Plank uplift and topography at the central Baikal Rift (SE Siberia): A test of kinematic models for continental extension

Uplifted flanks at intracontinental rifts are supported by flexural isostasy, as shown by the pattern of isostatic residual gravity anomalies associated with them. Models for flexural rift flank uplift differ in their kinematic description of extension, with respect to the asymmetry of lifting and the importance of brittle versus elastic upper crustal deformation. In this paper, I test different kinematic models of continental extension by comparing their predictions of rift flank topography and crustal structure with observations from the Baikal rift (SE Siberia). The rift is characterized by prominent flank topography on both sides of lake Baikal. The flanks reach similar elevations but differ in their structure: the tilt of the footwall flank is away from the basin, whereas the basin-ward part of the hanging wall flank tilts toward the basin center. Fission track data indicate that very little erosion affected the Banks since lifting started; geomorphological and sedimentological observations suggest that prerift relief was minor. Thus, the present-day topography reflects rift-related tectonic uplift. Pure-shear ''necking'' and pure-shear/simple-shear ''detachment'' models of extension predict the topographic and Bouguer gravity anomaly patterns observed along a profile across the central Baikal rift equally well. They do not permit to discriminate between different scenarios that have been proposed for the central Baikal rift; that is, half-graben versus full graben development; rifting at a continuous rate since the Oligocene versus a large increase in extension rate since the Pliocene. The models predict that the kinematics of lifting in Baikal are controlled by a midcrustal (20 km) depth of necking and/or a mid to lower crustal (20-30 km) detachment level; best-fit elastic thicknesses are in the range 30-50 km. These predictions are in agreement with results from coherence studies of Bouguer gravity and topography, as well as with the rheology of the lithosphere underneath Baikal as inferred from heat flow, seismic refraction and seismological observations. In contrast, a ''flexural cantilever'' model with low (< 10 km) elastic thickness predicts topographic patterns which are very different from those observed, for a wide range of rifting scenarios. Significant (> 3 km) erosion of the footwall flank is required to fit the topography if a flexural cantilever model is applied; this is incompatible with the fission track data. Thus, the kinematics of extension at deep and narrow intra-continental rifts such as Baikal appear to be controlled by a strong elastic lithosphere and require significant brittle deformation of the upper crust, as suggested by dynamic models for continental rifting.