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  • 1
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    In:  Earth planet. Sci. Lett., Kunming, China, D. Reidel Publishing Company, vol. 120, no. 3, pp. 221-238, pp. L09303, (ISSN: 1340-4202)
    Publication Date: 1993
    Description: Strongly mylonitic rocks associated with the regionally extensive Norumbega fault zone in south-central Maine provide an excellent opportunity for testing the effects of mylonitization on argon isotopic systems in muscovite. 40Ar/39Ar muscovite age spectra from samples outside the zone of mylonitization are relatively undisturbed and have well defined Early Carboniferous plateau ages. In contrast to these nonmylonitized samples, all age spectra for muscovite from the mylonites are highly discordant. They are characterized by young ages at low extraction temperatures, which systematically increase to ages that equal the plateau ages for muscovite collected outside the mylonite zone. Detailed petrographic observations suggest that these systematic discordances reflect a mixing of argon components from older, relict, muscovite porphyroclasts and fine-grained white mica aggregates that recrystallized during mylonitic deformation. Total gas ages of five different grain size fractions separated from the same mylonite sample become progressively younger with decreasing grain size; indicating a larger component of the recrystallized grains in the finer grain size fractions. Although the three finest grain size fractions give different total gas ages and do not overlap in age for most of their release spectra, their initial increments do coincide, at approximately 290 Ma. This indicates a minimal older age contribution from the relict porphyroclasts in the initial increments and suggests the 290 Ma age provides a good estimate for the time of mineral growth associated with mylonitic deformation. These data, combined with kinematic analysis, reveal that the segment of the Norumbega fault zone studied, the Sandhill Corner fault, is a Late Carboniferous-Early Permian dextral strike-slip fault. A lack of significant offset in regional Early Carboniferous mineral age patterns across the fault suggests that displacement was probably less than 30 km. This study demonstrates that 40Ar/39Ar dating methods can be used to date deformational events effectively, as long as several important criteria are met. First and foremost, samples must be well characterized prior to analysis. Dynamic recrystallization must have occurred at or below the closure temperature of the mineral to be analyzed. Regional cooling patterns must also be established through detailed thermochronology so that mineral ages and age spectra from the deformed rocks can be compared to regional cooling ages of the same mineral. Finally, the effects of excess argon must be negligible.
    Keywords: Fracture ; Geol. aspects ; Fault zone
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  • 2
    Publication Date: 1993-04-30
    Description: The preservation, age, and stratigraphic relation of an in situ ashfall layer with an underlying desert pavement in Arena Valley, southern Victoria Land, indicate that a cold-desert climate has persisted in Arena Valley during the past 4.3 million years. These data indicate that the present East Antarctic Ice Sheet has endured for this time and that average temperatures during the Pliocene in Arena Valley were no greater than 3 degrees C above present values. One implication is that the collapse of the East Antarctic Ice Sheet due to greenhouse warming is unlikely, even if global atmospheric temperatures rise to levels last experienced during mid-Pliocene times.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Marchant, D R -- Swisher, C C 3rd -- Lux, D R -- West, D P Jr -- Denton, G H -- New York, N.Y. -- Science. 1993 Apr 30;260(5108):667-70.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/17812227" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 3
    Publication Date: 1992-11-27
    Description: The South Tibetan detachment system separates the high-grade metamorphic core of the Himalayan orogen from its weakly metamorphosed suprastructure. It is thought to have developed in response to differences in gravitational potential energy produced by crustal thickening across the mountain front. Geochronologic data from the Rongbuk Valley, north of Qomolangma (Mount Everest) in southern Tibet, demonstrate that at least one segment of the detachment system was active between 19 and 22 million years ago, an interval characterized by large-scale crustal thickening at lower structural levels. These data suggest that decoupling between an extending upper crust and a converging lower crust was an important aspect of Himalayan tectonics in Miocene time.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Hodges, K V -- Parrish, R R -- Housh, T B -- Lux, D R -- Burchfiel, B C -- Royden, L H -- Chen, Z -- New York, N.Y. -- Science. 1992 Nov 27;258(5087):1466-70.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/17755108" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 4
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 323 (1986), S. 794-797 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Regional metamorphism in zones of continent-continent collision can be modelled mathematically as a thermal consequence of large-scale crustal thickening and subsequent erosion2"5. Such models depend on a previous knowledge of the burial and erosion history of the region. In addition, it is assumed ...
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1525-1314
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Following the Middle Devonian Acadian deformation an extensive belt of high grade metamorphism was formed in New England. In south-western Maine, at the northern end of this belt, there occurs a transition along the strike from regional low-pressure/high-temperature metamorphism to contact metamorphism in low-grade rocks. Petrological studies indicate that this transition occurs along a surface plunging to the north-east at about 3.5°, with respect to the Middle-to-Late Devonian erosion surface. In addition, detailed petrological mapping has defined a history of temporally separate, localized metamorphic events associated with plutonism and occurring at increasingly deeper levels to the south-west. Geochronological studies constrain ambient temperatures in the transition zone at the time of metamorphism to be less than 300° C in the north-east and between 350° C and 500° C in the south-west. They also establish a pattern of diachronous cooling due to differential uplift and erosion, with cooling occurring later and most rapidly to the south-west. Geophysical evidence suggests that along with this spatial variation in metamorphic style the shapes of the plutons in Maine undergo a transition from laterally extensive sheet-like bodies in the high grade terrane to more equant-shaped bodies in the low-grade terrane. Using the results of these petrological, geochronological and geophysical studies, as well as those of stratigraphical and structural studies we construct a thermal model for the transition zone. The model suggests that the Acadian metamorphism in south-western Maine is a result of deep-level contact metamorphism near laterally extensive granitic sills dipping to the north-east with respect to the present erosion surface. The plutons themselves are interpreted to be a result of lower crustal melting in response to crustal thickening in the presence of normal or slightly augmented mantle heat flux.
    Type of Medium: Electronic Resource
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  • 6
    Publication Date: 1992-11-27
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 7
    Publication Date: 1993-04-30
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 8
    Publication Date: 1986-10-01
    Print ISSN: 0028-0836
    Electronic ISSN: 1476-4687
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Published by Springer Nature
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  • 9
    Publication Date: 2013-06-29
    Description: We report results from comprehensive mapping and multi-technique geochemical characterization of obsidian from the Alca source in the Peruvian Andes (15.3°S, 72.7°W), aimed at understanding patterns of extraction and trade in one of the world’s centers of complex civilization. Alca obsidian was among the most economically important and widely distributed volcanic glasses used for stone tool making in South America from ca. 13 ka until recently, yet the geologic source of this material has never been studied comprehensively. Our work establishes Alca as one of the largest obsidian sources in South America and the only Peruvian source known to have patterned intrasource geochemical variability. There are six geochemically distinct Alca subsources exposed from 2710 to 5165 m elevation over 〉330 km 2 of the highlands, an area nearly seven times larger than previously known. Our results now permit provenance determination of artifacts to specific outcrops. We analyzed 252 geologic samples using energy-dispersive X-ray fluorescence, neutron activation analysis, wavelength-dispersive X-ray fluorescence, and nondestructive, portable energy-dispersive X-ray fluorescence. All techniques distinguish the same six Alca subsources, establishing analytical comparability between geochemical methods. Discrimination of Alca obsidian to the subsource level using portable X-ray fluorescence represents a major advance in nondestructive provenance analysis. Further nondestructive analysis of robust sets of obsidian artifacts within Peru will allow high-resolution study of the evolution of central Andean exchange systems.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
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