Plio-pleistocene tectonic evolution of Southern Sardinia

Up to now there are not detailed studies about the structures and the kinematic related to the Plio-Pleistocene tectonics in southern Sardinia, although tectonic activity during the latest million years is long since known in the Island. Then, the general aim of this research is to improve the understanding of the recent geological evolution of the southern Sardinian block, trying to calculate the extension affecting the crust, to reconstruct the paleostress fields and to define a model of the tectonic evolution of the southern Sardinia during this period, considering the new data obtained from this research and the data available from literature on the geodynamic evolution of the western Mediterranean, in particular about the opening of the Tyrrhenian Sea. To achieve this purposes, the research focuses on the study of the most important structure related to the Plio-Pleistocene tectonics: the "so-called" Campidano graben. This structure has been studied mainly interpreting the seismic lines acquired in the Campidano plain by SAIS in the early sixties and by the joint venture AGIP-Progemisa S.p.A. in the early nineties that have been made available for this research by the Regione Autonoma della Sardegna and Progemisa S.p.A. The continuation of the Campidano in the Cagliari Gulf has been studied interpreting the ministerial seismic lines aquired by AGIP in the seventies in the offshore of Cagliari. The main results achieved from the research are: - the 3D model of the Middle Pliocene erosional surface at the base of the Samassi formation detected in the subsurface of the Campidano plain; - the extension values and the deformation rates affecting the Campidano area during Plio-Quaternary times; - the reconstruction of the paleostress field active during Pliocene time in the Campidano graben; - a comparison between the structural setting of the Campidano and the structural setting of the southeastern Sardinian margin; - the integration of the data achieved during this research with the data available from literature on the geodynamic evolution of the western Mediterranean, in particular about the opening of the Tyrrhenian Sea. The 3D model of the Middle Pliocene erosional surface has been built using the two-way time structural maps of this surface carried out in both the northern and southern Campidano. This surface shows a gently rolling landscape, typical of the erosion and planation surface developed in continental environments, both in the subsurface of the southern and northern Campidano plain. In the Campidano of Oristano, the maximum depth of the bottom of the Samassi formation is approximately -1100 m, while in the southern it is roughly -900 m (mean velocity assigned 2 km/s). The two-way time structural maps show that both the northern and the southern Campidano graben are bounded in the western edges by master faults dipping to the east, but differences arise in the trends of the faults: mostly N-S in the northern Campidano and NW-SE in the southern Campidano. N-S normal faults affected the whole Samassi formation and the within-plate basalts that post-date it, so we may admit that an important extension occurred after its deposition, that is in Pleistocene time. The structures from seismic interpretation, that characterized the Plio-Quaternary evolution of the so-called Campidano graben have been validated during the drafting of the two-way time maps and with the structural and stratigraphic data available on surface. This allowed to construct a validated 3D model from which achieved the most possible reliable values of extension and paleostress. To quantify the Plio-Quaternary deformation in southern Sardinia the 3D model of the Middle-Pliocene erosional surface (roughly 3.5 Ma in age) has been considered. The extension has been calculated restoring the cross-sections achieved from the 3D model. In the southern Campidano, extension values range from 205 m to 596 m and the percentage of extension from 1.65% to 5.72%. In the northern Campidano extension values range from 171 m to 465 m and percentage of extension from 0.84% to 2.50%. Although the Plio-Pleistocene time-markers (3.5 Ma the erosional surface at the base of the Samassi formation and 2.5 Ma the surface sealed by the basalt lava flows in northern Campidano and a reflector calibrated with the Campidano1 well in the southern Campidano) are not very well constrained, they allow to estimate the vertical slip rate (0,6 mm/yr) and the extension rate (0,3 mm/yr in the northern Campidano and 0,4 mm/yr in the southern Campidano). Considering the erosional surface sealed by the basalt-lava flows of the "giare" to the east of Campidano that occurs up to 550 m above the sea level and the age of 3.0 Ma of the basalts that seal it, an uplift rate of 0.18 mm/yr can be calculated if a marine origin for this surface is confirmed. Furthermore, the difference in elevation between the higher basalts that crop out in the central Sardinia (approximately 700 m in the Orroli basalt plateau) and those detected in the subsurface (-440 m in the northern Campidano) is related to tectonic activity that is at least Pleistocene in age. Therefore, during the whole Plio-Quaternary tectonics, considering the Middle Pliocene erosional surface at the base of the Samassi formation detected at -1100 m in the northern Campidano, vertical displacements on the order of 1800 m can be inferred. The paleostress field active during Pliocene time in the Campidano area has been reconstructed using the movement vector on the fault plane achieved from the restoration of the Middle Pliocene erosional surface in both the northern and southern Campidano using two different methods that showed almost the same results. In the northern Campidano, an extension oriented roughly E-W, with the principal stress axes: σ1=263/77; σ2=356/1; σ3=86/13; has be inferred. In the southern Campidano, the extension is oriented roughly ENE-WSW, with the principal stress axes: σ1=105/85; σ2=332/3; σ3=242/3. The evolution during Pliocene in the southern Sardinia seems to be strictly related with the evolution of the sector of the southern Tyrrhenian basin located roughly south of the 40° parallel. This can be infer from the similarities in ages and stuctures in the eastern Sardinian margin and Campidano area. It seems that the extensional tectonics started in the Upper Miocene in the Sardinian Basin and Cornaglia Terrace and migrated westward in Sardinia and eastward in the Vavilov Basin. Concerning the Pliocene tectonics affecting Sardinia, therefore, it seems to be directly related to the beginning of the extension in the Tyrrhenian Basin, due to the eastward roll back of the subducting Adriatic plate. It seems, instead, that the Pleistocene tectonics recognized in Sardinia can not be related directly with the evolution in the Tyrrhenian Basin, because, although extensional tectonics continued in the Marsili Basin until now, no active tectonics occurs in the Sardinian Basin and Cornaglia Terrace. Thus, the recent uplift affecting the southern Sardinia is most likely related to the lithospheric structure inherited from the pre-Pliocene geodynamic evolution of the western Mediterranean, during which the lowering of the density in the mantle lithosphere could be caused an uplift up to 1620 m, if a normal thick mantle lithosphere (90 km) is considering as start point, or up to 720 m, considering the present day thinned mantle lithosphere (40 km), or by the thinning of the mantle lithosphere connected with process of intensive convective heating (thermal thinning).