Résumé
The Gobi-Altay massif in southwestern Mongolia recorded one of the largest intracontinental earthquakes during the XXth century (04.12.57, Mw nearly equal 8). This left-lateral strike-slip earthquake ruptured a 260 km-long section along the eastern part of the Bogd fault. About 100km of additional ruptures (essentially reverse) were also documented along subsidiary faults. Previous morphological and paleoseismological investigations along the eastern Bogd Fault determined a long-term slip rate of approximately 1 mm/yr and a mean recurrence interval of 3000-4000 years for events similar to the 1957 earthquake. The active tectonics of the western part of the Gobi-Altay massif proves to be more complex than the eastern section. Deformation is occurring along two strike-slip fault systems, the Western Bogd fault and the Valley-of-Lakes fault. This raises the question of the distribution of the deformation in space and time. In order to determine slip rates and dates of past events along the Western Bogd and Valley of Lakes faults, we carried out tectonic geomorphology and paleoseismological investigations. Preliminary results from a first expedition in 2014 allow estimating a slip rate of 0.3 mm/yr along the Valley-of-Lakes fault during the past 150 ka. Further analyses from a second expedition in summer 2015, should allow testing whether this rate remained stable through time, notably over the Holocene period. They will also enable determining the slip rate and the age of the most recent surface-rupturing event along the Western Bogd fault.