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

We present a detailed morphotectonic analysis of late Quaternary faulting i n the North Baikal Rift (NBR), a region characterized by ranges and basins distributed over more than 800 km along strike in eastern Siberia. Remote s ensing techniques (SPOT, METEOR scenes, and aerial photographs) are used to map the active fault network which displays a general en echelon distribut ion from the northern Lake Baikal to the easternmost basin, with similar to 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 c rustal fabric strongly influences the overall geometry of the rifted basins . We use 54 C-14 ages of postglacial terraces near the foot scarps of the M uya basin to date offsets measured inside alluvial fans. The last main post glacial event in this area appears to be the early Holocene optimum dated a t similar to 10 +/- 2 ka, following the onset of deglaciation at similar to 13 ka. Using these time constraints, a detailed leveling across two terrac es offset by the Taksimo fault (West Muya basin) shows consistent minimum v ertical slip rates of 1.6+/-0.6 mm yr(-1). Using 30 other active scarps ana lyzed 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 directio ns of slip vectors from Holocene field data and slip directions from earthq uake fault plane solutions: although local discrepancies appear, the mean d irections of lesser horizontal stress (sigma(3)) inverted from theses value s are similar to N130 degrees E and similar to N155 degrees E, respectively , which are comparable within uncertainties and favor a rifting obliquity o f similar to 30 degrees-40 degrees. Extrapolating our Holocene rates, we es timate basin ages younger than those generally believed (less than 7 Ma) an d propose a spatial and temporal evolution of rifted basins consistent with experimental models of oblique rifting. Total amounts of extension and ver tical throw (similar to 7 and similar to 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 zon e 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.