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1.
The Yulong Mountain massif is tectonically active during Quaternary and contains the southernmost glacierized mountains in China, and all of Eurasia. Past glacial remnants remain preserved on the east and west sides of the Yulong Mountains. A ridge of moraine protruded into the Jinsha River at the Daju Basin, damming the river, and forming a lake at the head of the Jinsha River. Cosmogenic 10Be and 26Al provide exposure age dates for the moraine-based fluvial terraces left behind after the dam breached, and for moraine boulders on both the eastern and western sides of the Yulong Mountains. Our results yield exposure ages for the terraces that range from 29 ka to 8 ka, and a downcutting rate of 7.6 m/ka. The preservation of the remaining dam for over 10,000 years suggests stability of the moraine dam and gradual erosion of the dam during drainage of the dammed lake. From the relationship between exposure ages and elevations of the fluvial terraces located in different walls of the Daju fault, we obtain a late Quaternary dip-slip rate of about 5.6 m/ka for the Daju fault. The exposure ages of 10.2 ka and 47 ka for moraine boulders located in the east and west sides of the Yulong Mountains, respectively, coincide with warm periods in the late Quaternary. This implies that precipitation provides the major control for glaciations on the Yulong Mountains, a domain of the southwest Asian monsoon.  相似文献   

2.
Many earlier studies have shown the very large spatial variability of rainfall in the Sahel at all time steps, from the event to the season. Often, the meteorological network in these countries is sparse, with one to five rain gauges per 10 000 km2. It is thus difficult to calculate accurate estimates of the mean rainfall over such a large area. To improve the knowledge of Sahelian systems and the spatial distribution of rainfall, a dense network was set up in an area of 16 000 km2 in southwestern Niger between 1991 and 1996. The aim was to calculate accurate rainfall spatial means over an area of 12 000 km2 at different time steps (from the season to the ten-day period). With the spatialisation method used (kriging), it was possible to calculate curves of estimation errors of mean rainfall versus the rain-gauge network density. Operational abacuses of the standard estimation error as a function of the spatial mean of rainfall and the network density are proposed.  相似文献   

3.
This study investigates the hydraulic conductivity field and the groundwater flow pattern as predicted by a calibrated steady state groundwater flow model for the Keta Strip, southeastern Ghana. The hydraulic conductivity field is an important parameter in evaluating aquifer properties in space, and in general basin-wide groundwater resources evaluation and management. This study finds that the general hydraulic conductivity of the unconsolidated unconfined aquifer system of the Keta Strip ranges between 2 m/d and 20 m/d, with an average of 15 m/d. The spatial variation in horizontal hydraulic conductivity appears to take the trend in the variations in the nature of the material in space. Calibrated groundwater recharge suggests that 6.9–34% of annual precipitation recharges the shallow aquifer system. This amount of recharge is significant and suggests high fortunes in terms of groundwater resources development for agriculture and industrial activities in the area. A spatial distribution of groundwater recharge from precipitation is presented in this study. The spatial pattern appears to take the form of the distribution in horizontal hydraulic conductivity, and suggests that the vertical hydraulic conductivity takes the same pattern of spatial variation as the horizontal hydraulic conductivity. This is consistent with observations in other areas. The resulting groundwater flow is dominated by local flow systems as the unconfined system is quite shallow. A general northeast – southwest flow pattern has been observed in the study area.  相似文献   

4.
Monsoonal climates at low latitudes (< 32°N) are an inevitable consequence of seasonal migrations of the Inter-tropical Convergence Zone (ITCZ), but the character of these monsoons depends on continental configuration, orographic expression and the strength of Hadley circulation. To explore the evolution of monsoon systems across southern Asia we compare climate signatures archived in ten Paleogene floras from northern India, Tibet and southern China, occupying low palaeolatitudes at a time of extreme global warmth and elevated CO2. Fossil leaf form reveals that under such 'hothouse' conditions megathermal early Eocene to earliest Miocene forests were exposed to strong monsoonal climates typical of those experienced today arising from annual migrations of the ITCZ, possibly enhanced by a lower equator-to-pole temperature gradient. Throughout the Paleogene an elevated Tibetan highland produced no discernable modification of this ITCZ monsoon, although rainfall seasonality similar to that of the modern South Asia Monsoon (SAM) is observed in northern India as early as the beginning of the Eocene, despite its near-equatorial palaeoposition. In South China rainfall seasonality increased progressively achieving modern monsoon-like wet season/dry season precipitation ratios by the early Oligocene. Despite evidencing weak rainfall seasonality overall, fossil leaves from South China have exhibited monsoon-adapted morphologies, comparable to those seen in today's Indonesia-Australia Monsoon, for at least 45 million years. Together, the Indian and South China fossil leaf assemblages show that the evolution of megathermal ecosystems across southern Asia has been influenced profoundly by monsoonal climates for at least the last 56 million years. The Paleogene ITCZ-driven monsoon system strongly impacted India as it transited the Equator likely eliminating Gondwanan taxa not able to adapt to seasonal precipitation extremes. Furthermore, powerful seasonally-reversing winds, and associated surface ocean currents, are likely to have facilitated two-way biotic transfer between India and Eurasia long before closure of the Tethys Ocean.  相似文献   

