关键词: 第四纪, 地壳运动, 虚地磁极, 鄂尔多斯地块

Abstract: In this paper we describe our efforts to determine relative motion of different blocks duringthe Quaternary period by using paleomagnetic method. The Ordos and Longxi blocks,on whichLuochuan and Jiuzhoutai loess profiles lie respectively,are selected for the study.Luochuan andJiuzhoutai loess profiles are complete and comparable in age of loess-paleosol sequences;Samplesfor paleomagnetic measurement were collected from 4 loess layers of these two profiles:upperpart of L₁,bottom of L₁, bottom of L₅ and bottom of L₁₅.The age of these layers is 15 ka B.P.,85 ka B.P.,and 1.1Ma B.P.respectively.The Ordos block is considered to be motionless;The virtual geomagnetic pole(VGP)posi-tion of different periods was obtained from paleomagnetic measurements of each layer on Lu-ochuan profile and its location. Paleomagnetic declinations expected(D_ex)of Lanzhou were calcu-lated from VGP position and the location on Jiuzhoutai profile.There are differences between de-clination expected(D_ex)and declination observed(D_ob )in different periods. The differences are-0.7±5.2°,-1.8±2.4°,-4.1±3.8°and-9. 0±4.7°。It indicates that Longxi Block hasundergone clockwise rotation relative to the Ordos Block since 1.1 Ma B. P. The mean rotationrate is about 0.5′/ka.

Key words: Quaternary period, Crustal movement, Virtual geomagnetic pole, Ordos fault