Quantification of the transient response to base-level fall in a small mountain catchment: Sierra Nevada, southern Spain

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112,, doi: /2006jf000524, 2007 Quantification of the transient response to base-level fall in a small mountain catchment: Sierra Nevada, southern Spain L. J. Reinhardt, 1,2 Paul Bishop, 1 T. B. Hoey, 1 T. J. Dempster, 1 and D. C. W. Sanderson 3 Received 13 April 2006; revised 29 January 2007; accepted 29 June 2007; published 26 September [1] We integrated optically stimulated luminescence dating and 10 Be cosmogenic nuclide measurements to quantify short-to-medium-term ( years) catchment dynamics and response to active tectonics. In the 27 km 2 Río Torrente catchment, Sierra Nevada, southern Spain, rapid base-level fall has triggered knickpoint migration up both trunk and tributary channels, resulting in two distinct geomorphic zones: (1) a steep lower catchment with concordant rates of hillslope erosion and channel incision over both short (100 years: 5 8 mm yr 1 ) and medium (12 ka: 5 ± 1 mm yr 1 ) timescales and (2) a low-relief soil-mantled upland topography with uniformly low bedrock and hillslope erosion rates (0.05 ± 0.02 mm yr 1 ). The uniformity of erosion in this upland surface indicates that catchment topography was previously in steady state. Rapid river incision below the channel knickpoints has resulted in the development of steep landslidedominated hillslopes that are essentially tracking the incising channel. The magnitude of base-level fall required to generate these steep hillslopes is >100 m; at least 50 m of this base-level fall occurred during the past 21 ka. These steep hillslopes are eroding back into the low-angled upland surface at a much slower rate than the channel knickpoint. Consequently, the trunk channel knickpoint has already reached the catchment headwaters while hillslopes continue to adjust to the new base level, indicating that the channel profile will regain equilibrium form long before hillslopes. Thus hillslopes are the limiting factor for the duration of landscape transience in this small mountain catchment. Citation: Reinhardt, L. J., P. Bishop, T. B. Hoey, T. J. Dempster, and D. C. W. Sanderson (2007), Quantification of the transient response to base-level fall in a small mountain catchment: Sierra Nevada, southern Spain, J. Geophys. Res., 112,, doi: /2006jf Introduction [2] The growth in studies in bedrock channel processes and evolution over the last decade reflects the fundamental importance of bedrock river channels in determining the relief structure of tectonically active areas [e.g., Howard et al., 1994; Whipple and Tucker, 1999]. Bedrock channel incision is the principal mechanism by which changes in base level are communicated to catchments. Thus elucidating bedrock river response to surface uplift is fundamental to understanding catchment-wide responses to perturbation, landscape recovery times and sediment fluxes from catchments that experience base-level changes. The importance of the latter issue stems from the fact that the channel is a key control on sediment flux, providing the base level for 1 Department of Geographical and Earth Sciences, University of Glasgow, Glasgow, UK. 2 Center for Earthquake Research and Information, University of Memphis, Memphis, Tennessee, USA. 3 Scottish Universities Environmental Research Centre, East Kilbride, Glasgow, UK. Copyright 2007 by the American Geophysical Union /07/2006JF hillslope processes and hence for the generation of sediment from a catchment [e.g., Howard et al., 1994]. [3] It is widely accepted that in tectonically active mountain ranges hillslopes respond to rapid river incision essentially instantaneously by steepening and (re)attaining the critical slope angle for rock failure [Howard et al., 1994; Burbank et al., 1996] Thus, in active mountain belts experiencing high rates of rock uplift, such as the Karakoram [Burbank et al., 1996; Hancock et al., 1998], the Southern Alps [Adams, 1985; Hovius, 2000], and Taiwan [Dadson et al., 2003, 2004], high rates of rock uplift are matched by rapid river incision and adjacent hillslopes are maintained by landsliding at the critical hillslope angle for rock failure [Hovius et al., 1997; Dietrich et al., 2003; Roering et al., 1999, 2005]. For convenience we term these landscapes as type 1 landscapes (Figure 1a). In situations where a change in the rate of base-level lowering is communicated through the drainage net by migrating knickpoints hillslopes can only respond instantaneously to base level after a knickpoint(s) migrates past the base of the hillslope [Bigi et al., 2006]. Thus the duration of transience in such landscapes is determined by the rate of knickpoint migration [Whipple and Tucker, 1999; Crosby and Whipple, 2006]. Knickpoints are here defined as upstream migrating, 1of20

2 Figure 1. (a) Diagrammatic cross section of a type 1 steady state landscape in which channels are incising by knickpoint retreat and hillslopes are lowering at the same rate. Note that the landscape consists of hillslopes (valley sides) at the characteristic slope angle for the lithology s failure by landsliding. Drainage spacing, d 1 and d 2, affects landscape morphology via its determination of slope length, but otherwise drainage density does not affect landscape morphology. (b) Diagrammatic cross section of a type 2 landscape (T 0 ) experiencing a large increase in the rate of base-level lowering. The soil-mantled steady state landscape experiences a rapid increase in the rate of base-level lowering at time T 1, triggering channel incision by knickpoint propagation and the establishment of new hillslopes at the critical slope angle for rock failure by landsliding. These steep time-invariant hillslopes erode back into the low-angled soil-mantled surface during times T 1 and T 2. A break in slope (points of hillslope inflection) marks the boundary between the steeper, landsliding-dominated lower hillslopes and the gentler, soil-mantled and diffusion-dominated upper slopes. By time T 3, a new type 1 steady state topography has developed. Geometry means that the duration of transience between type 2 and type 1 landscapes is a function of the rate at which the point of inflection moves laterally away from the channel (a function in turn of the rate of channel incision and the