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11.
Matthew E. Brueseke Jeffrey A. Benowitz Jeffrey M. Trop Kailyn N. Davis Samuel E. Berkelhammer Paul W. Layer Bethany K. Morter 《地学学报》2019,31(1):59-66
The Oligocene to Present Wrangell Volcanic Belt (WVB) extends for ~500 km across south‐central Alaska (USA) into Canada at a volcanic arc‐transform junction. Previously, geochemistry documented mantle wedge and slab‐edge melting in <12 Ma WVB volcanic rocks; new geochemistry shows that the same processes characterized ~18–30 Ma WVB magmatism in Alaska. New 40Ar/39Ar ages demonstrate that WVB magmatism in Alaska initiated at ~30 Ma due to flat‐slab subduction of the Yakutat microplate and that the dextral Totschunda fault was active at this time. Our results, together with prior studies, show that Alaskan WVB magmatism occurred chiefly due to subduction and should be considered a volcanic arc (e.g. the Wrangell Arc). The WVB provides a long‐term geological record of subduction, strike‐slip and magmatism. Slab‐edge upwelling, flat‐slab defocused fluid‐flux and faults acting as magma conduits are likely responsible for the exceptionally large volcanoes and high eruption rates of the Wrangell Arc. 相似文献
12.
Influences of the Clearness Index on UV Solar Radiation for Two Locations in the Tibetan Plateau——Lhasa and Haibei 总被引:1,自引:0,他引:1
HU Bo WANG Yuesi and LIU Guangren State Key Laboratory of Atmospheric Boundary Layer Physics Atmospheric Chemistry 《大气科学进展》2008,(5)
Ultraviolet (UV) solar radiation has a significant influence on human health, the environment and climate. A series of measurements, including UV radiation (290-400 nm) and global solar radiation (Rs), were continuously recorded from August 2004 at the Lhasa and Haibei sites on the Tibetan Plateau. Both observation sites' altitudes are above 3000 m and have similar meteorological conditions. The data from 2005-2006 was used to identify the varying characteristics of UV radiation. It's relation to the clearness index Ks, the relative optical mass mr, and Rs were established. The annual mean values of total daily UV radiation are 0.92 and 0.67 MJ m-2 at Lhasa and Haibei, respectively. The UV radiation in Lhasa represented 4.6% of the global solar radiation while in Haibei this percentage was 4.2%. In the case of clear days (Ks > 0.8), these percentages ranged between 4.0% and 4.5% in Lhasa and between 5.1% and 5.5% in Haibei. In the case of cloudy days (Ks < 0.4), these percentages ranged from 4.4% to 6.8% in Lhasa and from 5.1% to 5.5% in Haibei. The maximum values of UV radiation for each relative optical mass diminished exponentially with mr. Thus, for Lhasa and Haibei, UV=46.25m-r 1.29, and UV=51.76m-r 1.42, respectively. The results of this study can be used to obtain more UV radiation data for the study of UV climate characteristics, the effects of UV on ecological processes and the feedback of the thinning of the stratospheric ozone, from more routine measurements Rs data. 相似文献
13.
Anna O. Volynets Tatiana G. Churikova Gerhard Wörner Boris N. Gordeychik Paul Layer 《Contributions to Mineralogy and Petrology》2010,159(5):659-687
New 40Ar/39Ar and published 14C ages constrain voluminous mafic volcanism of the Kamchatka back-arc to Miocene (3–6 Ma) and Late Pleistocene to Holocene
(<1 Ma) times. Trace elements and isotopic compositions show that older rocks derived from a depleted mantle through subduction
fluid-flux melting (>20%). Younger rocks form in a back arc by lower melting degrees involving enriched mantle components.
The arc front and Central Kamchatka Depression are also underlain by plateau lavas and shield volcanoes of Late Pleistocene
age. The focus of these voluminous eruptions thus migrated in time and may be the result of a high fluid flux in a setting
where the Emperor seamount subducts and the slab steepens during rollback during terrain accretions. The northern termination
of Holocene volcanism locates the edge of the subducting Pacific plate below Kamchatka, a “slab-edge-effect” is not observed
in the back arc region. 相似文献
14.
Pedro Oyhantçabal Siegfried Siegesmund Klaus Wemmer Paul Layer 《International Journal of Earth Sciences》2010,99(6):1227-1246
The Sierra Ballena Shear Zone (SBSZ) is part of a high-strain transcurrent system that divides the Neoproterozoic Dom Feliciano
Belt of South America into two different domains. The basement on both sides of the SBSZ shows a deformation stage preceding
that of the transcurrent deformation recognized as a high temperature mylonitic foliation associated with migmatization. Grain
boundary migration and fluid-assisted grain boundary diffusion enhanced by partial melting were the main deformation mechanisms
associated with this foliation. Age estimate of this episode is >658 Ma. The second stage corresponds to the start of transpressional
deformation and the nucleation and development of the SBSZ. During this stage, pure shear dominates the deformation, and is
characterized by the development of conjugate dextral and sinistral shear zones and the emplacement of syntectonic granites.
