Ion-microprobe U–Pb zircon dating of lower-crust metasedimentary granulite are reported on samples from two localities in Europe in order to determine (a) how this environment recorded the Variscan and eo-Alpine events, and (b) whether the transition between the two orogenic cycles was continuous or separated by a gap. The samples come from enclaves hosted by Miocene volcanoes at Bournac in the French Massif Central, and from the granulitic metasedimentary basement of the Alpine Santa Lucia nappe in Corsica, on the South European paleomargin of the Ligurian branch of the Tethys Sea. The zircon ages from Bournac range between 630 and 430 Ma and between 380 and 150 Ma with a major frequency peak at 285 Ma; the zircons older than 430 Ma are interpreted as detrital, whereas those younger than 380 Ma are considered to have formed by metamorphic processes after burial in the lower crust. Zircon ages from Santa Lucia range from to 356 to 157 Ma, with exception of one inherited Archean grain, and are interpreted like the younger Bournac zircons as having been formed by metamorphic processes.
In a granulite metamorphic environment, as opposed to an anatectic environment, new zircon growth can occur in the solid state. Once Zr has been incorporated into zircon, however, it is difficult to remobilize without dissolution; thus Zr available for new zircon growth must result from the breakdown of Zr-bearing minerals during prograde and/or retrograde events. In this light, the U–Pb zircon-age probability curves are interpreted as markers for major tectonometamorphic events, as suggested by the close correspondence between peaks in the curve and geological events recorded in the upper-crust, such as magma emplacement and basin subsidence.
Evidence of a tectonometamorphic gap between the Variscan and Alpine orogeneses is provided by the Santa Lucia zircon-age probability curve, which reveals a probable interlude during the Variscan–Alpine transition between 240 and 210 Ma. Here, the peak at 240 Ma is interpreted as the very beginning of crustal extension and the low at 210 Ma as a period of quiescence prior to the formation of an active margin and oceanization. 相似文献
Based on the cost-benefit data (1980-2002) of farm products and China Agriculture Yearbooks, this paper studies the regional disparity in the changes of the agricultural land use in China during the period 1980-2002 from three aspects such as the degree of intensity, the sown area and the abandoned farmland. The results show that: (1) The degree of intensity of land use in the westena region during 1980-2000 has a strong uptrend, but in the eastern and central regions the degree of intensity descends obviously and has shown a continuous downtrend since 1997. (2) The total sown area shrinks notably in the eastern region, while it enlarges constantly in the western region. (3) The sown area in the eastern, central and western regions has gone through a similar cyclic process: down (1980-1985)-up (1985-1991)-down (1991-1994)-up (1994-1999)-down (1999-2002). However, there are obvious differences in amplitude variation and tendency among them. The sown area has shrunk in the eastern region and expanded in the central and western regions especially before 1999. (4) The most cases of abandoned farmland are reported in the central region, the second in the eastern region and the least in the western region. The abandonment phenomena chiefly occurred during 1992-1995 in the eastern region, and during 1998-2002 in the central region. 相似文献
The paper deals with stress-release effects induced by man-made cuts or excavations into natural stiff clay slopes that experienced erosion in response to valley deepening. The study was focused on the Monte Mario hill in Rome (Italy), which formed part of an area of recent urban expansion. The methodology of the study relied on a reference engineering-geology model, which was developed on the basis of site and laboratory data and stress–strain analyses. The latter analyses were carried out with the finite-difference code FLAC 4.0. Numerical modelling was based on a sequential approach, taking into account the main evolutionary stages of the Tiber river valley in Romeȁ9s urban area and then making cuts at the bottom of the slope located south of the Monte Mario Astronomical Observatory. The simulation revealed the stress-release effects that fluvial erosion and excavation fronts have caused on the investigated slopes and their consequent gravitational instabilities. These processes appear with metre-scale displacements, followed by stress-release cracks (actually observed on the slopes under review). In quantifying stress-release deformations, the simulation took into account the possible role of creep in the observed retardation of stress-release effects.Research activities were carried on with the co-operation of Dȁ9Arcangelo A. (Consorzio TREESSE, andrea.darcangelo@libero.it) and Moretti S. (IMG S.r.l. Servizi Tecnici per lȁ9Ingegneria e lȁ9Ambiente, moretti@img-srl.it) 相似文献
The elastic properties of coexisting natural 3T and 2M1 phengite samples (Cima Pal, Sesia Zone; Val Savenca; Western Alps, Italy) with similar chemical compositions have been studied by room temperature–high pressure powder diffraction, using synchrotron radiation on the ID9A beam-line at ESRF (Grenoble, France). The P–V curves have been modelled by the Birch–Murnaghan model; a third-order expansion fitted to the experimental data yields for 3T and 2M1K0=60.4(±0.7) GPa, K′=5.79(±0.11) at V0=703.8851 Å3, and K0=57.3(±1.0) GPa, K′=6.97(±0.24) at V0=938.8815 Å3, respectively. The relative stability of 3T vs. 2M1 has been explored as a function of pressure and temperature in terms of configuration and deformation contributions to the Gibbs energy, using the elastic properties determined here and other thermodynamic parameters from earlier investigations. The results presented agree with the hypothesis of stability of the 3T polytype in the high pressure regime. 相似文献
Based on ammonites, Upper Kimmeridgian sediments are first established in the Crimean Mountains. The Kimmeridgian-Tithonian boundary recognizable in a continuous section is placed inside the Dvuyakomaya Formation of uniform largely clayey sediments. Assemblages of Kimmeridgian ammonites Lingulaticeras cf. procurvum (Ziegler), Pseudowaagenia gemmellariana Oloriz, Euvirgalithacoceras cf. tantalus (Herbich), Subplanites sp.) and Tithonian forms (?Lingulaticeras efimovi (Rogov), Phylloceras consaguineum Gemmellaro, Oloriziceras cf. schneidi Tavera, and Paraulacosphinctes cf. transitorius (Oppel) are described. A new biostratigraphic scheme proposed for the upper Tithonian-Berriasian of the Crimean Mountains includes the following new biostratigraphic units: the Euvirgalithacoceras cf. tantalus Beds of the upper Kimmeridgian, ?Lingulaticeras efimovi Beds of the lower Tithonian, and Oloriziceras cf. schneidi and Paraulacosphinctes cf. transitorius beds of the upper Tithonian. The middle Tithonian is proposed to consist of the fallauxi and semiforme (presumably) zones. The ammonities found determine the early Kimmeridgian-Berriasian age of the Dvuyakornaya Formation that is most likely in tectonic contact with the underlying Khutoran Formation. 相似文献