Giovanni Muttoni

Evolution of Pangea: paleomagnetic constraints from the Southern Alps, Italy

Abstract A new early Late Triassic paleopole for Adria has been obtained from the Val Sabbia Sandstone in the Southern Alps. As Early Permian and Jurassic-Cretaceous paleomagnetic data from para-autochthonous regions of Adria such as the Southern Alps are consistent with ‘African’ APWPs[1–2], paleomagnetic data from this region can be used to bolster the West Gondwana APWP in the poorly known Late Permian-Triassic time interval. The Southern Alpine paleopoles are integrated with the West Gondwana and Laurussia APWPs of Van der Voo [1] and used to generate a tectonic model for the evolution of Pangea. The Early Permian overall mean paleopole for West Gondwana and Adria, in conjunction with the coeval Laurussia paleopole, support Pangea B of Morel and Irving [3]. The Late Permian/Early Triassic and the Middle/Late Triassic paleopoles from Adria and Laurussia support Pangea A-2 of Van der Voo and French [4]. The phase of transcurrent motion between Laurasia and Gondwana[5] that caused the Pangea B to A-2 transition occurred essentially in the Permian (at the end of Variscan orogeny) with an average relative velocity of approximately 10 cm/yr. Finally, the Late Triassic/Early Jurassic paleopoles from West Gondwana and Laurussia agree with Pangea A-1 of Bullard et al. [6], the widely accepted Pangea configuration at the time of the Jurassic breakup.

THE PRATI DI STUORES/STUORES WIESEN SECTION (DOLOMITES, ITALY): A CANDIDATE GLOBAL STRATOTYPE SECTION AND POINT FOR THE BASE OF THE CARNIAN STAGE

The Prati di Stuores/Stuores Wiesen section (Dolomites, Italy) is proposed as a candidate Global Stratotype Section and Point for the base of the Carnian Stage. In addition to being a famous, richly fossiliferous locality, it includes the type-section of the Cordevolian substage. The section is located near Pralongia, along the southern slope of the crest separating the Badia/Abtei and Cordevole valleys. Below the levels with Trachyceras aon , the section contains a rich ammonoid fauna that chacterizes the lower part of the Regoledanus Subzone and subsequently records the first appearances of the mid-high latitude genus Daxatina ( Daxatina sp., D. cf. canadensis ) and of traditional Trachyceras with species different from T. aon . Moreover, the Daxatina cf. canadensis Subzone is recognised above the Regoledanus Subzone. Very rare conodonts of the Budurovignatus group and species of Gladigondolella from the diebeli Assemblage Zone occur. Gondolella polygnatyformis, already known from the Aon Subzone, is absent. Palynomorphs, foraminifers, gastropods, bivalves, brachiopods, microcrinoids and holothurian sclerites were studied. Variations in frequency and taxonomic diversity of these faunas suggest anaerobic-disaerobic bottom conditions for the lower-middle part of the section (0-105 m), followed by a more stable oxygen content in the upper portion. Magnetostratigraphy showed four intervals with normal polarity and three intervals with reversed polarity. The Daxatina cf. canadensis Subzone falls close to the normal polarity interval S2n. The present study proposes the FAD of the cosmopolitan genus Daxatina as a marker of the base of the Carnian Stage, placing it at a lower stratigraphic level than previously indicated in the Stuores area. The Prati di Stuores section is proposed as GSSP of the Ladinian-Carnian boundary.

Paleomagnetic evidence for Neogene tectonic rotations in the northern Apennines, Italy

Abstract Paleomagnetic analysis was carried out in the northern Apennines on Eocene to Pliocene Epiligurian units. Five Early Miocene and two Middle Miocene sites yielded dual polarity site-mean directions which show signs of clustering after correction for bedding tilt. These likely primary magnetizations, in conjunction with data from the literature, give an overall mean Late Oligocene–Middle Miocene paleomagnetic pole which shows a large and significant counterclockwise rotation of 52° (±≈8°) with respect to the Africa reference paleopoles (or a similar amount of rotation with respect to the coeval Europe reference paleopole). However, this paleopole falls close to the roughly coeval paleopole for Corsica–Sardinia, which is here calculated by averaging data from the literature. Three additional Early Miocene sites from an area west of Parma affected by Pliocene tectonics yielded site-mean directions which pass the fold test and are rotated counterclockwise by a lesser amount than the rest of the Miocene sites. Most of the remaining sites bear paleomagnetic directions acquired after tilting during a recent phase of remagnetization. We suggest that the large-scale rotation observed in the northern Apennines was associated with the motion of the Corsica–Sardinia block within the general context of the Africa–Europe relative motions. A compilation of published data from the central Apennines also shows a differential rotation of the northern relative to the southern Umbria belt which occurred after the motion of Corsica–Sardinia and may have been due to pivoting of the northern Umbria belt against a deep-seated lineament during the non-rotational opening of the Tyrrhenian Sea.

Middle Triassic magnetostratigraphy and biostratigraphy from the Dolomites and Greece

Abstract Magnetostratigrapic and biostratigraphic data across the Anisian/Ladinian (Middle Triassic) boundary were obtained from the Frotschbach/Seceda section from the Dolomites region of northern Italy, and the Vlichos section from the Greek island of Hydra, where the Aghia Triada published section was also resampled. The Frotschbach/Seceda section includes two radiometrically dated (U Pb) tuff levels and covers two of the three chief candidates for the position of the base of the Ladinian, namely at the base of the Secedensis Zone or the subsequent Curionii Zone. The Aghia Triada section yields biochronological evidence for the base of the Secedensis Zone, whose significance is, however, critically discussed in the light of the magnetostratigraphic correlation with Frotschbach/Seceda. The Vlichos section can be correlated with Aghia Triada and Frotschbach/Seceda by means of magnetic polarity stratigraphy and sparse fossil occurrences. The satisfactory correlation of the magnetozones allows us to construct a composite geomagnetic polarity sequence tied to Tethyan ammonoid and conodont biostratigraphy for about a 2.4 Myr interval across the Anisian/Ladinian boundary.

“Wasp-waisted” hysteresis loops from a pyrrhotite and magnetite-bearing remagnetized Triassic limestone

Remagnetized samples of the Triassic Prezzo Limestone from northern Italy contain a mixture of pyrrhotite and magnetite, as deduced by thermal unblocking characteristics of triaxial isothermal remanent magnetizations (IRMs) and low-temperature cycling of saturation IRMs. Hysteresis loops are commonly “wasp-waisted” and remanent coercivity curves contain a break in slope, as a result of the contrast in coercivity between remanence-carrying pyrrhotite and magnetite. The relative proportion of the high to low remanent coercivity fractions, as deduced by the study of the remanent coercivity curves, seems to control the degree of “wasp-waistedness” of the hysteresis loops. Samples that are dominated by one of the two coercivity end members have lower Bcr/Bc values as well as hysteresis loops that have a less pronounced constricted waist compared to samples with higher Bcr/Bc values. Maximum Bcr/Bc values (and thus maximum degrees of “wasp-waistedness” in the hysteresis loops) are attained when the low remanent coercivity fraction contributes 15–35% to the bulk remanent coercivity curves.

The late Norian-Hettangian stratigraphic and paleogeographic evolution of the Bergamasc Alps

.....................................................................5