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991.
The Sultepec-Goleta area in southern Mexico hosts one of the major Eocene silicic volcanic centers that make up the Cenozoic
volcanic record of the north-central part of the Sierra Madre del Sur. This center is represented by a partially exhumed NNE-SSW
trending ignimbritic field that covers an area of ∼400 km2 with a preserved volume of ∼200 km3. A remarkable feature of volcanic center is the exposure of pyroclastic dike complexes up to 1 km wide that extend almost
continuously along the western and southern flanks of the volcanic field. The ignimbritic record comprises four units with
different textural features as well as variable proportions of the main components (i.e. phenocrysts, pumice and lithic clasts).
The most extensive unit is the basal Goleta Ignimbrite, of which the middle and upper members are richer in phenocrysts than
the lower member. It has a thickness that ranges from ∼200 m in the north to at least 600 m in the south of the volcanic center.
The overlying ignimbrite units (Cienega, Lobera and Potrero ignimbrites) are phenocryst-poor and are distributed in the northern
sector of the volcanic field. The semi-curvilinear trend of the pyroclastic dikes along the southern flank of the Goleta Range,
coupled with the greater thickness of the Goleta Ignimbrite encircled by the dikes, is indicative of the development of a
partial-collapse caldera. Based on spatial relationships as well as analogies in the nature and abundances of the components,
it has been recognized that the central and southern dike complexes fed the relatively phenocryst-rich Goleta Ignimbrite,
whereas the phenocryst-poor units of the northern sector were extruded through the pyroclastic conduits distributed around
Sultepec. We suggest that the stirring produced by the partial collapse of the magma chamber roof beneath the southern sector
of the study area triggered the tapping of phenocryst-rich portions of the zoned magma chamber, resulting in an increase of
phenocryst content in the upper members of the Goleta Ignimbrite. The presence of breccias and lithic-rich facies dominated
by wallrock fragments at the borders of the pyroclastic dikes, including those of the northern extra-caldera sector, indicate
that erosion of the conduit walls above the fragmentation level contributed to widening of the conduits and the construction
of wide dike complexes. 相似文献
992.
Teresa Grabowska Grzegorz Bojdys Miroslav Bielik Kristián Csicsay 《Acta Geophysica》2011,59(3):526-560
The paper presents 2D density and magnetic models of the crust and upper mantle along the DSS line profile of the CELEBRATION
2000 project that crosses the most important geological units in Central Europe. These are the Alps-Carpathians-Pannonian
(ALCAPA) region, the SE part of the Paleozoic Platform (PLZ), the Trans-European Suture Zone (TESZ) and a fragment of the
SW portion of the East European Craton (EEC). The density and magnetic models were constructed on the basis of a 2D model
of P-wave velocity converted into density model, geological data as well as geothermal data and the results of integrated geophysical
modelling for the lithosphere-asthenosphere boundary. 相似文献
993.
Germán Aguilar Rodrigo Riquelme Joseph Martinod José Darrozes Eric Maire 《地球表面变化过程与地形》2011,36(13):1736-1748
We quantify erosion rates in the higher sectors of the Huasco Valley, in the Main Cordillera of the semi‐arid Andes of Chile, using elevation differences between three successive geomorphic markers (pediments and paleo‐valleys) and the present day valley. Available Ar‐Ar ages of Neogene pediments are used to estimate mean erosion rates for the three periods (16 to 13 My, 13 to 8 My, and following 8 My). The landscape of the Huasco Valley is in a transient state, as indicated by well‐preserved pediment surfaces in interfluves, valleys deeply incised by fluvial and glacial erosion and scarped head‐valleys that represent the current knickzones. Higher erosion rates (45–75 m/My) are calculated for the more recent period (< 8 My) during which deep incision developed compared to previous periods (6–31 m/My). Quantitative data indicate that glaciers had a much higher erosional capability than fluvial activity in the higher sectors of the Main Cordillera. Comparison with erosion rates calculated in other drainage basins of the Chilean Andes suggests that the variability of erosion rates depends on the landscape's transient erosive state. The landscape's geomorphologic response to the uplift of the Main Cordillera results in the retreat of a knickzone, for which retreat velocity depends on precipitation rate pattern and glacial erosion intensity. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
994.
Diana Prado Garzón Christiano G.M. Brum Ezequiel Echer Néstor Aponte Michael P. Sulzer Sixto A. González Robert B. Kerr Lara Waldrop 《Journal of Atmospheric and Solar》2011,73(11-12):1568-1574
We analyze the data obtained using the Arecibo incoherent scatter radar to examine the response of the topside ionosphere to a moderate geomagnetic storm that occurred during the period March 7–11, 2008. During this time period a magnetic storm with a non-monotonic main phase decrease in the Dst index occurred. The recovery phase also exhibited a secondary Dst decrease. During the initial phase of the storm, Te and Ti increased coincident with the arrival of the solar wind. The main phase registered an increase in proton concentration proportional to Ne while temperatures reached the lowest values. Variations in O+ concentration were not significant but a reduction in helium fraction was observed. Soon after the peak of the storm, the transition height between the topside ionosphere and the protonosphere, where H+ ions dominate composition, was lower than would be expected during quiet conditions and this behavior lasted for approximately 12 h. 相似文献
995.
996.
