Hints of active deformation in the southern Adriatic foreland: Holocene tectonics along the Apulia offshore (Italy)

Abstract

The western Adriatic margin was intensely deformed during the Meso-Cenozoic evolution of the Adriatic region from a passive margin to a foreland basin. In the offshore area north and south of Gargano Promontory, several deformation belts develop parallel or cross-strike to the margin. Based on high-resolution seismic data, deformation along these inherited tectonic structures continued during the Quaternary, resulting in small-scale faults and folds affecting the upper-most 50-100 m of the sedimentary succession. This stratigraphic interval corresponds to the last ca. 450 ka and is composed of four depositional sequences, each recording 100 ka glacio-eustatic cycles, overlain by transgressive and highstand units of the last deglacial interval (the last ca. 20 ka). Locally, faults propagate through Holocene deposits and offset the sea floor; vertical displacements of reflectors is variable along the faults, ranging from few metres up to ca. 15-20 m, but usually decreases up-section, to less than 1 m within Holocene units. In some cases, active deformation along inherited tectonic lineaments is confirmed by recent seismicity. On the western margin of Adria, seismicity is mostly concentrated along the Apennines. However, in the Adriatic Sea, an overall W-E trending seismic belt extends offshore Gargano Promontory. The existence of this cross-strike seismicity belt, that also encompasses the Tremiti Islands, is documented by moderate but significant earthquakes. More in general, based on instrumental records, the offshore area north of Gargano Promontory appears more seismic than the area south of it, where instrumental seismicity is reduced, while it is more frequent on the Gargano Promontory. We focus on a deformation belt extending NE of Gargano Promontory, within the offshore area yielding significant instrumental seismicity. The NE-Gargano deformation belt comprises: 1) a faulted anticline on the sloping southern side of the Pelagosa sill, 2) a fault system on the outer shelf and 3) a syncline on the inner shelf. The anticline on the slope is the most remarkable feature within this deformation belt, and clearly affects seafloor relief. The set of sub-vertical faults that dissect the anticline also displace the sea floor, delimiting a graben-like feature. The comparison of high-resolution tectono-stratigraphic reconstructions and seismicity records can give information on the active deformation of “Adria”, and provide new insight on the existence of potential seismo-genic structures in the Adriatic offshore, where also evidence of slope instability is diffused. More in general, these results show the importance of using very high-resolution geophysical data and sequence-stratigraphic reconstructions to constrain present-day active tectonics.