青藏高原南缘新生代磨拉石的沉积特征

青藏高原南缘前陆盆地中的磨拉石主要沉积在小喜马拉雅带和次喜马拉雅带中 ,次之出现在冲积平原带。最老的新生代磨拉石为分布于 Potwar高原 (巴基斯坦 )的 Murree组 ,时代为始新世中期 -早中新世末。前陆盆地中分布最广泛的为 Siwalik群 ,它占据了次喜马拉雅带的绝大部分 ,时代主要为中新世 -上新世早期。更新的磨拉石主要分布在山前冲积平原带中 ,为第四纪的冲积物。总的来看 ,前陆盆地中的新生代磨拉石表现出向上变粗变厚、向南变细变新的变化趋势 ,并有盆地西部先沉积的特征。结合前人关于古印度扇、印度扇、孟加拉扇的研究资料 ,可以发现青藏高原南缘的造山作用具有向南生长的特点 ,同时该区出现早期东高西低、后期西高东低的地形变化过程。

FEATURE OF CENOZOIC MOLASSE AT THE SOUTH EDGE OF QINGHAI-TIBETAN PLATEAU

The Himalayan Tectonics Zone to the south of Indo-Yarlung Zangbo suture zone is divided into: Tethys Himalayan zone, Higher Himalayan zone, Lesser Himalayan zone, Sub-Himalayan zone and Alluvial Plain zone. Cenozoic molasse at the south edge of Qinghai-Tibetan Plateau was mostly deposited in the zones of Lesser-Himalaya and Sub-Himalaya, others in Alluvial Plain zone. The oldest Cenozoic molasse is Murree Formation. It outcrops in Potwar plateau (Pakistan) and was deposited from middle-Eocene to earlier-Miocene. The Siwalik Group (Miocene to Pliocene) has the widest distribution and covers most part of Sub-Himalaya zone. The youngest Molasse is Quaternary fluvial sediment. It is exposed in Alluvial Plain zone. In the summary, the Cenozoic molasse at the south edge of the plateau has a coarsing and thickening sequence upward. It becomes finer and younger southward and is older in the west than in the east. All these indicate that after the collision of India and Euro-Asia blocks, the orogenic zone grew southward and topography changed from higher to lower in the east at the south edge of Qinghai-Tibetan Plateau. This conclusion can be proved by the discovery of the palaeo-Indus delta-fan complex, the studies of the Indus fan and Bengal fan and the topographic feature of the Himalaya at present.