Cooperative Institute for Research in Environmental Sciences

Brian Yanites collects a sample for optically stimulated luminescence dating by pounding a hollow tube into a fluvial sand deposit.

This bridge over the Peikang River has been partly buried as a result of earthquake- and typhoon-triggered landslides, which fed massive volumes of sediment to the river following the 1999 Chichi earthquake.

Tectonics and Topography in Taiwan

Erosional Forcing of Late Quaternary Compressive Strain, West Central Taiwan

Taiwan’s rugged mountains are among the fastest growing on planet Earth. Taiwan owes its geologic vigor to plate tectonics: a collision between the Eurasian plate and a massive undersea ridge known as the Luzon Arc beginning about five million years ago has thrust rocks upward to build the island. The collision continues to this day, with the result that earthquakes are perennial hazards to the island’s 23 million residents. In 1999, a particularly powerful earthquake struck the west-central region of Taiwan, killing 2,400 people and leaving more than 100,000 homeless. Earthquakes are not the only natural hazards in Taiwan. Its steep relief and wet, typhoon-prone climate make it one of the most rapidly eroding landscapes on the planet. Much of that erosion occurs in the form of floods and landslides; a tragic landslide in August 2009, for example, buried an entire village.

Taiwan’s ferocious geological and meteorological activity has drawn a great deal of scientific interest. Our project is focused on understanding how rivers cope with this unusual mix of earthquakes and typhoons, and on what Taiwan’s rivers can reveal about processes in the crust below. Recent Ph.D. graduate Brian Yanites, together with Greg Tucker, CU-Boulder professor Karl Mueller, and a team of researchers from Taiwan and New Zealand, undertook a study of the Peikang Shi, a powerful river that slices across a series of active faults on Taiwan’s western flank. The river valley is lined by numerous bedrock terraces, which represent preserved segments of ancient floodplain that were left behind as the river continued to slice through the rocks below. By determining the age of these terraces using the method of optically stimulated luminescence dating, the team was able to estimate the rates of river incision over periods of thousands of years. The dates revealed a distinctive spatial pattern of river incision: just upstream of certain faults, the river has been slicing much more rapidly than in other locations, implying that the faults are tectonically active. The results provide useful new information for understanding geologically recent patterns of seismicity and rock deformation. Not surprisingly, the data also revealed a correlation between the rate of incision and the river’s hydraulic power.

What was more surprising was the manner by which the river appears to adjust its hydraulic power. In some stretches, variations in power occur because of changes in the river’s width: a narrower channel leads to higher fluid stresses along the bed. In others stretches, however, increased hydraulic power is associated with a steeper gradient rather than a narrower channel. This finding prompted us to develop a new mathematical model to explain why bedrock-incising rivers “choose” a particular combination of width and channel gradient. The model appears to explain not only the Taiwan data, but also recent data from bedrock-incising rivers in various locations around the globe.

Supported by NSF EAR-0510971, PI Karl Mueller with Co-investigators Greg Tucker and Phaedra Upton, 2006-2010

References:

Chen, Y.G., Hung, J., Lai, K., Lin, Y., Wilcox, T., and Mueller, K., "River terrace development in response to folding above active wedge thrusts in Houli, Central Taiwan", Journal of Asian Earth Sciences, p. 240-250 , vol. 31, Issue 3, (2007).

Hsu, H.L., B.J. Yanites, C.C. Chen, Y.G. Chen. (2010) Bedrock detection using 2D electrical resistivity imaging along the Peikang River, central Taiwan. Geomorphology, 114, n.3 (406-414), http://dx.doi.org/10.1016/j.geomorph.2009.08.004

Upton, P., K. Mueller, and Y. Chen (2009), Three-dimensional numerical models with varied material properties and erosion rates: Implications for the mechanics and kinematics of compressive wedges, J. Geophys. Res., 114, B04408, doi:10.1029/2008JB005708.

Yanites*, B.J., and Tucker, G.E. (2010) Controls and limits on bedrock channel geometry: Journal of Geophysical Research, v. 115, F04019, doi:10.1029/2009JF001601.

Yanites, B.J., Tucker, G.E., and Anderson, R.S. (2009) Numerical and analytical models of cosmogenic radionuclide dynamics in landslide-dominated drainage basins: Journal of Geophysical Research, v. 114, F01007, doi:10.1029/2008JF001088.

Yanites, B.J., Tucker, G.E., Mueller, K.J., and Chen Y.-G. (2010) How rivers react to large earthquakes: Evidence from central Taiwan. Geology, v. 38, no. 7, p. 639-642, doi:10.1130/G30883.

Yanites, B.J., Tucker, G.E., Mueller, K.J., Chen Y.-G., Wilcox, T., Huang S.-Y., and Shi K.-W. (2010) Incision and channel morphology across active structures in the Peikang River, central Taiwan: Implications for the importance of channel width. Geological Society of America Bulletin, v. 122, p. 1192-1208.