Average Denudation Rate Research Articles

Variability of Quaternary glacial erosion rates – A global perspective with special reference to the Eastern Pyrenees

Glaciers erode bedrock but are also efficient conveyors of debris supplied during a cycle of glaciation by processes other than basal erosion. In this dual capacity as both an eroding and a transporting agent lies the ambiguity of ‘glacial erosion’ as a geomorphic process, with implications for methods of measuring the removal of rock mass by glaciers in the geological past, and for interpreting what exactly the consequences have been on topography and elevation change. A global review of ∼400 Quaternary glacial denudation rates estimated from five different measurement techniques provides values ranging between 10 −4 and 10 mm yr −1. We investigate the causes of such wide variability by examining the respective influences of environmental setting and methodological bias. A reference frame chosen for assessing these issues is the Massif du Carlit (Pyrenees, France), where a quantified mass balance of the well preserved glacial, periglacial and paraglacial deposits was made possible by detailed geomorphological mapping and terrestrial cosmogenic nuclide dating of extant erosional and depositional landform sequences. Resulting age brackets helped to define three main episodes of ice-cap growth and decline, each characterized by a volume of debris and a mappable source area. Erosion rates were expressed in two ways: (i) as spatially averaged denudation rates ( D) during the successive stages of glacial advance to the line of maximum ice extent (MIE), post-MIE ice recession, and Lateglacial cirque readvance, respectively; and (ii) as cirque-wall recession rates ( R) where moraine facies criteria indicated a supraglacial provenance of debris. Results indicate low erosion ( D ≈ 0.05 mm yr −1) during the ice advance phase, probably because of thin or passive ice covering the low-gradient subglacial topography that occurs just above the late Pleistocene equilibrium line altitude (2.2–2.4 km). Erosion rates peaked ( D ≈ 0.6 mm yr −1 and R ≈ 2.4–4.5 mm yr −1) during the main transition to ice-free conditions, when deglacial debuttressing promoted the rapid response of freshly exposed slope systems to new equilibrium conditions in the steep crest zone. Lateglacial D- and R-values declined to 0.2–0.3 mm yr −1, with indications of spatially variable R controlled by lithology. In this environment glaciers overall behaved more as conveyors of debris supplied by supraglacial rock exposures in the mountain crest zone than as powerful modifiers of subglacial topography. This explains the widespread preservation of deep, in situ preglacial weathering profiles on relict Cenozoic land surfaces in the deglacierized part of the Eastern Pyrenees. When plotted on the global data set analyzed and discussed in the review, the East Pyrenean erosion rates stand out as being amongst the lowest on record.

Erosion rates on different timescales derived from cosmogenic 10Be and river loads: implications for landscape evolution in the Rhenish Massif, Germany

We determined erosion rates on timescales of 101–104 years for two catchments in the northeastern Rhenish Massif, in order to unravel the Quaternary landscape evolution in a Variscan mountain range typical of central Europe. Spatially averaged erosion rates derived from in situ produced 10Be concentrations in stream sediment of the Aabach and Mohne watersheds range from 47 ± 6 to 65 ± 14 mm/ka and integrate over the last 9–13 ka. These erosion rates are similar to local rates of river incision and rock uplift in the Quaternary and to average denudation rates since the Mesozoic derived from fission track data. This suggests that rock uplift is balanced by denudation, i.e., the landscape is in a steady state. Short-term erosion rates were derived from suspended and dissolved river loads subsequent to (1) correcting for atmospheric and anthropogenic inputs, (2) establishing calibration curves that relate the amount of suspended load to discharge, and (3) estimating the amount of bedload. The resulting solid mass fluxes (suspended and bedload) agree with those derived from the sediment volume trapped in three reservoirs. However, resulting geogenic short-term erosion rates range from 9 to 25 mm/ka and are only about one-third of the rates derived from 10Be. Model simulations in combination with published sediment yield data suggest that this discrepancy is caused by at least three factors: (1) phases with higher precipitation and/or lower evapotranspiration, (2) rare flood events not captured in the short-term records, and (3) prolonged periods of climatic deterioration with increased erosion and sediment transport on hillslopes.

