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Research about Mexico

Source-linked reports with geographic coverage including Mexico.

At least 253 records · Page 14Linked to original sources
USGS1974

Tectonic evolution of the southern Gulf of Mexico

A detailed magnetic survey in the southern Gulf of Mexico shows subdued irregular magnetic anomalies that are similar in wave length to those attributed to sea-floor spreading on present-day oceanic rises. The small amplitude of these anomalies, about 75γ, would be compatible with an oceanic basalt source at a depth of 10 km, and previous seismic refraction studies in this area have shown that layer 2 of the oceanic crust (presumed to be basalt) does indeed lie at approximately that depth. Palynomorphs in Deep Sea Drilling Project samples of cap-rock material from the Sigsbee Knolls have shown the associated salt to be Jurassic. A crustal model, based on seismic refraction evidence and on new gravity data, suggests that, whereas the salt-dome belt of the southern gulf is underlain by a thick layer with a density of 2.2 g per cm 3 (presumed to be halite rock), adjacent deeper areas of the basin seem to lack this low density layer. In northeastern Mexico, Triassic red beds fill grabens that are correlative with the Newark Group of the Atlantic Coast and suggest that the Gulf of Mexico originated at the time of the initial rifting of the North Atlantic. When the gulf was about half-opened during the Jurassic, oceanic circulation was restricted; and thick deep-basin evaporite deposits, analogous to those found in the Mediterranean Sea by the Deep Sea Drilling Project, were laid down. Further opening established normal salinity and led to the development of salt-free areas of oceanic crust that separated the Sigsbee Escarpment (together with adjacent ridges of offshore Mexico) from the Sigsbee Knolls and salt domes of Cuba. The subsequent structural evolution of the Gulf of Mexico basin is believed to have been mainly a result of interaction between it and tectonic plates of the Pacific area. After opening of the gulf, subduction began along the Cuban arc, where Atlantic rifting had created a nearly uninterrupted tract of oceanic crust extending from the Pacific between Yucatán and Colombia. Then, folding and gravity sliding associated with Laramide deformation on the west side of the gulf led to salt anticlines that underlie the ridges offshore from Mexico. Except for continued diapirism and subsidence associated with sediment loading, present tectonic activity is confined to the southwestern corner of the gulf, where volcanism and intermediate-focus earthquakes are a distant manifestation of subduction along the Middle America Trench on the Pacific side of Mexico.

Gulf of Mexico↗
USGS1973

Stages in the P-T path of ascending basalt magma: an example from San Quintin, Baja California

Late Pleistocene or Recent lavas from San Quintin, Baja California are basanitoids and alkali basalts. The surface quench temperatures of the lavas average 1 005° C with log f O 2 "> f O 2 fO2 =−11.4, as deduced from the groundmass Fe-Ti oxides. Spinel lherzolite xenoliths and megacrysts of augite and andesine have been found in lava flows and cinder deposits. Using analytical data on the rocks and minerals and simple thermodynamic expressions, the pressures and temperatures of equilibration of lavas and xenoliths, megacrysts and phenocrysts have been calculated. The lavas could have been in equilibrium with lherzolite at 1 330–1 410° C and 27.5–31.6 kb, the more silica-poor liquid having the higher values. The basanitoid could have equilibrated with the megacrysts at about 10.5 kb and with phenocrysts at about 1.4 kb and 1130° C. The variation in composition of the lavas may be explained by a rising zone of melting within the mantle, the most silica-poor liquid having the deepest source. The source of the San Quintin basalts is probably related to spreading of the ocean floor in the Gulf of California.

