Abstract: In the interior of the Tibetan Plateau, the active tectonics are primarily marked by conjugate strike slip faults and north-trending rifts, which represent the E-W extension since late Cenozoic of the plateau. The conjugate faults are mainly composed of NE-trending left-lateral strike-slip faults in Qiangtang terrane and NW-trending right-lateral strike-slip faults in Lhasa terrane. While, the rifts mainly strike N, NNW and NNE within southern Tibet. However, it is still a debate on the deformational style and specific adjustment mechanism of E-W extension. One of key reasons causing this debate is the lack of detailed investigation of these active faults, especially within the northwestern plateau. Recently, we found a 20km long, NNW-trending active fault at Bero Zeco in northwestern Tibet. This fault is presented as fault sag ponds, channel offsets and fault scarps. Displacement of channels and geomorphic features suggested that the Bero Zeco Fault(BZF)is a dextral strike-slip fault with a small amount of normal slip component, which may result from the E-W extensional deformation in the interior of Tibet. BZF strikes N330°~340°W, as shown on the satellite image. The main Quaternary strata in the studied area are two stages alluvial fans around the Bero Zeco. From the satellite images, the old alluvial fans were cut by the lake shoreline leaving many of lake terraces. And the young fans cut across the lake terraces and the old fans. By contrasting to the "Paleo-Qiangtang Huge Lake" since late Quaternary, these old alluvial fans could be late Pleistocene with age ranging from 40ka to 50ka. And the young fans could be Holocene. The sag ponds along the BZF are distributed in the late Pleistocene alluvial fans. Also, the BZF displaced the late Pleistocene fans without traces within Holocene fans, suggesting that the BZF is a late Pleistocene active fault. The fault scarps are gentler with the slope angle of around 10° and the vertical offset is about 2m by field measurement. Reconstruction of the offset of channels suggested that the accumulated dextral offset could be about 44m on the late Pleistocene alluvial fans. Therefore, we infer that the dextral slip-rate could be around 1mm/a showing a low-rate deformation characteristic. The angle between the strike of BZF and principal compressive stress axis(σ₁)is around 30°, which is significantly different to the other faults within the conjugate strike-slip fault zones that is 60°~75°. Now, the deformation mechanisms on these conjugate faults are mainly proposed in the studies of obtuse angle between the faults and σ₁, which is likely not applicable for the BZF. We infer that the BZF could be the northward prolongation of the north-trending rifts based on the geometry. This difference suggests that the conjugate strike-slip faults may be formed by two different groups:one is obtuse angle, which is related to block extrusion or shear zones in Lhasa and Qiangtang terranes possibly; the other is acute angle, which may represent the characteristics of new-born fractures. And more studies are needed on their deformation mechanisms.

Key words: E-W extensional deformation, active fault, conjugate strike-slip faults, Bero Zeco Fault

摘要： 由于调查研究资料有限，目前对藏北高原内部近EW向的伸展变形样式和具体调节机制一直存在诸多争议。最新开展的地表调查在藏北高原西部的别若则错新发现了1条长约20km、走向近NNW的走滑断裂。该断裂表现出断塞塘、水系错动及断层崖等典型的走滑断裂变形标志。水系错动及构造地貌显示，别若则错断裂是以右旋走滑运动为主、兼具明显正断分量的张扭性断层，是高原内部近EW向伸展变形的产物。通过与羌塘古大湖进行对比分析，认为该断裂错断的最新地貌体是晚更新世的冲洪积扇，未错断全新世扇体，且断崖坡角已显著变缓，表明其最新活动时间可能为晚更新世。综合分析地表调查和遥感影像的错断位移恢复结果，发现最新一次断裂活动的最大右旋走滑位移约2~3m。晚更新世早-中期冲洪积扇体的累积最大右旋走滑位移约44m，垂直错动约2m，由此推测该断裂晚第四纪以来走滑速率约1mm/a，显示弱走滑变形特征。别若则错断裂近NNW的走向与印度和欧亚板块碰撞的主压应力轴（σ₁）的夹角约30°，而已发现的区域性共轭走滑断裂与σ₁呈约60°~75°的较大夹角，两者显著不同，表明藏北地区共轭走滑断裂带的组合方式可能存在不同的样式：一种是钝角，可能与拉萨和羌塘地体内的剪切作用或块体挤出有关；另一种是锐角，可能代表着新生破裂特征，推测其可能与高原内部近SN向正断层的N向的延伸有关，其成因机制仍需进一步研究。

关键词: 近EW向伸展变形, 活动断裂, 共轭走滑断裂, 别若则错断裂