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1.
Nigel Harris Alison Hunt Ian Parkinson Andrew Tindle Magisuren Yondon Samantha Hammond 《Contributions to Mineralogy and Petrology》2010,160(1):67-81
Garnet-bearing mantle xenoliths have been recovered from Quaternary alkali basalts, both within and peripheral to the Hangay
dome of central Mongolia. Microfabric analysis and thermobaromery, combining empirical thermobarometers and the self-consistent
dataset of THERMOCALC, indicate that garnet websterites from the Shavaryn-Tsaram volcanic centre at the dome core were formed
in the spinel-lherzolite upper mantle at pressures of 17–18 kbars and temperatures of 1,070–1,090°C, whereas garnet lherzolites
were derived from greater depths (18–20 kbars). Garnet lherzolites from the Baga Togo Uul vents near the dome edge were formed
at 18–22 kbars under significantly cooler conditions (960–1,000°C). These xenoliths reveal reaction coronas of (1) orthopyroxene,
clinopyroxene, plagioclase and spinel mantling garnets; (2) spongy rims of olivine replacing orthopyroxene and (3) low-Na,
low-Al clinopyroxene replacing primary clinopyroxene. Trace-element abundances indicate that clinopyroxene from these coronas
is in chemical equilibrium with the host magma. The thermobarometric and textural data suggest that lherzolite xenoliths from
both sites were derived from depths of 60–70 km and entrained in magma at 1,200–1,300°C. The average rate of ascent, as determined
by olivine zoning, lies in the range 0.2–0.3 m s−1. The contrast in thermal profiles of the upper mantle between the two sites is consistent with a mantle plume beneath the
Hangay dome with elevated thermal conditions beneath the core of the dome being comparable to estimates of the Pleistocene
geotherm beneath the Baikal rift. 相似文献
2.
César Viseras Juan Fernández Fernando García-García Jesús M. Soria María Luisa Calvache Pedro Jáuregui 《Environmental Geology》2009,56(7):1353-1369
An analysis has been made of sedimentary systems involved in the rapid silting of a reservoir constructed in 1974 in Alhama
de Granada (S. Spain); in only 30 years the storage capacity of the reservoir has shrunk by 80% and its perimeter has decreased
by 64%. A study of sediment lithofacies identified in a series of shallow trenches and of georadar facies identified in a
series of almost 900 m lines of ground penetrating radar (GPR) images, together with a survey of surface geology, has identified
3 alluvial systems (2 transversal systems and a longitudinal system) whose deltas have filled in the reservoir. Thus, there
are three phases in the evolution of the reservoir siltation: (1) an initial stage (1974–1977) typified by northward progradation
of the longitudinal river delta of about 100 m year−1 and an eastward progradation of the transversal system delta of about 20 m year−1; (2) an intermediate stage (1977–1984) in which the longitudinal river delta progradation slowed to 25 m year−1 and the axial drainage became obstructed due to the considerable eastward progradation of the transversal delta; and (3)
a final phase (1984–present) in which there have been few changes in the areal distribution of the deltas apart from a southeastward
expansion of the transversal delta. Generally, aggradational growth patterns (vertical accretion) have dominated in this final
phase. The lithology of the source area, the slope and precipitation distribution has a significant effect not only on the
sediment supply, but also indirectly on the creation of accommodation space and on the evolution of stratal growth patterns. 相似文献
3.
Rainfall-induced landslide event of May 2010 in the eastern part of the Czech Republic 总被引:3,自引:2,他引:1
Tom��? P��nek Rudolf Br��zdil Jan Klime? Veronika Smolkov�� Jan Hradecky Pavel Zahradn��?ek 《Landslides》2011,8(4):507-516
More than 150 landslides originated in the eastern part of the Czech Republic (region of the Flysch Outer Western Carpathians—hereinafter,
OWC) due to soil saturation caused by antecedent precipitation and long lasting and intensive rainfalls on 16–18 May 2010
(>300 mm as measured by some stations). As a consequence, a multitude of small failures originated 88% of which was smaller
than 104 m2. Most landslides are characterised as shallow (<10 m) or middle–deep (10–30 m) incipient (rather short travel) landslides,
debris slides and soil slips spatially clustered to a geological domain underlain by rather weak thin-bedded flysch and unconsolidated
Quaternary deposits. An exception to this is represented by a kilometre-long rockslide (∼2–3 mil m3) affecting tectonically weakened and weathered claystone/mudstone-dominated flysch on the southern slope of Mt. Girová (the
Beskydy Mountains). The rockslide is one of the largest long runout landslides in the territory of the Czech Republic activated
over the past few decades as it reaches the dimensions of the largest documented Holocene long runout landslides in the Czech
part of the OWC. A majority of the May 2010 landslide events developed inside older (Holocene or historic) landslide terrains,
which points to their spatial persistency and recurrent nature. In spite of the fact that the May 2010 landslide event was
not as destructive as some previous landslide activisation in the OWC region (e.g. July 1997 event), it left many slope failures
at the initial stage of their potential future reactivation. 相似文献
4.