Effects of terrain smoothing on topographic shielding correction factors for cosmogenic nuclide‐derived estimates of basin‐averaged denudation rates

AbstractEstimation of spatially averaged denudation rates from cosmogenic nuclide concentrations in sediments depends on the surface production rates, the scaling methods of cosmic ray intensities, and the correction algorithms for skyline, snow and vegetation shielding used to calculate terrestrial cosmogenic nuclide production. While the calculation of surface nuclide production and application of latitude, altitude and palaeointensity scaling algorithms are subjects of active research, the importance of additional correction for shielding by topographic obstructions, snow and vegetation is the subject of ongoing debate. The derivation of an additional correction factor for skyline shielding for large areas is still problematic. One important issue that has yet to be addressed is the effect of the accuracy and resolution of terrain representation by a digital elevation model (DEM) on topographic shielding correction factors. Topographic metrics scale with the resolution of the elevation data, and terrain smoothing has a potentially large effect on the correction of terrestrial cosmogenic nuclide production rates for skyline shielding. For rough, high‐relief landscapes, the effect of terrain smoothing can easily exceed analytical errors, and should be taken into account. Here we demonstrate the effect of terrain smoothing on topographic shielding correction factors for various topographic settings, and introduce an empirical model for the estimation of topographic shielding factors based on landscape metrics. Copyright © 2008 John Wiley and Sons, Ltd.

Increase in late Neogene denudation of the European Alps confirmed by analysis of a fission-track thermochronology database

A sharp increase in deposited sediment volume since Pliocene times has been observed worldwide and in particular around the European Alps. This phenomenon has been linked to a rise in denudation rates controlled by an increase of either climatic or tectonic forcing. Observation of in-situ cooling histories for orogens is critical to assess the reality of the inferred increase in denudation rates, and to determine whether this phenomenon is widespread or localized at active tectonic structures. We exploit the unique density of fission-track ages in the Western European Alps to reconstruct cooling isoage surfaces and to estimate exhumation rates on the orogen scale between 13.5 and 2.5 Ma. Our novel technique is based on the association of isoage contours with age–elevation relationships. It uses map-view interpolation, enabling a spatio-temporal analysis of exhumation rates over the entire Western Alps. The resulting exhumation histories reconstructed for eight areas of the Western Alps display strong similarities in timing and rates with orogen-wide average denudation rates inferred from sediment volumes. This consistency validates the use of both techniques for the study of an orogen characterized by strong relief and high recent exhumation rates. We conclude that exhumation rates in the Western Alps have increased more than twofold since Late Miocene times. This increase may have been locally modulated by the distinct response of different tectonic units.

Historical soil erosion and land-use change during the last two millennia recorded in lake sediments of Frickenhauser See, northern Bavaria, central Germany

Based on palaeolimnological investigations on a 1275 cm long lacustrine sediment record, sediment yields (SY) and denudation rates (DR) for the last 2000 years were reconstructed from Frickenhauser See, a small lake in central Germany. SY and DR generally correspond with the human land-use history derived from pollen analysis and reveal a drastic interruption of natural processes starting in the eleventh century AD. SY increased over 350-fold from 0.9 t/km2 per yr to 328 t/km2 per yr in response to deforestation and subsequent soil erosion. The average denudation rate within the catchment area is 81 mm for the last 2000 years, most of which occurred between AD 1100 and 1870. Taking into account that only one-third of the catchment area is suitable for agriculture, the cumulative soil loss from agricultural fields is around 240 mm (0.31 mm/yr). Historical events such as a widespread abandonment period (AD 1300—1450) and the Thirty Years War (AD 1618—1648) are reflected in the pollen record but cause only minor fluctuations in reconstructed sediment yields. This study demonstrates that by applying a multiproxy approach lacustrine sediments provide an excellent archive to reconstruct the effects of human land use on the geomorphological process system and to identify human—environment interactions.

