La Angostura Research Articles

Nota sobre la dieta de Ameiva edracantha (Squamata, Teiidae) en el Parque Nacional Cerros de Amotape, Tumbes, Perú

The diet of Ameiva edracantha Bocourt 1874, a terrestrial diurnal teiid lizard distributed in northwestern Peru and southwestern Ecuador is described for the first time. Stomach contents of fifteen individuals collected from quebrada La Angostura were analyzed. Lepidopteran larvae, coleopterans, orthopterans and spiders were the dominant preys in the lizard’s diet. Snout-vent length and head length, width and height were not correlated to prey length, width and volume. Other parameters such as profitability and prey target selection could be involved in prey selection by A. edracantha.

Costumer-based brand equity for tourism destinations in Patagonia

We conducted empirical research into brand equity for two mountain tourism destinations, Villa la Angostura (VLA) and Villa Pehuenia (VP), in the Patagonia region of Argentina. Brand equity was measured through four dimensions: brand awareness, perceived image, perceived quality and loyalty. A structured survey was administered to a sample of 400 respondents living in Neuquén city, an important nearby source market for the destinations. Findings indicated high familiarity with both tourism brands. The Neuquén tourism market perceived both brands as nature and snow destinations, with beautiful landscapes and a range of attractions. High levels of perceived quality and positive loyalty attitudes and behaviours were also detected, although evidence was found of a differentiated perceived image for each destination. VLA was identified as a destination with a range of activities and services and better accessibility, while VP was identified as a destination with very hospitable hosts. VP seemed to be a more relaxing and peaceful destination but at the same time less entertaining compared to VLA. This research contributes to the local literature on equity brand measurement; recommendations for destination marketers are also presented.

Paleomagnetism of the Todos Santos and La Silla Formations, Chiapas: Implications for the opening of the Gulf of Mexico

We report paleomagnetic data for the Lower to Middle Jurassic La Silla and Todos Santos formations of southern Mexico, in west-central Chiapas and the Tehuantepec Isthmus region. Volcanic rocks and red beds of these formations were deposited prior to or during the early stages of Gulf of Mexico opening. Dual-polarity characteristic magnetizations reside primarily in hematite and pass intraformational conglomerate, regional tilt, and reversal tests; they are thus interpreted as primary magnetizations. Our sampling sites are concentrated in three localities; around La Angostura Lake, 17 accepted sites yield a tilt corrected mean of declination (Dec) = 325°, inclination (Inc) = 4.6° ( k = 11.9, α 95 = 10.8°); in the Matias Romero region, the mean is Dec = 312.9°, Inc = 3.2° (based on only seven sites); and in the Custepec area, Jurassic andesitic dikes intruding rocks of the Permian Chiapas Massif yield a corrected mean of Dec = 335.0°, Inc = 5.0° (six sites). The mean directions are discordant with respect to expected North America reference directions, and indicate a counterclockwise rotation of 35° to 40°. Inclinations indicate deposition or emplacement at near equatorial paleolatitudes (2.1°N ± 3.4°). This paleolatitude estimate is statistically indistinguishable from those previously observed in the La Boca Formation of the Huizachal Group in northeast Mexico. The localities we sampled in southern Mexico are separated by ∼150 km, suggesting that the paleomagnetic signature of these rocks reflects regional-scale rather than local deformation. These Jurassic paleomagnetic directions support a rotational origin for the Gulf of Mexico. The data are also consistent with an Early to Middle Jurassic reconstruction that places the Chiapas Massif offshore the Tamaulipas state in the western Gulf of Mexico, and an Euler rotation pole for the Maya Block for this time period in the eastern gulf. The apparent polar wander path defined by paleomagnetic poles for the Chiapas Massif and Jurassic rocks reported here suggests that relative motion between North America and the Maya Block occurred between Late Permian and Early Jurassic time, during a protracted rifting phase, and then in the Late Jurassic in association with seafloor formation in the Gulf.

