Showing papers by "Paul W. Layer published in 2010"

The Sierra Ballena Shear Zone in the southernmost Dom Feliciano Belt (Uruguay): evolution, kinematics, and deformation conditions

Abstract: The Sierra Ballena Shear Zone (SBSZ) is part of a high-strain transcurrent system that divides the Neoproterozoic Dom Feliciano Belt of South America into two different domains. The basement on both sides of the SBSZ shows a deformation stage preceding that of the transcurrent deformation recognized as a high temperature mylonitic foliation associated with migmatization. Grain boundary migration and fluid-assisted grain boundary diffusion enhanced by partial melting were the main deformation mechanisms associated with this foliation. Age estimate of this episode is >658 Ma. The second stage corresponds to the start of transpressional deformation and the nucleation and development of the SBSZ. During this stage, pure shear dominates the deformation, and is characterized by the development of conjugate dextral and sinistral shear zones and the emplacement of syntectonic granites. This event dates to 658–600 Ma based on the age of these intrusions. The third stage was a second transpressional event at about 586 to <560 Ma that was associated with the emplacement of porphyry dikes and granites that show evidence of flattening. Deformation in the SBSZ took place, during the late stages, under regional low-grade conditions, as indicated by the metamorphic paragenesis in the supracrustals of the country rocks. Granitic mylonites show plastic deformation of quartz and brittle behavior of feldspar. A transition from magmatic to solid-state microstructures is also frequently observed in syntectonic granites. Mylonitic porphyries and quartz mylonites resulted from the deformation of alkaline porphyries and quartz veins emplaced in the shear zone. Quartz veins reflect the release of silica associated with the breakdown of feldspar to white mica during the evolution of the granitic mylonites to phyllonites, which resulted in shear zone weakening. Quartz microstructures characteristic of the transition between regime 2 and regime 3, grain boundary migration and incipient recrystallization in feldspar indicate deformation under lower amphibolite to upper greenschist conditions (550–400°C). On the other hand, the mylonitic porphyries display evidence of feldspar recrystallization suggesting magmatic or high-T solid-state deformation during cooling of the dikes.

Mafic Late Miocene–Quaternary volcanic rocks in the Kamchatka back arc region: implications for subduction geometry and slab history at the Pacific–Aleutian junction

Abstract: New 40Ar/39Ar and published 14C ages constrain voluminous mafic volcanism of the Kamchatka back-arc to Miocene (3–6 Ma) and Late Pleistocene to Holocene ( 20%). Younger rocks form in a back arc by lower melting degrees involving enriched mantle components. The arc front and Central Kamchatka Depression are also underlain by plateau lavas and shield volcanoes of Late Pleistocene age. The focus of these voluminous eruptions thus migrated in time and may be the result of a high fluid flux in a setting where the Emperor seamount subducts and the slab steepens during rollback during terrain accretions. The northern termination of Holocene volcanism locates the edge of the subducting Pacific plate below Kamchatka, a “slab-edge-effect” is not observed in the back arc region.

Late-Pleistocene flank collapse triggered by dome growth at Tacaná volcano, México-Guatemala, and its relationship to the regional stress regime

Abstract: During late Pleistocene time, the extrusion of an andesitic dome at the summit of Tacana volcano caused the collapse of its northwestern flank. The stratocone collapse was nearly parallel to the σ min stress direction suggesting that failure was controlled by the regional stress field. The event produced a debris avalanche that was channelized in the San Rafael River and moved 8 km downstream. The deposit covered a minimum area of 4 km2, had a volume of 0.8 ± 0.5 km3, with an H/L (vertical drop to horizontal transport distance ratio) of ~0.35, defining a degree of mobility that is atypical for volcanic debris avalanches. The flank failure undermined the summit dome leading to its collapse and the generation of a series of block-and-ash flows that were emplaced in quick succession and covered the avalanche surface. The collapse event left a 600-m-wide summit amphitheatre with a 30-degree opening to the northwest, and >200 m thick debris that blocked the San Rafael River. Remobilization of this material produced debris flows that eroded the primary deposits and cascaded into the Coatan River. After the collapse, the activity of Tacana continued with the emission of the Agua Zarca lava flow dated at 10 ± 6 ka (40Ar/39Ar), and pyroclastic surges dated at 10,610 + 330/−315 yr BP (14C), which provide a minimum age for the collapse event. During the Holocene, Tacana has been very active producing explosive and effusive eruptions that ended with the extrusion of two summit domes that today occupy the amphitheatre. The 1950 and 1986 phreatic outbursts occurred along the Pleistocene collapse scar. Currently ~300,000 inhabitants live within a 35 km radius of Tacana, and could conceivably be impacted by future events of similar magnitude.

Estudio vulcanológico y estructural de la secuencia estratigráfica Mil Cumbres y del campo geotérmico de Los Azufres, Mich

Abstract: El proposito de este estudio fue actualizar y detallar el conocimiento geologico de superficie y del subsuelo de Los Azufres para realizar propuestas de localizacion de nuevos pozos geotermicos y solucionar diversos problemas que pue dan presentarse en el campo. El estudio cubri o una superficie de 984 km 2 . Se identificaron tres tendencias principales de fracturamiento : NN O - SSE, NE - S O y E - O . El primer tipo de fracturamiento corresponde a una deformacion del Mioceno con geometria semi - ve rtical que afecto a l basamento, mientras los otros dos se formaron con el Cinturon Volcanico Mexicano , tienen geometria semi - vertical y sub - horizontal, y afectaron regionalmente a rocas miocenicas del basamento y a rocas cuaternarias que afloran en el camp o geotermic o . Las fallas mas importantes para la localizacion de nuevos pozos resu ltaron ser Maritaro y Agua Fria, de direccion E - O. Mediante estudios vulcanologico s, diversos estudios de laboratorio y correlaciones del subsuelo y superficie se reconstruyo la secuencia lito - estratigrafica de la zona identificando al menos 64 tipos de aparatos volcanicos y los limites de sus respectivos depositos, asi como seis secuencias volcanicas principales de la region. Los estudios vulcanologicos no aportan argumentos para sostener la existencia de una caldera volcanica de gran dimension como se ha bia manejado anos atras, sino mas bien definen a Los Azufres como un campo volcanico complejo. El producto final fue un mapa vulcanologico estructural digital geo - referenciado , elaborado en ArcGis 9.2, que ofrece muchas ventajas respecto a los antiguos mapas , y un modelo conceptual actualizado y bases de datos con informacion confiable

Reconnaissance study of the Taylor Mountains pluton, southwestern Alaska

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