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Verbeeck, T. Camelbeeck, M. De Batist, F. Oggioni, O. Zurita Hurtado, W. Versteeg, H. Jomard, J. Collier, S. Gupta, A. Trentesaux, K. On April 6 one of the most destructive earthquakes of northwestern Europe took place in the Dover Strait Pas de Calais. The epicentre of this seismic event, the magnitude of which is estimated to have been about 6.
The location of this and two other moderate magnitude historical earthquakes in the Dover Strait suggests that the North Artois shear zone or some of its fault segments may be presently active. In order to investigate the possible fault activity in the epicentral area of the AD earthquake, we have gathered a large set of bathymetric and seismic-reflection data covering the almost-entire width of the Dover Strait.
These data have revealed a broad structural zone comprising several subparallel WNW—ESE trending faults and folds, some of them ificantly offsetting the Cretaceous bedrock. The geophysical investigation has also shown some indication of possible Quaternary fault activity. However, this activity only appears to have affected the lowermost layers of the sediment infilling Middle Pleistocene palaeobasins.
This indicates that, if these faults have been active since Middle Pleistocene, their slip rates must have been very low. Hence, the AD earthquake appears to be a very infrequent event in the Dover Strait, representing a good example of the moderate magnitude earthquakes that sometimes occur in plate interiors on faults with unknown historical seismicity.
Assessing the seismic hazard associated with plate-interior tectonic structures is generally a very difficult task. For instance, the seismicity in these areas is generally too low to be studied by means of classic seismotectonic approaches. It is thus necessary to adopt palaeoseismic methods in order to understand and characterise the activity of intraplate faults and hence the possible present-day seismic hazard related to them.
This study focuses on the poorly known offshore Sangatte Fault, which traverses the marine Dover Strait Pas de Calais from Sangatte northeastern France to Folkestone southeastern England; Figs 1 and 2. This fault is part of the North Artois shear zone Fig. However, nothing is presently known about any active fault located in the Dover Strait.
The Bouguer anomaly was calculated using the gravity database of the Royal Observatory of Belgium. The density reduction used for the calculation of the Bouguer anomaly on land was 2. Above the sea, the free-air anomaly was used. Apart from the AD earthquake, the Sangatte fault has only been associated with two other seismic events of moderate magnitude.
The presence of major infrastructures and the densely populated cities within the area of influence of an earthquake similar to the AD event make it very important to assess the nature of the Sangatte Fault and its tectonic activity over time. In order to do so, we have gathered a large set of seismic-reflection and bathymetric data from the epicentral area of this earthquake.
In this paper, we will present and discuss the interpretation of these geophysical data, which resulted in the first high-resolution bathymetric map of the Dover Strait and provided a clear visualization of the main tectonic structures and their activity over time. The present-day geometry of this fault zone from several phases of post-Palaeozoic deformation that have induced different reactivations of structures inherited from the Variscan Orogeny e.
Chadwick et al. Since its formation, the North Artois shear zone has passed from the original compressional setting Variscan Orogeny through extension related to the opening of the Tethyan and Atlantic ocean basins during Jurassic and Cretaceous times, and again to compression during the tectonic inversion that started in the early Palaeogene epoch due to the Alpine Orogeny Vandycke et al.
Based on their gravity map, Camelbeeck et al. Not only their activity, but the precise geometry of these faults is still poorly known. This is especially true for the mostly submarine Sangatte Fault, on which there has been no specific study published until now; despite the fact that it corresponds to the most likely geological structure capable of having generated the AD event e.
Camelbeeck et al. The Quaternary activity of the North Artois shear zone is still debated, as no conclusive field evidence of recent tectonic deformation has yet been associated with any of its fault segments. The latter suggest right-lateral strike-slip deformation, which is in good agreement with the NNW—SSE orientation of the maximum horizontal stress measured near Boulogne by Froidevaux et al.
On the offshore geological maps published in and by the British Geological Survey BGS and the compilation of geological data performed by James et al. Elsewhere Quaternary sedimentary features are mainly limited to a of major stable and minor mobile Holocene sandbanks Mellet et al. Recent tectonic movements affecting the latter features will not be preserved for long due to the dynamic conditions this area has been subjected to during the Holocene, with strong sediment reworking, erosion and sediment starvation e. Hamblin et al. On the other hand, if the Sangatte Fault reaches the surface and has ruptured several times in earthquakes similar to the AD since the Fosse Dangeard and the Lobourg Channel were formed, the cumulated deformation produced on these erosional features should be measurable.
