Earth Surface Processes and Landforms EarthFluvial Surf. bedrock Process. incision Landforms in Taiwan 30, 955–971 (2005) 955 Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/esp.1256 Fluvial bedrock incision in the active mountain belt of Taiwan from in situ-produced cosmogenic nuclides M. Schaller,1* N. Hovius,1 S. D. Willett,2 S. Ivy-Ochs,3 H.-A. Synal4 and M.-C. Chen5 1 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK 2 Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA 3 Institute of Particle Physics, ETH Hönggerberg, CH-8093 Zürich, and Department of Geography University of Zürich – Irchel, CH-8057 Zürich, Switzerland 4 Paul Scherrer Institut, c/o Institute of Particle Physics, ETH Hönggerberg, CH-8093 Zürich, Switzerland 5 Taroko National Park Headquarters, Fusu Village, Hualien, Taiwan R.O.C. *Correspondence to: M. Schaller, Abstract Department of Geological Sciences, University of Michigan, The concentration of cosmogenic nuclides in rocks exposed at the Earth’s surface is propor- 2534 C.C. Little Building, 1100 tional to the total duration of their exposure. This is the basis for bedrock surface exposure N, University Ave., Ann Arbor, dating and has been used to constrain valley lowering rates in the Taroko gorge, eastern MI 48109-1005, USA. Central Range, Taiwan. Taroko gorge contains a uniquely complete geomorphic record of E-mail: [email protected] 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 36Cl 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 ± 3mma−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 long- term gorge development governed by tectonic uplift is superimposed by short-term varia- Received 1 September 2004; tions in incision rates caused by climatic or regional tectonic changes. Copyright © 2005 Revised 28 February 2005; John Wiley & Sons, Ltd. Accepted 17 April 2005 Keywords: fluvial incision; cosmogenic nuclides; tectonics; climate; Taiwan Introduction Where hillslopes and valley floors are effectively coupled, landscape lowering is driven by fluvial incision into uplifting rock mass, and hillslopes follow. This is the case in most active mountain belts. Independent assessments of fluvial incision and its controls (Tinkler and Wohl, 1998) are fundamental to the understanding of erosional landscape evolution (Howard and Kerby, 1983), crustal deformation (e.g. Beaumont et al., 1992; Koons, 1989; Willett, 1999), basin fill (e.g. Clift and Gaedicke, 2002), and ocean chemistry and atmospheric composition and circulation (France- Lanord and Derry, 1997; Raymo and Ruddiman, 1992). Rates of fluvial bedrock incision are necessarily influenced by climatic and tectonic processes (Whipple et al., 1999). Models of physical processes of fluvial incision are conten- tious, but many arguments have been made that incision is proportional to water discharge or discharge variability (Snyder et al., 2003; Tucker, 2004; Tucker and Bras, 2000) and may be enhanced by temperature-dependent weather- ing (Gaillardet et al., 1999), thus linking incision to climatic processes. In addition, tectonic processes perturb river channel slopes (Snyder et al., 2000) and/or cross-sections (Lavé and Avouac, 2000) and rock mass properties, thus setting the bed shear stress of a given flow, and the erodibility of its substrate. The effective evaluation of climatic and tectonic controls on river incision requires quantitative constraints on erosion rates and patterns on timescales of climate change and tectonic forcing. Dated strath terraces are commonly used for this purpose (e.g. Amorosi et al., 1996; Burbank et al., 1996). Straths are planar remnants of old bedrock channel floors, isolated above an incising river. They are separated by erosional steps and do not normally permit the Copyright © 2005 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms 30, 955–971 (2005) 956 M. Schaller et al. reconstruction of an incision history at high temporal resolution (Pratt et al., 2002). Taroko gorge in east Taiwan contains a rare, continuous record of river incision, consisting of extensive, fluvially sculpted sections of the marble gorge wall. Using in situ-produced cosmogenic 36Cl in marble samples collected from a 200 m high cliff section, we have dated fluvially sculpted features in the gorge wall, and reconstructed the Holocene incision history of the Liwu River. Existing erosion data for the Liwu catchment include annual measurements of fluvial bedrock wear at selected sites (Hartshorn et al., 2002), decadal suspended sediment transport estimates from hydrometric measurements (Water Resources Agency, 1970–2003; Dadson et al., 2003), millennial valley lowering estimates from dated river terraces (Liew, 1988), and fission track estimates of rock exhumation over a million-year