Место издания: Florence , Год издания: 2004

The Baikal Rift Zone is located in Siberia at the centre of the world's largest continental area. It offers a unique opportunity for studying the processes of intra-cratonic rifting. The BEST project (Baikal Explosion Seismic Transects) comprises two deep seismic profiles at the southern end of Lake Baikal. The field project was carried out in October 2003 after a pilot project in September 2002. The aim of the project is to determine the crustal and upper mantle seismic velocity structure. The two profiles are: (1) a 360 km long, NS-trending profile across the rift zone from the Mongolian border to Cheremhovo, and (2) a 360 km long EW-trending profile along-strike of the rift zone at the northern shore of the lake into the Tunka depression to the Mongolian border. 180 Texan one-component seismographs were deployed along each profile. The primary seismic sources were 13 explosions in boreholes, each with a 500 to 3000 kg charge. Supplementary sources were airgun shots in the lake (one 100 l airgun on profile 1, one 30 l airgun on profile 2) and the supervibrator located at Babushkin near the cross point between the two profiles at the shore of the lake. We present the preliminary modelling results of the BEST data from tomographic inversion of first arrival times and 2D ray tracing modelling of the seismic velocity structure of the crust and uppermost mantle. The models show: (1) A pronounced sedimentary graben structure to depths of at least 10 km; (2) No significant crustal thinning below the rift zone; (3) Strong seismic reflectivity underneath the rift zone, indicative of pronounced magmatic intrusion into the lower crust, despite of the non-volcanic appearance of much of the rift zone; (4) No sign of significant reduction of the seismic Pn wave velocity; and (5) No clear indication of continuation of the cratonic crust underneath the rift zone in disagreement with a recently published model of the rifting process. Instead the model is in agreement with a model in which the rift zone developed in the suture between the Siberian Craton and the Palaeozoic fold belt to the SE of Lake Baikal.