Riser Diachroneity, Lateral Erosion, and Uncertainty in Rates of Strike-Slip Faulting: A Case Study from Tuzidun along the Altyn Tagh Fault, NW China

Accurate slip-rate measurements provide the observational foundation for evaluating the extent to which strain rates along faults vary in both time and space. At timescales from 1-100 kyr, offset fluvial terrace risers are among the most common landforms used to determine rates of strike-slip faulting. A previously unexplored source of uncertainty associated with these markers centers on the correlation of diachronous risers across the fault. To evaluate the impact of such diachroneity, we investigated the Tuzidun site (37.7N, 86.72E) along the Cherchen He reach of the central Altyn Tagh Fault (ATF). Tuzidun is located where a south flowing, ephemeral drainage intersects the ATF at a high angle. The east bank of the channel is flanked by inset fluvial terraces showing left-slip, as documented by our neotectonic mapping. While the present offset is 54±3 m, geochronologic data and analysis of riser topography indicate that the displaced riser segments are diachronous. The downstream riser segment formed at ~6 ka, while the upstream riser segment may have been laterally refreshed as recently as ~0.5 ka. A valley wall on the west bank of the channel places a maximum limit of 38 m on the amount of possible lateral erosion of the upstream riser. This bound, in turn, limits the total offset since formation of the downstream riser to range from 54-89 m. Together, our observations bracket the ATF slip-rate to range from 9-16 mm/yr since ~6 ka. This result is consistent with geodetic, paleoseismic, and additional morphochronologic studies along the central ATF. More generally, this investigation shows that an observed riser displacement does not correlate with the age of either segment in cases where one riser segment is displaced while the other is subjected to lateral erosion. If this diachroneity goes undetected, erroneous slip-rate measurements are likely to result.