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@ut.ee

Outline

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

Figure from Dypvik et al. 2008

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.

Söderström (1966) suggested that the bay was a caldera structure related to the Alnö complex.

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).

Examples of rock typesin the outcrop.

Photos by J. Kirs

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.

Methods

• AF (alternating field) demagnetization.

• Thermal demagnetization.

• Standard component analysis (Kirschvink 1980; Fisher 1953).

• Mineralogical studies and X-ray diffractometry.

Examples of demagnetization curves

Low coercivity and low-temperaturecomponent (D = 50.0º; I = 66.3º; k = 194.7, a95 = 8.9º).

Paleomagnetic results

Alnö complex has similar low unblocking and low coercivitypaleomagnetic componentfrom 15 dikes (D = 51.2º; I = 70.2º; k = 22.0, a95 = 8.3º)(Meert et al. 2007).

J TrP

C

DS

C bn

P t

O

3 0 0 Eo 0 Eo 6 0 Eo 1 2 0 Eo

6 0 So

9 0 No

3 0 No

0 o

3 0 So

6 0 No

9 0 So

A ln p o leöF en p o le

Å v ike p o le

*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.

Mineralogical studiesPolished sample of carbonatitic breccia (same matrix composition as in Alnö)

Thin-section view -“breccia in breccia”

Photos by L. Saksing

1 mm

2 mm

PDFs in quartz

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

0

1

2

3

4

1 3 5 7 9 11 13 15 170 10 20 30 40 50 60 70 80 900

1

2

3

4 {10

13}

{10

12}

{224

1}

11 sets in11 grains

Num

ber

of

obse

rvat

ions

Angle c-axis (degrees)

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.

What next?

• More detailed study of breccias – to find PDFs.

• Geochemical studies.

• Drilling in the central mound.

• ...

Thank you!