Jurassic volcanic and sedimentary rocks of the La Silla and Todos Santos Formations, Chiapas: Record of Nazas arc magmatism and rift-basin formation prior to opening of the Gulf of Mexico

Stratigraphic relationships, detrital zircon provenance, U-Pb and 40 Ar/ 39 Ar geochronology, and trace element geochemistry in volcanic and sedimentary rocks of the Sierra homocline of central Chiapas near La Angostura reservoir in Mexico document an extensive pulse of Early–Middle Jurassic arc magmatism in rocks that overlie and intrude the Permian–Triassic Chiapas massif. Upper Jurassic rift-basin strata unconformably overlie the volcanic rocks and the massif. A Pliensbachian U-Pb (zircon) SHRIMP (sensitive high-resolution ion microprobe) age from porphyritic andesite (191.0 ± 3.0 Ma), Early to Middle Jurassic 40 Ar/ 39 Ar dates from andesitic dikes, U-Pb grain ages of detrital zircons in overlying strata (196–161 Ma), and previously reported K-Ar dates indicate that subduction-related magmatism occurred in the western portion of the Maya block from Early to latest Middle Jurassic time. We assign the volcanic rocks to the La Silla Formation, which correlates with the informal Pueblo Viejo andesite of the Cintalapa and Uzpanapa regions to the northwest. La Silla magmatism predates opening of the Gulf of Mexico Basin. The Todos Santos Formation, which overlies La Silla Formation, was deposited in extensional basins during the early stages of gulf opening. We recognize a lower El Diamante Member of the Todos Santos, consisting of red fluvial sandstone, mudstone, and minor conglomerate containing primarily volcanic-lithic detritus; this member is characterized by a nearly unimodal Jurassic detrital zircon age population that indicates a Callovian or younger depositional age. Volcanic activity continued into the upper part of the El Diamante Member, but with a more mafic character. We also recognize an upper member, which we term the Jerico Member. This member is characterized by thickly bedded, coarse-grained pebbly arkose intercalated with several thick intervals (tens of meters) of conglomerate and pebbly sandstone. Sandstone petrology indicates a source in the granitic rocks of the Chiapas massif, with a tendency to show deep-seated sources and a diverse zircon population in the upper part of the section. The upper Todos Santos Formation in the study area is gradational into the overlying San Ricardo Formation (Kimmeridgian–Tithonian). The La Silla Formation was deposited in volcanic-complex environments, with a clear lack of differentiated volcanic rocks. Fluvial strata of the El Diamante Member were deposited in a mud-rich sinuous river system. The Jerico Member was deposited in large, sand-rich fluvial systems, which probably represent deposits of rift-axis trunk streams; conglomerate facies were deposited in adjacent and interfingering alluvial fan systems. We suggest that the stratigraphic record of the western Maya block records a transition from volcanic arc to intra-arc basin and subsequently to rift basin during Pliensbachian to Oxfordian time.

Effect of solar UV‐B exclusion on the phytoplankton community in a sub‐tropical mountain reservoir: a mesocosm study

A mesocosm experiment was conducted over a 4 week period to investigate the response of a phytoplankton community in a mountain freshwater reservoir to solar UV-B exclusion. The mesocosms were filled with water taken from the depth of 0.30 m in the reservoir, and exposed to near-ambient solar UV-B (+UV-B) and solar UV-B exclusion (−UV-B). The resulting chlorophyll-a (Chl-a), carotenoids, soluble sugars, algal composition and algal abundance were analysed. The study results indicated that carotenoids, carbohydrates and species distribution were affected under the −UV-B treatment. The carotenoid concentration was generally higher under the +UV-B treatment than under the −UV-B treatment. The sucrose and glucose concentrations were affected differently by the UV-B treatments, although both sugars strongly decreased under the +UV-B treatment by the end of the experiment. The sucrose and glucose concentrations exhibited similar distribution patterns to those of carotenoids for the +UV-B treatment, suggesting that these sugars could be directly involved in secondary metabolism. In contrast, the fructose content did not exhibit significant differences between UV-B treatments. The Chl-a concentration exhibited a progressive decrease during the experiment for both UV-B treatments. Although no significant impact of UV-B exclusion on the Chl-a concentration was observed in this study, a slightly higher concentration was observed for the −UV-B treatment. The total phytoplankton biovolume (BV) exhibited a pattern similar to that of the Chl-a concentration. Analysis of the phytoplankton composition revealed 52 species. However, because some species and classes were extremely difficult to identify, only 46 species from seven classes (Cyanophyceae, Chlorophyceae, Bacillariophyceae, Euglenophyceae, Charophyceae, Dinophyceae, Xanthophyceae) were unambiguously identified and considered for the UV-B treatments. Chlorophyceae, Bacillariophyceae and Cyanophyceae comprised about 78% of the identified species. Aulacoseira granulata and Cyclotella meneghiniana, followed by Ceratium hirundinella, were the most abundant species. Principal component analysis, using sampling periods and algal densities (PCA-1), indicated a heterogeneous phytoplankton assemblage, whereas PCA-2, with algal densities and biochemical variables (soluble carbohydrates, Chl-a and carotenoids), indicated that biochemical parameters could provide qualitative information about La Angostura phytoplankton community responses to quality of the sunlight.

