CHICKEN OR EGG: CLIMATE AND MANTLE TECTONICS ALONG THE HIMALAYA

For researchers interested in climate-tectonic interactions, the Himalaya presents a chicken-and-egg problem. The only Himalayan tectonic model involving climate - i.e., the channel flow model - invokes the South Asian monsoon as the major driver for the major Miocene rise of the Himalayan mountains and their crystalline core. However, climatic models include either the high Tibetan plateau or the high Himalayan mountains as a necessary component for monsoon generation. Something has to be the trigger, unless both the mountain-building and the monsoon intensified simultaneously, perhaps as a dynamically coupled system. This positive feedback mechanism is a common hypothesis, but timing constraints are not entirely satisfying: major mountain-building is thought to have occurred during the period ~23-16 Ma, whereas the monsoon intensifies from ~25 Ma up to a peak at ~15-10 Ma. Therefore, we have formulated a new model: the lateral migration of slab detachment from both ends of the Himalaya (corresponding to the edges of the Indian indentor) to the east-central Himalaya during the period of ~26-25 Ma to ~14-12 Ma led to a corresponding dynamic rebound of the Himalayan mountains, which in turn intensified the South Asian monsoon. This period was preceeded by a period in which the Himalayan mountains moved northwards relative to the down-going Indian slab (i.e., relative slab rollback, eventually leading to the slab tearing), and followed by a period of resumed underthrusting of the Indian plate below the Tibetan plateau (leading to the modern configuration). These concepts provide new explanations of major tectonometamorphic developments in the Himalaya, e.g., the rollback of the slab decreased the Himalayan sole thrust, initiating deep crustal duplexing that generated the Himalayan crystalline core and the shift from "Eo-" to "Neo-" Himalayan metamorphism. Similarly, shallowing of crustal duplexing upon resumed underthrusting of the Indian slab generates the Lesser Himalayan duplex systems. In summary, this new model suggests that for the case of the Himalayan and the South Asian monsoon, slab tectonics is the cause and climate is the effect.