A NEW ESTIMATE OF THE RATE OF ACTIVE DEFORMATION AND THE DISCOVERY OF AN ANCIENT EARTHQUAKE RUPTURE IN THE MONGOLIAN ALTAY MOUNTAINS

Sweeping across the Mongolian, Chinese, Kazakh, and Russian borders, the Altay Mountains encompass a distal region of orogenesis resulting from the continuing indentation of India into Central Asia. This NW-SE trending range is actively deforming along right-lateral strike-slip and oblique-slip reverse faults. Likely through a combination of transpressional faulting and vertical axis rotations, the Altay accommodate approximately 7 mm/yr of NE directed shortening, estimated from GPS measurements. In order to understand the correlation between these modern geodetic strain measurements and the rate and style of deformation over tens of thousands of years, we aim to quantify average slip rates along Altay faults. There are at least five main, parallel fault systems that continuously extend through the central Altay, separated by 10 to 40 km and generally over 200 km long. Slip rates can be determined by dating a surface that has a measureable amount of offset from cumulative fault motion, but this has not been done for most of the Altay faults. At a site on the Ölgiy fault in the Central Altay, we sampled boulders for cosmogenic ¹⁰Be surface dating from an alluvial fan with two streams prominently displaced by right-lateral fault motion. This measured displacement will be combined with forthcoming ¹⁰Be data to calculate a slip rate for the Ölgiy fault. Also important for understanding the rates and styles of deformation in the Altay are well preserved records of ancient earthquake ruptures. The Altay faults are capable of producing some of the largest earthquakes recorded in a continental setting. During summer 2008, we discovered and documented evidence for an ancient earthquake along the Ölgiy fault system. The ‘Sair Uul’ rupture is clearly expressed for ~80 km and along the rupture are streams offset by ~5 m. These dimensions are consistent with a M_w 7.5 earthquake, and when the degree of erosion and rupture characteristics are compared with features along a similar and well dated rupture on the Jid fault to the east, we estimate that the Ölgiy fault ruptured ca. 1000–500 years ago. In the context of our work on other faults in the Altay, these observations and new data will help to quantify deformation rates over longer time scales and lead to an interpretation of the structural evolution of this active, intracontinental range.