首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Lidar measurements of upper atmospheric sodium were made at Alcântara (2°20′ S, 44°30′ W), Brazil, on 8 days during the month of November 1996. These are the first sodium measurements to be reported from an almost equatorial location. The average sodium distribution at Alcântara was found to be very similar to the annual mean for São José dos Campos (23° S), but about 1 km higher than the November mean value of 92 km for the latter site. Only one, weak, sporadic sodium layer was observed during more than 26 h of observations, an occurrence frequency much less than that seen at low latitude sites in the northern hemisphere. A survey of the observations of sporadic layers at various locations suggests that both magnetic dip and declination angles might influence their rate of occurrence.  相似文献   

2.
本文利用2014年9月到2017年8月全球高时空分辨率TEC数据对北半球四个经度带电离层中纬槽的发生率和槽极小位置的变化进行了统计研究.基于Kp指数,我们引入了一个包含地磁活动变化历史效应的地磁指数(Kp 9)来分析中纬槽位置变化与地磁活动水平的关系.通过与其他地磁活动指数的对比,发现槽极小纬度与Kp 9指数的相关性最好.此外,本文重点分析了中纬槽发生率及槽极小纬度的经度差异、季节变化、地方时变化以及与地磁活动强度等的关系.结果表明,中纬槽的发生率与经度关系不大,主要受到季节、地方时与地磁活动的影响.午夜中纬槽发生率在夏季较低,其随地方时的变化则呈现出负偏态分布的特点,在后半夜发生率更高,而地磁活动增强对中纬槽的发生具有明显的促进作用.对于槽极小纬度,其在四个经度带的分布差异不大,但月变化各不相同,其中-120°经度带呈单峰分布,在夏季槽极小纬度更高,而0°经度带夏季槽极小纬度更低.槽极小的位置显著依赖于地磁活动、地方时以及季节变化.一般说来,地磁活动越强,中纬槽纬度越低.中纬槽位置随地方时的变化有明显的季节差异,冬季昏侧槽极小纬度随地方时变化较快,弱地磁活动条件下22∶00 LT前即达到最低纬度,其后位置几乎保持不变,而两分季槽极小纬度从昏侧至午夜都在降低,夏季槽极小纬度从昏侧连续下降至03∶00 LT左右.  相似文献   

3.
We examine seismic and eruptive activity at Fuego Volcano (14°29′N, 90° 53′W), a 3800-m-high stratovolcano located in the active volcanic arc of Guatemala. Eruptions at Fuego are typically short-lived vulcanian eruptions producing ash falls and ash flows of high-alumina basalt. From February 1975 to December 1976, five weak ash eruptions occurred, accompanied by small earthquake swarms. Between 0 and 140 (average ≈ 10) A-type or high-frequency seismic events per day with M > 0.5 were recorded during this period. Estimated thermal energies for each eruption are greater by a factor of 106 than cumulative seismic energies, a larger ratio than that reported for other volcanoes.Over 4000 A-type events were recorded January 3–7, 1977 (cumulative seismic energy ≈ 109 joules), yet no eruption occurred. Five 2-hour-long pulses of intense seismicity separated by 6-hour intervals of quiescence accounted for the majority of events. Maximum likelihood estimates of b-values range from 0.7 ± 0.2 to 2.1 ± 0.4 with systematically lower values corresponding to the five intense pulses. The low values suggest higher stress conditions.During the 1977 swarm, a tiltmeter located 6 km southeast of Fuego recorded a 14 ± 3 microradian tilt event (down to SW). This value is too large to represent a simple change in the elastic strain field due to the earthquake swarm. We speculate that the earthquake swarm and tilt are indicative of subsurface magma movement.  相似文献   

