氟铈硅磷灰石—产自加拿大Mont St.Hilaire的新矿物     

古阶祥  G. Y. Chao  汤素仁 《岩石矿物学杂志》1996,15(1):40-45

新矿物氟铈硅磷灰石产自加拿大魁北京省的ＭｏｎｔＳｔ．Ｈｉｌａｉｒｅ地区，赋存于霞石正长岩、大理岩捕虏体，方钠石正长岩捕虏体和伟晶岩岩墙中。  相似文献   

Science and the St Elias: an evolving framework for sustainability in North America's highest mountains     

Ryan K Danby  David S Hik  D Scott Slocombe†  Andrew Williams‡ 《The Geographical journal》2003,169(3):191-204

The past, present, and future contributions of science in the St Elias Mountains, and its relationship with regional development, resource management, and traditional ecological knowledge is examined. Science has evolved from an early foundation of exploration, through stages of resource inventories and surveys, to deductive scientific research and, more recently, a promising reconnection with traditional knowledge. Directly and indirectly, events such as the Klondike Gold Rush, construction of the Alaska Highway, creation of the Arctic Institute of North America's Kluane Lake Research Station, and establishment of protected areas have helped foster scientific activities in the region. In turn, this scientific perspective has influenced regional development by providing detailed information that has been utilized, to varying degrees, in resource use, planning, and decisionmaking. Over the past decade, management of the region has become less sectoral and more cooperative in nature, due partly to the implementation of co-management agreements, regional land use planning, and settlement of first nations' land claims. Incorporating both science and traditional knowledge into this process through collaborative endeavours such as long-term ecological monitoring, adaptive management, and information integration will contribute to ecosystem-based management of the St Elias and ensure that both perspectives play an integral role in sustainable development of the region.  相似文献   

Synthesis of magnitude and focal mechanism computations for the M ≥ 4.5 earthquakes in France for the period 1995–2000     

Hugues Dufumier 《Journal of Seismology》2002,6(2):163-181

This paper proposes a synthesis of thestudies made in terms of source parametersevaluation for the last earthquakes oflocal magnitude greater than 4.5 whichoccurred in or nearby France during thelast five years. Focal mechanisms andseismic moments have been computed for thethree most important events, largely feltby the population: St Paul de Fenouillet(February 18^th 1996, M_L 5.6),Annecy-Epagny (July 15^th 1996, M_L5.2) and St-Béat (October 4^th1999, M_L 4.8). These focal mechanismshave been obtained either by regionalmoment tensor inversion or from firstmotion polarities and are compared withcomplementary studies made on theseearthquakes. In addition, for the otherearthquakes of local magnitude greater than4.5 which occurred nearby French borderssince the beginning of the recording ofbroadband data by the RéNaSS(Réseau National de SurveillanceSismique, French Seismological Survey) inmid–1995, several magnitude calculationsconcerning the following earthquakes arepresented: Pamplona (25/2/1996, M_L4.7), Aoste (31/03/1996, M_L 4.6),Imperia (24/2/1997, M_L 4.5),Barcelonette (31/10/1997, M_L 4.8),Pamplona (27/10/1998, M_L 4.9), andBonifacio (26/4/2000, M_L 4.5). Localmagnitudes are usually higher thanthe M_b magnitudes reported by the PDE(Preliminary Determination of Epicenters),while the extension of the Msz scale toregional magnitudes and the Mw magnitudesderived from seismic moments give smallervalues. The relative importance of thevarious earthquakes in terms of surfacewave magnitude or seismic moment does notalways agree with that implied by localmagnitudes.  相似文献   

Post-eruption erosion and deposition in the 1980 crater of Mount St Helens,Washington, determined from digital maps     

Hugh H. Mills 《地球表面变化过程与地形》1992,17(8):739-754

The crater of Mount St Helens shows one of the world's highest known rates of mass wasting. On many summer days, rockfall is almost continuous, and many large rock and dirty-snow avalanches have travelled several kilometres from their sources on the crater walls. Since formation of the crater on 18 May 1980, talus cones exceeding 100 m in thickness have formed at the base of the unstable 600 m high crater walls. To estimate rates of erosion and deposition, a series of digitized topographic maps made from aerial photographs taken of the crater in 1980, 1981, 1983, 1986 and 1988 were analysed using a geographic information system. Between 1980 and 1988, 30 × 10⁶ m³ of rock were eroded from the crater wall, representing a mean retreat rate of 2.1 m yr^?1. To account for the volume increase that occurs when bedrock is transformed into scree, this volume is multiplied by 4/3; this provides an estimate of the rock-debris volume supplied to the crater floor of 40 × 10⁶ m³. The actual volume of deposits that accumulated during this 8 year period, however, is 68 × 10⁶ m³. The difference of 28 × 10⁶ m³ is presumably the volume of snow intercalated between insulating layers of rock debris. Similar calculations for each of four time intervals between 1980 and 1988 suggest that wall erosion and thus talus accumulation rates are declining, but that rates will probably remain high for decades to come.  相似文献   