5.
New zircon U–Pb data, along with the data reported in the literature, reveal five phases of magmatic activity in the Tengchong Terrane since the Early Paleozoic with spatial and temporal variations summarized as Cambrian–Ordovician (500–460 Ma) to the east, minor Triassic (245–206 Ma) in the east and west, abundant Early Cretaceous (131–114 Ma) in the east, extensive Late Cretaceous (77–65 Ma) in the central region, and Paleocene–Eocene (65–49 Ma) in the central and western Tengchong Terrane, in which the Cretaceous–Eocene magmatism migrated from east to west. The increased zircon εHf(t) of the Early Cretaceous granitoids from − 12.3 to − 1.4 at ca. 131–122 Ma to − 4.6 to + 7.1 at ca. 122–114 Ma, identified for the first time in this study, and the magmatic flare-up at ca. 53 Ma in the central and western Tengchong Terrane indicate increased contributions from mantle- or juvenile crust-derived components. The spatial and temporal variations and changing magmatic compositions over time in the Tengchong Terrane closely resemble those of the Lhasa Terrane in southern Tibet. Such similarities, together with the data of stratigraphy and paleobiogeography, enable us to propose that the Tengchong Terrane in SW Yunnan is most likely linked with the Lhasa Terrane in southern Tibet, both of which experienced similar tectonomagmatic histories since the Early Paleozoic.  相似文献   

6.
The article presents a newly created precipitation data set for the African continent and describes the methodology used in its creation. It is based on a combination of proxy data and rain gauge records. The data set is semi-quantitative, with a “wetness” index of ? 3 to + 3 to describe the quality of the rainy season. It covers the period AD 1801 to 1900 and includes data for 90 geographical regions of the continent. The results underscore a multi-decadal period of aridity early in the nineteenth century.  相似文献   

7.
Hydrography of the Bay of Bengal is highly influenced by the river runoff and rainfall during the southwest monsoon. We have reconstructed δ18Osw, sea surface salinity and sea surface temperature (SST) changes in the Bay of Bengal by using paired measurements of δ18O and Mg/Ca in a planktonic foraminifera species Globigerinoides ruber from core SK218/1 in the western Bay of Bengal in order to understand the rainfall variability associated with southwest monsoon over the past 32 kyr. Our SST reconstructions reveal that Bay of Bengal was ～3.2 °C cooler during the LGM as compared to present day temperature and a ～3.5 °C rise in SST is documented from 17 to 10 ka. Both SST and δ18Osw exhibit greater amplitude fluctuations during MIS 2 which is attributable to the variability of NE monsoon rainfall and associated river discharge into the Bay of Bengal in association with strong seasonal temperature contrast. On set of strengthening phase of SW monsoon was started during Bølling/Allerød as evidenced by the low δ18Osw values ～14.7 ka. δ18Osw show consistently lower values during Holocene (with an exception around 5 ka), which suggests that the freshening of Bay of Bengal due to heavy precipitation and river discharge caused by strong SW monsoon. Results of this study signify that the maximum fluctuations of the NE monsoon rainfall during MIS 2 appear to be controlled by the strong seasonality and boundary conditions.  相似文献   

