宁夏海原园河最高基座阶地的古地磁测年

Paleomagnetic Dating to the Highest Terrace of the Yuanhe River,Ningxia,Northeastern Margin of the Tibetan Plateau

宁夏海原园河位于青藏高原的东北缘,穿越强烈活动的香山—天景山和海原断裂带,两断裂带均具有规模大、挤压变形强、左旋走滑量大和发震强度大等显著特点。通过野外对园河阶地沉积及上覆黄土-古土壤系列的取样,古地磁开放实验室的超导磁力仪测试,并与古土壤进行了对比,确定了园河最高阶地的B/M界限、贾拉米洛事件和古地磁年龄为1.04Ma B.P.。结合园河最老基座阶地古地磁测年结果和阶地距现代河流西岸距离为7km,固可得海原断裂带自早更新世晚期以来平均水平走滑速率为6.73mm/a。T6开始发育的时间是1.04MaB.P.,并推测T5上、下限时间为0.593-0.146MaB.P.,时代为中更新世(布容期);T4形成时代属晚更新世,时间为146-49KaB.P.。由此推断早更新世晚期以来海原断裂带是以左旋走滑为主,1.04Ma可能是海原山前断裂带活动的转折点。自1.04Ma以来,园河开始形成了一系列基座阶地,反映海原断裂带北盘在向西运动同时,至少还发生了阶段性的抬升,形成了园河的三级基座阶地。

Located in the Haiyuan County, northeastern margin of the Qinghai-Tibet Plateau, the Yuanhe River flows across very active seismic fault zones, the Xiangshan-Tianjingshan and Haiyuan Fault Zones, with largescale thrusting and sinistral faulting, The Yuanhe River, as the largest and oldest one in the Yuanhe River System, should hint the longest records of both fault zones. Thus, study of landform of the Yuanhe River is of important significance. Although former researchers had measured the river terraces, terrace ages were still unsolved problems. By detail sampling and paleomagnetic measurement to the highest terrace, T6, bed-rock seated terrace, of the Yuanhe River, and the overlying loess-paleosoil series, using superconducting magnetometer, B/M boundary, Jaramillo event, and paleomagnetic age （ 1.04Ma B. P. ） for the highest terrace are determined. Based on the above results, and supposing even strike-slipping of the Haiyuan Fault Zone, together with the width of the asymmetrical valley, the sinistral rate of the Haiyuan Fault Zone and other two lower terraces ages can be estimated to 6. 73mm/a, 0. 593 ～0. 146MaB. P. , and 146 -49KaB. P. , respectively.