Eine vergleichende Studie über Methoden zur quantitativen Bestimmung von Abrundung und Form an Sandkörnern (Im Hinblick auf die Verwendbarkeit an Dünnschliffen.)     

Paula Schneiderhöhn 《Contributions to Mineralogy and Petrology》1954,4(1-2):172-191

Zusammenfassung Kurze Darlegung der zur Verwendung an Sandkornern geeigneten Methoden zur Ermittlung von Abrundung und Form. Vergleichende Betrachtung der an 25 Kernprojektionen mit den verschiedenen Methoden erzielten Untersuchungsergebnisse. Anwendung der gemachten Erfahrungen auf den Dünnschliff unter Erörterung der hierbei auftretendon besonderen Fragen und Schwierigkeiten.Herrn Professor Dr.Carl W. Correns zum 60. Geburtstag gewidmet.  相似文献   

GEOALGAR Project: First Results on Seismicity and Fault-plane Solutions   总被引：1，自引：0，他引：1  

F.?CarrilhoEmail author  P.?Teves-Costa  I.?Morais  J.?Pagarete  R.?Dias 《Pure and Applied Geophysics》2004,161(3):589-606

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Palaeomagnetic data from Ediacaran (Vendian) sediments of the Arkhangelsk region, NW Russia: An alternative apparent polar wander path of Baltica for the Late Proterozoic–Early Palaeozoic     

María Paula Iglesia Llanos  Jennifer Alice Tait  Viktor Popov  Alexandra Abalmassova 《Earth and Planetary Science Letters》2005,240(3-4):732-747

The study area is situated along the Zolotica river in NW Russia, located within the Kola–Dvyna Rift System in the Baltic Shield that developed during Meso and Neoproterozoic times. A 9-m thick section made up of shallow marine sediments of Upper Ediacaran age was sampled in this locality. Two volcaniclastic levels from the middle part of the section yielded an age of 556 Ma. (U/Pb SHRIMP-II on zircons). Two magnetic components were successfully isolated, component A (Decl = 157.1, Incl = 68.0, ₉₅ = 1.9°, N = 575 in situ) carried by magnetite and component B (Decl = 120.3, Incl = − 31.7, ₉₅ = 3.9°, N = 57, bedding corrected), carried by haematite. While component A is thought to represent a younger overprint direction, the in situ direction for component B on the other hand, is dissimilar to any expected younger direction and is considered to be primary magnetisation in origin, acquired during or soon after deposition of the sediments in the Late Ediacaran. The corresponding palaeomagnetic pole for component A in situ is located at Lon = 55.4°E, Lat = 31°N, A₉₅ = 2.7° and for component B at Lon = 110°E, Lat = 28.3°S, A₉₅ = 3.8°, N = 57. Combined with other palaeomagnetic poles of the same tectonostratigraphic unit an alternative apparent polar wander path for the Late Proterozoic–Early Palaeozoic of Baltica is proposed. Such an alternative path shows that after the mid Cryogenian (750 Ma), the poles that were situated over South Africa (p.d.c.) moved to the east until they reached Australia during the Late Ediacaran (555 Ma) where they remained approximately stationary until the beginning of the Cambrian (545 Ma). Finally, they moved to the northwest until they reached the Arabian Peninsula in the Early Ordovician. Palaeolatitudes indicate that Baltica situated near the equator from the Cryogenian through to the Ediacaran moving gradually to the south at c. 1 cm/yr. During the Late Early Ediacaran, the plate suddenly began to drift northward at c. 8 cm/yr and in the boundary with the Cambrian it was positioned in low to intermediate latitudes. Finally, Baltica began to move back to the south at c. 13 cm/yr until in the Early Ordovician, reaching intermediate to high southern latitudes.  相似文献   

40Ar/39Ar and K–Ar geochronology of magmatic and hydrothermal events in a classic low-suphidation epithermal bonanza deposit: El Peñon, northern Chile     

Gloria Arancibia  Stephen J. Matthews  Paula Cornejo  Carlos Pérez de Arce  José I. Zuluaga  Stabro Kasaneva 《Mineralium Deposita》2006,41(5):505-516

