The Tristan da Cunha mantle plume and its role in the continental breakup and opening of the South Atlantic (insights from geophysical, petrological and geochemical studies along the Walvis Ridge hotspot track)

Wolfram Geissler (Alfred Wegener Institute Helmholtz Center for Polar and Marine Research, Germany)

Abstract
Tristan da Cunha is a hotspot in the South Atlantic Ocean, located ~450 km east of the Mid-Atlantic Ridge. The intraplate volcanoes and seamounts that form the Tristan da Cunha archipelago are connected to the Cretaceous (~132 Ma) Etendeka continental flood basalt province in Namibia via the aseismic Walvis Ridge. The ridge is built-up by seamounts chains and submarine volcanic plateaus that show a clear age progression and extend from the Namibian continental margin (northeast) to the volcanic islands of Tristan da Cunha and Gough (southwest). This age-progressive distribution of volcanic rock samples collected from the Walvis Ridge and the Rio Grande Rise west of the Mid-Atlantic Ridge provide evidence for the volcanism at Tristan da Cunha and the formation of Cretaceous flood basalts in Namibia and Brazil to be due to a common hotspot source, with the Walvis Ridge and the Rio Grande Rise documenting the hotspot tracks. The Tristan da Cunha-Walvis Ridge system is one of the few examples of a complete hotspot track, and thus the it is generally assumed to be a surface expression of a long-lasting mantle plume. However, a debate continues about whether the mantle plume beneath Tristan da Cunha is an expression of convection of the whole mantle or of shallower plate-driven convection. The hypothesis of a deep mantle plume origin of the hotspot volcanism at the Tristan da Cunha archipelago is supported by anomalous geochemical data, geochronological constraints, and global seismic tomography. The first proof of the Tristan da Cunha mantle plume activity are the Cretaceous Etendeka-Parana flood basalts, that erupted shortly before the South Atlantic finally opened. Therefore, a debate exists about whether the mantle plume triggered the continental breakup or played at least a strong role in facilitating the breakup. NW Namibia, the Walvis Ridge, Tristan da Cunha, and the South Atlantic Ocean in general were target regions for various international geophysical and geochemical projects over the past two decades, including a recent IODP expedition to the Walvis Ridge. In my talk, I will summarize the recent findings on the Tristan da Cunha mantle plume activity and its role during the breakup of the South Atlantic Ocean.

Title: Seismological evidence for hidden crustal fluid diffusion transients in California

Abstract:
Earthquake swarms are primarily a consequence of aseismic driving processes that occur transiently, but the details of these processes are still unclear. We use deep learning algorithms to produce a significantly enhanced catalog of seismicity for Southern California and better resolve the details of swarms. Near the San Jacinto fault zone, a major strike-slip fault system, we identify nearly one hundred swarms that have durations in the range 0.5-6 years. These swarms are not directly driven by tectonic processes yet constitute about 1/4 of the total seismicity. We find that about half of the swarms exhibit ultra-slow diffusive migration patterns, along with propagating backfronts, in which a shut-down of seismicity propagates away from the source region. These observations are all consistent with expectations for natural fluid injection processes. Furthermore, we find evidence that permeability along the fault zone increases exponentially with time elapsed. The chronology of the swarms indicates that these aseismic driving processes were active in some part of the region throughout 2008–2020. The observations challenge common views about the nature of swarms, which would characterize any one of these sequences as anomalous. The regional prevalence of these sequences suggests that transient fluid injection processes play a key role in crustal fluid transport.

Title: Seismic radiation from regions sustaining brittle rock damage

Abstract:

Analytical results indicate that dynamic changes of elastic moduli in source volumes produce “damage-related-radiation” associated with tensorial products of the changes of elastic moduli and the elastic strain components at the source. Decreasing elastic moduli (as produced by brittle deformation of low-porosity rocks and explosions) increase the radiation to the surrounding medium, while increasing moduli (which may be produced during the formation of compaction bands in porous rocks) decrease the radiation to the bulk. A tensorial decomposition analysis indicates that the damage-related-radiation has a large isotropic component. While the total radiation may still be dominated by shear, the dynamic dilation associated with the isotropic radiation can significantly reduce the heat generation near the rupture front and produce a pulse-type rupture and rock fragmentation. Numerical simulations confirm these expectations. The damage-related-radiation involves small length scales (proportional to the rupture width) and is therefore difficult to observe in the far field. However, analyses of earthquake waveforms using seismic data recorded close to earthquake ruptures reveal elevated P-wave radiation and appreciable explosive isotropic source terms (e.g., 5-15% of the total event potencies). Geological studies of deeply exhumed faults in dry granulite rocks show evidence of pulverization and delayed heating in earthquake rupture zones consistent with strong dynamic dilation at the rupture fronts. The high potential importance of damage-related-radiation to the local physics of earthquake ruptures motivates additional theoretical, experimental, and observational research.