陕西新构造活动概要

陕西新构造活动强烈,主要表现为地块的拱起和隆升。区内断裂带、断陷盆地、夷平面及阶地十分发育。六盘山及渭河盆地为地堑式凹陷,受北山和秦岭大断裂的控制,陆相沉积厚达7000m,该区地震活动频繁,地热资源很丰富。陕北黄土高原区,在新生代曾多次发生不均衡性拱起,第三纪的抬升导致黄河改道,沿拱起边缘向北流入内蒙古一带,更新世和全新世分别发育了西南和东北部掀斜区。秦岭—巴山断块隆升区,发育了山间湖盆10余个,河流的袭夺与改道也很频繁,例如:丹江与洛河的分流,汉江分解为东、西二条水系,汉江的二次向南袭夺等。全区发育有Ⅰ—Ⅳ级夷平面,其海拔高度依次为3000m、2000m、2000～1600m,1600～600m,它们形成的时代分别为古新世,始新世、渐新世及中—上新世。此外,在盆地及河谷区,一般均发育了3～5级阶地。

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.