The epithermal El Peñon gold–silver deposit consists of quartz–adularia veins emplaced within a late Upper Paleocene rhyolitic dome complex, located in the Paleocene–Lower Eocene Au–Ag belt of northern Chile. Detailed K–Ar and ⁴⁰Ar/³⁹Ar geochronology on volcano–plutonic rocks and hydrothermal minerals were carried out to constrain magmatic and hydrothermal events. The Paleocene to Lower Eocene magmatism in the El Peñon area is confined to a rhomb-shaped basin, which was controlled by N–S trending normal faults and both NE- and NW-trending transtensional fault systems. The earliest products of the basin-filling sequences comprise of Middle to Upper Paleocene (～59–55 Ma) welded rhyolitic ignimbrites and andesitic to dacitic lavas, with occasional dacitic dome complexes. Later, rhyolitic and dacitic dome complexes (～55–52 Ma) represent the waning stages of volcanism during the latest Upper Paleocene and the earliest Eocene. Lower Eocene porphyry intrusives (～48–43 Ma) mark the end of the magmatism in the basin and a change to a compressive tectonomagmatic regime. ⁴⁰Ar/³⁹Ar geochronology of hydrothermal adularia from the El Peñon deposit yields ages between 51.0±0.6 and 53.1±0.5 Ma. These results suggest that mineralization occurred slightly after the emplacement of the El Peñon rhyolitic dome at 54.5±0.6 Ma (⁴⁰Ar/³⁹Ar age) and was closely tied to later dacitic–rhyodacitic bodies of 52 to 53 Ma (K–Ar ages), probably as short-lived pulses related to single volcanic events.  相似文献   

Hydrodynamic response of subduction zones to seismic activity: A case study for the Costa Rica margin     

Paula A. Cutillo  Shemin Ge  Elizabeth J. Screaton   《Tectonophysics》2006,426(1-2):167

Thermal anomalies in tectonically active areas are often attributed to sub-seafloor fluid circulation and faulting mechanisms, particularly in subduction zones where the largest thrust earthquakes occur. Postseismic fluid flow is enabled by the poroelastic response of the fault system to the earthquake's strain field, as well as by the rupturing of permeability barriers in the vicinity of the fault zone. We investigated the relative importance of these mechanisms on postseismic pore-pressure diffusion and advective heat transport in the subduction zone setting. A two-dimensional numerical fluid flow and heat transport model was developed for the Costa Rica subduction zone offshore of the Nicoya Peninsula. The flow and transport model was coupled with an earthquake strain model to quantify the effects of coseismic strain and permeability enhancement on fluid pressures and temperatures within the Costa Rica margin. Coseismic changes in pore pressure and postseismic pore-pressure diffusion were found to be sensitive to the compressibility of the porous medium, and patterns of pore-pressure recovery were more complex than that predicted by theoretical faulting models. Coseismic contraction and extension of the crust produced high fluid pressures close to the fault, while the inflow of fluid from depth increased fluid pressures several years following the simulated fault slip. Crustal deformation alone was not observed to perturb the temperature field. Laterally extensive permeability increases of two orders of magnitude along the décollement were required to produce small changes in heat flow. Local permeability changes in the upper slope region of least five orders of magnitude were necessary to noticeably affect heat flow. The results of the numerical simulations may help to refine conceptual faulting models and provide guidance for locating long-term hydrologic monitoring sites at Costa Rica and other subduction zones.  相似文献   

Ca-Fe-Mg olivines: phase relations and a solution model     

Paula M. Davidson  Dilip K. Mukhopadhyay 《Contributions to Mineralogy and Petrology》1984,86(3):256-263

