Structural, seismic and hydrothermal features at the border of an active intermittent resurgent block: Ischia Island (Italy)

Resurgent blocks are uplifted parts of volcanoes where most of the deformation (uplift) is accommodated along peripheral faults. The island of Ischia, Italy, has an active resurgent block uplifted ∼1000 m in the last ∼30 ka, which allows us to investigate the deformation associated with a strong uplift in volcanic areas. Air-photo interpretation was matched with field work (1:5000 scale mapping and structural analysis) throughout the resurgent block. These results have been matched with a review of the seismic and hydrothermal activity at Ischia. The results show that the block has a polygonal shape in map view and is characterized by a trapdoor uplift, which results in a ∼15° tilt towards the SE. The northern, most uplifted part is bordered by E–W- and NE–SW-trending faults, arranged in parallel segments over an area ∼1.5 km wide. I0≥5 MCS degree shallow (1–1.6 km depth) earthquakes focused in correspondence with the outermost set of faults on the northernmost part of the block. Here, variations in the hydrothermal activity have been observed during seismic events. Previously performed analogue models show that resurgent blocks are bordered by inward-dipping high-angle reverse faults (outermost fault system) and outward-dipping normal faults (innermost fault system). We propose a model of trapdoor uplift of a block which induces reverse faulting, responsible for seismicity and hydrothermal variations along the most uplifted part. Outward-dipping normal faults, in the innermost part, form as a consequence of the gravitational sliding of the volumes bordered by the reverse faults. The further reactivation of the outermost reverse faults triggers a new cycle of intermittent trapdoor uplift of the block.