characteristic slope angle for landsliding on that lithology) and drainage spacing. step-like steepenings in the channel profile initiated by a base-level change [cf. Gardner, 1983; Hayakawa and Matsukura, 2003; Bishop et al., 2005; Crosby and Whipple, 2006]. Theoretical derivations and field data imply that the rate of knickpoint propagation is strongly dependent upon stream gradient and drainage area (i.e., discharge) in both bedrock rivers [Whipple and Tucker, 1999; Hayakawa and Matsukura, 2003; Bishop et al., 2005] and alluvial systems [e.g., Begin, 1988]. Accordingly, rates of knickpoint retreat decline as knickpoints propagate headward, reflecting decreasing catchment area. It follows that the final stages of knickpoint retreat in small headwater catchments may be very slow [Clark et al., 2005, 2006; Crosby and Whipple, 2006]. [4] A second situation is associated with knickpoint propagation into a topographic steady state landscape of the type described by Hack [1960, 1975]. Slopes in this type 2 landscape reflect, not rock strength and the propensity of the rock slopes to fail by landsliding, but soil strength and the corresponding equilibrium morphology of soil-mantled slopes (Figure 1b). As in type 1 landscapes, type 2 landscapes are in topographic steady state (i.e., they lower at a uniform rate across the landscape), but hillslopes are generally soil mantled and not prone to slope failure by 2of20

3 the rate of base-level lowering is being communicated into the drainage net by upstream migrating knickpoints. Figure 2. Topographic map of the western Sierra Nevada and Río Torrente catchment, generated from 30 arc sec GTOPO30 digital elevation data. The Padul normal fault that marks the mountain front where the Río Torrente leaves the mountain block is highlighted. bedrock landsliding. These hillslopes also tend to be convex as the rate of sediment flux is linearly related to hillslope angle [Gilbert, 1909; Ahnert, 1976, 1987; Fernandes and Dietrich, 1997; Roering et al., 1999, 2001]. Such hillslopes respond to increases in the rate of base-level lowering by slowly increasing the rate of sediment production and hillslope convexity and/or through lateral hillcrest migration [Fernandes and Dietrich, 1997; Mudd and Furbish, 2005]; landsliding is generally restricted to the soil mantle. [5] If the rate of base-level lowering in a type 2 landscape increases to the high rates seen in type 1 landscapes (millimeters per year), hillslopes steepen to the critical angle for bedrock landsliding (Figure 1b). In such situations, the progressive attainment of the new steady state across the landscape is marked by the lateral propagation of the break in slope (or point of inflection) between the steep steady state landslide-dominated lower hillslopes and the lowerangled steady state soil-mantled upper hillslopes (Figure 1b). Until steady state is attained everywhere, the landscape is a transient mosaic of largely diffusive, soil-mantled, steady state upland surfaces, with low to moderate slope angles, and steep, bedrock landsliding-controlled steady state valley sides. Clark et al. [2005] suggested that such transient landscapes may persist in active orogens for up to several tens of millions of years, but detailed data on the rate(s) of attainment of topographic steady state (and therefore the durations of the transient state) following, for example, an increase in the rate of base-level fall are rare. This is especially the case for small upland and headwater streams, which being in the upper reaches of the drainage net are the last parts of the landscape to adjust to a new state. [6] Our focus in this study is the nature and duration of transience in a small type 2 mountain catchment experiencing an abrupt increase in the rate of base-level fall. We use optically stimulated luminescence (OSL) and cosmogenic isotope analysis to provide quantitative data on transient dynamics in the Rio Torrente, a small mountain catchment in the semiarid Sierra Nevada, Spain. The abrupt change in 2. Río Torrente, Sierra Nevada [7] The Spanish Sierra Nevada is an 80 km 40 km eastwest trending mountain block in the westernmost extension of the Alpine orogeny [Dewey et al., 1989] (Figure 2). Uplift of the Sierra Nevada began 9 Ma[Braga et al., 2003], culminating in the present mountain belt with a maximum elevation of 3479 m, the highest in mainland Spain. The topography of the western Sierra Nevada is characterized by rounded mountain tops and deep valleys: mean valley relief is 1640 ± 600 m [Reinhardt et al., 2007a]. The long-term denudation history of the western Sierra Nevada indicates that over million year timescales denudation is controlled by deep-seated (<1.5 km) structural detachments, which also act to limit relief [Schmidt and Montgomery, 1995; Sanz de Galdeano and López-Garrido, 1999; Reinhardt et al., 2007a]. [8] The Río Torrente study area is a small semiarid catchment in the western Sierra Nevada (Figure 3) with a surface area of 27 km 2. No relict glacial or periglacial features are observed in this 3000 m high catchment, consistent with the view that late Pleistocene glaciation of the Sierra Nevada was minor [Lhénaff, 1977; Sánchez et al., 1990; Gomez-Ortiz et al., 1996; Schulte, 2002]; a Mediterranean climate was established in this region 12 ka [Jalut et al., 2000]. The catchment is divided into two lithological sectors along a NE SW axis by a major shear zone, the Betic Movement Zone (BMZ) [Platt and Vissers, 1989]. The schistose Nevado-Filábride unit lies SE of this axis and the Alpujárride thrust sheet complex to the NW (Figure 3b). The Alpujárride is composed of >1 km thick succession of NW dipping low-grade dolomitic marbles with occasional thin units of phyllite and/or schist 10 m thick. Quartz is unevenly distributed in the Alpujárride part of the catchment and we therefore did not measure 10 Be concentrations or investigate erosion rates within this area. Instead, the focus of our work was the graphiticmica-schist Nevado-Filábride unit SE of the BMZ where quartz veins are ubiquitous and the percentage of quartz grains recovered from fluvial sediments are uniform, i.e., 11 ± 2% [Reinhardt, 2005], thereby enabling 10 Be mean catchment-wide erosion rate measurement. [9] The Nevado-Filábride part of the Río Torrente catchment comprises two geomorphic zones (Figures 3 and 4). The upper, headwater parts of the catchment are characterized by relatively low slope angles (tan slopes <0.5) and a thin soil cover with dense low-lying shrub [Sánchez- Marañón et al., 1996]. The lower catchment is steeper (tan slopes >0.5) and more sparsely vegetated. The valley side NW of the trunk channel comprises the BMZ and Alpujárride metacarbonates, which form a near-vertical cliff subject to rockfalls and rock slides (Figure 5). On the left-bank (SE) valley wall the highly schistose Nevado-Filábride dips NW toward the trunk channel at 40. The strong schistosity of the Nevado-Filábride promotes shallow rock and debris slides. High rainfall can transform these into debris flows generating high-sediment fluxes in these steep channels (Figures 5 and 6). The gradient of the trunk channel in the lower catchment ranges between 0.1 and 0.5, well within the range of channel 3of20

