Present-day stress field changes along the Baikal rift and tectonic implications

Intraplate extension, in a frame of a global compressional stress field, seems linked to local lithospheric perturbations (lithospheric thinning or thickening) able to modify the resulting state of stress [Zoback, 1992]. The Baikal Rift Zone (BRZ), Siberia, is located north of the India-Asia collision zone and exhibits no direct communication with any oceanic domain. It can thus be fully considered as an area of continental extension, dominated by the ''global compressional intraplate stress field'' resulting from plate driving forces. In order to address the problem of its dynamics and kinematics and their links with the India-Asia collision, a comprehensive: stress tensor analysis is presented, based on 319 focal mechanisms of earthquakes located along the whole Baikal rift. The stress field is varying at different scales of observation: when looking at central Asia (several thousands kilometers), the maximum horizontal stress S-Hmax directions remain rather constant (with a fan-shape geometry) when the tectonic regime goes from compressional (Himalayas) to extensional (Baikal). When observing the Baikal rift (about 1000 Km long), clear variations of the stress regime are observed, from an extensional regime in the central part of the rift to wrench ones in its northern and southern ends. Finally, at the scale of 100 km, systematic S-Hmax reorientations occur close to major rift faults. We thus infer that the interaction between collisional processes and inherited structures may have a strong influence on rift dynamics. We then use computed stress tensors to predict slip vectors on major rift faults. Deformation patterns show two distinct parts of the rift: the South Baikal Rift (SBR) is characterized by a constant trending (around N100 degrees E) slip vector, meanwhile the North Baikal Rift (NBR) exhibits a complex block rotation behavior involving at least three crustal blocks. We propose to interpret these surficial structures and motions as the result of an interaction between-the regional compression coming from the India-Asia collision and the geometry of the hardly deformable Siberian platform. This particular setting can explain most of the surficial deformation patterns, which suggest a large-scale cracking of the lithosphere in the Baikal region. Other possible sources of stress could also be considered, like deep mantellic upwelling, or trench suction linked to the Pacific subduction.