The structural control of the methane venting area in the Southern basin of Lake Baikal, Siberia

Gas hydrates have been found in the near-bottom sediments within gas venting structures in the southern basin of Lake Baikal. The Baikal basin consists of three subbasins, separated by accommodation zones: the Academician Ridge between the north and central basins, the Selenga Delta Accommodation Zone between the central and south basins. The venting structures are located south of the Posolskiy Bank, which is a basement ridge with a thick sequence of sediments on top, trending NE, oblique to the main border fault. The Posolskiy Bank belongs to the Selenga Delta Accommodation Zone. It is delimited at its south-eastern side by the Posolskiy Fault with a displacement of more than 1000 metres. The venting structures are located close to an ENE trending fault, parallel to the Posolskiy Fault. Besides the fault which is delimiting the venting area at its northern side, and which has a vertical offset of its northern footwall of 25-30 metres, a fault with a vertical offset of its eastern footwall is not more than 5 metres. The venting area corresponds to a zone of local elevation of the lake floor, which is delimited at its northern side by the ENE trending fault and which is gently dipping towards SW in its southern part. It consequently appears as a zone of updoming probably related to a fluid and gas flow from depth, finally disrupted along its northern and eastern sides. The updoming is interpreted as the consequence of the disruption of the deep gas hydrate layer resulting from fluid overpressure. This interpretation is favoured by the regional pattern of the BSR, which shows strong variations in depth over this part of the south Baikal basin: it reaches minimum depths in the venting area and drops abruptly down to significantly larger depths north of the fault delimiting the venting area. The upward migration of the BSR is interpreted as a consequence of a regional fluid flow at depth, resulting in heat transport, consequent upward migration of the gas hydrates stability zone, updoming of the area and finally disruption along a fault zone. The origin of the fluid flow is discussed. Folding structures in the lake sediments along a NW-SE to WNW-ESE trend have been described in different parts of the central and southern basins. These folding structures are assumed to be related to left-lateral strike-slip movements along the eastern side of the basin. In this assumption, the NE trending fault associated with the venting structures and the spatial alignment of the structures themselves would correspond to the direction of extension associated with the left-lateral strike-slip, whereas the NW oriented direction of compression would correspond to the fluid flow