Showing papers in "Geofisica Internacional in 1986"

Post-paleozoic tectonics of northeast mexico and its role in the evolution of the gulf of mexico

Abstract: Se muestra la evolucion sedimentaria mesozoica del noreste de Mexico a traves de varios mapas paleogeograficos regionales. Tambien, con base en diferentes estilos de plegamiento, se propone un modelo alternativo para la deformacion del Terciario Temprano del noreste de Mexico, asi como una serie de paleorreconstrucciones de tectonica de placas, basados principalmente en la existencia de varias fallas sinistrales de transcurrencia, orientadas al noroeste y una falla dextral transcurrente (Falla Tamaulipas-Oaxaca) orientada al nor-noroeste, a lo largo de la cual el bloque de Yucatan se movio hacia el Sur (Jurasico Tardio) desde su antigua position en el sitio que hoy ocupa el Golfo de Mexico.

The petrology of the lower crust and upper mantle beneath southeastern chihuahua, mexico: a progress report

Abstract: Granulite facies metamorphic rocks and ultramafic nodules, of lower crustal and upper mantle origin, respectively, occur as xenoliths in a host alkali basalt at the La Olivina peridot mine, approximately 150 km southeast of Ciudad Chihuahua. The La Olivina ultramafic xenoliths are of three compositional groups that are distinguished by clinopyroxene chemistry: a high magnesian (Mg/Mg+Fe = 0.90), high Cr 2 O 3 group (Group I); a moderately-magnesian (Mg/Mg+Fe = 0.76), high A1 2 0 3 , high TiO 2 group (Group II); and a group compositionally intermediate between these two (Transitional Group). Analyses of primary minerals show that these xenoliths are similar to ones from San Carlos, Arizona (USA), from Xalapasco de La Joya, San Luis Potosi (Mexico), and from Kilbourne Hole, New Mexico (USA). The La Olivina mantle xenoliths are of three textural groups: allotriomorphic granular (igneous?), porphyroclastic (metamorphic?), and granoblastic (metamorphic?). All Group I xenoliths are granoblastic, and most nodules of Group II and the Transitional Group are allotriomorphic granular. Porphyroclastic texture is less common than the other textures in the rocks studied, but it is found in some Transitional Group rocks. The La Olivina lower crust xenoliths are predominantly pelitic gneisses and pyroxene granulites. The pelitic gneisses are uniform in mineral assemblage with garnet + quartz + plagioclase + sanidine + sillimanite + rutile + graphite. Most of the pyroxene granulites are plagioclase-bearing, but some contain scapolite in place of plagioclase. These lower crustal samples are identical in mineral assemblage and very similar in mineral chemistry to xenoliths described from Kilbourne Hole. The granulites and pelitic gneisses have model Nd ages of 1.1 to 1.2 b.y. using a chondritic initial ratio, or 1.6 b.y. using a depleted source model. The latter age is in excellent agreement with those from Kilbourne Hole. Thus it appears that Precambrian craton, similar in age, metamorphic history, and protolith composition to that which underlies Kilbourne Hole, extends into Mexico at least as far southeast as La Olivina.

Crustal thickness, heat flow, arc magmatism, and tectonics of mexico – preliminary report

Abstract: Se presentan mapas preliminares de espesor cortical y de flujo termico y una discusion breve sobre la estructura cortical, tectonica y provincias magmaticas en Mexico. Las isopacas siguen aproximadamente la linea de la costa en el margen Pacifico y en el Golfo de Mexico. La corteza es mas gruesa (≥40 km) en la porcion central-sur del Altiplano y en la porcion central-este de la Faja Volcanica Trans-Mexicana. El flujo termico es generalmente mayor que 1.5 UFT, excepto sobre los terrenos Precambricos-Paleozoicos del sur de Mexico y en la porcion norte de la planicie costera del Golfo de Mexico (~ 1.0 UFT). Se observan valores altos de flujo termico en la Sierra Madre Occidental y en la Faja Volcanica Trans-Mexicana, las cuales representan provincias magmaticas del Mesozoico-Cenozoico Temprano y del Cenozoico, respectivamente.