4.
The Jemez Mountains volcanic field (JMVF), located in north-central New Mexico, has been a site of basaltic to rhyolitic volcanism since the mid-Miocene with major caldera forming eruptions occurring in the Pleistocene. Eruption of the upper Bandelier Tuff (UBT) is associated with collapse of the Valles Caldera, whereas eruption of the lower Bandelier Tuff (LBT) resulted in formation of the Toledo Caldera. These events were previously dated by K-Ar at 1.12 ± 0.03 Ma and 1.45 ± 0.06 Ma, respectively. Pre-Bandelier explosive eruptions produced the San Diego Canyon (SDC) ignimbrites. SDC ignimbrite “B” has been dated at 2.84 ± 0.07 Ma, whereas SDC ignimbrite “A”, which underlies “B”, has been dated at 3.64 ± 1.64 Ma. Both of these dates are based on single K-Ar analyses.40Ar/39Ar dating of single sanidine crystals from these units indicates revision of the previously reported dates. Isochron analysis of 26 crystals from the UBT gives a common trapped 40Ar/36Ar component of 304.5, indicating the presence of excess 40Ar in this unit, and defines an age of 1.14 ± 0.02 Ma. Isochron analysis of 26 crystals from the LBT indicates an atmospheric trapped component and an age of 1.51 ± 0.03 Ma. An age of 1.78 ± 0.04 Ma, based on the weighted mean of 5 individual analyses, is indicated for SDC ignimbrite “B”, whereas 3 analyses from SDC ignimbrite “A” give a weighted mean age of 1.78 ± 0.07 Ma. Evidence for xenocrystic contamination in the SDC ignimbrites comes from analyses of a correlative air-fall pumice unit in the Puye Formation alluvial fan giving ages of 1.75 ± 0.08 and 3.50 ± 0.09 Ma. The presence of xenocrysts in bulk separates used for the original K-Ar analyses could account for the significantly older ages reported.Geochemical data indicate that SDC ignimbrites are early eruptions from the magma chamber which evolved to produce the LBT, as compositions of SDC ignimbrite “B” are virtually identical to least evolved LBT samples. Differentiation during the 270-ka interval between eruption of SDC ignimbrite “B” and the LBT produced an array of high-silica rhyolite compositions which were erupted to form the LBT. Mixed pumices associated with eruption of the LBT indicated an influx of more mafic magma into the system which produced shifts in some incompatible trace-element ratios. Lavas and tephras of the Cerro Toledo Rhyolite record the geochemical evolution of the Bandelier magma system during the 370-ka interval between eruption of the LBT and the UBT.The combined geochronologic and geochemical data place the establishment and evolution of the Bandelier silicic magma system within a precise temporal framework, beginning with eruption of the SDC ignimbrites at 1.78 Ma, and define a periodicity of 270–370 ka to ash-flow eruptions in the JMVF. These intervals are comparable to those in other multicyclic caldera complexes and are a measure of the timescales over which substantial fractionation of large silicic magma bodies occur.  相似文献   

5.
Basic properties of the mid-latitude traveling ionospheric disturbances (TIDs) during the maximum phase of a major magnetic storm of 6–8 April 2000 are shown. Total electron content (TEC) variations were studied by using data from GPS receivers located in Russia and Central Asia. The nightglow response to this storm at mesopause and termospheric altitudes was also measured by optical instruments FENIX located at the observatory of the Institute of Solar-Terrestrial Physics (51.9°N,103.0°E), and MORTI located at the observatory of the Institute of Ionosphere (43.2°N, 77.0°E). Observations of the O (557.7 and 630.0 nm) emissions originating from atmospheric layers centered at altitudes of 90 and 250 km were carried out at Irkutsk and of the O2(b1∑g+−X3∑g−) (0-1) emission originating from an atmospheric layer centered at altitude of 94 km was carried out at Almaty. Our radio and optical measurement network observed a storm-induced solitary large-scale wave with duration of 1 h and a wave front width of no less than 5000 km, while it traveled equatorward with a velocity of 200 m/s from 62°N to 38°N geographic latitude. The TEC disturbance, basically displaying an electron content depression in the maximum of the F2 region, reveals a good correlation with growing nightglow emission, the temporal shift between the TEC and emission variation maxima being different for different altitudes. A comparison of the auroral oval parameters with dynamic spectra of TEC variations and optical 630 nm emissions in the frequency range 0.4–4 mHz (250–2500 s periods) showed that as the auroral oval expands into mid-latitudes, also does the region with a developed medium-sale and small-scale TEC structure.  相似文献   

