transport processes and basin infilling style described for the Strava graben can be used as a predictive model for the early synrift stage of ancient submarine intrabasinal structures, in which the major sediment source area is the bounding fault scarps and not drainage basins in the hinterland.     

Seismic reflection imaging of active offshore faults in the Gulf of Corinth: their seismotectonic significance

High resolution seismic reflection surveys over one of the most active and rapidly extending regions in the world, the Gulf of Corinth, have revealed that the gulf is a complex asymmetric graben whose geometry varies significantly along its length. A detailed map of the offshore faults in the gulf shows that a major fault system of nine distinct faults limits the basin to the south. The northern Gulf appears to be undergoing regional subsidence and is affected by an antithetic major fault system consisting of eight faults. All these major faults have been active during the Quaternary. Uplifted coastlines along their footwalls, growth fault patterns and thickening of sediment strata toward the fault planes indicate that some of these offshore faults on both sides of the graben are active up to present. Our data ground‐truth recent models and provides actual observations of the distribution of variable deformation rates in the Gulf of Corinth. Furthermore they suggest that the offshore faults should be taken into consideration in explaining the high extension rates and the uplift scenarios of the northern Peloponnesos coast. The observed coastal uplift appears to be the result of the cumulative effect of deformation accommodated by more than one fault and therefore, average uplift rates deduced from raised fossil shorelines, should be treated with caution when used to infer individual fault slip rates. Seismic reflection profiling is a vital tool in assessing seismic hazard and basin‐formation in areas of active extension.     

Quaternary tectonics in the Gulf of Patras, western Greece

Air gun seismic and 3.5 kHz profiling data from the Gulf of Patras, western Greece, show that it is occupied by a small asymmetric graben with several geometric similarities to the larger-scale graben in the Gulf of Corinth to the east. Major listric faulting characterizes the southern flank of the graben whilst the northern flank represents an associated rollover structure affected by antithetic and synthetic faulting. The present phase of subsidence is of Holocene age, but buried growth faults suggest earlier subsidence in the Gulf. The average rate of subsidence through the Holocene is estimated to be 10 mm/year.The Gulf of Patras graben, together with the Gulf of Corinth graben and the Megara basin, represent a continuous system of WNW-ESE trending grabens in a broad zone of intense seismicity within the Aegean domain. Individual grabens are offset and are interconnected by NE-SW trending fault systems.     

A string of large and deep gas-induced depressions (pockmarks) offshore Killini peninsula, western Greece

A string of five large depressions (pockmarks) has been discovered offshore Killini peninsula in western Greece. They can be considered as some of the deepest (61-m maximum detected depth), if not the deepest observed on the seafloor world-wide. The pockmarks erode deeply buried compacted sediments and are modified by sliding. Their formation is attributed to gases which were detected in the subbottom sedimentary layers. Gas origin is considered to be both biogenic and thermogenic as gas was detected geochemically in exploratory wells in the neighbouring Katakolon area. Although uncertainties concerning their mode of formation, age and activity still remain, it is believed that the high seismicity and salt tectonics of the region contributed to the violent eruption and escape of large quantities of gases through small faults or joints which enabled erosion deep into the seafloor.     

Active seepage in two contrasting pockmark fields in the Patras and Corinth gulfs, Greece

Two pockmark fields, located along the coastal zone of the Patras and Corinth gulfs, Greece were surveyed in detail. The pockmark fields, which are 30 km apart, are formed in shallow waters at depths of 20–40 m and are about 0.5–1 km from the shoreline. The oceanographic data suggest that two different mechanisms were responsible for their formation. The pockmark field in the Patras Gulf appears to have been formed as a result of methane seepage from the seabed, whereas the field in the Corinth Gulf appears to have resulted from groundwater seepage.