AN INTEGRATED MODELING STUDY OF THE CENTRAL AND NORTHERN BAIKAL RIFT - EVIDENCE FOR NONUNIFORM LITHOSPHERIC THINNING

We employ a kinematic model of continental extension to assess the lit hospheric structure across the Baikal rift zone. The model includes si mple shear extension of the upper crust along detachment faults and in dependent pure-shear thinning of the lower crust and mantle. Model pre dictions of topography, gravity anomalies and surface heat how are com pared with observations along two sections, one crossing the central B aikal rift and the other across northern Baikal. For the central Baika l profile, topography and gravity anomaly patterns are fit by a unifor m extension model: upper crustal extension of approximately 19 km is e qualled by a predicted mantle thinning factor delta < 2. In contrast, the high flank topography and strongly negative gravity anomalies acro ss the northern Baikal basin require the approximately 12 lan of crust al extension there to be accompanied by much higher mantle thinning fa ctors (delta = 4-6). A model of non-uniform mantle thinning below nort hern Baikal is supported by P-T estimates for Miocene and Quaternary I herzolite xenoliths from the Vitim volcanic field. The rate of extensi on has a minor influence on the predicted topography and gravity anoma ly patterns. None of the models predicts a significant (>10 mW m(-2)) increase in regional surface heat flow. The short-wavelength, large-am plitude variability of the observed surface heat flow requires crustal redistribution processes involving large-scale ground water how. The modelled topography and gravity anomaly patterns suggest that a possib le asthenospheric upwarp is more likely to be situated below the north ern than below the central Baikal basin, in contrast with earlier infe rences from teleseismic and gravity studies. (C) 1998 Elsevier Science B.V. All rights reserved.