4 Figure 3. Topographic, geological, and slope maps of the Torrente catchment generated from a 3 m digital elevation model that was constructed from aerial photographs. (a) Slope map of the Torrente catchment highlighting the sharp transition (inflection in hillslope) between the upper, low-angle (<25 ) portion of the catchment and the steep (>25 ) landslide-dominated portion at lower elevations. The zone of increased surface roughness in the southeast quadrant of this map is an artifact of the DEM generation process. Spatial autocorrelation analysis indicates that this artifact has a negligible influence upon catchment topography; the Moran autocorrelation indices of this rough area and adjacent surfaces are almost identical: A = and B = [Goodchild, 1986]. The white lines (excluding 2a) denote transect lines along which hillslope profiles were plotted in Figure 7. The black dashed lines extending from transects 1, 2a, 3, 4, and 5 denote lines along which the distance between hillslope inflection and catchment drainage divide were measured, as described in section 5.2 (Table 4). (b) Combined geological and geomorphic map of the Río Torrente catchment showing the locations of all 10 Be and OSL samples. The locations of the following are also shown: the debris flow terrace detailed in Figure 6 and the channel cross section detailed in Figure 9. BMZ is the Betic Movement Zone. 4of20

5 Figure 4. Northeasterly looking view of the upper Torrente catchment. Knickpoints are currently migrating up the Río Torrente and tributary channels (see also Figure 11). The head of the knickpoint in each channel is highlighted with arrows. The location of the trunk channel knickpoint A is shown in Figure 7. gradients expected to be dominated by debris flows (i.e., gradient >0.1) [Montgomery and Foufoula-Georgiou, 1993; Stock and Dietrich, 2003, 2006]. [10] The Torrente s upper and lower catchments are separated by knickpoints that are propagating up both trunk and tributary channels from the mountain front (Figures 4 and 7). These knickpoints are associated with an abrupt change in local hillslope gradient, and a transition from shallow alluvial (vegetated) channels in the upper catchment to deeply incised bedrock channels subject to frequent bedrock landslides in the lower catchment. The catchment exit coincides with the Padul Fault which is part of a major crustal-scale fault array marking the SW edge of the Sierra Nevada block (Figures 2 and 7). This fault array is responsible for 420 m of vertical displacement since Ma in the Pleistocene Alhambra Fm. alluvial fan sediments 10 km NW of the Torrente catchment [Sanz de Galdeano, 1996; Keller et al., 1996; Sanz de Galdeano and López-Garrido, 1999]. We take this long-term average rock uplift rate of mm yr 1 on the Padul fault to be the rate of baselevel fall in the Torrente catchment. The rate of base-level fall is much greater over shorter timescales. The Río Torrente has recently incised through 100 m depth of alluvial fan sediments at the mountain front. The upper surface of these alluvial fan sediments (termed the Nigüelas Formation) is preserved close to the catchment exit and the sediments have been dated to 16 ± 6 ka using OSL [Reinhardt, 2005]. It is likely that this 100 m thick piedmont fan developed during a regional aggradational phase beginning 21 ka [Reinhardt, 2005]. Subsequent movement of the Padul fault has led to the development of a 50 m high bedrock fault scarp at the mountain front; this fault has propagated into the alluvial fan disrupting its surface generating a similar 50 m high fault scarp within the fan sediments (Figures 7 and 8). Thus the rate of base-level fall at the mountain front has been extremely rapid over the past 21 ka (see section 5.2 for a discussion of this topic). We consider it likely that this 50 m of vertical fault movement is driving upstream knickpoint migration in the Torrente. 3. Methods [11] We quantified the spatial and temporal pattern of erosion and deposition in the Torrente catchment using estimates of bedrock exposure age or erosion rate from 10 Be concentrations in bedrock outcrops [Lal, 1991], and mean erosion rate estimations from 10 Be concentrations in fluvial sediment [Brown et al., 1995; Bierman and Steig, Figure 5. Downstream view of the middle reaches of the Río Torrente. Hillslopes are dominated by shallow bedrock landsliding. The two dams shown here were built in 1981 and were completely backfilled with sediment in less than 20 years, providing compelling evidence of the large volumes of sediment being produced and transported through this catchment. 5of20

6 Figure 6. Northward view of the debris flow terrace, the largest incised sedimentary deposit observed upstream of the Torrente mountain front (volume 4000 m 3 ); the location of this deposit is shown in Figure 3b. Inset shows the poorly stratified, poorly sorted deposits that form this terrace and that we interpret as debris flow deposits, using Costa s [1984] criteria. The location of OSL sample site SUTL 1423 (Figure 3b), which was scraped from the surface of an active sandbar, is also shown. Figure 7. Channel and hillslope profiles in the Torrente catchment. The trunk channel profile evidences many abrupt changes in channel steepness; some of these knickpoints are actually dams that have backfilled with sediment (Figure 5). The locations of the following are shown: channel knickpoints A, B, and C (Figures 4 and 11); the OSL dated fill terrace (Figure 9); and the 50 m high fault scarp at the mountain front. Also shown are the locations of zones A, E, G, and F where mean erosion rates have been estimated (Table 1) and a series of hillslope profiles (see Figure 3a for locations of hillslope profiles 1 6). Both hillslope and tributary channel profiles are drawn at the same scale as the trunk channel profile but occupy a different x-y space. Hillslope inflection points were identified on profiles 1 6 by the change from slopes of 0.5 on lower hillslopes to slopes of <0.5 on the upper hillslopes. The preincision hillslope surface is visible in Figure 11. 6of20