Supracrustal faults of the St. Lawrence rift system, Québec: kinematics and geometry as revealed by field mapping and marine seismic reflection data     

Alain Tremblay  Bernard Long  Manon Mass 《Tectonophysics》2003,369(3-4):231-252

The St. Lawrence rift system from the Laurentian craton core to the offshore St. Lawrence River system is a seismically active zone in which fault reactivation is believed to occur along late Proterozoic to early Paleozoic normal faults related to the opening of the Iapetus ocean. The rift-related faults fringe the contact between the Grenvillian basement to the NW and Cambrian–Ordovician rocks of the St. Lawrence Lowlands to the SE and occur also within the Grenvillian basement. The St. Lawrence rift system trends NE–SW and represents a SE-dipping half-graben that links the NW–SE-trending Ottawa–Bonnechère and Saguenay River grabens, both interpreted as Iapetan failed arms. Coastal sections of the St. Lawrence River that expose fault rocks related to the St. Lawrence rift system have been studied between Québec city and the Saguenay River. Brittle faults marking the St. Lawrence rift system consist of NE- and NW-trending structures that show mutual crosscutting relationships. Fault rocks consist of fault breccias, cataclasites and pseudotachylytes. Field relationships suggest that the various types of fault rocks are associated with the same tectonic event. High-resolution marine seismic reflection data acquired in the St. Lawrence River estuary, between Rimouski, the Saguenay River and Forestville, identify submarine topographic relief attributed to the St. Lawrence rift system. Northeast-trending seismic reflection profiles show a basement geometry that agrees with onshore structural features. Northwest-trending seismic profiles suggest that normal faults fringing the St. Lawrence River are associated with a major topographic depression in the estuary, the Laurentian Channel trough, with up to 700 m of basement relief. A two-way travel-time to bedrock map, based on seismic data from the St. Lawrence estuary, and comparison with the onshore rift segment suggest that the Laurentian Channel trough varies from a half-graben to a graben structure from SW to NE. It is speculated that natural gas occurrences within both the onshore and offshore sequences of unconsolidated Quaternary deposits are possibly related to degassing processes of basement rocks, and that hydrocarbons were drained upward by the rift faults.  相似文献   

Physical properties of the 1980 Mount St. Helens cryptodome magma     

M. Alidibirov  D. B. Dingwell  R. J. Stevenson  K.-U. Hess  S. L. Webb  J. Zinke 《Bulletin of Volcanology》1997,59(2):103-111

 Physical properties of cryptodome and remelted samples of the Mount St. Helens grey dacite have been measured in the laboratory. The viscosity of cryptodome dacite measured by parallel–plate viscometry ranges from 10.82 to 9.94 log₁₀ η (Pa s) (T=900–982  °C), and shrinkage effects were dilatometrically observed at T>900  °C. The viscosity of remelted dacite samples measured by the micropenetration method is 10.60–9.25 log₁₀ η (Pa s) (T=736–802  °C) and viscosities measured by rotational viscometry are 3.22–1.66 log₁₀ η (Pa s) (T=1298–1594  °C). Comparison of the measured viscosity of cryptodome dacitic samples with the calculated viscosity of corresponding water-bearing melt demonstrates significant deviations between measured and calculated values. This difference reflects a combination of the effect of crystals and vesicles on the viscosity of dacite as well as the insufficient experimental basis for the calculation of crystal-bearing vesicular melt viscosities at low temperature. Assuming that the cryptodome magma of the 18 May 1980 Mount St. Helens eruption was residing at 900  °C with a phenocryst content of 30 vol.%, a vesicularity of 36 vol.% and a bulk water content of 0.6 wt.%, we estimate the magma viscosity to be 10^10.8 Pa s. Received: 25 August 1996 / Accepted: 19 July 1997  相似文献   

Bimodal Density Distribution of Cryptodome Dacite from the 1980 Eruption of Mount St. Helens,Washington     

Richard P. Hoblitt  Russell S. Harmon 《Bulletin of Volcanology》1993,55(6):421-437

The explosion of a cryptodome at Mount St. Helens in 1980 produced two juvenile rock types that are derived from the same source magma. Their differences-color, texture and density-are due only to vesicularity differences. The vesicular gray dacite comprises bout 72% of the juvenile material; the black dacite comprises the other 28%. The density of juvenile dacite is bimodally distributed, with peaks at 1.6 g cm^-3 (gray dacite) and 2.3 g cm^-3 (black dacite). Water contents, deuterium abundances, and the relationship of petrographic structures to vapor-phase crystals indicate both rock types underwent pre-explosion subsurface vesiculation and degassing. The gray dacite underwent a second vesiculation event, probably during the 18 May explosion. In the subsurface, gases probably escaped through interconnected vesicles into the permeable volcanic edifice. We suggest that nonuniform degassing of an initially homogeneous magma produced volatile gradients in the cryptodome and that these gradients were responsible for the density bimodality. That is, water contents less than about 0.2–0.4 wt% produced vesicle growth rates that were slow in comparison to the pyroclast cooling rates; greater water contents produced vesicle growth rates that were fast in comparison to cooling rates. In this scheme, the dacite densities are bimodally distributed simply because, following decompression on 18 May 1980, one clast population vesiculated while the other did not. For clasts that did vesiculate, vesicle growth continued until it was arrested by fragmentation.  相似文献   

