Large Counterclockwise Rotation of the Inner West ... - Science Direct

Phys. Chem. Earth (A), Vol. 24, No. 8, pp 645- 649, 1999 Pergamon

© 1999 Elsevier Science Ltd All rights reserved 1464-1895/99/$ - see front matter PII: S 1464-1895(99)00094-0

Large Counterclockwise Rotation of the Inner West Carpathian Paleogene Flysch .........Evidence from Paleomagnetic Investigations of the Podhale Flysch (Poland) E. M~irton l, L. Mastella 2 and A. K. Tokarski 3 t E 6 t v 6 s L o r g n d G e o p h y s i c a l Institute o f H u n g a r y , P a l e o m a g n e t i c L a b o r a t o r y , H - 1 1 4 5 B u d a p e s t , C o l u m b u s 17-23, H u n g a r y 2Faculty o f G e o l o g y , W a r s a w U n i v e r s i t y , Z w i r i k i i W i g u r y 93, 0 2 - 0 8 9 W a r s z a w a , P o l a n d 3Institute o f G e o l o g i c a l S c i e n c e s , P o l i s h A c a d e m y o f S c i e n c e s , S e n a c k a 1-3, 31-002 K r a k 6 w , P o l a n d

Received 2 May 1998, revised 1 October 1998, accepted 20 October 1998 Abstract. We are reporting the first paleomagnetic results fiom the Podhale Flysch, which crops out in the area between the Pieniny Klippen Belt and the Yatra Mts., where claystones and mudstones were drilled at 10 localities, mainly fi'om subhori,zontal strata. In all cases, the magnetic thbric was found to be typical of undeformed sedfinents, with well developed magnetic tineation (afigned with the sedimentm T transport direction) at some of the localities; the dominant magnetic mineral was identified as magnetite, accompanied by iron sulphides. For six of the localities, with one exception for those with poorly developed lineation, we obtained statistically well-defined paleomagnetic mean directions, on AF or on combined AF and thermal demagnetization. The overall-mean paleomagnetic direction is D=298 ° 1=53 ° k=121, a95=6 °, in tectonic coordinates. Similar direction was observed for Inner Carpathian flysch from the Levo~:a basin (Slovakia). We conclude, that the flysch of the two basins must have travelled a few hundred kilometres to the North, after the early Miocene tectonic phase: this displacement was accompanied by about 60 '~ counterclockwise rotation with respect to Stable Europe. © 1999 Elsevier Science Ltd. All rights reserved 1. Introduction The Podhale Flysch is a fragment of the Paleogene cover of the Inner West Carpathians, situated between the Tatra Mts. and the Pieniny Klippen Belt (Fig.l). It is underlain by conglomerates and limestones of the Numulitic Eocene complex, which in its turn, discordantly overlies folded and eroded Mesozoic rocks of the Tatra Mts. The Uppermost Eocene - Oligocene Podhale Flysch (Gedl, 1998) is more than 3000 m thick. It comprises four complexes (Fig. 2a). The lowermost and the uppermost of them consist of thick-bedded sandstones and mudstones with minor intercalations of conglomerates and claystones. Correspondence to: Era6 Mzlrton

Fig. 1. Simplified geological map of the Podhale Flysch (after Mastella el al. in: Stupnicka, 1989, fig 9.15. modified) with an index map ill the bonom left comer and the paleomagnetic sampling localities Arrows indicate the sense of throw along the Bialka and Bialy Dunajec river fauk zones A, B -geological cross-section shown in Fig2b

The Zakopane beds are mostly thin-bedded claystones with minor intercalations of thin bedded sandstones and local intercalations of ferruginous dolomites. The Chocholdu beds are composed of thin-bedded ctaystones, mudstones and sandstones, with local intercalations of thick-bedded sandstones. Within the whole Podhale Flysch the sotemarks are oriented mostly E-W (N 70-100 °) indicating eastward directed sedimentary, transport (Radomski, 1958). Within the lowermost complex sole marks oriented N-S also occur. indicating transport directed southward (Radomski, 1958. Krysiak, 1976). The last observation is confirmed by results ofpalynofacies distribution analysis (Gedl, 1997). The Podhale Flysch is folded into an asymmetrical synclinorium (Fig.2b), which is disturbed by E-W trending zones of reverse faults and hectometre-scale folds close to its contacts with the Tatra Mts. and the Pieniny Klippen Belt. The northern limb of the synclinorium, close to its axial part, is cut by a zone of diverging, subverticat reverse faults. This z o n e a p p e a r s to be r o o t e d into a hor~l in ~hc