Variability of fluvial transport of the upper Wieprz river

Variability of fluvial transport of the upper Wieprz river This paper shows variability of fluvial transport of the Wieprz river. The catchment of the upper Wieprz river to profile at Guciów located in Central Roztocze Region. In Wieprz during the hydrological years 1996-2006 average annual flow is 1.32 m3·s-1, which correspond to the outflow - 41.5 Mm3·a-1. Extreme values of the flows varied from 0.5 m3·s-1 to 13,1 m3·s-1. Outflow the Wieprz river has feature for the snow-rainy hydrological regime. During the study the annual average suspended load amounted 737 Mg. The annual average dissolved load amounted 10 343 Mg. The bed load estimated for less than 10% of the dissolved load and 1% of the suspended load. The monthly average suspended load and dissolved load were least in the November and were highest in the April. During the period of researches, the average mechanical denudation rates 2.4 Mg·km-2·a-1. Chemical denudation rates were much higher - 36.1 Mg·km-2·a-1. Their differentiation are similar in other regions of Poland.

中央ネパール，カトマンドゥ・ナップの上昇機構―放射年代学的アプローチ―

The Himalayan orogen represents a fold-and-thrust belt characterized by a series of foreland-propagating thrust systems with an out-of-sequence thrust (OST). Geological and geomorphological evidence implies that the Himalayan upheaval started from the north and shifted gradually southward with time. The uplift seems to have been accelerated in the Higher Himalaya since the Pliocene. To investigate the role of OST in recent uplifts of the Higher Himalaya and to estimate the denudation rate of the latter, we carried out fission track (FT) dating on zircon and apatite of high-grade pelitic and granitic gneisses from both sides of an inferred OST north of the Kathmandu nappe and along a traverse across Gosainkund Lekh in the northern root zone of the Kathmandu nappe.Nine zircon FT ages of sites in the Gosainkund Lekh area range from 1.5 Ma (at 1525 m) to 2.8 Ma (at 5045 m). Ages of four samples from the northern slope of the area increase linearly with elevation, and yield an average denudation rate of 2.4 mm/y. On the other hand, six samples from the southern slope yield identical ages (2.5-2.8 Ma) regardless of their sample elevations (1150-5045 m). The zircon ages of the area suggest that rocks on the southern slope passed evenly the depth of closure temperature of zircon at around 2.6 Ma. Following this, the northern part of the southern slope (the highest part of Gosainkund Lekh) and the northern slope were uplifted more rapidly than the southern slope.Eight zircon FT ages and seven apatite FT ages from rocks around the Sheopuri zone range from 4.4 Ma to 6.9 Ma and 4.3 Ma to 5.5 Ma, respectively. These ages vary irrespective of their sample elevations (1200-2400 m). Furthermore, zircons and apatites from both sides of the OST (Trisuli-Likhu fault) yield almost the same FT ages, suggesting that the OST has been inactive since the latest Miocene. It is noteworthy that zircon FT ages from the sample elevations between 1200 m and 1800 m in the present area together with zircon FT ages (ca. 9 Ma) reported from early Paleozoic granites in the southern part of the Kathmandu nappe suggest a general trend of younging towards the north between 9 Ma and 1.5 Ma. On the basis of such a trend, two possible uplift mechanisms of the Kathmandu nappe and its northern root zone in the Higher Himalaya are presented. Both mechanisms might be related to crustal ramp due to northward wedging of Indian mid-crust beneath the Himalaya.

The control mechanisms of erosion and weathering at basin scale from cosmogenic nuclides in river sediment

