THE GROWTH OF TIBET AND ITS IMPACT OF CLIMATE IN ASIA (Invited Presentation)

Much evidence suggests that Tibet and its surroundings underwent changes at ~15-10 Ma, long after the collision of India with the rest of Eurasia. Normal faulting. east-west extension, and crustal thinning across the plateau began; incision of the eastern margin of the plateau accelerated, presumably in response to surface uplift; increased sedimentation and a pulse of thermochronological dates suggest accelerated of erosion of high terrain in surrounding areas of the Tien Shan and Mongolia, consistent with their emergence as high terrain, etc. Collectively, these observations are consistent with removal of mantle lithosphere beneath Tibet, an abrupt surface uplift of ~1000 m, and an increased horizontal force per unit length of 3-4 TN/m applied to surrounding terrain. These apparent changes in surface elevation of Tibet and surroundings occurred approximately concurrently with climate changes in eastern Asia, but no single, simple mechanism links surface uplift to all climate changes. First, the widely espoused (by me among many) strengthening of the South Asian monsoon at ~10 Ma is an artifact of emergence of the Owen Ridge above the Calcite Compensation Depth. In fact, evidence from vertebrate paleontology suggests that northwestern India and northern Pakistan became more arid, possibly in response to a rise of eastern Tibet that induced subsidence of air over the arid region. The height of Tibet seems relatively unimportant to the modern South Asian monsoon; if heating over Tibet affects the monsoon, it apparently does so only early and late in the monsoon season. Enhanced loess deposition on the Loess Plateau in China beginning near 10 Ma may have resulted from the emergence of high terrain in Mongolia, but the connection of Tibet to this aspect of Asian climate would then be strictly geodynamic: the increased in force per unit length led to the construction of high terrain across which lee cyclogenesis could kick up sufficient dust. One size does not fit all connections between climate changes and a rise of Tibet’s surface.