Baja California, San Quintin↗
USGS1972

Acoustic-reflection profiles, Bay of Campeche: International Decade of Ocean Exploration, U.S. Geological Survey, Leg 1, 1971 cruise, UNITEDGEO I

Introduction As part of a cooperative marine research program designated the International Decade of Ocean Exploration (IDOE), the United States Geological Survey is participating in an investigation of the geologic framework and resource potential of continental margins and small ocean basins in the Gulf of Mexico, Caribbean Sea, and west African continental shelf areas. These studies are funded through the National Science Foundation. Acoustic-reflec­tion profiles of Leg 1, described in this report concern mainly a 200 X 250 kilometer project area in the Bay of Campeche in the southwestern part of the Gulf of Mexico. The cruise leg began at Corpus Christi, Texas, and ended at Veracruz, Mexico. Other geophysical records collected during the cruise include magnetometry, gravimetry, and bathymetry. Visiting scientists from the Institute of Geophysics of the National University of Mexico, the Oceanographic Commission of the Mexican Navy, and the Mexican Institute of Petroleum joined the research vessel UNITEDGEO I throughout the cruise in the Bay of Campeche. The cruise offered an excellent opportunity for scientific interchange between investigators from Mexico and the United States and permitted collaboration in the instru­mentation, operational techniques, and interpretation of acoustic-­reflection records aboard the research vessel.

Bay Of Campeche;Gulf Of Mexico↗
USGS1971

Surf-beat origin for pulsating bottom currents in the Rio Balsas submarine canyon, Mexico

A previously unreported process was observed at the head of a tributary to the Rio Balsas submarine canyon system in Mexico. During a period of large surf, river discharge deflected a pulsating longshore current [peaking at over 7 km/hr (2 m/sec)] seaward over the tributary heading in the surf zone. This pulsating flow occasionally entered the river mouth, causing rhythmic fluctuations with amplitudes of at least 30 cm and a period of about 3 minutes within the mouth, as recorded by a partially filtered tide gage. In diving to the bottom of the tributary at a depth of 18 m, we encountered current pulses with estimated velocities of 4 km/hr (more than 1 m/sec) transporting large amounts of suspended sand down an axial slope of 26°. This bottom flow was at least 3 m thick, and was characterized by pulses separated by quiet periods in phase with the surface rip current. The upper 20 cm of the canyon fill during this time consisted of sand smoothly laminated parallel to the bottom, indicating net deposition on the steeply sloping floor. Estimated water budget suggested that the entire water column below the rip current at 18 m was not flowing seaward during the pulses. The bottom flow landward of the dive site probably separated from the surface flow and was propagated downslope as a turbidity current. Its magnitude was sufficient to erode the canyon walls. These observations substantiate that rip currents play a role in the formation of some submarine canyons. Surf-beat induced rhythmic flushing of the river mouth, however, did not cause density currents in the main canyon head. The need for future canyon studies under extreme conditions is pointed out.

Cañón de la Necesidad, Rio Balsas submarine canyon↗
USGS1970

Submarine basalt from the Revillagigedo Islands region, Mexico

Ocean-floor dredging and submarine photography in the Revillagigedo region off the west coast of Mexico reveal that the dominant exposed rock of the submarine part of the large island-forming volcanoes (Roca Partida and San Benedicto) is a uniform alkali pillow basalt; more siliceous rocks are exposed on the upper, subaerial parts of the volcanoes. Basalts dredged from smaller seamounts along the Clarion fracture zone south of the Revillagigedo Islands are tholeiitic pillow basalts. Pillows of alkali basalts are more vesicular than Hawaiian tholeiitic pillows collected from the same depths. This difference probably reflects a higher original volatile content of the alkali basalts. Manganese-iron oxide nodules common in several dredge hauls generally contain nucleii of rhyolitic pumice or basalt pillow fragments. The pumice floated to its present site from subaerial eruptions, became waterlogged and sank, and was then coated with manganese-iron oxides. The thickness of palagonite rinds on the glassy pillow fragments is proportional to the thickness of manganese-iron oxide layers, and both are a measure of the age of the nodule. Both oldest basalts (10–100 m.y.) and youngest (less than 1 m.y.) are along the Clarion fracture zone, whereas basalts from Roca Partida and San Benedicto volcanoes are of intermediate age.