Geomorphological evidences of post-LGM glacial advancements in the Himalaya: A study from Chorabari Glacier,Garhwal Himalaya,India 总被引:2,自引:0,他引:2
MANISH MEHTA ZAHID MAJEED D P DOBHAL PRADEEP SRIVASTAVA 《Journal of Earth System Science》2012,121(1):149-163
Field geomorphology and remote sensing data, supported by Optical Stimulated Luminescence (OSL) dating from the Mandakini
river valley of the Garhwal Himalaya enabled identification of four major glacial events; Rambara Glacial Stage (RGS) (13
± 2 ka), Ghindurpani Glacial Stage (GhGS) (9 ± 1 ka), Garuriya Glacial Stage (GGS) (7 ± 1 ka) and Kedarnath Glacial Stage
(KGS) (5 ± 1 ka). RGS was the most extensive glaciation extending for ~6 km down the valley from the present day snout and
lowered to an altitude of 2800 m asl at Rambara covering around ~31 km2 area of the Mandakini river valley. Compared to this, the other three glaciations (viz., GhGS, GGS and KGS) were of lower
magnitudes terminating around ~3000, ~3300 and ~3500 m asl, respectively. It was also observed that the mean equilibrium line
altitude (ELA) during RGS was lowered to 4747 m asl compared to the present level of 5120 m asl. This implies an ELA depression
of ~373 m during the RGS which would correspond to a lowering of ~2°C summer temperature during the RGS. The results are comparable
to that of the adjacent western and central Himalaya implying a common forcing factor that we attribute to the insolation-driven
monsoon precipitation in the western and central Himalaya. 相似文献
5.
The landslides in the Three Gorges Reservoir Region,China and the effects of water storage and rain on their stability 总被引:5,自引:0,他引:5
According to the statistics, there were about 1,736 landslides with the total volume of 1.339 × 1011 m3 in the 100 km2 area of upper reaches of the Yangtze River, in which about 94% of landslides are triggered by rain and water storage. Thus,
based investigation of the results of the landslides in this region, this paper completed a systematic study on the effects
of rainfall and water storage on the landslides in the region and found that the distribution of landslides over time and
space are well correlated with the precipitation distribution in this region; landslides are also strictly controlled by the
stratum, geological structure, river valley form, shore type, etc. Meanwhile, the pattern of water table variation of the
three gorges reservoir will do have very serious impacts on the stability of the slopes in this region. All research results
show that water storage and rainfall are the two important inducing factors that can cause a lot of large-scale landslides.
Therefore, some effective control measures for water storage and rainfall should be taken so as to minimize the impacts of
water on the stability of the slopes in the Three Gorges Reservoir Region of Yangtze River, China. 相似文献
6.
Terra rossa and eutric cambisol soils were surveyed in Slovenia. At both sites, 6–13 boreholes were drilled in a regular 24 m × 24 m
square grid. Soil samples from various depths were taken for gamma spectrometric analysis, and radon in soil gas was measured
at a depth of 80 cm using an AlphaGuard instrument. The following ranges of activity concentration (Bq kg−1) were obtained for 238U, 226Ra, 228Ra, 40K and 137Cs: in terra rossa, 64–74, 70–84, 45–49, 293–345, 20–30 and, in eutric cambisol, 55–80, 132–147, 50–57, 473–529, 106–272.
Radon activity concentrations in both soils ranged from about 100 kBq m−3 to 370 kBq m−3. 相似文献
7.
