Showing papers in "Geofisica Internacional in 1985"

Volcanic stratigraphy of the guadalajara area, mexico

Abstract: Two ignimbrites of petrologic interest provide distinctive time horizons in the volcanic succession near Guadalajara. The older of the two, referred to as San Gaspar ignimbrite, is approximately 4.8 million years old and has the composition of dacitic andesite. The younger ignimbrite, called Guadalajara ignimbrite because of its use as the characteristic building stone in the city, is more siliceous and approximately 3.3 million yearsold. In the formation of both ignimbrites, two contrasting magmas were erupted simultaneously, for each rock contains glass fragments of two distinct compositions. In some fiamme, the two glasses are intricately intermixed. The San Gaspar ignimbrite is widespread, thin, firmly welded throughout, and characterized by numerous fiamme of porphyritic dark glass containing abundant phenocrysts of plagioclase, augite, hypersthene, hornblende and biotite. Dark glass (R. I.= 1.521) is the predominant phase in the rock, but all samples contain shards and small fiamme of colorless glass (R.l_.= 1.510) that contains approximately 5 percent more SIO₂ and significantly less CaO, MgO, and FeO than the dark glass. Microprobe analyses of ferromagnesian phenocrysts indicate pre-eruptive magma temperaturas of about 1000°C. Conversion of common hornblende to basaltic hornblende at the top of the ignimbrite signifies an emplacement temperature greater than 800°C. Late vesiculation of the larger fiamme, following compaction and welding, is characteristic. The Guadalajara ignimbrite is characterized by abundant fiamme of two distinct compositions. Vitric facies consist of two different glasses in nearly equal proportions, one colorless and aphyric, the other dark-colored with sparse phenocrysts of alkali feldspar. Generally, however, the ignimbrite has been completely. devitrified, the light-colored fiamme being axiolitic and the darker porphyritic fiamme cryptocrystalljne and vesicular. East and southeast of Guadalajara, the terrane beneath the San Gaspar ignimbrite consists largely of basaltic flows, but to the northwest rhyolitic rocks are widespread. An olivine basalt distinguished by megaphenocrysts of plagioclase overlies the ignimbrite north of Guadalajara and provides a recognizable horizon about 4 million years old. To the west, the younger Guadalaiara ignimbrite is overlain by siliceous ash flows and dacitic lavas about 3 million years old. Younger volcanism has been concentrated along a northwest-trending zone where activity has continued since late Pliocene time, culminating at Sierra La Primavera during the last 140 000 years with eruptions of high-silica rhyolite. Elsewhere the young eruptions have produced flow of basalt and basaltic andesite

Mislocation of mexican earthquakes as reported in international bulletins

Abstract: La comparacion de las localizaciones de temblores determinadas desde sismografos de campo o de estudios especiales con aquellos reportados en los boletines de PDE e ISC, sugieren que los temblores costeros superficiales en Mexico estan sistematicamente mal localizados en estos boletines. En general, los epicentros tienen un corrimiento de cerca de 35 km hacia N 350°- 45°E. ISC, con mayor frecuencia que PDE, consigna profundidades de foco basadas en fases de profundidad. Estas profundidades, aunque mayores que las estimadas mediante modelos sinteticos o datos de campo, son mas precisas que las de PDE. Sin embargo, por lo que concierne a la localizacion epicentral, hay poco que escoger entre los dos boletines. Un pequeno aumento en el numero de lecturas de estaciones mexicanas enviadas a PDE y ISC mejora aparentemente las localizaciones cerca de 10 km. Los errores en la localizacion se deben probablemente a la velocidad alta de la placa de Cocos bajo Mexico. Estos errores sistematicos deben de tomarse en cuenta en el uso de estos boletines.

Zoneamient0 composicional y mineralogic o en los productos eruptivos del centro volcanico de los humeros, puebla, mexico

Abstract: Los Humeros volcanic center, located 180 km east of Mexico City, is one of several silicic volcanic centers located in the “back-arc” portion of the Mexican Neovolcanic Belt. During the last 460 000 years, three major plinian eruptions and two major episodes of lava flow emplacement periodically “sampled" the magma Chamber. The eruptive products of these events are compositionally zoned and indicate that the magma chamber was zoned from rhyolitic upper-most levels to andesitic or perhaps even basaltic lower levels. Strong compositional zonation persisted throughout the lifetinre of the system. In general, the most silicio products of each plinian eruption are either aphyric or are characterized by the lowest phenocryst contents and sirnplest phenocryst assemblages (plagioclase, Fe-Ti oxides, and biotite or orthopyroxene). Clinopyroxene and olivine make their appearance in progressively more mafic products of each eruption. Ca/Na ratios of plagioclase and Mg/Fe ratios of the mafic phenocrysts increase as the eruptive products become more mafic. Phenocryst content increases progressively, reaches a maximum in rhyodacitic and andesitic compositions, and then decreases in basaltic andesite compositions. Changes in phenocry st assemblages and abundances are probably a consequence of superirnposed gradients in composition, temperature, and volatile content in the magma Chamber. Temperaturas calculated from Fe-Ti-oxide geothermometry range from 800 to 875°C for the upperrnost rhyolitic levels of the chamber, 860 to 940°C for the rhyodacitic portions, and 920 to 1000°C for the andesitic portions.

