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- Buchan, Kenneth L., et al.
(författare)
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Rodinia : the evidence from integrated palaeomagnetism and U-Pb geochronology
- 2001
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Ingår i: Precambrian Research. - 0301-9268 .- 1872-7433. ; 110:1-4, s. 9-32
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Tidskriftsartikel (refereegranskat)abstract
- Of many hundreds of well-defined palaeomagnetic poles that have been reported from cratons around the world in the 1700-500 Ma period, only a few are precisely dated. However, such ‘key' palaeopoles are a prerequisite for establishing rigorous palaeomagnetic reconstructions in order to chart the assembly, drift and breakup of the postulated late Precambrian supercontinent of Rodinia. Most key palaeopoles are derived from mafic dykes and sills that have been dated by U-Pb techniques. Most are from Laurentia, the largest and best studied of the continental fragments that are thought to have comprised Rodinia. Thirteen key Laurentia palaeopoles form an incomplete reference set that can be used for comparison with key palaeopoles from other cratons as they become available. Currently, there are four key palaeopoles for Baltica between 1700 and 500 Ma, although only one allows a direct comparison with a similar aged pole from Laurentia. The 1265 Ma match between Baltica and Laurentia is consistent with reconstructions in which Baltica is adjacent to present-day east Greenland, with the ca. 1700-1500 Ma Gothian and Labradorian belts aligned. Few key palaeopoles are yet available from other cratons. However, recent U-Pb dating of dykes, sills, or volcanic rocks in the Siberian, Australian and Kalahari cratons and in Coats Land of Antarctica constrains the ages of individual palaeopoles from each of these areas. Most of these are not key palaeopoles because they have not been conclusively demonstrated to be primary, or local tectonic rotations have not been ruled out. Nevertheless, they are useful in testing Rodinia reconstructions. In this paper, a U-Pb baddeleyite age is reported from the late Gardar magmatic rocks of southwest Greenland. Along with the previously published palaeopole for this unit, this age helps constrain the Mesoproterozoic location of southwest Greenland relative to North America.
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| 2. |
- Nisca, Dan H., et al.
(författare)
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The lycksele structure, a huge ring formation in Northern Sweden : result of an impact?.
- 2000
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Ingår i: S T A R (Online). - 1548-8837. ; 38
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The current database of impact structures in Fennoscandia reveals 28 proven impact craters of various ages and sizes. Recently, we started to search for large and old impact structures, which may show traces of the impact in their shape or in their rocks and minerals and which can be diagnostically identified through the masking effects of postimpact deformations. Since we are dealing with strongly eroded and often deformed structures, the classical criteria for proving an impact origin are not tenable. Through a combined analysis of topography, drainage, gravity, magnetic, and petrophysical data, a circular structure has been identified in northern Sweden. The structure is characterized by a circular system of faults, arc-shaped contacts between rocks, and a circular distribution of granitic intrusions. The fault zone at the edges is defined from gradients in the gravity data, from magnetic data by arc-shaped anomalies, and from topographic data by an arc-shaped relief, which is also reflected in the drainage pattern. The zone is characterized by vertical faults that also cut the granitic intrusions at the edge of the formation, and from interpretation of gravity data an uplift of high density rocks (about 2850 kg/m(exp 3)) is indicated in the central part. The age of granites and pegmatites ranges from 1.82 to 1.78 Ga. Therefore, based on age data and on paleomagnetic results of various generations of dikes that cut and are truncated by the structure, the age of the formation lies within 1.80-1.26 Ga. The rocks have not been studied petrologically to see if any traces of shock metamorphism can be found. At present, four models for the origin of the structure are possible: (1) basement doming, (2) meteorite impact, (3) large buried pluton, or (4) fault-bounded block. An impact origin, however, seems most plausible.
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| 3. |
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| 4. |
- Thunehed, Hans, et al.
(författare)
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Lycksele structure in northern Sweden: Result of an impact?
- 2000
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Ingår i: Large meteorite impacts and planetary evolution II. - Boulder, Colo : Geological Society of America. - 0813723396 ; , s. 125-30
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- The Lycksele structure in northern Sweden is a large circular structure with a diameter of approximately 130 km. The structure has been defined from a combined analysis of topography, gravity, and magnetic data and is characterized by a circular system of faults, arc-shaped contacts between rocks, a quasi-circular system of downfaulted lowdensity granitic rocks, and an uplift of high-density rocks in the center. On the basis of ages of granite intrusions and a thrust zone that is cut by faults of the ring, together with paleomagnetic data, the age of the structure is between 1.80 and 1.26 Ga. The Bouguer gravity anomaly is similar to that associated with other known large impact structures on Earth. The rim of the structure, defined from the gravity gradients, magnetic data, and topography, is not restricted to specific rock types but cuts regional geologic structures as well as smaller intrusions, which may be expected for an impact structure in Precambrian deformed target rocks. Therefore, an impact origin seems most plausible for this structure, but to confirm this idea we need identifications of shock metamorphic features in the rocks.
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