Tristan da Cunha hotspot : Mantle plume or shallow plate tectonics?

Tristan da Cunha is a small volcanic island in the South Atlantic Ocean close to the Mid- Atlantic Ridge. It is part of an area, which is characterized by widely scattered seamounts and small islands at the western end of the Walvis Ridge - Tristan/Gough hotspot track. Tristan da Cunha represents the end member of a classical hotspot track with an underlying plume: The active volcanic island Tristan da Cunha at the youngest end of the track is linked to the Cretaceous Etendeka flood basalt province in northwestern Namibia at its oldest end. But the genesis of the island itself has so far been puzzling. It is hotly debated if the island sits actually above a deep-seated mantle plume or if it is caused by shallow plate tectonics. To understand the Tristan da Cunha hotspot, a multi-disciplinary geophysical study has been conducted in 2012 and 2013 on board the German research vessel Maria S. Merian to acquire marine magnetotelluric and seismological data. The aim was to reveal the upper mantle structure with electrical density and velocity perturbations. Within this study I focused on the seismological dataset. At first, I performed a P-wave finite-frequency tomography with cross-correlated travel time residuals of teleseismic earthquakes. This allows to resolve the upper mantle structure beneath the island in terms of velocity perturbations and clarifies the existence of a mantle plume. I also investigated the local seismicity in the Tristan region to identify tectono-magmatic processes at the Mid-Atlantic Ridge and Close to the islands and seamounts. Moreover, I compared and combined my tomographic results with electromagnetic results to identify zones of partial melt and to understand plume processes in the upper mantle beneath the Tristan da Cunha hotspot. The tomographic results provide evidence for the existence of the Tristan conduit southwest of the archipelago. Its shape is cylindrical with a radius ca. 100 km down to a depth of 250 km. The structure ramifies in narrow veins below that depth. A recent link from the conduit to a seamount chain shows, that melt is channelled towards seamounts and islands in the study area. High seismicity within an oceanic plate segment north of Tristan da Cunha can be related to the internal stresses of the fragment. Differently directed forces act at the northern boundary of this plate. An earthquake free zone coincides spatially with the location of the Tristan mantle plume. This indicates a ductile regime in the lithosphere above the plume. Furthermore, hints for an incipient ridge jump towards a parallel line to the actual location of the Tristan plume were found. Several earthquakes were localised close to the archipelago of Tristan da Cunha. The locations of these earthquakes are related to young surface eruptions like small volcanic cones or seamounts

The seismically active Mt. Hochstaufen, Bad Reichenhall (Germany)

For centuries, the Mt. Hochstaufen/Bad Reichenhall region in the Northern Alps has been affected by single earthquakes with magnitudes up to M3.2 or by irregular swarms. Most of the swarms occur during summer, often accompanied by strong precipitation, suggesting a direct correlation of seismicity and rainfall. A swarm in 2019 comprised many earthquakes with high magnitudes but a synchronous strong rain event is missing. Consequently, precipitation and tectonic background stress cannot be the sole explanation for the unusual high local seismicity. A trans-disciplinary study combining seismological analysis, meteorological and geodetic observations of the last years was proposed to identify the set of acting forces. Here, we present the first results of the seismological analyses and of the ground-based radar measurement performed at three measuring points surrounding Mt Hochstaufen. The relocation and clustering of earthquakes of the last decade, enables to distinguish between one-time occurrence of fault mechanisms limited to one swarm and faults reactivated with a distinct (yearly) periodicity. Beside a subsidence of the Bad Reichenhaller basin west of the Saalach river, the ground-based radar measurements show the opening of prominent fractures at both flanks of the mountain

Seismicity in the vicinity of the Tristan da Cunha hotspot: Particular plate tectonics and mantle plume presence

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 one year. We relate intra-plate 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

