A BRIEF ACCOUNT OF THE NEOTECTONIC ACTIVITY IN SHAANXI

Neotectonic activity has been strong in Shaanxi, which is mainly manifested by doming and uplifting of blocks. Fracture zones, down-faulted basins and multi-level planation surfaces and terraces are well developed in the region. The Liupan Mountains and Weihe basin are graben-type depressions, controlled by major piedmont fractures of the Beishan and Qinling Mountains. There terrestrial sediments are as thick as 7000m, seismic activity occurred frequently and there are very rich geothermal resources. In the loess plateau area of northern Shaanxi, there have occurred inhomogeneous and intermittent doming many times since the Cenozoic. Tertiary uplifting caused most of the strata to be absent. The Yellow River was also forced to change its course and flew northwards through Inner Mongolia. The old course of the Yellow River was occupied by the Weihe River. In the Pleistocene a tilted area was formed in the southwestern part and in the Holocene a tilted was developed in the northeastern part. More than ten intermontane lake basins were formed in the Qinling-Bashan fault block area, where the capture and change of river courses were of common occurrence, for example, the diversion of the Danjiang and Luohe rivers, the division of the Hanjiang River into the eastern and western water systems and two southward captures of the Hanjiang River. Four levels of planation surfaces are developed in the whole region: Ⅰ level, 3000 m above sea level （formed in the Paleocene）; Ⅱ level, 2000 m above sea level （Eocene）; Ⅲ level, 2000—1600 m （Oligocene）; Ⅳ level, 1600—600 m above sea level （Miocene-Pliocene）. Besides there generally occur 3 to 5 levels of terraces in basins and valleys.