Magnetostratigraphy of the Hettangian Langmoos section (Adnet, Austria): evidence for time-delayed phases of magnetization

Abstract

S U M M A R YStudy of the magnetostratigraphy of the earliest Jurassic (mainly Hettangian) section of Langmoos near Adnet in Austria (Northern calcareous Alps) provides a sequence of 10 magnetic polarity intervals. These intervals are often evident in the natural remanent magnetization. However, rather than a true magnetostratigraphic sequence, palaeomagnetic analyses reveal evidence for successive phases of magnetization within the section. Evidence includes (1) changes in lithology associated with changes in magnetic polarity, (2) different combinations of components of both polarities, and (3) largely non-antipodal directions. Considering the palaeomagnetic directions, we suggest that the apparent magnetostratigraphy is the result of two or three phases of magnetization which occurred at different stages of the tectonic emplacement of the Northern Calcareous Alps.

New evidence for block and thrust sheet rotations in the central northern Calcareous Alps deduced from two pervasive remagnetization events

“…2. In most cases (about 80%) and before any tectonic correction, J3 and J2 components display NE‐E declinations and positive inclinations (Figures 4a and 5 and Table 2), in agreement with previous data [ Mauritsch and Frisch , 1978; Channell et al , 1990, 1992; Heer , 1982; Gallet et al , 1993].…”

“…Most part of the data set belongs to the former Stauffen‐Höllengebirge and Berchtesgaden nappes (lower and upper Tirolic units, respectively, in the new classification by Frisch and Gawlick [2003]) and part to the Göll‐Lammer unit [ Tollmann , 1973, 1985, 1987; Plöchinger , 1995], redefined as Juvavic‐Hallstatt Mélange [ Frisch and Gawlick , 2003]. Previous paleomagnetic data were mostly obtained in the north central part of the NCA [ Mauritsch and Frisch , 1978; Smathers , 1987; Channell et al , 1992; Heer , 1982; Gallet et al , 1993], mainly in the Liassic formations because of their well‐know magnetic signal (Table 1 and Figure 2); primary components of magnetization, which are characterized by dual polarities and carried by hematite were identified. Secondary directions were only poorly described or ignored.…”

“… a First column is number of site on Figure 1; Site, name of the original site; Ref, references (M&F, Mauritsch and Frisch [1978]; Ch, Channell et al [1992], Heer [1982], and Gall, Gallet et al [1993]); Long, longitude; Lat, latitude; Locality, geographic names; Strike (right‐hand rule), dip; DD, dip direction; Obs, samples or sites; n/N, number of samples considered/number of samples analyzed; D, I, declination and inclination of the paleomagnetic vector (BAC, before any correction (in situ); ABC, after bedding correction); question marks indicate nonsense results; Pol, polarity; α 95 , K, Fisher [1953] statistical parameters. …”

“…The northern Calcareous Alps (NCA) belong to the upper Austroalpine megaunit (AU) of the Eastern Alps and represent one of the largest tectonostratigraphic units of the Alps. The application of paleomagnetism in this part of the orogen started very early [ Hargraves and Fischer , 1959], and continues until today [ Soffel , 1975; Mauritsch and Frisch , 1978, 1980; Heer , 1982; Becke and Mauritsch , 1985; Soffel and Wohl , 1986; Mauritsch and Becke , 1987; Channell et al , 1990, 1992; Channell and Stoner , 1994; Gallet et al , 1993, 1994, 1996, 1998; Haubold et al , 1999; Thöny et al , 2006]. These studies had different goals, such as the reconstruction of the nappe system evolution (characterization of rotations) or the platform configuration of this margin of the Tethys Ocean (Triassic and Jurassic magnetostratigraphy and paleogeographical reconstructions).…”

“…To shed some light on these problems, the central part of the NCA (south of Salzburg) has been examined in detail by studying 77 new sites and four magnetostratigraphic profiles. Previous data in the area (45 sites from Mauritsch and Frisch [1978], Heer [1982], Becke and Mauritsch [1985], Mauritsch and Becke [1987], Channell et al [1990, 1992], and Gallet et al [1993]) were also included in the interpretation.…”