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991.
Katrin Monecke Flavio S. Anselmetti Arnfried Becker Michael Sturm Domenico Giardini 《Tectonophysics》2004,394(1-2):21-40
Deformation structures in lake sediments in Central Switzerland can be attributed to strong historic earthquakes. The type and spatial distribution of the deformation structures reflect the historically documented macroseismic intensities thus providing a useful calibration tool for paleoseismic investigations in prehistoric lake sediments.The Swiss historical earthquake catalogue shows four moderate to strong earthquakes with moment magnitudes of Mw=5.7 to Mw=6.9 and epicentral intensities of I0=VII to I0=IX that affected the area of Central Switzerland during the last 1000 years. These are the 1964 Alpnach, 1774 Altdorf, 1601 Unterwalden, and 1356 Basel earthquakes. In order to understand the effect of these earthquakes on lacustrine sediments, four lakes in Central Switzerland (Sarner See, Lungerer See, Baldegger See, and Seelisberg Seeli) were investigated using high-resolution seismic data and sediment cores. The sediments consist of organic- and carbonate-rich clayey to sandy silts that display fine bedding on the centimeter to millimeter scale. The sediments are dated by historic climate and environmental records, 137Cs activity, and radiocarbon ages. Deformation structures occur within distinct zones and include large-scale slumps and rockfalls, as well as small-scale features like disturbed and contorted lamination and liquefaction structures. These deformations are attributed to three of the abovementioned earthquakes. The spatial distribution of deformation structures in the different lakes clearly reflects the historical macroseismic dataset: Lake sediments are only affected if they are situated within an area that underwent groundshaking not smaller than intensity VI to VII. We estimate earthquake size by relating the epicentral distance of the farthest liquefaction structure to earthquake magnitude. This relationship is in agreement with earthquake size estimations based on the historical dataset. 相似文献
992.
H. El Arabi J. Canrot B. Ouahhabi A. Charrire S. Kerchaoui 《Journal of African Earth Sciences》2004,39(3-5):393
A comparative study of the Middle Liassic series in the south western part of the Middle Atlas system leads to the definition of three new formations called Fellat 1, 2 and 3. They mainly involve gravity-flow deposits related to the breakup of the regional carbonate platform in a subsiding area, the Selloum Basin. The creation of this basin provides evidence for a rifting stage during the Carixian–Domerian interval, inducing the separation of the Selloum unit with respect to the Middle Atlasic trough and its possible connection with the High Atlasic sedimentary domain. 相似文献
993.
994.
四川大渡河金矿田位于扬子地台西缘金成矿带北段,受大渡河剪切带控制。本文以该矿田黄金坪、白金台子、黑金台子金矿为例,根据对黄铁矿流体包裹体氦氩同位素、黄铁矿硫同位素以及与黄铁矿共生的石英流体包裹体的氢氧同位素组成测定,讨论了大渡河金矿田成矿流体的来源。结果显示,该金矿田黄铁矿流体包裹体中的~3He/~4He变化较小,为0.16~0.86Ra,而~(40)Ar/~(36)Ar的变化较大,为298~3288;而黄铁矿δ~(34)S同位素变化范围较窄,一般为0.7‰~4.2‰,集中于2.5‰~3‰,显示硫地幔来源的特点;石英流体包裹体的氢、氧同位素分别为-2.6‰~ 3.64‰和-39.13‰~-108.23‰,说明成矿流体为岩浆水和大气降水的混合流体。本文认为大渡河金矿田成矿流体是地幔流体与地壳流体的混合作用的结果,而以地壳流体占主导地位。其中,地幔流体为与下伏隐伏岩体有关的岩浆水,而地壳流体端元则是含有一定放射成因Ar的大气降水,并且温度小于200℃。 相似文献
995.