Sediment depth in the northern colima graben from 3-d interpretation of gravity

Abstract: The N-S trending Colima Graben is one of three large graben systems which intersect 50 km SSW of Guadalajara, Mexico. Results from a gravity survey conducted within the northern. part of the Colima Graben indicate that a 20 mgal Bouguer gravity anomaly is associated with graben infilling sediments, relative to gravity measured on plateaux on both sides of the graben. Three-dimensional, two-body modeling of the gravity data indicate that the sediment depth is as much as 900 m. An estimate of 2 1/2 km of vertical offset within the northern Colima Graben is obtained from combining the predicted sedirnent depth with the topographic relief.

Contemporaneous eruptions of calc-alkaline and alkaline magmas along the volcanic front of the mexican volcanic belt

Abstract: According to general models for subduction-related volcanic arcs, magma cornpositions become progressively richer in K₂0 and incompatible elements with increasing distance from the trench and volcanic front and with increasing depth to the Benioff zone. Arc-type magmas are typically hypersthene- and quartz-normative. Nepheline-normative alkali basalts are expected to erupt, if at ah, only at great distances from the trench. Along the volcanic front of the Mexican Volcanic Belt, however, hydrous, nepheline-normative, basic alkalic magmas have erupted at two different locations during the late-Quaternary Period: in the southern Colima graben and in the southern Michoacan-Guanajuato Volcanic Field. In both places, hypersthene- and quartz-nor-mative calc-alkaline magmas with typical `volcanic front' characteristics were erupting contem-poraneously. These contrasting magma suites can not be related by any simple mechanism; they appear to record distinct melting events within the upper mantle.

Geochemistry of tertiary igneous rocks from arandas-atotonilco área, northeast jalisco, mexicos.

Abstract: Se presentan los analisis de elementos mayores para diez muestras de basalto y andesita, seleccionadas de diez sitios paleomagneticos en el noreste de Jalisco, Mexico. Se reportan tambien los datos de fechas radiometricas K-Ar y de Tierras Raras (TR) en cuatro muestras, asi como de contenidos de elementos alcalinos (K, Rb y Cs) y tierras alcalinas (Ba y Sr) y razones isotopicas de estroncio (⁸⁷Sr/⁸⁶Sr) en ocho muestras. Se presenta evidencia de actividad volcanica hace 11 Ma en esta area. Las razones iniciales de ⁸⁷Sr/⁸⁶Sr varian desde 0.7033 hasta 0.7039 y no se correlacionan con los elementos mayores o trazas ni con las razones de elementos. Las razones de ⁸⁷Sr/⁸⁶Sr son similares a las observadas en otras areas del Cinturon Volcanico Mexicano (CVM) y son compatibles con un origen sub-cortical para los magmas jalicienses. Major elernent analyses for ten samples of basalt and andesite, selected one each from ten paleo-magnetic sites in northeast Jalisco, Mexico are presented. K-Ar dates and REE data on four samples and contents of alkali (K, Rb and Cs) and alkaline earth (Ba and Sr) elements and ⁸⁷Sr/⁸⁶Sr ratios on eight samples are also reported. Evidence of volcanic activity at about 11 Ma is shown. Initial ⁸⁷Sr/⁸⁶Sr ratios range from 0.7033 to 0.7039 and are not corre/ated with major or trace elements or element ratios. ⁸⁷Sr/⁸⁶Sr ratios are similar to those observed in other areas of the Mexican Volcanic Belt and are compatible with a sub-crustal origin for the Jalisco magmas

Investigaciones geofisicas y caracteristicas geoquimicas de las aguas hidrotermales: volcan poas, costa rica

Abstract: A geophysical and geochemical preliminar research has been carried out in the active caldera of Poas volcano during 1980-82. The water of the intra-caldera lake has a very strong acidity (pH <1) due to the high content of Cl-and SO 4 -2 . The concentration of metal cations is considerably high as well. Temperature calculated (T = 229°C), by means of chemical geothermometer for the mixing of phreatic waters with hot fluids, are in the range of those detected in geothermal systems associated to andesitic rocks. The temperature increase from 92°C (December, 1980) to 960°C (March-November, 1981) in the N-E section of the lava dome seems to be related to the fracturing (probably connected to the seismic crisis of July, 1980) of the upper plug of the cooling magma body at depth. This mechanism induced the exolution of relatively high temperature gaseous phases from below. Near surface oxidation phenomena might have been effective, thus contributing to increase the temperature of gases. Magnetic measurements confirm that the temperature increase was superficial and confined within a small area surrounding the central dome. In the light of the data hereby presented we suggest that the local temperature increase (from 92°C to 960°C) does not have any implication with magma ascent.

