小型拉分盆地的生长与走滑断层的位移速率--以青藏高原东南缘则木河断裂带为例

GROWTH OF A SMALL PULL-APART BASIN AND SLIP RATE OF STRIKE-SLIP FAULT:WITH THE EXAMPLE OF ZEMUHE FAULT ON THE SOUTHEASTERN MARGIN OF THE TIBETAN PLATEAU

活动断层滑动速率可以用来定量比较不同断裂带或同一断裂带不同时段的活动性,同时还是地震危险性评价的重要参数,合理评估活动断层的滑动速率主要受限于两个参数的可靠性,即断层的累积位移量和相应的活动时间。传统上较理想的用于评估走滑断层滑动速率的地貌体一般为阶地、冲洪积扇、冲沟等的位错测量和相应活动累积时间的确定,文中则尝试通过青藏高原东南缘则木河断裂带大箐梁子段三维组合探槽揭示的小型三角状拉分盆地的生长模式来约束断层的水平位移速率,基于野外详细的调查、拉分盆地底部14C测年以及实时差分GPS精确测量,约束了则木河断裂带全新世以来的断层平均左旋滑动速率为(2.4±0.2)～3.6mm/a,这比前人给出的滑动速率偏小,但却与古地震资料及GPS观测结果吻合得较好。

Slip rate of active faults is deterministic to compare active earthquake behaviors among different faults or different segments along a fault,and also it is a key parameter for seismic hazard assessment.Geologically reliable estimation on slip rate is subject to two active tectonic parameters,the cumulative displacement produced by multiple surface-rupturing seismic events and the corresponding true ages,respectively.Generally,for strike-slip faults,we carefully measure geomorphic expressions,such as deformation or offset produced by multiple faulting on river terrace,alluvial-fluvial fans or gullies,and then integrate geochronological constraint from dating on these geomorphic expressions.Based on the above two crucial parameters,we further determine slip rates along faults.However,this paper is attempted to use another deformation of geomorphic expression,a growth model for a small triangular pull-apart basin（sag pond）,to constrain fault slip rate at the Daqingliangzi section of Zemuhe Fault on the southeastern margin of the Tibetan Plateau.Based on several three-dimension trench excavations,reliable radiocarbon dating at the bottom of stratigraphic unit in the triangular pull-apart basin（sag pond）,detailed field investigation along the Daqingliangzi section of Zemuhe Fault and accurate RTK（GPS）survey,we suggest that Holocene average left-lateral slip rate of the Zemuhe Fault is constrained between 2.4±0.2mm/a and 3.6mm/a,which is a little smaller than those estimated by other geoscientists,however this strike-slip rate is much more accordant with paleoseismic recurrence behaviors and present velocity field obtained from GPS measurement across the Zemuhe Fault.