hochwert bm n - unileoben.ac.atgeophysik.unileoben.ac.at/.../gruber_2010_pangeo_leoben.pdf · 2012....
TRANSCRIPT
-
Directional correlation of petrophysical anisotropy
patterns with rock deformation during alpine tectonic
evolution (Eastern Alps, Austria) Karin Gruber (Leoben)1, Robert Scholger (Leoben)1
Acknowledgements The study was funded by the Austrian Academy of Sciences (ÖAW) in the frame of the Geophysics of the Earths Crust Programme.
Authors address (1) Department of Applied Geological Sciences and Geophysics, Chair of Geophysics, Univ. of Leoben, Paleomagnetic Laboratory, Leoben,
Austria.
References Peresson , H., Decker, K., 1997. The Tertiary dynamics of the northem Eastem Alps (Austria): changing palaeostresses in a collisional plate boundary. Tectonophysics
272, 125-157.
Plöchinger B., 1995. Tectonics of the Northern Calcareous Alps: a review. Mem Sci Geol 47, 73-86.
Pueyo, E.L., Mauritsch, H.J., Gawlick, H.-J., Scholger, R., Frisch, W., 2007. New evidence for block and thrust sheet rotations in the central northern Calcareous Alps
deduced from two pervasive remagnetization events. Tectonics, 26, doi:10.1029/2006TC001965, 1-25.
Wessely, G., 2006. Geologie der Österreichischen Bundesländer – Niederösterreich. Geologische Bundesanstalt, 1-416, Wien.
www.agico.com/software/anisoft
Introduction
Recent geodynamic investigations in the
Eastern Alps and surrounding areas
gave rise for large neotectonic
movements, which are dated younger
than 5 Ma. The main objective of the
study involves petrophysical analyses of
textures in Alpine rocks in order to
observe changes of the regional
deformation pattern.
Measurements of anisotropy of magnetic
susceptibility (AMS) were carried out on
samples from 34 sites collected in the
Eastern Alps. Samples spanning an age
range from U. Permian to Mesozoic
were taken along a North-South transect
from Scheibbs in the North to
Kapfenberg in the South, comprising
Helvetic and Penninic Flysh units, most
of the Northern Calcareous Alps (NCA)
nappes, as well as the Greywacke zone.
AMS analysis was carried out on two to
six sites per thrust sheet or nappe for a
structural investigation of the relationship
between magnetic fabrics and tectonic
strain in the investigated area. The study
focuses on the directional correlation of
the AMS axes patterns.
The palaeostress results of Peresson &
Decker (1997) give evidence of six
tectonic events during alpine evolution
which we use as reference frame for our
strain correlation:
• T1: NW – SE compression (post –
Early Eocene)
• T2: N – S compression (Early to Middle
Miocene)
• T3: NE – SW compression (Middle
Miocene)
• T4: E – W extension (Middle Miocene)
• T5: E – W compression (Late Miocene,
Pannonian)
• T6: N – S extension (Late- to post-
Miocene).
Anisotropy of magnetic susceptibility
Particularly within the NCA, AMS is inapplicable on most rocks for strain
analysis as they are diamagnetic and/or isotropic. Even so, some sites
are anisotropic and yield oblate and triaxial magnetic fabrics. In some
cases even tendency to prolate magnetic fabrics can be observed. The
AMS data of the research area show three main characteristics of
magnetic fabrics related to strain. In the Flysh units and Greywacke
zone, the AMS principal axes document a weaker tectonic deformation
than in the NCA.
Figure A:
AMS parameter, rose diagrams displaying the mean direction of the
maximum susceptibility axes (kmax) and susceptibility values are
presented along a North – South profile of the eastern NCA.
Rock and paleomagnetic results
Northern Calcareous Alps Flysh units Greywacke zone
Conclusion
N
90
180
270
TILT CORR
(Wulf)
Down
Up
OT01 - C1N
90
180
270
TILT CORR
(Wulf)
Down
Up
N
90
180
270
GEO
(Wulf)
Down
Up
OT05
N
90
180
270
TILT CORR
(Wulf)
Down
Up
N
90
180
270
GEO
(Wulf)
Down
Up
RG01 - C1N
90
180
270
TILT CORR
(Wulf)
DownUp
MU09 - C1
N
90
180
270
GEO
(Wulf)
DownUp
N
90
180
270
GEO
(Wulf)
Down
Up
N
90
180
270
GEO
(Wulf)
DownUp
MU08
SN - C1
N
90
180
270
TILT CORR
(Wulf)
Down
Up
N
90
180
270
GEO
(Wulf)
Down
Up
OT01 - C2
N
90
180
270
TILT CORR
(Wulf)
Down
Up
N
90
180
270
GEO
(Wulf)
Down
Up
RG01 - C2N
90
180
270
TILT CORR
(Wulf)
DownUp
N
90
180
270
GEO
(Wulf)
Down
Up
MU09 - C2
N
90
180
270
GEO
(Wulf)
Down
Up
SN - C2
N
90
180
270
GEO
(Wulf)
Down
Up C
Figure C:
Stepwise thermal demagnetization defines two magnetic directions
carried by phases with low coercivity and low unblocking temperatures:
C2, 250°C and C1, 580°C. Clockwise and anticlockwise vertical axes
rotation (Middle - Late Miocene) with values ranging from 2° to 57° were
recorded by the remanence vector C2 (Figure D). The remanence
vector C1 also documents an older blockrotation with higher rotation
values than C2 but the remagnetization and rotation ages cannot be
given due to lack of data.
