We present a new compilation of estimates of modern rates of mechanical and chemical denudation for externally drained basins exceeding 5×105 km2 in area. These estimates are based on sediment and solute load data selected in order to represent natural rates as far as possible. Chemical denudation rates have been calculated by deducting the nondenudational component of solute load. Mechanical denudation rates range from 1 mm kyr−1 for the St. Lawrence and Dnepr basins to 670 mm kyr−1 for the Brahmaputra basin. Chemical denudation rates vary from 1 mm kyr−1 (Kolyma, Niger, Nile and Rio Grande basins) to 27 mm kyr−1 (Chiang Jiang basin). The Kolyma basin has the lowest (4 mm kyr−1), and the Brahmaputra basin the highest, overall rate of denudation (688 mm kyr−1). Relationships between denudation rates and a range of morphometric, hydrologic, and climatic variables are investigated through correlation and regression analysis. Morphometric variables, such as mean local relief, are accurately calculated for large basins for the first time by using the National Geophysical Data Center 10‐minute topographic database. Variables expressing basin relief characteristics and runoff are found to be most strongly associated with both mechanical and chemical denudation rates, with more than 60% of the variance in total denudation being accounted for by basin relief ratio and runoff. Basin area, runoff variability, and mean temperature, however, are only weakly associated with rates of denudation. Although direct comparisons cannot be made, it appears that rates of basin denudation derived from present‐day mass flux estimates are not, overall, significantly different from estimates of long‐term rates based on sediment volume and thermochronologic data. It therefore appears that the key factors identified as controlling denudation rates here are also applicable to the geological time spans relevant to the interaction between tectonic and denudational processes.

a b s t r a c tTectonics and erosion are the driving forces in the evolution of mountain belts, but the identification of their relative contributions remains a fundamental scientific problem in relation to the understanding of both geodynamic processes and surface processes. The issue is further complicated through the roles of climate and climatic change. For more than a century it has been thought that the present high topography of western Scandinavia was created by some form of active tectonic uplift during the Cenozoic. This has been based mainly on the occurrence of surface remnants and accordant summits at high elevation believed to have been graded to sea level, the inference of increasing erosion rates toward the present-day based on the age of offshore erosion products and the erosion histories inferred from apatite fission track data, and on over-burial and seaward tilting of coast-proximal sediments.In contrast to this received wisdom, we demonstrate here that the evidence can be substantially explained by a model of protracted exhumation of topography since the Caledonide Orogeny. Exhumation occurred by gravitational collapse, continental rifting and erosion. Initially, tectonic exhumation dominated, although erosion rates were high. The subsequent demise of onshore tectonic activity allowed slow erosion to become the dominating exhumation agent. The elevation limiting and landscape shaping activities of wet-based alpine glaciers, cirques and periglacial processes gained importance with the greenhouse-icehouse climatic deterioration at the Eocene-Oligocene boundary and erosion rates increased. The flattish surfaces that these processes can produce suggest an alternative to the traditional tectonic interpretation of these landscape elements in western Scandinavia. The longevity of western Scandinavian topography is due to the failure of rifting processes in destroying the topography entirely, and to the buoyant upward feeding of replacement crustal material commensurate with exhumation unloading.We emphasize the importance of differentiating the morphological, sedimentological and structural signatures of recent active tectonics from the effects of long-term exhumation and isostatic rebound in understanding the evolution of similar elevated regions.

Studies across a broad range of drainage basins have established a positive correlation between mean slope gradient and denudation rates. It has been suggested, however, that this relationship breaks down for catchments where slopes are at their threshold angle of stability because, in such cases, denudation is controlled by the rate of tectonic uplift through the rate of channel incision and frequency of slope failure. This mechanism is evaluated for the San Bernardino Mountains, California, a nascent range that incorporates both threshold hillslopes and remnants of pre-uplift topography. Concentrations of in situ-produced cosmogenic 10 Be in alluvial sediments are used to quantify catchment-wide denudation rates and show a broadly linear relationship with mean slope gradient up to ~30°: above this value denudation rates vary substantially for similar mean slope gradients. We propose that this decoupling in the slope gradient-denudation rate relationship marks the emergence of threshold topography and coincides with the transition from transport-limited to detachment-limited denudation. The survival in the San Bernardino Mountains of surfaces formed prior to uplift provides information on the topographic evolution of the range, in particular the transition from slope-gradient-dependent rates of denudation to a regime where denudation rates are controlled by rates of tectonic uplift. This type of transition may represent a general model for the denudational response to orogenic uplift and topographic evolution during the early stages of mountain building.

No abstract

[1] 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 36 Cl-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.

No abstract

[1] We employ a numerical surface processes model to study the controls on postbreakup landscape development and denudational history of the southeast African margin. Apatite fission track data, presented in the companion paper, suggest that the Drakensberg Escarpment formed by rapid postbreakup river incision seaward of a preexisting drainage divide, located close to its present position, and subsequently retreated at rates of only $100 m m.y.À1 . Numerical modeling results support such a scenario and show that the prebreakup topography of the margin has exerted a fundamental control on subsequent margin evolution. The rheology of the lithosphere, lithological variations in the eroding upper crust, and inland base level falls provided secondary controls. A relatively low flexural rigidity of the lithosphere (T e % 10 km) is required to explain the observed pattern of denudation as well as the observed geological structure of the southeast African margin. Lithological variations have contributed to the formation of flat-topped ridges buttressing the main escarpment, as well as major fluvial knickpoints. Both these features have previously been interpreted as supporting significant Cenozoic uplift of the margin. An inland base level fall, possibly related to back-cutting of the Orange River drainage system and occurring 40-50 m.y. after breakup, explains the observed denudation inland of the escarpment as well as the development of inland drainage parallel to the escarpment. Our model results suggest that in contrast to widely accepted inferences from classical geomorphic studies, the southeast African margin has remained tectonically stable since breakup and escarpment retreat has been minimal (<25 km).

No abstract