Linking erosion and the sediment record : detrital thermochronology in the Taiwan

Whether tectonic activity or climate is the driving force behind the major uplift of mountains during Cenozoic times is a lively modern controversy in tectonic geomorphology. Since this chicken or egg debate was instigated by Peter Molnar and Philip England in 1990 much effort has been invested in determining whether tectonic activity has driven long term climate change or vice versa. However we are only just beginning to understand some of the complex interplay between climate and tectonics and how this shapes the landscape that surrounds us. What is required is greater resolution to solve the problem through an integrated, cross-disciplinary approach. I propose to use both the modern and ancient record of sediment supply from the Central Ranges of eastern Taiwan to investigate long term erosion rates and document whether climate or tectonic activity dominates the erosion record in this active orogen. End member scenarios where climate or tectonic activity dominates the signal will be modelled independently using a surface dynamic model thereby potentially fingerprinting climate versus tectonic forcing in ancient sedimentary records. To tackle this problem I intend to focus on Taiwan due to the high rates of rock uplift and erosion recorded there which reflect both the convergent tectonic setting and sub-tropical climate. Taiwan although small has a major global impact as it contributed almost 2% of the total global suspended sediment entering the ocean in the last 30 years. Such high rates of erosion are not sustainable over prolonged periods of time, as no high grade metamorphic rocks have been exhumed in the core of the orogen, yet average erosion rates since the beginning of collision in the Late Miocene are comparable with average historic erosion rates. Understanding what drives erosion in this setting through time is the ambition of this project. Ultimately the results will help determine how the effects of climate and/or tectonic forcing are preserved in ancient sediment archives and in future help to evaluate the importance of climate versus tectonic forcing in the sedimentary record over geological time. The fluvial response to exhumation in an antecedent river system will be documented and long term erosion rates in both modern and ancient detrital sediments determined using low temperature thermochronometers or geological clocks. The clock concept can be used as a basic tool in investigating what happens to a rock as it first moves from depth towards the surface and then is eroded and finally deposited in a basin. The concept as applied here is based on the fact that minerals become closed to the diffusion of particular elements at particular temperatures. Once closed the clock start to tick and information on the cooling history of the sample is accumulated. If the depositional age of the sample is known, from either magnetostratigraphy or nanofossils, then the time taken by the sample to pass through closure and be exhumed from depth and deposited in the basin can be calculated. The erosion rate can then be inferred by thermal modelling. Current investigations tend to be either model biased or field biased and not well integrated. As a result, once I have collected the data concerning erosion rates both in the Pliocene and Quaternary sequences I aim to apply the surface dynamic model CHILD to modelling sediment flux and fluvial response to long term exhumation forced by climate and/or tectonic variation. The model results will be generated to closely mirror what is preserved in the sediment archive and help distinguish between climate and tectonic forcing in the erosion record. If this is achieved I shall have improved our understanding of how these processes shape the Earth's surface and taken a large step towards linking erosion history with the sediment record.