Transition from mildly-tholeiitic to calc-alkaline suite: the case of Chichontepec volcanic centre, El Salvador, Central America     

Silvio G. Rotolo  Francesca Castorina 《Journal of Volcanology and Geothermal Research》1998,86(1-4)

The Chichontepec volcano is a Plio-Pleistocene composite volcano that erupted lavas ranging from high-alumina basalts to dacites. It experienced a caldera-forming paroxysmal eruption during the early Pleistocene. Pre-caldera lavas are mildly tholeiitic and they evolved mainly by low pressure crystal fractionation, notwithstanding the fact that most mafic lavas (low-MgO high-alumina basalts) retain traces of polybaric evolution. Conversely, post-caldera lavas, which are mainly pyroxene andesites, are clearly calc-alkaline, having evolved by open-system crystal fractionation. Sr–Nd isotopic data and trace elements characteristics indicate that the same mantle source was involved in the petrogenesis of these series. Modelling the AFC process showed that it did not play any role in the petrogenesis of these rocks; a crystal fractionation model is considered to be more relevant. A slight variation in the fractionating assemblage could have caused the transition from an early mildly tholeiitic trend to a late calc-alkaline one. Mineralogical evidence, mass-balance calculations and elemental chemistry support this hypothesis, assuming that the greater amount of pyroxene on the liquidus is at the expense of plagioclase; this would have prevented the trend in iron enrichment.  相似文献   

Microdeformation of lacustrine laminite sequences from Late Miocene formations of SE Spain: an interpretation of loop bedding   总被引：2，自引：0，他引：2  

Jose P. Calvo  Miguel Rodriguez-Pascua  Silvia Martin-Velazquez  Sergio Jimenez  & Gerardo De Vicente 《Sedimentology》1998,45(2):279-292

Lacustrine laminated sediments (laminites) present in Late Miocene formations of the Híjar Basin, SE Spain, display well developed loop bedding, a structure consisting of bundles of laminae that are sharply constricted at intervals, giving a morphology of loops or links of a chain. The laminite sequences, which are interbedded with turbidite marlstones, were accumulated on the bottom of a permanently stratified lake developed in a rapidly subsiding basin limited by 010° and 105° normal faults. As deduced from both macro- and microdeformational analyses, the basin evolved under an extensional stress field throughout the Late Miocene. Four main types of loops, simple and complex loops with subcategories, have been recognized within the laminite sequence. Simple loops of type 1 show the best definite pattern, quite similar to ' pinch and swell structures ', a type of boudinage typical of stretching of alternating beds where the competence contrast is not strongly marked. The remaining loop types display contortion and occasional breakage of laminae (microfaulted edges) indicative of microdeformation near the boundary between the ductile-brittle deformational fields. The distribution of the various loop types across the laminite sequence reflects an interplay between progressive lithification of the laminites as sedimentation progressed and tectonic stresses which affected the sediment sequence. Accordingly, a mechanism of deformation under an extensional stress field, ultimately related to the creep movement of the main basin faults which resulted in successive seismic shocks of low magnitude, is proposed to explain the formation of loop bedding in the laminites.  相似文献   

Geophysical Discovery of a New LNAPL Plume at the Former Wurtsmith AFB, Oscoda, Michigan     

Jose L. Bermejo  William A. Sauck  Estella A. Atekwana 《Ground Water Monitoring & Remediation》1997,17(4):131-137

Alight nonaqueous phase liquid (LNAPL) ground water contaminant plume has been discovered by purely geophysical means at the former Wurtsmith Air Force Base (AFB) near Oscoda, Michigan. It is located near another plume called FT-02, which is a well-studied area undergoing natural bioremediation. The plume was discovered by ground penetrating radar (GPR) profiling while extending a long line from FT-02 to establish background variability around that plume. The new plume was apparent because of a high-conductivity "shadow' or GPR reflection attenuation observed below the conductive zone at the top of the aquifer, identical to the pattern observed at the FT-02 plume. Further GPR surveys were conducted by students of a Western Michigan University geophysics field course to outline the proximal part of the plume. The GPR survey was supplemented by an electromagnetic induction (EM) survey which showed a group of four cables crossing the area. Finally, a magnetometer survey was conducted to search for any buried steel objects which might have been missed by the EM survey. The results of the three geophysical surveys were then used by students of a University of Michigan field course to guide subsurface soil and fluid sampling, which verified the presence of residual LNAPL product and ground water with conductivities 2.5 to 3.3 times above background. The plume source is in the vicinity of a vaulted underground storage tank (UST) formerly used for the collection of waste solvents and fuels for subsequent use in the fire training exercises at FT-02. This newly discovered LNAPL plume, along with other "mature' plumes, fits the electrical model which predicts conductive ground water below the decomposing but electrically resistive LNAPLs. Finally, this is a fine example of the cooperative use of a dedicated research site for training by students of two different universities.  相似文献   

