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.