关键词: 断裂滑动速率, 河流阶地演化, 阶地废弃年龄, 阶地沉积年龄

Abstract: Slip rates along major active faults are important components of quantitative studies of active tectonics. Slip rates can be directly used to seismic potential evaluation of active faults and seismic safety assessments of major engineering. In principle,dividing total displacement by its initial time yields slip rate along the fault. But,accurate determination of slip rate along a particular fault is not a simple task in practices for which the rates may deviate as much as 3 times among different researches and different methods. We argue that offset terrace risers that are protected by topography upstream of them are more closely dated by the age of the upper terrace than by that of the lower terrace. In some cases,valleys upstream of the fault have been incised into bedrock,and few if any terrace risers can be seen within the valleys. Such streams debouch onto alluviated floodplains or fans that become incised,presumably during climate changes,to create terrace risers. The terrace risers are then displaced so that they lie downslope from bedrock ridges on the upstream side of the fault,and thus the risers become protected from further incision. In such cases,dates of upper terraces should more closely approximate the ages of the risers than those of lower terraces. As noted above,whether the age of the upper or of the lower terrace more closely approximates the age of the riser will depend upon how the stream flowing over the flood plain that becomes the lower terrace alters the riser,and therefore at least in part on whether the offset riser moves into the path of the active stream or becomes shielded from it. Of cause,the age of the riser should be neither greater than the age of the upper terrace nor smaller than the age of the lower terrace. In an ideal situation,the ages of both would be sufficiently similar that they would place nearly equal upper and lower bounds on the slip rate. In many regions,however,the ages of the two terraces are so different that the bounds that they place on the slip rate are too large to be useful. We propose three methods to determine slip rate based on offsets of terrace risers. The first is to use both upper and lower terraces to constrain the maximum and minimum age of the offset of the riser. The second is to use the abandonment age of upper terrace as the initial age of the offset on the side of stream moving away from the river course. The third is to use the inception of sedimentary deposition on the lower strath terrace as the initial age of terrace riser offset. We use these methods to study slip rates along the Haiyuan Fault and the Altun Fault. The results show consistency of slip rates among different time scales,and are also consistent with other independent studies.

Key words: fault slip rate, strath terrace formation, abandonment age of terraze, sedimentary age of terrace