The study of a sample of river sediment enables the determination of spatially averaged denudation rates that provide an exceptional perspective on erosion and weathering processes that have taken place within a landscape. These measurements are done with in-situ-produced cosmogenic nuclides (e.g., 10Be, 26Al), mostly in quartz from alluvial sediment. Cosmogenic nuclides are produced when secondary cosmic rays interact with the very uppermost layer of the Earth's surface. They are produced within a characteristic depth scale of about 1 m, which means that the measured concentrations record an integrated denudation history while material passed through this depth interval. Depending on the denudation rate the resulting integration time scales are 103 to 105 years, and one obtains a robust long-term estimate of natural denudation that is relatively insensitive to short-term changes. The last 10 years have seen significant research activity using these methods, and an array of fascinating tectono-geomorphologic and geochemical insights are emerging. Amongst these is the ability to identify the physical and chemical processes with which a landscape responds to tectonic activity or climate change. A compilation of world-wide denudation rates in non-glaciated areas, that however, does not yet include some of the world's most active mountain belts, has resulted in the following findings, some of which have been unexpected: (1) No obvious relationship between precipitation or mean annual temperature and total denudation is apparent. (2) Topographic relief alone does not result in high rates of denudation. (3) Denudation rates are high in areas of landscape rejuvenation; that is triggered and controlled by tectonic activity (faulting, escarpment formation and retreat, rifting, surface uplift). (4) Rates of weathering (using a combination of cosmogenic nuclides and zirconium-normalised cation loss balances) co-vary primarily with physical erosion rates and much less with temperature or precipitation. (5) In some areas of high land use short-term rates (from river load gauging) exceed those from cosmogenic nuclides by several orders of magnitude, which serve to highlight the severity of geomorphic change caused by human action. In the future, the control mechanisms over denudation will be determined on all spatial scales, ranging from the single soil section to entire river basins. The same analysis can be done back through time on well-dated terraces, lake records, and marine sediment cores, which is possible with 10Be for the past 1–2 My. The rates obtained will be used to develop a quantitative understanding of tectonic, geomorphologic, and geochemical landscape processes, which in turn is a prerequisite to design and calibrate models of the response of landscapes to tectonic, climate, and anthropogenic forcing.

Thermal evolution of leucogranites in extensional faults: implications for Miocene denudation rates in the Himalaya

Abstract The crustally derived High Himalayan leucogranites (HHL) are characterized by strong isotopic heterogeneity and occurrence of magmatic muscovite. Such attributes indicate that the HHL were non-convecting magma bodies and crystallized at pressure-equivalent depths of more than 8.5 km. We have performed one-dimensional thermal modelling in order to simulate the process of incremental growth of a laccolith whose roof is tectonically removed during intrusion, in a context of crustal exhumation due to channel flow. The objective is to define under what conditions HHL laccoliths emplaced close to active normal faults may be built without convecting while crystallizing muscovite. The results indicate that for a HHL thickness in the range 5–10 km, denudation rates cannot be higher than 4 mm a −1 , and are more likely below 3 mm a −1 . At such denudation rates, the intrusion process needs to start at depths of c. 22 km, except when the final laccolith thickness is 10 km, in which case the depth of first-emplaced magmas cannot exceed 18 km. Thick HHL laccoliths (>7 km) may require a minimum denudation rate, on the order of 1 mm a −1 , to prevent wholesale convection and allow muscovite crystallization. Yet, emplacement of such thick HHL laccoliths during normal faulting implies that the top part of the leucogranite nearly reaches the surface while its base is still fed by active intrusions. Overall, such relatively low denudation rates suggest that, when HHL were intruded, the overlying crustal column was not undergoing vigorous erosion. Within the framework of a crustal channel flow, this suggests that the zone of focused erosion during the Miocene was located to the south of the current exposures of the HHL belt. Our results also show that to explain the steep cooling histories documented in many HHL, denudation must have been active after HHL solidification, especially when they were intruded close to their source region. However, to preserve the HHL from exhumation and erosion until the present time, the average denudation rate after emplacement cannot have exceeded 0.5 mm a −1 .

The control mechanisms of erosion and weathering at basin scale from cosmogenic nuclides in river sediment

