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 (k max ) 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 - C1 N 90 180 270 TILT CORR (Wulf) Down Up N 90 180 270 GEO (Wulf) Down Up OT01 - 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 Intensität [A/m] OT0104B 2.5 Tesla 0.5 Tesla 0.1 Tesla B 3D I S T IRM Figure A: Oblate fabrics are dominant with a quite low bulk susceptibility In-situ orientations of k max 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 k max 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) k max was aligned parallel to σ 3 . 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 k max displays a more suitable correlation with σ 3 . 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 k max yields two groups: (1) k max scattered around a NW – SE trending girdle normal to the bedding and (2) k max 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 k max 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 Grauwackenzone TL k 1 Dek KP02 k 1 Dek GB01 k 1 Dek Grauwackenzone 0 45 90 135 180 225 270 315 0 1 2 3 4 5 Ötscher OT05 k 1 Dek OT01 k 1 Dek RG01 k 1 Dek Ötscher Decke 0 45 90 135 180 225 270 315 0 1 2 3 4 Mürzalpen Decke MU08 k1 Dek SN k1 Dek MU04 k 1 Dek MU09 k1 Dek Mürzalpen Decke 0 45 90 135 180 225 270 315 0 1 2 RA03 RA03 k 1 dec Reisalpen Decke 0 45 90 135 180 225 270 315 0 1 2 3 Flysch FL01 k 1 Dek FL02 k 1 Dek FL03 k 1 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 projection N=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 B C 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-area projection N=6 K1 K2 K3 B 1.000 1.380 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 B C 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 Volumensuszeptibilität [10 -6 SI] FL03 FL02 FL01 Flysch 288000 280000 272000 Hochwert BMN 0 200 400 600 800 Volumensuszeptibilität [10 -6 SI] SN MU08 MU04 MU09 Mürzalpen Decke 300000 296000 Hochwert BMN 0 200 400 600 800 Volumensuszeptibilität [10 -6 SI] OT01 OT05 RG01 Ötscher Decke Grauwackenzone 280000 260000 Hochwert BMN 0 200 400 600 800 Volumensuszeptibilität [10 -6 SI] KP02 GB01 TL 306600 306400 Hochwert BMN 0 200 400 600 800 Volumensuszeptibilität [10 -6 SI] RA03 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 I S T) identify magnetite and haematite as well as goethite as the main magnetic minerals. Figure B: IRM and 3D I S T of a sample from the Ötscher nappe yielding magnetite as magnetic carrier mineral.