Volcanismo riolitico en el eje neo volcanico mexicano

Abstract: The Mexican Neovolcanic Belt is a plio-quaternary volcanic belt, of east-west orientation, that bisects central Mexico. It is formed by large andesitic stratovolcanoes, silicic volcanic centers, and extensive basaltic and andesitic cinder cone fields. Five silicic volcanic centers have been recognized to date in the Neovolcanic Belt: La Primavera, Jai., Los Azufres, Mich., Amealco, Qro., Huichapan, Hgo., and Los Humeros, Pue., These centers are located along the northern portion of the Belt, "behind" the front of the active andesitic stratovolcanoes. The volumes of rhyolitic and rhyodacitic magmas erupted in each one of these centers range from 10 to 150 km 3 . In most of these centers some of the individual eruptive units are strongly zoned in composition, and in some instances span the range rhyolite to andesite. An exception is La Primavera, where only high-silica rhyolitic magma was erupted. In addition to these five centers, there are numerous other occurrences of rhyolitic volcanism throughout the Neovolcanic Belt; some of them might be related to large but as yet unrecognized volcanic centers, but others seem to represent isolated events. The silicic volcanism of the Neovolcanic Belt has two important economic implications: volcanic risk and geothermal energy. Among the known silicic centers, La Primavera represents the major potential hazard on account of its youth and proximity to the city of Guadalajara. With respect to geothermal energy, Los Azufres is a geothermal field currently under production, and La Primavera and Los Humeros are prospects currently under exploration. In all three cases the reservoirs are located in zones of secondary permeability within the local volcanic basement.

Breve revisión sobre la evolución tectónica de México.

Abstract: New concepts about the lithospheric movility have conduced during the last year to review the main tectonic and paleogeographic problem of Mexico The Mexican and Central American geological structure reveals one of the most complex histories of crustal integration and evolution, involving periods of diverse geodynamic processes and major minor continental blocks interaction

Estudio del factor anelastico q de la coda de los terremotos correspondientes a las regiones central y oriental del eje volcanico de mexico

Abstract: Using the coda of several earthquakes situated in Central and Oriental regions of the Volcanic Axis of Mexico, and recorded in the OXM seismographic station of the Servicio Sismologico Nacional, the following results have been obtained: Empezamos lista con letra para terminar Q is strongly frequency dependent in the form Q = Q 0 (f/f 0 ) v , being the values of v= 0.4 for the Central region and 0.6 for the Oriental one, where f varies between 0.6 Hz and 1.0 Hz. The values of Q 0 obtained for a reference frequency of f 0 = 1 Hz are 322 for the Central region and 129 for the Oriental one. The S-wave attenuation coefficients obtained are 0.0023 km -1 and 0.0102 km -1 for the Central and Oriental parts respectively. The coda excitation coefficients obtained are 3.56 x 10 -25 cm s/dyne cm for the Central region and 1.85 x 10 24 cm s/dyne cm for the Oriental one. The above results may indicate a higher degree of faulting and/or a higher concentration of magmatic chambers under the Oriental region than under the Central one.

Estructura de la corteza al oriente de la sierra madre occidental, mexico, basada en la velocidad de grupo de las ondas de rayleigh

Abstract: Dispersion of fundamental mode Rayleigh waves for two large nuclear explosions from Nevada, USA, recorded at UNM, seismological station in Mexico City, was used to estimate the shear wave velocity structure along a profile running mainly along the eastern flank of the Sierra Madre Occidental, Mexico. The observed group velocity dispersion data in the period range of 10 to 21 seconds can be explained by 5 homogeneous horizontal elastic layers over a half space. The average thickness of the crust is of the order of 40 km. The shear velocities and thicknesses obtained are as follows: Sediments, V s1 = 1.5 km/s, h 1 = 1.2 km; Granite, V S2 = 2.90 km/s, h 2 = 2.2 km and V S3 = 3.56 km/s, h 3 = 26 km; Basalt, V S4 =4.10 km/s, h 4 = 10.0km; Upper Mantle, V s5 =4.5 km/s, h 5 = 10.0 km; Mantle, V s6 = 4.15 km/s, h 6 = ∞. A ratio of 1.78 is assumed for V p /V s . The velocity model suggests that the structure of the eastern flank of the Sierra Madre Occidental is transitional between the Alpine and the Basin and Range types. The model agrees satisfactorily with earlier results based on refraction studies of the crust in the surrounding regions, and supports the ideas about the Cenozoic tectonic evolution of the western North American Continent.

