Geology and permian paleomagnetism of the Val-di-Non area W. Dolomites, N. Italy

Abstract

During the summers of 1957 till 1959 geological investigations were carried out in the western part of the Dolomites (southern Alps), where these are separated from the central Alps by the Judicaria fault. A long time before the principal displacements occurred along this fault - late oligocene - early miocene its presence was indicated by a zone of weakness (lineament), which caused facies differences at both sides (e.g. Lombardian and Venetian facies) starting from the rhaetian. Even the permian boundary between Collio and volcanic series might be reduced to this zone of weakness. The development of facies in the Val-di-Non area allows many correlations with the well-known and much studied stratigraphy of the central Dolomites. During the upper-cretaceous and the eocene, the activity of the Judicaria zone is felt also in our area by breccias and conglomerates which developed parallel to this fault. The originally chaotic tectonic picture shows different directions offolding, dipping b-axes, and subhorizontal overthrusting; the area is cut by faults which demonstrate many different mechanisms of displacement. Curving of fault planes and sub-recent collapse structures of great extension are frequently met with. Thanks to the lucky circumstance, that the Judicaria fault is not running parallel to the trend of the midtertiary alpine foldings in the Val-di-Non area, an exact, relative dating of the various deformation phases could be established. Three mid-tertiary orogenic subphases, with very marked characteristics, appear to be responsible for the deformation pattern. The first subphase shows a gravitative decollement of the sedimentary epidermis from the rising central Alps towards the SSE. Secondly, the Judicaria fault originated as a steep normal fault, along which the southern Alps sank relatively to the central Alps, which continued their rising to enormous heights. Shortly after, a tonalitic magma made its way upwards, using mainly the previously formed Judicaria fault. In the adjoining south-alpine regions too, these three subphases may be found again, though the peri-adriatic fault - which the Judicaria fault forms part of - runs generally parallel with the alpine trend there, by which the relative age of the subphases becomes less conspicuous. The assumption of a central alpine geanticline offers a mechanically acceptable synthesis of these phenomena and their sequence. It was caused by the buoyancy of an increasing accumulation of relatively light mobile material at the base of the crust. This directly affirms van Bemmelen's views on the mechanism of the mid-tertiary orogeny in the eastern Alps (van Bemmelen 1958, 1960b and 1960c); he proposed a primary tectogenesis (rising of the central alpine geanticline), followed by a secondary tectogenesis at various levels in the crust (gravitative decollement of the epidermis from the flanks of the geanticline, vertical shearing-off of the southern Alps from the central Alps, and the lateral intrusion along this fault of granitic magma, which previously caused the rising of this geanticline). The sudden change of direction, displayed by the Judicaria fault (NNE-SSW) as a part of the periadriatic suture - the regular E-W extension is interrupted over a distance of 80 km - induced many authors to assume a sinistral strike-slip faulting along the Judicaria fault, by which the 'Dolomitic Unit' should have been shoved further northward than the 'Bergamasc Unit', which lagged behind. In the field, however, no traces are found of any accompanying (north-directed) foldings, nor of any sinistral torsion along the supposed strike-slip faults. This change of direction (Judicaria fault) of the periadriatic suture is to be regarded rather as a local adaption of this suture to a zone of weakness of the crust, which appears to be present there since triassic times, as may be concluded from the sedimentary history of the Judicaria zone. The paleomagnetic properties of the lower-permian volcanic series of Bolzano have been investigated. In most cases unstable directions of magnetization could be removed by progressive demagnetization. The stable direction of magnetization furnishes a lower-permian pole, situated at 118,6° Wand 51,4° N; this position deviates considerably from the average pole position, as obtained from other European permian samples. This deviation might be due to geotectonic causes.