8.
《Quaternary Science Reviews》2004,23(3-4):225-244
We present new paleoclimate reconstructions for the time period of 6000 yr BP (6 ka) that permit a quantitative comparison to corresponding mapped patterns of the newest version of the Canadian CCCma AGCM2 climate model. July temperature reconstructions were estimated using the modern analog technique employing fossil pollen records from the North American Pollen Database. We present a new method of optimizing critical choices within the application of the MAT through modern calibration. The pattern of 6 ka temperature anomalies generated by the CCCma AGCM2 generally agree with those of the pollen-based reconstruction, particularly in the presence of a band of positive temperature anomalies across the continent from the Yukon through the Great Lakes region and along the St. Lawrence. This agreement suggests that both the model and data are reconstructing a contraction in the polar vortex over North America at 6 ka. Major areas of disagreement between the pollen-based reconstruction and model simulation are also areas where the data are sparse and edge effects produce less reliable reconstructions. The CCCma AGCM2 simulates drier conditions in a west–east band stretching from Alaska through the Canadian prairies and Midwest United States, in agreement with low lake-level observations at 6 ka. Simulated and reconstructed precipitation–evaporation are also in agreement over the southwest United States. With the exception of north central Africa, the CCCma AGCM2 fails to capture the northward and eastern extension of the Afro-Asian monsoon belt at 6 ka. General agreement between CCCma AGCM2 P-E anomalies and lake status change is found throughout Europe at 6 ka.  相似文献   

9.
With the aim of better understanding the history of ocean closure and suturing between India and Asia, we conducted a geologic investigation of a siliciclastic matrix tectonic mélange within the western Yarlung suture zone of southern Tibet (Lopu Range region, ~ 50 km northwest of Saga). The siliciclastic matrix mélange includes abundant blocks of ocean plate stratigraphy and sparse blocks of sandstone. Metapelite and metabasite blocks in the mélange exhibit lower greenschist facies mineral assemblages, indicating that they were not deeply subducted. We obtained detrital zircon U-Pb geochronologic and sandstone petrographic data from sandstone blocks in the mélange and sandstone beds from Tethyan Himalayan strata exposed to the south of the suture. The sandstones from both units are all similar in U-Pb detrital zircon age spectra and petrography to the nearby Tethyan Cretaceous–Paleocene Sangdanlin section, which records the earliest appearance (at ~ 59 Ma) of arc-affinity strata deposited conformably on Indian-affinity strata. Two Paleocene sandstones, one of which is a schistose block incorporated in the siliciclastic matrix mélange, yielded indistinguishable maximum depositional ages of ~ 59 Ma. Mesozoic Asian-affinity sandstone blocks previously documented in the siliciclastic matrix mélange 200–500 km along strike to the east are notably absent in the Lopu Range region. We documented a gradational transition in structural style from the block-in-matrix mélange in the northeast to the south-vergent Tethyan thrust belt in the southwest. Blocks of Tethyan Himalayan strata increase in size and the volumetric proportion of matrix decreases from northeast to southwest. We conclude that no arc-affinity sandstone blocks were incorporated into the subduction complex until India-Asia collision at ~ 59 Ma when the Xigaze forearc basin became overfilled and Tethyan Himalayan strata entered the trench. As collision progressed, there was a gradual transition in structural style from block-in-matrix mélange formation to imbricate-style thrust belt formation.  相似文献   

10.
The Dashuang complex in Zhejiang Province of southeast China is composed of two distinct lithologies: syenite in the west and quartz monzonite in the east. They record similar zircon U–Pb ages of 224 ± 3 Ma (syenite), and 226 ± 2 Ma and 227 ± 1 Ma (quartz monzonite), respectively, but are notably different in petrography, magnetic susceptibility, whole-rock chemistry, zircon Hf isotope and zircon trace element characteristics. The west Dashuang syenitic pluton (the west body) has high modal alkali feldspar, high zircon saturation temperatures, high whole-rock and zircon MREE/HREE ratios, low Fe–Mg–Ti contents, and is depleted in Ba, Sr and Eu. It also has low magnetic susceptibilities, belongs to the ilmenite-series, and is a peraluminous and ferroan granitoid. The east Dashuang quartz monzonitic pluton (the east body) has abundant K-feldspar megacrysts, with hornblende, titanite and biotite being the major ferromagnesian minerals. In contrast to the west body, the east body has lower zircon saturation temperatures, lower whole-rock and zircon MREE/HREE ratios, higher Fe–Mg–Ti contents, and shows no depletion in Ba, Sr or Eu. The east body has higher magnetite contents, high magnetic susceptibilities and belongs to the magnetite-series. It is a metaluminous and magnesian granitoid of arc-affinity. Zircon Hf isotopic data reveal that both bodies were derived from partial melting of Paleoproterozoic igneous protoliths in the lower crust, but the east body possibly incorporated subducted terrigenous sediments. Both bodies have higher melting temperatures and pressures than adjacent Cretaceous granitoids, reflecting their origin in a thickened, hotter lower crust. The most feasible model to explain their differences is variations in water content during crustal melting, resulting in different melting and crystallization behaviors. Such melting in a Triassic thickened crust with variable water involvement, followed by Cretaceous magmatism in an extensional setting, is consistent with the flat-slab subduction model proposed for South China. The model involves crustal thickening and partial melting, with mantle and lower crustal metasomatism during flat-slab propagation in the Triassic–Early Jurassic, and crustal thinning and extension from the mid-Jurassic to the Cretaceous.  相似文献   

