关键词: 塔拉斯-费尔干纳断裂, 西天山, GPS速度场, 运动速率, 断层分段

Abstract: The Talas-Fergana Fault(TFF)with a total length of more than 1 000km is a large dextral strike-slip fault across the West Tianshan Mountains in the northwest direction. The fault plays an important role in accommodating deformation in Central Asia and has attracted much attention by geologists due to the huge controversy in its strike-slip rate and kinematic pattern. Previous studies indicated that its average dextral strike-slip rate is 8~20mm/a since Late Holocene based on offset ephemeral stream valleys and ¹⁴C dating method. Some researchers recently updated the strike-slip to 2.2~6.3mm/a by the application of multiple dating methods(¹⁰Be, ²⁶Al, ³⁶Cl, luminescence, and radiocarbon)and satellite images with higher precision. But the strike-slip rates derived from modern GPS velocity field are only~2mm/a or even as low as 0.8mm/a. Thus, there is a substantial divergence between geological results and geodetic results in the strike-slip rate of the TFF. Some scholars believe that the huge difference between the geological rate and the rate obtained by geodetic measurements is caused by fault locking. In this study, the updated GPS data was used to establish velocity field of the West Tianshan Mountains relative to the stable Eurasian framework and the velocity field without self-rotation. The velocity field shows that the Tianshan Mountains are under intense crustal shortening and deformation. Moreover, for the TFF, as an important boundary fault in the western Tianshan Mountains, whether the far velocity field or the near velocity field, the differential movement of the crust is not obvious. And far-field velocity vectors away from the TFF show that there is minor difference of crustal movement along the fault. The TFF does not have the typical characteristic of locked fault that there is a big difference in velocities of far-field vectors, but a small difference in that of near/mid-field vectors. Thus, the activity of the fault is weak actually.
To further illustrate the overall low slip rate of the TFF, we compare the maximum shear strain rate and its distribution characteristics along the Altyn Fault and the Haiyuan Fault with large slip rates with the results of the TFF. The maximum shear strain rates along the Altyn Fault and the Haiyuan Fault are concentrated along the fault, and are as high as~60nano strain/a and~40nano strain/a, which are much larger than the overall maximum shear along the TFF. This shows that the sliding rate of the TFF is much lower than the strike-slip rate of the Altyn Fault of 9~15mm/a, and even slightly lower than the sliding rate of the Haiyuan Fault of 4~8mm/a. Therefore, we are more certain that the current activity rate of the TFF is far less than 8~20mm/a estimated by some geological methods.
The half-space elastic dislocation model is used to more rigorously re-constrain the current strike-slip rate of the TFF. The results show that the fault is divided into three segments. The TFF dextral strike-slip rate increases from the northwest section to the middle section and decreases from the middle section to the southeast section. And the strike-slip rates of the northwestern, middle and southeastern segments are(2.1±0.7)mm/a, (3.3±0.4)mm/a and(2.4±0.7)mm/a, respectively. The TFF is dominated by strike-slip motion, but there is also a weak dip-slip motion in the middle section of the TFF, with a magnitude of about 1mm/a.
The above results confirm the current low strike-slip rate of the TFF obtained by GPS which is much less than the strike-slip rate of 8~20mm/a estimated by geological methods. And through the GPS results, it is certain that the TFF presently has a low fault activity rather than a locked fault. To reconcile the high geological strike-slip rates and the geodetic results, a new deformation pattern of the West Tianshan Mountains may be needed. And more detailed GPS observations are required to explore whether the TFF has penetrated into the southern foreland basin of the West Tianshan Mountains.

Key words: Talas-Ferghana Fault, West Tienshan Mountains, GPS velocity field, strike-slip rate, fault segmentation