Late mesozoic-cenozoic evolution of the northwestern mexico magmatic arc zone

Abstract: Late Mesozoic - Cenozoic radiometric data on igneous rocks from the extensive magmatic province of north-western Mexico are used to investigate on the evolution of the continental margin. Results show that magmatic activity changed systematically with time from about 140 my B.P. to the present. Between about 140 and 60 my B.P. the trench-arc gap increased from about 100 to 400 km with a velocity varying from 1 to 12 cm/yr. The area covered by magmatic products also increased up to ~540 km, which, combined with the trench-arc gap changes, resulted in a ~1200 km wide spatial magmatic arc. Magmas were dominantly calc-alkaline and tectonism was predominantly compresional. Between about 60 and 35 - 30 my B.P., magmatic activity developed at points farthest from the trench, giving high-K calc-alkalic and alkaline rock suites. Tectonism changed from mainly compressional (Laramide orogeny, ~80-45 my B.P.) to mainly extensional (mid-Tertiary orogeny, ~35 - 15 my B.P. Finally, between about 35 - 30 and 10 - 0 my B.P., magmatic activity was rapidly displaced towards the trench and tectonism was mainly extensional. Bimodal volcanism developed including the emplacement of a thick ignimbritic sequence, and apparently continued long after cesation of active plate subduction. Major plate reorganization occurred during this interval, associated with the encounter of segments of the spreading center and trench, which resulted in replacement of plate subduction by transform motion. Major recent events include the development of the Gulf of California, about 15 my ago, with active spreading occurring mainly during the last 4.5 my. The changes observed in the trench-arc gap and in the width of the zone of magmatic manifestations are interpreted in terms of a model of variable subduction zone dips and variable depth and range of magma generation. Major factors involved were the change of age of the plate being subducted (progressively younger), rate of plate convergence, absolute motion of upper plate, sediment accretion, obduction, subduction of oceanic plateaus and tectonic extension. Relative importance of these factors, however, remains to be quantified.

Origin of voluminous mid-tertiary ignimbrites of the batopilas region, Chihuahua: implications for the formation of continental crust beneath the Sierra Madre Occidental.

Abstract: The voluminous rhyolitic ignimbrites of the Batopilas region of the northern Sierra Madre Occidental are interlayered with minor amounts of andesitic and dacitic lava flows. Several lines of evidence indicate that this andesite - dacite - rhyolite series is comagmatic. For example, the series is chemically distinct from older and younger igneous rocks found in the same geographical region; it lacks systematic variations in isotopic compositions; and trace element variations within the series can be modeled by Rayleigh fractionation calculations. Variations in Sr concentrations are inconsistent with a partial melting origin for the series, and the voluminous rhyolites of the Batopilas region appear to be the product of essentially closed-system fractional crystallization of magmas from a subcrustal source. Additional published data on mid-Tertiary rhyolites from four other areas of the Sierra Madre Occidental establish that these rocks are isotopically heterogeneous compared to those from Batopilas and that some of them clearly contain a crustal component. The subcrustal source region beneath the Sierra Madre Occidental is believed to have 87 Sr/ 86 Sr between about 0.7042 and 0.7050, and € Nd near 0. Most analyzed rhyolites have initial 87 Sr/ 86 Sr < 0.7070 and their isotopic compositions can be explained by an assimilation - fractional crystallization model that involves less than 25% of a crustal component. The extrusion of the Upper Volcanic sequence and the emplacement of the underlying batholith marked a major crust forming event in western Mexico.

Hydrothermal activity detected by self-potential measurements at the n-s volcanic axis between the volcanoes “nevado de colima” and “volcan de fuego de colima” (mexico)

Abstract: Measurements of electrical self-potential (SP), taken on the north-facing slope of the Fuego de Colima volcano, show a large-scale anomaly non-correlated with the cone which is active at present. This may arise due to hydrothermal activity which originates in a highly fissured zone above a heat source, located deep below the surface of the earth. It is presumed that this anomaly is linked with the N-S volcanic axis, running from Nevado, the oldest volcano in the Colima volcanic chain, to Fuego, the site of the present activity.

