The velocity structure of the upper lithosphere in the Northern Baikal rift (NBR), Siberia, is analyzed in order to bring some constraints on the early stages of continental rifting. A simultaneous hypocenter-velocity inversion is performed using a set of local and regional seismological data recorded in the NBR. First-arrival times of 3,492 direct and refracted P and S waves allow to investigate the crustal velocity structure and the Moho geometry. Results of inversion reveal an unbalanced crust made of two adjacent parts of different thickness, delimited by a rather sharp discontinuity following approximately the norther edge of the rift zone. This structure is interpreted as the result of ante-rift deformation phases creating a weakened zone where rifting deformation may easily occur. Oblique rifting is invoked to explain the formation of sedimentary basins without significant crustal thinning at depth. ¿ American Geophysical Union 1995