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Figure 19.3. Cenozoic alluvial sediments tilted by recent tectonic activity, Xinjiang
Province, north-west China.
In winter, opposite conditions apply, with cooling of the high-altitude plateau surface
leading to high pressure and cold, dry winds blowing out across India, blocking any
incoming moist air and minimising rainfall at that time ( Figure 19.2 ). Because the
Tibetan Plateau plays such an important role in generating the summer and winter
monsoons, the history of the monsoon must be closely allied to that of the Tibetan
Plateau, especially to when it attained sufficient elevation to have a major influence
on the seasonal wind patterns.
Estimates for the inception of the monsoon extend as far back as the end of the
Oligocene, with successive changes in monsoon activity postulated at 15-13, 9-8 and
3.6-2.6 Ma (An et al., 2001 ;Lietal., 2011 ). We saw in Chapter 18 that marine cores
off the west coast of the Sahara show evidence of more humid conditions on land
at 20-18, 14-13, 9.5-7.5 and 5.3-3.2 Ma (Sarnthein et al., 1982 ). There is a broad
similarity between both sets of ages, suggesting that the summer monsoon influence
may have extended as far as North Africa during those times.
The precise timing of uplift of the Tibetan Plateau is still a focus of research, and
a variety of ingenious methods have been used in an attempt to clarify when uplift
occurred in different parts of the plateau. Amano and Taira ( 1992 ) investigated the
heavy minerals and rates of accumulation in early Miocene to Quaternary sediments
in the Bay of Bengal deposited by rivers flowing from the Higher Himalayas. They
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