11.
The Indian subcontinent comprises accreted cratonic fragments that underwent subsequent rifting–subduction–collision processes along major mobile belts. It is also a storehouse of a prolonged history of crustal evolution involving repeated episodes of magmatism, volcanism, sedimentation, and metamorphism, and the formation of intracratonic and foreland basins. The geologic–geomorphic evolution of the Peninsula is recorded in relic faults/suture zones that confine Paleozoic, Mesozoic, and Cenozoic sedimentary basins and landforms. In addition, escarpments, plateaus, waterfalls, deltas, planation surfaces, and strandlines are either aligned parallel and/or formed over them. Asymmetric relief across the western–eastern parts of the Peninsula, orographic effect on the precipitation pattern and climatic zones provide important constrains on the geologic and geomorphic evolution of the region. Though extensive continental and coastal deposition occurred during Permo-Triassic and Early Cretaceous, only the downwarped grabens of Gondwana basins preserved the sedimentary records. Occurrences of extensive Cenozoic deposits offshore and Neogene–Holocene deltaic deposits along the entire east coast as opposed to the absence of comparable deltaic sequences in the west coast are linked to intense and recurrent episodes of denudation indicating first-order control of basement structures over landscape evolution.Inheritance and continuum of basement structures over climate, and landscape evolution of Peninsular India can also be witnessed by the unique and diverse drainage patterns of major rivers. While most of the major rivers follow a general easterly direction, the landscape morphology of their catchments indicates youthful character and tectonically active nature. The east flowing rivers show an asymmetry between the areal extents of drainage basin size and delta, whereas the west-flowing rivers are short and straight and their catchments are miniscule in comparison with the rivers flowing toward east. The Western Ghats Escarpment forms an orographic barrier and acts as the main divide between these two drainage systems. There is a disparity between the quanta of sediment discharge as well. In addition, the eastern and western parts of India, across the Western Ghats Escarpment, show significant differences in terms of climatic zones (humid to per-humid on the west while it is arid to subhumid on the east). The west coast is characterized by > 3000 mm/year annual rainfall, that often reaches > 5000 mm/year, dominance of SW monsoon winds, occurrence of narrow (< 60 km), rocky, crenulated and coastal cliffs and pocket beaches, and net southerly long shore sediment transport. In contrast, the east coast is characterized by several hundred kilometers wide deltaic low lands built by major rivers. These rivers show trellis to dendritic stream patterns and follow major basement faults that are associated with geologic–historic–recent seismic activity. The sedimentary basins and delta heads, strandlines, and active delta lobes whose western boundary is always limited by basement faults have shown activity since the formation of the Gondwanan supercontinent. The eastern coastal region is also different from the west in terms of active NE monsoon, long shore sediment transport (toward north), and the formation of extensive lagoon-beach barrier systems since Neogene that in turn continue to occur until recent.Here we propose a conceptual tectono-geomorphic model demonstrating the inheritance and continuum of Gondwanan structural trends on the transient landscape of Peninsular India.  相似文献   

