Seismicity in the Vicinity of the Tristan Da Cunha Hot Spot: Particular Plate Tectonics and Mantle Plume Presence

Abstract

Earthquake locations along the southern Mid‐Atlantic Ridge have large uncertainties due to the sparse distribution of permanent seismological stations in and around the South Atlantic Ocean. Most of the earthquakes are associated with plate tectonic processes related to the formation of new oceanic lithosphere, as they are located close to the ridge axis or in the immediate vicinity of transform faults. A local seismological network of ocean‐bottom seismometers and land stations on and around the archipelago of Tristan da Cunha allowed for the first time a local earthquake survey for 1 year. We relate intraplate seismicity within the African oceanic plate segment north of the island partly to extensional stresses induced by a bordering large transform fault and to the existence of the Tristan mantle plume. The temporal propagation of earthquakes within the segment reflects the prevailing stress field. The strong extensional stresses in addition with the plume weaken the lithosphere and might hint at an incipient ridge jump. An apparently aseismic zone coincides with the proposed location of the Tristan conduit in the upper mantle southwest of the islands. The margins of this zone describe the transition between the ductile and the surrounding brittle regime. Moreover, we observe seismicity close to the islands of Tristan da Cunha and nearby seamounts, which we relate to ongoing tectono‐magmatic activity.

Crustal and uppermost mantle structure of the NW Namibia continental margin and the Walvis Ridge derived from ambient seismic noise

“…The shape (morphology) of the hotspot track strongly varies from the Namibian coast towards the present-day hotspot position close to Tristan da Cunha Island. The initial volcanism formed a massive ridge, with up to 35-km-thick igneous crust, while the present-day active hotspot region around the Tristan Island is characterized by scattered seamounts (O'Connor & Duncan 1990;Milner et al 1995;Ewart et al 2004;O'Connor & Jokat 2015;Jokat & Reents 2017;Schlömer et al 2017). While the deep seismic sounding (seismic wide-angle and refraction) data provide a detailed insight into the general offshore crustal structure along some 2-D sections, it remained an open question if and how the crustal structure varies in 3-D (Fig.…”

Seafloor evidence for pre-shield volcanism above the Tristan da Cunha mantle plume

“…Nevertheless, we speculate that the two areas west-northwest of Inaccessible Island, each about 20 × 40 km across, with cones and lava flows causing high backscatter in comparison to surrounding sediment-covered undisturbed seafloor, might represent the incubators of future large seamounts or even islands. Perhaps, consistent with this, Schloemer et al 32 detected earthquake activity below the western part of area B, possibly indicating active tectono-magmatic processes.…”

“…These events suggest that the plumbing system of the volcano may still be active. Deeper earthquakes (up to 30 km depth) were located closer to the TTFZ by Schloemer et al 32 .…”

“…7 ). Global 52 and regional 32 seismicity data show local earthquake activity close to the westernmost mapped volcanic cones.…”

“…5b ). Schloemer et al 32 located two earthquakes in 2012 in the close vicinity of Isolde Seamount, at focal depths of 5–15 km. These events suggest that the plumbing system of the volcano may still be active.…”