Gregory R. Brooks 《地球表面变化过程与地形》2005,30(13):1617-1632
Slow earth sliding is pervasive along the concave side of Red River meanders that impinge on Lake Agassiz glaciolacustrine deposits. These failures form elongated, low‐angled (c. 6 to 10°) landslide zones along the valleysides. Silty overbank deposits that accumulated during the 1999 spring freshet extend continuously along the landslide zones over hundreds of metres and aggraded the lower slopes over a distance 50 to 80 m from the channel margin. The aggradation is not obviously related to meander curvature or location within a meander. Along seven slope profiles surveyed in 1999 near Letellier, Manitoba, the deposits locally are up to 21 cm thick and generally thin with increasing distance from, and height above, the river. Local deposit thickness relates to distance from the channel, duration of inundation of the landslide surface, mesotopography, and variations in vegetation cover. Immediately adjacent to the river, accumulated overbank deposits are up to 4 m thick. The 1999 overbank deposits also were present along the moderately sloped (c. 23 to 27°) concave banks eroding into the floodplain, but the deposits are thinner (locally up to c. 7 cm thick) and cover a narrower area (10 to 30 m wide) than the deposits within the landslide zones. Concave overbank deposition is part of a sediment reworking process that consists of overbank aggradation on the landslide zones, subsequent gradual downslope displacement from earth sliding, and eventually reworking by the river at the toe of the landslide. The presence of the deposits dampens the outward migration of the meanders and contributes to a low rate of contemporary lateral channel migration. Concave overbank sedimentation occurs along most Red River meanders between at least Emerson and St. Adolphe, Manitoba. © Her Majesty the Queen in right of Canada. 相似文献
996.
Aroldo Misi Sundaram S.S. Iyer Carlos Eduardo S. Coelho Colombo C.G. Tassinari Washington J.S. Franca-Rocha Ion de Abreu Cunha Adriana S. Rocha Gomes Tolentino Flvio de Oliveira Joo Batista G. Teixeira Valter Mnaco C. Filho 《Ore Geology Reviews》2005,26(3-4):263-304
The Proterozoic sediment-hosted Zn–(Pb) sulfide and non-sulfide deposits of the São Francisco Craton, Brazil, are partially syn-diagenetic and epigenetic and were probably formed during extensional events. The majority of the deposits occur within shallow water dolomites. The Pb isotopic data of sulfides are relatively homogeneous for individual deposits and plot above the upper crust evolution curve of the Plumbotectonic model. Some of the deposits are characterized by highly radiogenic lead (206Pb/204Pb ≥ 21) originating from the highly radioactive crust of the São Francisco Craton. Pb and S isotopic data suggest the sources of metal and sulfur for the deposits to be the basement rocks and seawater sulfates in the sediments, respectively. The relatively high temperatures of formation (100 to 250 °C) and moderate salinity (3% to 20% NaCl equiv.) of the primary fluid inclusions in the sphalerite crystals suggest the participation of basinal mineralizing fluids in ore formation. The steep paleo-geothermal gradient generated by the radioactively enriched basement rocks probably assisted in heating up the circulating mineralizing fluids. 相似文献
997.
Stratigraphical and palynological appraisal of the Late Quaternary mangrove deposits of the west coast of India 总被引:1,自引:0,他引:1
K.P.N. Kumaran K.M. Nair Mahesh Shindikar Ruta B. Limaye D. Padmalal 《Quaternary Research》2005,64(3):418-431
The organic deposits derived from the mangrove swamps form reliable stratigraphic markers within the Late Quaternary sequence of Kerala–Konkan Basin. Three generations of such deposits have been identified. The older one is dated to around 43,000–40,000 14C yr B.P., with a few dates beyond the range of radiocarbon. The younger ones date from the Middle Holocene to latest Pleistocene (10,760–4540 14C yr B.P.) and the Late Holocene (<4000 14C yr B.P.). Pollen analyses confirm that the deposits are mostly derived from the mangrove vegetation. Peat accumulation during the period 40,000–28,000 14C yr B.P. can be correlated with the excess rainfall, 40–100% greater than modern values, of the Asian summer monsoon. The low occurrence of mangrove between 22,000 and 18,000 14C yr B.P. can be attributed to the prevailing aridity and/or reduced precipitation associated worldwide with Last Glacial Maximum, because exposure surfaces and ferruginous layers are commonly found in intervals representing this period. The high rainfall of 11,000–4000 14C yr B.P. is found to be the most significant as the mangrove reached an optimum growth around 11,000 14C yr B.P. but with periods of punctuated weaker monsoons. From the present and previous studies, it has been observed that after about 5000 or 4000 14C yr B.P., the monsoons became gradually reduced leading to drying up of many of the marginal marine mangrove ecosystems. A case study of Hadi profile provided an insight to the relevance of magnetic susceptibility (χ) to record the ecological shift in Late Holocene. 相似文献
998.
