Geometry and rate of faulting in the North Baikal Rift, Siberia

We present a detailed morphotectonic analysis of late Quaternary faulting in the North Baikal Rift (NBR), a region characterized by ranges and basins distributed over more than 800 km along strike in eastern Siberia. Remote sensing techniques (SPOT, METEOR scenes, and aerial photographs) are used to map the active fault network which displays a general en échelon distribution from the northern Lake Baikal to the easternmost basin, with ∼30-km-spaced overstepping segments of 10-80 km in length. Most faults have a dominant dip-slip component over their Cenozoic history. The inherited crustal fabric strongly influences the overall geometry of the rifted basins. We use 54 ¹⁴C ages of postglacial terraces near the foot scarps of the Muya basin to date offsets measured inside alluvial fans. The last main postglacial event in this area appears to be the early Holocene optimum dated at ∼10 ± 2 ka, following the onset of deglaciation at ∼13 ka. Using these time constraints, a detailed leveling across two terraces offset by the Taksimo fault (West Muya basin) shows consistent minimum vertical slip rates of 1.6±0.6 mm yr^-1. Using 30 other active scarps analyzed in the field, we find a lower bound for horizontal velocity of 3.2±0.5 mm yr^-1 across the NBR, a rate close to the one found in the southern rift from Global Positioning System measurements. We then compare directions of slip vectors from Holocene field data and slip directions from earthquake fault plane solutions: although local discrepancies appear, the mean directions of lesser horizontal stress (σ₃) inverted from theses values are ∼N130°E and ∼N155°E, respectively, which are comparable within uncertainties and favor a rifting obliquity of ∼30°-40°. Extrapolating our Holocene rates, we estimate basin ages younger than those generally believed (less than 7 Ma) and propose a spatial and temporal evolution of rifted basins consistent with experimental models of oblique rifting. Total amounts of extension and vertical throw (∼7 and ∼12 km, respectively) across major faults appear rather constant from the central to the northern rift. These results favor a progressive development of asymmetric grabens in a rift zone that widens with times and they indicate a strong rheological control on deformation which seems enhanced by other contributions than the far-field effects of the Indo-Eurasian collision.