6.
Sixty-six K---Ar dates from igneous rocks in the central Chilean Andes between 33° and 38°S are reported in this study. From these results and observed field relations, major Cenozoic volcanic and intrusive rock units are divided into chronologic groups representing igneous events.Volcanic units of Oligocene (33.3–27.9 m.y.) and Early Miocene (20.2 m.y.) age have been dated west of the present range at 33°S but neither the magnitude nor extent of these volcanic events has yet been established. Extensive Middle to Late Miocene volcanism (15.3–6.4 m.y.) followed by regional folding is recognized in the map area between 35° 20′ and 36°S. Partly contemporaneous Middle Miocene volcanism (18.4–13.7 m.y.) also followed by regional folding is recorded in the Andes between 37° 30′ and 38°S. General volcanic quiescence from 6.4 to 2.5 m.y. is observed in the map area but whether this volcanic hiatus is of regional significance is not known.The majority of the K---Ar dates document a history of nearly continuous volcanism throughout the last 2.5 m.y. in the map area. The abundant and diverse sequences of volcanic strata formed during this time, have been divided into four successive age groups which as map units show the evolution and distribution of latest volcanic activity.Landforms preserved by this volcanic series show that topographic relief similar to the present has prevailed during this time. Deep incision of rivers into young volcanic terrain, estimated to be on the order of 1–2 m/1000 years, has produced a complex volcanic and morphologic record.Four plutons dated in this study give ages of 62.0, 41.3, 19.5, and 7.0 m.y. No spatial pattern of emplacement is observed in the map area where three of these plutons are represented.Similarities in structural style, orientation and degree of deformation of Miocene and Mesozoic strata suggest that Late Miocene regional folding may have accounted for a significant part of the observed deformation in older basement strata previously ascribed to earlier orogenies.A regional comparison of ages of recognized igneous and tectonic event at different latitudes in the central and southern Andes shows the gross chronology of Cenozoic events which can be correlated with sea-floor spreading and subduction events.  相似文献   

7.
An all-sky CCD imager designed to measure wave structure of the OH, O2b(0,1) and OI557.7 nm airglow emission layers in the mesosphere and lower thermosphere (MLT) region has been operated near the equatorial region at São João do Cariri (Cariri), Brazil, (7.5°S, 36.5°W). A large number of gravity wave was observed from September 2000 to September 2002 and among them 64 wave events were identified as mesospheric bores. The bore front shows a horizontal extension greater than 1000 km, and observed in the airglow layers as a complementary brilliance between the three emissions. At the first time mesospheric bore events were observed and analyzed in the equatorial region. Their predominant characteristics, occurrence, local time dependency, morphology and propagation direction will be presented and discussed.  相似文献   

8.
We carried out thermomagnetic susceptibility analyses of fault rocks from core samples from Hole B of the Taiwan Chelungpu Fault Drilling Project (TCDP) to investigate the cause of high magnetic susceptibilities in the fault core. Test samples were thermally and mechanically treated by heating to different maximum temperatures of up to 900 °C and by high-velocity frictional tests before magnetic analyses. Thermomagnetic susceptibility analyses of natural fault rocks revealed that magnetization increased at maximum heating temperatures above 400 °C in the heating cycle, and showed three step increases, at 600 to 550 °C and at 300 °C during the cooling cycle. These behaviors are consistent with the presence of pyrite, siderite and chlorite, suggesting that TCDP gouge originally included these minerals, which contributed to the generation the magnetic susceptibility by thermomechanical reactions. The change in magnetic susceptibility due to heating of siderite was 20 times that obtained by heating pyrite and chlorite, so that only a small fraction of siderite decomposition is enough to cause the slight increase of the susceptibility observed in the fault core. Color measurement results indicate that thermal decomposition by frictional heating took place under low-oxygen conditions at depth, which prevented the minerals from oxidizing to reddish hematite. This finding supports the inference that a mechanically driven chemical reaction partly accounts for the high magnetic susceptibility. A kinetic model analysis confirmed that frictional heating can cause thermal decomposition of siderite and pyrite. Our results show that decomposition of pyrite to pyrrhotite, siderite and, to some extent, chlorite to magnetite is the probable mechanism explaining the magnetic anomaly within the Chelungpu fault zone.  相似文献   