Reversed phase equilibrium experiments in the system (Ca, Mg, Fe)₂SiO₄ provide four tielines at P?1 bar and 1 kbar and 800° C–1,100° C. These tielines have been used to model the solution properties of the olivine quadrilateral following the methods described by Davidson et al. (1981) for quadrilateral clinopyroxenes. The discrepancy between the calculated phase relations and the experimentally determined tielines is within the uncertainty of the experiments. The solution properties of quadrilateral olivines can be described by a non-convergent site-disorder model that allows for complete partitioning of Ca on the M2 site, highly disordered Fe-Mg cation distributions and limited miscibility between high-Ca and low-Ca olivines. The ternary data presented in this paper together with binary solution models for the joins Fo-Mo and Fa-Kst have been used to evaluate two solution parameters: $$\begin{gathered} F^0 \equiv 2(\mu _{{\rm M}o}^0 - \mu _{{\rm K}st}^0 ) + \mu _{Fa}^0 - \mu _{Fo}^0 = 12.660 (1.6) kJ, \hfill \\ \Delta G_*^0 \equiv \mu _{{\rm M}gFe}^0 + \mu _{FeMg}^0 - \mu _{Fo}^0 - \mu _{Fa}^0 = 7.030 (3.9) kJ. \hfill \\ \end{gathered} $$ . Ternary phase quilibrium data for olivines tightly constrain the value of F⁰, but not that for ΔG _* ⁰ which describes nonideality in Fe-Mg mixing. From this analysis, we infer a function for the apparent standard state energy of Kst: $$\begin{gathered} \mu _{{\rm K}st}^0 = - 102.79 \pm 0.8 - (T - 298)(0.137026) \hfill \\ + (T - 298 - T1n(T/298))(0.155519) \hfill \\ + (T - 298)^2 (2.8242E - 05)/2 \hfill \\ + (T - 298)^2 (2.9665E + 03)/(2T(298)^2 ) kJ \hfill \\ \end{gathered} $$ where T is in Kelvins and the 298 K value is relative to oxides.  相似文献   

Anthropogenic and climatic influences on the diatom flora within the Fallen Leaf Lake watershed,Lake Tahoe Basin,California over the last millennium     

Briana E. Johnson  Paula J. Noble  Alan C. Heyvaert  Sudeep Chandra  Robert Karlin 《Journal of Paleolimnology》2018,59(2):159-173

This study addresses the effects of climate, land-use, and atmospheric nitrogen (N) deposition on the Fallen Leaf Lake watershed, Lake Tahoe Basin, through diatom and geochemical analyses of sediment cores. Four diatom zones are recognized from a core taken at Fallen Leaf Lake (FLL), a site moderately impacted by human activities: (1) Pre-Little Ice Age Zone (840–1385), (2) Little Ice Age Zone (1385–1810) characterized by Stephanodiscus alpinus and Aulacoseira subarctica, (3) Transitional Zone (1810–1950) of warming and anthropogenic influence with increased Lindavia rossii-ocellata group and Discostella stelligera and decreased Pseudostaurosira brevistriata, and (4) Anthropogenic Zone (1950–2010) characterized by a rapid increase of mesotrophic diatoms of the Fragilaria tenera-nanana group, Tabellaria flocculosa strain IIIP, and Nitzschia gracilis. The Transitional Zone increases in elemental Co, Zn, and Sn that may be attributed to an increase in coal burning and smelting activities in California and Nevada. Beginning around 1910 and accelerating in the 1940s, increased building, land-use, and recreation around FLL caused an increase in terrestrial sedimentary input. Down core proxies for atmospheric N deposition in the FLL watershed are, at best, weakly expressed and appear to be overshadowed by stronger signals. Lack of support for N deposition includes the asynchrony in the appearance of the N-sensitive diatom Asterionella formosa in FLL and a lower impact site at Gilmore Lake, and an uninformative δ¹⁵N record. Asterionella formosa is a dominant component in the FLL water column today, but has been present in similar abundances for at least the last 1200 years. Asterionella formosa is present in the water column at Gilmore Lake and absent from the sediment, indicating a very recent appearance. The data collected show that the FLL record is sensitive to climatic cooling during the Little Ice Age and to anthropogenic activities commencing in the 1800s that increased throughout the latter half of the twentieth century; however the effects of anthropogenic N deposition in these lakes could not be substantiated.  相似文献   

Source Parameters of Old Earthquakes: Semi-Automatic Digitization of Analog Records and Seismic Moment Assessment     