Clementine was a technology demonstration mission jointly sponsored by the Department of Defense (DOD) and NASA that was launched
on January 25th, 1994. Its principal objective was to use the Moon, a near-Earth asteroid, and the spacecraft’s Interstage
Adapter as targets to demonstrate lightweight sensor performance and several innovative spacecraft systems and technologies.
The design, development, and operation of the Clementine spacecraft and ground system was performed by the Naval Research
Laboratory. For over two months Clementine mapped the Moon, producing the first multispectral global digital map of the Moon,
the first global topographic map, and contributing several other important scientific discoveries, including the possibility
of ice at the lunar South Pole. New experiments or schedule modifications were made with minimal constraints, maximizing science
return, thus creating a new paradigm for mission operations. Clementine was the first mission known to conduct an in-flight
autonomous operations experiment. After leaving the Moon, Clementine suffered an onboard failure that caused cancellation
of the asteroid rendezvous. Despite this setback, NASA and the DOD applied the lessons learned from the Clementine mission
to later missions. Clementine set the standard against which new small spacecraft missions are commonly measured. More than
any other mission, Clementine has the most influence (scientifically, technically, and operationally) on the lunar missions
being planned for the next decade. 相似文献
999.
Tropical chemical weathering produces extensive lateritization and formation of deep weathering profiles. Both processes are fundamental to landscape evolution and slope instability. The Aburrá Valley of the northern Colombian Andes is characterized by tropical conditions. The valley slopes are mostly covered by hillslope deposits originating from four basement rock suites which comprise contrasting granitoid, volcanic–sedimentary, ophiolitic, and metamorphic sources, respectively. Tropical chemical weathering of the Aburrá hillslope deposits and their respective bedrock were examined using X-ray fluorescence and X-ray diffraction analysis, to document and quantify their chemical weathering profiles, compositions, and mineralogical properties. The Chemical Index of Alteration (CIA), loss on ignition (LOI), and the Mobiles index (Imob) were used to quantify the degree of weathering of hillslope deposits and bedrock source. Weathering trends were analyzed using A–CN–K and A–CNK–FM diagrams. The material mantling the slopes in the Aburrá Valley records an intense weathering history. Chemical weathering is characterized by increased development of clay minerals (kaolinite, halloysite) and iron and aluminum sesquioxides. Lateritization characterizes the final stage of the weathering profiles. Concentrations of CaO, Na2O, K2O decrease markedly in the weathering products compared to the fresh bedrock source, whereas concentrations of Al2O3, Fe2O3, and MgO increase significantly. CIA ratios of matrix slope deposits derived from all four sources near 100, whereas those of boulder slope deposits and saprolites are lower, but exceed source rock values. Different A–CN–K weathering paths are evident for each lithotype, validating the correlation established between the hillslope deposits and their various parents. Chemical weathering indices in some samples are strongly influenced by the presence of sesquioxides, as reflected by high LOI, anomalously low CIA, and varying enrichment trends on the A–CNK–FM diagrams. Consequently, different chemical indices based on different criteria need to be combined to obtain best results, as illustrated here by the combination of LOI, CIA, and Imob. The overall results suggest that tropical conditions have dominated for a long time in the northern Colombian Andes, leading to uniformly high weathering indices in matrix slope deposits irrespective of parent lithotype. Prolonged warm and humid conditions could thus be responsible for the weathering and remobilization of extensive old hillslope deposits during the Quaternary. However, in addition to the influence of climatic factors, tectonism has also undoubtedly influenced slope evolution in the Aburrá Valley. 相似文献
1000.
This study investigates the origin and regional tectonic implications of high-altitude Plio (?)–Quaternary fluvial deposits developed over the Bozdağ horst which is an important structural element within the horst–graben system of western Anatolia, Turkey.A total of 23 deposits occur near the modern drainage divide comprising fluvial to occasionally lacustrine deposits. The deposits are all elongated in N–S direction with a width / length ratio of 1 / 10. The largest of them is of 13 km in length with a maximum observable thickness of about 100–110 m. Morphological, lithological, deformational characteristics of these deposits and the drainage system of the area all suggest that the deposits were formed due to uplift and southward tilting of the Bozdağ horst. This tilting which is estimated as 1.2° to 2.2° caused accumulation of the stream load along channels flowing from south to north. All the deposits were later dissected by the same streams with the exception of one deposit which still preserves its original lake form. These deposits are of Quaternary age, which corresponds to the latest N–S directed extensional tectonic phase in the region. 相似文献