9.
For the Fiji-Tonga-Kermadec area and for the period from January 1977 to July 2003, the Harvard CMT catalogue lists 1022 shallow, 410 intermediate and 633 deep earthquakes of moment magnitude from 4.9 to 8.0. The magnitude threshold, above which the catalogue is complete, is 5.3–5.4, and the number of earthquakes of magnitude above this value is 691 for shallow, 329 for intermediate and 476 for deep events, respectively. The proportion of earthquakes associated with doublets and multiplets against the total number of earthquakes is approximately the same in both data sets and therefore all earthquake pairs were considered regardless of their magnitude. We investigated all the pairs of earthquakes that occurred at a centroid distance of less than 40, 60 or 90 km from each other and within a time interval of 200, 300 or 450 days, depending on their magnitude. We found 208 pairs of shallow, 31 of intermediate and 92 of deep events. To ascertain whether these earthquakes in pairs are not connected by chance, the possibility of their occurrence in an uncorrelated Poissonian catalogue was considered. It was assumed that in such a catalogue the inter-event time is exponentially distributed, the earthquake magnitude follows the Gutenberg-Richter relation, and the distribution of centroid distances between the events in pairs is controlled by its non-parametric kernel estimate. The probability of the appearance of the observed proportion of doublets of shallow earthquakes in the Poissonian catalogue was found to be very low. The low probability of occurrence in a semi-random catalogue, created by randomising centroid locations in the actual data set, also indicates major importance of the distance criterion used for a doublet specification. In general, shallow earthquakes tend to form pairs at shorter distances and within shorter time intervals than deep earthquakes. Both the distance and the time intervals do not depend on the magnitude of involved events. The largest number of pairs of deep earthquakes is observed at a depth of about 600 km, and the proportion of deep events associated with doublets against the number of all events increases with depth. From comparison of the focal mechanism of earthquakes in pairs, measured by the 3-D rotation angle, it follows that deep earthquakes forming pairs have a more diverse focal mechanism than shallow events; the rotation angle for three quarters of shallow pairs and only for about one third of deep pairs is reasonably small. The azimuth between two events forming a doublet is in about 60–65% of cases close to the strike of one of nodal planes of the first or the second event.  相似文献   

10.
Continuous MF and meteor radar observations allow detailed studies of winds in the mesosphere and lower thermosphere (MLT) as well as temperatures around the mesopause. This height region is characterized by a strong variability in winter due to enhanced planetary wave activity and related stratospheric warming events, which are distinct coupling processes between lower, middle and upper atmosphere. Here the variability of mesospheric winds and temperatures is discussed in relation with major and minor stratospheric warmings as observed during winter 2005/06 in comparison with results during winter 1998/99.Our studies are based on MF radar wind measurements at Andenes (69°N, 16°E), Poker Flat (65°N, 147°W) and Juliusruh (55°N, 13°E) as well as on meteor radar observations of winds and temperatures at Resolute Bay (75°N, 95°W), Andenes (69°N, 16°E) and Kühlungsborn (54°N, 12°E). Additionally, energy dissipation rates have been estimated from spectral width measurements using a 3 MHz Doppler radar near Andenes. Particular attention is directed to the changes of winds, turbulence and the gravity wave activity in the mesosphere in relation to the planetary wave activity in the stratosphere.Observations indicate an enhancement of planetary wave 1 activity in the mesosphere at high latitudes during major stratospheric warmings. Daily mean temperatures derived from meteor decay times indicate that strong warming events are connected with a cooling of the 90 km region by about 10–20 K. The onset of these cooling processes and the reversals of the mesospheric circulation to easterly winds occur some days before the changes of the zonal circulation in the stratosphere start indicating a downward propagation of the circulation disturbances from the MLT region to the stratosphere and troposphere during the stratospheric warming events. The short-term reversal of the mesospheric winds is followed by a period of strong westerly winds connected with enhanced turbulence rates and an increase of gravity wave activity in the altitude range 70–85 km.  相似文献   