Large-scale ancient landslides of the area of more than 5 km2 and volume exceeding 200 × 106 m3 are characteristic features of the valleys incised in the northern periphery of the Crimean Mountains (Ukraine). The largely
affected area is located in the outermost cuesta range of the Crimean Mountains which consists of rigid Sarmatian limestones
overlying weak Middle Miocene and Upper Palaeogene deposits. A giant landslide arose in the Alma water gap as a reflection
of several coincident preparatory factors such as suitable bedrock stratification, smectite-rich bedrock exposed to swelling
activity, presence of faults parallel to the valley trend, and river capture event which preceded the landslide event. The
occurrence of such ancient megaslides is particularly interesting in the area which is characterized by low precipitation
(<500 mm/year) and weak contemporary seismicity. It probably reflects a more dynamic environment in humid phases of the Holocene;
however, seismic triggering along the Mesozoic suture zone cannot be rejected. Compressional features such as gravitational
folds in the central and distal parts of the landslide, which probably correlate with the whole landslide genesis or its significant
reactivation, arose, according to the radiocarbon dating, during the Holocene climatic optimum in the Atlantic period. The
slope deformation has been relatively quiescent since that time, except minor historic reactivization which took place in
the frontal part of the landslide. We suppose that the studied landslide could be classified as a transitional type of slope
deformation with some signs of spreading and translational block slides. 相似文献
8.
Long Lake, located near Lake Michigan within the dune-complexes of Indiana Dunes National Lakeshore, USA, was formed some
time during the Pleistocene and Holocene epochs. A surficial aquifer underlies Long Lake, which is either a source or sink
for the later. The hydrologic processes in the lakeshore and surrounding environs have been significantly altered during the
agricultural, municipal, and industrial development of the region. Limited data suggest that the organisms of Long Lake have
elevated levels of several contaminants. This study attempts to quantify seepage within the lake to assess the potential threat
to groundwater quality. Seepage measurements and minipiezometric tests were used to determine seepage within the lake. Seepage
measurements and minipiezometric tests suggest that water seeps out of Long Lake, thus recharging the groundwater that flows
southwest away from the lake. There is a great deal of variability in the seepage rate, with a mean of 11.5×10–4±11.2×10–4 m d–1. The mean seepage rate of 0.3 m yr–1 for Long Lake is greater than the 0.2 m yr–1 recharge rate estimated for the drainage basin area. The Long Lake recharge volume of 2.5×105 m3 yr–1 is approximately 22% of the volume of the lake and is significant when compared to the total surface recharge volume of 4.8×105 m3 yr–1 to the upper aquifer of the drainage area. There is a potential for contamination of the groundwater system through seepage
from the lake from contaminants derived from aerial depositions.
Received: 16 August 1995 · Accepted: 18 September 1995 相似文献
9.
Wilson Yetoh Fantong Hiroshi Satake Festus T. Aka Samuel N. Ayonghe Kazuyoshi Asai Ajit K. Mandal Andrew A. Ako 《Environmental Earth Sciences》2010,60(1):107-120
Unplanned exploitation of groundwater constitutes emerging water-related threats to MayoTsanaga River Basin. Shallow groundwater
from crystalline and detrital sediment aquifers, together with rain, dams, springs, and rivers were chemically and isotopically
investigated to appraise its evolution, recharge source and mechanisms, flow direction, and age which were used to evaluate
the groundwater susceptibility to contamination and the basin’s stage of salinization. The groundwater which is Ca–Na–HCO3 type is a chemically evolved equivalent of surface waters and rain water with Ca–Mg–Cl–SO4 chemistry. The monsoon rain recharged the groundwater preferentially at an average rate of 74 mm/year, while surface waters
recharge upon evaporation. Altitude effect of rain and springs show a similar variation of −0.4‰ for δ18O/100 m, but the springs which were recharged at 452, 679, and 773 m asl show enrichment of δ18O through evaporation by 0.8‰ corresponding to 3% of water loss during recharge. The groundwater which shows both local and
regional flow regimes gets older towards the basins` margin with coeval enrichment in F− and depletion in NO3
−. Incidentally, younger groundwaters are susceptible to anthropogenic contamination and older groundwaters are sinks of lithologenic
fluoride. The basins salinization is still at an early stage. 相似文献
10.
Source and distribution of trace metals and nutrients in Narmada and Tapti river basins,India 总被引:2,自引:1,他引:1
The study was designed to establish the distributions of trace metals, dissolved organic carbon, and inorganic nutrients as
well as to assess the extent of anthropogenic inputs into the Narmada and Tapti rivers. Water and sediment qualities are variable
in the rivers, and there are major pollution problems at certain locations, mainly associated with urban and industrial centers.
