Vertical and lateral collapses on Tenerife (Canary Islands) and other volcanic ocean islands: Comment

Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu

Rocas volcánicas submarinas en la base de la Formación Cañadas. Macizo de Tigaiga (N. de Tenerife)

A volcanic submarine formation crops out in the north coast of Tenerife, east of San Juan de la Rambla. This unit is formed by massive and pil/owed lavas, pillow-breccias, hyaloc/astites and palagonitic tuffs and is interpreted as consisUng of subaerial lavas that flowed into shal/ow marine waters. The lavas are amphibolic trachybasalts. An unaltered sample coming fram the most massive zone has yielded a whole rack 2.60±0.17 Ma K-Ar age. These submarine lavas are underlying the oldest formations of the Las Cañadas Series dated at 1.24 Ma in that same area. The submarine lavas correspond to a time period for which very scarce information was yet available in Tenerife

Repeated debris avalanches on Tenerife and genesis of Las Cañadas caldera wall (Canary Islands)

Geologic evidence on Tenerife, Canary Islands, indicates six successive north-directed debris avalanche events, including: the Anaga and Teno (ca. 6 Ma) events that affected the old basaltic series, and the Tigaiga (>2.3 Ma), Roques de García (possibly 0.6–0.7 Ma), Orotava (ca. 0.6 Ma), and Icod (<0.15 Ma) avalanche events that affected the Cañadas and Dorsal volcanic edifices. The approximate total volume (>1000 km3) inferred for these events can account for the volume of previous estimates of offshore volcanic debris. These repeated flank failures can also account for the present morphology of Las Cañadas caldera wall, which partly bounds a multiepisodic lateral-collapse structure 25 km wide

Un episodio volcánico pre-Cañadas en la Isla de Tenerife

Based on new radiometric data, a Pre-Cañadas volcanic building of 3.5-3.0 Ma has been defined. This new building is mainly compared of basaltic rack cropping out in several gorges to the S of Cañadas Building, inside of the «Pared de Las Cañadas», «Boca de Tauce», and «La Grieta» and «La Angostura» valleys. Estimated minumum diameter and maximun high is 23 Km and 2300 m respectively. The destruction of this volcanic structure could contribute in the formation of the volcanic breccia ocurring underneath the Tigaiga massif

Evolution of the eastern volcanic ridge of the Canary Islands based on new K-Ar data

New age determinations from Tenerife, together with those previously published (93 in all), provide a fairly comprehensive picture of the volcanic evolution of the island. The oldest volcanic series, with ages starting in the late Miocene, are formed mainly by basalts with some trachytes and phonolites which appear in Anaga, Teno and Roque del Conde massifs. In Anaga (NE), three volcanic cycles occurred: one older than 6.5 Ma, a second one between 6.5 and 4.5 Ma, with a possible gap between 5.4 and 4.8 Ma, and a late cycle around 3.6 Ma. n Teno (NW), after some undated units, the activity took place between 6.7 and 4.5 Ma, with two main series separated by a possible pause between 6.2 and 5.6 Ma. In the zone of Roque del Conde (S), the ages are scattered between 11.6 and 3.5 Ma. Between 3.3 and 1.9 Ma, the whole island underwent a period of volcanic quiescence and erosion

Données geochronologiques sur les granites de La Cabrera

El Plutón de La Cabrera constituye la intrusión granítica tardihercínica más oriental del Sistema Central español. Su carácter es netamente alóctono y su emplazamiento se verifica con posterioridad a los procesos metamórficos y deformativos de este ciclo. La composición petrológica predominante es granítica (adamellítica), existiendo localmente facies marginales granodioríticas, y una gran proporción de rocas aplíticas y de grano fino muy diferenciadas. Este último tipo de rocas tiene relaciones de contactos variables frente a los granitos de grano grueso, a los que puede estar cortando nítidamente o bien presentar limites difusos y transicionales, generalmente en los afloramientos de mayores dimensiones. Con el fin de establecer sus relaciones cronológicas se seleccionaron dos series, una de granitos de grano grueso y otra de la facies microgranuda sobre las que se han hecho determinaciones de Rb-Sr. Ambas series dan buenas isocronas con los valores de 310 ±14 MA. y 87Sr/86Sr inicial de 0,7094 para la facies de grano grueso, y 287 ± S MA. y 87Sr/68Sr inicial de 0,7073 para la facies de grano fino. Se discuten las diferencias encontradas en estos resultados

Relaciones entre ortogénesis y series volcano-sedimentarias en el Macizo de El Caloco (Guadarrama central)