Flow and sediment processes in a cutoff meander of the Danube Delta during episodic flooding     

Laura Jugaru Tiron  Jrme Le Coz  Mireille Provansal  Nicolae Panin  Guillaume Raccasi  Guillaume Dramais  Philippe Dussouillez 《Geomorphology》2009,106(3-4):186-197

This article analyzes the water and suspended solid fluxes through a straightened meander of the southern branch of the Danube Delta (the St. George branch) during episodic flooding. The Mahmudia study site corresponds to a vast natural meander which was cut off in 1984–1988 by an artificial canal opened to shipping. The meander correction accelerated fluxes through the artificial canal and dramatically enhanced deposition in the former meander. After his formation, the cutoff meander acted as sediment storage locations, essentially removing channel and point bar sediments from the active sediment budget of the main channel. Increases in slope and stream power in reaches upstream and downstream have also occurred, but to a lesser degree. During the one-hundred-year recurrent flood in April 2006, bathymetry, flow velocity and discharge data were acquired across several sections of both natural and artificial channels with an acoustic Doppler current profiler (aDcp Workhorse Sentinel 600 kHz, Teledyne RDI) in order to investigate the distribution of the flow and sediment and his impact on sedimentation in a channelized reach and its adjacent cutoff. The contrasting hydro-sedimentary processes at work in both channels and bifurcation/confluence nodal points are analyzed from the measured flux distribution, morphological profiles and velocity and concentration patterns. In the cutoff, a diminishing of the intensity of the flow velocity (c. 50%) and of the SSC was observed correlated with the aggradation of the river bed. In the bifurcation/confluence nodal points and in the artificial canal were observed the most intensive hydrodynamic activity (high flow velocity, SSC concentration, degradation of the river bad). Both the event-scale and long-term morphological trends of the alluvial system are discussed analyzing the boundary shear stress and SSC variability. Excess boundary shear stress in the sub-reaches directly affected by cutoffs resulted in scour that increased downstream bed material load. These high sediment loads play a key role in driving morphological adjustments towards equilibrium in the cutoff channel.The approach followed in this paper combines detailed episodic in-situ aDcp measurements and robust numerical 1D modeling in order to provide a practical comprehension of the relevant morphodynamical processes. The 1D model reproduces robustly the continuity of hydrodynamical variables along the streamwise axes of the two-channel network. The simulated are used in the paper for highlighting reach-scale morphological processes, at both event and long-term scales.  相似文献   

A whole-basin stratigraphic record of sediment and phosphorus loading to the St. Croix River,USA     

Laura D. Triplett  Daniel R. Engstrom  Mark B. Edlund 《Journal of Paleolimnology》2009,41(4):659-677

Lake St. Croix is a natural impoundment of the lowermost 37 km of the St. Croix River in Minnesota and Wisconsin, making this one of a few large river systems in the world possessing a long-term depositional basin at its terminus. The river’s relatively pristine condition led to its designation as a National Scenic Riverway in 1968, but increasing urbanization in its lower reaches has raised concerns about impacts on water quality. This study was initiated to reconstruct historical loadings of suspended sediment and phosphorus (P) from the sediment record in Lake St. Croix. Twenty-four piston cores, with an average length of 2 m, were collected along eight transects of the lake. Dated chronologies from ²¹⁰Pb, ¹³⁷Cs and ¹⁴C were used to calculate the rate of sediment accumulation in the lake over the past 100+ years. Diatom microfossil analysis was used to reconstruct historical lakewater P concentrations over the same time period, and sediment P analysis quantified the amount of P trapped in lake sediments. Using a whole-lake mass balance approach, the loading of sediment and P to Lake St. Croix over the last 100+ years was calculated. Beginning in 1850, sediment accumulation increased dramatically to a peak in 1950–1960 of eight times background rates prior to European settlement. The peak is driven largely by sediment contributions from small side-valley catchment tributaries to the downstream half of the lake. The total P load to the lake increased sharply after 1940 and remains high, at around four times the level of pre-European settlement conditions. The timing of peak sediment and P loading to the lake shows that early settlement activities, such as logging and the conversion of forest and prairie to agricultural land between 1850 and 1890, had only modest impacts on the lake. By contrast, the mid-1900s brought major increases in sediment and P loading to the lake, suggesting that relatively recent activities on the landscape and changes to nutrient balances in the watershed have caused the current eutrophic condition of this important recreational and natural resource. This is one of eight papers dedicated to the “Recent Environmental History of the Upper Mississippi River” published in this special issue of the Journal of Paleolimnology. D. R. Engstrom served as guest editor of the special issue.  相似文献   

Budget and sources of suspended sediment transported in the St. Lawrence River,Canada     

Bernard Rondeau  D. Cossa  P. Gagnon  L. Bilodeau 《水文研究》2000,14(1):21-36

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