Solid as a rock: Tectonic control of graben extension and dike propagation

The 2014–2015 CE rift event associated with the Bárðarbunga eruption at Holuhraun, Iceland, offers a unique opportunity to study the spatial and temporal evolution of a rift graben. We present the first four-dimensional (three-dimensional plus time) monitoring of the formation and evolution of a graben during active magma transport using a suite of digital elevation models spanning from shortly before the eruption throughout 6 months of magma transport and up to 4.5 years after the eruption. This multiscale data set enables investigations of how magma supply and eruption dynamics affect tectonic structures that feed eruptions. After formation (time scale of a few days), the graben is remarkably stable throughout the eruption and for years beyond. It is unaffected by large changes in eruptive activity and effusion and seismicity rates within the plumbing system. These data document that (1) there was no direct feedback between eruptive dynamics and graben topography, and (2) graben formation is near instantaneous on tectonic time scales. These results challenge the overarching role ascribed to magma transport in recent studies of tectonomagmatic relationships in rift events, favoring regional tectonics as the fundamental driving force.

Potential Eruption and Current Activity of Anak Krakatau Volcano, Indonesia

Anak Krakatau Volcano is located in the Sunda Strait known for its paroxysmal eruption in 1883. During the January - November 2019 period, seismicity was dominated by types of quakes which indicated the occurrence of magma supply (VA and VB), near-surface volcanic activity (LF, Hybrid, Harmonic Tremors), and volcanic activity above the volcanic surface (eruptions, emission, and continuous tremors). In the period December 2019 - July 2020, there was an increase in the types of quakes near the surface (LF, Hybrid) and the types of quakes on the surface (emission and continuous tremors). Volcanic deformation monitors changes in tilt over the 2019-2020 period associated with pressure releases before, during and after the eruption. The results of GPS data modeling, the shallow pressure source is at a depth of 0.22 km below sea level. Volcanic activity until July 2020 was dominated by activity near and above the volcanic surface associated with the growth of lava domes. The volcanic system of Anak Krakatau is currently an open system, with the potential for eruptions. Strengthening the early warning system for the eruption of Anak Krakatau is important in mitigating efforts and understanding its eruption potential

Translithospheric magma plumbing system of intraplate volcanoes as revealed by electrical resistivity imaging

The magma plumbing systems of volcanoes in subduction and divergent tectonic settings are relatively well known, whereas those of intraplate volcanoes remain elusive; robust geophysical information on the magma pathways and storage zones is lacking. We inverted magnetotelluric data to image the magma plumbing system of an intraplate monogenetic volcanic field located above the stagnant Pacific slab in northeast China. We identified a complex, vertically aligned, low-resistivity anomaly system extending from the asthenosphere to the surface consisting of reservoirs with finger- to lens-like geometries. We show that magma forms as CO2-rich melts in a 150-km-deep asthenospheric plume crossing the whole lithosphere as hydrated melt, inducing underplating at 50 km depth, evolving in crustal reservoirs, and erupting along dikes. Intraplate volcanoes are characterized by low degrees of melting and low magma supply rates. Their plumbing systems have a geometry not so different from that of volcanoes in subduction settings.

A heterogeneous subcontinental mantle under the African–Arabian Plate boundary revealed by boron and radiogenic isotopes

The northern and northwestern margins of the Arabian Plate are a locus of a diffuse and long-lasting (early Miocene to Pleistocene) Na-alkali basaltic volcanism, sourced in the asthenosphere mantle. The upwelling asthenosphere at the Africa–Arabia margin produces very limited magma volumes in the axial zone. Therefore, portions of hot, fertile mantle continue their eastward migration and are stored at shallower depths under the 100-km thick Arabian lithosphere, which is much thinner than the African one (≈175 km): this causes the occurrence and 20-Ma persistence of magma supply under the study area. Erupted basalts sampled a continuous variation of the mantle source, with a striking correlation among temperature, pressure and isotopic composition shifting between two end members: a 100 km-deep, more depleted source, and a 60 km-deep, more enriched one. In particular, we observed an unusual variation in boron isotopes, which in the oceanic domain does not vary between more depleted and more enriched mantle sources. This study shows that, at least in the considered region, subcontinental mantle is more heterogeneous than the suboceanic one, and able to record for very long times recycling of shallow material.

On magma supply and spreading modes at slow and ultraslow mid-ocean ridges

<p>The availability of magma is a key to understand mid-ocean ridge tectonics, and specifically the distribution of the two contrasted spreading modes displayed at slow and ultraslow ridges (volcanically-dominated, and detachment fault-dominated). The part of the plate divergence that is not accommodated by magma emplaced as gabbros or basaltic dikes is taken up by normal faults that exhume upper mantle rocks, in many instances all the way to the seafloor. </p><p>Magma is, however, more than just a material that is, or is not, available to fill the gap between two diverging plates. It is the principal carrier of heat into the axial region and as such it may contribute to thin the axial lithosphere, hence diminishing the volume of new plate material formed at each increment of plate separation. Magma as a heat carrier may also, however, if emplaced in the more permeable upper lithosphere, attract and fuel vigorous hydrothermal circulation and contribute instead to overcooling the newly formed upper plate (Cochran and Buck, JGR 2001). </p><p>Magma is also a powerful agent for strain localization in the axial region: magma and melt-crystal mushes are weak; gabbros that crystallize from these melts are weaker than peridotites because they contain abundant plagioclase; and hydrothermally-altered gabbros, and gabbro-peridotite mixtures, are weaker than serpentinites because of minerals such as chlorite and talc. As a result, detachment-dominated ridge regions that receive very little magma probably have a stronger axial lithosphere than detachment-dominated ridge regions that receive a little more magma. </p><p>Because magma has this triple role (building material, heat carrier, and strain localization agent), and because it is highly mobile (through porosity, along permeability barriers, in fractures and dikes), it is likely that variations in magma supply to the ridge, in time and space, and variations in where this magma gets emplaced in the axial plate, cause a greater diversity of spreading modes, and of the resulting slow and ultraslow lithosphere composition and structure, than suggested by the first order dichotomy between volcanically-dominated and detachment-dominated spreading. </p><p>In this talk I illustrate these points using results of recent studies at the Mid-Atlantic and Southwest Indian ridges.</p>