Marine Geology↗
USGS1970

Rapid changes in the head of the Rio Balsas Submarine Canyon system, Mexico

The investigation of a river delta and the heads of several nearby submarine canyons in western Mexico produced evidence for rapid changes in the configuration and depth of the nearshore portions of canyon tributaries. General scarcity of data on the rates of submarine canyon formation and the relationship to river discharge should make these results of special interest. The Rio Balsas, one of Mexico's largest rivers, empties into the ocean near the heads of a large submarine canyon that terminates in the Middle America Trench. One of the distributaries of the Rio Balsas presently is discharging at the head of Cañón de la Necesidad, which is being eroded actively. Two inactive canyons are related to former discharge channels of the river. Their heads lie at some distance from shore and are being filled with sediment. The Cañón de Petacalco, not now receiving sediment directly from a Rio Balsas distributary, has remained active because the shoreline has not retreated far. Until about 100 years ago its head was being filled with fine-grained and highly organic sediments from a nearby rivermouth, while the coarse portion of the sediment supply joined the canyon via a tributary farther seaward. Since then the river has shifted away from this canyon, and the horizontally stratified sediments in the canyon head have been incised as much as 20–30 m, as evidenced by three 14 C dates of organic material exposed in the steep to overhanging canyon walls. The changes in the shallow portion of the Rio Balsas submarine canyons seem to be related to changes in river discharge pattern, either directly or indirectly. A shifting point source of sediment supply either activates a pre-existing, partly filled canyon, or erodes a new one near the new river mouth, whereas the canyon at the abandoned river mouth is deactivated following retreat of the shoreline. The heads of the different tributaries form a dendritic pattern in Holocene unconsolidated sediment. Subaerial processes are not involved in the formation of these submarine canyons. Thus, a dendritic pattern of submarine canyons is not necessarily indicative of subaerial erosion.

Rio Balsas Submarine Canyon system↗
USGS1970

Photogeologic and thermal infrared reconnaissance surveys of the Los Negritos-Ixtlan de los Hervores geothermal area, Michoacan, Mexico

New techniques, involving interpretation of panchromatic, ektachrome and ektachrome infrared aerographic photogaphs and thermographic infrared imagery recording emission from the earth's surface in middle and far infrared wavelengths (3–5μm and 8–14μm), are being introduced in geothermal investigations in Mexico to identify outstanding structural and geologic features in a rapid and economical manner. The object of this work is to evaluate the new airborne infrared techniques and equipment as a complement to the data obtained from panchromatic aerial photography. This project is part of the Mexican remote sensing program of natural resources carried out under the auspices of the Comision Nacional del Espacio Exterior and in which the Research Institute (Instituto de Investigaciones de la Industria Electrica) is actively participating. The present study was made cooperatively with the U.S. National Aeronautics and Space Administration and the U.S. Geological Survey. The Los Negritos-Ixtlan de los Hervores geothermal fields are located east of Lake Chapala at the intersection of the Sierra Madre occidental and the west-central segment of the neovolcanic axis of Mexico. The two principal zones of hydrothermal activity occur in a tectonic trench filled with lake sediments of the Quaternary intercalated with Quaternary and Holocene volcanic rocks and characterized by an intricate system of block-fault tectonics, part of the Chapala-Acambay tectonic system, along which there has been volcanic activity in modern time. Surface manifestations of geothermal activity consist of relatively high heat flow and hot springs, small geysers and small steam vents aligned along an E-W axis at Ixtlan, possibly at the intersection of major fault trends and mud volcanoes and hot pools aligned NE-SW at Los Negritos. More than 20 exit points of thermal waters are shown on infrared imagery to be aligned along an extension of the Ixtlan fault between Ixtlan and El Salitre. A narrow zone of hydrothermal alteration and deposition at the surface is identifiable on the infrared imagery of this area, closey related spatially to a resistivity low at depth. Extinct geothermal areas near El Salitre, Ixtlan, and farther west at San Gregorio are clearly delineated on both infrared images and infrared ektachrome photographs. Predawn infrared images also show high-angle fault zones suggesting the dominance of block tectonics in much of the area. Special image enhancement techniques applied to the original magnetic tape records will be required for more precise identification of warm ground zones and for a qualitative or semiquantitative estimate of ground radiance associated with anomalously high convective heat flow.