7 Figure 8. View of the Padul fault scarp, taken looking NW from the village of Nigüelas. This fault scarp has developed within both bedrock and alluvial fan sediments at the Torrente mountain front. The development of this 50 m high fault scarp is the most likely trigger for the knickpoints currently migrating upstream in the catchment (Figure 7). 1996; Granger et al., 1996; Reinhardt et al., 2007b]. The deposition of a sedimentary deposit that has subsequently been incised during knickpoint propagation was dated using OSL Optically Stimulated Luminescence [12] A fill terrace m above the modern Río Torrente bed was dated using OSL (Figures 3b, 7, and 9). This 5 m thick and 10 m long terrace has been preserved at the top of the cliffs forming the NW side of a narrow canyon 3 km upstream of the mountain front (Figures 3b and 9). Terrace deposition is not part of the deposition of the much larger (Nigüelas) alluvial fan at the mountain front as both the axial fan slope (0.05) and the presence of fan fragments in the lower reaches of the Río Torrente indicate that the Nigüelas Fm backfilled to <1 km upstream of the mountain front [Reinhardt, 2005]. The OSL-dated fill terrace comprises a m thick coarse basal conglomerate overlain by a well-stratified sequence of sands and imbricated gravels and cobbles, composed entirely of Nevado-Filábride lithologies. This homogenous composition is striking as there are no glacial moraines or Nevado- Filábride bedrock exposures on the fill terrace side of the trunk channel (Figure 9). Thus we interpret the geomorphic setting of these coarse sediments, perched as cliff-top remnants above the adjacent gorge, to mean that the adjacent canyon did not exist when the fill terrace was deposited. The canyon was either filled with alluvium at this time or had not yet been cut (Figure 9). We believe it likely that the Río Torrente incised into the fill terrace and underlying bedrock after the fill terrace had been deposited because: (1) the coarse basal layer of this terrace is typical of proximal fluvial deposits overlying bedrock; (2) the terrace returned an OSL age of 12 ka (see section 4.2) whereas channel wall bedrock at elevations below the fill terrace returned a very young 10 Be minimum surface exposure age of 1.2 ka (see section 4.1.2), indicating that the Río Torrente incised into bedrock only during the very latest Holocene; and (3) there are no correlative alluvial deposits in this reach (neither upstream, downstream nor on the opposite bank) as would reasonably be expected if a 60 m thick alluvial deposit had filled the adjacent gorge during deposition of the material now forming the terrace. Thus we interpret the depositional age of the fill terrace to correspond to the maximum age of the onset of incision of both the terrace and the underlying bedrock. [13] We collected an OSL sediment sample from the middle of a 55 cm thick sand layer within the fill terrace by driving a copper tube into the sediment and sealing both ends with tape to prevent light exposure. We also scraped three water-saturated samples from the surface of modern sandbars in the lower reaches of the Torrente to assess the potential strength of any residual (unbleached) dose that may have survived during transport and deposition of the fill terrace sediment (Figure 3b): these surface samples had been lying on the channel floor in full direct sunlight prior to sampling and should therefore have zero OSL. Quartz was separated from the mm fraction of each sample using sodium polytungstate for density separation and immersion in 40% HF for 40 min to dissolve feldspars and to etch the quartz. Each quartz sample was split into grain aliquots dispensed onto stainless steel discs. During OSL measurement of the modern sandbar samples we discovered that the Torrente quartz has low sensitivity and therefore dispensed 45 discs for the fill terrace sample to allow for a high recycling failure rate. We calculated the mean natural stored dose of each set of discs, arising from background radiation within the buried sediments by following the SAR protocol of Murray and Wintle [2000, 2003] using an automated Risø DA15 reader fitted with GaN Figure 9. Schematic geological cross section of the Río Torrente channel showing the position of the OSL dated fill terrace. The Río Torrente channel here follows the contact between the Nevado-Filábride and Alpujárride metamorphic complexes (Figure 3b). The lower part of the Alpujarride thrust sheet is marked by high-grade metamorphic rocks that rapidly decrease in grade away from the contact. The thrust fault marked in Figure 3b follows the contact between these high-grade lithologies and dolomite. The fill terrace is composed solely of Nevado-Filábride clasts, and there are no Nevado-Filábride bedrock exposures on the fill terrace side of the trunk channel, meaning that terrace deposition must predate channel incision. The location of this cross section is shown in Figure 3b. 7of20

