Late Quaternary deformation of the sub-Himalaya on 100 kyr timescales
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<p>In mountain accretionary wedges, it is generally considered that the preservation of a topography in mechanical equilibrium is modulated by the activation of faults, sometimes internal to the prism, sometimes frontal. The folds of the Himalayan foothills correspond to the most frontal structures of the Himalayan prism. Understanding the timing of the initiation and the activity of these frontal folds can provide valuable information on the deformation sequences within the range (reactivation of the MCT, prograde sequences and transfer to frontal folds, ...) in response to tectonic and climatic forcing. Late Cenozoic climatic changes, including glaciations, might have impacted the denudation of the Himalayan range. The study of recent deformation rates is thus key for understanding lateral variations in deformation along the entire Himalayan arc, which will bring new constraints on the interactions between tectonics and surface processes at different scales time, as well as deepen our understanding of the seismic behaviour of the range.</p><p>&#160;</p><p>Here we quantify exhumation rates in the Himalayan foothills using luminescence thermochronometry, which is a recently developed very-low temperature thermochronometer applicable between tens of years and a few hundred kyr. In contrast to classical methods, it can resolve thermal histories from the upper few km of the Earth&#8217;s crust, allowing spatial variations in exhumation rates across the Himalayas to be deciphered on sub-Quaternary timescales. An extensive data set of more than 40 Siwalik rock samples, from Western Nepal to Eastern Bhutan, was measured to complement other thermochronometric data and understand the sub-Quaternary deformation on the Himalayan foreland.</p><p>&#160;</p><p>The results show along-strike variations in exhumation rates in the Himalayan foothills during the late Quaternary, with exhumation rates across the sub-Himalaya varying locally independently of precipitation trends and changes in the modern convergence rates. These along-strike variations may suggest that over the last 300 kyr, Himalayan shortening has not only been accommodated by the most frontal faults along the Himalayan range.</p>Keywords:
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Denudation
The eastern range of Tian Shan begins in the Bogeda Mountains in the west, continues to the Barkul Mountains, and meets the Harlik mountains in the east. There are two basins, the Hami basin and Turpan basin, which collectively are known as the Tuha basin, lying in the southern foothills of the eastern range of Tian Shan. Yiwu county lies in the northern foothills of the Harlik mountains, while Barkul county lies in the northern foothills of Barkul Mountains. In the northern foothills of the Bogeda Mountains are Jimusaer, Qitai, and Mulei counties, which are known as the Eastern three counties for short. In the southwestern foothills of the Bogeda Mountains is the Wutu Corridor ranging from Urumqi to the Turpan region.
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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.
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This paper describes the development of settlements and roads in the Southeastern Zagreb Foothills from the 13th to the 16th century. The Foothills are a microregion bordered by the river Sava in the south, Zagreb in the west, the Medvednica in the north, and the Zelina Foothills in the east. In the High and Late Middle Ages and the early modern period, this microregion was located in the eastern parts of the Zagreb County and did not have any specific name, unlike today, when it is colloquially known as the Sesvete Foothills, the Foothills, or the Sesvete Area, after the Sesvete district of Zagreb. Today, it is the most densely inhabited area where the foothills meet the Sava plain, while its central hilly parts, with numerous mountain stream valleys and their tributaries, were the most densely populated area in the High and Late Middle Ages and the early modern period. The first settlements in this area developed below fortresses or seats of parishes in the 13th century in the hilly parts of the Zagreb Foothills, but later settlements, in the late 15th and early 16th centuries, developed mostly along the main plains roads. These roads connected the Austro-German lands in the west with the Slavonian and Hungarian counties in the Sava-Drava interfluve and the Podunavlje to the east, and the central parts of the Kingdom of Hungary with the eastern Adriatic ports. All settlements were part of four large estates, which encompassed almost the entire Southeastern Zagreb Foothills area.
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This paper evaluates the denudation thickness on the denudation surface in the deep part of Songliao Basin using Dow method and its correction method basing on vitrinite reflectance restore calculation.The result shows that the denudation thickness obtained by Dow method is less than 1 100～3 000 m except Well Yushen 1 with the d/s denudation thickness of 1 534.7 m.The denudation thickness obtained by Dow method is less than 730 m.Comparing to this value, the deep denudation thickness obtained by the correction method considering the reburial effect is 1 100~3 000 m.The analysis shows that Dow method may correspond to the lower limit of the denudation thickness value,while the correction method may correspond to the upper value.
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Vitrinite reflectance
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Denudation
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The main purpose of this research is to clarify morphological characteristics and fanning processes of undulating denudation surface on the Eastern Coast. For this research, undulating denudation surface around Kangnung and Sokcho are surbeyed. 1) Denudation landfrnm are distributed at the altitude of 20 to 150 meters along eastern slopes of the Taebaek mountains. Bedrocks of denudation surface consist of granite and are deeply-weathered. The thickness of the weathered crust presumed to be 15meters at least. 2) At present, the denudation surfaces are dissected into undulating hills revealing concordant summit level. From this fact, it might be concluded that the undulating hills of concordant summit level should be remnant of former denudation surface. This can be interpreted that more flatted denudation surface had been exist before the unduating denudation surface is developed. It is assumed that the dissection of the denudation surface took place during the pleistocene epoch. 3) There are no deposits on denudation surface. This fact imply that fluvial erosion did not occurred actually. Corestones, usually from about 0.5 to 5m in diameter, are included in weathered crust of denudation surface, and sometimes they are exposed in the earth`s surface. Dome-shaped hills that are similar to inselberg stand in isolation admist undulating denudation surface. Accordingly, it is estimated that denudation surface had been developed as a result of deep weathering of granite and the denudation of weathered products.
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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.
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Alpine climate
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Among the species of Mycena and related genera of the Himalayan foothills described in this paper, the following are proposed as new: Mycena abietina, M. bathyrrhiza, M. cinnabarina, M. coalita, M. gentilis, Hydropus eburneus, and Xeromphalina aspera.
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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.
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Alpine climate
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The valleys of the Brahmaputra and the uplands around the valley meet each other in the foothills of Northeast India. This entry explores, through pre-colonial history and current day situations, the role that the foothills have in the relationship between the hills and the plains. Historically, the foothills were both borders and sites of negotiations between the different kingdoms of the Brahmaputra Valley and the residents of the uplands surrounding the valley, with the duars standing out as feats of diplomacy between the two regions. These negotiations concerned religious identity, control over trading areas and resources, political alliances, ethnicity, identity amongst other aspects of life. In today's Northeast India, the foothills continue to mediate between the hills and plains, albeit in a different political and economic landscape. More importantly, these sub-montane regions have acquired their own specificities, forged out of their roles as mediators and sites of negotiations. It is these specificities that distinguish the foothills from the other regions of the Northeast. This entry seeks to highlight the importance of the 'foothills' and set them apart as an area that deserves to be studied in its own right.
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