Preliminary paleomagnetic study of rocks from the possible impact structure at vikebukten Bay, centr

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Preliminary paleomagnetic study of rocks from the
possible impact structure at Åvikebukten Bay,
central Sweden

• Preeden Ulla (1), Plado Jüri (1), Puura Väino
  (1), Kirs Juho (1) and Flodén Tom (2)
• (1) Department of Geology, University of Tartu
• (2) Department of Geology and Geochemistry,
  Stockholm University
• E-mail ulla.preeden_at_ut.ee

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Outline

• Location
• Origin
• How the rocks look like?
• Methods
• Paleomagnetic results
• Short mineralogical overview
• What next?

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Location

• There are quite a lot of impact structures on
  Fennoscandian Shield.
• Several of them are studied by geological,
  geophysical and geochemical methods.
• However, there are some structures that require
  further studies.

Figure from Dypvik et al. 2008
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• Söderström (1966) suggested that the bay was
  a caldera structure related to the Alnö complex.

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Impact structure at Åvikebukten Bay (Henkel and
Lilljequist 2001 Henkel et al. 2005)

• Extraordinarily circular topography.
• Diameter of 9.5 km at the present erosional
  level.
• Submarine central mound.
• Polymict breccia.
• Kink banded biotite, planar fractures in quartz
  and feldspar (PDFs in quartz).

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Examples of rock types in the outcrop.
Photos by J. Kirs
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Paleomagnetic method

• The essence of paleomagnetism is that the rock
  will lock in a fossil record of ancient (or
  paleo) magnetic field.
• Rocks are not closed systems once they have been
  formed - later geological processes may partially
  or completely overprint primary NRMs.
• Shock remanent magnetization is a significant
  mode of alteration of the intensity and direction
  of magnetization in rocks subjected to the
  dynamic and thermochemical effects associated
  with meteorite impact.

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Methods

• AF (alternating field) demagnetization.
• Thermal demagnetization.
• Standard component analysis (Kirschvink 1980
  Fisher 1953).
• Mineralogical studies and X-ray diffractometry.

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Examples of demagnetization curves
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Paleomagnetic results
Low coercivity and low-temperature component
(D  50.0º I  66.3º k  194.7,
a95  8.9º).
Alnö complex has similar low unblocking and low
coercivity paleomagnetic component from 15 dikes
(D  51.2º I  70.2º k  22.0,
a95  8.3º)(Meert et al. 2007).
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Colored line marks the APW path of Baltica
(after Torsvik et al. 1996 Torsvik and Rehnström
2001). Black circle 583 Ma Fen pole (Meert et
al. 1998). Red circle denotes Alnö complex of
age 584 Ma (Meert et al. 2007). Blue circle is
our study.
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Mineralogical studies
Polished sample of carbonatitic breccia (same
matrix composition as in Alnö)
Thin-section view - breccia in breccia
1 mm
2 mm
Photos by L. Saksing
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PDFs in quartz

• In 3 out of studied 25 thin-sections planar
  microdeformation features were observed.

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Opinions

• Geophysical data are consistent with the
  Åvikebukten Bay being an impact structure (Henkel
  et al. 2005).
• Radial on-shore fracture pattern.
• One system of PDFs.
• The impact can be NO younger than the Ediacaran.
  Moreover, the pole is similar to what is found
  from Alnö dikes.

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What next?

• More detailed study of breccias to find PDFs.
• Geochemical studies.
• Drilling in the central mound.
• ...

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Thank you!