Geothermics↗
USGS1970

4 Earthquake: Major offshore earthquakes recall the Aztec myth

Long before the sun clears the eastern mountains of April 29, 1970, the savanna highlands of Chiapas tremble from a magnitude 6.7 earthquake centered off the Pacific coast near Mexico’s southern border. Then, for a few hours, he Isthmus of Tehuantepec is quiet.

Earthquake Information Bulletin (USGS)↗
USGS1969

Chemical composition of the hydrothermal fluids responsible for the lead-zinc deposits at Providencia, Zacatecas, Mexico

The chemical composition of 22 samples of primary fluid inclusions in quartz, calcite, and sphalerite from Providencia, Mexico, has been determined. Samples were prepared, crushed, and leached as described by Roedder et al. (1963). Cation analyses of leachates were made by atomic absorption spectroscopy, and the anions were analyzed by a special X-ray fluorescence technique. The Na, K, Ca, and C1 concentrations show a considerable range that reflects the widespread salinity variations in the hydrothermal fluids observed by Sawkins (1964). The K/Na atomic ratios of most samples range from 0.18 to 0.43 and the ratios tend to increase with the salinity of the inclusions. The temperatures indicated by the K/Na ratios from the published curves of K/Na ratios vs. temperature are mostly at least 100 ø C greater than corresponding filling temperatures. The Ca/Na atomic ratios of inclusions in sphalerite range from 0.12 to 0.61. The K/Na and Ca/Na data agree well with previous δO¹⁸ data (Rye, 1966; Rye and O'Neil. 1968) which indicate that the hydrothermal fluids were derived from the magma related to the Providencia granodiorite stock and that the fluids reacted only slightly with the crystalline stock during the late phases of ore deposition. The data also indicate that the salinity variations in the hydrothermal fluids probably occurred at the source of the fluids and may have resulted from boiling in the magma chamber. Mg concentrations are generally no more than a few hundred parts per million. Most of the Mg in the hydrothermal fluids was evidently removed during the formation of calcium-magnesium silicates in the lower levels of the ore pipes. Mg/Ca atomic ratios are generally less than 0.08 and are consistent with the paucity of dolomite in the area. Zn and Cu concentrations in water leaches of inclusions in calcite are generally less than 50 ppm. Maximum base-metal concentrations from water leaches of two samples of inclusions in quartz are 890 ppm Zn and 530 ppm Cu. Chloride is the major anion in the fluids, and C1 concentrations vary with corresponding cation concentrations. With two exceptions, sulfur concentrations of the inclusions in calcite and quartz, analyzed as SO₄, are below the level of detection

Zacatecas↗
USGS1969

Dilemma posed by uranium-series dates on archaeologically significant bones from Valsequillo, Puebla, Mexico

In an attempt to date stone artifacts of Early Man excavated from several sites at the Valsequillo Reservoir, a few kilometers south of Puebla, Mexico, Szabo applied the uranium-series method on bone samples known to be either from the same geologic formation as the sites or in direct association with the artifacts. The geologic context of the bones was studied by Malde, and the archaeological sites were excavated by Irwin-Williams. A date determined for bone associated with an artifact (Caulapan sample M-B-6, see below) agrees with a radiocarbon date for fossil mollusks in the same bed and indicates man's presence more than 20 000 years ago. However, some of these bone dates exceed 200 000 years. Because such dates for man in North America conflict with all prior archaeological evidence here and abroad, we are confronted by a dilemna — either to defend the dates against an onslaught of archaeological thought, or to abandon the uranium method in this application as being so much wasted effort. Faced with these equally undesirable alternatives, and unable to decide where the onus fairly lies (if a choice must be made), we give the uranium-series dates as a possible stimulus for further mutual work in isotopic dating of archaeological material. A sample from the Lindenmeier archaeological site north of Fort Collins and another from a Pleistocene terrace along the Arkansas River, both in Colorado, were also dated.

