Abstract: Ganyanchi (Salt Lake)basin, located in the central part of the Haiyuan Fault, northeastern corner of the Tibetan plateau, is the largest pull-apart basin along this fault. Due to its location in northeastern Tibet, the Ganyanchi Basin preserves an important sedimentary record of tectonism and climate change associated with progressive growth of the Tibetan plateau. The sediments of this basin also contain abundant information regarding the deformational history of the bounding strike-slip fault, i.e., the Haiyuan Fault. Therefore, a detailed study on the depository history of the Ganyanchi Basin is of great importance. Earlier studies only focused on regional geological mapping and paleoseismic research, however, no sedimentologic or chronological work has been done in the Ganyanchi pull-apart basin. To address this problem, we drilled a 328m-deep borehole, named HY-C8, at the south of the cross-basin fault and near the active depocenter, and employ magnetostratigraphic analyses and seismic reflection data to constrain the age and to deduce the evolving history of the basin. The deep borehole profile shows that the stratigraphy of the basin can be divided into three main units (Unit Ⅰ, Ⅱ and Ⅲ), which began to deposit at about 2.76, 2.33 and 1.78Ma, respectively. The grain size of the deposits manifests an upward thinning trend, which probably implies the profile is a characteristic retrogradational sequence. The magnetic susceptibility results indicate that the playa lake probably was formed at about 1.78Ma ago, the corresponding playa-lake deposits recorded more than eight high susceptibility sections, which are most likely due to the iron sulfides (such as melnikovite, pyrrhotine etc.)that were usually produced in high-lake-level and reduction conditions. A combination of boreholes and shallow seismic reflection data indicates that the Ganyanchi Basin is mainly controlled by the cross-basin fault and its northern boundary fault, and the depocenter, probably deeper than 550m, lies in between these two faults. Finally, the sedimentary facies of the Ganyanchi Basin experienced a four-stage evolving history:eluvial facies (before~2.76Ma)to alluvial fan facies (about 2.76~2.33Ma)to distal alluvial fan facies (2.33~1.78Ma)to playa lake facies (1.78Ma~present). Based on accumulation rates, the stage of playa lake can be divided into two subchrons, and the depositional rates of subchrons 2 (about 0.78Ma~present)is as high as 232.5m/Ma, which probably was caused by the activity along the cross-basin fault in the Ganyanchi Basin.

Key words: Ganyanchi pull-apart basin, sedimentary facies, magnetic susceptibility, basin evolution, Haiyuan Fault

摘要： 干盐池拉分盆地为青藏高原东北边界海原断裂带内最大的拉分盆地，对揭示青藏高原NE向生长、海原断裂的运动变形及古环境演变有着重要的意义。前人主要开展了区域地质填图和古地震方面的研究，而该盆地的地层、年龄、形成演化等皆不清楚。针对以上问题，在干盐池盆地开展了浅层人工地震勘探、深孔钻探及样品测试等工作。深孔剖面显示，干盐池盆地大约形成于距今2.76Ma前，盆地内堆积了3段地层，第Ⅰ、Ⅱ、Ⅲ段地层分别约于距今2.76Ma、2.33Ma和1.78Ma前开始堆积，沉积物的粒度呈现出"向上逐渐变细"的趋势，表现为1个典型的"退积型"沉积序列；磁化率结果表明，干盐池盆地大约于距今1.78Ma前进入积水期，湖相层记录了多个高磁化率区间，这有可能是因为自生的亚铁磁性硫化铁矿物所致；钻孔及浅层人工地震勘探资料表明，干盐池盆地主要受南-西华山与唐家坡-干盐池断层的控制，其沉降中心介于这2条断层之间，最深处可能超过了550m。干盐池拉分盆地的沉积过程经历了由残积相（距今约2.76Ma前）—扇根相（距今2.76～2.33Ma）—扇中相（距今2.33～1.78Ma）—盐湖相（距今约1.78Ma—现今）的演变。盐湖相期以B/M界线为界又可细分为2个阶段，其中，第2阶段（距今约0.78Ma—现今）的沉积速率急剧加快，高达232.5m/Ma，这有可能是受盆地中央断层的活动影响所致。

关键词: 干盐池拉分盆地, 沉积相, 磁化率, 盆地演化, 海原断裂