8 blue LEDs. The effective dose rate of each sediment sample was calculated from (1) the internal beta dose rate of each sample measured from 20 g subsamples [Sanderson, 1988] and calculated from laboratory gamma spectrometry results, (2) the gamma dose rate of sediments adjacent to each sample, measured in the field and laboratory, and (3) cosmic dose rates calculated following Prescott and Hutton [1994]. The depositional age of each sample was then calculated by dividing the mean natural dose by the effective dose rate (mgy yr 1 ) The 10 Be Erosion Rate and Exposure Age Estimation [14] We used one of the best constrained cosmogenic nuclide systems, 10 Be in quartz [Gosse and Phillips, 2001], to quantify the pattern and rate of erosion in the Río Torrente catchment. Three types of 10 Be sample were analyzed. (1) Seven bedrock samples were collected from hillslope outcrops and used to estimate hillslope erosion rates. Six of these bedrock samples (B1, B2, B5, B11, B39 and B41) were collected from the slowly eroding upper catchment and one (B5/8) from the more rapidly incising lower catchment. For each hillslope bedrock sampling site 3 10 individual samples of vein quartz were amalgamated from up to three widely spaced outcrops up to 100 m apart at the same altitude [Reinhardt et al., 2007b]. (2) A minimum exposure age was estimated from channel wall bedrock at one location in the rapidly incising lower catchment. At this sample site (B5/1) a single vein quartz sample was collected from channel wall bedrock 10 m above the Río Torrente channel bed. (3) Catchment-wide average erosion rates were estimated from the 10 Be concentrations of modern fluvial sediments ( mm) collected from seven river reaches in both the upper and lower catchments (Figure 3b). At each sediment-sampling site 5 kg of mm sediment was collected from the armor layer (<0.2 m depth) along a m reach. Armor layer sediments are here considered representative of the full depth of transportable material as a result of the very high sediment supply rate [Lisle, 1995]. In order to interpret the 10 Be content of fluvial sediments in terms of mean upstream erosion rates [Brown et al., 1995; Bierman and Steig, 1996; Granger et al., 1996], we used a 3 m resolution DEM to calculate for every cell the 10 Be production rate [Granger and Smith, 2000; Stone, 2000; Granger et al., 2001] and corrected for topographic shielding [Codilean, 2006]. [15] We measured the 10 Be content of quartz taken from both bedrock outcrops and fluvial sediments; quartz was extracted using standard preparation methods [Bierman et al., 2002]. All samples for 10 Be analysis were treated with 30% HCl and passed though a Frantz magnetic separator. The nonmagnetic fraction was lightly crushed and ultrasonically etched (repeatedly, if necessary) in 2% HF/2% HNO 3 to isolate pure quartz, with Al <100 ppm as measured by atomic absorption spectroscopy [Kohl and Nishiizumi, 1992]. A 250 mg spike of 9 Be was added and the 10 Be/ 9 Be ratio measured by AMS at the Department of Nuclear Physics, Australian National University, Canberra [Fifield, 1999]; a 3% AMS measurement error is incorporated into all calculations. As a test of AMS measurement reproducibility, samples B1 and B2 were each split into two representative subsamples and processed and measured independently. In both cases measurement reproducibility was within 1s (B1; ± atm g 1 versus ± atm g 1 ; B2, ± atm g 1 versus ± atm g 1 ). The weighted mean of these two paired measurements was used in erosion rate estimation (Table 1a). 4. Results 4.1. The 10 Be Erosion Rate and Exposure Age Estimation [16] In situ bedrock and fluvial sediment measurements reveal consistently high 10 Be concentrations in the upper catchment, and typically two orders of magnitude lower concentrations in the lower (Tables 1a and 1b). This pattern of 10 Be concentrations is consistent with our qualitative observation that the lower part of the Torrente catchment is experiencing active incision, with low 10 Be concentrations in this lower catchment indicating young surface exposure ages and/or high erosion rates. In other words, the two orders of magnitude variation in 10 Be contents reflect the contrasting geomorphic processes in the upper and lower catchments. The interpretation of these 10 Be concentrations in terms of steady state erosion rate or surface exposure age is subject to a number of complicating factors [Lal, 1991; Bierman and Steig, 1996; Granger et al., 1996; Small et al., 1997; Gosse and Phillips, 2001; Vance et al., 2003; Niemi et al., 2005]. Requirements for estimating steady state erosion rates from 10 Be concentrations in bedrock and fluvial sediment samples collected in the Torrente catchment were assessed by Reinhardt et al. [2007b] and judged to have been met; see Appendix A for a summary Estimating Mean Zonal Erosion Rates [17] The 10 Be method applied to sediments measures the mean erosion rate of all quartz-contributing areas upstream of the sediment sampling site [Brown et al., 1995; Bierman and Steig, 1996; Granger et al., 1996]. Almost all of the quartz in the Torrente catchment comes from the Nevado- Filábride area to the southeast of the BMZ, and hence it is in this area that we assessed catchment response to the relatively rapid incision indicated by the knickpoints that separate the lower and upper catchments (Figures 3 and 4). The mean erosion rates measured from samples MRS 14, MRS 17, MRS 21A and MRS 21B in the steep lower catchment are an integration of all upstream areas, including the extensive and slowly eroding upper catchment (Figure 3b). The latter area exerts a strong influence on the mean erosion rates determined from sediments collected in the lower catchment because of the combined effects in the upper catchment of a relatively slow erosion rate and a doubling of 10 Be production (arising from the upper catchment s 1000 m higher elevation). To remove this excessive influence of the upper catchment, so that we could investigate the spatial pattern of erosion in the Torrente, we subdivided the catchment into eight subcatchments/zones: A to D actively incising, dominated by landsliding; and D 1 to G dominated by slow diffusive processes (Figure 10). The contribution to the cosmogenic isotope concentrations of samples MRS 14, MRS 15, MRS 17, MRS 21A and MRS 21B from high altitude slowly eroding areas can be removed by subtracting the total sediment contribution of subcatchments in the upper catchment 8of20