11.
High-pressure and high temperature experiments at 20 GPa on (Mg,Fe)SiO3 have revealed stability fields of two types of aluminium-free ferromagnesian garnets; non-cubic garnet and cubic garnet (majorite). Majorite garnet is stable only within a limited compositional variation, 0.2 < Fe/(Mg + Fe)< 0.4, and in the narrow temperature interval of 200°C around 2000°C, while the stability of non-cubic garnet with more iron-deficient compositions persists up to higher temperatures. These two garnets show fractional melting into iron-deficient garnet and iron-rich liquid, and the crystallization field of cubic garnet extends over Fe/(Mg + Fe)= 0.5. The assemblage silicate spinel and stishovite is a low-temperature phase, which also occurs in the iron-rich portion of the MgSiO3—FeSiO3 system. The sequence as given by the Fe/(Mg + Fe) value for the coexisting phases with the two garnets at 2000°C and 20 GPa is: silicate modified spinel aluminium-free garnets silicate spinel.Natural majorite in shock-metamorphosed chondrites is clarified to be produced at pressures above 20 GPa and temperatures around 2000°C. Similar shock events may cause the occurrence of non-cubic garnet in iron-deficient meteorites. Non-cubic garnet could be a stable phase in the Earth's mantle if a sufficiently low concentration of aluminium is present in the layer corresponding to the stable pressure range of non-cubic garnet. The chemical differentiation by melting in the deep mantle is also discussed on the basis of the present experimental results and the observed coexistence of majorite garnet with magnesiowüstite in chondrites.  相似文献   

12.
We report results from an investigation of the geologic processes controlling hydrothermal activity along the previously-unstudied southern Mid-Atlantic Ridge (3–7°S). Our study employed the NOC (UK) deep-tow sidescan sonar instrument, TOBI, in concert with the WHOI (USA) autonomous underwater vehicle, ABE, to collect information concerning hydrothermal plume distributions in the water column co-registered with geologic investigations of the underlying seafloor. Two areas of high-temperature hydrothermal venting were identified. The first was situated in a non-transform discontinuity (NTD) between two adjacent second-order ridge-segments near 4°02′S, distant from any neovolcanic activity. This geologic setting is very similar to that of the ultramafic-hosted and tectonically-controlled Rainbow vent-site on the northern Mid-Atlantic Ridge. The second site was located at 4°48′S at the axial-summit centre of a second-order ridge-segment. There, high-temperature venting is hosted in an  18 km2 area of young lava flows which in some cases are observed to have flowed over and engulfed pre-existing chemosynthetic vent-fauna. In both appearance and extent, these lava flows are directly reminiscent of those emplaced in Winter 2005−06 at the East Pacific Rise, 9°50′N and reference to global seismic catalogues reveals that a swarm of large (M 4.6−5.6) seismic events was centred on the 5°S segment over a  24 h period in late June 2002, perhaps indicating the precise timing of this volcanic eruptive episode. Temperature measurements at one of the vents found directly adjacent to the fresh lava flows at 5°S MAR (Turtle Pits) have subsequently revealed vent-fluids that are actively phase separating under conditions very close to the Critical Point for seawater, at  3000 m depth and 407 °C: the hottest vent-fluids yet reported from anywhere along the global ridge crest.  相似文献   

13.
Local seismic activity consisting of sharp earthquakes accompanied by thunderous noise was reported starting in late December 1985 around Tacaná volcano (15.13°N, 92.10°W). Portable seismic stations were established in the area by late January 1986 and sampling of the only known thermal spring on the volcano flanks started at the same time. A marked increase in SO42− concentration in the spring water preceded by two months the occurrence of a seismic swarm crisis and a small phreatic explosion. A model involving a crystalline basement fractured by tectonic stresses is proposed to explain the chemical and seismic anomalies, and the consequences on risk of volcanic activity are briefly discussed in terms of the observed behaviour.  相似文献   

14.
UPb age determinations by ion microprobe reveal multiple episodes of zircon growth and recrystallization within a single sample of tonalitic gneiss from the Ancient Gneiss Complex. The oldest episode at3644 ± 4Ma(2σ) produced the dominant type of zircon, characteristically purplish-brown and massive in texture; this probably constitutes unaltered zircon precipitated from the original magma. Recrystallization accompanied (and obscured) by early Pb loss took place within the oldest grains at3504 ± 6Ma and3433 ± 8Ma. Whole new grains grew at these times also. We interpret the post-3644 Ma growth as due to episodic deformational and metamorphic events that transformed the tonalite pluton into foliated banded gneiss. In addition, many grains are visibly overgrown by two layers of younger zircon of different colour and texture, dated at2986 ± 20Ma and2867 ± 30Ma. Euhedral, finely-zoned whole grains having the 2986 Ma age are present also, evidently contributed by very thin felsic veins associated with the nearby Lochiel granite. The age of3644 ± 4Ma combined with precise zircon UPb dating of volcanics from the Onverwacht Group reported elsewhere demonstrates that at least part of the Ancient Gneiss Complex is older than the Barberton Greenstone Belt.  相似文献   

