An integrated modelling study of the central and northern Baikal rift: evidence for non-uniform lithospheric thinning?

Abstract

We employ a kinematic model of continental extension to assess the lithospheric structure across the Baikal rift zone. The model includes simple shear extension of the upper crust along detachment faults and independent pure-shear thinning of the lower crust and mantle. Model predictions of topography, gravity anomalies and surface heat flow are compared with observations along two sections, one crossing the central Baikal rift and the other across northern Baikal. For the central Baikal profile, topography and gravity anomaly patterns are fit by a uniform extension model: upper crustal extension of approximately 19 km is equalled by a predicted mantle thinning factor δ < 2. In contrast, the high flank topography and strongly negative gravity anomalies across the northern Baikal basin require the approximately 12 km of crustal extension there to be accompanied by much higher mantle thinning factors (δ = 4–6). A model of non-uniform mantle thinning below northern Baikal is supported by P-T estimates for Miocene and Quaternary lherzolite xenoliths from the Vitim volcanic field. The rate of extension has a minor influence on the predicted topography and gravity anomaly patterns. None of the models predicts a significant (> 10 mW m −2) increase in regional surface heat flow. The short-wavelength, large-amplitude variability of the observed surface heat flow requires crustal redistribution processes involving large-scale ground water flow. The modelled topography and gravity anomaly patterns suggest that a possible asthenospheric upwarp is more likely to be situated below the northern than below the central Baikal basin, in contrast with earlier inferences from teleseismic and gravity studies.