Gravity tectonics in the Northwestern Dolomites (N. Italy)

Abstract

The NW Dolomites (see fig. 1 and 2) have an area of approximately 1500 sq. km and are situated on the southern flank of the east alpine mountain range. The region consists mainly of permo-triassic and some younger mesozoic strata with a maximal thickness of about 3000 m. This sedimentary series was deposited unconformably on the basement of quartz phyllites, which had been metamorphosed during the hercynian orogenesis. This thesis deals especially with the tectonic evolution of the area. The greater part of the tectonic structures of the permo-triassic strata have previously been explained by most authors, excepting Diener, Accordi and Signorini, as a result of regional, tangential compression related to the alpine orogenesis. But a study of the crucial points of the tectonics showed that an explanation of the structural evolution by means of more locally restricted gravity tectonics should be preferred. The more general principles of the tectonic evolution of the eastern Alps, as expounded by Van Bemmelen (1960 a and b), are accepted as the tectonic setting for the structural evolution of the NW Dolomites in cenozoic time. The NW Dolomites are the northwestern corner of the larger unit of the Dolomites. The latter unit of the southeastern Alps lagged behind during the rise of the east alpine geanticline in tertiary time. This Dolomites-block has been separated from the more elevated, central parts of the geanticline by the large Judicaria and Pusteria faults (with a vertical throw of at least 5 km). Between this fault system and the area of the Dolomites a zone of relative subsidence is intercalated, which has a graben-like or synclinal character (Brenta Alps, Val di Non area, Sarntal Alps, Pusteria Valley). From San Candido this zone extends SE-ward to San Stefano di Cadore, this tract (Valle di Sesto) separates the Dolomites from the Carnian Alps. Parts of this depressed zone have been compressed subsequently by the gravitative spreading of the adjacent higher areas to the NW, N, and NE. (Dietzel, 1960; Van Hilten, 1960; Agterberg 1961). These authors have shown that this marginal belt along its NW, N, and NE side has been subjected to tectogenesis as a result of gravitational reactions to the rise of the east alpine geanticline. The central part of the NW Dolomites, however, has been more or less shielded from this gravitational stress field radiating from the central alpine uplift, by the deformations of this marginal belt. In this central part of the NW Dolomites the tectonic deformations of the sedimentary cover are merely an indirect result of the alpine orogeny. The tertiary uplift of the area caused a strong erosion. This erosion created a considerable relief with more local stress fields due to relief energy. The complicated tectonic structures of the NW Dolomites appear to be almost entirely the result of the local relief-energy produced by differential erosion in the course of the cenozoic time (such as the removal of the soft La Valle and San Cassiano strata between the ladino-carnian reef masses).