The lycksele structure, a huge ring formation in Northern Sweden : result of an impact?.

• 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.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geofysik (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geophysics (hsv//eng)

Nyckelord

Tillämpad geofysik

Applied Geophysics

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