Sedimentary infill at the western tip of the GulfofCorinth during the last 120 ka: Evidence for an acceleration of the subsidence
Arnaud Beckers 1, 2, @ , Aurélia Hubert-Ferrari 1, @ , Christian Beck 3, @ , Dimitri Sakellariou 4, @ , Efthymios Tripsanas 4, @ , Marc De Batist 5, @
Characteristics and frequency of large submarine landslides at the western tip of the GulfofCorinth based on a grid of 600 km high-resolution seismic profiles
Aurelia HUBERT-FERRARI1, Arnaud BECKERS2, Christian BECK3, George PAPATHEODOROU4, Marc de BATIST5, Dimitris SAKELLARIOU6, Efthymios TRIPSANAS7 & Alain DEMOULIN1
5.2. Along strike changes in rift structure and rift polarity
Several studies have been devoted to the mapping of offshore faults in the GulfofCorinth ( Stefatos et al., 2002; Moretti et al., 2003; McNeill et al., 2005a; Lykousis et al., 2007a; Sakellariou et al., 2007a, 2007b; Bell et al., 2008, 2009; Taylor et al., 2011; Charalampakis et al., 2014 ). This study at the western tip of the Gulf results in the completion of the fault map, up to the Rion sill to the west ( Fig. 15 ). The relatively low penetration of acoustic waves used in this area does not allow us to es- timate the age of the deformation. However, the observed structure and the spatial changes in rift polarity over the Late Quaternary period (0 –130 ka) can be compared to the ones described in the other parts of the rift. At a regional scale, the present physiography of the Gulf implies an asymmetry between the northern subsiding coastline and the southern uplifting margin. This asymmetry has been con ﬁrmed at the time scale of the whole gulf history by the southward dip of its base- ment ( Taylor et al., 2011 ). However, the Late Quaternary isochore map (3rd sequence of Taylor et al., 2011 ) and sea ﬂoor morphology show a more complex pattern of subsidence with several along strike changes in rift polarity ( Fig. 15 ).
A peculiar characteristic of the western GulfofCorinth is the background microseismic activity, which appears very intense and concentrated in a narrow band at 6–10 km depth and is distributed along a trend dipping approximately 15 ◦ northwards (Rigo et al. 1996). Rigo et al. (1996) have postulated that this microseismicity is related to either a low-angle active fault or a low-angle detach- ment zone lying at 9–11 km depth. In this model, Rigo et al. (1996) suggest that microseismicity mainly occurs at the intersection be- tween the major steeper normal faults that crop out in the south- ern part of the gulf and the low-angle north-dipping detachment zone. The extension of major steep north-dipping faults down to 8– 12 km depth was also hypothesized by Doutsos & Poulimenos (1992), following the model suggested by King et al. (1985) for the eastern GulfofCorinth. On the contrary, Sorel (2000) proposes a model in which an active low-angle detachment fault (the Kelmos fault) represents the major extensional structure of the rift, whereas the other steeper normal faults cropping out on the Peloponnesus are only secondary listric structures, which branch the master fault at a relative shallow depth. The presence of active low-angle normal faults beneath the gulf has been confirmed by both cluster anal- ysis of microearthquakes (Rietbrock et al. 1996) and aftershock studies of large events, such as those from the Galaxidi earthquake (Hatzfeld et al. 1996) and the Aigion earthquake (Bernard et al. 1997). However, in the rotating domino model proposed by Hatzfeld
The study area is the GulfofCorinth, Greece. It is a 120 km long, 869 m deep depression located in central Greece (Fig. 1). This basin is connected to the Mediterranean Sea at its western tip through a 62 m deep, 2 km wide sill. Numerous damaging EQs are known in GulfofCorinth region for at least 2000 years by historical sources (Fig. 1), with largest estimated magnitudes approaching 7 (Ambraseys & Jackson, 1997). In the western part of the Gulf, that is investigated in the present study, tsunamis and submarine landslides have been triggered by historical EQs (Schmidt, 1887; Papatheodorou & Ferentinos, 1997), but these phenomena also happened without any EQ (Heezen et al., 1966). Only one study has been dedicated to the identification of offshore historical (before 1990) earthquake-related deposits in this area (Lykousis et al., 2007). These authors investigated the basin floor of the Western Gulf with short box cores and identified a widespread 6-10 cm thick sandy deposit attributed to the 1861 earthquake. This result highlights the potential of this area for developing offshore paleoseismological methods.
the topset and the foreset beds (e.g., the 1963 slide in the Er- ineos fan delta).
