Deception Island (Bransfield Strait, Antarctica): an example of a volcanic caldera developed by extensional tectonics

Abstract

Abstract Deception Island has traditionally been considered as a collapse caldera formed by subsidence into a magma chamber of a group of overlapping volcanoes along arcuate and radial faults. In fact, the morphological features of Deception Island (horseshoe shape, location of post-caldera vents apparently along concentric faults, existence of a depression in the centre of the island, concentration of post-caldera activity along the ‘ring fault’, etc.) support this idea. However, a detailed revision of the structure of Deception Island combining field geology, high resolution seismic profiles and analysis of local and regional seismicity, indicates that most of the structural features identified as evidence for supporting the model of caldera formed by collapse of a magma chamber have been misunderstood. Post-caldera volcanic activity is not restricted to the border of the depression, but appears both inside (submarine volcanism) and outside (subaerial volcanism) the hypothetical ring fault. Nearly all the post-caldera vents are located on linear faults, and seismic profiles indicate that most of these linear faults, which are parallel or normal to the spreading axis of Bransfield Strait, can be traced outside of the island. The postulated ring fault results from these intersecting linear faults in the interior of the island where they define a depression. No arcuate faults were identified on the border of the depression. The structure of the depressed sector of the island is also defined by several blocks limited by a nearly orthogonal network of normal faults. This geometry is also indicated by the epicentral location of more than 100 seismic events, which define a linear distribution of earthquakes following the main regional normal faults across the entire island. This contrasts with the circular pattern found in collapse calderas with well defined ring fault systems (i.e. Long Valley, Rabaul, Campi Flegrei). Moreover, the presence of radial faults, postulated by previous authors, has not been proved. The existence of a well developed system of radial faults reflects the pushing action of a shallow magma chamber and should normally imply the intrusion of dykes through these fractures. Very few dykes, always of basaltic composition, have been identified in Deception Island and were intruded along linear regional faults. Because the central depression exists we cannot deny the existence of the caldera depression at least in a morphological sense. However, we propose a mechanism of caldera formation contrasting to the one proposed by previous authors. The tectonic features of Deception Island indicate the existence of a tensional stress field in all directions at the surface, compatible with the regional stress field which has characterized the Bransfield Strait at least during the last 4 Ma. This favoured the normal faulting of the island and the subsidence of the central blocks without the formation of any ring fault system. After the formation of the depression at the centre of the island submarine and subaerial volcanic activity has continued along the nearly orthogonal network of normal faults