Génération d'échantillons synthétiques des volumes mensuels écoulés vers deux barrages selon la méthode de Svanidze modifiée / Synthetic sample generation of monthly inflows into two dams using the modified Svanidze method

A modification of Svanidze's fragments method was applied to generate synthetic samples of monthly inflows to the La Angostura and Malpaso dams, located on the Grijalva River, in the state of Chiapas, Mexico. This modification allows generation of the monthly runoff of several dams in a series and preservation of their cross-correlations. Comparing statistics of generated and historical series, a good agreement between them was observed, especially for the autocorrelations between consecutive months and for cross-correlations between both dams, except for slight differences in the standard deviation and the skewness during the dry period.

Trend detection in the water regime of the main rivers of the Province of Mendoza, Argentina

Hydrologic systems are potentially sensitive to climate change. The objective of this paper is to detect possible changes in the runoff regime of mountain rivers in the Province of Mendoza, Argentina. The methodology is based on parametric (Student’s t-test) and non-parametric (Mann-Kendall) statistical tests applied to time series of monthly, maximum and minimum flows, and of typical daily flows derived from the flow duration curve. Both tests yielded similar results for a 5% significance level. No trend was detected in monthly, annual, maximum, minimum and typical flow series on the Diamante River at La Jaula and the Grande River at La Gotera. Trends rose with late spring and summer flows in the Mendoza River at Guido. In the Atuel River, at La Angostura, flow trends increased in all months, with the exception of the December through February period. The annual flow of the Tupungato and Mendoza at Guido, the daily minimum of Vacas and Atuel at La Angostura, and the drought flow of Vacas, Tunuyan and Atuel have significantly increasing trends. Puente Sosneado on the Atuel is the only one with negative trends in the flow series corresponding to the February–April period, and to annual and typical flows exceeded 90, 180, 270 and 355 days a year. In summary, of the 210 time series analyzed, only 40 exhibit changes and most of them are associated with low-water periods.

Upper Cambrian carbonate sequences of the Argentine Precordillera and the Steptoean C-Isotope positive excursion (SPICE)

Carbon and Sr-isotope profiles in Upper Cambrian platformal carbonate Formations in the Precordillera, western Argentina (Zonda, La Flecha and La Silla Formations), were constructed for three representative sections: (a) Quebrada de la Flecha, Eastern Precordillera, (b) Cerro La Silla, Central Precordillera and (c) Quebrada de La Angostura, northern part of the Central Precordillera. At Quebrada de La Angostura, upper part of the La Flecha Formation, δ 13C carb varies continuously up-section from − 2.0 to + 5.6‰ (PDB) and records the SPICE anomaly (+ 5‰) reported for the first time in South America. The peak of this excursion is characterized by intercalated 2 m thick beds of black shale with marl and limestone that record the onset of a sea-level change. The Steptoean Zonda Formation dolomites at the Quebrada de la Flecha exhibit a total δ 13C range from − 2.7 to + 0.6‰ with discrete positive anomaly about 200 m from the transition to the overlying Sunwaptan La Flecha Formation. Pronounced C-isotope anomaly (− 5.6‰) is observed in the La Flecha Formation at about 300 m below the transition to the La Silla Formation. At the Cerro La Silla section, the Zonda Formation exhibit δ 13C values of ∼ − 1‰, increasing slightly at the transition to the La Flecha Formation (− 1 to 0‰). The transition of the La Flecha to the La Silla Formations is characterized by alternation of black shales and dolomitic limestone with a discrete positive C-isotope excursion, probably corresponding to the SPICE. At the Quebrada de La Flecha, 87Sr/ 86Sr for the Zonda Formation varies from 0.70924 to 0.70955 and for the La Flecha Formation from 0.70908 to 0.70942. At Cerro La Silla this ratio varies from 0.70914 to 0.70923 for the La Flecha Formation, and from 0.70898 to 0.70980 for the La Silla Formation. At the Quebrada de La Angostura, ratios for the La Flecha carbonates range from 0.70918 to 0.70993. The overall variation of 87Sr/ 86Sr is consistent with globally reported Upper Cambrian seawater values at ca. 500 Ma. The unambiguous record of SPICE in the La Flecha Formation at the Quebrada de La Angostura supports a Steptoean age for its deposition and allows precise local, regional, and global stratigraphic correlation. The pronounced negative C-isotope excursion recorded in the La Flecha Formation carbonates at the Quebrada de La Flecha is likely equivalent to that registered in Sunwaptan carbonates of North America and Australia, and might be tied to a global event, as a valuable tool in stratigraphic correlation (SNICE, acronym for Sunwaptan negative isotope carbon excursion).