0 500 1000 1500 2000 2500
Magnetfeld [mT]
-1
-0.5
0
0.5
1
-400 -300 -200 -100 0
OT0104B
0 200 400 600
Temperatur [°C]
0
0.4
0.8
1.2
1.6
Inte
nsi
tät
[A/m
]
OT0104B
2.5 Tesla
0.5 Tesla
0.1 Tesla
B 3D IST IRM
Figure A:
Oblate fabrics are dominant with a quite low bulk susceptibility
In-situ orientations of kmax are well aligned in a subhorizontal NE
– SW direction with a separate cluster in the South. The
magnetic fabrics differ slightly from sedimentary fabrics which
indicates a weak stage of deformation.
Figure D:
The magnetic fabrics indicate a relationship to the tectonic event
T1 – NW-SE compression but direct comparison is not possible
due lack of strain data in the Flysh units.
Figure A:
Some sites reveal weakly deformed magnetic fabrics with very
low susceptibility values and the directions of kmax vary locally.
Oblate to triaxial magnetic fabrics are dominant. In some
samples prolate magnetic fabrics are observed.
Figure D:
During NE-directed compression (T3 - Middle Miocene) kmax was
aligned parallel to σ3. NW – SE directed reverse faults and
sinistral ENE – WSW directed strike-slip faults are significant for
phase T3 (Peresson and Decker, 1997). After restoration of the
pre-Miocene situation kmax displays a more suitable correlation
with σ3.
All sampled lithologies of the Flysh units and Greywacke
zone yield well defined AMS patterns whereas in the NCA
only carbonatic sediments accumulated in sub-oxic and
reducing environments with terrigenous influence are
suitable.
The magnetic fabrics document weak deformation. The
orientations of the susceptibility tensor can be correlated
to strain directions during neotectonic events from Early
to Middle Miocene. Remagnetization vectors record very
young (< 5 Ma) vertical axes blockrotations within the
NCA.
N
T1: NW – SE Compression (post – early Eocene) T2: N – S compressionen (Early to Middle Miocene) T3: NE – SW compression ( Middle Miocene) D
modified after Wessely (2006)
Figure A:
Oblate magnetic fabrics are dominant. The in-situ orientation of
kmax yields two groups: (1) kmax scattered around a NW – SE
trending girdle normal to the bedding and (2) kmax is aligned in a
E – W direction. The bulk susceptibility reaches its highest
values in the transect.
Figure D:
In group (2) the alignment of kmax can be related with the N-
directed compression (T2) during Early to Middle Miocene
(Peresson and Decker, 1997). Group (1) represents a tectonic
block with similar magnetic fabric orientation as the NCA.