9 Table 1a. Bedrock Erosion Rates a Altitude, m Sample Thickness, cm Shielding Correction Factor In Situ 10 Be Production Rate, atoms g 1 yr 1 10 Be/ 9 Be Ratio, B1 c ± 0.0 NA 40.2 ± ± 4.5, 113 ± 4.5 B2 c ± ± ± ± 3.5, 70.3 ± Be/ 9 Be Procedural Blank Quartz Ratio, Mass, g Upper Slowly Eroding Catchment 1.4 ± 0.6, 0.55 ± ± 0.32, 0.79 ± ± 0.04, 0.55 ± ± 0.03, ± Be Concentration, 10 3 atoms g 1 SiO ± 21, 497 ± ± 21, 408 ± 77 Number of Samples Amalgamated b Chipping Depth, b m Standard Error as a Function of Chipping Depth b Bedrock Erosion Rate, b mm yr 1 5 < ± d 10 < ± d B ± ± ± ± ± ± ± ± B ± 0.0 NA 23.3 ± ± ± ± ± >0.01 ± e B ± ± ± ± ± ± ± ± B ± ± ± ± ± ± ± ± Lower Rapidly Eroding Catchment B5/ ± ± ± ± ± ± ± 1 1 N/a N/a <8 ± 2 e B5/ ± ± ± ± ± ± ± ± 0.3 a The 10 Be production rate (muon and spallation) at sea level and high latitude (5.1 ± 0.3 atoms g 1 yr 1 ) were scaled to the appropriate latitude (37 ) and altitude using the scaling parameters of Stone [2000]. The in situ bedrock 10 Be production rates cited here were corrected for sample thickness and exposure geometry using the shielding correction factors cited (attenuation length = 160 g cm 2 ) and a rock density of 2.71 g cm 3 [Dunne et al., 1999]. The 10 Be concentrations were calculated using the measured 10 Be/ 9 Be ratios and a carrier mass of 260 mg. Catchment-wide 10 Be production rates were calculated from a 3 m digital elevation model and were corrected for topographic shielding [Codilean, 2006]. The erosion rate formulations of Granger and Smith [2000] and Granger et al. [2001, equation (1)] were used to calculate mean catchment and bedrock erosion rates from (1) the 10 Be concentration of ( mm) fluvial quartz collected from sandbars along a m reach and (2) multiple quartz veins collected from one or more closely spaced bedrock exposures. Uncertainties represent one standard error measurement uncertainty; production rate and other errors [Gosse and Phillips, 2001] were fully propagated and were added in quadrature. NA means not applicable. b The error associated with applying the steady state erosion model [Lal, 1991] to episodically eroding bedrock exposures was quantified using the model of Reinhardt et al. [2007b], where the variability in erosion rate estimates is a function of chipping depth for all erosion rates >0.01 mm yr 1. c Samples B1 and B2 were each split into two representative subsamples and were processed and measured independently, as discussed in section 3.2. The weighted mean 10 Be concentration of each paired sample was used to calculate the erosion rate. d The erosion rate calculated from sample B11 (>0.01 ± mm yr 1 ) must be interpreted as a minimum value because the sample had a cosmic ray exposure geometry >2p [Dunne et al., 1999]. e This erosion rate is an estimate of the rate of river incision calculated from the exposure age of river channel bedrock (>1200 ± 200 years) by dividing the height of the sample above the riverbed by the exposure age as described in Appendix A. 9of20

10 Table 1b. Mean Erosion Rates Calculated From 0.25 to 0.5 mm Size Sediment a,b,c,d,e Altitude Range Upstream of Sampling Site, m Mean 10 Be Production Rate, Total Catchment atoms g 1 yr 1 Area, 10 3 m 2 10 Be/ 9 Be Ratio, Be/ 9 Be Procedural Blank Ratio, ) Quartz Mass, g 10 Be Concentration, 10 3 Mean Catchment atoms g 1 SiO 2 Erosion Rate, mm yr 1 Upper Slowly Eroding Catchment MRS ± ± ± ± ± ± 0.01 MRS 12B MRS ± ± ± ± ± ± ± ± ± ± ± ± MRS 15B MRS 14 MRS 17 MRS 21A MRS 21B Lower Rapidly Eroding Catchment ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.4 a The 10 Be production rate (muon and spallation) at sea level and high latitude (5.1 ± 0.3 atoms g 1 yr 1 ) were scaled to the appropriate latitude (37 ) and altitude using the scaling parameters of Stone [2000]. The in situ bedrock 10 Be production rates cited here were corrected for sample thickness and exposure geometry using the shielding correction factors cited (attenuation length = 160 g cm 2 ) and a rock density of 2.71 g cm 3 [Dunne et al., 1999]. The 10 Be concentrations were calculated using the measured 10 Be/ 9 Be ratios and a carrier mass of 260 mg. Catchment-wide 10 Be production rates were calculated from a 3 m digital elevation model and were corrected for topographic shielding [Codilean, 2006]. The erosion rate formulations of Granger and Smith [2000] and Granger et al. [2001, equation (1)] were used to calculate mean catchment and bedrock erosion rates from (1) the 10 Be concentration of ( mm) fluvial quartz collected from sandbars along a m reach and (2) multiple quartz veins collected from one or more closely spaced bedrock exposures. Uncertainties represent one standard error measurement uncertainty; production rate and other errors [Gosse and Phillips, 2001] were fully propagated and were added in quadrature. 10 of 20

11 Figure 10. The 10 Be-based erosion rate estimates in the Torrente catchment. The catchment was subdivided into eight subcatchments/zones: A D, upland relatively low angle soil-mantled slopes; and D 1 G, zones of rapid river incision and hillslope erosion. The bias introduced into sediment samples collected in the lower rapidly eroding portion of the catchment by sediments sourced in high-altitude, slowly eroding areas is removed by subtracting the total sediment contribution of upland subcatchments, as described in equation (1). Bedrock erosion rate estimates are also shown. (Table 2) and recalculating mean erosion rates as described in equation (1): e l ¼ A te t A u e u ð1þ A l where A = area and e = erosion rate, and subscripts l, u and t refer to the lower, the upper and the total catchments, respectively. The upper catchment erosion rates are calculated from samples MRS 3, MRS 12b, MRS 18 (see Table 2) Spatial Pattern of Erosion in the Torrente [18] We combined sediment-based erosion rates with bedrock erosion rate estimation to characterize the pattern of erosion within and between different parts of the catchment (Tables 1a and 1b and Figure 10). In the upper catchment both bedrock outcrops (0.04 ± 0.01 mm yr 1 : n = 5) and soil-mantled slopes (0.06 ± 0.01 mm yr 1 :n=3) are eroding at uniformly low rates over the characteristic timescale of 12 ka (i.e., the time required to erode 0.6 m during which time 10 Be accumulation is most sensitive to surface erosion). This concordance in erosion rate is the key evidence that the upper catchment topography is in topographic steady state, as will be discussed in section 5.1. Rates of erosion in the lower, landslide-dominated catchment are two orders of magnitude higher than in the upper catchment (Figure 10). The rate of erosion of the bedrock outcrop (B5/8) on the ridge bounding subcatchment E close to, but downstream of, the head of active incision (the hillslope inflection highlighted in Figure 3) is 1.0 ± 0.3 mm yr 1 similar to the mean erosion rate in zone E (2.3 ± 0.3 mm yr 1 ). Mean erosion rates in the other lower catchment zones are similarly high (D 1 : 12.2 ± 1.5 mm yr 1 ;F 1 :4.3±3.1mmyr 1 ;F 2 : 0.6 ± 2.0; G: 6.5 ± 0.9 mm yr 1 respectively). [19] Very rapid rates of bedrock channel incision characterize the late Holocene history of the lower catchment. Sample B5/1 was collected from the bedrock channel wall between 9 and 11 m above the channel bed; the uncertainty in sample height is due to the burial of the river channel by sediment accumulating behind a 20-year-old dam. The 10 Be Table 2. Mean Zonal Erosion Rates a Zone Surface Area, m 2 Data Used to Calculate Erosion Rate Erosion, mm yr 1 Upper Catchment A 6,830,000 MRS ± 0.01 B 2,010,000 Mean of MRS 12B and MRS ± C 2,910,000 MRS 12B 0.03 ± D 506,000 Mean of MRS 12B and MRS ± Lower Catchment D 1 77,000 MRS 15B - D 12.2 ± 1.5 E 1,800,000 MRS 17 - C 2.3 ± 0.3 F 1 2,100,000 MRS 21A - (B + MRS 14) 0.6 ± 2.0 F 2 2,400,000 MRS 21B - (B + MRS 17 + MRS 14) 4.3 ± 3.1 G 2,100,000 MRS 14 - A 6.5 ± 0.9 a The lower catchment erosion rates D 1,E,F 1,F 2, and G were calculated from equation (1), using the data presented here and the total catchment areas and erosion rates of MRS 15B, MRS 17, MRS 21A, MRS 21B, and MRS 14 cited in Table 1b. Uncertainties represent one standard error measurement uncertainty. 11 of 20

12 Table 3. OSL Depositional Age Estimates a Sample ID b Estimated Dose, Gy Recycling Ratio Effective Dose Rate, mgy yr 1 Apparent Age, years Residually Corrected Age, ka SUTL ± ± ± ± 80 NA SUTL ± ± ± ± 100 NA SUTL ± ± ± ± 80 NA Fill terrace 38.7 ± 5.7 <10% 3.00 ± ,800 ± ,000 ± 1000 a The OSL depositional age of stored sediments was determined through application of the single aliquot regenerative dose (SAR) protocol of Murray and Wintle [2000, 2003]. The natural dose of each sample arising from background radiation in the buried sediment was calculated from the OSL response to a series of laboratory-based regenerative beta doses. In this procedure, OSL signals are normalized by a beta test dose that is applied after each regenerative dose. The recycling ratio was calculated by dividing the first and last normalized regenerative doses, and where this ratio deviates from unity by >10%, recycling is said to have failed and the sample is rejected. Uncertainties represent one standard error measurement uncertainty. NA means not applicable. b The locations of these samples are shown in Figure 2b. concentration of this sample may be interpreted either as a channel wall erosion rate (0.6 mm yr 1 )or,assumingno channel erosion since the onset of rapid river incision, as providing a minimum exposure age of 1200 ± 200 a (Table 1a) and thereby implying a maximum river incision rate of 8 ± 2mmyr 1 (see section A3). This maximum river incision rate is of the same order of magnitude as the mean zonal erosion rate in this reach (F: 4.3 ± 3.1 mm yr 1 ). There are considerable uncertainties both in the rates of zonal erosion discussed previously and in this rate of channel incision; nonetheless, all of the 10 Be data consistently imply erosion rates of the order of several millimeters per year OSL Dating of Alluvial Sediments [20] OSL dating of the alluvial fill terrace m above the modern Río Torrente bed was used to estimate the medium term (10 4 years) rate of river incision. We began our OSL analysis by assessing whether or not modern Río Torrente sediments have a residual (unbleached) stored dose, arising from incomplete optical zeroing of quartz grains during transport. The residual signals of three samples taken from the surfaces of modern sandbars in the lower Torrente are strikingly similar and correspond to estimated ages of 750 ± 80 years, 760 ± 110 years and 730 ± 80 years, with a weighted mean age of 740 ± 50 years (Table 3). The magnitude and consistency of these nonzero values imply that quartz from the Torrente catchment is insensitive to optical bleaching. The causes of poor sensitivity in quartz remain elusive but impurities and defect concentrations within the lattice perhaps in conjunction with variable thermal and radiation histories are implicated [Murray and Roberts, 1998; Wallinga et al., 2001]. We used a residual age correction of 740 ± 50 years in calculating the depositional age of the alluvial fill terrace sample. [21] We used 17 quartz aliquots to estimate the stored dose of the sample of the fill terrace m above the modern river bed; all but two of the quartz aliquots lie within 2s of each other. Rejecting these two outliers and assuming that the remaining aliquots represent a single population, or depositional event, allows the calculation of a weighted mean estimated stored dose of ± 5.70 Gy, corresponding to a burial age of ± 1000 years BP (1s; Table 3). Accounting for the presence of a residual (unbleached) stored dose equivalent to 740 ± 50 years, the corrected age of this fill terrace is c ± 1000 years BP. We have argued in section 3.1 that the terrace was deposited on bedrock prior to channel incision and so the terrace s depositional age implies a minimum channel incision rate of 5±1mmyr 1 averaged over the past 12 ka (Figure 9). This estimate of the medium-term (12 ka) channel incision rate is consistent with the short-term 10 Be-based rate of river incision (8 ± 2 mm yr 1 ) estimated from channel wall bedrock in the same reach, confirming that channel incision has been extremely rapid over the past 12 ka. 5. Discussion 5.1. Rates of Pretransient and Posttransient Landscape Erosion [22] A major wave of erosion [cf. Whipple and Tucker, 1999] has propagated 8.3 km upstream of the mountain front in the Río Torrente, generating very high rates of hillslope erosion and channel incision in the lower catchment (Figure 10). Channel incision rates over both short (1 ka) and medium (12 ka) timescales are concordant at 8±2mmyr 1 and 5 ± 1 mm yr 1 respectively, and mean zonal erosion rates in the lower catchment are similarly high: (E: >2 mm yr 1 ) (Figure 10). These mean (sedimentbased) erosion rates characterize hillslope erosion as hillslope erosion dominates the sediment flux. An equivalence between rates of mean hillslope erosion and rates of channel incision in steep, rapidly incising landscapes is characteristic of a steady state system where hillslopes develop the critical angle for landsliding and thereafter track the incising river channel. We follow Burbank et al. [1996] and use the term critical hillslopes for hillslopes that are tracking the incising river channel at the critical angle for