Finite element modelling of crustal deformation in the Baikal rift zone: new insights into the active-passive rifting debate

The origin of the forces responsible for crustal extension in the Baikal rift zone, Siberia, is the object of a debate between 'passive rifting' models, where crustal extension is primarily caused by horizontal forces related to the kinematics of Asia (India-Eurasia collision), and 'active rifting' models, where crustal extension is primarily caused by a diapiric mantle upwelling. In this work, we used a two-dimensional visco-elastic finite element model in order to determine whether horizontal forces alone can account for the present-day deformation in the Baikal rift zone. We tested a number of kinematic boundary conditions and compared predictions of various models against the observed stress and strain field deduced from seismotectonic data (earthquake focal mechanisms and microtectonic analyses). By adjusting the kinematic boundary conditions and using a three-plate model with a differential displacement between the Mongolian and Amurian plates, we found a best-fit model that correctly accounts for the observed strain and stress field over the entire Baikal rift zone. The fact that our model does not take into account vertical forces but still explains most of the observed deformation suggests that the present-day opening of the Baikal rift is essentially controlled by horizontal forces related to the regional kinematics. These forces could have their origin in the India-Eurasia collision zone further south. This result does not imply that the asthenosphere played no role in the rifting process, in particular before the 'fast rifting' stage of the Baikal rift evolution (3-4 Ma), but might suggest a recent (Plio-Quaternary) triggering effect of the India-Asia collision on the deformation in the Baikal rift. (C) 1998 Elsevier Science B.V. All rights reserved.