5.3 Significance of the sliding events
The data suggest that large submarine landslides have been triggered during six short periods of time over the last 130 ka. These sliding events include variable numbers of clustered MTDs, from one (SE F) to 8 (SE A). During three sliding events (C, D, F), a particularly large MTD accumulated at the basin floor, and it has been shown that these large MTDs resulted from several possibly synchronous slope failures. Similar MTD distributions have been observed in lakes in the Alps and in the Chilean Andes (Strasser et al., 2013; Moernaut et al., 2007). In these studies, the correlation of MTDs into a same sliding event was supported by radiocar- bon dating and a simultaneous triggering has been proposed. Correlations between the mass wasting records of neighbor- ing lakes and the historical seismicity revealed that most of these sliding events had been triggered by large earthquakes (Strasser et al., 2006; Moernaut et al., 2007). In the western- most GulfofCorinth, neither coring nor dating is available to confirm our correlations between MTDs. Moreover, the oc- currence of frequent turbidity currents (Heezen et al., 1966; Lykousis et al., 2007) and small-scale submarine landslides perturbs the sediment layering and induces discontinuities in the seismic reflections, which makes MTD correlations based on the seismic stratigraphy less accurate there than in many lakes.
Characteristics and frequency of large submarine landslides at the western tip of the GulfofCorinth
Arnaud Beckers (1), Aurélia Hubert-Ferrari (1), Christian Beck (2), George Papatheodorou (3), Marc de Batist (4), Dimitris Sakellariou (5), and Alain Demoulin (1)
By extrapolating an average sedimentation rate to hemipelagic in- tervals downcore, we also extrapolated the associated uncertainties. This gave very large ranges of ages for event deposits located at the base of the cores. To reduce this uncertainty, we tested the use of a par- ticular event deposit that has been observed in every core as an anchor point. This event deposit is dated from the 19th century, and is assumed to be related to a major earthquake. The 1861 CE Aigion earthquake is the largest reported earthquake in and around the study area. This earthquake is the only one with a large surface rupture, and also the only one that induced high macroseismic intensities (larger than or equal to VII), both on the northern and on the southern coast of the western GulfofCorinth (see the macroseismic map in the Supplemen- tary Materials). It also triggered numerous liquefactions and coastal landslides in the Aigion area ( Schmidt, 1879 ), and a sediment density ﬂow in the basin ﬂoor ( Lykousis et al., 2007 ). For these reasons, it is as- sumed that event deposits associated with this earthquake are wide- spread in the study area. Consequently, in each core where an event- deposit likely related to this earthquake was found, this event-deposit was used as an anchor point to improve the age-depth curves. Thus, for each coring site, two age-depth curves are presented: the ﬁrst is only based on 210 Pb
Figure 1. Bathymetry map and shot lines. Bathymetry data were collected during the cruise using the Hydrosweep DS2 multibeam sonar of the R /V Ewing. Contour interval for bathymetry is 50 m. Heavy black lines, which are actually closely spaced dots, show shot points used in this study. Every 100th shot point is identified (numbers next to white circles). Numbers within squares identify the lines. Thin black lines beneath, and extending beyond, the shot points show the extent of each 2-D model. X symbols near the bottom of the plot represent the model origins for the north–south lines; the origin for line 1 is off this map (at 22.20292 ◦ E, 38.33004 ◦ N). GOC, GulfofCorinth; GOI, Gulfof Itea. Inset map shows the regional setting for this study. Rectangle marks region of main map. Broken white line shows trend of the Hellenic mountain belt. Numbered arrow indicates direction and rate (mm yr −1 ) of extension in the Aegean region relative to Eurasia. The African Plate, not shown on this map, is moving north at approximately 6 mm yr −1 relative to Eurasia (McClusky et al. 2000). A, Athens; NAF, North Anatolian fault.