landsliding. Hillslopes in the lower catchment are steep (slope = 0.6 ± 0.14: Table 4) and straight (Figure 7) as expected for critical hillslopes in which the nonlinear process of landsliding efficiently transports sediment downslope at a constant hillslope angle [Roering et al., 1999, 2001]. Rates of hillslope erosion in the lower Torrente may therefore be dependent upon the rate of channel incision. Accordingly we infer that the three trunk valley hillslope erosion rates (G, F 1,F 2 ) are from the same population enabling calculation of a weighted mean erosion rate of 5.4 ± 0.8 mm yr 1, integrated over the characteristic timescale of 100 years (i.e., the time required to erode 0.6 m during which time 10 Be accumulation is most sensitive to surface erosion). The striking similarity between this short-term hillslope erosion rate and the medium term rate of river incision estimated from OSL dating (12 ka: 5 ± 1 mm yr 1 ) lends support to our view that trunk valley hillslopes are tracking the 12 of 20

13 Table 4. Data Generated From a 3 m Resolution DEM of the Torrente Catchment, as Shown in Figure 13 a Hillslope Profile Number b Head of the trunk channel knickpoint c In-Channel Distance Between Head of Knickpoint and Base of Hillslope, c m Upstream Drainage Area, m 2 Drainage Area Between Head of Knickpoint and Hillslope, c m 2 Lateral Distance, Ld, Between Channel and Hillslope Inflection, d m Lateral Distance, Ld, Between Hillslope Inflection and Drainage Divide, e m Critical Slope Angle, Tan Ø Preincision Hillslope Angle, Tan m Vertical Distance Between Channel and Hillslope Inflection, m Estimated Fall in Base-Level Required to Generate Hillslope Inflection, f m NA NA 0 NA NA 467 1,406, , NA g NA 622 1,480, , NA g NA 641 1,481, , NA g NA 766 1,50, , NA g NA ,623, , NA g NA ,999,000 4,150, h , ,571,000 4,723, NA h NA ,337,000 5,489, NA h NA ,063,000 7,215, NA h NA 2a ,977,000 8,129, h , ,932,000 9,084, h , ,862,000 11,013, h , ,602,000 11,753, h , NA 0.54 * NA NA 0.57 * NA NA 0.51 * NA Catchment exit NA NA NA NA NA NA NA a Distances and upstream drainage areas were measured from along 16 hillslope transects following lines of steepest decent; the base of each hillslope is identified in Figure 3a. The predicted hillslope inflection traveltime to catchment drainage divide was calculated along five transects following the approximate line of steepest decent (Figure 3a), using equation (3). NA means not applicable. b Hillslope profiles are from Figures 3a and 7. c The head of the knickpoint is labeled A in Figure 7. d Lateral distance is distance traveled from the base of hillslope (Figure 3a) to hillslope inflection between time Tc 0 and time T c 1 (Figure 13). e Lateral distance is distance traveled by hillslope inflection between time Tc 1 and time T c 2 (Figure 13). f The predicted fall in base level was calculated using equation (2), as shown in Figure 13. These data are plotted in Figure 14b. g Preincision hillslope angle is calculated in a 3 m DEM by drawing a line between the upper edges of a narrow canyon close to the head of knickpoint A, assuming that the slope of this line equals the preincision/ precanyon hillslope angle. h The preincision hillslope angle is assumed to equal the mean slope of the relict (triangular) surface shown in Figure 11. Predicted Hillslope Inflection Traveltime to Drainage Divide, years 13 of 20

14 Figure 11. View of the southeast Torrente catchment highlighting the striking contrast between the slowly eroding upper catchment terrain and the rapidly eroding hillslopes of the lower catchment. An obvious inflection in hillslope denotes the transition between these two terrains, as highlighted by the dashed line (see also Figures 3, 12, and 13). The locations of channel knickpoints are indicated by arrows; knickpoints B and C are identified in Figure 7. Two tributary channel knickpoints have migrated upstream and met to form a narrow ridge above a relict preincision surface. This triangular-shaped surface is bounded above and below by inflections in hillslope (Figure 7). incising channel. If we add the two tributary valley erosion rates (D 1 : 12.2 ± 1.5 mm yr 1 ; E: 2.3 ± 0.3 mm yr 1 )tothe calculation the weighted mean lower catchment erosion rate becomes 3.0 ± 0.3 mm yr 1. This revised estimate remains consistent with the OSL-derived rate and does not change our interpretation even though we consider that the high erosion rate estimate for zone D 1 is potentially unrepresentative of mean catchment rates because of the very small size of this subcatchment (Figure 10). [23] Rates of hillslope erosion and bedrock erosion area are also concordant in the upper catchment, where a mean erosion rate of 0.05 ± 0.02 mm yr 1 (n = 8) was calculated. This concordance is important as bedrock exposures are generally found to erode more slowly than surrounding soilmantled hillslopes [e.g., Small et al., 1999; Heimsath et al., 1999, 2000]. Bedrock exposures erode at the same rate as soil-mantled hillslopes only when topography is in steady state. Thus our data indicate that hillslopes, bedrock outcrops and channels are in steady state in the upper catchment also. We infer therefore that the entire catchment was in steady state prior to the change in rate of base-level fall that has triggered knickpoint migration into the upper catchment Nature of the Transient Response to Base-Level Lowering [24] Hillslopes in the lower catchment have developed the critical angle for bedrock landsliding and are now propagating laterally into the lower-angled slopes of the upper catchment (Figure 11). The head of each critical hillslope forms an obvious inflection (or break) in slope where it intersects with this lower-angle topography (Figures 3, 7, 11, and 12). Simple geometry dictates that for a given slope Figure 12. Schematic cross section of the northwest facing hillslopes in the Torrente catchment, showing indistinguishable rates of channel incision and hillslope erosion; error is cited at 1-sigma. The steep lower hillslopes are at the critical angle for landsliding and essentially track the incising channel. An inflection in hillslope forms where these critical slopes intersect the low-angled hillslopes of the upper catchment. Continued channel incision will result in the lateral migration of this hillslope inflection toward the drainage divide (see Figure 13). 14 of 20