FINITE-ELEMENT MODELING 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 Baik al rift zone, Siberia, is the object of a debate between 'passive rift ing' models, where crustal extension is primarily caused by horizontal forces related to the kinematics of Asia (India-Eurasia collision), a nd 'active rifting' models, where crustal extension is primarily cause d by a diapiric mantle upwelling. In this work, we used a two-dimensio nal visco-elastic finite element model in order to determine whether h orizontal forces alone can account for the present-day deformation in the Baikal rift zone. We tested a number of kinematic boundary conditi ons and compared predictions of various models against the observed st ress and strain field deduced from seismotectonic data (earthquake foc al 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 n ot take into account vertical forces but still explains most of the ob served deformation suggests that the present-day opening of the Baikal rift is essentially controlled by horizontal forces related to the re gional kinematics. These forces could have their origin in the India-E urasia collision zone further south. This result does not imply that t he 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.