P. Teves-Costa  J. F. Borges  I. Rio  R. Ribeiro  C. Marreiros 《Natural Hazards》1999,19(2-3):205-220

Recovering seismic information contained in old analog records could increase our knowledge of seismic source characteristics and the seismicity of a region. This is particularly important in zones with low to moderate seismicity.To extract the available information, it is necessary to digitize the seismic records. This is not an easy task especially owing to the generally poor quality of the original records, with illegible or missing parts. However some exceptions were found in the records of a few seismological stations in Germany, Sweden and France.This paper presents an example of the recovering of source parameters and can be divided in two parts: the first one presents a simple semi-automatic technique for digitization of old analog seismic records, developed using commercial software on a PC; the second part describes the methodology of assessing the seismic moment, using empirical and theoretical relations, as well as the seismic source dimensions.The earthquake selected to illustrate this procedure is the 23 April 1909 Benavente (Portugal) earthquake. This earthquake occurred in the Lower Tagus Valley region and caused great destruction in the meizoseismal area. It is the biggest earthquake that occurred during this century in the central part of the country and its magnitude has been estimated between 6.6 and 7.6.The digitization procedure allowed the recovery of seismic information contained in old analog records, in particular, the seismic moment estimation. The results obtained indicated that 7.6 was a very high value for the magnitude of the 23 April 1909 earthquake, suggesting that the magnitude reported in the Portuguese catalogue is overestimated. The estimated moment magnitude is 6.0.  相似文献   

Systematic increase in model complexity helps to identify dominant streamflow mechanisms in two small forested basins     

Paula C. David  Debora Y. Oliveira  Fernando Grison  Masato Kobiyama 《水文科学杂志》2019,64(4):455-472

This study shows how the use of increasing model complexity allows us to hypothesize about dominant streamflow mechanisms in two small Brazilian forested basins. Nine different structures from SUPERFLEX, an objective framework to systematically increase hydrological model complexity, were tested and we extended the flexible modelling methodology to error models as well. We show that applying a rigorous methodology in a model evaluation framework, with residual analysis and control of model complexity, is essential for testing a model as a hypothesis for dominant hydrological controls. Our results indicate that the model architecture was more important than the increase in the number of model parameters. Better performing models were those with a parallel structure, which confirms our a priori belief about the dominant runoff mechanisms of the studied catchments, characterized by a rapid response to rainfall, but also a constant river discharge fed by water storage on the thick soil layer.  相似文献   

Decadal-scale changes in suspended wood after riparian recruitment in managed stands in headwater streams of coastal British Columbia,Canada     

Felipe Rossetti de Paula  John S. Richardson  Alex C.Y. Yeung  Stephen J. Mitchell  Devesh Bahuguna 《地球表面变化过程与地形》2020,45(9):1974-1989

Large wood (LW) affects several ecological and hydrogeomorphic processes in streams. The main source of LW is riparian trees falling inside channels. However, in confined valley floors, falling trees are more likely to be suspended above the channel. Eventually, these suspended trees will decompose and break to finally fall into the channel to better provide functions for streams. We evaluated changes in wood decay, length, diameter, and suspended status (suspended or non-suspended) 17 years post-harvest and nine years after the first sampling occurred in 2006 in 12 headwater streams of coastal British Columbia, Canada. We also evaluated whether changes differed among riparian management treatments (no-harvest buffers of 10 and 30 m in width, thinning, and unharvested reference sites), and identified the factors affecting wood changes and suspended status. Wood pieces advanced in decay, became shorter, and 34% of them (n = 108) changed status from suspended to non-suspended. Non-suspended wood pieces were more decayed and shorter than suspended wood. Suspended wood was longer, thicker, less decayed, and represented 46.5% (n = 147) of the wood sampled in 2006. Our findings revealed limited influences of riparian management on many aspects of wood changes considered in this study. Changes in wood characteristics were more likely for pieces that were smaller in diameter, longer, and suspended closer to the water. The transition from suspended to non-suspended LW can be a long-term process that can increase wood residence time and reduce LW in-stream functions particularly in confined stream valleys. The suspended stage is also an important mechanism underlying time lags in stream ecosystem responses to riparian tree fall. © 2020 John Wiley & Sons, Ltd.  相似文献