Puebla↗
USGS1969

Relationships of the avifauna of San Esteban Island, Sonora

Seven species of geographically variable birds have been reported as part of the resident avifauna of San Esteban Island in the Gulf of California, Sonora. Two of these, the Curvebilled Thrasher and the Blue-gray Gnatcatcher, may not actually have breeding populations there, at least at the present time. Of the seven, only two, the Verdin and the doubtfully resident thrasher, can be used in a determination of the relationships of the avifauna, and both show affinities to that of Sonora. The Ash-throated Flycatcher and the House Finch are represented by the same form on San Esteban and in both Sonora and Baja California at the latitude of the island. Neither the Ladder-backed Woodpecker nor the endemic Black-throated Sparrow shows greater similarity to the Baja California or Sonoran avifauna. The slight balance of evidence indicates that the avifauna of San Esteban Island is best considered with that of Sonora.

San Estéban Island↗
USGS1969

Taxonomy of the common dolphins of the eastern Pacific Ocean

Delphinus bairdii Dall is a species of dolphin distinct from D. delphis Linnaeus, with which it has usually been synonymized. D. bairdii has a longer rostrum relative to the zygomatic width of the skull; the ratio of these measurements falls at 1.55 or above for bairdii and 1.53 and below for delphis. In the eastern Pacific Ocean, D. bairdii is found in the Gulf of California and along the west coast of Baja California, Mexico; D. delphis is presently found in the waters off California. Until approximately the beginning of the present century, bairdii occurred farther north in the eastern Pacific Ocean, at least to the Monterey Bay area of California. Restriction of bairdii to more southerly waters, probably as an indirect result of a change in water temperature, may have permitted delphis to move into inshore Californian waters. The Pacific population of D. delphis has a somewhat shorter rostrum than the Atlantic population, and is perhaps subspecifically different. A thorough analysis of the entire genus Delphinus is needed before the relationship of all the populations can be understood and names properly applied.

Baja, California, Gulf of California↗
USGS1967

The Peromyscus guardia-interparietalis complex

Peromyscus interparietalis is considered to be specifically distinct from P. guardia , of which it was originally considered a subspecies. Each species includes three subspecifically distinct populations, each endemic to an island in the northern Gulf of California, Baja California, Mexico. Two subspecies of P. interparietalis and one of P. guardia are newly described here. The mice are widespread on the islands, apparently favoring no particular microhabitat. Reproduction takes place in early spring.

Gulf of California↗
USGS1963

Investigations of the principal fluorspar districts of Mexico

As a result of the increasing need for fluorspar, especially in North America, the Mexican fluorspar industry expanded greatly is the 8-year period, 1952-1960. Since 1956, Mexico has been the world's largest producer and exporter of this commodity. From 1956 through 1959 Maltice imported about 90 percent of the fluorspar production to the United States; these exports seemed about 330,000 tons a year. Districts that yielded about 95 percent of the Mexican fluorspar production in 1959 were among those visited in the spring of 1960. Brief visits Imre made to 36 fluorspar mines in 9 states of Mexico. Measured, indicated, and inferred reserves in the fluorspar districts investigated total about 15 million tons of ore averaging about 65 percent CaF 2 . These reserves and potential future sources indicate a long-continued production of fluorspar, adequate to meet the domestic seeds and to supply other North American requirements for many years.

Open-File Report↗