White rectangle indicates the area shown in Fig. The yellow line represents the location of the Fosse Dangeard according to James et al. Merged bathymetry, single- dark blue lines and multichannel white lines seismic-reflection profiles gathered for this study, and boreholes archived at BRGM. Hb location of the single-channel seismic-reflection profile interpreted by Hamblin et al. The Fosse Dangeard is a kilometre-scale network of sediment-filled palaeobasins that are incised several tens of metres into bedrock e. Destombes et al. The origin of these buried palaeobasins is presently uncertain, although most authors agree that they were probably formed during the Middle Pleistocene.
Initially, they were attributed to glacial processes Destombes et al. Nevertheless, most of the recent studies seem to agree that none of the Pleistocene ice sheets extended that far south e. Clark et al. However, considering the amount of deformation necessary to create such large basins, similar offsets should be observable in Quaternary deposits along the on-shore continuation of these structures, which is not the case e.
Minguely et al. A tectonic origin of the fosse is thus very unlikely. Presently, it is widely accepted that these palaeobasins have been originally carved into the bedrock by a mega-flood produced during the breaching of the Weald-Artois ridge, which used to separate the southern North Sea from the English Channel Smith ; Gupta et al. Dating of the Fosse Dangeard is currently limited to pollen extracted from a 50 m borehole collected from one of its sediment-filled palaeobasins Destombes et al.
Based on this dating, Destombes et al. In addition, the sedimentary infill of these palaeobasins presents several erosional surfaces Destombes et al. These authors thus proposed that the Fosse Dangeard was formed and firstly filled-up with sediments during the Saalian glaciation 0.
This feature may actually be much older, as considering the mega-flood hypothesis postulated by Smith and Gupta et al. The exact age of this event is presently unknown. However, it is generally accepted e. Gibbard ; Toucanne et al. This palaeochannel extends to the southwest into a complex anastomosing system of valleys that can be mapped continuously from a few kilometres northeastward of the Dover Strait to the western approaches e. Smith ; Lericolais et al.
Presently, two main hypotheses are proposed to explain this major erosional feature. Some authors e. Gibbard ; Lericolais et al. Others authors have proposed that this feature may have been created by several episodes of Middle Pleistocene catastrophic flooding e. Smith ; Gupta et al. In any case, both groups of authors agree that the Lobourg Channel formed following the breach of the Weald-Artois ridge. This palaeochannel was probably active during all the major Middle and Upper Pleistocene marine low stands, being finally submerged at the end of the last glaciation—beginning of the Holocene epoch e.
This study is based on the interpretation of several sets of bathymetric and seismic-reflection data Fig. These campaigns consisted of the acquisition of 17 multichannel total line km and 48 single-channel total line km seismic-reflection profiles over the entire width of the strait and more than km 2 of multibeam bathymetric data from its central part. The seismic dataset mainly consists of parallel lines. This was due to limitations of collecting lines across the Dover Strait due to strong tides and shipping lane restrictions.
The high quality bathymetric data was therefore invaluable to correlate between the seismic profiles. Notably, the high resolution of this horizontal resolution 1.
Specific technical details on the vertical and horizontal resolutions of the geophysical data, as well as on their acquisition and processing, can be consulted in the electronic supplement attached to this paper. In this study, correlations between geology and seismic stratigraphy were first done in the northwesternmost seismic-reflection profile collected for this study and correlated afterwards southeastward from one profile to another.
The interpretation of the first profile is based on the equivalence established by Hamblin et al.
We complemented this interpretation with several interpreted boreholes Fig. Correlations between seismic profiles were performed using the few available cross-lines and the merged bathymetry. The latter proved invaluable in correlating between the seismic profiles, as many of the sedimentary formations form distinct geomorphological features on the seafloor that can be followed along the entire width of the strait see Figs 56 and 8.
The geological interpretation presented in this study is limited to the first m below the seabed, as we were not able to completely remove the first seabed multiple in the multichannel seismic-reflection data. Hence, we have very little information on the structure below the Wealden Beds of Lower Cretaceous age Fig. Structural map derived from the interpretation of the seismic reflection and multibeam bathymetric datasets collected for this study.Job dating nord pas de calais 2014
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PICUM Bulletin — 19 December