139736eo.pdf (20MB) - Japan Oceanographic Data Center

The ophiolite suite along the Indus Suture Zone and in Tsang Po Valley marks the formersubduction zone along which the Indian Plate was being consumed under the Eurasian Plate. Majorthrusting in the Himalayas began at the close of Eocene epoch (36-37 Ma), and this time may alsomark the more intimate contact between the two plates. The large scale vertical movements on Eurasia,which was previously a stable platform, began at this time and many of the eastern China's basins andgrabens have developed since then (MOLNAR and TAPPONNIER, 1975).The junction of India with Eurasia had serious repercussions for the Asian mainland. Over1500 km of crustal shortening and deformation of the continental lithosphere has taken place since thebeginning of the collision. The deformation zone extends up to 3000 km northeast of the Himalayas.The absence of a long and shallow-dipping fault zone between India and the overlying Tibetan crustprecludes underthrusting of India along the Indus Suture Zone at present (MOLNAR andTAPPONNIER, 1975). According to these writers, crustal shortening by underthrusting of the IndianPlate beneath Himalayas and Tibet in the geologic past accounts for only 300 to 700 km. Another200-300 km can be accounted for by crustal thickening and thrusting in the Pamirs, Tien Shan, Ataiand Nan Shan areas. A major fraction of this convergence occurred along major east-west trendingstrike-slip faults in China and Mangolia and may account for up to a 1000 km of crustal shortening.Such large strike-slip faulting may be a characteristic feature of collisions between continents(MOLNAR and TAPPONNIER, 1975).Early Cenozoic Oceanography and ClimatesDuring the late Cretaceous and early Cenozoic, the northward migration of India progressivelycreated more open ocean conditions south of the Indian Plate and between other pieces of the ancientEast Gondwanaland. The tectonic activity of this period and changing bottom water circulation,however, caused a very discontinuous or patchy stratigraphic record (KIDD and DAVIES, 1978).Pelagic clays continued to be deposited in the deeper parts of the basins and calcareous sediments inthe shallower areas. In the Tethyan Seaway thick carbonates still accumulated in Paleocene and mostof the Eocene. However, as the seaway became narrower and shallower the nature of carbonateschanged accordingly.First major influx of sediments in the northern Arabian Sea occurred in the middle and lateEocene, after the initial uplift of the Himalayas. First Indus-derived sediments in this area and themajor buildup of the Bengal Fan began in the Oligocene (KIDD and DAVIES, 1978). CURRAY andMOORE (1974) suggested that the Bengal Fan was already building up, from a western river source,as far back as the early and late Cretaceous.The mid Paleocene paleoceanographic patterns (Fig. 7) show that the source of at least somedeeper waters was probably still in the mid to low latitudes, although by this time Pacific deep watersmay have been of mixed high and low latitudes origin. The Tethys Current dominated thecircum-global circulation. In the Indian Ocean south of the drifting Indian Plate the cell was mostlikely deflected southwestward, hugging the western Australian margin during the return floweastward.The position of India in the earlier Eocene did not severely restrict the flow of the TethysCurrent north of the Indian Plate. During the generally high seastand times of the early middle Eocenethis passage may have remained open to the westward Tethyan flow. In late middle Eocene the generaldrop in sea level and the further convergence of India and Asia, reduced the passage more severely andthe main flow shifted to the west of the Indian Plate (Fig. 8). This is indicated by the continuedpresence of neritic marine and marginal sediments of middle and late Eocene age in Pakistan(McGOWRAN, 1978) and their general absence further northeast. The Tethys Current becamesharply restricted to a narrow southwestern passage by early Oligocene (ca 35 Ma).Near the Eocene/Oligocene boundary (ca 37-36 Ma) there is widespread evidence of thedevelopment or intensification of bottom water activity and a general climatic cooling in all oceanbasins. Oxygen-isotopic data from the Indian Ocean (Fig. 9) show generally high temperatures in thelate Cretaceous and Paleocene, reaching a peak in the late Paleocene-early Eocene interval. Thesepatterns follow global trends, and this interval may have been the warmest in the Cenozoic (HAQ,1982). First temperature drop occurred in the middle Eocene, followed by another period of relativestability, until the Eocene/Oligocene transition, when a dramatic drop both in the surface and bottomwater temperatures took place. This event manifests itself in the presence of widespread erosionalhiatuses in the sedimentary record throughout the Indian and southwestern Pacific Oceans. The62