This event dates to 658–600 Ma based on the age of these intrusions. The third stage was a second transpressional event at
about 586 to <560 Ma that was associated with the emplacement of porphyry dikes and granites that show evidence of flattening.
Deformation in the SBSZ took place, during the late stages, under regional low-grade conditions, as indicated by the metamorphic
paragenesis in the supracrustals of the country rocks. Granitic mylonites show plastic deformation of quartz and brittle behavior
of feldspar. A transition from magmatic to solid-state microstructures is also frequently observed in syntectonic granites.
Mylonitic porphyries and quartz mylonites resulted from the deformation of alkaline porphyries and quartz veins emplaced in
the shear zone. Quartz veins reflect the release of silica associated with the breakdown of feldspar to white mica during
the evolution of the granitic mylonites to phyllonites, which resulted in shear zone weakening. Quartz microstructures characteristic
of the transition between regime 2 and regime 3, grain boundary migration and incipient recrystallization in feldspar indicate
deformation under lower amphibolite to upper greenschist conditions (550–400°C). On the other hand, the mylonitic porphyries
display evidence of feldspar recrystallization suggesting magmatic or high-T solid-state deformation during cooling of the
dikes. 相似文献
15.
Dwight C. Bradley Julie Dumoulin Paul Layer David Sunderlin Sarah Roeske Bill McClelland Anita G. Harris Grant Abbott Tom Bundtzen Timothy Kusky 《Tectonophysics》2003,372(1-2):23-40
Evidence is presented for a previously unrecognized late Paleozoic orogeny in two parts of Alaska's Farewell terrane, an event that has not entered into published scenarios for the assembly of Alaska. The Farewell terrane was long regarded as a piece of the early Paleozoic passive margin of western Canada, but is now thought, instead, to have lain between the Siberian and Laurentian (North American) cratons during the early Paleozoic. Evidence for a late Paleozoic orogeny comes from two belts located 100–200 km apart. In the northern belt, metamorphic rocks dated at 284–285 Ma (three 40Ar/39Ar white-mica plateau ages) provide the main evidence for orogeny. The metamorphic rocks are interpreted as part of the hinterland of a late Paleozoic mountain belt, which we name the Browns Fork orogen. In the southern belt, thick accumulations of Pennsylvanian-Permian conglomerate and sandstone provide the main evidence for orogeny. These strata are interpreted as the eroded and deformed remnants of a late Paleozoic foreland basin, which we name the Dall Basin. We suggest that the Browns Fork orogen and Dall Basin comprise a matched pair formed during collision between the Farewell terrane and rocks to the west. The colliding object is largely buried beneath Late Cretaceous flysch to the west of the Farewell terrane, but may have included parts of the so-called Innoko terrane. The late Paleozoic convergent plate boundary represented by the Browns Fork orogen likely connected with other zones of plate convergence now located in Russia, elsewhere in Alaska, and in western Canada. 相似文献
16.
STRUCTURAL FEATURES OF THE MEIYU FRONT SYSTEM 总被引:10,自引:0,他引:10
A new subtropical front near the periphery of the West Pacific subtropical anticyclone isfound,which is never revealed in previous studies.The coupling of the subtropical front andMeiyu front forms a Meiyu front system (MFS) and is the most direct synoptic system for theMeiyu precipitation along the Mid-lower Reaches of Yangtze River (MRYR) in China.In thispaper.The detailed structural features and cloud features of the MFS in 1998 and 1999 areanalyzed,which manifests that the MFS is an objective phenomenon over the period of Meiyu alongMRYR and the Southwest Japan.Generally.the subtropical front is mainly located between 850hPa and 500 hPa.The moist southwest monsoon is transported in the passageway between theMeiyu front and the subtropical front.The vertical motion ascends in the passageway and descendson both sides of the MFS.forming the MFS's secondary circulation.A lower TBB band indicatedthat obvious convective activities are also located in the passageway of MFS.The horizontal windof MFS is vertically asymmetric. 相似文献
17.
