It is generally agreed (Gwinner, 1971) that the Northern Calcareous Alps (together with the Northern Grauwackenzone) rest allochthonously on top of a Penninic basement and have been transported into their present position from the South after Upper Eocene. Their complicated structure is characterized by a series of nappes of various lateral extensions and thicknesses. A sketch map of the Western part of the Northern Calcareous Alps (Fig. 1) shows the main nappes (after Gwinner, 1971).
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Soffel, H.

Journal of Geophysics

|00000457||J. Geophys. 45, 447--450, 1979 Short Communication Journal of Geophysics Palaeomagnetism of Permo-Triassic Red Sandstones From the Northern Calcareous Alps H. Soffel lnstitut fiir Allgemeine und Angewandte Geophysik, Theresienstrasse 41, Universitiit Miinchen, D-8000 Miinchen 2, Federal Republic of Germany Key words: Northern Calcareous Alps - Permo-Triassic sandstones. 1. Introduction It is generally agreed (Gwinner, 1971) that the Northern Calcareous Alps (toge-ther with the Northern Grauwackenzone) rest allochthonously on top of a Penninic basement and have been transported into their present position from the South after Upper Eocene. Their complicated structure is characterized by a series of nappes of various lateral extensions and thicknesses. A sketch map of the Western part of the Northern Calcareous Alps (Fig. 1) shows the main nappes (after Gwinner, 1971). Earlier palaeomagnetic measurements on rocks from the Northern Cal-careous Alps gave contradictory results. Hargraves and Fischer (1959) studied Liassic red limestones and radiolarites near Lofer (Lo) and deduced a slight (20°) clockwise rotation with respect to Central Europe. A much larger clockwise rotation (40°) was concluded by Mauritsch and Frisch (1978) from the study of Upper Jurassic red limestones of the Osterhorngruppe (Os). However, an anticlockwise rotation (60°) was deduced by Soffel (1975) from the palaeomag-netism of Upper Permian to Lower Triassic effusives in Vorarlberg (Vo). None of these data seem to have a general validity for a rotation of the Northern Calcareous Alps as a whole during their transport by gravity gliding. Along the southern margin of some of the nappes, Permo-Triassic (Skythian) red sandstones are exposed which are in some places in an undisturbed tectonic context with overlying Triassic limestones and dolomites. Several localities of these brown to violet coloured fine grained sandstones have been sampled. (In: Innsbruck, Rumer Spitze, 1 site, 10 samples; Wo: Hopfgarten near Worgl, 5 sites, 36 samples; Le: Leo gang near Saalfeld, 4 sites, 29 samples.) All localities and their corresponding position within the different nappes are shown in Fig. 1. 0340-062X/79/0045/0447 /$01.00 448 H. Soffel Vo In Wo Lo Le Os Fig. I. Western part of the Northern Calcareous Alps. L Lechtaldecke; I Inntaldecke; SH Staufen-Hollengebirgsdecke; K Kaiserdecke; B Berchtesgadener Schubmasse; W Werfener Schuppenzone; D Dachsteindecke; G Grauwackenzone; FH Flysch, Helvetikum ; C Kristallin, Schieferhiille. Br Bregenz; To Bad Tolz; Sa Salzburg. For sampling localities see text. Measured remanence directions (thick) a nd reference directions with regard to Cent ral Europe (thin lines). Arrows indicate rotation. 2. Palaeomagnetic Results Fig. 2. NRM directions in equal area projection. Star: present geomagnetic field direction. Triangle : site In. Squares: sites Wo. DoJS : sites Le Figure 2 shows the NRM directions, all being normal. For meaning of the symbols see legend. With exception of site In, a ll directions differ significantly from the present local geomagnetic field direction (star). AF demagnetization up to 2,000 Oe and thermal demagnetization up to 650° C revealed the presence of two anti parallel remanence components in all three localities: a reversed component which could be destroyed with AF fields up to 100 Oe and thermal treatment up to 300° C, and a normal component stable up to 2,000 Oe and thermal treatment up to 560° C. At higher temperatures the directions scattered randomly and revealed no indication of a consistent component with higher blocking temperatures. Largest precision parameter was obtained for thermal demagnetization at 300° C. Figure 3 a and b show the site mean and locality mean directions after thermal cleaning and tectonic correction in equal area projection. For details see legend. The locality means are as follows: In: N= 10, R=9.82, D=359.0°, Palaeomagnetism of Permo-Triassic Sandstones 449 In .J. - '\ ' ' ;.