Control on off-rift magmatism: A case study of the Baikal Rift Zone

The Baikal Rift Zone (BRZ) is an active intracontinental rift zone with almost no rift-related magmatism along the rift axis. Most magmatism is observed outside the rift center, and e.g. the major Vitim volcanic field is displaced more than 200 km from the rift center. The reasons for this regional distribution of the magmatism are enigmatic, in particular regarding the off-rift magmatism. We present results of numerical modeling of rift structures similar to BRZ that develop in the transition between craton and orogenic belt. Geophysical evidence suggests that pre-existing weak zones control the location of the BRZ. The models therefore include a pre-set fault (weak zone) within the transition zone between the craton and the orogenic belt as an initial condition in the model. If the pre-existing fault is close to the craton, partial melting in the mantle, due to sub-crustal extension, is offset from the surface rift graben by over 200 km. A horizontal shear zone in the lower crust transfers the extension from the shallow crust to the lithospheric mantle far from the rift graben. A pre-set fault close to the orogenic belt zone is required to explain magmatism close to the rift center in the southernmost BRZ. We conclude that the location of the pre-existing fault zone relative to a change in lithosphere thickness may shift the magmatism away from the surface expression of the rift graben and may explain the location of the off-rift magmatism, e.g. the basaltic Vitim plateau. The predicted geometry of the sedimentary graben along the rift strike is consistent with seismic observations in the northern and central BRZ.