645

E. Mfirton et al.: Rotation of the Inner West Carpathian Paleogene Flysch

646

synclinorium basement. The axial zone of the synclinorium is folded into numerous E-W trending, hecIometre-scale folds. East of the Biatka River (Fig. 1), the axes of these folds ptunge 3-9 ° eastward, whereas west from the Biaty Dunajec River, the fold axes plunge about 6 ° westward. Between Bialka and Bialy Dunajec rivers the fold axes are horizontal. The described structures were formed in a N-S oriented compressional field, during the Early Miocene (Mastella, 1975), when, according to Birkemnajer (1986), the contact between the Podhale Flysch and the Pieniny KIippcn Belt was the locus of the left-lateral strike-slip nlovement,

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(Fig.2), half of them in the area where the fold axes are horizontal (Fig.l). Here as well as in the areas where the fold axes are plunging, strata with dips steeper than 30-35 ° were avoided. The samples were cut and subjected to laboratory measurements in the Paleomagnetic LaboratoLy of the E6tv6s Lorfind Geophysical Institute of Hungary, using the following instruments: cryogenic magnetometer (GM--400), spinner magnetometers (JR-4), alternating field demagnetizers (Schonstedt GSD-1 and an other built at the Technical University, Budapest), thermal demagnetizer (Scbonstedt TSD-1), low field susceptibility bride complete with Curie-point measuring device (KLY-2 and CS-1 ) and a pulse magnetizer (Molspin). The laboratory processing included low field susceptibility measurements, NRM and susceptibility measurements in the natural state, stepwise demagnetization of the NRM by thermal or AF method or the combination of the two, remeasurements of the NRM after each demagnetization step, remeasurements of lhe susceptibility after each heating step; IRM acquisition experiments and stepwise thermal demagnetization of the three-component IRM (Lowrie, 1990) were carried out on rep,esentative samples for each locality, in order to identity, the magnetic minerals. Low field magnetic susceptibility anisotropy was also measured and evaluated with a computer program which is based on Jelinek's tensor statistics (Jelinek, 1977, 1978)

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Fig.2. l.llostratigraphycat scheme showing the positions of the sampling [oca[iHes(a) and geological c~oss-secfion (b) of the Podbale Flysch

Numerous NW and NE striking strike-slip and normal faults cut the synclinorium. Two zones of normal faults of mid-Miocene age follow the Biatka and Biaty Dunaiec rivers respectively. Towards the north, both faull zones pass imo the PJeniny Klippen Bell. The sense of throw changes along both zones (Mastella, 1975; Mastella et al., 1988). Due to these changes, the block between the fauh zones is tilted northwards. The first results of a diagenetic maturity study of clay minerals (Kotarba, 1998} show that the degree of maturity is not uniform in the Podhale Flysch, and the highest temperature reached did not exceed 165°C_

1RM experiments show that the magnetic minerals are soft. Thermal demagnetization of the three-component IRM (acquired in fields of 1.0, 0.36 and 0.20 T, respectively) reveals that the dominant magnetic mineral is magnetite and it is accompanied by minor amounts of magnetic iron sulphides (medium component is demagnetized between 300 ° and 400°C) and afso by pyrite, since susceptibility. dramatically increases on heating from 360°C on (Fig.3).

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2. Sampling and laboratory measurements Paleomagnetic samples were drilled and oriented in situ at 10 localities in the Zakopane and Chochot6w complexes

Fig. 3. Identifications of magnetic minerals. From top to bottom; IRM acquisition diagrams; decay of the soft {circle), medium (dot) and hard (square) component of the IRM on slepwise thermal demagnetization; normalized susceptibility versus temperature diagrams.

E. M~rton et at.: Rotation of the Inner West Carpathian Paleogene Flysch Susceptibilities in the natural state are in the order of 104 SI, intensities of the natural remanent magnetization are 12 times 10-4 A/m. Full demagnetization of the NRM is achieved either by AF method or by thermal method (up to 400°C) followed by AF treatment (Fig.4).

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Though the dominant feature of the fabric is foliation, ranging from 2.0 to 8.7 per cent and lineation is subordinate (0.3 to 1.3 per cent) there are important differences in the degree of clustering of the maximum susceptibility directions: there are localities where the clustering is excellent, at others it is poor, while at three localities only a tendency for clustering is observed (Fig.5). Concerning the paleomagnetic directions, 6 localities yielded statistically fairly well defined mean directions, which significantly depart from the present Earth's magnetic field direction at the sampling area. For one locality, where an outcrop-scale fold was sampled, the fold test is positive.

e,J i ~ 4. Discussion and conclusions S, down

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In the sampled Oligocene complexes of the Podhale Flysch the sedimentary transport direction, known from geological observations, and the directions of the clustered maximum susceptibility directions correlate (Fig.6). The minimum N