1,720,967 research outputs found
Fault gouge analyses: K-Ar illite dating, clay mineralogy and tectonic significance-a study from the Sierras Pampeanas, Argentina
No full textWe introduce a method for the detailed interpretation of K-Ar illite fine-fraction ages of fault gouges from non-sedimentary host rocks. Ages are cross-evaluated with several independent parameters, e.g. illite crystallinity, illite polytype quantification, grain size, mineralogical observations, K-Ar muscovite and biotite host-rock cooling ages as well as low-temperature thermochronological data (AFT, AHe, ZHe). This interpretation approach is applied to a regional study in order to constrain the 'deformation path' of the Eastern Sierras Pampeanas in NW Argentina. In the course of this study, a large number of gouge-bearing fault zones were systematically sampled and analysed. Obtained K-Ar illite fine-fraction ages range from Devonian to Cretaceous times, documenting a long-lasting brittle fault activity in this region. Ages > 320 Ma are synchronous with a period of intra-Carboniferous compressional tectonism, whereas Permo-Triassic ages are contemporaneous to a flat-slab subduction episode of the Farallon plate beneath the South American plate. Middle to Late Permian and Early Triassic ages as well as Early Jurassic to Middle Cretaceous ages correlate with extensional tectonics in this region. Additionally, K-Ar illite ages reveal a propagation of brittle deformation from north to south in the Sierras de Crdoba and San Luis. Data integrity and consistency with other chronometers and geological evidence show that the here suggested interpretation is valid and can provide a powerful tool to evaluate cooling and deformation histories. Despite of that, we could show that the reliability of fault gouge data strongly depends on the regional cooling.German Science Foundation (DFG) [SI 438/31-1]; DAAD [D/08/48018
Cenozoic exhumation and uplift of the Sierra de Pie de Palo constrained by Thermochronological methods
No full textThe Sierra de Pie de Palo located between 67 degrees 30'-68 degrees 30' W and 31 degrees 00'-32 degrees 00' S in the Argentine Western Sierras Pampeanas in Argentina is a distinct basement range, which lacks thermochronological data deciphering its exhumation and uplift history below 200 degrees C. Integrated cooling histories constrained by apatite fission-track data as well as (U-Th)/He measurements of zircon and apatite reveal that the structural evolution of this mountain range commenced during the Late Paleozoic and was mainly controlled by tectonically triggered erosion. Following further erosional controlled exhumation in a more or less extensional regime during the Mesozoic, the modern topography was generated by denudation in the Paleogene during the early stage of the Andean deformation, whereupon deformation propagated towards the west since the Late Mesozoic to Paleogene. This evolution is characterised by a total of 3.7 -4.2 km vertical rock uplift and by 1.7-2.2 km exhumation with a rate of 0.03-0.04 mm/a within the Sierra de Pie de Palo since ca. 60 Ma. Onset of uplift of peak level is also referred to that time resulting in a less Pliocene amount of uplift than previously assumed. (C) 2013 Elsevier Ltd. All rights reserved.German Research Council (DFG) [SI 438/31-1
Is the exhumation of the Sierras Pampeanas only related to Neogene flat-slab subduction? Implications from a multi-thermochronological approach
No full textThis paper presents new thermochronological data and reviews a set of recently published data for the Sierras Pampeanas in central and northwestern Argentina, which constitutes a distinct morphotectonic feature between 27 degrees S and 33 degrees S. Thermochronological data, derived from zircon and apatite (U-Th)/He, as well as apatite fission-track dating, reveal that cooling below 200 degrees C commenced locally during the Carboniferous period. In Permo-Triassic times, pronounced cooling propagated from east to west in the Southern Sierras Pampeanas, being time-equivalent and spatially equivalent to a flat-slab subduction period at these latitudes. Mesozoic rifting, accompanied by sedimentation and burial re-heating, only affected the thermal history of sampled rocks locally, suggesting that substantial sedimentary thicknesses were only accumulated along narrow and spatially-restricted Cretaceous rift basins. Final cooling in the northern Pampean ranges occurred during the Miocene. Contrastingly, in the Southern and Southwestern Sierras Pampeanas, cooling to near-surface temperatures occurred between the Late Cretaceous and the Paleogene, supporting the idea that a positive topography already existed in these areas before the Neogene. This contradicts the previous hypothesis that the uplift of the Pampean ranges is completely related to the Neogene flat-slab subduction. Instead, this process just accentuated a preexisting relief built up by diachronously developed Mesozoic land surfaces. Calculated long-term denudation rates, varying between 0.010 and 0.024 km/Ma, also support the idea of diachronous surfaces preserved since Mesozoic times. (C) 2013 Elsevier Ltd. All rights reserved.German Science Foundation (DFG) [SI 438/31-1]; DAAD [1461 D/08/48018
Exhumation and uplift of the Sierras Pampeanas: preliminary implications from K–Ar fault gouge dating and low-T thermochronology in the Sierra de Comechingones (Argentina)
