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

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

Costa Recta beach, Deception Island, West Antarctica: a retreated scarp of a submarine fault?

“…Martí & Baraldo (1990) suggested a passive collapse of the caldera along orthogonal faults as the origin of the inner great bay, Port Foster. The influence of the regional major faults was postulated by Smellie (1989), Rey et al (1995) and Martí et al (1996). suggested a volcanotectonic evolution and collapse of the caldera.…”

Analysis of the Temporal Occurrence of Seismicity at Deception Island (Antarctica). A Nonlinear Approach

“…It is presently the most active volcano of the South Shetland Islands-Antarctic Peninsula group, with eruptions known to have taken place in 1848, 1967, 1969 and 1970. The main part of Deception Island is composed of a largely unexposed shield structure, formed of basaltic lava flows and palagonitized tuffs, which underlies a thick and continuously exposed sequence of subaerial pyroclastic flow and surge deposits of andesitic composition. In contrast, recent volcanism is represented by several small strombolian and phreatomagmatic vents which have acted independently inside and outside the central volcano-tectonic depression (MARTI et al, 1996). The tectonic structure of Deception Island is characterized by the existence of a dense network of normal faults, many crossing the entire island (Marti et al, 1996).…”

“…In contrast, recent volcanism is represented by several small strombolian and phreatomagmatic vents which have acted independently inside and outside the central volcano-tectonic depression (MARTI et al, 1996). The tectonic structure of Deception Island is characterized by the existence of a dense network of normal faults, many crossing the entire island (Marti et al, 1996). A NE-SW oriented tectonic trend, parallel to the expansion axis of the Bransfield Strait, is clearly predominant on the island.…”

Local influences of geothermal anomalies on permafrost distribution in an active volcanic island (Deception Island, Antarctica)

“…This anomalous warm behaviour in the site should be the result of the volcanic-hydrothermal processes occurring at depth (Rey et al, 1995;Martí et al, 1996). Thus, they inhibit permafrost development in the coastal fringe, as well as ground frost below 0.20 m, even when the air temperature falls below 0°C.…”