Relações interétnicas, processos de construção da identidade e estratégias etnopolíticas mapuches no Departamento Los Lagos, Neuquén, Argentina

This research aims at analizing the in-dentity ethnic construction process within familieswith Mapuche ancestors in Villa la Angostura (LosLagos Department) in the province of Neuquénin relation to the policies carried out by the Na-tional and Provincial States. For this, we proposeas a working hypothesis different national scenariosfrom which during the 20th century, the “ethnic”identity caused a prejudice to the situation of thesedomestic groups in terms of their right to the land.For this we analyze the role played by the stateinstitutions in charge of regulating this right. Laterwe wonder about the social movilization processwhich arose in 2003 focused on the economic con-text which envolves the city. In that field we studythe different state policies with an aim to promotingthis kind of claim and we account for their inciden-ce when visualizing the protest and revaluation ofthe Mapuche identity.

Temporal dynamics of soil erosion and rainfall erosivity as geoindicators of land degradation in Northwestern Argentina

The subtropical region of Northwestern Argentina displays severe environmental deterioration due to a combination of factors, including seasonally contrasting weather, highly erodable loessic soils, a topography with varied slopes, and natural vegetation coverage restricted by deforestation and agricultural advance, in addition to exploitation of natural resources by people. The aim of the present research was to determine the landscape degradation, using as geoindicators the increase in erosion by water and the pluviometric gradient variations through time. Two regions were chosen as sample areas in the province of Tucumán: the eastern foothills and plain of the Southeastern Ranges (Western Chaco); and the intermountain valley of Tafí. Erosion increase was quantified by measuring the length of gullies and ravines; and the sedimentation in the artificial lake (La Angostura) located in the center of the valley of Tafí. Climate changes were analyzed by means of the rainfall erosivity variations in both sample areas during the last 30 years. The lineal increase in erosion is closely related to the pluvial dynamics. Nevertheless, this erosive intensification is also dependent on the interaction of environmental factors and human activities (deforestation, land use changes, unrestricted agriculture).

In situ Responses of Phytoplankton from the Subtropical Lake La Angostura (Tucumán, Argentina) in Relation to Solar Ultraviolet Radiation Exposure and Mixing Conditions

In situ experiments were conducted at various depths in the water column to determine the effects of solar ultraviolet radiation (UVR, 280–400 nm) on photosynthesis of natural phytoplankton assemblages from the subtropical Lake La Angostura (Argentina, 26°45′ S; 65°37° W, 1980 m asl.). Water samples were taken daily and incubated under three radiation treatments: (a) Samples exposed to UVR + Photosynthetic Available Radiation (PAR) – PAB treatment (280–700 nm); (b) Samples exposed to ultraviolet-A radiation (UV-A) + PAR – PA treatment (320–700 nm), and, (c) Samples exposed to PAR only – P treatment (400–700 nm). Additionally, depth profiles were done to determine different physical (i.e., temperature and underwater radiation field) and biological characteristics of the water column – photosynthetic pigments, UV-absorbing compounds, cell concentration, deoxyribonucleic acid (DNA) and cyclobutane pyrimidine dimers (CPDs). The effects of UVR on natural phytoplankton assemblages were significant only in the first 50 cm of the water column, causing a decrease in photosynthetic rates of 36 and 20% due to UV-A and ultraviolet-B radiation (UV-B), respectively; below this depth, however, there were no significant differences between radiation treatments. Concentration of CPDs per mega base of DNA in natural phytoplankton was low, <27 CPDs MB−1 between 0 and 4 m. Data on net DNA damage, together with that on mixing conditions of the water column, suggest that mixing can favour phytoplankton by allowing cells to be transported to depths where active repair can take place. This mechanism to reduce UVR-induced DNA damage would be of great advantage for these assemblages dominated by small cyanobacteria and chlorophytes where UV-absorbing compounds that could act as sunscreens are virtually absent.