The metal concentrations of samples of the aquatic compartments investigated were close to the maximum permissible concentration
for the survival of aquatic life, except for higher values of Cu (5–763 μg l−1), Pb (24–376 μg l−1), Zn (24–730 μg l−1), and Cr (70–740 μg l−1) and for drinking water except for elevated concentrations of metals such as Pb, Fe (850–2,060 μg l−1), Cr, and Ni (20–120 μg l−1). In general, the concentrations of trace metals in the rivers vary down stream which may affect the “health” of the aquatic
ecosystem and may also affect the health of the rural community that depends on the untreated river water directly for domestic
use. The assessment of EF, I
geo, and PLI in the sediments reveals overall moderate pollution in the river basins. 相似文献
11.
Geochemistry,dissolved elemental flux rates,and dissolution kinetics of lithologies of Alaknanda and Bhagirathi rivers in Himalayas,India 总被引:1,自引:1,他引:0
Alaknanda and Bhagirathi (AB) river basins in the Himalayan region in India expose lithologies comprising mainly of granites,
low–high-grade metamorphics, shales and carbonates which, in conjunction with the monsoon rains and glacial melt, control
water chemistry and dissolved elemental flux rates. In the present study, we monitored two locations: (a) Srinagar on the
Alaknanda river and (b) Maneri on the Bhagirathi river for daily variations in total suspended sediments, major ions and dissolved
silica over one complete year (July 2004–June 2005). Based on long-term discharge data, discharge-weighted composition and
dissolved elemental flux rates (with respect to Ca, Mg, HCO3, Si) of the river were estimated. The information thus obtained has substantially added up to the existing chemical data
of these rivers and has refined the flux rates. Our high-frequency samples provide informations such as (a) water chemical
compositions that show a large temporal and spatial variation and (b) carbonate lithology that controls water chemistry predominantly.
The dissolution kinetics of various lithologies namely leucogranite, gneiss, quartzite, phyllite and shale of the AB river
basins were studied through batch experiments at controlled temperature (25 and 5°C) and pH (8.4) condition. In laboratory,
these lithologies undergo slow rates of dissolution (10−13 to 10−15 mol/m2 s), while field weathering rates based on dissolved elemental flux rates in the AB rivers are much higher (10−8 to 10−9 mol/m2 s). Extremely high physical weathering rates in AB rivers, which enhance chemical weathering significantly, mainly attribute
this wide discrepancy in laboratory-derived rates of representative basin rocks and dissolved elemental fluxes in the field.
However, laboratory-simulated experiments facilitate to quantify elemental release rates, understand the kinetics of the dissolution
reactions, and compare their roles at individual level. 相似文献
12.
《Sedimentology》2018,65(6):1973-2002
Dolomitization is commonly associated with crustal‐scale faults, but tectonic rejuvenation, diagenetic overprinting and a fluid and Mg mass‐imbalance often makes it difficult to determine the dolomitization mechanism. This study considers differential dolomitization of the Eocene Thebes Formation on the Hammam Faraun Fault block, Gulf of Suez, which has undergone a simple history of burial and exhumation as a result of rifting. Stratabound dolostone bodies occur selectively within remobilized sediments (debrites and turbidites) in the lower Thebes Formation and extend into the footwall of, and for up to 2 km away from, the Hammam Faraun Fault. They are offset by the north–south trending Gebel fault, which was active during the earliest phases of rifting, suggesting that dolomitization occurred between rift initiation (26 Ma) and rift climax (15 Ma). Geochemical data suggest that dolomitization occurred from evaporated (ca 1·43 concentration) seawater at less than ca 80°C. Geothermal convection is interpreted to have occurred as seawater was drawn down surface‐breaching faults into the Nubian sandstone aquifer, convected and discharged into the lower Thebes Formation via the Hammam Faraun Fault. Assuming a ca 10 Myr window for dolomitization, a horizontal velocity of ca 0·7 m year−1 into the Thebes Formation is calculated, with fluid flux and reactivity likely to have been facilitated by fracturing. Although fluids were at least marginally hydrothermal, stratabound dolostone bodies do not contain saddle dolomite and there is no evidence of hydrobrecciation. This highlights how misleading dolostone textures can be as a proxy for the genesis and spatial distribution of such bodies in the subsurface. Overall, this study provides an excellent example of how fluid flux may occur during the earliest phases of rifting, and the importance of crustal‐scale faults on fluid flow from the onset of their growth. Furthermore, this article presents a mechanism for dolomitization from seawater that has none of the inherent mass balance problems of classical, conceptual models of hydrothermal dolomitization. 相似文献
13.
