Tectonic Geomorphology And Late Quaternary Slip Rate Of The Fuyun Fault Zone, Altai Mountains, Central Asia

The Altai Mountains in central Asia generally trend NW-SE, and accommodate the shortening of NNE-SSW direction caused by the India-Eurasia convergence. We focused on the Fuyun fault, a right-lateral strike-slip fault bounding the southwest edge of the Altai Mountains, and conducted a preliminary study to constrain the slip rate of this fault. Field investigations were carried out by Trimble 5700 GPS using the real-time kinematic method to obtain the spatial distribution of fluvial terraces in the piedmont region of the Altai Mountains in western China. The OSL (Optically Stimulated Luminescence) dating results for the alluvium constituting those fluvial terraces are obtained to determine the abandonment ages of these fault displaced terraces. We find there exists five levels of fluvial terraces, termed Tp, T3, T2, T1 and To from high to low elevations. Tp and T3 are climatic-change induced fill terraces and formed in the middle Pleistocene and the late Pleistocene, indicating the Penultimate Ice Age and the Last Ice Age of the Fuyun area terminated in the middle Pleistocene and the end of the late Pleistocene. The fill-cut terraces T2 and T1 formed in the Holocene as complex responses to climatic changes and tectonic activities along the Fuyun fault. The highest terrace of Tp might be abandoned at the end of the middle Pleistocene (uppermost OSL age of 150.4±8.1 ka) with an offset of 203.1 +151.9/-75.3m, and this yields a minimum dextral slip rate of 1.35+1.1/-0.5 mm/yr along the Fuyun fault. Considering a convergence rate of-6 mm/yr GPS velocity across the Altai Mountains, we inferred about 22% of strike-slip component of this convergence has been accommodated by the Fuyun fault zone since the Late Pleistocene. Based on the offset magnitudes of individual Holocene terraces and the time since the abandonment of higher-level terraces, we obtained an upper bound slip rate of 23.3+2.3/-2.5 mm/yr. The great discrepancies between the upper bound and lower bound slip rates are likely because the lateral erosion on the T3/T2, T2/T1, and T1/To risers downstream from the active fault lead to the offsets being overestimated despite they have been displaced away from the stream channel, and therefore indicate the lateral erosion of terrace risers which are displaced away from stream channel should be taken into consideration in reconstruction of faulted terraces, especially in degradational terraces.