Early Rise Followed by Late Reactivation of Distal Salt Diapirs in the Western Mediterranean
Virginie Gaullier, Bruno C. Vendeville, Françoise Sage, and Laurence Droz
Some of the world’s most spectacular salt structures are located along continental margins along which large volumes of sediments have prograded seaward, implying significant amounts of downslope displacement of both salt and its cover. During the Messinian Salinity Crisis, a sea-level drop of more than 1500 m allows the deposition of thick evaporitic series in the deep basins. The mobile salt is post-rift since the opening of the basin formed during the Oligo-Aquitanian age (i.e., the main margin tilting related to rifting had already happened, the salt base is often sub-horizontal). Plio-Quaternary tectonics in the western Mediterranean is dominated by huge thinskinned salt tectonics with gravity spreading and/or gliding above the Messinian décollement, typically characterized by proximal extension, mid-slope translation, and distal shortening. The distal region comprises numerous diapirs, whose rise was traditionally attributed to combined distal shortening and sediment loading contemporary of the proximal extension. However, new seismic data and re-interpretation of older ones show that salt tectonics started very early in the deepest areas, immediately after deposition of the mobile salt, as attested by lateral thickness changes and internal angular unconformities within the Messinian Upper Unit, while this Upper Unit is isopach in the extensional region. We therefore propose two steps to explain the initiation of distal salt diapirs. First, salt in the deepest regions was mobilized by gravity spreading triggered by deposition of sediment fans. Second, gravity gliding started later, as was recorded by upslope fan-shaped Early Pliocene deposits adjacent to normal growth faults, some of the pre-existing diapirs being reactivated by distal contraction. This scenario explains why the onset of updip salt diapirism was not coeval with downdip gliding and how so many diapirs can have pierced through in a brittle, i.e., strong, sedimentary cover, by simple downbuilding. The tilting responsible for this late slope could be due to thermal subsidence, to the rapid Pliocene re-flooding of the Mediterranean or to kinematics changes at that time in this geodynamically complex area. Some analogue models validate this new interpretation.
AAPG Search and Discovery Article #90161©2013 AAPG European Regional Conference, Barcelona, Spain, 8-10 April 2013