Effects of el niño on panama rainfall

Abstract: The effect of El Nino on the rainfall in Panama has been studied. Thirteen episodes of El Nino during the period, 1920 to 1983, were analyzed. The results show that El Nino produces below normal rainfall in almost all regions of Panama. The average annual rainfall anomaly, based on all occurrences of El Nino, is 8% below normal. In the case of the strong El Nino episodes of 1976 and 1982, the corresponding anomalies are 28% and 24% below normal, respectively. The driest month of the year 1982 (December) has a rainfall anomaly of about 60% below normal. The results of the study also showed that there is a considerable geographical variation in the rainfall anomalies. In the case of the El Nino of 1976, the largest magnitudes of the negative anomalies are located in the southwestern part of Panama, just south of the central cordillera on the Pacific side. On the other hand, El Nino had the opposite effect (positive anomalies) north of the cordillera in the Atlantic coastal region. The relatioship between the time variations of the annual rainfall anomalies and sea surface temperature anomalies in the Eastern Pacific was studied. A high negative correlation was found between the rainfall anomalies and the sea surface temperature anomalies during the preceding months. This high correlation has been used to devise a method for long range forecasting of rainfall.

Paleomagnetic constraints on the volcanic history of iztaccihuatl

Abstract: Las direcciones de magnetizacion remanente de los 24 flujos de lava del volcan Iztaccihuatl muestran polaridad magnetica normal, sugiriendo que el presente cuerpo del volcan se formo durante el periodo de Brunhes Chron. Esta juventud relativa de la montarla ha sido substanciada por medio de recientes pruebas de K-Ar para comprobar la edad geologica. Los analisis de dispersion de V. G. P. conceden una estimacion de la desviacion secular de 14.20°. Este valor es semejante al predicho en la latitud de Iztaccihuatl por recientes modelos de variacion secular y a una media de variacion paleosecular determinada por la distribucion mundial de los torrentes de lava. La concordancia de la cantidad estimada de dispersion con la esperada indica que la erupcion de las lavas de este volcan ocurrio probablemente durante un intervalo de tiempo bastante grande, como para haber obtenido un ejemplo representativo de variacion secular; lo que significa que el volcanismo se extendio por un periodo de tiempo mas corto que el de Brunhes Chron pero mas largo o comparable a la escala temporal de la variacion secular. La media de V. G. P. en el volcan Iztaccihuatl es estadisticamente indistinguible del polo geografico.

The climate of ten thousand years ago: a numerical simulation

Abstract: A thermodynamic model that includes the annual cycle, an ocean mixed layer and variable cryosphere is applied to simulate the climate of 10 000 years ago. It is shown that in lower latitudes the insolation anomalies due to the orbital variations were the main factor that produced the departures from the present climate. However, besides the insolation anomalies, the permanent ice sheets also affected the climate of 10 kyr BP in middle and higher latitudes. The average computed land surface temperature in the Northern Hemisphere is 1.9°C warmer than present summer and colder during the other seasons, with an anomaly as large as 2.1°C in winter. The annual average anomaly being -0.7°C. The computed surface ocean temperature remains below the present values during the entire year with an average value of -0.8 in winter, -0.2 in summer and -0.5°C for the whole year. It is shown that when present values of surface ocean temperature are prescribed, a warmer climate is obtained, with average land surface temperature anomalies equal to 2.2, -1.6 and -0-3°C for summer, winter and annual values respectively. A study on the importance of the inclusion in the model of the annual cycle, the ocean mixed layer and the variable cryosphere is carried out.

Frictional damping of the tide in the gulf of california

Abstract: The effect of friction on the tide in the Gulf of California is evaluated, using a one dimensional schematization of the basin; it is found to be minor. This permits to take account of the interaction between the tidal components implied by the quadratic law of friction, by assuming it to be a perturbation on the basic motion and to rewrite the equation of motion in a form which is linear but in which the coefficient of friction depends on the maximum velocity at each point and on the relative amplitude of the harmonic component being considered, besides the value given to the friction coefficient r= g/C 2 . A simple one dimensional model using the value r = .0036 reproduces adequately the measurements presently available on the semidiurnal and diurnal tides felt along the Gulf.