Gas migration in Holocene sediments in Greece identified by very-high-resolution seismic data

International audience

English. Gases, and fluids in general, play a key role in tectonically active areas and sedimentarybasins, thus, their characterisation and quantification are crucial. This case study is locatedin Central Greece, to the north of the Corinth rift, within the Sperchios – North-EviaGulf rift. This very young rift ( ˜3.6 Ma to present-day) results from the Aegean back-arcextension initiated by the African slab roll-back. Moreover, the westward propagation of theNorth-Anatolian Fault into the Aegean Sea could influence the recent deformation withinthe rift system. Numerous hydrothermal systems have been documented inland, especiallythree very hot springs (> 60◦C) in North Evia and three warm springs (30-40◦C) alongthe Sperchios Basin in continental Greece. These CO2-rich thermal springs present intensebubbling and important travertine formation. According to recent studies, the very hotsprings are related to the local Plio-Pleistocene volcanic edifice belonging to the active Aegeanvolcanic arc. The warm springs are located along one of the major fault systems of the rift.No gas accumulation or emission have been reported offshore so far. From a recently acquiredvery-high-resolution seismic dataset, we examined the seismic markers of gas trapped in therecent sediments within the rift. The gas-related structures in the Sperchios – North-EviaGulf rift can be classified in three categories: (1) gas accumulations characterised by enhancedstratigraphic reflectors, acoustic turbidity and blanking, (2) gas chimneys recognized fromlocalized enhanced reflections, acoustic blanking columns and hyperbole artefacts, and (3)gas leakage along fault planes highlighted by blanking, hyperbole artefacts and enhancedreflectors. Moreover, water column echoes suggest gas is released in the water column.Most of these gas-related features are present within the main Holocene depocentres. Ahigh concentration of these structures appears within the Kandili sub-basin, where somepublished high-resolution tomographic data reveal the occurrence of a magmatic chamber atc. 8 km depth. Thus, the gas in this area could be thermogenic and related to this magmaticbody. The gas leakages related to faults take place along E-W-striking faults. In the region,this direction corresponds to the youngest normal faults triggered by the N-S extension ofthe Aegean Domain.