Preliminary paleomagnetic study of rocks from the possible

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Transcript Preliminary paleomagnetic study of rocks from the possible

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: [email protected]
Outline
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Location
Origin
How the rocks look like?
Methods
Paleomagnetic results
Short mineralogical overview
What next?
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
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 types
in 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
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).
o
90 N
Alnö pole
Fen pole
60oN
Cbn
O
o
30 N
0
Åvike pole
o
D
S
Pt
C
o
30 S
Tr
J
P
o
60 S
o
90 S
o
300 E
o
0E
*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).
o
o
*Blue circle
60 Eis our study.
120 E
Mineralogical studies
Polished sample of carbonatitic breccia
(same matrix composition as in Alnö)
Thin-section view “breccia in breccia”
1 mm
Photos by L. Saksing
2 mm
PDFs in quartz
{2241}
{1012}
{1013}
44
33
11 sets in
11 grains
22
11
80
17
15
13
11
40 50 60 70
c-axis (degrees)
9
20 30
Angle
7
10
5
0
3
00
1
Number of observations
• In 3 out of studied 25 thin-sections planar
microdeformation features were observed.
90
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!