US EPA

7475252 

Journal Article 

Oligo-Miocene volcanic sequences and rifting stages in Sardinia: a review 

Lecca, L; Lonis, R; Luxoro, S; Melis, E; Secchi, F; Brotzu, P 

1997 

1 

Periodico di Mineralogia
ISSN: 0369-8963 

66 

1-2 

7-61 

English 

The orogenic Oligo-Miocene volcanism of Sardinia (28-15 Ma) is distributed mainly along the main branches of the Sardinia Rift system (MBVZ) and, subordinately, in the Sulcis area (SWVZ). This paper stratigraphically redefines the volcanic sequences on the basis of the volcanological style, K-Ar (mainly on mineral pairs) and last but not least, contact field relationships with biostratigraphically known marine markers. From a geodynamical point of view, the studied volcanism as well as the whole Sardinian Rift system are interpreted as activated by drifting movements resulting mostly from indentation of Sardinia palaeomicroplate with Maghrebian orogen during upper Oligocene-middle Miocene kynematic processes in the western Mediterranean. These processes reactivated the late-Hercynian faults, probably with minor rotational components of the main sub regional blocks. The relative mobility of the sub regional blocks appears to have played a major role in driving the volcanic activity and favouring transtensional and extensional fault zones that promoted the uplift of magmas in the crust and the formation of magma reservoirs in the upper crust as inferred by hornblende barometer. Significantly, the explosive activity is preferentially distributed in central-northern Sardinia where the Aquitanian-Burdigalian major fragmentation of the Sardinia block is recorded along the main NE trending faults of Ottana graben and Logudoro. Ignimbrites from the eastern sectors of the MBVZ are commonly characterized by strongly porphyritic biotite-amphibole ash and pumice flow deposits, followed by pyroxene olivine phyric welded pyroclastic sequences. These characters are interpreted as indicating different magmatic reservoirs localized in different position in the upper crust than to zoned magma reservoirs. In summary, the rifting stages of the Sardinian Rift may be as follows: (a) Pre-Rift phase characterized by strike-slip and/or transpressive regime during collisional phases (upper Eocene-Oligocene); (b) 1st Rift phase (Aquitanian-Burdigalian); strike-slip → transtensive up to extensional regimes that reactivated the late-Hercynian faults that produced an earlier proto-Rift phase with emplacement of pre-Aquitanian andesitic sequences (28-24 Ma). Volcanic activity marks a progressive increase of explosive characters in MBVZ accompanied by the structuration and progressive development of epicontinental marine basins (1st Rift phase). Particularly, the marine basin may be slightly older in the southern than northern areas and show different geological evolution as indicated by quite different subsidence rates. In detail, tectonic evolution produced three main marine (southern, western and northern) basins. c) 2nd Rift phase (upper Burdigalian-Langhian); extensional collapse phase linked to the eastward deplacing of the Apennine thrust front and Sardinia-Corsica palaeomicroplate drifting with formation in northern Sardinia of wide structural lows infilled by marine epicontinental sediments that reach a more basinal character (upper Burdigalian-Langhian). Important changes in the chemical characters of volcanics are documented by the occurrence of very primitive basalts (e.g. high-Mg basalts of Montresta area and Arcuentu dykes; 17 Ma) and peralkaline rhyolites of Sulcis area (15 Ma). The ignimbrites from Sardinia have an important geodynamic significance for the evolution of the western Mediterranean area. Indeed, K-Ar radiometric ages of explosive activity of MBVZ indicate that it is confined to 21-19 Ma that is indistinguishable from the time span defined by Montigny et al. (1981) for the climax rotation of the Sardinia-Corsica palaeomicroplate and also straddle the age of volcanoclastites from northern Apennines belonging to the 'siliceous zone'. It must be emphasized that there is general consensus that the Sardinia volcanism was the source of volcanoclasatites which are widespread in the Apennine foredeep and piggy-back basins. This suggests a tectonic back-arc extension in Sardinia Rift as the counterpart of shortening processes recorded in the frontal zone of the northern Apennine chain in Aquitanian-lower-middle Burdigalian time. 

backarc basin; Miocene; Oligocene; rifting; tectonics; volcanism; Italy, Sardinia