Palaeomagnetic and rockmagnetic properties of the Permian volcanics in the western Southern Alps

Abstract

A palaeomagnetic and rockmagnetic investigation has been carried out in the western Southern Alps. At 31 sites in the Permian volcanics from four regions 349 samples were drilled: 1. the region of Arona (SW of Lago Maggiore), 2. the porphyry district of Lugano and Ganna, 3. the Valle Brembana (N of Bergamo) and 4. the Auccia volcanics (N of Brescia). AF-cleaning as well as continuous and progressive thermal demagnetization reveal, in most of the igneous rocks studied, the presence of a stable characteristic remanent magnetization (ChRM) representing the magnetization acquired at the time of formation of these rocks. Microscopic observations, IRM-acquisition curves and Curie point measurements indicate the ChRM to be carried by Ti-poor magnetite and titanohematite. Stable secondary magnetizations due to oxidation may be present. Their directions, however, are very similar to the primary thermoremanent magnetization (TRM). Therefore it is inferred that the oxidation probably took place shortly after acquisition of the primary TRM. The magnetization directions within individual sites are well grouped (α₉₅ usually < 10°), but the site mean directions are dispersed, due to regional and local tectonic complications. At Arona, Ganna, and Auccia a suitable tilt correction can be made. Since the consistent directions from these sites are very similar to the well defined results from the Bolzano porphyries (Zijderveld et al., 1970), it is suggested that the western and eastern Southern Alps have, on a large scale, behaved as a single tectonic unit. The Southern Alpine block has been rotated anticlockwise by about 50° relative to extra-alpine Europe since the Early Mesozoic. The Permian Southern Alpine palaepoles are situated close to the Permian part of the African polar wander path. Therefore the palaeomagnetic data support geological and sedimentological arguments which consider the Southern Alps as originating on the southern margin of Tethys and forming a parautochthonous extension of the African plate since the Early Mesozoic.

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