Xiangchun Quan Yulai Wang Weicong Xiong Mengchang He Zhifeng Yang Chunye Lin 《Environmental Earth Sciences》2010,61(8):1725-1734
The Daliao River watershed, an important industrial base of China, has been heavily influenced by anthropogenic activities.
Microbes in sediments play an important role in recycling of organics and nutrients, and knowledge of the microbial composition
and community structure in river sediments can help us to understand the contribution of microorganisms to environmental processes
and their response to environmental perturbation. In this study, 11 surface sediment samples and 1 core sediment sample were
collected from the Daliao River water system and its estuary, and their microbial abundance and community compositions were
investigated using fluorescence in situ hybridization. Results showed that total cell numbers in surface sediments from different
locations ranged from 4.2 × 108 to 16.2 × 108 cells cm−3. Domain bacteria represented 58–82% with α-, β-, and γ-proteobacteria as the major subgroups summing up to 40%. Total cell
numbers along the core sediment profile ranged from 7.9 × 108 to 20.1 × 108 cells cm−3, with relatively higher total cell numbers in the upper (0–6 cm) and middle (21–30 cm) layers. In the core sediment, domain
bacteria represented 62–85% and archaea 1.0–11.8%. α-, β- and γ-Proteobacteria were three major phylogenetic groups of bacteria
in the core sediment also with γ-proteobacteria as the most abundant subgroup accounting for 9.8–40.8% of total cells. 相似文献
14.
Sediment instabilities are common on the prodeltas of the seismically active continental margins of Western Greece. Sediment
failures on the low-angle (0.5°–2°) prodelta slopes manifest themselves as successions of peripheral rotational block slumps
restricted to the foresets of the late highstand systems tract (HST). The individual slump blocks are about 80–150 m long
and are bounded by growth faults acting as curved slip planes that extend to a mean depth of 10–15 m below seafloor. Shear
planes develop in the lower part of muddy and/or gas charged HST foresets. Deeper basal transparent muddy layers of the early
HST bottomset, together with the late Pleistocene transgressive systems tract sequences (TST), are mostly unaffected. On the
steeper (2°–6°) fan delta slopes of the western Gulf of Corinth debris flows and avalanches with a significant retrogressive component dominate
slope destabilisation. Sediment cores taken from landslide scarps and slide planes penetrated gas bubble releasing sediments
thereby indicating that failure planes are in the late HST foresets/upper part of the early HST bottomsets gas charged zone.
The foresets of the HST prodelta deposits display high water content (30–80%), low bulk density (1.4–1.9 g cm−3) and relatively low values of undrained shear strength (3–20 kPa). The water content of the HST distal muddy bottomsets is
relatively higher (50–110%) and bulk density relatively lower (1.3–1.7 g cm−3) with low values of shear strength (2–10 kPa). The shear strength of the gas releasing sediment layer displays lower values
(2–9 kPa) relative to the overlying, post failure, muddy sediments of the late 100–300 years. Slope stability was calculated
using the normalised soil parameter (NSP) method under undrained conditions for normally consolidated prodelta sediments.
This analysis indicates that instabilities could be induced by critical earthquake ground accelerations of 26.6–29.6% g for
the HST foresets and 12.4–14.1% g for the basal transparent layer belonging to the early HST bottomsets. Consequently the
early HST bottomsets has to be considered a potentially unstable layer since the regional peak ground accelerations (PGAs)
for the next 50 years are expected to range from 19 to 30% g. Moreover, our results show that new glide planes in the prodeltaic
sediment bodies of the seismically active continental margins of Western Greece will likely develop from the gas charged sediments
of the lower part of the HST foresets to the upper part of early HST bottomsets. 相似文献
15.