timescale (Liu et al., 2001; Willett et al., 2003), making this an optimal location to study (fluvial) erosion and its controls. Placed in the context of these other estimates of incision and erosion rates, our data reveal that Holocene incision by the Liwu River has been considerably faster than the Quaternary average, and that incision rates may have varied within this interval. Study Area Taiwan Taiwan is possibly the best documented collision orogen dominated by fluvial processes. The formation of the orogen results from collision of the Luzon Arc, on the Philippine Sea plate, and the Asian continental margin (Teng, 1990; Figure 1), and links opposite-dipping subduction systems at the Manila and Ryukyu trenches. Obliquity between the Manila trench and the Asian continental margin has led to southward propagation of the collision over the past c. 5 Ma. This propagation manifests as a progression from submarine accretionary prism building at the Manila trench south of Taiwan, through full-scale arc–continent collision represented by the subaerial mountain belt, to orogen Figure 1. The catchment of the Liwu River in eastern Taiwan. The Liwu River flanks the Central Range from the main divide (3500 m a.s.l.) to the Pacific Ocean and drains approximately 600 km2 of steep terrain. Where the Liwu River crosses a 6 km thick sequence of marbles and gneisses, the river forms the deep Taroko gorge. Erosion and incision rates from different studies and the approximate study locations are indicated. Copyright © 2005 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms 30, 955–971 (2005) Fluvial bedrock incision in Taiwan 957 destruction in response to Ryukyu back-arc extension in the far north of Taiwan (Teng, 1996). Across central Taiwan, metamorphic grade increases from poorly consolidated, Late Tertiary sediments in the Western Foothills thrust belt, through slates in the Hsuehshan and western Central Ranges, to greenschist-grade pre-Tertiary metasediments in the eastern Central Range. The current rate of convergence between the Philippine Sea plate and Asia is 80 mm a−1 (Yu et al., 1997), and rock uplift rates of 5–7 mm a−1 have been calculated from Holocene coastal platforms (Bonilla, 1977; Liew et al., 1990). Rapid rock uplift has resulted in the construction of up to 4 km of subaerial relief, but due to its low latitudinal position, Taiwan has never experienced extensive glaciation (Ho, 1988). The main drainage divide of the mountain belt runs parallel to, and c. 25 km west of, the range-bounding Longitudinal Valley fault. Regularly spaced transverse rivers drain the mountain belt to the east, cross-cutting the structural grain. Westward drainage is by larger rivers that follow structural trends. Straight slopes, mostly around 35° (less in weak sedimentary rocks in the Western Foothills) and with thin (<1 m), discontinuous regolith cover, flank the montane valleys (Hovius et al., 2000). Valley floors are in bedrock, mantled by discontinuous, coarse-grained lag deposits. In these cascading channels little material is available for fluvial transport unless provided by hillslope mass wasting (Montgomery and Buffington, 1997). Taiwan has a subtropical climate with an average of four typhoons per year and mean annual precipitation of 2500 mm (Wu and Kuo, 1999). Precipitation is orographically enhanced at high elevation, but, otherwise, is symmetri- cally distributed across the mountain belt. Runoff in the main streams draining the mountain belt reflects the seasonality of precipitation. The bulk of the water discharge occurs between June and October. During typhoon passage, daily rainfall rates in the region can top 400 mm, causing peak discharges of >100 times the annual average. Runoff variability is greatest along the east flank of the Central Range, which is exposed to direct impact of typhoons moving off the Pacific Ocean, and in the southwest of Taiwan. The pollen record of Sun Moon Lake, central Taiwan, indicates that the climate during the Last Glacial Maximum was substantially colder and drier (Kuo and Liew, 2000) (Figure 2). Liwu catchment The Liwu catchment is situated towards the northern end of the compressional Taiwan orogen (Figure 1). The catchment flanks the Central Range from the main divide (3500 m a.s.l.) to the Pacific Ocean and drains approxi- mately 600 km2 of steep terrain underlain by metasediments, mainly schists, gneisses and marbles (Figure 3). Mean annual precipitation is around 2200 mm throughout the catchment and precipitation rates have reached up to 600 mm/ day during typhoon passage. A palaeoclimate record is not available for the catchment, but it is believed that general climate trends are shared with the Sun Moon Lake location (Kuo and Liew, 2000) across the main divide.