The study of a sample of river sediment enables the determination of spatially averaged denudation rates that provide an exceptional perspective on erosion and weathering processes that have taken place within a landscape. These measurements are done with in-situ produced cosmogenic nuclides (e.g. 10Be, 26Al), mostly in quartz from alluvial sediment. Cosmogenic nuclides are produced when secondary cosmic rays interact with the very uppermost layer of the Earth's surface. They are produced within a characteristic depth scale of about 1 m, which means that the measured concentrations record an integrated denudation history while material passed through this depth interval. Depending on the denudation rate the resulting integration time scales are 10 3 to 10 5 years, and one obtains a robust long-term estimate of natural denudation that is relatively insensitive to short-term changes. The last 10 years have seen significant research activity using these methods, and an array of fascinating tectono-geomorphologic and geochemical insights are emerging. Amongst these is the ability to identify the physical and chemical processes with which a landscape responds to tectonic activity or climate change. A compilation of world-wide denudation rates in non-glaciated areas, that however, does not yet include some of the world's most active mountain belts, has resulted in the following findings, some of which have been unexpected: (1) No obvious relationship between precipitation or mean annual temperature and total denudation is apparent. (2) Topographic relief alone does not result in high rates of denudation. (3) Denudation rates are high in areas of landscape rejuvenation; that is triggered and controlled by tectonic activity (faulting, escarpment formation and retreat, rifting, surface uplift). (4) Rates of weathering (using a combination of cosmogenic nuclides and zirconium-normalised cation loss balances) co-vary primarily with physical erosion rates and much less with temperature or precipitation. (5) In some areas of high land use short-term rates (from river load gauging) exceed those from cosmogenic nuclides by several orders of magnitude, which serves to highlight the severity of geomorphic change caused by human action. In the future, the control mechanisms over denudation will be determined on all spatial scales, ranging from the single soil section to entire river basins. The same analysis can be done back through time on well-dated terraces, lake records, and marine sediment cores, which is possible with 10Be for the past 1–2 My. The rates obtained will be used to develop a quantitative understanding of tectonic, geomorphologic, and geochemical landscape processes, which in turn is a prerequisite to design and calibrate models of the response of landscapes to tectonic, climate, and anthropogenic forcing.

Analysis of landslide frequencies and characteristics in a natural system, coastal British Columbia

AbstractTwo hundred and one debris slides and debris flows were analyzed in a 286 km2 study area on the west coast of Vancouver Island, British Columbia, Canada. The study area remains essentially untouched by humans and therefore affords a natural setting in which to examine slope processes. Landslides were identified and characterized on aerial photographs from 1:15 000 to 1:31 680, and were then mapped and transferred to a GIS for analysis. Based on detailed landslide surveys, we propose a new method to accurately determine volume of landslides of this type by measured total area. Results indicate average denudation rates of 56 m3 y−1 km−2, and higher natural rates of failure than analogous regions in coastal British Columbia. In contrast, the landslide rates are substantially less than those from forested watersheds. Landslide distribution is spatially clustered in air photograph epochs, and we propose intense storm cells within regional events as the causal mechanism. Further, failures occurred preferentially over the West Coast Crystalline Complex (by 1·4 times), a metamorphic assemblage of gabbros, schists and amphibolites, but 1·5 times less often over the Island Plutonic Suite, a granitic intrusive formation. The former result represents a new finding, while the latter corroborates findings of previous authors. We examined magnitude–frequency relationships of the data set and present for the first time a strong argument that the rollover effect is not merely an artefact, but is instead a consequence of the physical characteristics of the landslides themselves. We subsequently analyzed magnitude–frequency relationships from two other complete data sets from coastal British Columbia and produced a family of curves corroborating this result. Copyright © 2004 John Wiley & Sons, Ltd.

Denudation rates of crystalline rock in the Huangling anticline of the Three Gorges of the Yangtze River in China since the Eocene

In the crystalline rock region of the Huangling anticline center located in the eastern part of the Three Gorges of the Yangtze River, 6 typical geological topographical section lines were selected and their paleotopographies were reconstructed. Based on the difference between the present topography and the paleotopography, the denudation thicknesses of different topographical positions could be calculated. According to the principle of denudation correlated with deposition, the initial denudation age of the crystalline rock was inferred from the sedimentary strata deposited in the Jianghan basin near the Huangling anticline, including crystalline rock gravel from the Huangling anticline, which was about 56.5 MaBP. The widespread and intensive denudation took place at about 2.4 MaBP. On the basis of the above-mentioned initial denudation age (56.5 MaBP) and the denudation thicknesses, the denudation rates of different topographical summit levels, different topographical terrace levels and various watershed systems were calculated. The greatest denudation rate is 49.56 mm/ka. The average denudation rate is 16.97 mm/ka in the crystalline rock region of the Huangling anticline.