15.
Microthermometric analyses of fluid inclusions on a suite of hydrothermally altered gabbros recovered just south of the eastern intersection of the Kane Fracture Zone and the Mid-Atlantic Ridge, record the highest homogenization temperatures yet reported for mid-ocean ridge hydrothermal systems. Fluid salinities in the high temperature inclusions are more than ten times that of seawater. Multiple generations of fluid inclusions entrapped along healed microfractures exhibit three distinct temperature-compositional groups. We interpret these populations as having been trapped during three separate fracturing events.The earliest episode of brittle failure in the gabbros is represented by coplanar, conjugate vapor-dominated and brine-dominated fluid inclusion arrays in primary apatite. Vapor-dominated inclusions exhibit apparent homogenization temperatures of 400°C and contain equivalent salinities of 1–2 wt.% NaCl. These inclusions are interspersed with liquid-dominated, sulfide-bearing inclusions containing salinities of 50 wt.% NaCl equivalent. These high salinity inclusions remain unhomogenized at temperatures greater than 700°C.Compositional and phase relationships of the fluid inclusions may be accounted for by two-phase separation of a fluid under 1000–1200 bars pressure. These pressures require that fluid entrapment occurred under a significant lithostatic component and indicate a minimum entrapmentdepth of 2 km below the axial valley floor. This depth corresponds to a minimum tectonic uplift of 3 km, in order to emplace the samples at the 3100 m recovery depth. The microfracture networks within magmatic apatites represent fluid flow paths for either highly modified, deeply penetrating seawater or a late stage magmatic aqueous fluid. The inclusions may have formed close to the brittle-ductile transition zone adjacent to an active magma chamber.Following collapse of the high temperature front, lower temperature fluids of definite seawater origin circulated through the open fracture networks, pervasively altering portions of the gabbros. This stage is represented by low-to-moderate (1–7 wt.% NaCl equivalent) salinity inclusions in plagioclase, apatite, epidote, and augite, which homogenize at temperatures of approximately 200–300°C and 400°C. Formation of hydrous mineral assemblages, under greenschist to lower amphibolite facies conditions, resulted in sealing of the vein system and may have resulted in modification of seawater salinities by as much as a factor of two. During or following these later stages of hydrothermal activity the gabbros were emplaced high on the axial walls by differential uplift attending formation of the flanking mountains.  相似文献   

16.
In this study we assessed changes in the contribution of terrigenous organic matter (OM) to the Gulf of Mexico over the course of the last deglaciation (the last 25 kyr). To this end, we combined optical kerogen analyses with bulk sedimentary, biomarker, and compound-specific carbon isotope analyses. Samples were obtained from core MD02-2550 from Orca Basin (2249 m water depth at 26°56.77N, 91°20.74W) with temporal resolution ranging from multi-decadal to millennial-scale, depending on the proxy. All proxies confirmed larger terrigenous input during glacial times compared to the Holocene. In addition, the kerogen analyses suggest that much of the glacial OM is reworked (at least 50% of spores and pollen grains and 40% of dinoflagellate cysts). The Holocene sediments, in contrast, contain mainly marine OM, which is exceptionally well preserved. During the deglaciation, terrigenous input was generally high due to large meltwater fluxes, whereby discrepancies between different proxies call for additional influences, such as the change in distance to the river mouth, local productivity changes, and hydrodynamic particle sorting. It is possible that kerogen particles and the terrigenous biomarkers studied here represent distinct pools of land-derived OM with inputs varying independently.  相似文献   