Hydrogeochemical study of the thermal and mineralized waters of the Banaz (Hamamboğazi) area,western Anatolia,Turkey 总被引:1,自引:1,他引:0
The Hamamboğazi spa in western Turkey was built around natural hot springs with discharge temperatures in the range of 30–54°C;
the waters have near neutral pH values of 6.50–7.10 and a TDS content between 2,694 and 2,982 mg/l. Thermal water with a temperature
of 47.5–73°C has been produced at 325 l/s from five wells since 1994, causing some springs to go dry. A management plan is
required in the study area to maximize the benefits of this resource, for which currently proposed direct uses include heating
in the district and greenhouses, as well as balneology in new spas in the area. The best use for the water from each spring
or well will depend on its temperature, chemistry and location. The thermal waters are mixed Na–Mg–HCO3–SO4 fluids that contain a significant amount of CO2 gas. The chemical geothermometers applied to the Hamamboğazi thermal waters yield a maximum reservoir temperature of 130°C.
Isotope results (18O, 2H, 3H) indicate that the thermal waters have a meteoric origin: rainwater percolates downward along fractures and faults, is heated
at depth, and then rises to the surface along fractures and faults that act as a hydrothermal conduit. The basement around
the Banaz Hamamboğazi resort is comprised of Paleozoic metamorphic schist and marbles exposed 8 km south and 15 km north of
Banaz. Mesozoic marble, limestone and ophiolitic complex are observed a few km west and in the northern part of Banaz. These
units were cut at a depth of 350–480 m in boreholes drilled in the area. Overlying lacustrine deposits are composed of fine
clastic units that alternate with gypsum, tuff and tuffites of 200–350 m thickness. The marble and limestones form the thermal
water aquifer, while lacustrine deposits form the impermeable cap. 相似文献
16.
V. B. Che M. Kervyn G. G. J. Ernst P. Trefois S. Ayonghe P. Jacobs E. Van Ranst C. E. Suh 《Natural Hazards》2011,59(1):47-74
Limbe town and surrounding areas, on the SE foot slopes of the active Mt Cameroon Volcano, have experienced numerous small-scale
shallow landslides within the last 20 years. These resulted in the loss of ~30 lives and significant damage to farmland and
properties. Landslides and their scars are identified in the field, and their geometry systematically measured to construct
a landslide inventory map for the study area. Specific landslides are investigated in detail to identify site-specific controlling
and triggering factors. This is to constrain key input parameters and their variability for subsequent susceptibility and
risk modeling, for immediate local and regional applications in land-use planning. It will also enable a rapid exploration
of remediation strategies that are currently lacking in the SW and NW regions of Cameroon. Typical slides within the study
area are small-scale, shallow, translational earth, and debris slides though some rotational earth slides were also documented.
The depletion zones have mean widths of 22 m ± 16.7 m and lengths of 25 ± 23 standard deviation. Estimated aerial extents
of landslide scars and volume of generated debris range from 101 to 104 m2 and 2 to 5 × 104 m3, respectively. A key finding is that most slope instabilities within the study area are associated with and appear to be
exacerbated by man-made factors such as excavation, anarchical construction, and deforestation of steep slopes. High intensity
rainfall notably during localized storms is the principal triggering factor identified so far. The findings from this case
study have relevance to understanding some key aspects of locally devastating slope instabilities that commonly occur on intensely
weathered steep terrains across subtropical Africa and in the subtropics worldwide and affecting an ever denser and most vulnerable
population. 相似文献
17.
