西秦岭岷县-宕昌地区洮河和岷江阶地特征对比研究——兼论中国南北构造带在西秦岭的地貌响应

Study and Comparision on the Terrance of Taohe River and Minjiang River in Minxian-Dangchang Area of the Western Qinling——Discussion on Geomorphological Respones of the SN Tectonic Zone in China

西秦岭岷县-宕昌地区位于我国环绕青藏高原东北缘的重力异常和地壳增厚梯度带,同时又位于贺兰-川滇南北构造带与秦岭造山带交汇迭加复合(西秦岭-松潘大陆)构造结内。特殊的大陆动力学环境形成了特定构造-地貌转换区域,区内独特的河流格局和河流地貌研究可以为认识青藏高原东北缘隆升最新过程和南北构造带构造活动的地貌响应提供重要依据。笔者通过对该区洮河和岷江两条河流阶地特征的对比研究,结合区域地貌过程分析,提出在晚更新世中-晚期(30～50kaBP),洮河与岷江曾为同一条河流,笔者称之为古岷江,其中岷县以西现今洮河的临潭-卓尼-岷县段为古岷江上游,而岷县以北洮河的木寨岭南坡-岷县段则为古岷江之支流。30～50kaBP以来地壳快速隆升形成的北北东向分水岭切断了古岷江上下游的水力联系,阻塞了古岷江下泄,且在岷县一带形成汇水盆地,沉积了近30m的河湖相堆积,而后木寨岭南坡-岷县的古岷江的支流溯源侵蚀,切穿了木寨岭分水岭,与西秦岭北坡的洮河水系贯通,逐渐形成现今的河流格局。该地区晚更新世以来形成的北北东向地貌隆起带导致的洮河和岷江水系之间的切断和袭夺,可以解释为印度板块和欧亚板块的碰撞造成的青藏高原隆升过程中东北缘高原物质向东扩展受到阻挡和南北构造带深部热物质上涌的地貌学响应。

Minxian-Wudu area in the western Qinling is just located at the intersection of Qinling-Qilian Kulun EW tectonic zone and Helan-Chuandian SN tectonic zone and the northeastern fringe of uplifted or uplifting Qinghai-Tibet Plateau, where is in the gravitational gradient zone and crust thickness steep gradient zone circling Qinghai-Tibet Plateau. Its geomorphological characteristics, in particular, its stream geomophological evolution, can provide the latest messages on the interaction of uplift of Qinghai-Tibet Plateau and activity of SN active tectonic zone. Based upon the comparision for terrances of Taohe River（TR） and Minjiang River（MR） in this area and analysis for regional geomorphology it is suggested that this two rivers had been a river before the late Pleistocene （about 0.03 Ma BP）, called ancient Mingjiang River（AMR）, and Lintan-Zhuoni-Minxian sector of the present TR was the upper reach of AMR, Muzhailing-Minxian sector of TR was one of tributary streams of AMR. The geologically historical relation- ship between the TR and MR above is interpreted as the result of the NNE strike divide ridge appearing 0.03 Ma BP, which cut the hydraulic relation between the TR and MR. The NNE strike divide ridge in this area is suggested to be the geomorphological response of thermo-tectonic activity of the Helan-Chuandian SN tectonic zone and the external diffusion and rapid uplift of Qinghai-Tibet Plateau.