Full text linkThe Sierras Pampeanas in central and north-western Argentina constitute a distinct morphotectonic feature between 27°S and 33°S. The last stage of uplift and deformation in this area are interpreted to be closely related to the Andean flat-slab subduction of the Nazca plate beneath the South American plate. K–Ar fault gouge dating and low-temperature thermochronology along two transects within the Sierra de Comechingones reveal a minimum age for the onset of brittle deformation about 340 Ma, very low exhumation rates since Late Paleozoic time, as well as a total exhumation of about 2.3 km since the Late Cretaceous. New Ar–Ar ages (7.54–1.91 Ma) of volcanic rocks from the San Luis volcanic belt support the eastward propagation of the flat-slab magmatic front, confirming the onset of flat-slab related deformation in this region at 11.2 Ma. Although low-temperature thermochronology does not clearly constrain the signal of the Andean uplift, it is understood that the current structural relief related to the Comechingones range has been achieved after the exhumation of both fault walls (circa 80–70 Ma)
Refined exhumation history of the northern Sierras Pampeanas, Argentina
No full textThe Sierra de Aconquija and Cumbres Calchaquies in the thick-skinned northern Sierras Pampeanas, NW Argentina present an ideal setting to investigate the tectonically and erosionally controlled exhumation and uplift history of mountain ranges using thermochronological methods. Although these ranges are located along strike of one another, their spatiotemporal evolution varies significantly. Integrating modeled cooling histories constrained by K-Ar ages of muscovite and biotite, apatite fission track data as well as (U-Th)/He measurement of zircon and apatite reveal the structural evolution of these ranges beginning in the late stage of the Paleozoic Famatinian Orogeny. Following localized rift-related exhumation in the central part of the study area and slow erosion elsewhere, growth of the modern topography commenced in the Cenozoic during Andean deformation. The main activity occurred during the late Miocene, with varying magnitudes of rock uplift, surface uplift, and exhumation in the two mountain ranges. The Cumbres Calchaquies is characterized by a total of 5-7km of vertical rock uplift, around 3km of crestal surface uplift, and a maximum exhumation of 2-4km since that time. The Sierra de Aconquija experienced 10-13km of vertical rock uplift, similar to 4-5km of peak surface uplift, and 6-8km of exhumation since around 9Ma. Much of this exhumation occurred along a previously poorly recognized fault. Miocene reactivation of Cretaceous rift structures may explain along-strike variations within these ranges. Dating of sedimentary samples from adjacent basins supports the evolutionary model developed for the mountain ranges
From feeder dykes to scoria cones: the tectonically controlled plumbing system of the Rauðhólar volcanic chain, Northern Volcanic Zone, Iceland
No full textThe Rauoholar volcanic chain, located in the Northern Volcanic Zone of Iceland, has been variably eroded such that, in the northern part, the original scoria cones are preserved, while the central and southern parts expose their shallow feeders. The chain thus offers insight into the inner workings of the near-surface feeder system of scoria cones. The volcanic chain was mapped in 3D using GPS. The en echelon-arranged volcanic chain can be divided into three parts: The southernmost part contains only plugs and necks with a thin pyroclastic cover as well as multi-tiered lava flows. The central part combines partially eroded scoria cones, (feeder) dyke intersections, and welded scoria interbedded within rootless and clastogenic lava flows; the welded scoria is composed of different kinds of lithics and bombs. The northern part preserves almost intact, overlapping scoria cones with voluminous lapilli- sized scoriaceous deposits. The overall dyke trend is orthogonal but shows radial patterns in individual cone complexes. Feeder dykes observed to depths of about 200 m below the volcanic chain are up to 8 m thick and flare in to conduits in the uppermost 20-50 m. The exposed shallow plumbing system shows that magma pathways through the volcanic edifice are very complex with incremental, repeated intrusions. We interpret the arcuate shape to be the result of a local change in the orientation of the stress field because the Rauoholar volcanic chain is located within a major relay structure between volcanoes on the eastern Fremrinamur rift arm and a rift extension with grabens on the western periphery
Exhumation history and landscape evolution of the Sierra de San Luis (Sierras Pampeanas, Argentina) - new insights from low - temperature thermochronological data
Full text linkThis paper presents low-temperature thermochronological data and K‑Ar fault gouge ages from the Sierra
de San Luis in the Eastern Sierras Pampeanas in order to constrain its low-temperature thermal evolution and exhumation
history. Thermal modelling based on (U-Th)/He dating of apatite and zircon and apatite fission track dating point
to the Middle Permian and the Triassic/Early Jurassic as main cooling/exhumation phases, equivalent to ca. 40-50% of
the total exhumation recorded by the applied methods. Cooling rates are generally low to moderate, varying between
2-10 °C/Ma during the Permian and Triassic periods and 0.5-1.5 °C/Ma in post-Triassic times. Slow cooling and, thus,
persistent residence of samples in partial retention/partial annealing temperature conditions strongly influenced obtained
ages. Thermochronological data indicate no significant exhumation after Cretaceous times, suggesting that sampled
rocks were already at or near surface by the Cretaceous or even before. As consequence, Cenozoic cooling rates are
low, generally between 0.2-0.5 °C/Ma which is, depending on geothermal gradient used for calculation, equivalent to a
total Cenozoic exhumation of 0.6-1.8 km. K-Ar fault gouge data reveal long-term brittle fault activity. Fault gouge ages
constrain the end of ductile and onset of brittle deformation in the Sierra de San Luis to the Late Carboniferous/Early
Permian. Youngest K-Ar illite ages of 222-172 Ma are interpreted to represent the last illite formation event, although
fault activity is recorded up to the Holocene
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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