Arctic, Antarctic, and Alpine Research, Vol. 31, No. 2, 1999, pp. 121-127 Denudationof Small Alpine Basins, Nanga Parbat Himalaya, Pakistan John F. Shroder, Jr.,* Abstract Rebecca A. Scheppy, t Thirty-three debris fans and five small alpine basins on the south side of the and Michael P. Bishop* rapidly uplifting Nanga Parbat Himalaya of northern Pakistan were assessed to determine how much alpine processes contribute to the overall denudation of the *Departmentof Geographyand massif. A high-resolution digital elevation model was used to measure the volume Geology, Universityof Nebraskaat Omaha,Omaha, Nebraska 68182, of the small alpine fans and a few basins in the Rupal valley. These volumetric U.S.A. estimates, coupled with time-constraining dates from cosmogenic-nuclides of gla- [email protected] cially exposed rocks and infrared-stimulated luminescence of sediments, indicate that estimated denudation rates in these have been -2 mm tDepartmentof Geology, University average systems yr-~ of Kansas,Lawrence, Kansas 66045, over the past 4600 to 6000 yr since the last major deglaciation. This is similar to U.S.A. other estimates of rates of denudation in the Himalaya. Introduction yr-' and can be as high as 5.2 cm yr-~ close to the zones of most active faulting and deepest river incision. In addition, lo- BACKGROUND calized rates of denudation at Nanga Parbat for several large glacier and river basins range from 0.7 to 2.5 cm yr-'. Cata- The Parbat at 8125 m altitude is the Nanga massif, (Fig. 1), strophic floods, resulting from the breaking of repetative land- ninth mountain in the and an area of ero- highest world, rapid slide dams, can result in rates of denudation as high as 12 cm sional Burbanket unroofing (Zeitler, 1985; Shroder, 1989, 1993; yr-~. Such rapid and localized, but still short-termand episodic, The Parbat from which this is al., 1996). Nanga Project, paper denudation at Nanga Parbat appears responsible for the extreme of the tectonics a product, is a multidisciplinary investigation relief at a multitude of scales (Shroder et al., 1998; Shroder and and of the massif. Members of the are as- topography project Bishop, in press). sessing very young, high-grade metamorphism,partial melting, Denudation processes on Nanga Parbat are controlled in fold and fault strain et and the high (Zeitler al., 1993), rapid part by strong climatic gradients (Hewitt, 1993). Because of its denudation mass and by movement, glaciers, rivers, catastrophic great height and bulk, the Nanga Parbat massif serves as a cli- breakout floods (Bishop et al., 1998a, 1998b, 1999; Shroder, matic divide between the continental air masses of cold and arid 1998; Shroder et al., 1998). Analysis of the topography and de- central Asia and the maritime air masses of the Arabian Sea, nudation of the massif is critical to the understanding petrologic leaving in rainshadow various regions of the massif. This varies and tectonic evidence of which has been denudation, sufficiently locally depending upon aspect and relief, but in general, the to induce and at rapid significant pressure temperaturechanges northern regions of the massif are more arid than the southern depth. (Scott, 1992). Year-round, orographic precipitation provides the continental collision of India with Asia -50 Following snowfall on the peaks in excess of 2000 mm yr- , whereas valley continued to move north to force Ma ago, the Indian plate up floors at lower altitudes can receive as little as 200 mm yr-' of chain from to Burma. Parbat the Himalaya Afghanistan Nanga precipitation, with temperaturesup to 50?C in summer months is an up-faulted, pop-up or flower structure engirdled with in- (Gardner,1986). Debris fans, however, can be covered with thick that ward-dipping reverse faults (Seeber and Pecher, 1999) avalanche snows for much of the summer season. For example, stands near the western end of the High Himalaya chain and has in 1996 following a winter of heavy snows, five fans with a an anomalous north-northeasttrend between its main bounding northern aspect in the mid-Rupal valley between Bhazin and faults of similar direction. The felsic basement gneisses of Nanga Chungparglaciers (Fig. 1) were completely snow-covered in late Parbatrepresent the northernmostexpression of the Indian plate July. At the same time in the Chungpar valley, fans with an that has thrust up and through the previously overlying mafic eastern aspect still had substantial snow from avalanches. rocks of the Kohistan-Ladakhisland arc caught between the col- 1985). liding plates (Zeitler, PURPOSE Zeitler et al. (1989) and Winslow et al. (1994) estimated an uplift or exhumation rate of the Nanga Parbatarea of 4 to 8 mm The geomorphic processes responsible for the denudation yr- , on the basis of geochronologic evidence that the massif had of the alpine valleys on Nanga Parbat can be subdivided into been exhumed 10 km in the past 10 