Fluvial bedrock incision in the active mountain belt of Taiwan from in situ‐produced cosmogenic nuclides

Abstract

The concentration of cosmogenic nuclides in rocks exposed at the Earth's surface is proportional to the total duration of their exposure. This is the basis for bedrock surface exposure dating and has been used to constrain valley lowering rates in the Taroko gorge, eastern Central Range, Taiwan. Taroko gorge contains a uniquely complete geomorphic record of fluvial valley lowering: continuous, fluvially sculpted surfaces are present in the lower 200 m of this marble gorge. Assuming no post-fluvial erosion of the gorge wall, the concentration of in situ-produced cosmogenic 36 Cl measured in gorge wall marbles reveals exposure ages from 0·2 ka in the active channel to 6·5 ka at 165 m above the present river. These ages imply an average fluvial incision rate of 26 ± ± ± ± ± 3 mm a − − − − −1 throughout the middle and late Holocene. Taking into account lateral gorge wall retreat after initial thalweg lowering would give rise to calculated older exposure ages. Without considering gorge wall retreat, our estimates therefore represent maximum incision rates. Estimated maximum Holocene incision rates are higher than the long-term exhumation rates derived from fission track dating. The longterm gorge development governed by tectonic uplift is superimposed by short-term variations in incision rates caused by climatic or regional tectonic changes.

Exceptional river gorge formation from unexceptional floods

“…Numerous studies have examined rates of gorge formation and knickpoint retreat 2,[5][6][7][8][9] , and the controls on those rates via bedrock erodibility 10,11 , the effectiveness of mechanisms (bedload abrasion/plucking) 8,12,13 and the role of hillslope processes 14 . Most findings are based on conceptual and empirical models 11,12,15,16 or long-term landscape analysis, typically using dating techniques and flood reconstruction [5][6][7]17,18 . These are plausible approaches but inherently involve unknowns and assumptions.…”

Long‐term landscape evolution: linking tectonics and surface processes

“…Rapidly eroding, tectonically active settings are characterized by massive transfers of crustal material, both as rock is tectonically advected into the orogen, and as rock (sediment) is advected out of the orogen by surface processes; these fluxes provide one of the great attractions of research in these areas. Thus sediment fluxes out of active mountain belts correspond to rates of river incision of 2-12 mm a −1 in the Himalayan Indus (Burbank et al, 1996;Hancock et al, 1998), 10-20 mm a −1 in the Southern Alps (New Zealand) (Tippett and Hovius, 2000), 2-6 mm a −1 in the Spanish Sierra Nevada (Reinhardt et al, in review) and 3-7 mm a −1 (and locally up to 60 mm a −1 ) in Taiwan (Dadson et al, 2003(Dadson et al, , 2004Schaller et al, 2005). These rates, which for steady-state landscapes correspond to rates of landscape lowering, may be compared with rates of <0·01 mm a −1 in the PCM settings described above.…”

“…A large proportion of recent research documenting bedrock river incision and channel-hillslope linkages has focused on large rivers flowing through seismically active orogens experiencing rapid rock uplift and very high-magnitude, monsoonal or typhoon-driven storm events, such as in the Himalaya or Taiwan. These large, sediment-laden bedrock rivers are able to incise bedrock rapidly, at rates that are generally understood to keep pace with rock uplift (Southern Alps, Adams, 1980Adams, , 1985the Indus, Burbank et al, 1996;Hancock et al, 1998;Taiwan, Hartshorn et al, 2002;Schaller et al, 2005), and to maintain valley sides at the critical slope angle for rock failure (Burbank et al, 1996;Hovius et al, 1997;Hovius, 2000;Dietrich et al, 2003;Roering et al, 1999Roering et al, , 2005 (Figure 12). In numerical models, steadystate topography is attained in a time that scales with the uplift rate, width of the mountain belt and physical parameters of the erosion model (Kooi and Beaumont, 1996;Willett et al, 2001).…”

Direct Measurements of Bedrock Incision Rates on the Surface of a Large Dip-slope Landslide by Multi-Period Airborne Laser Scanning DEMs

“…The evaluation of the erosion rate is important in the study of landform evolution, slope and channel interactions, landslide effects, sediment transport processes, and so on. The methods for calculating the erosion rate include the cross section measurement method [15,16], the erosion pin method [17], radiometric dating [18][19][20][21][22][23][24][25], the river suspended-sediment discharge technique [26,27], dendrogeomorphological methods (based on tree-ring analysis from exposed roots) [28,29], interferometric synthetic aperture radar (InSAR) [6],…”