An integrated geophysical study of the Ionian Basin western margin : new insights on its Permo-Triassic origin

Abstract

The origin and evolution of the Ionian Basin of the Central Mediterranean have been the subject of intense study and debate in the last three decades. The principal unsettled issues include the nature of the crust underlying the basin, the location of the presumed continent ocean boundary in the west, the timing of its opening and subsequent spreading, and the exact mechanism of its evolution to the present-day configuration. The structural complexity of its northern and eastern subduction margins has added to the difficulty in fully understanding this enigmatic province. The crust underlying the basin has been variously described as being either continental, oceanic or transitional. Earlier models for its evolution postulated a thinned continental crust linking the Pelagian Shelf to the west and the Apulian Shelf to the northeast to form a promontory of the north African plate. However, more recent seismic reflection and refraction surveys and potential field studies conducted in the 1980's and 1990's reveal features that are more reminiscent of a basin underlain by oceanic crust. This would imply the presence of two conjugate continent-ocean boundaries separating the crust underlying the basin from the Pelagian and Apulian Shelves. The location of the western boundary is uncertain although it is often considered to coincide with the Sicily-Malta Escarpment and its southeast continuation towards the Libyan coast. The eastern boundary is located offshore southwest Puglia in Italy in the direction of Albania and Greece. The age of the basin is even more controversial, with a time for the initiation of rifting that could be as early as the Permo-Triassic, the early Jurassic or as recent as the middle Cretaceous. This thesis presents new geophysical data from a study area covering regions of the Ionian Basin in its westernmost bathymetric limits with the Pelagian Shelf. Gravity modelling confirms the oceanic nature of the crust beneath the basin and also shows a progressive increase in crustal density from the Pelagian Shelf and the Sirt Rise towards the Ionian Basin. The crustal thickness is in agreement with published results from deep-penetrating seismic surveys. Moreover, the relatively high density of the crust of the Shelf suggests that it is made up of oceanic accretions rather than continental-derived terranes. Interpretation of regional gravity and seismic data indicates that the location of the western continent-ocean boundary lies significantly to the east of the Sicily-Malta Escarpment and extends in a southeast direction towards the Cyrenaica Ridge. North of the Medina Ridge, east-west dextral wrenching in the Plio-Quaternary is deemed to have shifted the boundary further east with respect to the boundary in the south. Integrated seismic and well data show the presence of a series of en echelon fault blocks, igneous mounds, syn-rift deposits of probable late Palaeozoic to early Mesozoic age and a thick sequence of late Mesozoic to Tertiary post-rift sediments coinciding with the western margin of the basin. The age of the syn-rift sediments suggests that although the foundering of the basin to its present bathymetric depths may have started in the late Miocene, its opening occurred much earlier and has an origin in the Permo-Triassic during the :fragmentation of the north African margin in the break-up of Pangaea.