S U M M A R Y
A multichannel seismic and bathymetry survey of the central and eastern GulfofCorinth (GoC), Greece, reveals the offshore fault geometry, seismic stratigraphy and basin evolution of one of Earths most active continental rift systems. Active, right-stepping, en-echelon, north- dipping border faults trend ESE along the southern Gulf margin, significantly overlapping along strike. The basement offsets of three (Akrata-Derveni, Sithas and Xylocastro) are linked. The faults are biplanar to listric: typically intermediate angle ( ∼35 ◦ in the centre and 45–48 ◦ in the east) near the surface but decreasing in dip and/or intersecting a low- or shallow-angle (15–20 ◦ in the centre and 19–30 ◦ in the east) curvi-planar reflector in the basement. Major S-dipping border faults were active along the northern margin of the central Gulf early in the rift history, and remain active in the western Gulf and in the subsidiary Gulfof Lechaio, but unlike the southern border faults, are without major footwall uplift. Much of the eastern rift has a classic half-graben architecture whereas the central rift has a more symmetric w- or u-shape. The narrower and shallower western Gulf that transects the >40-km-thick crust of the Hellenides is associated with a wider distribution of overlapping high-angle normal faults that were formerly active on the Peloponnesus Peninsula. The easternmost sector includes the subsidiary Gulfs of Lechaio and Alkyonides, with major faults and basement structures trending NE, E–W and NW. The basement faults that control the rift architecture formed early in the rift history, with little evidence (other than the Vrachonisida fault along the northern margin) in the marine data for plan view evolution by subsequent fault linkage. Several have maximum offsets near one end. Crestal collapse graben formed where the hanging wall has pulled off the steeper onto the shallower downdip segment of the Derveni Fault. The dominant strikes of the Corinth
3 Laboratoire de Sismologie Exp´erimentale, D´epartement de Sismologie, Institut de Physique du Globe de Paris, Paris, France
Accepted 2005 March 14. Received 2004 December 19; in original form 2004 May 5
S U M M A R Y
The GulfofCorinth (GOC), Greece is a continental rift with high rates of seismicity and ex- tensional strain. How this strain is accommodated in the crust and whether there are variations in the mechanism along strike remain open questions, in part because of a lack of wide-angle reflection/refraction studies that constrain crustal velocity structure. In 2001, an extensive mul- tichannel seismic survey was conducted within the GOC, one component of which included the wide-angle recording of sources from within the gulf at stations on land surrounding the gulf. In this paper we use wide-angle data in two separate, but allied, studies to constrain crustal velocities and depth to the Moho. A 2-D inversion of refraction and reflection traveltimes along an axial profile through the GOC constrains the shallow crustal velocity structure, images the Moho at 29 km depth in the east, dipping to 39 km in the west, and images the eastward subducting African slab beneath the western GOC at a depth of 74 km. The 1-D average of the 2-D velocity model was used in a tomographic inversion of PmP reflection times to solve for depth to the Moho throughout the Corinth region. This model shows generally thick, isostati- cally compensated crust (≥37 km) beneath the Hellenide Mountains, except immediately south of the GOC, and a singular region of thin crust ( <30 km) beneath the Perahora Peninsula at the eastern end of the gulf. A comparison with Moho depths derived from gravity inversion shows a general agreement with crust thickening from east to west, but a number of differences in detail. The 3-D crustal thickness variations are more complex than those predicted by either pure shear or simple shear models of continental extension and suggest significant pre-rift structural variability.
3.2. Simulation for a Convective Karst
[ 33 ] The effect of the convection in the karst below the
fault is studied by reusing the numerical simulation detailed in the auxiliary material. Intuitively, because of the convec- tion, the temperature within the karst is uniform. This isothermal volume disturbs the isotherms around the karst. In the configuration of Figure 2, this would induce a local increase in temperature gradient in the hanging wall. The convection in the karst is modeled by assigning a thermal conductivity k = 100 W/(K m). In practice, the exact value of this thermal conductivity is not important: the karst is isothermal and its temperature is controlled by the heat flow at the base of the model and the temperature at the surface. To model the convection in the karst, we do not add any parameters.