Development and erosion in the Brazilian Amazon: A geochronological case study     

Dr. Bruce Forsberg  Dr. Jose Marcus Godoy  Dr. Reynaldo Victoria  Dr. Luiz Antonio Martinelli 《GeoJournal》1989,19(4):399-405

A ²¹⁰Pb chronology of sedimentation in a small floodplain lake in the Jamari river basin (Rondonia, Brazil) provided insight into the relationship between development and erosion in the Brazilian Amazon. Positive correlations between increased sedimentation in the lake and levels of deforestation and mining in the Jamari basin suggested a major increase in erosion associated with these activities. This increase in erosion could diminish agricultural yields in the basin and significantly reduce the useful life of a hydro-electric plant recently built on the river.  相似文献   

Geology and tectonics of the basin of Mexico and their relationship with the damage caused by the earthquakes of September 1985     

Ricardo Jose Padilla Y Sanchez 《Geotechnical and Geological Engineering》1989,7(1):17-28

Summary The earthquakes of 19 September 1985 (18.2° N, 102.7° W and a magnitude of 8.1 Richter scale) and of 20 (17.6° N, 101.8° W and a magnitude of 7.5 Richter scale) September 1985, caused the total or partial destruction of more than 2000 structures in Mexico City. The most affected areas are located along the fringes of and bordering old roadways, earthworks (dikes), aquaducts and pre-hispanic population centres. Ancient construction artificially modified the sedimentation in the basin of the Mexico Valley Lakes making the sub-soil of Mexico City more rigid near to the surface, and producing deviations of the surface seismic waves (Rayleigh waves and Love waves). Also, when earthquakes occur on the Pacific coast, seismic waves travel quickly through plutonic, metamorphics and continental and marine rocks of different ages, having high seismic velocities. When the seismic waves enter the poorly consolidated lake sediments having low seismic velocities in the Mexico City Basin, they produce an energy buildup that causes the phenomenon called magnification.There exists a direct relation between the amplification mentioned above and the presence of rigid bodies that are buried in the sub-soil. The length of these bodies is of the order of tens of kilometres horizontally with thicknesses less than 50 metres. These Rigid Barriers produce reflections and refractions of the surface waves along their borders with destructive consequences for the buildings. A correlation between the buildings and the houses damaged and destroyed and the location of the prehispanic construction on the sub-soil has been made which shows that the most damage happened in the borders of old roadways (i.e. Tlalpan road), perimeter walls (i.e. San Lazaro), aqueducts (i.e. Chapultepec Avenue), pyramids (i.e. Templo Mayor) and population centres (i.e. Tlaltelolco).  相似文献   

Early Paleozoic Structural and Metamorphic Evolution of Western Sierra de San Luis (Argentina), in Relation to Cuyania Accretion     

Pablo Diego GonzAlez  Ana Maria Sato  Eduardo Jorge Llambias  Miguel A.S. Basei  Silvio R.F. Vlach 《Gondwana Research》2004,7(4):1157-1170

Structural, metamorphic and isotopic data obtained from the Nogoli Metamorphic Complex of western Sierra de San Luis indicate that the Early Paleozoic Famatinian Orogeny overprinted an already structured and metamorphosed older basement. The older geological features are relict NW trending fabric associated with high-grade (amphibolite facies) regional metamorphism preserved within thin strips of schists and paragneisses and in the core of mafic to ultramafic lenses. Arc magmatism, medium P (Barrovian type)/high T (amphibolite to granulite facies) regional metamorphism and penetrative NNE to NE trending foliation are related to the building of the Famatinian orogenic belt. The P-T conditions of the Famatinian prograde metamorphism reached a pressure peak of ca. 8 kb, with a thermal peak from -750°C up to -820°C. U-Pb conventional and chemical dating and Ar-Ar plateau ages constrain the peak of the main orogenic phase related to the Famatinian belt to 470–457 Ma (Early to Mid-Ordovician). Greenschist facies retrograde metamorphism closely associated with shear zones and secondary Ar-Ar plateau and Sm-Nd ages suggest that a late to post-orogenic phase of the Famatinian belt was active at least since -445 Ma. This phase continued during the Silurian to Late Devonian times through multiple reactivation of early shear zones. The Famatinian Orogeny reset a previous thermal history and therefore, the timing of the relict fabric could not be constrained conclusively with radiometric dates. Despite this difficulty, a range of 520 to 490 Ma suggests some inheritance from Pampean events registered by the older NW-SE fabric. The Early to Mid-Ordovician regional metamorphism and ductile deformation of the western Sierra de San Luis is interpreted as the orogenic effects of the collision of the allochthonous Cuyania terrane with the autochthonous proto-Pacific margin of Gondwana during the Famatinian Orogeny.  相似文献   

Photoelectrons in the upper atmosphere: A formulation incorporating effects of transport     

Michael J. Prather  Michael B. McElroy  Jose Rodriguez 《Planetary and Space Science》1978,26(2):131-138

An efficient scheme is described for computation of photoelectron energy spectra. The method, similar to one used in radiative transfer theory, incorporates effects due to spatial and angular redistribution of electrons. These effects play a dominant role at heights above 400 km.  相似文献   