Descripcion preliminar de la geologia y mecanica de emplazamiento del complejo ultrabasico del cretacico de loma baya, guerrero, mexico

Abstract: A la altura de Papanoa, la costa de Guerrero corta a un cinturon magmatico del Cretacico Medio, dentro del cual se emplazan cuatro complejos basicos-ultrabasicos. Uno de estos, el de Loma Baya cubre una area de 0.75 Km 2 , y aparece encajonado por rocas volcanicas e hipabisales de composicion intermedia. En orden de abundancia, este nucleo esta compuesto por clinopiroxenitas de olivino, wherlitas, clinopiroxenitas, dunitas, y en menor cantidad, pegmatitas de homblenda y cromititas. El olivino es Fo 90 en promedio, y los clinopiroxenos son diopsida y augita diopsidica. El ortopiroxeno esta tipicamente ausente. Quimicamente, el conjunto ultrabasico es mas rico en FeO que sus simulares de tipo ofiolitico y aun de las intrusiones cordilleranas del oeste de Canada, E. U. A. y Alaska. Entre los oxidos analizados por medio de microsonda, se identificaron magnetitas y ferrocromitas primarias seudoestratificadas, y secundarias formadas durante la serpentinizacion del conjunto. Estos ultimos se diferencian claramente de los primarios por los altos valores de SiO 2 y MgO, tipicamente entre 8 y 20% y q4 y 24%, respectivamente. Con respecto a otras localidades, las cromitas masivas presentan un mayor contenido de Mg y menor de Cr. Los rasgos estructurales predominantes presentan una tendencia vertical, un intenso brechamiento interno, con una foliacion promedio de 73°, mientras que el cuerpo en conjunto parece haberse emplazado con una inclinacion de 53° W. Se explica la intrusion forzada del cuerpo, como debida al efecto combinado del aumente de volumen (30% ?) y la perdida aproximada de un tercio de la densidad de la masa ultrabasica durante la serpentinizacion, lo cual puede producir el movimiento ascendente del nucleo ultrabasico. Asi, se considera que el amplazamiento del cuerpo ocurrio durante el Cretacico Superior, mediante un mecanismo de tipo diapirico.

Acid rain over México City valley and surrounding rural areas

Abstract: 173 determinations of pH were made during sequential rain fractions of single rain events in Mexico City and surrounding rural areas from June to September 1983. Results indicate acidic pH values in the majority of rain samples collected in the urban area, even in those samples taken at mountainous locations (pH values less than 4.5). The variability in the pH values could be attributed to: transport mechanisms of pollutants, prevailing meteorological conditions, geographic location and the rain fraction sequential number.

Palaeomagnetic study of mesozoic rocks from ixtapan de la sal, mexico

Abstract: Se reportan datos paleomagneticos para 25 muestras de tres localidades de una secuencia de rocas metavolcanicas expuestas en el area de Ixtapan de la Sal, Mexico. Las muestras presentan grados variables de alteracion hidrotermal y metamorfismo, con dos sitios de esquistos verdes y un sitio de andesitas alteradas. Las muestras fueron sometidas a tratamiento con campos magneticos alternos decrecientes y con altas temperaturas. Las direcciones tienen polaridades normal y reversa, son estables con la desmagnetizacion y proporcionan una posicion polar media de 54.1°N, 139.1°E (dp= 10°, dm= 7°). Las magnetizaciones son llevadas por minerales magneticos de alta coercitividad, posiblemente hematita secundaria. Dataciones radiometricas de K-Ar en muestras de dos sitios dan edades de 108 ± 5 ma y 125 ± 5 ma, las cuales se relacionan con el evento de compresion y alteracion regional que afecto la zona. Se sugiere que las magnetizaciones remanentes observadas fueron adquiridas durante el Jurasico Tardio o Cretacico Tardio. La posicion polar derivada de las rocas metavolcanicas diverge de otros polos paleomagneticos reportados para otras areas de Mexico del Mesozoico, lo que sugiere la posible ocurrencia de movimientos tectonicos de esta area.

