Abstract: Recognition of major tectonic events and tectonic cycles in Quaternary is an important topic that attracts more and more attention of many geologists. Recently, the research in this aspect has focused mainly on Quaternary sedimentation and geomorphologic evolution, while little attention has been paid to the study of fault evolution, which is directly related to tectonic cycle or tectonic event. This is mainly because of the lacking of geological evidence and the limitation of dating technique. Nevertheless, significant progress has been made in loess study, especially the characteristics and dating of loess-paleosoil sequences, which provide a time scale for regional correlation and timing of loess deposits. In this paper, an attempt has been made to discuss the migration and variation of Quaternary activity of fault zone through the analysis of loess deposition along the zone by using this time scale. The main purpose of this study is to provide direct evidence for the division of tectonic cycles in Quaternary. The present study deals mainly with the Lintong-Chang'an Fault and the Lishan mountain front fault on the southern margin of Weihe basin, as well as the Kouzhen-Guanshan Fault on the northern margin of the basin. The loess deposits along these fault zones have been studied in detail, while the main unconformities in loess sequences were identified in the geological sections across the fault zones and dated by using the loess-paleosoil time scale. The results show that tectonic unconformity presents broadly along the Lintong-Chang'an Fault and the Lishan mountain front fault, appearing as discordant contact of the S 8 paleosoil layer with the underlying strata. The underlying strata are offset significantly by the fault, but the overlying strata of S 8 are offset inconsiderably. Along the Kouzhen-Guanshan Fault zone, the S 1 paleosoil layer discordantly contacts with the underlying strata, which are significantly offset by the fault, but the overlying strata of S 1 layer are inconsiderably offset. In term of the loess-paleosoil time scale, the following conclusion can be drawn from the result of this study: The activity of the Lintong-Chan'an Fault zone on the southern margin of the Weihe basin was markedly changed at 800～900ka B.P. At the same time, the migration of activity occurred along the Lishan mountain front, while strong activity started to occur along the Weinan Yuan front Fault and the whole Weinan Yuan began to be uplifted. At 120ka B.P., the activity of the Kouzhen-Guanshan Fault zone began weakened. Thus, the variation of fault activity in this area may indicate two major tectonic events in mid-late Quaternary. This result may provide basic material for regional correlation of Quaternary tectonic events and for the research of the tectonic manifestation of tectonic events.

Key words: tectonic event, active fault, Quaternary, Weihe Basin

摘要： 研究了渭河盆地南缘和北缘几条断裂第四纪活动性的迁移变化.利用横跨断裂的构造地层剖面,结合断裂上覆黄土地层的年代学研究结果,对断裂活动强度迁移变化的演化阶段进行了研究.结果表明,在大约80～0万年前,渭河盆地南缘的临潼-长安断裂带的活动性发生了显著变化,骊山山前断裂也有活动性迁移现象,渭南塬前断裂开始强烈活动,渭南塬全面抬升;12万年前左右,渭河盆地北缘的口镇-关山断裂活动性明显减弱.该区断层活动的这些变化是对第四纪中晚期的2次重要构造事件的反映.

关键词: 构造事件, 活断层, 第四纪, 年代, 渭河盆地