Pleistocene/Holocene climate change, re‐establishment of fluvial drainage network and increase in relief in the Swiss Alps

ABSTRACT Data are presented about modern sediment discharge of the Swiss rivers and related to the size of catchments. The information reveals that the Central Alps have experienced denudation rates of ≈0.15 mm yr−1 in the foreland, and ≈0.5 mm yr−1 in the Alpine core. Mapping, however, indicates that modern erosion only affects 30–50% of the Alpine surface, and that fluvial and associated hillslope processes have focused erosion in 50–200‐m‐deep valleys. These valleys are incised into the glacial surface. If this limited spatial extent of erosion is considered, then effective erosion rates are significantly higher than average denudation rates. These effective rates equal or locally exceed modern rates of rock uplift. This implies that the modification of erosional processes related to the Pleistocene/Holocene climate change has resulted in an increase in the relief at a local scale. At a drainage basin scale, however, the relief appears not to change at present.

Denudational history along a transect across the Drakensberg Escarpment of southern Africa derived from apatite fission track thermochronology

The denudational history of a ∼500 km long transect across the Drakensberg Escarpment on the high‐elevation passive margin of SE Africa is quantified on the basis of thermal history modeling of apatite fission track data for 15 deep borehole samples, supplemented by an additional 10 outcrop samples. A minimum of 4.5 km of denudation since formation of the margin ∼130 Myr ago is estimated for the coastal zone, with a marked Early Cretaceous episode of accelerated denudation broadly coincident with continental breakup. Samples from the Swartberg borehole (SW 1/67) located ∼30 km seaward of the present position of the Drakensberg Escarpment indicate a total depth of denudation of 3.1 ± 1.2 km since ∼91 Myr, with a phase of accelerated denudation of 2.1 ± 0.9 km at a mean rate of 95 ± 43 m/Myr between ∼91 and 69 Myr. Samples from the Ladybrand borehole (LA 1/68) west of the Lesotho Highlands indicate 1.7 ± 0.5 km of denudation since ∼78 Myr, with a phase of accelerated denudation at 82 ± 43 m/Myr from ∼78 to 64 Myr. Average denudation rates declined to about 10 m/Myr during much of the Tertiary. Although the apatite fission track data do not provide any direct constraints on the denudational history of the Lesotho Highlands, interpolation between the two boreholes, constrained by geological evidence and extrapolated in situ‐produced cosmogenic 36Cl‐derived denudation rate estimates, suggests a pattern of denudation compatible with numerical modeling studies of escarpment evolution involving rapid river incision seaward of a preexisting inland drainage divide. These patterns of denudation are incompatible with constant retreat of the Drakensberg Escarpment from an initial position near the present coast. We suggest that the Drakensberg Escarpment formed by rapid post‐breakup river incision seaward of a preexisting drainage divide located just east of the present escarpment location and became pinned at this divide with subsequent retreat rates of only 100–200 m/Myr.

The post-Variscan thermal and denudational history of Ireland

Abstract The thermal and denudational history of Ireland is evaluated using an extensive new apatite fission-track (AFT) dataset derived from surface samples. Modelled thermal histories are used to construct maps of denudation for a number of time slices from Triassic time to 10 Ma using a time-dependent palaeogeotherm. The maps illustrate the spatial variability of denudation and subsidence within each time slice. The patterns of denudation are complex, showing considerable variability at the length scale of 10 1 –10 2 km, with especially high denudation rates found over known igneous centres such as the Mournes of County Down. Based on the onshore AFT data alone, there is no definitive signature of an Irish Sea Dome extending significantly across Ireland in Early Tertiary time. The cumulative amount of denudation during Tertiary time varies depending on the AFT annealing model used, but is generally in the region between 1 and 2 km and without clear spatial trends. High amounts of denudation have been mapped over the Tertiary intrusions in County Down, reflecting their unroofing since emplacement in Paleocene time. The cumulative denudation from Triassic time to 10 Ma shows relatively low amounts of denudation (<2 km) in the Irish Midlands and the extreme NE of the island, consistent with the observation that Mesozoic-Tertiary sediments and igneous products are preserved in the Ulster Basin. The western flank of Ireland and the region between Dublin and County Down show high cumulative amounts of denudation (<4 km), the latter being consistent with the high amounts of denudation interpreted for the Irish Sea region. This denudation pattern explains in part the outcrop of Precambrian and Lower Palaeozoic rocks in these areas. The spatial integration of the denudation over the entire landmass gives the average denudation rate and the sediment discharge from Ireland as a function of time. Average denudation rates are moderately high in Triassic time, falling to low values in Cretaceous time, and increasing substantially in Tertiary time. However, the total volumetric discharge of sediment in Tertiary time is an order of magnitude smaller than the preserved solid volume of Tertiary sediment in the basins offshore western Ireland.

