The rise of the huge Tibetan Plateau, with an average elevation of 5 km, presented a major barrier to atmospheric circulation, perhaps one of the largest that has ever existed. With its latitude close to the down flow of the tropical Hadley cells, it has had an effect on the Asian monsoon in particular, strengthening its effects. Many climatologists believe that Tibet has played a major role in global climatic change towards the end of the Cainozoic. So, timing the uplift is critical in assessing the modelled effects in relation to detailed climate records of the Neogene. This is by no means easy, for the late-Tertiary sediments are terrestrial in origin.
A team of Australian and Chinese geologists focussed on the sedimentary record in the Tarim Basin, north of the Kunlun mountains that form the northern flank of the Tibetan Plateau (Zheng, H. et al., 2000. Pliocene uplift of the northern Tibetan Plateau. Geology, v. 28, p. 715-718). Sediments there change from redbeds deposited in gently sloping flood plains to coarse debris laid down by flash floods at a rising mountain front; exactly the relationship that records the beginning of uplift in northern Tibet. Dating this is no easy matter, however. The technique that the team used is magnetostratigraphy, based on highly sensitive measurements of the polarity of 2500 samples of weakly magnetized sediments.
The change in facies spans a period when the Earth’s magnetic field was reversed – the Gilbert reversed chron – which occurred between 4.5 to 3.5 Ma ago. The maximum age for the beginning of Tibetan uplift in the north is therefore 4.5 Ma, in the Pliocene. This contrasts with the accepted age of Oligocene – Miocene for uplift of the Himalaya and southern Tibet, and with models that postulate climatic change that followed it. Whereas the Tarim Basin today is arid, the sediments indicate that until the Pliocene abundant water flowed at the surface, to deposit great thicknesses of fine alluvium.