Idael Francisco Blanco-Quintero C. Lázaro A. García-Casco J. A. Proenza Y. Rojas-Agramonte 《Contributions to Mineralogy and Petrology》2011,162(2):395-413
Whole-rock compositions of muscovite-bearing amphibolite, trondhjemite, pegmatite and quartz-muscovite rocks from Sierra del
Convento and La Corea mélanges (eastern Cuba), as well as mineralogy, record complex circulation of Ba-rich fluids and melts
in the subduction environment. Partial melting of fluid-fluxed, MORB-derived amphibolite produced trondhjemite magmas that
crystallized at depth, in some cases evolving into pegmatites. Qtz-Ms rocks probably crystallized from primary fluids derived
from subducted sediments. All these rocks have elevated concentrations of large-ion lithophile elements, especially Ba (up
to 11,810 ppm), presumably released from slab sediments by fluids and/or melts. Fluid–rock interaction produced crystallization
of phengite in parental amphibolites. The phengite crystallized in all types of rocks is rich in Ba, with concentric zoning,
characterized by Ba-rich cores and Ba-poor rims, indicating a compatible behaviour of Ba in the studied systems. Zoning in
phengite is governed primarily by the celadonite (tschermak) exchange vector ((Mg,Fe)Si-(Al)-2), with more moderate contributions of the celsian (BaAl-(KSi)-1) and paragonite (NaK-1) exchange vectors. Late remobilization of Ba at relatively low temperature formed retrograde celsian. The compatible behaviour
of Ba in the studied rocks strengthens the importance of the stability of phengite for the transfer of LIL elements from the
subduction to the volcanic arc environments. 相似文献
997.
The paper presents a time-dependent 2D numerical model which has been developed with the purpose of highlighting the effects of the slope foot evolution on the behaviour of slow-moving landslides. The model allows to quantitatively analyse how foot mass variations can influence the stability and the movement rates of the landslide.The landslide body is modelled as composed of two rigid blocks sliding on two different planes and interacting through a common boundary, which position is assumed fixed during the analysis. A finite difference approach is used to discretize the time. For each time increment, changes in model parameters are allowed, including variations in shearing resistances, groundwater level and block masses (in order to simulate foot erosion). During each iteration, the overall stability of the system is checked computing the safety factor, assuming that the percentage of mobilized shear strength is the same for the three surfaces. If the system is not stable or if it is not at rest, the velocity of the system is computed solving the momentum equations for the two blocks taking into account destabilising and resisting forces, the mechanical interaction between the blocks and viscous components. Computed velocities are iteratively used to compute mass accumulation at the foot, which in turn results in a change of the stability condition of the system.Examples are provided in order to highlight the roles of the model parameters. Finally, the application to a real landslide (Vallcebre, Spain) is presented which shows the advantage of considering the foot mass in the analysis of the displacement trends and the possibility to take into account the foot erosion in the long-term behaviour of the slope. 相似文献
998.
Mourad Essalhi Stanislas Sizaret Luc Barbanson Yan Chen France Lagroix François Demory José M. Nieto Reinaldo Sáez M. Ángeles Capitán 《Mineralium Deposita》2011,46(8):981-999
In the Rio Tinto district of the Iberian Pryrite Belt of South Spain, the weathering of massive sulfide bodies form iron caps,
i.e., true gossans and their subsequent alteration and re-sedimentation has resulted in iron terraces, i.e., displaced gossans.
To study the stucture and evolution of both types of gossans, magnetic investigations have been carried out with two foci:
(1) the characterisation and spatial distribution of magnetic fabrics in different mineralised settings, including massive
sulfides, gossans, and terraces, and (2) paleomagnetic dating. Hematite has been identified as the suceptibility carrier in
all sites and magnetic fabric investigation of four gossans reveals a vertical variation from top to bottom, with: (1) a horizontal
foliation refered to as “mature” fabric in the uppermost part of the primary gossans, (2) highly inclined or vertical foliation
interpreted as “immature” fabric between the uppermost and lowermost parts, and (3) a vertical foliation interpreted to be
inherited from Hercynian deformation in the lowermost part of the profiles. In terraces, a horizontal foliation dominates
and is interpreted to be a “sedimentary” fabric. Rock magnetic studies of gossan samples have identified goethite as the magnetic
remanence carrier for the low-temperature component, showing either a single direction close to the present Earth field (PEF)
direction or random directions. Maghemite, hematite, and occasionally magnetite are the remanence carriers for the stable
high-temperature component that is characterized by non PEF directions with both normal and reversed magnetic polarities.
No reliable conclusion can be yet be drawn on the timing of terrace magnetization due to the small number of samples. In gossans,
the polarity is reversed in the upper part and normal in the lower part. This vertical distribution with a negative reversal
test suggests remanence formation during two distinct periods. Remanence in the upper parts of the gossans is older than in
the lower parts, indicating that the alteration proceeded from top to bottom of the profiles. In the upper part, the older
age and the horizontal “mature” fabric is interpreted to be a high maturation stage of massive sulfides’ alteration. In the
lower part, the age is younger and the inherited “imature” vertical Hercynian fabric indicates a weak maturation stage. These
two distinct periods may reflect changes of paleoclimate, erosion, and/or tectonic motion. 相似文献
999.
259 stream water and 21 bulk precipitation samples have been studied between April 2008 and December 2010 in order to assess baseline hydrochemical conditions in a rural area of NW Spain. Stream waters are related to five nested catchments (<10 km2) with apparent low mean residence times. Their hydrochemical characteristics are closely related to bulk precipitation although clear evidences of seasonal (flushing, dilution, sediment desorption, etc.), lithological (schist vs. granite substrate) and anthropogenic (land use, contamination) effects are also described. 相似文献
1000.