22 In general, these methods have also been partially successful as they rely on high-precision local data collocated in space and time, which are not always available for the times of SAR acquisitions. However, they are better suited for estimating the turbulent and coherent short-scale component of the tropospheric term than phase-based methods, and as such produce better results. Each method has its strengths and weaknesses, for example GNSS ground stations in most areas are sparsely distributed and tropospheric delay data can only be used at the exact location of each station, especially where there is significant topographical and/or meteorological variability. It is usually not adequate to use GNSS data as a standalone correction technique, but rather in combination with additional datasets, e.g. spectrometer measurements [Puysségur et al., 2007]. Also, spectrometers can only provide precise data under cloud-free and daylight conditions. However, their ability to measure Precipitable Water Vapour (PWV) accurately at high spatial resolution (250-1000 meters) makes them a highly efficient technique, under suitable conditions. The MERIS PWV accuracy has been estimated close to 1 mm, equivalent to 6 mm of Zenith Wet Delay (ZWD) for each epoch, or 9 mm between two epochs [Li et al., 2006b]. This is equivalent to approximately 1 cm in radar line-of- sight for ENVISAT data with an incidence angle of 23°. With respect to MODIS, PWV accuracy has been estimated at best equal to that of MERIS, and at worst twice that of MERIS [Bekaert et al., 2015b].
Pierre Briole , Simon Bufféral, Dimitar Dimitrov, Panagiotis Elias, Cyril Journeau, Antonio Avallone, Konstantinos Kamberos, Michel Capderou, and Alexandre Nercessian
Abstract—We assess the accuracy and the precision of the
TanDEM-X digital elevation model (DEM) of the western GulfofCorinth, Greece. We use a dense set of accurate ground coordinates obtained by kinematic Global Navigation Satellite Systems (GNSS) observations. Between 2001 and 2019, 148 surveys were made, at a 1 s sampling rate, along highways, roads, and tracks, with a total traveled distance of ∼25 000 km. The data are processed with the online Canadian Spatial Reference System precise point positioning software. From the output files, we select 885 252 co- ordinates from epochs with theoretical uncertainty below 0.1 m in horizontal and 0.2 m in vertical. Using specific calibration surveys, we estimate the mean vertical accuracy of the GNSS coordinates at 0.2 m. Resampling the DEM by a factor of 10 allows one to compare it with the GNSS in pixels of metric size, smaller than the width of the roads, even the small trails. The best fit is obtained by shifting the DEM by 0.47 ± 0.03 m upward, 0.10 ± 0.1 m westward, and 0.36
Keywords: Delphi / GulfofCorinth / archaeology / building materials
Résumé – Les pierres du sanctuaire de Delphes, Marge nord du Golfe de Corinthe, Grèce. Le choix des pierres de construction par les anciens Grecs reste à ce jour une question ouverte. Si l’emploi de matériaux locaux semble dominer, les faciès allochtones sont aussi présents. Ce travail propose une vue exhaustive des pierres mises en œuvre dans le sanctuaire de Delphes. Situé dans la zone du Parnasse, au pied d’une faille normale liée à l’ouverture du Golfe de Corinthe, le sanctuaire d’Apollon est majoritairement construit en calcaires, brèches et marbres. Des sédiments peu consolidés, récents, d’origine et de faciès variés appelés pôros dans les sources textuelles antiques, appellation souvent reprise dans la littérature moderne, sont également utilisés. L ’identiﬁcation des différents faciès a été menée sur le site, ainsi que dans les carrières locales, dans le but de retrouver la provenance des différents matériaux. Les calcaires sont clairement locaux et proviennent de la plate-forme carbonatée, d’âge Jurassique Supérieur à Crétacé, de la Téthys, reprise en compression lors de la formation de la chaîne des Hellénides. Ces calcaires du Massif du Parnasse correspondent à la majeure partie du volume de roche mis en œuvre ; un faciès spéciﬁque de calcaire maastrichtien, appelé le calcaire de Saint-Élie, a été utilisé pour les édiﬁces les plus prestigieux tels
Accepted 2018 June 28. Received 2018 February 9; in original form 2018 June 26
S U M M A R Y