12.
South-East Greenland forms part of the North Atlantic Craton and is characterized by migmatitic orthogneisses, narrow bands of mafic granulite, ultramafic and possible meta-sedimentary rocks, and alkaline-carbonatitic intrusive rocks. Mafic granulite, meta-sedimentary and ultramafic rocks form the basement for the emplacement of granitic intrusions at ca. 2865 Ma that lasted episodically until ca. 2790 Ma and continuously during 2750–2700 Ma. The area is structurally complex with evidence of at least seven deformation events including reclined and mushroom-like fold interference patterns. An older (> 2790 Ma) foliation formed in granitic rocks and the basement during the Timmiarmiut Orogeny (DT). Deformation associated with the ca. 2790–2700 Ma Skjoldungen Orogeny folded this early foliation, and is associated with a penetrative foliation that is refolded progressively in a northeast–southwest oriented stress field. The orientation of the stress field progressively rotated into a northnorthwest–southsoutheast orientation during the last stages of the orogeny. The orogeny is also characterized by syn-deformational anatexis at granulite-facies (at approximately 800 °C and 5–8 kbar, ca. 2790–2740 Ma), which decreased to the amphibolite-facies at ca. 2730 Ma.The late- to post-tectonic granite and alkaline rocks assigned to the Skjoldungen Alkaline Province intruded the central-northern part around 2710 Ma. This was followed by north–south extensional deformation during the Singertat Stage forming discrete shear-zones at greenschist-facies grades, which is coeval with the emplacement of pegmatite, ijolite, and carbonatite emplacement during ca. 2680–2650 Ma.Similar lithology and tectonic processes in the Tasiusarsuaq Terrane of southern West Greenland and the Lewisian Complex in Scotland suggest a possibly large Archaean terrane at that time, which, taking the present size, at least covered around 500–600 km in an east–west direction and approximately 200 km in a north–south direction.  相似文献   

13.
A conceptual groundwater flow model was developed for the crystalline aquifers in southeastern part of the Eastern region, Ghana. The objective was to determine approximate levels of groundwater recharge, estimate aquifer hydraulic parameters, and then test various scenarios of groundwater extraction under the current conditions of recharge. A steady state groundwater flow model has been calibrated against measured water levels of 19 wells in the area. The resulting recharge is estimated to range from 8.97 × 10?5 m/d to 7.14 × 10?4 m/d resulting in a basin wide average recharge of about 9.6% of total annual precipitation, which results in a basin wide quantitative recharge of about 2.4 million m3/d in the area. This compares to recharge estimated from the chloride mass balance of 7.6% of precipitation determined in this study. The general groundwater flow in the area has also been determined to conform to the general northeast–southwest structural grain of the country. The implication is that the general hydrogeology is controlled by post genetic structural entities imposed on the rocks to create ingresses for sufficient groundwater storage and transport. Calibrated aquifer hydraulic conductivities range between 0.99 m/d and over 19.4 m/d. There is a significant contribution of groundwater discharge to stream flow in the study area. Increasing groundwater extraction will have an effect on stream flow. This study finds that the current groundwater extraction levels represent only 0.17% of the annual recharge from precipitation, and that groundwater can sustain future increased groundwater demands from population growth and industrialization.  相似文献   

14.
表层岩溶带调蓄系数定量计算--以湘西洛塔赵家湾为例   总被引：2，自引：1，他引：2  
本文以湖南洛塔赵家湾表层岩溶系统为例,综合利用水位、降雨量、蒸发量及泉流量等长观资料,分别对不同降雨特征下的表层岩溶带调蓄系数进行了定量计算,并对影响表层岩溶带调蓄能力的因素进行了分析。计算得到丰水期赵家湾表层岩溶带调蓄系数为0 14～0 2 8,明显小于枯水期调蓄系数0 44 ;但丰水期久旱后表层岩溶带的调蓄能力接近于枯水期;在丰水期,场雨和连续降雨时的调蓄能力比间歇降雨时的小近1倍。实例计算表明:赵家湾表层岩溶带具有一定的调蓄能力,其地下迳流滞后于降雨至少3d ,调蓄系数确实可以用来定量评价表层岩溶带的调蓄能力。  相似文献   

