4:30 PM - 4:45 PM
[SSS15-24] Slip rate of the Chaman fault: estimates based on 10Be exposure dating for offset geomorphic surfaces in Afghanistan
Keywords:Chaman fault, 10Be exposure dating, offset geomorphic surface , Afghansitan
The fault offset older and younger alluvial fans and stream deposits of late Pleistocene and Holocene age. Along the fault, relative ages of geomorphic surfaces are known but their absolute ages are unknown. In the absence of absolute ages for faulted deposits, it is not possible to determine the fault slip-rate. Therefore we have conducted cosmogenic dating as was done in Pakistan side to provide age controls for fluvial deposits in the study area. Terrestrial Cosmogenic Nuclide (TCN) uses the interactions between cosmic rays and nuclides in transported boulders, and therefore is an excellent method to directly date alluvial deposits. TCN is particularly useful in Afghanistan due to lack of terrestrial organic materials for radiocarbon dating. Our study focuses on three different areas located at Wardak Province. One of the sites is Ghat Bandakul south of Kabul where the distal part of alluvial fan deposited at the mouth of a stream channel is offset left-laterally. The fan slopes 3-5° westward for ~1 km from the foot of the Syah-Tapa Mountain. The deposits are composed of clay, sandstone and granitic clasts originating from bedrocks. Analysis of ALOS images and field work indicate that younger and older terrace risers are displaced left-laterally along the fault for 165 m and 235 m respectively. We collected 6 samples from the top 1-3 cm of each desired sandstone boulders that were >1m in diameter and well-embedded in surface to ensure their long-term stability. The sandstone has large amounts of quartz mineral, which yields 10Be TCN nuclide ideal for dating deposits over Quaternary timescale. Samples were processed at the laboratories of DPRI, Kyoto University and MALT, University of Tokyo.
Based on ~165 m displaced alluvial fan with a 10Be exposure age of ~46 kyr, we obtained the first geomorphologically determined slip-rate of 3.5 mm/yr along the Chaman fault. This slip-rate is considerably smaller than the geologically and geomorphologically defined slip-rates of 19–24 mm/yr and 33 mm/yr along the fault in the border region of Afghanistan and Pakistan. Along the southern section at Ghazni and Zabul Provinces, we mapped east-dipping thrust faults as well as the multiple fault splays indicating a complex interplay between left-lateral faulting and northwest compression, which may cause slower slip-rate at the northern end of the fault. We plan to date two other alluvial surfaces in the southern part of the study area to determine exact slip-rate of the fault in Afghanistan.