GNGTS 2014 - Atti del 33° Convegno Nazionale

54 GNGTS 2014 S essione 3.1 event of about 0.6 s TWT for the underlying reflectors, that are probably related to the LU (Fig. 2A). The hypothesized scenario is represented in the scheme of Fig. 2D and takes in account mean interval velocities of 2600 m/s and 4400 m/s for the Plio-Quaternary sequence and for the MU, respectively. They were chosen on the basis of both literature works and determined velocity function discussed above. In Figs. 2B and 2C are reported, for comparison, the details of the outcomes of PSDM1 and PSDM2 in correspondence to the pull up event and the related velocity functions picked on semblance panels derived from CDP 800 (VELGEOD–PSDM1) and 820 (VELGEOD2-PSDM2) gathers (Figs. 2E, 2F). It can be noticed that in the first outcome (PSDM1) no evidence of pull up flattening is present, while in the second outcome (PSDM2) the pull up seems to be restored. A strong reflection in correspondence with the salt diapir of the detail of PSDM2 is present at 6.8-6.9 km of depth (Fig. 2C). Its seismic facies is similar to that of the bottom MU interpreted in correspondence of the little sedimentary basin on the left side of the line. Although the PSDM2 is affected by artefacts (probably due to large lateral velocity variations), the information given around the CDP 820, in correspondence of 6.8-6.9 km depth, could confirm the interpretation of the pull up event previously done on the post stack time migrated data. Furthermore, the resultant size of the salt diapir is almost 4 km high. Seismic interpretation and mapping. Among the interpreted dataset, in the deep basin the MSC units and the Plio-Quaternary (PQ) sequence were generally recognised with the same acoustic characters defined by literature, although some variabilities in the seismic response are locally present. Furthermore, different numbers of reflections characterise the UU and the PQ units among the analysed dataset. This is because the WMR dataset offers relatively a medium- high vertical resolution which is higher than that of the CROP dataset (aiming at a deep crustal investigation) and lower than that of the WS dataset (supplying in any case a medium-high resolution). Thus, for example, the autochthonous salt layer “s” is imaged on the WMR and WS data, but it is not detected by the CROP profiles (Fig. 3A). Halokinesis is particularly intense and generates high diapirs that often intrude the UU and the lower part of the PQ sequence (Fig. 3A), but sometimes reach the sea bed (Figs. 2A, 2B, 2C). The UU is locally characterized by growth strata. This demonstrates that the halokinetic movements started during the late stage of the MSC event, as suggested by Geletti et al. (2014). This could be due to the increasing of the sedimentary and water column loading related to several cycles (Krijgsman and Meijer, 2008) of inflow water during the deposition of the UU. However, growth strata are frequently present within the lower and middle Pliocene units, testifying that the halokinesis developed further and more intensely during the Pliocene, when the complete reflooding of the Mediterranean was reached, accompanied by relatively thick sedimentation of pelagic deposits. In the part of the basin which is adjacent to the toe of the slope, coalescent salt diapirs almost completely disrupt the UU, which is disarticulated in numerous small packages: sometimes they are difficult to recognize, either because they may be little over the limit of resolution or because, being incorporated into the salt as it moved vertically, their seismic responses are attenuated. At the toe of the slope and on the lower slope, roll over structures are often present. They are sometimes associated with growth strata of the UU and/or the PQ sequence (whose thickness is considerably reduced in respect to the north-western part of the basin). Halokinetic normal and listric faults bound the dislocated blocks of the overburden and in some cases they also affect the sea bed. This intense halokinesis may be related to a local sliding of the salt toward the basin on the inclined West Sardinian lower slope. The process produced local compressive stresses that acted on the adjacent part of the basin, also deforming of the post-Messinian salt sedimentary sequence. MU shows the minimum thickness (less than 200 ms) on the sector adjacent to the Western Sardinian lower slope, where it flowed inside or above the UU as allocthonous salt bodies. In correspondence of the lower slope, the thickness of MU is further reduced and the unit pinches