Applying optically stimulated luminescence to determine the slip-rate of part of the Har-Us-Nuur Fault



Optically Stimulated Luminescence (OSL) is currently one of the most important methods for dating minerals during the Quaternary. This method plays an important role in studies related to Paleoseismology and tectonic activities, particularly in arid and semi- arid regions. OSL dates the last exposure to sunlight; therefore, it can directly find the ages of the coseismic or post seismic evidences.
The aim of this study was to employ OSL method to date samples collected from alluvial fans around the Har- us- Nuur fault in Mongolia. Mongolia is an arid zone and therefore, OSL dating should be able to provide reliable ages for this region. The Har- us- Nuur is one of the most important faults in the eastern margin and depression of Great Lake of Altai Mountain in western Mongolia. This fault is an active and right lateral strike slip fault and source of large earthquakes in the region. Two alluvial fans A2, F1 which were cut by this fault were selected for slip rate determination. Displacements were estimated by differential GPS and landsat images. The displacements were 130 ± 40 m and 15 ± 10 m for A2 and F1 fans, respectively.
Two OSL samples from fan A and one OSL sample from fan F1 were collected using steel food cans. The samples were wrapped in several layers of black plastic to prevent them from being exposed to light. Under dim red light in the lab, we unwrapped the cans. The sediments from both sides of metal cans were used for ICP measurements. We calculated alpha, beta and gamma dose rates using the conversion factors of Aitken (1985), Bell (1980) and Mejdahl (1979). Cosmic rate was determined following the method of Prescott and Hutton (1988). The moisture content was determined by drying at 40°C. The radioactive materials (U, Th and K40), moisture content, cosmic rate and other parameters were employed to calculate the annual dose (Table 1).
The middle part of the sediments inside the cans was sieved (90-250 µm) under the water, using plastic sieves. Each plastic sieve was disposed after it was used for one sample. The sample was then treated in the laboratory with 1N HCl for 2 days and H2O2 to remove carbonates and organic matters, respectively. Following removing heavy mineral greater than 2.72 g cm-3, using heavy liquid separation method, the sediments were etched with 48% HF for one hour. Then, the remaining quartz was rinsed in distilled water, treated with 10% Hcl and again rinsed in distilled water. After this the wet quartz were dried in oven and dry-sieved (90µm) before mounting as a monolayer on 10 mm aluminum disks using Silko-Spray silicone oil. Between 10 to 14 aliquots were prepared and measured for De determination for each sample. The De was calculated employing the SAR method (Table 1) of Murray and Wintle (2000). The detailed experimental condition was similar to what is outlined by Fattahi et al (2006). Three regeneration dose points was used for making dose growth curves. One point was employed to estimate the recuperation effect and recycling point was used to check the reliability of sensitivity corrections. Accepted individual De values (in Gray) for samples A2a, A2b and F1 were 7, 6 and 11 aliquots (Figure 6). The average De for each sample which was calculated using histogram method of analyst program, were 97.23 ± 43.52, 71.03 ± 28.84 and 33.97 ± 13.66 Gy (Table 1).
OSL provided ages of 18.89 ± 7.72, 26.28 ± 11.83 and 7.47 ± 3.02Ka for A2a, A2b, and F1, respectively. Finally, the slip rates were determined by dividing the displacement by ages which provided 3.48- 8.68mmyr-1 and 0.79-3.23 mmyr-1 for A2 and F1 surface, respectively (Table 3).