PRESENT-DAY STRESS-FIELD CHANGES ALONG THE BAIKAL RIFT AND TECTONIC IMPLICATIONS

Intraplate extension, in a frame of a global compressional stress fiel d, seems linked to local lithospheric perturbations (lithospheric thin ning or thickening) able to modify the resulting state of stress [Zoba ck, 1992]. The Baikal Rift Zone (BRZ), Siberia, is located north of th e India-Asia collision zone and exhibits no direct communication with any oceanic domain. It can thus be fully considered as an area of cont inental extension, dominated by the ''global compressional intraplate stress field'' resulting from plate driving forces. In order to addres s the problem of its dynamics and kinematics and their links with the India-Asia collision, a comprehensive: stress tensor analysis is prese nted, based on 319 focal mechanisms of earthquakes located along the w hole Baikal rift. The stress field is varying at different scales of o bservation: when looking at central Asia (several thousands kilometers ), the maximum horizontal stress S-Hmax directions remain rather const ant (with a fan-shape geometry) when the tectonic regime goes from com pressional (Himalayas) to extensional (Baikal). When observing the Bai kal rift (about 1000 Km long), clear variations of the stress regime a re observed, from an extensional regime in the central part of the rif t to wrench ones in its northern and southern ends. Finally, at the sc ale of 100 km, systematic S-Hmax reorientations occur close to major r ift faults. We thus infer that the interaction between collisional pro cesses and inherited structures may have a strong influence on rift dy namics. We then use computed stress tensors to predict slip vectors on major rift faults. Deformation patterns show two distinct parts of th e rift: the South Baikal Rift (SBR) is characterized by a constant tre nding (around N100 degrees E) slip vector, meanwhile the North Baikal Rift (NBR) exhibits a complex block rotation behavior involving at lea st three crustal blocks. We propose to interpret these surficial struc tures 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 e xplain most of the surficial deformation patterns, which suggest a lar ge-scale cracking of the lithosphere in the Baikal region. Other possi ble sources of stress could also be considered, like deep mantellic up welling, or trench suction linked to the Pacific subduction.