--...;,, Wo ~Oz',', ' 'V· ••.J .. ' ' ... ~ .. ;,, 60° 30° ' \ ., 60° 30° a b Fig. 3a and b. RM directions after thermal cleaning with 300° C and tectonic correction. Star: present geomagnetic field direction. a Diamond: site In. Square : sites Le, 3 closely grouped . Triangle: sites Le, all. b Square: sites W 6, 4 closely grouped 180° Fig. 4. Pole positions and part of European polar wander path. For deta ils see text. Diamond : Yornrlberg after Soffel (1975) ; S tar : Innsbruck; Square : Leogang, 3 closely grouped sites ; Triangle: Leogang, all sites. Full circle : Worgl 1=49.9°, k = 50.l, i:x95 = 6.3°; W6: N = 4, R = 3.97, D = 44.3°, 1= 59.2°, k = 102.7, tx95 = 6.9° (omitting site 1 because of uncertain tectonic position); Le: N=3, R=2.995, D = 31.5°, I= 19.6°, k = 414.8, i:x95 = 4.0° (omitting site 3 because of too large deviation from the remaining group). The corresponding pole positions are: In: 194.5°E, 73.4°N; W6 : 98.9°E, 57.6°N; Le: 147.0°E, 44. l 0 N. The mean geographic coordinates are 12° C, 47.5° N, however the exact coordinates were used for the computation of the pole positions. They are shown in Fig. 4 together with a part of the European polar wander path after McElhinny (1973). The Jurassic pole is from Heller ( 1977), the Lower Cretaceous-Upper Jurassic pole was taken from Schweitzer ( 1975). 3. Discussion and Conclusions The presence of two a lmost exactly antiparallel remanence components (with fortunately not too much overlapping coercivity and blocking temperature spec-|00000460||450 H. Soffel tra) indicates a complicated magnetization history of these sandstones. The too steep inclinations for rocks of Early Triassic age (except for site Le) suggests a later overprint, perhaps during Upper Cretaceous or Lower to Middle Tertiary (early stage of the alpine orogeny). On the other hand, the presence of a third magnetization component with shallover inlination could not be found during the demagnetization experiments. From polished section studies the carrier of remanence seems to be detrital magnetite to some degree oxidized to hematite. From the petrology of the rocks there is no evidence for major mineralogical changes or reheating. The exact age of remanence of sites In and Wo remains therefore uncertain. Only the Leogang sandstones have a pole position which agrees with the Triassic pole for Central Europe indicating no rotation during the transport. Supposing an at least pre-Oligocene age of remanence of the two other sandstone sites, a slight clockwise rotation (15°) must be assumed for the Worgl and a small anticlockwise rotation (20°) for the Innsbruck sandstones. The amounts of rotation are indicated in Fig. 1, where the corre-sponding rotations from Vorarlberg (Vo), Lofer (Lo) and Osterhorngruppe (Os) are also shown. The pattern of rotations (anticlockwise in the West, clockwise in the East) does in my view not reflect any bending of the Northern Calcareous Alps. Unless more data from the whole extend of the Northern Calcareous Alps are available I tend to regard the observed rotations as local effects which may be typical for a specific nappe or a part of it. They seem to reflect the sense and amount of relative rotation of the different units during their post Eocene northward transport by gravity gliding. Acknowledgement. Dr. H. Bogel (Munich) and Dr. R. Brandner (Innsbruck) are kindly acknowledged for their advice and help during the collection of the samples. Thanks are also due to N. Grohmann, Dr. A Schult, Professor Dr. G. Angenheister and Dr. N. Petersen for many interesting discussions and to the Deutsche F orschungsgemeinschaft for the financial support. References Gwinner, M.P.: Geologie der Alpen. p. 237ff. Stuttgart: E. Schweizerbart'sche Verlagsbuchhandlung, 1971 Hargraves, R.B., Fischer, A.G.: Remanent magnetism in jurassic red limestones and radiolarites from the Alps. Geophys. J. R. Astron. Soc. 2, 34-41, 1959 Heller, F.: Palaeomagnetism of upper jurassic limestones from Southern Germany. J. Geophys. 42, 475-488, 1977 Mauritsch, H.J., Frisch, W.: Palaeomagnetic data from the central part of the Northern Calcareous Alps, Austria. J. Geophys. 44, 623-637, 1978 McElhinny, M.W.: Palaeomagnetism and Plate Tectonics. p. 206. Cambridge: University Press 1973 Schweitzer, Ch.: Vergleich mehrerer Methoden zur Bestimmung der Intensitiit des Erdmagnetfeldes an rezenten Laven und ihre Anwendung auf mesozoische und palaozoische magmatische Ge-steine. Universitiit Miinchen: Dissertation 1975 Soffel, H.: The Palaeomagnetism of the Permian Effusives near St. Anton, Vorarlberg (Austria) and the Anticlockwise Rotation of the Northern Calcareous Alps through 60°. Neues Jahrb. Geo!. Palaeontol. Monatsh. 6, 375-384, 1975 Received May 2, 1978; Revised Version April 5, 1979; Accepted June 19, 1979 

Palaeomagnetism of Permo-Triassic red sandstones from the Northern Calcareous Alps

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Palaeomagnetism of Permo-Triassic red sandstones from the Northern Calcareous Alps

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