天水盆地晚新生代构造演化——对青藏高原北东向扩展的指示意义

Late Cenozoic Tectonic Evolution of Tianshui Basin:Implications for the Northeast Growth of Tibetan Plateau

天水盆地是一个位于青藏高原东北缘的晚新生代盆地,西秦岭北缘断裂穿盆而过。盆地内充填了较为完整的晚新生代地层,记录了该区晚新生代以来的构造变形历史,对研究青藏高原北东向扩展的构造响应具有重要意义。本文基于详细的野外构造变形分析与测量,结合已有的年代学与沉积学研究,初步提出天水盆地晚新生代以来构造变形序列与构造应力场,重建其晚新生代构造演化历史。详细研究表明,天水盆地晚新生代以来主要经历了3期构造演化:即中新世早-晚期NW-SE向构造伸展,沉积盆地发育,并伴随碱性超基性火山岩喷发和金刚石矿床形成;中新世晚期-早、中更新世NE-SW向挤压,盆地发生构造反转,其动力学背景可能源于晚新生代青藏高原的北东向扩展,指示高原物质扩散开始显著影响到西秦岭地区;晚更新世以来受近N-S向伸展作用控制,盆地发生向东有限挤出并伴随顺时针旋转,主要由于青藏高原向北东扩展过程中,区域构造挤压应力方向发生顺时针偏转所致。

The Tianshui Basin is located in the northeastern margin of the Tibetan Plateau, and the junction of the N-S-striking Helanshan-Liupanshan structural belt and the Qinling-Dabie Orogen, which is cut through by the north frontal fault of the western Qinling. This basin was filled with the Late Cenozoic sediments that recorded the deformation process of the area and bore important implications for the northeastern expanding of the Tibet Plateau. On the basis of field structural measurement and analysis, coupling with previous chronologic and sedimentologic studies, we suggest tentatively a deformation process and tectonic stress field sequences in this basin, and further, rebuild the Late Cenozoic tectonic evolution history. A three-stage tectonic evolution sequences are proposed as follows： The first episode（the Early Miocene to Late Miocene） presented by the formation of sedimentary basin due to NW-SE extension, which was accompanied by the eruption of alkaline ultramafic volcanics. The subsequent NE-SW compression, which is likely to be related to the northeastward growth of the Tibetan Plateau, led to the basin inversion in the Late Miocene-Early or middle Pleistocene. Finally, a transpressional regime（nearly N-S extension and E-W compression） became dominated in this region since the Late Pleistocene, which resulted in the limited eastwards extrusion with clockwise rotation intrigued by the rotation of regional shortening direction.