Unroofing history of the Sillai Patti granite gneiss, Pakistan: constraints from zircon fission-track dating

A group of alkaline igneous rocks is exposed in the north of the Peshawar Plain, extending from Tarbela in the east up to Loe–Shilman near the Pakistan–Afghanistan border in the west. The alkaline rocks consist mainly of granites, syenites, gabbros, ijolites and carbonatites. Granitic gneisses of Paleozoic age are exposed at the Malakand and further westward at Sillai Patti. However, the fission-track dating studies on zircon, based on the present work, indicate that the age of the Sillai Patti granite gneiss is less than the absolute age of granite gneisses. Therefore, the zircon fission-track age of 24.28±2.97 Ma of the Sillai Patti granite gneiss, represents a time of post-metamorphic denudation history of the area, when these rocks passed through the 210°C isotherm, corresponding to a depth of about 6.7 km inside the earth's crust from their present position if a paleogeothermal gradient of 30°C/km is assumed to have prevailed. Our average fission-track zircon age of 24.28±2.97 Ma is very similar to the average fission-track zircon age of 25.4±0.7 Ma of Mansehra granites. Average cooling rates of the Mansehra and Sillai Patti granite gneisses have been computed to be (8.00±0.22)°C/Ma and (8.00±0.98)°C/Ma, while the average denudation rates of the Mansehra and Sillai Patti granite gneisses have been computed to be (0.262±0.007) and (0.274±0.034) mm/ yr , respectively, on the basis of zircon fission-track ages for the period between 25 Ma and the present time. This indicates that the two complexes have experienced similar average cooling/uplift-induced denudation histories during the last 25 Ma or so.

Denudation of Small Alpine Basins, Nanga Parbat Himalaya, Pakistan

Thirty-three debris fans and five small alpine basins on the south side of the rapidly uplifting Nanga Parbat Himalaya of northern Pakistan were assessed to determine how much alpine processes contribute to the overall denudation of the massif. A high-resolution digital elevation model was used to measure the volume of the small alpine fans and a few basins in the Rupal valley. These volumetric estimates, coupled with time-constraining dates from cosmogenic-nuclides of glacially exposed rocks and infrared-stimulated luminescence of sediments, indicate that estimated average denudation rates in these systems have been ~2 mm yr−1 over the past 4600 to 6000 yr since the last major deglaciation. This is similar to other estimates of rates of denudation in the Himalaya.

Denudation and uplift at passive margins: the record on the Atlantic Margin of southern Africa

The onshore region of a passive margin forms an integral part of the geological evolution from continental break–up to later sedimentation in the offshore basins. The dominant surface process in the onshore region is denudation, which acts to remove any direct evidence of surface uplift. However, denudation can be constrained on geological time–scales through low temperature thermochronological data, such as that obtained through apatite fission track analysis. Here, we present a suite of such data from the Atlantic Margin of southern Africa. The data have been modelled in terms of their temperature histories since the Jurassic. These temperature histories have been combined with heat–flow data to estimate the equivalent depth of denudation over these time–scales. Average denudation rates are of the order of a few tens of metres per million years, but show considerable variations both temporally and spatially. These results demonstrate that passive margins experience complex patterns of denudation. Three landscape evolution models are considered. Our results imply the downwarping model is inappropriate. The other two models, scarp retreat and pinned drainage divide, predict trends similar to those observed but the complexities inherent in the data and the evolution of passive margin topography do not allow us to resolve one from the other. In practice, both models probably operate to some extent as a margin evolves. Estimates of palaeotopography have been made, assuming simple isostatic response models to denudational unloading. Flexural models with effective elastic thickness (EET) of 25 km predict elevations 2 km and more above the present day values, while models with EET of 0 km predict elevations up to 750 m higher than the present–day. These models ignore any post–break–up tectonic uplift and need independent constraints on surface elevation to assess their validity.

