Topographic and lithologic control on catchment-wide denudation rates derived from cosmogenic 10Be in two mountain ranges at the margin of NE Tibet
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Denudation
Lithology
Colluvium
Thrust fault
Low-temperature thermochronology, such as that provided by apatite fission-track analysis, provides a valuable means of establishing the timing of major denudational events and associated rates of denudation over geological time-scales of 106–108 Ma. Care must be taken, however, in deriving denudation rates from the crustal cooling histories documented by thermochronologic techniques, especially in rapidly eroding terrains, since, in such cases, apparent denudation rates derived from thermochronologic data will usually overestimate true rates if the advective effect of denudation is not included. This is likely to be resolvable where the rate of denudation exceeds 300 m Ma−1 and when the depth of denudation occurring at these rates exceeds several kilometres prior to the sample cooling below the appropriate closure temperature. Because the time at which a sample cools below a particular closure temperature is relatively insensitive to advection, the initiation of denudation can be accurately established, even given uncertainties in the estimation of depths and rates of denudation. Where thermal events originate from a source within or below the lower crust, the cooling through denudation will dominate the low-temperature history of the shallow crust if denudation occurs coevally with the subsurface heating. © 1997 by John Wiley & Sons, Ltd.
Denudation
Thermochronology
Fission track dating
Closure temperature
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Abstract Evidence from the Wellington region, the lower Pareora Gorge, and the Otago schist belt testifies clearly to the importance of lithology in determining response of rocks to metamorphism and tectonic forces. The order of decreasing response appears to be: volcanic argillites (claystones), argillites (siltstones), lavas, greywackes (sandstones), jaspers and cherts. It is ,essential in regional metamorphic studies to distinguish between original lithology, metamorphic lithology, degree of internal reconstitution, and mineralogical changes. Both textural and mineralogical metamorphic rank should be based on the response of one rock type (greywacke). The “normal” or “standard” metamorphic sequence is that in which textural change of the selected rock type is accompanied by a full succession of mineralogical changes, and this is more nearly attained in south-cast Nelson than in Otago. Just as metamorphic textural :md mineralogical rank should be based on greywacke so too should interpretations of regional structure be based on one lithology.
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Mt. Nagi apperes to present a subdued form, and on its southern side it goes down to the basin of Nihonbara as a scarp of the Nagisan fault. There are some low hills and dissected fans which seems to be of Tertiary age. A zone of soil of usual apperence extends in a W-E direction between the moutainland and the flat parts of the Tertiary beds and the alluvium, which the author thinks is colluvium. The term “colluvium” originally refered to a soil possesing certain petrographic characteristics. The author, however, used the word here for a certain soil accumlation based on geological and topographical characteristics. The colluvium consists of angular gravels of diameters less than 10cm, and probably covers to a certain extent the fault plane of Mt. Nagi. The inclination of the colluvium, which is 100 to 200‰, is steeper than that of ordinally alluvial fans and elevated deltas, but more gradual than that of talus. If the breadth of the valley of the upper stream is wide, the colluvium is accumulated in the form of a fan. In a series of valleys of smaller breadth, the colluvium forms a continuous surface, which should be called colluvial base. When the mountain at the back is of subdued form, that is, the elevation of the mountainland is proceeding at a very slow rate, and the conditions of the rock is suitable for the formation of the colluvium, it may be concluded that conditions are right for the deposition and accumulation of the colluvium.
Colluvium
Alluvial fan
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Annual denudation rates were calculated using data on the suspended sediments runoff module of the rivers. These rates vary widely: from less than 0.005 to more than 1.75 mm/year in the Caucasus, Pamir-Alai and Alps; from 0.002 to 0.4 mm/year in the Tien Shan. The most significant factors of denudation cutting of the mountains are topography, climate and composition of rocks. The maximum rates of denudation (more than 1 mm/year) occur in the highest and most humid parts of the mountains (Western Alps, Western Caucasus, etc.). The minimum values of annual denudation rate (less than 0.1 mm/year) are observed on the lower mountain slopes with little rainfall and with resistant to denudation sediments (eastern slopes of the Lesser Caucasus, the interior of the Tien Shan, etc.). Vegetation and human activities can both enhance and suppress exogenous processes and thus the denudation rate. In all these mountains tectonic uplift prevails over the denudation.
Denudation
Alpine climate
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In the alpine Hrafndalur catchment denudation rates are relatively high, with mechanical denudation dominating over chemical denudation due to high mechanical weathering rates of the low-resistant rhyolites in this area. Both the Kidisjoki and the Latnjavagge catchments are characterised by low contemporary denudation rates where chemical denudation dominates over mechanical fluvial denudation. In Latnjavagge, the low intensity of mechanical fluvial denudation is mainly due to a closed and stable vegetation cover. The low-relief area of Kidisjoki is characterised by very low mechanical fluvial denudation rates.
Denudation
Alpine climate
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Contemporary stream sediment loads are dubious indicators of regional denudation. Recent analysis of ten river basins in the southeastern United States indicates that of the material eroded from upland slopes since European settlement, only about 5 percent has been exported. The remainder, alluvium and colluvium, will probably not be exported because of extensive reservoir impoundments.
Denudation
Colluvium
Settlement (finance)
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Denudation
Thrust fault
Thermochronology
Main Central Thrust
Fission track dating
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