Structural Outline Of Lake Baikal And The Venting Structures

Lake Baikal consists of three distinct bathymetric basins: the South Baikal (SBB), Central Baikal (CBB) and North Baikal basins (NBB). Each of these also corresponds to a well-defined sedimentary basin, bounded by steep border faults, which are best expressed along the northwestern side of the rift. The three basins are at least moderately asymmetric and tend to adopt a half-graben shape (Zonenshain et al., 1992 ; Hutchinson et al., 1992 ; Scholz et al., 1993). They are separated by active high-relief morphostructural ridges, or accomodation zones s.l.. The Selenga Delta Accomodation Zone consists of a broad bathymetric saddle between the SBB and CBB. Although the saddle is largely obliterated by the Selenga delta sediments, it is underlain by several elevated basement blocks (Hutchinson et al., 1992 ; Zonenshain et al., 1992 ; Colman et al., 1993 ; Scholz et al., 1993) (Fig. 1).

The area south of the Selenga delta is complicated in its central part by the Posolsky Bank, a morphological ridge, striking SW-NE, transverse to the basin shape (Fig. 2). It is assumed to be a large back-tilted fault block, belonging to the Selenga Delta Accomodation Zone (Scholz and Hutchinson, 2000) and separated at its south-eastern side from the SBB by a relatively narrow fault zone with a vertical displacement of about 900 m. The southeastern limit of the Posolsky Bank is delimited by a steep slope corresponding to the Posolsky fault transiting abruptly to the deep-water SBB at depths of about 1100 m. The basin floor gradually deepens to about 1300 m when reaching the steep slope of the main eastern border fault. The sediments of this deep basin are not affected by major faults (Logatchev, 1993 ; Zonenshain et al., 1992 ; Mats, 1993).

The venting area is located immediately south of the structural high of Posolsky Bank. It is delimited at its northern side by a small, secondary fault, antithetic to the Posolsky fault (Fig. 3) (Van Rensbergen et al., 2002). The venting structures are aligned almost parallel to the small antithetic fault (Fig. 2), which offsets the lake floor by about 20 m. Some of the venting structures themselves are also faulted.

The present or recent activity of the venting structures is testified by the freshness of their morphology as observed on side-scan sonar mosaics (Fig. 2), and by the presence of 10-25 m high acoustically non-transparent plumes in the water column above them or along their rims. The venting structures coincide with "gas chimneys"-or fluid-flow conduits- (Figs. 3 and 4) in the sub-surface that are observed on several seismic profiles (e.g. Van Rensbergen et al., 2002; 2003).

Figure 2. Location map of the study area within the SBB near the Selenga River delta, showing the bathymetry and the main faults (Scholz & Hutchinson, 2000; Mats et al., 2000). The area with inferred hydrate occurrences in the SBB and CBB (Golmshtok et al., 1997) is indicated in grey. Stars indicate areas with anomalously shallow and disrupted BSR and/or with mud volcanoes. A: General location of the study area within Lake Baikal. B: Available data from the Posolsky Bank study area. C. Side-scan sonar mosaic of the Pososlky Bank venting area, with clear indication of the venting structures (i.e. Bolshoy, Stari, Malyutka and Malenki).The venting area is located immediatelhy south of the structural high of Posolsky Bank. It is delimited at its northern side by a small, secondary fault, antithetic to the Posolsky fault (Fig. 3) (Van Rensbergen et al., 2002). The venting structures are aligned almost parallel to the small antithetic fault, which offsets the lake floor by about 20 m. Some of the venting structures themselves are also faulted.

Figure 2. Location map of the study area within the SBB near the Selenga River delta, showing the bathymetry and the main faults (Scholz & Hutchinson, 2000; Mats et al., 2000). The area with inferred hydrate occurrences in the SBB and CBB (Golmshtok et al., 1997) is indicated in grey. Stars indicate areas with anomalously shallow and disrupted BSR and/or with mud volcanoes. A: General location of the study area within Lake Baikal. B: Available data from the Posolsky Bank study area. C. Side-scan sonar mosaic of the Pososlky Bank venting area, with clear indication of the venting structures (i.e. Bolshoy, Stari, Malyutka and Malenki).The venting area is located immediatelhy south of the structural high of Posolsky Bank. It is delimited at its northern side by a small, secondary fault, antithetic to the Posolsky fault (Fig. 3) (Van Rensbergen et al., 2002). The venting structures are aligned almost parallel to the small antithetic fault, which offsets the lake floor by about 20 m. Some of the venting structures themselves are also faulted.

GAHY088

Figure 3. Seismic profile GAHY088 crossing the Stari and Malyutka venting structures, showing the irregular behaviour of the "BSR" and the presence of fluid-flow conduits adjacent to the small fault.

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