15.
The summer monsoon rainfall over Orissa occurs mostly due to low pressure systems (LPS) developing over the Bay of Bengal and moving along the monsoon trough. A study is hence undertaken to find out characteristic features of the relationship between LPS over different regions and rain-fall over Orissa during the summer monsoon season (June-September). For this purpose, rainfall and rainy days over 31 selected stations in Orissa and LPS days over Orissa and adjoining land and sea regions during different monsoon months and the season as a whole over a period of 20 years (1980-1999) are analysed. The principal objective of this study is to find out the role of LPS on spatial and temporal variability of summer monsoon rainfall over Orissa. The rainfall has been significantly less than normal over most parts of Orissa except the eastern side of Eastern Ghats during July and hence during the season as a whole due to a significantly less number of LPS days over northwest Bay in July over the period of 1980-1999. The seasonal rainfall shows higher interannual variation (increase in coefficient of variation by about 5%) during 1980-1999 than that during 1901-1990 over most parts of Orissa except northeast Orissa. Most parts of Orissa, especially the region extending from central part of coastal Orissa to western Orissa (central zone) and western side of the Eastern Ghats get more seasonal monsoon rainfall with the development and persistence of LPS over northwest Bay and their subsequent movement and persistence over Orissa. The north Orissa adjoining central zone also gets more seasonal rainfall with development and persistence of LPS over northwest Bay. While the seasonal rainfall over the western side of the Eastern Ghats is adversely affected due to increase in LPS days over west central Bay, Jharkhand and Bangladesh, that over the eastern side of the Eastern Ghats is adversely affected due to increase in LPS days over all the regions to the north of Orissa. There are significant decreasing trends in rainfall and number of rainy days over some parts of southwest Orissa during June and decreasing trends in rainy days over some parts of north interior Orissa and central part of coastal Orissa during July over the period of 1980-1999  相似文献   

16.
利用1961-2011年我国东北地区(包括黑龙江、吉林、辽宁、内蒙古东部)的冬季降水资料,分析了东北地区冬季降水量的时空分布和冬季降水日数的时空分布,对比分析了东亚冬季风对我国东北地区冬季降水的影响. 结果表明：东北地区冬季降水量和降水日数分布有明显的区域差异,以内蒙古东南部、辽宁西北部、吉林西部、黑龙江西南部为低值中心,外围辽宁东部,吉林南部,黑龙江东部、中北部、北部,内蒙古东北部等地区为高值区;冬季降水量年际变化呈增加的线性趋势,1980年代中期以后冬季降水量的高值和低值都有明显的增大;年降雪日数年际变化呈线性增加趋势. 1948-2011年东亚冬季风强度指数的结果表明,东亚冬季风呈明显的线性减弱趋势,弱东亚季风主要集中在20世纪80年代中期以后,除内蒙古东南部等少数区域外,我国东北大部分地区的冬季降水量都和东亚冬季风呈负相关. 对应地,东北地区冬季降水量增大,年际变化的幅度变化增大,降水日数增量较小,这可能与东北地区冬季极端降水天气和干旱天气增加有关. 在东亚范围内,我国东北地区冬季降水多年200 hPa U风增强、500 hPa高压减弱、850 hPa东海南风增强,冬季降水少年则相反.  相似文献   

17.
Different continental collision belts show contrasting metamorphic trend along their length, including the distribution of extreme metamorphism; i.e., ultrahigh-pressure (>100 km depth) and ultrahigh-temperature (900–1150 °C) metamorphisms. However, no previous study has succeeded in explaining these trends. The present study investigates the main factors that control the metamorphic trends along collision belts, with reference to the Dabie–Hongseong collision belt between the North and South China blocks and the Himalayan collision belt between the Indian and Asian blocks. In the Dabie–Hongseong collision belt, collision began in the east before 245 Ma and propagated westward until ca. 220 Ma. In the eastern part of the belt, the amount of oceanic slab that subducted before collision was insufficient to pull down the continental crust to the depths of ultrahigh-pressure metamorphism; however, ultrahigh-pressure metamorphism occurred in the western part of the belt. Slab break-off also migrated from east to west, with a westward increase in the depth of break-off (from ca. 10 kbar in the west to ca. 35 kbar in the east). These lateral trends along the belt resulted in a westward change from ultrahigh-temperature (915–1160 °C, 9.0–10.6 kbar) to high-pressure (835–860 °C, 17.0–20.9 kbar) and finally ultrahigh-pressure metamorphism (680–880 °C, 30–40 kbar). In the Himalayan collision belt, collision started from the west at 50 Ma and propagated eastward. The amount of oceanic slab subducted prior to collision was sufficient to pull down the continental crust to the depths of ultrahigh-pressure metamorphism in the west, but not in the east. Slab break-off started in the west at ca. 46 Ma and propagated eastward, with an eastward decrease in the depth of slab break-off from 27–29 to 17–18 kbar. Consequently, the metamorphic trend along the belt changes eastward from ultrahigh-pressure (690–750 °C, 27–29 kbar) to high-pressure and finally high-pressure granulite facies metamorphism (890 °C, 17–18 kbar). The differences in metamorphic trend between the Dabie–Hongseong and Himalayan collision belts reflect the amount of oceanic crust subducted prior to collision and the depth and timing of slab break-off along each belt.  相似文献   