Quantifying denudation and sediment–accumulation systems (open and closed lakes): basic concepts and first results

The quantitative aspects of denudation in the drainage areas and sediment accumulation in the corresponding closed or open lakes are approached using two methods: (1) from known mean rates of mechanical and chemical denudation and the ratio of the drainage/lake area, average terrigenous and potential chemical (biogenic and evaporites) sedimentation rates in the lake are determined; or (2) from known sedimentation rates the average denudation rate is found. In closed systems the total mass denuded during a certain unit of time is deposited in the lake as terrigenous, biogenic and evaporitic sediments, whereas open lake systems lose a major part of the dissolved river input derived from chemical denudation. Applying elementary equations describing these relationships and using ratios between mechanical and chemical denudation (weathering ratio) established in other regions, e.g. chemical weathering in the lake drainage can be estimated. Likewise, the knowledge of the `bio-index' (ratio of biogenic sediment/total dissolved mass) allows the calculation of the loss of dissolved matter through the outflow from the lake, etc. In a first step, existing data from modern lake systems are evaluated. Later, idealized lake systems in various climatic and morphotectonic zones can be simulated and the results utilized to better understand ancient lake systems. Modern Alpine lakes reflect increased mechanical denudation rates (100 to ≥300 mm/ka) in highly elevated drainage areas of the Central Alps (mainly crystalline rocks) and high chemical denudation (up to ca. 100 mm/ka) in carbonate rocks of the Northern Calcareous Alps. Mechanical denudation is significantly enhanced by glaciation, but transport of the detritus into lakes may be delayed until the time immediately after glacial retreat. Average rates of terrigenous lake sedimentation amount to 5 to ≥15 mm/a (crystalline rocks, high relief), or are ≤5 mm/a (carbonate rocks). Delta outbuilding was a main factor in the filling of many Alpine lakes. Lakes draining low-altitude, semi-arid parts along the eastern margin of the Alps (Lake Balaton in Hungary) have low sedimentation rates and reflect the influence of increased temperature and vegetation. In East Africa, the transition from the Late Pleistocene (arid) to the early Holocene (humid) and again back to somewhat drier conditions caused substantial rises in lake levels and changes from closed to open lake systems. In such cases, the calculation of denudation from accumulation rates is biassed as a result of intermittent sediment storage at the lake margins (low sedimentation rates in the lake centre during lake highstands) or redeposition of sediment during lowstands (lowstand shedding). The modern denudation rates of East African rift lake systems vary by a factor of ≥15 (Lake Tanganyika, crystalline rocks and more dense vegetation cover, total denudation rate ca. 4 mm/ka; Lake Turkana, young volcanic rocks and tephra, sparse vegetation, mechanical denudation rate ca. 60 mm/ka).

Exhumation of the Pyrenean orogen: implications for sediment discharge

Apatite fission track analyses of 21 samples from the central and eastern Pyrenees are modelled to generate time–temperature plots for the post 110±10 °C cooling history over the 40–10 Ma time interval. Modelled thermal histories have been converted into exhumation plots through the application of the present‐day geothermal gradient in the Pyrenees. The documented geology of the Pyrenees allows us to assume no significant extensional unroofing and subvertical exhumation trajectories, thus enabling exhumation to be translated into erosional denudation. Maps of denudation have been constructed for six, 5‐Myr time intervals. Denudation varied with a 20–50‐km length scale, and does not appear to have been related to the major structural zones of the mountain belt. Spatially averaged denudation rates for the six time intervals ranged from 34 to 61 mm kyr−1 assuming the present‐day geothermal gradient. Maximum rates of 240 mm kyr−1 occurred in the interval 35–30 Ma, in the region of the Querigut‐Millas massif.Assuming the denudation resulted primarily from erosion, the denudation maps can be used to calculate sediment discharge through time to the neighbouring foreland basins. Using a series of rectangular drainage basins with a 2:1 aspect ratio (based on modern linear mountain belts) and a location of the main drainage divide based on the mean present‐day position, it is possible to evaluate the potential for spatial and temporal variations in sediment discharge as a function of denudation. The results show along‐strike variations in sediment discharge between drainage basins of 500%, and temporal variations from individual basins of >300%. A comparison of total sediment discharge per year to the Ebro and Aquitaine basins, assuming a fixed drainage divide, shows that the discharge to the south is likely to have been between 1.5 and 2.8 times greater than to the north.