South Shetland margin BSR

The first evidence of BSRs along this margin was documented by Lodolo et al (1993), on multichannel seismic data acquired in 1989. In 1997, a high-

resolution multichannel seismic survey was designed to purposely map the BSRs distribution and study its nature (Lodolo et al., 1998; Tinivella et al., 1998). Seismic data analysis has shown that the BSR, which bears the characteristic of reverse polarity with respect to the seafloor reflector, is widespread in NE sector of the South Shetland margin, and is present in water depths ranging from 1000 to 4800 m; sub-bottom two-way travel time of the BSR generally varies from 500 ms at the shallowest water depth surveyed to 900 ms at a depth of 4800 m in the vicinity of the South Shetland trench. The BSR occurs in that part of the margin characterised by a very complex geological setting (see above).

i mn

Fig. 6 - Part of multichannel seismic profile (unmigrated) acquired on the Wilkes Land margin showing the presence of a prominent BSR cross-cutting a possible sediment drift (data from the SCAR Antarctic Seismic Data Library System). Location in Fig. 2.

i mn

Fig. 6 - Part of multichannel seismic profile (unmigrated) acquired on the Wilkes Land margin showing the presence of a prominent BSR cross-cutting a possible sediment drift (data from the SCAR Antarctic Seismic Data Library System). Location in Fig. 2.

The BSR is superimposed on the sedimentary configuration of the margin (Fig. 7) but it shows a significant difference in strength and lateral continuity in the surveyed area. In areas where the surface geology is dominated by folding, the BSR strength is low, but lateral continuity is preserved. This suggests that the distribution of temperature and pressure has reached an equilibrium to allow for a smoother base of the hydrate stability zone. In areas where faults of the superimposed extensional tectonic regime cut across older compressional structures, the BSR is stronger but less continuous and apparently offset by such faults. A structural discontinuity oriented North to South seems to separate the two zones of BSR with different strength. It is therefore suggested that a direct relationship exists between abundance of gas hydrates and structural complexity, which may in some ways influence the movements of fluids responsible of free gas accumulations and gas hydrate formation.

Reflection tomography techniques applied to some profiles from this region (see Carrion et al., 1993, for a rigorous description of the tomographic method), indicate a velocity trend from the seafloor to the BSR generally consistent with that of normally compacted marine sediments, with an abrupt decrease between the BSR and the underlying normal polarity reflector indicating presence of free gas in the sediment pore spaces (Base of Gas Reflector, BGR, Böhm et al., 1995). The calculated thickness of this gas-bearing layer is approximately 50 m. Conversely, local increments of compressional wave velocity above the BSR can be related either to cementation of normally compacted slope basin sediments by gas hydrate or to overcompaction in accreted sediments (Tinivella et al., 1998).

Was this article helpful?

0 0

Post a comment