Ma, 6 km of that in the past two large groups-glacial and nonglacial. Glacial processes ap- 1.3 Ma of the Quaternary.Burbank et al. (1996) used cosmo- pear to be highly effective in removing large amounts of sedi- genic-radionuclide exposure ages to estimate bedrock incision ment from the Nanga Parbat massif (Gardnerand Jones, 1993). rates by the antecedent Indus River of 2 to 12 mm yr-' across Large areas (302 km2) of the mountain are presently glacierized the Nanga Parbat uplift. We have made 15 preliminary mea- (Kick, 1980, 1994), and much more was covered with ice during surements of glacier and river incision rates below cosmogeni- the Pleistocene (Shroder et al., 1989; Scott, 1992; Khule, 1996). cally dated or correlated glacier erosion and deposition terraces Nonglacial processes affecting formation of alpine basins and on the massif. These maximum incision rates of bedrock at spe- fans at Nanga Parbatinclude mass movement, fluvial processes, cific locations are highly variable, but average -2.2 + 1.1 cm and catastrophicflood-flushing (also influenced by glaciation). ? 1999 Regents of the Universityof Colorado J. F. SHRODER ET AL. / 121 1523-0430/99 $7.00 * Alpine Fan Ridge Line 11 I Glacier IZI Bedrock AlpineBasin N J o 0 10 Kilometers FIGURE 1. Location of Naga Parbat study area and map of the Rupal Valley on the south side of the massif wherein 33 of 74 alpine basis and fans were analyzed. An earlier pilot project to assess denudation in two small alanches; (5) nonglacial fluvial action; (6) glacial meltwater out- basins above Biale in the Raikot valley on the north side of wash; and (7) polygenetic, ice-marginal ramp action (Kuhle, Nanga Parbat was unsuccessful. A variety of data sets was used 1990). to estimate rates of denudation, resulting in irreconcilably dis- The major factors controlling these alpine slope processes parate rates. This was attributedto use of the somewhat inac- are tectonics and climate. The extreme relief produced by the curate, old (1937) topographic map, the difficulty of accurate active coupling of denudation with tectonics (7 km in 21 km, measure of basin and fan volumes, the erosion of much fan sed- north side; 4.6 km in 6 km, south side) at Nanga Parbat leads iment by periodic debris flows dated with tree rings (Shroder et to increased steepening of the climatic-geomorphic zones and a al., 1996), and the experimental nature of the 3He cosmogenic tendency for distribution of process according to an altitudinal isotope signal used to model denudation (Phillips et al., 1996; gradient (Hewitt, 1993). Most of the polygenetic basins in this Phillips, 1997). study extend into Hewitt's zone II (humid, high alpine tundra) The purpose of the study described here was to use more and sometimes into zone I (humid, perennial ice), but the fans advanced techniques to characterizeand measure the volume of fall within zones III (subalpine montane) and IV (semiarid, sub- some of the smaller alpine basins and their polygenetic debris montane). Mass-movement processes dominate most basins and fans in the Rupal valley of Nanga Parbat in order to calculate fans, although the processes of formation are more commonly reasonable estimates of rates of denudation (Scheppy, 1997). transitionalthan distinct. Debris flows are one of the most common mass-movement on mechanisms in the Himalaya (Goudie et al., 1984; Shroder, Geomorphologyof Small Alpine Basins and Fans material that can be Parbat 1993), because of abundant loose source Nanga mobilized by torrentialrain or rapidly melting snow. Such debris The characteristicglaciated, U-shaped valley walls of Nan- at Nanga Parbatis provided by the extreme relief and associated ga Parbat are commonly dissected by alpine erosional basins high freeze and thaw on the higher slopes. Typical debris flows with polygenetic depositional fans at their mouths (Figs. 1, 2). on Nanga Parbathave exhibited one or more waves of wet debris The fans are one of the most common depositional forms in the through a channel and over the banks to leave levees of coarse valleys, and are rarely far enough apart to be completely sepa- clasts on channel sides. On the bigger fans at Nanga Parbat,large rate. Fans range in scale from isolated small slope failures to sieve lobes of rubble from debris-flow emplacement occur in extensive areas of polygenetic processes and complex slope many places. movements. Polygeneity of the fans is demonstrated by their Rockfall appears common to every fan in the study. It is varying slope gradients, many of which are low-angle, fluvial or due to the high relief and highly fracturedcrystalline rock of the debris-flow dominated alluvial fans, whereas others have a high massif. The rocks of the mountain are resistant crystallines but angle of repose with little vegetative cover characteristicof rock- their actual strength is controlled largely by the size and orien- fall deposition.