The Aksay segment of the northern Altyn Tagh fault: Tectonic geomorphology, landscape evolution, and Holocene slip rate

Abstract

Millennial slip rates have been determined for the Altyn Tagh fault (ATF) at three sites near Aksay (∼94°E) in northeastern Tibet by dating fluvial channels and terrace riser offsets with radiocarbon and 10 Be‐ 26 Al surface exposure dating. Up to nine main surfaces are defined on the basis of morphology, elevation, and dating. The abandonment age of some surfaces is constrained by radiocarbon dating, which typically coincides with the youngest cosmogenic ages for a particular surface. Older surface exposure ages are taken to represent the duration of terrace emplacement. Cumulative offsets range from 20 to 260 m and fall in distinct groups, indicative of climatically modulated regional landscape formation. Most samples are younger than ∼14 ka and postdate the Last Glacial Maximum. The end of the early Holocene optimum marks the boundary between the ages of the two main terrace levels at 5–6 ka. At this longitude the ATF is divided into a northern and southern branch. The northern ATF should thus yield a minimum rate for the ATF system. Slip rate estimates using the abandonment age of the overlying level for fill terraces or channels and the emplacement of the underlying level for strath terraces give 30 consistent results, yielding an average Holocene rate of 17.8 ± 3.6 mm/yr. It is ∼9 mm/yr less than the long‐term rate obtained near Tura at ∼87°E (26.9 ± 6.9 mm/yr), in keeping with the inference of an eastward decreasing rate on the ATF, due to increased thrusting to the south. However, it remains twice the rate determined by GPS studies.