Abstract: A Tertiary pull-apart basin named Laolongwan basin was found in the Haiyuan fault zone. Through interpretation of the satellite image and observations of stratigraphic profiles in the field, stratigraphic sequence, sediment distribution and character of relevant faults in the basin are studied. The Laolongwan basin developed at the largest step area in the Haiyuan fault zone, and the deposition in the Laolongwan basin is bounded by faults. The total thickness of deposition in the basin is about 4600m including boulder rock, gravel-sand rock and sand-mud rock, and breccia rock from the bottom to the top. Strata in the basin discordant contact with various strata outside the basin. Comparing stratigraphic sequence in the basin with regional Tertiary stratigraphical timetable, it is suggested that the Laolongwan basin started in the middle Miocene. Thus, based on above characters, the Laolongwan basin is identified as a Tertiary pull-apart basin in the Haiyuan fault zone. The Laolongwan pull-apart basin provides geological evidence of strike slipping on the northeast edge of Tibet in Tertiary time.

Key words: Haiyuan fault zone, Strike slipping, Pull-apart basin

摘要： 在海原断裂带内部发现一个第三纪拉分盆地,命名为老龙湾拉分盆地。根据卫片解译结果和野外地质调查,对该盆地内的地层沉积序列、地层分布、相关断裂等特征进行了研究。结果表明,老龙湾拉分盆地发育于海原断裂内的最大斜列部位,盆地沉积受边界断裂控制;盆地内部沉积了巨砾岩、杂砾岩及紫红色-灰绿色泥岩、桔红色角砾岩等地层,最大沉积厚度约4600m。老龙湾盆地内部地层不整合于不同的外围地层之上,根据对盆地内部沉积序列及外围第三系的区域对比,认为盆地沉积开始于中新世中期。由以上特征确定老龙湾盆地为海原断裂带内的第三纪拉分盆地。它为青藏高原东北缘第三纪的走滑断层活动的研究提供了地质证据。

关键词: 海原断裂带, 走滑断层活动, 拉分盆地