Evidences of the existence of an abnormal seismic signal attenuation in southern mexico

Abstract: The analysis of earthquakes occurred off the coast of Chiapas, Mexico, indicates the existence of an abnormal high seismic signal (P-waves) attenuation in a certain range of distances and azimuths. This feature is observed primarily in seismic stations located east of North America. In the present paper three possible explanations of this phenomenon origin are discussed based on our own observations and other studies.

New formulae for linear travel time inversion in 2-d heterogeneous media. theory and results

Abstract: Reliable formulae for the computation of travel time residuals and partial derivatives for the inverse problem in two dimensions based on ray tracing are developed. The partial derivatives are completely determined by the path and the travel time residuals are as accurate as the observed travel times; it is not necessary to solve the traditional `fixed ends problem'. A new kind of inversion is introduced that operates directly on the shape of the isovelocity lines, making possible the immediate visual inspection of the results of an iteration. The formulae were checked against synthetic data corresponding to a variety of models. The way to approach a particular problem depends both on the available data and the structure.

Sulfatos y nitratos en particulas atmosfericas y su relacion con algunos parametros opticos

Abstract: Approximate concentrations of sulfates and nitrates of atmospheric aerosol are determined and their relationships between visibility and turbidity in the visible and ultraviolet regions of the spectrum are obtained The results show that there is a high correlation of these radicals with the turbidity parameters It is possible to estimate the daily mean of visibility and Angstrom's turbidity coefficients by fitting a multiple-variable regression to the concentrations of sulfates and nitrates Nitrates play a more important role than sulfates with the optical parameters studied in this work in spite of the fact that the concentration of sulfates is larger than the nitrates

Bosquejo sismotectonico del sur de mexico

Abstract: The seismic zone of southern Mexico can be divided in four regions of different characteristics even though the seismic activity as a whole is considered as a result of the interaction among the Pacific, Northamerica, Cocos, Caribbean and Rivera plates. In this paper some of the differences are shown. The Isthmus of Tehuantepec and the coast of Chiapas are the best differenciated regions. For the remaining regions: Oaxaca, Guerrero-Michoacan and Jalisco-Colima some peculiarities resulting from the analysis of seismicity and some focal mechanisms solutions are shown. Thus, the Isthmus of Tehuantepec-Chiapas region seems to be a transition between a zone of normal subduction (Central America) and an anomalous one (Oaxaca) which is comparable to the central part of Peru in South America. The Guerrero-Michoacan region shows some complications that can be attributed to the proximity of the interception of the East Pacific Ridge, the Acapulco trench and the Rivera fault. The Jalisco-Colima region shows a marked reduction in the frequency of occurrence of seismic activity.

Balances de radiación y potenciales de productividad primaria en algunos ecosistemas terrestres de la Republica Mexicana

Abstract: Radiation balances (Q*) and net primary productivity potentials (NPP) are estimated from climatological data for thirteen locations with different vegetation patterns: Tropical deciduous forest, Pine-oak forest, grassland and “chaparral”, in the Mexican Republic. Radiation balances are determined from temperature, relative humidity, insolation and albedo expected values, for the ecosystems considered. Net primary productivity potentials estimates are obtained applying Rosenzweig (PER 1 ) Lieth and Box (PER 2 ) and Lieth’s models (PN and PT) which use actual evapotranspiration, precipitation and temperature, as environmental predictors. When precipitation (PN) and actual evapotranspiration (PER 2 ) are used, productivity predictions match with observed values of equivalent vegetation types; consequently, productivity variations respond more clearly to water availability. Determinations of the heating coefficient as an index of ecological significance prove that it has a closer relation to atmospheric conditions than to the surfaces considered. The relationship between net primary productivity predicted from actual evapotranspiration and precipitation, and the aridity index, estimated from radiation balances and precipitation values, coincide with Budyko’s conclusions, having the following expressions: PER 2 = 2480.7 (IA) -0.7958 PN= 2016.0 (IA) -0.83139