18.
The old Senhora das Fontes uranium mine, in central Portugal, consists of quartz veins which penetrated along fracture shear zones at the contact between graphite schist and orthogneiss. The mine was exploited underground until a depth of 90 m and was closed down in 1971. The ores from this mine and two others were treated in the mine area by the heap-leach process which ended in 1982. Seven dumps containing a total of about 33,800 m3 of material and partially covered by natural vegetation were left in the mine area. A remediation process took place from May 2010 to January 2011. The material deposited in dumps was relocated and covered with erosion resisting covers. Surface water and groundwater were collected in the wet season just before the remediation, in the following season at the beginning of the remediation and also after the remediation in the following dry season. Before, at the beginning and after the remediation, surface water and groundwater have an acid-to-alkaline pH, which decreased with the remediation, whereas Eh increased. In general, before the remediation, uranium concentration was up to 83 μg/L in surface water and up to 116 μg/L in groundwater, whereas at the beginning of the remediation it increases up to 183 μg/L and 272 μg/L in the former and the latter, respectively, due to the remobilization of mine dumps and pyrite and chalcopyrite exposures, responsible for the pH decrease. In general, after the remediation, the U concentration decreased significantly in surface water and groundwater at the north part of the mine area, but increased in both, particularly in the latter up to 774 μg/L in the south and southwest parts of this area, attributed to the remobilization of sulphides that caused mobilization of metals and arsenic which migrated to the groundwater flow. Uranium is adsorbed in clay minerals, but also in goethite as indicated by the geochemical modelling. After the remediation, the saturation indices of oxyhydroxides decrease as pH decreases. The remediation also caused decrease in Cd, Co, Cr, Ni, Pb, Zn, Cu, As, Sr and Mn concentrations of surface water and groundwater, particularly in the north part of the mine area, which is supported by the speciation modelling that shows the decrease of most dissolved bivalent species. However, in general, after the remediation, Th, Cd, Al, Li, Pb, Sr and As concentrations increased in groundwater and surface water at south and southwest of the mine area. Before and after the remediation, surface water and groundwater are contaminated in U, Cd, Cr, Al, Mn, Ni, Pb, Cu and As. Remediation caused only some improvement at north of the mine area, because at south and southwest part, after the remediation, the groundwater is more contaminated than before the remediation.  相似文献   

19.
The Asian Summer Monsoon (ASM) is the dominant climate system of South and East Asia. However, the history of monsoon intensification and the driving forces behind it are controversial. Wind-blown sediments in mid-latitude East Asia and fluvial-derived sediments in the northern South China Sea imply contrasting ASM patterns during the late Cenozoic. Here we use pollen records from the southwest South China Sea (International Ocean Discovery Program (IODP) Site U1433) to reconstruct the ASM evolution in low-latitude Southeast Asia. A slow increase in herbaceous plants since 8 Ma indicates a persistent weakening of precipitation in Indochina, which is dominated by the ASM. This signal is closely associated with a consistent coniferous plant record, indicating a continuous cooling trend that correlates well with Sea Surface Temperature (SST) decrease in the west Pacific Ocean. We propose that the monsoon weakening resulted in as much as a ~ 25% reduction in precipitation over the past 8 Ma in response to the Northern Hemisphere glaciation/global cooling, with some of the increase in conifers being linked to uplift of the Vietnamese Central Highland and the SE flank of the Tibetan Plateau in Yunnan and northern Vietnam.  相似文献   

20.
Sea-breeze-initiated convection and precipitation have been investigated along the east coast of India during the Indian southwest monsoon season. Sea-breeze circulation was observed on approximately 70–80% of days during the summer months (June–August) along the Chennai coast. Average sea-breeze wind speeds are greater at rural locations than in the urban region of Chennai. Sea-breeze circulation was shown to be the dominant mechanism initiating rainfall during the Indian southwest monsoon season. Approximately 80% of the total rainfall observed during the southwest monsoon over Chennai is directly related to convection initiated by sea-breeze circulation.  相似文献   

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