青藏高原东北部阶段性变形隆升：西宁、贵德盆地高精度磁性地层和盆地演化记录

青藏高原东北部的形成演化是检验高原隆升模型及其驱动季风-干旱环境形成假说的关键。青海贵德和西宁盆地新生代高精度磁性地层和盆地演化揭示出贵德和西宁盆地在早新生代两个盆地曾经为一个统一的、发育于东昆仑山前的弱挤压型陆内挠曲盆地或前陆盆地,可能包括兰州盆地、循化-化隆盆地和祁连山东部一些盆地在内的周边地区都向这个统一的盆地内注入水流和沉积物质,在西宁一带形成汇水中心,并在当时为行星风系的亚热带副高压带作用下形成巨厚的膏盐层。从约21Ma的中新世早期开始,前陆盆地挠曲下沉明显加剧,盆地早期地层被挤压变形,形成盆地中最显著的角度不整合,推测分隔贵德盆地东部的海宴—泽库右旋断裂强烈活动,分隔贵德和西宁盆地的拉脊山东部开始隆升,贵德盆地河流水系由北转向西流,至中中新世,隆升可能席卷整个拉脊山,贵德盆地水系明显南流,盆地挤压中心由早先的昆仑山前转移至拉脊山两侧。从约8Ma开始,拉脊山开始强烈阶段性幕式(3.6、2.6及1.8Ma)变形隆升,导致两侧断层以花状向盆地中心逐步扩展,断裂、掀斜和褶皱地层,盆地转变成山间盆地,并在约1.8Ma的强烈变形隆升后,黄河出现,紧接着形成上千米深切河谷和7级阶地,高原东北部现今构造地貌沉积格局最终形成。上述盆地形成演化过程总体揭示出印度板块碰撞早期最远端的高原东北部就已经开始变形隆升响应,这个过程阶段性由弱至强,至8Ma以来达到最大,反映了高原南北的同步变形隆升但幅度不同的动力学过程与形成模式,可能指示了脆性上地壳块体间柔性变形、块体内刚性挤压破裂变形和塑性下地壳连续变形增厚与流动的共同作用机制。

Cenozoic deformation and uplift of the NE Qinghai-Tibet Plateau: evidence from high-resolution magnetostratigraphy and basin evolution

The formation and evolution of the NE Qinghai-Tibetan Plateau are a key to test tectonic models of uplift of the plateau and the hypothesis of the plateau formation in relation to the Asian monsoon and the drying of Asia continent. High-resolution magnetostratigraphy from the Guide and Xining basins, Qinghai Province demonstrates that in the early Cenozoic the Guide-Xining basin was a unified weakly flexural or rejuvenated foreland basin, developed in the foothills of the eastern Kunun Mountains. The nearby Lanzhou and Xunhua-Hualong basins and the basins in the eastern Qilian Mountains may be also parts of a foreland basin. This may have caused the formation of very thick gypsum beds in Xining under paleo-subtropical conditions and wind system, before the plateau uplift. Since the early Miocene (ca. 21 Ma), the basin was subjected to flexural subsidence, accompanied by strong deformation of the early sedimentary rocks (Xining Group), and a change of river drainage from north-flowing to westerly, resulting in an unconformity between the early Xining Group and later Guide Group in the basin. This suggests that the Haiyan-Zeku dextral fault bordering the eastern Guide basin began a strong activation, which may have caused the uplift of the eastern part of the Laji Mountain, which now divides the Guide and Xining basins. In the mid Miocene, this uplift may have reached the whole Laji Shan, giving rise to south-flowing rivers and shifting the flexural subsidence center from the foothills of the Kunlun Mountains to the Laji Mountain. Since about 8 Ma, the Laji Shan began an episodic (~3.6, 2.6 and 1.8 Ma) rapid deformation and uplift, fault propagation into the Guide and Xining basins from both sides of the Laji Mountain. This caused the basin to become an intermontane basin. Since the episodic rapid uplift at about 1.8 Ma, the Yellow River appeared in the Guide Basin, forming a great (up to a thousand meters) incision, resulting in the deep valley, with its seven terraces, that are observed today. The evolution of the Guide-Xining basins demonstrates that the deformation from the early collision of India with Asia plates had reached the remote NE Qinghai-Tibet Plateau and caused an early uplift of the region. This process continued with increasing intensity, peaking at 8 Ma. This suggests a dynamic mechanism or model of a synchronous but magnitude-different deformation and uplift in the south and north Qinghai-Tibet Plateau.