A multi-proxy study of Argentina loess: Marine oxygen isotope stage 4 and 5 environmental record from pedogenic hematite

A 12 m loess–paleosol section (La Angostura) from Tafi del Valle, Tucumán, Argentina was studied using rock magnetic measurements, geochemistry, Mössbauer effect, particle size and Nd isotopes in order to elucidate changes in paleoenvironment at the site. Combining the results of these experiments allowed us to separate the effects of the two major controls (parent material and pedogenesis) on the measured parameters. Two behavior regimes were identified in the sediment: 1) in the lower half of the section pedogenic processes, though weak, dominate the variation of the sediment's physical properties, and 2) for the upper half of the section subtle changes in parent material control the majority of the measured parameters, even those typically controlled by pedogenesis, e.g., clay fraction. Applying rock magnetic, Mössbauer, and bulk geochemistry measurements in concert we calculated an iron budget for the loess. ∼80% of the iron in the sediment is in paramagnetic minerals; ∼20% of the iron is in hematite (∼1.6% of the sediment's mass), and ≪ 1% of the total iron is present as magnetite/maghemite (∼0.1% of the sediment's total mass). Neither the pedogenic formation nor the depletion of ferrimagnetic material with increasing pedogenesis was consistently observed in this sediment. Throughout the entire section the concentration of hematite is argued to be sensitive to in situ alteration, and it is further surmised to be sensitive to the available moisture at the site. Finally we present a curiosity: within the error of our chronology, the concentration of hematite seems to correlate with the coeval climate record from Antarctica (Vostok). It is possible that the sedimentary–magnetic record from La Angostura is a proxy for the strength of the South American summer monsoon.

Dataciones radiometricas (&lt;sup&gt;14&lt;/sup&gt;C y K/Ar) del Teide y el Rift noroeste, Tenerife, Islas Canarias

Teide volcano, the highest volcano on earth (3,718 m a.s.l., > 7 km high) after Mauna Loa and Mauna Kea in the Hawaiian Islands, forms a volcanic complex in the centre of the island of Tenerife. Its most recent eruptive activity (last 20 Ka) is associated with the very active NW branch of the 120" triple rift system of the island. Most of the eruptions of Tenerife during the past 20 Ka have occurred along these volcanic features, frequently in the production of extensive mafic and felsic lava flows, many of which reached the coast, crossing what is now one of the most densely populated areas of Tenerife and of any oceanic island in the world. However, despite numerous previous studies, very important basic geological information is still lacking, in particular dating of these flows to construct a geochronological framework for the evolution of the Teide-NW rift system, and a scientifically based, much needed volcanic hazard assessment. New carbon- 14 ages, obtained via coupled mass spectrometry (other in progress), provide important time constraints on the evoliition of Teide's volcanic system, the frequency and distribution of its eruptions, and associated volcanic hazards. Most of the eruptions are not related to the Teide stratovolcano, which apparently had only one eruption in the last 20 Ka about 1,240 f 60 years BP (between 1,287 CAL years BP and 1,007 CAL years BP, corresponding to a time interval between the VI1 and X centuries, 663 years AD to 943 years AD), but to the Pico Viejo volcano (17,570 f 150 years BP), flank parasitic vents (Mna. Abejera upper vent, 5,170 f 110 years BP; Mna. Abejera lower vent, 4,790 f 70 years BP; Mna. de La Angostura early, 2,420 f 70 years BP; Mna. La Angostura late, 2,010 f 60 years BP and Roques Blancos, 1,790 f 60 years BP) and the NW rift (Mna. Chio, 3,620 f 70 years BP). Although the volcanic activity during the past 20 Ka involved at least 7 voluminous phonolitic flank vents in the northem, more unstable slopes of Teide, it took place without any apparent response of the volcano; on the contrary, these eruptions seemed to progressively buttress and enhance the stability of Teide Volcano. Conversely, the occurrence of these flank eruptions, combined with the Pico Viejo and NW rift eruptions, poses a very high lava-flow risk to the now densely populated areas in north and west Tenerife, which have been almost entirely resurfaced during the past 20,000 years.

