Beth Cowen

2009 - Michael Keen Memorial Awards

There are growing lines of evidence for spatial and temporal correlations between rates of precipitation, surface erosion and deformation along active orogens, however field evidence for couplings and feedbacks between erosional, climatic and tectonic processes are still lacking. The coupled Himalayan orogen and Indian Summer Monsoon (ISM) is an ideal system for the study of these relationships. The Siwalik Group, at the toe of the Himalaya, was formed during the Miocene as the foreland sediment of the uplifting Himalaya. Himalayan orogen is the result of the collision and ongoing convergence of India and Eurasia since ~ 55 Ma. The ISM was established by approximately 12 Ma, as a moist air package moving northward from the Bay of Bengal up to the orographic barrier, formed by the Himalayan foothills, where it rose and cooled, causing a band of high precipitation along the orogenic front. The ISM is perturbed in the eastern Himalaya where the Shillong Plateau, an ~1600 m-high orographic barrier, is located along the northward pathway of the monsoonal circulation, causing high amounts of precipitation along its southern slope and creating a rain shadow in the Bhutan Himalaya to the north.

The uplift of the Shillong Plateau occurred at the Miocene-Pliocene transition, 6-7 Ma, therefore after the ISM had been established and potentially induced a drastic reduction in rainfall along the Bhutan Himalaya front. Meanwhile, low-temperature thermochronological data suggest that the cooling rates (i.e. erosion rates) slowed in Bhutan at the Miocene-Pliocene transition, while the plate convergence rates remained nearly constant, suggesting a temporal correlation between erosion and rainfall distribution.

Since the Siwaliks cover the period of the uplift of the Shillong Plateau, these sediments may carry information on the changes in precipitation patterns and the concomitant changes in erosion in the hinterland. In this study I focus on extracting the climate proxies from the Siwalik sediments to assess the regional climatic changes potentially induced by the uplift of the Shillong Plateau.

A river section in southeastern Bhutan offers ~ 2200 meters of continuous stratigraphic outcrop of the Siwaliks. In this study, measurements of oxygen and hydrogen isotopes in authigenic clays were used to estimate the precipitation within the Himalayan foreland basin at the time of the Shillong Plateau's uplift. Processes of evaporation, condensation, and rain-out of water from air masses passing over mountain belts will systematically affect the isotope composition of the precipitation and therefore of hydrous minerals formed by weathering. Isotopic results show an overall increase in d 18O from the Siwalik deposition to Recent. Since the Miocene there has been a general cooling trend as global ice volume has increased, which has also affected the Bhutan Himalaya through the weakening of the ISM since 2.7 Ma. This trend accounts for some of the isotopic change, but not all that has been observed in this study. Therefore the uplift of the Shillong Plateau is the most viable cause for the change in precipitation distribution since the Siwalik deposition. These results are the first continental paleoclimatic record for the eastern Himalaya, indicating higher precipitation within the foreland basin of eastern Bhutan during the deposition of the Siwalik Group. Accurate dating of the sediments (magnetostratigraphy, detrital thermochronology) will allow us to compare our isotopic data with the established marine record and provide a final interpretation.

Keywords: Himalayan orogen, foreland sediments, monsoon, paleoclimate, continental record

Pages: 75
Supervisor: Djordje Grujic

Awards

Earth Sciences University Medal (2012)
MacEachern-Ponsford Memorial Award (2010)
Michael Keen Memorial Award (2009)