La seconda fase del Rift Sardo: vulcanismo ed evoluzione dei sub-bacini di Ardara-Chilivani e Bonorva (Sardegna settentrionale)

The second stage of the Sardinian Rift: volcanism and evolution of Ardara-Chilivani and Bonorva sub-basins (Northern Sardinia). The Oligo-Miocene Sardinian Rift (27-15 Ma; CHERCHI & MONTADERT, 1982; LECCA et alii, 1997), is a typical intra arc-massif basin (sensu DICKINSON, 1974) made up of several sub-basins filled by thick orogenic volcanic sequences and marine sediments. This arc is linked to the subduction of the Neotethys oceanic plate under the European plate and to the Africa-Europe convergence. The displacement of the arc or, in other words, the drift of the Sardinia-Corsica Block, is related to the orogenesis of the northern Apennine and of the Maghrebid-Sicilian chain. Despite the key-role played by the Sardinia Rift in the knowledge of the geodynamic evolution of the western Mediterranean basin, the stratigraphy of volcanic sequences is still incomplete because large portions of this rift still require detailed mapping. At the regional scale, the Sardinia Rift shows marked differences with regard to volcanic and tectonic style. In northern Sardinia, the rift is arranged into several half-graben type sub-basins, related to tilted block and horst block systems. The basins are schematically attributable to the relative mobility of the main blocks with minor relative strike-slip to transtensional movements along NE-SW trending faults among the sub-blocks of eastern Sardinia. The Chilivani-Ardara and Bonorva sub-basins contain a poorly known thick pyroclastic sequence buried by epicontinental marine sediments. On the basis of field, petrographic and volcanological criteria, the volcanic pile is constituted, from bottom to the top, by early volcanics of broadly andesitic composition (M. Cuguttada Unit: MC), densely welded and rheomorphic ignimbrites (WI) followed by ash and pumice pyroclastic flow deposits named throughout this paper Pianu Ladu (PL), Pianu de Puma (IC) and Chilivani Unit (CH). Epiclastic deposits (EVL) are observed locally at different stratigraphic levels (the so-called «lacustre», VARDABASSO & ATZENI, 1962; PECORINI, 1962). Marine deposits onlap the volcano-sedimentary sequence in the upper Burdigalian-Serravallian interval. Published K-Ar (LECCA et alii, 1997) and Ar-Ar (EDEL et alii, 2001) radiometric data obtained for the pyroclastic flow deposits indicate a quite narrow time span in the range of 20-18 Ma. With regard to the regional stratigraphic picture proposed by LECCA et alii (1997), MC andesites belong to the A1 andesites; WI are the densely welded reomorphic ignimbrites, whereas the PL, IC and CH ash and pumice flow-Units constitute the AP2 group. In detail, pyroclastic units belonging to the latter group show the following: Pianu Ladu Unit (PL). The PL unit is a light grey-coloured ash and pumice pyroclastic flow deposit, outcropping with continuity at the northern edge of the Chilivani-Ardara sub-basin. It lies normally on WI ignimbrites or locally on reddish matrix-supported conglomerates made up of clasts of Palaezoic basement. Content of pumice and xenoliths (accidental and cognate), ranges from 20 to 10% moving from S. A. of Bisarcio towards M. Ladu, while size and degree of welding decrease. Pianu de Puma Unit (IC). The IC Unit constitutes discontinuous outcrops in the Bonorva basin as well as in the southern part of Chilivani- Ardara. It lies with nonconformity on the Palaeozoic basement and/or on the WI. Locally (Pianu de Puma), it is preceded by greenish to yellow coloured epiclastic deposits at least 5 m thick, containing rare clasts of the Palaeozoic basement dispersed in an ashy matrix. In the field, it is a poorly porphyritic greyish coloured ashy pyroclastic flow deposit ranging in thickness from a few metres to 30 m the greater thickness being observed eastward of Pranu Mannu. Epiclastic deposits (EVL). They constitute discontinuous interbeds among the PL, IC and CH Units; characterized essentially by ash, crystals and pumice-fragments and containing conglomerate beds formed at the expense of Palaeozoic basement and WI ignimbrites. In the field they commonly show a greenish colour and typical cross and parallel bedding structures. Chilivani Unit (CH). This is an ash and pumice compound ignimbrite made up of at least three different flow units well exposed in the Chilivani-Ardara sub-basin. It represents the volumetrically most important volcanic unit of the investigated area constituting wide plateaux dismembered by post-depositional faulting. The thickness of the unit CH ranges from 10 m (M. Salattu and M. Filigosu sectors), up to 100 m in M. Cordianu-M. S. Bernardo and in the west of Pranu Mannu (sub-basin of Bonorva). Macroscopically, it is easily recognizable because of its high porphyritic index. Petrographic characters suggest that the pyroclastic units recognized may be related to different magmatic reservoirs localized in the upper crust, along active fault zones. In a tectonic/volcanism feedback scenario, the progressive discharge of magmatic reservoirs gives rise to caldera-like structures favoured by a progressive extensional regime. Several lines of evidence indicate that the PL, IC and CH units mark the early supply in the studied basins, predating the marine ingression. They were erupted along regional volcanogenic faults observable westward in Bosa sub-basin, as well as in the Chilivani- Ardara and Bonorva sub-basins. The whole data set suggest that the volcanic products fill the inner regions of the subsiding sub-basins in the eastern Logudoro, prolonging the continental conditions that end with the volcanic activity. The volcanic activity of the sub-basins described is tectonically related to NE-SW trending sinistral transtensive faults which show a more pronounced extensional character with time. The sub-basins’ evolution can be summarized as progressive fault blocking and tilting of a block system, easily recognizable by the through the WI cover that represents a lithostratigraphic marker for northern Sardinia. Tectonic subsidence favoured the local deposition of fluvial conglomerates (Pianu Ladu) and finally ash and pumice flowdeposits (IC and PL) which mark the acme of sub-basin filling and assume a tectonic significance. In the general picture of the multiphase evolution of the Sardinian Rift, these sub-basins belong to the second rifting phase sensu LECCA et alii (1997), during which the transtensional-extensional faulting affects particularly north eastern Sardinia at about 18 Ma and predates the beginning of the extension in the north Tyrrhenian domain