A NEW FOSSIL PHYLLOTINE (RODENTIA: MURIDAE) FROM NORTHWESTERN ARGENTINA AND RELATIONSHIPS OF THEREITHRODONGROUP

Abstract We describe an extinct murid, assigned to the tribe Phyllotini, from the late Pleistocene (Tafi del Valle Formation) of La Angostura (26°55′30″S, 65°41′50″W; 1,900 m elevation) in northwestern Tucuman Province, Argentina. The new genus is characterized by very hypsodont molars with flat crowns and simplified occlusal structure, upper incisors with a mediolateral groove, a straight premaxillary–maxillary suture, and high zygomatic plate with a small spine on its anterodorsal edge. Phylogenetic analyses including fossil and living members of the Reithrodon group show that the new genus is the sister taxon to the Reithrodon–Neotomys–Euneomys clade. The paleoenvironmental and paleogeographic significance of the new genus is discussed within the context of the climatic changes that occurred during the late Pleistocene in southern South America. The new phyllotine would have lived in high-elevation grasslands, which today occur >1,000 m higher under cold and dry climatic conditions than those of the la...

Evolution of the Cañadas edifice and its implications for the origin of the Cañadas Caldera (Tenerife, Canary Islands)

The volcano-stratigraphic and geochronologic data presented in this work show that the Tenerife central zone has been occupied during the last 3 Ma by shield or central composite volcanoes which reached more than 3000 m in height. The last volcanic system, the presently active Teide-Pico Viejo Complex began to form approximately 150 ka ago. The first Cañadas Edifice (CE) volcanic activity took place between about 3.5 Ma and 2.7 Ma. The CE-I is formed mainly by basalts, trachybasalts and trachytes. The remains of this phase outcrop in the Cañadas Wall (CW) sectors of La Angostura (3.5–3.0 Ma and 3.0–2.7 Ma), Boca de Tauce (3.0 Ma), and in the bottom of some external radial ravines (3.5 Ma). The position of its main emission center was located in the central part of the CC. The volcano could have reached 3000 m in height. This edifice underwent a partial destruction by failure and flank collapse, forming debris-avalanches during the 2.6–2.3 Ma period. The debris-avalanche deposits can be seen in the most distal zones in the N flank of the CE-I (Tigaiga Breccia). A new volcanic phase, whose deposits overlie the remains of CE-I and the former debris-avalanche deposits, constituted a new volcanic edifice, the CE-II. The dyke directions analysis and the morphological reconstruction suggest that the CE-II center was situated somewhat westward of the CE-I, reaching some 3200 m in height. The CE-II formations are well exposed on the CW, especially at the El Cedro (2.3–2.00 Ma) sector. They are also frequent in the S flank of the edifice (2.25–1.89 Ma) in Tejina (2.5–1.87 Ma) as well as in the Tigaiga massif to the N (2.23 Ma). During the last periods of activity of CE-II, important explosive eruptions took place forming ignimbrites, pyroclastic flows, and fall deposits of trachytic composition. Their ages vary between 1.5 and 1.6 Ma (Adeje ignimbrites, to the W). In the CW, the Upper Ucanca phonolitic Unit (1.4 Ma) could be the last main episode of the CE-II. Afterwards, the Cañadas III phase began. It is well represented in the CW sectors of Tigaiga (1.1 Ma–0.27 Ma), Las Pilas (1.03 Ma–0.78 Ma), Diego Hernández (0.54 Ma–0.17 Ma) and Guajara (1.1 Ma–0.7 Ma). The materials of this edifice are also found in the SE flank. These materials are trachybasaltic lava-flows and abundant phonolitic lava and pyroclastic flows (0.6 Ma–0.5 Ma) associated with abundant plinian falls. The CE-III was essentially built between 0.9 and 0.2 Ma, a period when the volcanic activity was also intense in the `Dorsal Edifice' situated in the easterly wing of Tenerife. The so called `valleys' of La Orotava and Güimar, transversals to the ridge axis, also formed during this period. In the central part of Tenerife, the CE-III completed its evolution with an explosive deposit resting on the top of the CE, for which ages from 0.173 to 0.13 Ma have been obtained. The CC age must be younger due to the fact that the present caldera scarp cuts these deposits. On the controversial origin of the CC (central vertical collapse vs. repeated flank failure and lateral collapse of mature volcanic edifices), the data discussed in this paper favor the second hypothesis. Clearly several debris-avalanche type events exist in the history of the volcano but most of the deposits are now under the sea. The caldera wall should represent the proximal scarps of the large slides whose intermediate scarps are covered by the more recent Teide-Pico Viejo volcanoes.