1
1.05
1.1
1.15
1.2
1.25
L F P Pj
Box Whisker - FL01
1
1.05
1.1
1.15
1.2
1.25
L F P Pj
Box Whisker - FL02
1
1.05
1.1
1.15
1.2
1.25
L F P Pj
Box Whisker - FL03
1
1.1
1.2
1.3
1.4
L F P Pj
Box Whisker - OT01
1
1.05
1.1
1.15
1.2
1.25
L F P Pj
Box Whisker - OT05
1
1.05
1.1
1.15
1.2
1.25
L F P Pj
Box Whisker - RG01
1
1.05
1.1
1.15
1.2
1.25
L F P Pj
Box Whisker - MU09
1
1.2
1.4
1.6
1.8
L F P Pj
Box Whisker - MU04
1
1.05
1.1
1.15
1.2
1.25
L F P Pj
Box Whisker - MU08
1
1.05
1.1
1.15
1.2
1.25
L F P Pj
Box Whisker - SN
1
1.05
1.1
1.15
1.2
1.25
L F P Pj
Box Whisker - GB01
1
1.05
1.1
1.15
1.2
1.25
L F P Pj
Box Whisker - KP02
1
1.05
1.1
1.15
1.2
1.25
L F P Pj
Box Whisker - TL
N S
1
1.05
1.1
1.15
1.2
1.25
L F P Pj
Box Whisker - RA03
0
45
90
135
180
225
270
315
0 1 2 3 4
GrauwackenzoneTL k1 Dek
KP02 k1 Dek
GB01 k1 Dek
Grauwackenzone
0
45
90
135
180
225
270
315
0 1 2 3 4 5
ÖtscherOT05 k1 Dek
OT01 k1 Dek
RG01 k1 Dek
Ötscher Decke
0
45
90
135
180
225
270
315
0 1 2 3 4
Mürzalpen DeckeMU08 k1 Dek
SN k1 Dek
MU04 k1 Dek
MU09 k1 Dek
Mürzalpen Decke
0
45
90
135
180
225
270
315
0 1 2
RA03RA03 k1dec
Reisalpen Decke
0
45
90
135
180
225
270
315
0 1 2 3
FlyschFL01 k1 Dek
FL02 k1 Dek
FL03 k1 Dek
Flysch
N
90
180
270
Equal-area
projection
N=6
K1
K2
K3
B
C
FL01 in-situ
1.000 1.044 Pj
-1
1
T
N
90
180
270
FL02 - in-situ Equal-area
projectionN=6
K1
K2
K3
B
1.000 1.085 Pj
-1
1
T
N
90
180
270
FL03 in-situ Equal-area
projection
N=5
K1
K2
K3
B
1.000 1.035 Pj
-1
1
T
N
90
180
270
GB01 in-situ Equal-area
projection
N=6
K1
K2
K3
BC
1.000 1.019 Pj
-1
1
T
N
90
180
270
KP02 in-situ Equal-area
projection
N=6
K1
K2
K3
B
1.000 1.044 Pj
-1
1
T
N
90
180
270
TL in-situ Equal-area
projection
N=10
K1
K2
K3
B
1.000 1.248 Pj
-1
1
T
N
90
180
270
OT05 in-situ Equal-area
projection
N=9
K1
K2
K3
B
C
1.000 1.096 Pj
-1
1
T
N
90
180
270
OT01 in-situ Equal-areaprojection
N=6
K1
K2
K3
B
1.0001.380
Pj
-1
1
T
N
90
180
270
RG01 in-situ Equal-area
projection
N=6
K1
K2
K3
B
C
1.000 1.104 Pj
-1
1
T
1.000 1.065 Pj
-1
1
T
N
90
180
270
MU04 in-situ Equal-area
projection
N=8
K1
K2
K3
B
N
90
180
270
MU08 in-situ Equal-area
projection
N=6
K1
K2
K3
BC
N
90
180
270
SN in-situ Equal-area
projection
N=12
K1
K2
K3
B
C
N
90
180
270
MU09 in-situ Equal-area
projection
N=9
K1
K2
K3
B
1.000 1.050 Pj
-1
1
T
1.000 1.078 Pj
-1
1
T
1.000 1.616 Pj
-1
1
T
1.000 1.108 Pj
-1
1
T
324000 322000 320000
Hochwert BMN
0
200
400
600
800
Vo
lum
ensu
szep
tib
ilitä
t [1
0-6
SI]
FL03
FL02 FL
01
Flysch
288000 280000 272000
Hochwert BMN
0
200
400
600
800
Vo
lum
ensu
szep
tib
ilitä
t [1
0-6
SI]
SN MU
08
MU
04 MU
09
Mürzalpen Decke
300000 296000
Hochwert BMN
0
200
400
600
800
Vo
lum
ensu
szep
tib
ilitä
t [1
0-6
SI]
OT0
1
OT0
5
RG
01
Ötscher Decke
Grauwackenzone
280000 260000
Hochwert BMN
0
200
400
600
800
Vo
lum
ensu
szep
tib
ilitä
t [1
0-6
SI]
KP02 G
B01
TL
306600 306400
Hochwert BMN
0
200
400
600
800
Vo
lum
ensu
szep
tib
ilitä
t [1
0-6
SI]
RA
03
Reisalpen Decke
N
90
180
270
RA03 in-situ Equal-area
projection
N=5
K1
K2
K3
B
C
A
The software package AGICO Inc. Anisoft 4.2
was used for AMS data analyses
Isothermal Remanent Magnetization (IRM) measurements and stepwise
thermal demagnetization of 3-component Isothermal Remanent
Magnetization (3D IST) identify magnetite and haematite as well as
goethite as the main magnetic minerals.
Figure B: IRM and 3D IST of a sample from the Ötscher nappe yielding
magnetite as magnetic carrier mineral.