Cathodoluminescence petrography and isotope geochemistry of KT impact ejecta deposited 360 km from the Chicxulub crater, at Albion Island, Belize     

Bruce W. Fouke  Aubrey L. Zerkle  Walter Alvarez†  Kevin O. Pope‡  Adriana C. OcampO§  Richard J. Wachtman  Jose Manuel Grajales Nishimura¶  Phillipe Claeys  & Alfred G. Fischer†† 《Sedimentology》2002,49(1):117-138

The depositional and diagenetic history of Cretaceous–Tertiary (KT) impact ejecta deposited 360 km from the Chicxulub crater, at Albion Island, Belize, has been investigated using integrated cathodoluminescence and isotopic analyses. A quarry exposes 26 m of Upper Cretaceous Barton Creek Formation dolomitized marine limestone overlain by 16 m of dolomitized Albion Formation impact ejecta. The Albion Formation consists of a lower fine‐grained ≈1‐m‐thick spheroid bed and an upper 15‐m‐thick coarse conglomeratic diamictite bed. A 14‐event paragenetic sequence has been documented and used as a temporal framework to interpret chemostratigraphic trends in bulk rock δ¹⁸O, δ¹³C and ⁸⁷Sr/⁸⁶Sr. The uppermost surface of the Barton Creek Formation was subaerially exposed before the KT impact, as indicated by a brecciated palaeosol that caps upsection decreases in δ¹³C and δ¹⁸O. Small 1‐cm‐diameter spheroids in the spheroid bed exhibit vermicular crystalline textures but lack the concentric zonations common to accretionary lapilli. These spheroids are hypothesized originally to have been impact glass or reactive Ca and Mg oxide dusts that adhered to water vapour particles condensing from the cooling impact vapour cloud. The spheroids were dolomitized soon after deposition. The earliest dolomitization in the matrix sediments of the Albion Formation was also post‐depositional, replacing clays formed by devitrification of impact glass. Dolomite and clay ⁸⁷Sr/⁸⁶Sr exhibit a distinct symmetrical distribution in the spheroid bed ranging from 0·707745 to 0·707872. Although unproven, this may represent primary changes in the chemical composition of the impact glass. The limestone clasts in the diamictite bed were dolomitized before the KT impact and exhibit upsection decreases in bulk rock ⁸⁷Sr/⁸⁶Sr. This suggests that the clasts were excavated from strata equivalent in age or older than the Barton Creek Formation at locations closer to, or in, the Chicxulub crater.  相似文献   

Remote Sensing Of Three-Dimensional Winds With Elastic Lidar: Explanation Of Maximum Cross-Correlation Method     

William T. Buttler  Cecilia Soriano  Jose M. Baldasano  George H. Nickel 《Boundary-Layer Meteorology》2002,103(1):173-174

Maximum cross-correlation provides a method to remotely determine highly resolved three-dimensional fields of horizontal winds with elastic lidar throughout large volumes of the planetary boundary layer (PBL). This paper details the technique and shows comparisons between elastic lidar winds, remotely sensed laser Doppler velocimeter (LDV) wind profiles, and radiosonde winds. Radiosonde wind data were acquired at Barcelona, Spain, during the Barcelona Air-Quality Initiative (1992), and the LDV wind data were acquired at Sunland Park, New Mexico (N.M.), during the Border Area Air-Quality Study (1994). Comparisons show good agreement between the different instruments, and demonstrate the method useful for air pollution management at the local/regional scale. Elastic lidar winds could thus offer insight into aerosol and pollution transport within the PBL. Lidar wind fields might also be used to nudge or improve initialization and evaluation of atmospheric meteorological models.  相似文献   

Impact of a Saturation-Dependent Dissipation Source Function on Operational Hindcasts of Wind-Waves in the Australian Region     

Jose Henrique G. M. Alves  Diana J. M. Greenslade  Michael L. Banner 《Journal of Atmospheric & Ocean Science》2002,8(4):239-267

The impact of a modified parametric form of the dissipation source function S ds on the skill of an operational spectral wind-wave model is investigated. Numerical experiments are made with a version of the WAM model presently used operationally by the Australian Bureau of Meteorology to produce sea-state forecasts within the Australian region. Performances of wave hindcasts obtained using several alternative forms of this newly proposed dissipation source function are compared to those produced with forms of S ds commonly used operationally. A new ad hoc technique to compare modelled and observed wave spectra is introduced to assess the quality of calculated one-dimensional frequency spectra.

Our results indicate that wave model performance in terms of integral spectral parameters, such as the significant wave height H s , may benefit from improved parameterisations of the dissipation source term S ds . On the other hand, we also found that model performance was relatively poor in terms of predictions of the one-dimensional frequency spectrum, regardless of the chosen form of S ds . These results suggest that further refinements of the dissipation source term are strongly dependent on improved parametrisations of the nonlinear interactions source term S nl .  相似文献