Petrology of some Mexican mesozoic-cenozoic plutons: Sources and tectonic environments

In central and southern Mexico, three Late Mesozoic to Early Tertiary plutonic suites have been investigated. They record the evolution of the Mexican Pacific margin through space and time and more specifically its crustal thickness. In Central Mexico the La Angostura pluton (110-100 Ma) is calc-alkaline and intrudes a Late Jurassic-Early Cretaceous exotic arc, represented by the Guanajuato magmatic sequence. Both pluton and country rocks were affected by a greenschist facies metamorphism. Their high εNd(i) and low εSr(i) values are more or less similar to those of the Guanajuato arc tholeiitic rocks and suggest that both the calc-alkaline intrusions and the arc tholeiitic suite were derived from a mantle source essentially uncontaminated by a crustal component. The La Angostura calc-alkaline plutonic rocks could represent the remnants of the mature stage of this arc before its tectonic emplacement on the western margin of the North American craton. In the vicinity of the La Angostura pluton, the Eocene (ε50 Ma) Comanja and La Estancia granodiorites are chemically similar to volcanic arc granites and their low εNd(i) and high εSr(i) ratios suggest that they were derived from a mantle source highly contaminated by continental crust. Finally, the youngest plutonic suite (38 Ma), exposed near Zihuatanejo ranges from gabbro to granodiorite and displays calc-alkaline compositional features. Their εNd(i) and εSr(i) ratios indicate that the Zihuatanejo plutonic suite was derived from a mantle source which has been weakly contaminated by subducted sediments and/or continental crust. The contrasting petrology, chemistry and isotopic ratios of the Comanja-La Estancia granodiorites and the Zihuatanejo gabbro-granodiorite suite reflect a difference in thickness of their continental basement.

UPb zircon and monazite ages of the La Angostura granite and the orogenic history of the northwest Argentine basement

In the Cordillera Oriental of northwest Argentina, plutonic complexes of varying surface extent are aligned from NNE to SSW: Cañaní, Tipayoc, Fundición, Santa Rosa de Tastil, La Poma, Palermo, Cachi, La Paya, Molinos. They intrude the pelitic-psammitic series of the latest Precambrian-Early Cambrian Puncoviscana Formation in many places. Intrusion of the La Angostura granite (15 km south of Molinos) is believed to be post-kinematic to a second deformation (D 2) in the Sierra de Cachi. U-Pb isotopic ages of zircons and monazites have been determined on the La Angostura granite, formerly considered as Precambrian. The lower concordia intercept age of 453 + 25/−27 Ma is interpreted as the time of zircon crystallization during emplacement of the granitic complex. Within the limits of error, this Late Ordovician intercept age is confirmed by a monazite crystallization age of 462 ± 1 Ma. Thus, a Precambrian age for the granite at La Angostrua and for the minimum age of sedimentation of the Puncoviscana Formation in this area of the Cordillera Oriental must be excluded.

La Angostura dam underground powerhouse. Prediction and measurement of displacements during excavation : Arambula, JA Univ. Nac. Auton. Mexico, MEX Conference. 12F, 3T, 8R. Proc. Third Congress, Int. Soc. Rock Mech. Denver, 1974, V2, Part B, 1974, p1231–1241

La Angostura dam underground powerhouse. Prediction and measurement of displacements during excavation : Arambula, JA Univ. Nac. Auton. Mexico, MEX Conference. 12F, 3T, 8R. Proc. Third Congress, Int. Soc. Rock Mech. Denver, 1974, V2, Part B, 1974, p1231–1241