Orobic Alps Research Articles

Thrust kinematics and internal deformation in basement‐involved fold and thrust belts: The eastern Orobic Alps case (Central Southern Alps, northern Italy)

A new model, which integrates original detailed structural data and published geological data, is proposed for the Alpine (Cretaceous to present) convergent evolution of the northern portion of the Central Southern Alps. The Central Southern Alps are believed to have developed from the very beginning of their convergent evolution as a critical wedge, characterized by plastic deformation at depth. Clear evidence for the existence of a brittle‐plastic transition at depth within the Central Southern Alps wedge during pre‐Oligocene evolutionary stages comes from detailed structural data (original 1∶10000 form surface mapping). A regionally verified fabric sequence manifests the superposition of brittle to plastic to brittle structures developed during the path followed by the same volume of rocks within the wedge. On the other side no direct evidence exists in the rear part of the wedge for the occurrence of plastic deformation at depth during post‐Oligocene stages. However, considerations driven by the comparison of timing and amount of deformations in the Central Southern Alps and brittle wedge theory predictions strongly support this hypothesis. Different deformation mechanisms operating at different structural levels have been controlling a crustal scale strain partitioning between deep plastic and shallow brittle portions of the Central Southern Alps wedge. Plastic deformations have strongly controlled the overall mechanical behavior of the Central Southern Alps wedge, reducing sensitively the amount of internal deformation required in order to attain criticality of the wedge.

Aspects of Permian palaeobotany and palynology. XVI. Three-dimensionally preserved stromatolite-incrusted conifers from the Permian of the western Orobic Alps (northern Italy)

Freshwater stromatolites from a silty to sandy channel deposit in the Ponteranica Conglomerate (Permian) of the Orobic Alpe (province of Sondrio, Lombardy, northern Italy), contain very robust, three-dimensionally preserved large-leaved conifer shoots. Some of these shoots still show some details of their internal organization. These conifer remains cannot be assigned to any known genus of late Palaeozoic conifers. Therefore the new form-genus Cassinisia is established. Cassinisia orobica Kerp et al., sp. nov. is compared with other late Palaeozoic conifer taxa.

Cryptocrystalline Permian tourmalinites of possible metasomatic origin in the Orobic Alps, northern Italy

Strata-bound tourmaline-rich rocks, or tourmalinites, are relatively rare but important rock types that locally are associated with base metal, gold, tungsten, cobalt, and uranium deposits (e.g., Ethier and Campbell, 1977; Slack, 1982; Slack et al., 1984; Plimer, 1986, 1987; Bone, 1988; Sun and Ge, 1989; Nie et al., 1990; Modreski and Connor, 1991; Bhattacharya et al., 1992; Bandyopadhyay et al., 1993). Although their significance as guides to strata-bound mineral deposits is well established, the origin of most tourmalinites is still controversial (see Slack et al., 1993). Nearly all documented tourmalinites are of Proterozoic or early Paleozoic age and have undergone at least lower greenschist facies metamorphism that hampers interpretations of their origin because of changes in texture, mineralogy, and chemistry. This paper describes an important occurrence of finegrained tourmalinites from the Orobic Alps, northern Italy, in which metamorphic onditions did not exceed prehnite-pumpellyite grade. The Orobic tourmalinites are believed to be Permian and represent the youngest and most fine grained examples of such rocks known to date.

Contrasting thermomechanical evolutions in the Southalpine metamorphic basement of the Orobic Alps (Central Alps, Italy)

In the Southern Alps a progressive metamorphic zonation, with an increase in the geothermal gradient from NE to SW, has been widely proposed. However, recent investigations have shown that the greenschist metamorphic imprint of the low‐grade zone corresponds to a metamorphic retrogression following amphibolite facies conditions. On the other hand, in the medium‐grade zone, a later low‐pressure, high‐temperature (LPHT) metamorphic event has also been proposed. In an attempt to resolve these different interpretations, new petrological and partly new structural data have been obtained for two sectors of the Orobic Alps, traditionally attributed to different metamorphic zones. Thermobarometric determinations, supported by microstructural analysis, indicate the following different pressure‐retrograde paths in each sector: (1) in the Val Vedello basement (VVB) rocks, a first metamorphic imprint characterized by P= 7–9 kbar and T= 570–610°C was followed by a greenschist retrogression (P≤ 4 kbar and T≤ 500° C); (2) in the Lario basement (LB) rocks, the first detectable metamorphic stage, characterized by mineral assemblages indicating P= 7–9 kbar and T= 550–630° C, was followed by a LPHT event, synkinematic with F2 extensional deformation. A greenschist retrogression marks the final uplift of these rocks.Reinterpretation of the available geochronological data indicates a diachronism for the two thermomechanical evolutions. In the light of these data, we interpret the retrograde P–T–t path of the VVB rocks as a pre‐Permian post‐thickening uplift and the retrograde P–T–t evolution of the LB rocks as a Permo‐Mesozoic uplift related to the extensional tectonic regime of the Tethyan rifting.

Pre-alpine and alpine evolution of the South-alpine basement of the Orobic Alps

New K-Ar age determinations on the gneisses and schists of the south-Alpine basement of the Orobic Alps and their separated minerals have been carried out to supplement and support the previous study ofBocchio et al. (1981). The cooling ages of biotites and muscovites range from 218 to 331 my and from 170 to 330 my respectively, according to a bimodal distribution with modes at 222 and 298 my for biotite, 182 and 322 my for muscovite. The whole rock ages spread from 43 to 402 my with no significant cluster. Only a few samples give statistically significant internal isochrones, and these suggest two periods of equilibration, at 226–245 and 312–368 my. These age determinations support the conclusion drawn from regional petrologic studies (Bocchio et al., 1980;Crespi et al., 1980), that the south-Alpine basement underwent two main phases of metamorphism, now preferentially preserved in different areas: an old, Hercynian phase under a regime of intermediate pressure (staur-ky type), and a young, post-Hercynian phase under a regime of low pressure (staur-sill type) reactivated by the cover of the thick sedimentary pile on top of the basement. Furthermore, radiometric ages also point out the widespread but irregularly distributed occurence of an alpine overprint under the conditions of a very low metamorphic regime, that is documented by the growth of post-kinematic stilpnomelane in rocks of suitable bulk composition (Crespi et al., 1981, 1982), as well as by fine-grained phengite.