Julien Berger Renaud Caby Jean-Paul Liégeois Jean-Claude C. Mercier Daniel Demaiffe 《Contributions to Mineralogy and Petrology》2011,162(4):773-796
We show here that the Amalaoulaou complex, in the Pan-African belt of West Africa (Gourma, Mali), corresponds to the lower
and middle sections of a Neoproterozoic intra-oceanic arc. This complex records a 90–130-Ma-long evolution of magmatic inputs
and differentiation above a subducting oceanic slab. Early c. 793 Ma-old metagabbros crystallised at lower crustal or uppermost
mantle depths (25–30 km) and have geochemical characteristic of high-alumina basalts extracted from a depleted mantle source
slightly enriched by slab-derived sedimentary components ((La/Sm)N < 1; εNd: +5.4–6.2; 87Sr/86Sr: 0.7027–0.7029). In response to crustal thickening, these mafic rocks were recrystallised into garnet-granulites (850–1,000°C;
10–12 kbar) and subject to local dehydration–melting reactions, forming trondhjemititic leucosomes with garnet–clinopyroxene–rutile
residues. Slightly after the granulitic event, the arc root was subject to strong HT shearing during partial exhumation (detachment
faults/rifting or thrusting), coeval with the emplacement of spinel- and garnet-pyroxenite dykes crystallised from a high-Mg
andesitic parental magma. Quartz and hornblende-gabbros (700–660 Ma) with composition typical of hydrous volcanic rocks from
mature arcs ((La/Sm)N: 0.9–1.8; εNd: +4.6 to +5.2; 87Sr/86Sr: 0.7028–0.7031) were subsequently emplaced at mid-arc crust levels (~15 km). Trace element and isotopic data indicate that
magmas tapped a depleted mantle source significantly more enriched in oceanic sedimentary components (0.2%). Exhumation occurred
either in two stages (700–660 and 623 Ma) or in one stage (623 Ma) with a final exhumation of the arc root along cold P-T
path (550°C, 6–9 kbar; epidote–amphibolite and greenschist facies conditions) during the main Pan-African collision event
(620–580 Ma). The composition of magmas forming the Cryogenian Amalaoulaou arc and the processes leading to intra-arc differentiation
are strikingly comparable to those observed in the deep section of exposed Mezosoic oceanic arcs, namely the Kohistan and
Talkeetna complex. This evolution of the Amalaoulaou oceanic arc and its accretion towards the West African craton belong
to the life and closure of the Pharusian Ocean that eventually led to the formation of the Greater Gondwana supercontinent,
a similar story having occurred on the other side of the Sahara with the Mozambique Ocean. 相似文献
18.
Damian Steffen Fritz Schlunegger Frank Preusser 《International Journal of Earth Sciences》2010,99(8):1975-1989
This study investigates the connection between sediment aggradation, erosion and climate in a desert environment of the Majes
valley, southern Peru. Luminescence dating of terraces and fans shows that sediment aggradation correlates with wet time intervals
on the Altiplano, suggesting a climatic influence on the aggradation–degradation cycles. Major periods of aggradation occurred
between ~110–100, ~60–50 and 12–8 ka. More precipitation in the Majes catchment resulted in increased erosion and transportation
of sediment from the hillslopes into the trunk river. As a result, the sediment loads exceeded the transport capacity of the
Majes River and aggradation started in the lower reaches where the river gradient is less. Depletion of the hillslope sediment
reservoirs caused a relative increase in the capacity of the trunk river to entrain and transport sediment, resulting in erosion
of the previously deposited sediment. Consequently, although climate change may initiate a phase of sediment accumulation,
degradation can be triggered by an autocyclic negative feedback and does not have to be driven by climatic change. 相似文献
19.
Soil erosion modeling of a Himalayan watershed using RS and GIS 总被引:5,自引:1,他引:4
Ashish Pandey Abhisekh Mathur S. K. Mishra B. C. Mal 《Environmental Earth Sciences》2009,59(2):399-410
Employing the remote sensing (RS) and geographical information system (GIS), an assessment of sediment yield from Dikrong
river basin of Arunachal Pradesh (India) has been presented in this paper. For prediction of soil erosion, the Morgan-Morgan
and Finney (MMF) model and the universal soil loss equation (USLE) have been utilized at a spatial grid scale of 100 m × 100 m,
an operational unit. The average annual soil loss from the Dikrong river basin is estimated as 75.66 and 57.06 t ha−1 year−1 using MMF and USLE models, respectively. The watershed area falling under the identified very high, severe, and very severe
zones of soil erosion need immediate attention for soil conservation. 相似文献
20.
The prehistoric Mt Wilberg rock avalanche, Westland, New Zealand 总被引:1,自引:0,他引:1
The Mt Wilberg rock avalanche in Westland, New Zealand occurred before 1300 AD and may have occurred as a consequence of an
Alpine fault earthquake in ca. 1220 AD or earlier. Its ∼40 × 106 m3 deposit may have briefly obstructed the Wanganui River, but only about 25% of its surface morphology still survives, on terraces
isolated from river erosion. The landslide appears to have moved initially as a block, in a direction controlled by a strong
rock mass at the base of the source area, before disintegrating and spreading across terraces, fans, and floodplains. Rock
avalanche deposits in Westland have relatively short expected lifetimes in the rugged terrain and high rainfall of the area;
hence, the hazard from such events is under-represented by their current remnants. 相似文献