The geological development of southeastern Norrbotten, based on the Precambrian stratigraphy of the Luleå area

Lilljequist, Robert

doi:10.1080/11035898009450898

Cited by 11 publications

(2 citation statements)

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“…The contact with the Bothnian supergroup metasediments is not visible in the field along the profile, but macroscopic structures in outcrop in both metasediments and volcanic rocks in the vicinity are steeply west‐dipping. Calculations based on magnetic anomaly profiles indicate a shallower west‐dipping contact (Johan Söderman, SGU, personal communication) suggesting the sediments to have been thrusted upon the east‐lying volcanic complex (Lilljequist 1980; Öhlander et al 1993). One of the goals of the present investigation was to test this idea.…”

Section: Geological Settingmentioning

confidence: 99%

“…If the well‐defined lower crustal reflectivity below the Bothnian Bay was also observed on the test profile then the lack of lower crustal reflectivity elsewhere could possibly be attributed to geological factors and not necessarily to acquisition methods. Finally, the seismic test profile would also cross a major break in the magnetic anomaly pattern that may represent a Proterozoic southwest‐dipping thrust zone (Lilljequist 1980; Öhlander et al 1993). In order to image both upper crustal and lower crustal features the seismic test profile consisted of a small‐charge higher‐resolution shallow component and a larger‐charge lower‐resolution deep component.…”

Section: Introductionmentioning

confidence: 99%

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Crustal reflectivity near the Archaean-Proterozoic boundary in northern Sweden and implications for the tectonic evolution of the area

Juhlin¹,

Elming

Mellqvist³

et al. 2002

Geophysical Journal International

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Summary Sm–Nd isotope ratios of 1.9–1.8 Ga granitoids delineate the Archaean–Proterozoic boundary in northern Sweden, an important feature in the Fennoscandian Shield. The boundary strikes approximately WNW–ESE and is defined as a c. 20 km wide zone with juvenile Palaeoproterozoic rocks to the SSW and Archaean and Proterozoic rocks, derived to a large extent from Archaean sources, to the NNE. It therefore constitutes the strongly reworked margin of the old Archaean craton. Extrapolation of the boundary offshore into the Bothnian Bay and correlation with the marine reflection seismic BABEL Lines 2 and 3/4 indicates that the boundary dips to the south‐southwest, consistent with interpretation of the Sm–Nd data. In order to tie the BABEL results with onshore surface geology and obtain detailed images of the uppermost crust a short (30 km of subsurface coverage) pilot profile was acquired in the Luleå area of northern Sweden during August 1999. The profile consisted of a high‐resolution shallow component (1 kg shots) and a lower‐resolution deep component (12 kg shots). Both components image most of the reflective crust, with the deep component providing a better image below 10 s. Comparison of signal penetration curves with data acquired over the Trans‐Scandinavian Igneous Belt (a large batholith) indicate the transparent nature of the crust there to be caused by geological factors, not acquisition parameters. Lower crustal reflectivity patterns on the Luleå test profile are similar to those observed on the BABEL lines, suggesting the same lower crust onshore as offshore. Interpreted Archaean reflective upper crust in the NE extends below more transparent Proterozoic crust in the SW. This transparent crust contains a number of high‐amplitude reflectors that may represent shear zones and/or mafic rock within granite intrusions. A marked boundary in the magnetic field in the SW has been interpreted as being the result of a gently west‐dipping contact zone between meta‐sediments and felsic volcanic rocks, however, the seismic data indicate a near‐vertical structure in this area. By correlating the onshore and offshore seismic data we have better defined the location of the Archaean–Proterozoic boundary on the BABEL profiles. Our new interpretation of the crustal structure along the northern part of the BABEL Line 2 shows a more bi‐vergent geometry than previous interpretations. Comparison of the re‐interpreted crustal structure in northern Sweden with that found in the Middle Urals shows several similarities, in particular the accretion of a series of arcs to a stable craton. Based on this similarity and geological data, we deduce that a continental arc accreted to the southwestern margin of the Archaean craton at c. 1.87 Ga. Shortly thereafter, the Skellefte island arc underthrust the continental arc owing to a collision further to the southwest resulting in the bi‐vergent crustal structure observed today.

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Section: Geological Settingmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%