PALEOSTRESS RECONSTRUCTIONS AND GEODYNAMICS OF THE BAIKAL REGION, CENTRAL-ASIA, PART 2 - CENOZOIC RIFTING

Investigations on the kinematics of rift opening and the associated st ress field present a renewed interest since it has recently been shown that the control of the origin and evolution of sedimentary basins de pends to a large extent on the interplay between lithospheric strength and applied stresses. It appears that changes of stress field with ti me are an important factor that either controls or results from the ri fting process. The object of this paper is to study the changes of fau lt kinematics and paleostress field with time in the Baikal Rift Syste m during the Cenozoic. Reduced paleostress tensors were determined by inversion from fault-slip data measured in the central part of the rif t and its southwestern termination, between 1991 and 1995. Results sho w that the stress field varies as well in time as in space. Two major paleostress stages are determined, corresponding broadly to the classi cal stages of rift evolution: Late Oligocene-Early Pliocene and Late P liocene-Quaternary. The first paleostress stage is related to the rift initiation and the second to the major stage of rift development. Sim ilarities between the recent paleostress field and the present-day str ess field inverted from focal mechanisms indicate that the second pale ostress stage is still active. Therefore, we propose to use 'proto rif t' for the Late Oligocene-Early Pliocene stage and 'active rift' for t he Late Pliocene-Quaternary stage of rift development. During the 'pro to rift' stage, the stress held was characterized by a compressional t o strike-slip regime. A progressive change from transpression to trans tension is suspected for the central part of the rift (Baikal and Barg uzin basins) during this period. In the western termination of the rif t (Sayan Massif, Tunka depression), a strongly compressional stress fi eld with oblique thrusting kinematics is well constrained in the Late Miocene-Early Pliocene interval. The 'active rift' stage was initiated by a marked change in fault kinematics and stress regime in the Late Pliocene. In the central part of the rift, the stress regime changed i nto pure extension, while in the southwestern extremity, it changed in to pure strike-slip. Fault kinematics suggests that rifting was initia ted by an extrusion mechanism due to the interaction of far-held compr essional stress on a mechanically heterogeneous crust, with the southw ards-pointing wedge of the Siberian Craton acting as a passive indento r. The Cenozoic time-space evolution of the stress field is believed t o reflect the increasing influence of locally generated buoyancy exten sional stresses associated with density anomalies of the lithosphere, on intraplate stresses generated by the India-Eurasia convergence and the West-Pacific subduction.