Fault pattern of Nisyros Island volcano (Aegean Sea, Greece): structural, coastal and archaeological evidence

Abstract

Abstract There has been much debate about the fault pattern of Nisyros Island at the southeastern edge of the Aegean volcanic arc. The small active volcanic island, less than 200–100 ka old, is dominated by a well-developed caldera and by post-caldera domes that are less than 25 ka old and up to 600 m high. Detailed mapping of the tectono-volcanic features of Nisyros have revealed that faults have a clear radial pattern, and they are more abundant in the northwest of the island, where volcanic domes are also concentrated. In contrast to previous speculations that certain major faults control the tectono-volcanic development of Nisyros, this paper argues that the radial fracture pattern is expected where faulting is a secondary effect of volcanic doming. Structural, coastal and archaeological evidence supports this contention. Most faults have a short fault length and a ‘scissors-type’ geometry typical of magma or salt ascent dynamics, and variable throws that are too high to reflect simply tectonic effects. Elevated coastal marine fossils (vermetids) in the northwest of the island indicate rates of uplift too high to be explained solely by differential fault movements. In the same area, a fault that borders the fortifications of a fourth-century BC castle is inferred to be the source of seismic damage effects observed in its ramparts and responsible for the near-total destruction of its westernmost fortifications. Together, the evidence suggests that localized high rates of uplift, faulting and tilting reflect tectonvolcanic deformation effects, and remind us that associated fault activity is likely to have constituted a major threat for the island since antiquity.