A BROKEN PLATE BENEATH THE NORTH BAIKAL RIFT-ZONE REVEALED BY GRAVITYMODELING

We modelled a 1200 km long gravimetric profile in the North Baikal rif t to assess the mechanical behaviour of the lithosphere, using a numer ical model that accounts for realistic brittle-elasto-ductile theology . We use published seismicity and refraction data, a new 5'x7.5' free- air/Bouguer gravity and topography data set, and a detailed map of fau lts obtained from high resolution SPOT imagery. Analysis of the gravit y field over the North Baikal rift zone indicates significant asymmetr y of the mechanical processes governing the deformation of the divergi ng sides of the rift. These anomalies cannot be explained by a convent ional continuous plate undergoing extension beneath the rift zone, whe reas a strong mechanical discontinuity (wedge shaped detachment zone b eneath the rift arris) is able to reproduce observations. Such a disco ntinuous model provides a good fit to the gravity and crustal thicknes s data and explains the deep seismicity reported there.