El macizo de El Caloco está constituido por dos formaciones metamórficas de caracteres distintos. Una es fundamentalmente paraderivada (metapelitas con niveles carbonatados asociados), con tramos de origen volcano-sedimentarios o subvolcánico (gneises glandulares fémicos) y que se ha denominado «serie fémica heterogénea». Aparecen también metabasitas con clinopiroxeno, granate y plagioclasa, en parte anfibolitizadas. La otra formación es de naturaleza cuarzo-feldespática y está compuesta por ortogneises glandulares y leucogneises de origen metagranítico. En función de los caracteres petrológicos y geoquímicos interpretamos los ortogneises cuarzo-feldespáticos como representantes de antiguos granitos de dos micas con silicatos de aluminio (granitos tipo 8). El estudio geoquímico pone de manifiesto en las facies más diferenciadas (leucogneises con nidos de turmalina y/o granate) un trend evolutivo de carácter sílico-potásico. Existe un contacto neto y discordante entre ambos conjuntos, encontrándose localmente facies mosqueadas en los esquistos metapelíticos próximos al mismo

Evolución metamórfica hercínica de la región de Segovia (Sierra de Guadarrama)

La región de Segovia representa uno de los sectores más profundos de la Sierra de Guadarrama, donde el metamorfismo regional hercínico alcanza la mayor intensidad. La evolución tectonometamórfica de este sector coincide con la que se encuentra en los niveles meso corticales de las cadenas con fuerte engrosamiento cortical. Una primera etapa prograda de media P/T (MI), contemporánea con la tectónica tangencial más intensa, da paso a un estadio paroxismal marcadamente descompresivo (M2) y a una etapa final retrógrada (M3). Las condiciones metamórficas más extremas tuvieron lugar en la parte de alta temperatura de la facies de las anfibolitas. En las rocas pelíticas y cuarzo feldespáticas la desestabilización de la moscovita conduce a la aparición de una extensa zona con ortosa + sillimanita, dentro de la cual queda incluida toda la región considerada. En las metabasitas, las condiciones termales más extremas vienen definidas por la coexistencia de hornblenda, diopsido y plagioclasa, que definen texturas subgranoblásticas con bordes de grano netos

Volcanic evolution of the island of Tenerife (Canary Islands) in the light of new K-Ar data

New age determinations from Tenerife, together with those previously published (93 in all), provide a fairly comprehensive picture of the volcanic evolution of the island. The oldest volcanic series, with ages starting in the late Miocene, are formed mainly by basalts with some trachytes and phonolites which appear in Anaga, Teno and Roque del Conde massifs. In Anaga (NE), three volcanic cycles occurred: one older than 6.5 Ma, a second one between 6.5 and 4.5 Ma, with a possible gap between 5.4 and 4.8 Ma. and a late cycle around 3.6 Ma. In Teno (NW), after some undated units. the activity took place between 6.7 and 4.5 Ma, with two main series separated by a possible pause between 6.2 and 5.6 Ma. In the zone of Roque del Conde (S),the ages are scattered between 11.6 and 3.5 Ma. Between 3.3 and 1.9 Ma, the whole island underwent a period of volcanic quiescence and erosion. The large Caaadas volcano, made up of basalts, trachytes and phonolites, was built essentiaiiy between 1.9 and 0.2 Ma. To the NE of this'central volcano, linking it with Anaga, is a chain of basaltic emission centers, with a peak of activity around 0.8 Ma. The Cañadas Caldera had several collapse phases, associated with large ignimbrite emissions. There were, at least, an older phase more than 1 Ma old, on the western part of the volcano, and a younger one, less than 0.6 Ma old, in the eastern side. The two large "valleys" of Guimar and la Orotava were formed by large landslides less than 0.8 Ma ago, and probably before 0.6 Ma ago. The present Cañadas caldera was formed by another landslide, less than 0.2 Ma ago. This caldera was later filled by the huge Teide volcano, which has been active even in historic times. During the same period a series of small volcanoes erupted at scattered locations throughout the island. The average eruptive rate in Tenerife was 0.3 km3/ka, with relatively small variations for the different eruptive periods. This island and La Gomera represent a model of growth by discontinuous pulses of volcanic activity, separated by gaps often coinciding with episodes of destruction of the edifices and sometimes extended for several million years. The neighbouring Gran Canaria, on the other hand, had an initial, rapid "shield-building phase" during which more than 90% of the island was built, and a series of smaiier pulses at a much later period. A comparison between these three central islands indicates that the previously postulated westward displacement in time of a gap in the volcanic activity is valid only as a first approximation. Several gaps are present on each island, overlapping in time and not clearly supporting either of the models proposed to explain the evolution of the Canaries