We analyse the complete earthquake archive of the western Corinth Rift using both cross- correlations between pairs of event waveforms and accurate differential traveltimes observed at common stations, in order to identify small-scale fault structures at depth. The waveform database was generated by the dense Corinth Rift Laboratory network and includes about 205 000 events between 2000 and 2015. Half of them are accurately relocated using double- difference techniques. The novelty of this relocated catalogue is the integration of the recent westernmost earthquakes due to the extension of the network in 2010 to the western extremity of the Corinth Rift and the consideration of the whole database over more than 15 yr. The total relocated seismicity exhibits well-defined clusters at the root of the main normal faults mainly between 5 and 10 km depth in the middle of the gulf and illuminates thin active structure planes dipping north about 20 ◦ under the northern coast. Some seismicity is observed in the footwall of the main active faults, along the West and East Helike faults. We also built a multiplet database based on waveform similarity taking into account cross-correlation coefficients weighted by signal-to-noise ratios. Short-term multiplets are concentrated in the middle of the gulf along the Kamarai fault system, in a 1–2 km thick layer at 6–8 km depth, interpreted as a highly fractured geological layer. They are often associated to slow seismic migration velocities occurring in this zone during strong swarm episodes and are thus likely to be triggered by pore pressure variations. On the other hand, most long-term and regular multiplets are located deeper (7– 10 km), under the northern coast, within a layer less than 0.3 km thick. They occur at the border of nearly planar structures with low seismicity rate, which we identify as fault planes, and they may be
that the study area consists of sandy sediments with low organic matter content. Ben- thic respiration was well correlated to the chlorophyll a content of the top 1 cm of the sediment cores. Evidence was found for dissolution of CaCO 3 due to the acidification
of superficial sediments in relation to the production of CO 2 and the oxidation of H 2 S
We do not have reliable sea-level estimates for older features along the margin, but a first-order estimate can be made for the Arabian crystalline basement. The land surface at the northern end of the Red Sea was near to paleo-sea-level at the beginning of rifting ~23 Ma, as shallow marine deposits are recorded shortly after rift ini- tiation 6 . Most of the pre-rift stratigraphic section had been previously eroded away in the area where the Gulfof Aqaba would develop, with minor occurrences preserved in local fault-bounded inliers 51 . Although Red Sea rift shoulder uplift began soon after rift initiation 52 , 53 , the region of the future Aragonese Deep was tens of kilome- ters in-board from the active rift margin. We assume that the basement in the central Gulfof Aqaba remained nearly at sea level into the Middle Miocene, with minor uplift and erosion keeping pace. In Fig. 8 the high corner of the Aragonese Deep footwall block that was removed by erosion during post-Aqaba rift-initiation uplift has been restored. This is a simple projection of the rift shoulder that reflects the flexural rigidity of the crust and is a minimum estimate of the material that has been removed. The interpreted 1.8 km of uplift occurred since onset of strike-slip and associated secondary extensional movement on the Gulfof Aqaba transform boundary at 14–11 Ma. Paleo-sea-level at 14–11 Ma was thought to be no more than ~50–100 m above present-day 54 though more recent analyses suggest it may have only been ~25 m above present 55 . We therefore decrease our net tectonic uplift by 100 m to 1.7 km to cover the more extreme scenario. Using the range of initiation ages for the Gulfof Aqaba faulting (14–11 Ma) uplift rates of 0.012–0.016 cm/yr are obtained.
A major onshore implication of the northward migration of the southern rift margin was the emergence of successive tracts of land in the footwall of the active faults. There, up to 2800-m-thick synrift sediments have been uplifted and, subsequently, exposed to erosion (Rohais et al., 2007a). The resulting geomorphological sequence of the northern Peloponnese displays, from south to north (1) drainage networks strongly incised within former uplifted footwall blocks, with deep narrow gorges (e.g., the Vouraikos gorge) and valley depths of 500 to 800 m; (2) characteristic braided channels in the middle and lower river reaches, transferring huge amounts of sediments delivered by the eroding uplifted deltas; (3) coastal areas extending a few kilometres inland and showing poorly developed drainage systems and limited incision in the uplifting footwall of the currently active rift margin; and (4) development of large fan deltas along the modern shoreline. Owing to the proximity of the crestline of the northern Péloponnèse, distant by less than 50 km from the coastline, and to footwall backtilting, the river catchments are limited in size. With a length of 49 km and a catchment area of 366 km 2 , the Selinous is the largest Peloponnesian river draining to the Corinthgulf. Backtilting also caused drainage reversal toward poljes developed in the limestone basement cropping out in the south (Feneos, Stymfalia), forming endorheic basins at the expense of the north-flowing rivers (Rohais et al., 2007a). Furthermore, karstic phenomena are also present in the catchments of some of these rivers (e.g., the Asopos), significantly reducing discharge and stream power in the concerned reaches.