17.
Glaciers are major agents of erosion that increase sediment load to the downstream fluvial system. The Castle Creek Glacier, British Columbia, Canada, has retreated ~1.0 km in the past 70 years. Suspended sediment concentration (SSC) and streamflow (Q) were monitored independently at five sites within its pro‐glacial zone over a 60 day period from July to September 2011, representing part of the ablation season. Meteorological data were collected from two automatic weather stations proximal to the glacier. The time‐series were divided into hydrologic days and the shape and magnitude of the SSC response to hydro‐meteorological conditions (‘cold and wet’, ‘hot and dry’, ‘warm and damp’, and ‘storm’) were categorized using principal component analysis (PCA) and cluster analysis (CA). Suspended sediment load (SSL) was computed and summarized for the categories. The distribution of monitoring sites and results of the multivariate statistical analyses describe the temporal and spatial variability of suspended sediment flux and the relative importance of glacial and para‐glacial sediment sources in the pro‐glacial zone. During the 2011 study period, ~ 60% of the total SSL was derived from the glacial stream and sediment deposits proximal to the terminus of the glacier; during ‘storm’ events, that contribution dropped to ~40% as the contribution from diffuse and point sources of sediment throughout the pro‐glacial zone and within the meltwater channels increased. While ‘storm’ events accounted for just 3% of the study period, SSL was ~600% higher than the average over the monitoring period, and ~20% of the total SSL was generated in that time. Determining how hydro‐meteorological conditions and sediment sources control sediment fluxes will assist attempts to predict how pro‐glacial zones respond to future climate changes. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

18.
For the Central European area three-dimensional model techniques are used to estimate the temperature-depth distribution within the Earth's crust, the temperature at the crust-mantle boundary, the MOHO heat flow density, and the lateral variations of the heat production within the granitic upper crust. Regularization methods are used for the inversion of the surface heat flow density. The calculations were done for a 1 × 1 degree grid. Evidently, for single geothermal provinces the 200°, 400°, 600°, and 800° isotherms are running parallel to the crust-mantle boundary which implies that this discontinuity might be interpreted in terms of an isotherm.  相似文献   

19.
Paleomagnetic poles for Late Paleozoic-Early Mesozoic rocks exposed in stable and possible mobile zones of Western Argentina are reported. They suggest two interpretations.One of the interpretations suggests that a mobile zone situated at about 32.1°South 69.3°West, in the Central Andes, rotated about 60° clockwise after Late Paleozoic time.The other interpretation suggests that at least the Andean zone between 32.1° and 31.7° South is allochthonous and was situated in the Pacific, at tropical latitudes, in the Late Paleozoic. On this interpretation the accretion of this microplate to South America ocurred in Permo-Triassic times.  相似文献   

20.
The Tatun Volcano Group (TVG) is located at the northern tip of Taiwan, near the capital Taipei and close to two nuclear power plants. Because of lack of any activity in historical times it has been classified as an extinct volcano, even though more recent studies suggest that TVG might have been active during the last 20 ka. In May 2003 a seismic monitoring project at the TVG area was initiated by deploying eight three-component seismic stations some of them equipped with both short-period and broadband sensors. During the 18 months observation period local seismicity mainly consisted of high frequency earthquakes either occurring as isolated events, or as a continuous sequence in the form of spasmodic bursts. Mixed and low frequency events were also present during the same period, even though they occurred only rarely. Arrival times from events with clear P-/S-wave phases were inverted in order to obtain a minimum 1D velocity model with station corrections. Probabilistic nonlinear earthquake locations were calculated for all these events using the newly derived velocity model. Most high frequency seismicity appeared to be concentrated near the areas of hydrothermal activity, forming tight clusters at depths shallower than 4 km. Relative locations, calculated using the double-difference method and utilising catalogue and cross-correlation differential traveltimes, showed insignificant differences when compared to the nonlinear probabilistic locations. In general, seismicity in the TVG area seems to be primarily driven by circulation of hydrothermal fluids as indicated by the occurrence of spasmodic bursts, mixed/low frequency events and a b-value (1.17 ± 0.1) higher than in any other part of Taiwan. These observations, that are similar to those reported in other dormant Quaternary volcanoes, indicate that a magma chamber may still exist beneath TVG and that a future eruption or period of unrest should not be considered unlikely.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号