Isotope datings of amphibole-bearing mafics and metamafics in the northern part of the Anadyr-Koryak region allow clarification
of the time of magmatic and metamorphic processes, which are synchronous with certain stages of the geodynamic development
of the northwest segment of the Pacific mobile belt in the Phanerozoic. To define the 40Ar/39Ar age of amphiboles, eight samples of amphibole gabbroids and metamafics were selected during field work from five massifs
representing ophiolites and mafic plutons of the island arc. Rocks from terranes of three foldbelts: 1) Pekulnei (Chukotka
region), 2) Ust-Belaya (West Koryak region), and 3) the Tamvatnei and El’gevayam subterranes of the Mainits terrane (Koryak-Kamchatka
region), were studied. The isotope investigations enabled us to divide the studied amphiboles into two groups varying in rock
petrographic features. The first was represented by gabbroids of the Svetlorechensk massif of the Pekulnei Range and by ophiolites
of the Tamvatnei Mts.; their magmatic amphiboles show the distribution of argon isotopes in the form of clearly distinguished
plateau with an age ranging within 120–129 Ma. The second group includes metamorphic amphiboles of metagabbroids and apogabbro
amphibolites of the Ust-Belaya Mts., Pekulnei and Kenkeren ranges (El’gevayam subterranes). Their age spectra show loss of
argon and do not provide well defined plateaus the datings obtained for them are interpreted as minimum ages. Dates of amphiboles
from the metagabbro of the upper tectonic plate of the Ust-Belaya allochthon points to metamorphism in the suprasubduction
environment in the fragment of Late Neoproterozoic oceanic lithosphere in Middle-Late Devonian time, long before the Uda-Murgal
island arc system was formed. The amphibolite metamorphism in the dunite-clinopyroxenite-metagabbro Pekulnei sequence was
dated to occur at the Permian-Triassic boundary. The age of amphiboles from gabbrodiorites of the Kenkeren Range was dated
to be Early Jurassic that confirmed their assignment to the El’gevayam volcanic-plutonic assemblage. These data are consistent
with geological concepts and make more precise the available age dates. Neocomian-Aptian 40Ar/39Ar age of amphibolites from the Pekulnei and Tamvatnei gabbroids make evident that mafics of these terranes (varying in geodynamic
formation settings and in petrogenesis) were generated in later stages of the development of the West Pekulnei and Mainits-Algan
Middle-Late Jurassic-Early Cretaceous island arc systems, presumably due to breakup of island arcs in the Neocomian. 相似文献
18.
Stefan Löbens Frithjof A. Bense Klaus Wemmer István Dunkl Carlos H. Costa Paul Layer Siegfried Siegesmund 《International Journal of Earth Sciences》2011,100(2-3):671-694
The Sierras Pampeanas in central and north-western Argentina constitute a distinct morphotectonic feature between 27°S and 33°S. The last stage of uplift and deformation in this area are interpreted to be closely related to the Andean flat-slab subduction of the Nazca plate beneath the South American plate. K–Ar fault gouge dating and low-temperature thermochronology along two transects within the Sierra de Comechingones reveal a minimum age for the onset of brittle deformation about 340 Ma, very low exhumation rates since Late Paleozoic time, as well as a total exhumation of about 2.3 km since the Late Cretaceous. New Ar–Ar ages (7.54–1.91 Ma) of volcanic rocks from the San Luis volcanic belt support the eastward propagation of the flat-slab magmatic front, confirming the onset of flat-slab related deformation in this region at 11.2 Ma. Although low-temperature thermochronology does not clearly constrain the signal of the Andean uplift, it is understood that the current structural relief related to the Comechingones range has been achieved after the exhumation of both fault walls (circa 80–70 Ma). 相似文献
19.
A coupled model of RAMS3b(Regional Atmospheric Modeling System,Version 3b)andLSPM(a land surface process model),in which some basic hydrological processes such asprecipitation,evapotranspiration.surface runoff,infiltration and bottom drainage are included,has been established.With the coupled model,we have simulated the response of soil to the severeweather process which caused the disastrous flood in north italy during 4-7.November,1994,simultaneously compared with the observation and the original RAMS3b,which has a soil andvegetation parameterization scheme(hereafter,SVP)emphasizing on the surface energy fluxes,while some hydrological processes in the soil are not described clearly.The results show that the differences between coupling LSPM and SVP exist mainly in theresponse of soil to the precipitation.The soil in the SVP never saturates under the strong input ofprecipitation,while the newly coupled model seems better,the soil has been saturated for one dayor more and causes strong surface runoff,which constitutes the flood.Further sensitivityexperiments show that the surface hydrological processes are very sensitive to the initial soilmoisture and soil type when we compared the results with a relatively dry case and sandy soil.The coupled model has potentiality for simulation on the interaction between regional climateand land surface hydrological processes,and the regional water resources research concerningdesertification,drought and flood. 相似文献
20.
多尺度空气质量模式系统及其验证 II.东亚地区对流层臭氧及其前体物模拟 总被引:2,自引:0,他引:2
利用一个多尺度空气质量模式系统对东亚地区2001年春季对流层臭氧及其前体物的输送与演变过程进行了模拟。为了检验模式系统模拟臭氧演变过程的能力,将模拟的臭氧浓度及其与之紧密相关的化学物种浓度,如OH自由基、HO2自由基、一氧化氮、二氧化氮、一氧化碳、乙烷和乙烯,与TRACE-P期间两架飞机在西北太平洋上空获取的大量观测资料进行了比较。比较结果表明,模拟值的平均值与相应的观测值具有相当好的一致性,且绝大多数模拟值在相应观测值的2倍范围之内;模拟的化学物种浓度的时空分布与观测结果基本相符;模式系统对于非甲烷烃中不同化学物种的输送与转化过程的处理是基本合理的,反映了光化学生成和平流层输送等过程对对流层臭氧浓度分布的影响。 相似文献