UNIBRA / DSEBRA – the German seismological contribution to AlpArray

UNIBRA was a joint initiative of German universities to install and maintain 74 seismic broadband stations at the beginning of the international AlpArray project in 2015 when the proposal for the 100 station broadband array DSEBRA was not yet approved by DFG. In this way, full participation of German teams in the AlpArray project could be secured. Most of these stations were deployed in southern Germany and a few in Austria. After approval in 2017 and installation of DSEBRA in 2018, the UNIBRA stations were replaced and further DSEBRA stations were deployed east of the SWATH-D array and also in Hungary. At that time, DSEBRA made up about one third of AlpArray’s temporary stations. After deinstallation of SWATH-D in autumn 2019 DSEBRA stations were used to reoccupy some of SWATH0-D’s critical sites. In spring 2020, the Covid19 pandemic started in Europe and it became unfeasible to move the DSEBRA stations to new sites. Instead of deinstallation, DFG allowed us to use remaining investment funds to continue the operation of DSEBRA at the current sites. As collaboration partners from Austria, Czech Republic, Poland, Slovakia and Hungary had already relocated many of their AlpArray stations to new sites towards the north-east and east of the Alps before Covid19 started, DSEBRA became part of the PACASE deployment with 214 temporary stations operated by partners from these countries and University of Lausanne. In summer 2022, new funds from DFG could be acquired by RU Bochum and LMU München to move 42 DSEBRA stations to Greece and Northern Macedonia and further 19 stations to Albania, Kosovo and Montenegro as part of the new AdriaArray project. The remaining DSEBRA stations stayed in Austria and Hungary to form a major part of AdriaArray’s backbone circling the Adriatic plate. With little exceptions, the DSEBRA stations have been in the field now without interruption for nearly 6 years. They massively contributed to the collection of a unique, large-scale and long-term seismological dataset which has enabled investigations into the structure of the crust and mantle beneath the greater Alpine area using receiver functions, shear-wave splitting, teleseismic body and surface wave tomography, local earthquake tomography and teleseismic full waveform inversion. Moreover, they allowed new insights into the seismic activity and hazard of active faults. DSEBRA will continue to do so in the framework of AdriaArray as part of an even larger seismic network comprising about 1300 permanent and temporary stations and doubling the size of AlpArray. Noise at the DSEBRA stations on the vertical component stayed below the Peterson high noise model by 20 dB over the entire seismic frequency band. Noise on the horizontal components was partially higher, in particular at low frequencies below 1 Hz. Thanks to special measures to avoid failures of mobile communication and battery charging and efforts to keep the low-power data logger running as long as possible in case of power failures, data availability of the DSEBRA stations reached extremely high values of 98% to 100%. The data were archived and disseminated on the EIDA node at LMU München during the experiment and transferred to the GEOFON for long-term archiving

Der Tristan da Cunha Hotspot :Mantelplume oder flache Plattentektonik?

Tristan da Cunha is a small volcanic island in the South Atlantic Ocean close to the Mid- Atlantic Ridge. It is part of an area, which is characterized by widely scattered seamounts and small islands at the western end of the Walvis Ridge - Tristan/Gough hotspot track. Tristan da Cunha represents the end member of a classical hotspot track with an underlying plume: The active volcanic island Tristan da Cunha at the youngest end of the track is linked to the Cretaceous Etendeka flood basalt province in northwestern Namibia at its oldest end. But the genesis of the island itself has so far been puzzling. It is hotly debated if the island sits actually above a deep-seated mantle plume or if it is caused by shallow plate tectonics. To understand the Tristan da Cunha hotspot, a multi-disciplinary geophysical study has been conducted in 2012 and 2013 on board the German research vessel Maria S. Merian to acquire marine magnetotelluric and seismological data. The aim was to reveal the upper mantle structure with electrical density and velocity perturbations. Within this study I focused on the seismological dataset. At first, I performed a P-wave finite-frequency tomography with cross-correlated travel time residuals of teleseismic earthquakes. This allows to resolve the upper mantle structure beneath the island in terms of velocity perturbations and clarifies the existence of a mantle plume. I also investigated the local seismicity in the Tristan region to identify tectono-magmatic processes at the Mid-Atlantic Ridge and Close to the islands and seamounts. Moreover, I compared and combined my tomographic results with electromagnetic results to identify zones of partial melt and to understand plume processes in the upper mantle beneath the Tristan da Cunha hotspot. The tomographic results provide evidence for the existence of the Tristan conduit southwest of the archipelago. Its shape is cylindrical with a radius ca. 100 km down to a depth of 250 km. The structure ramifies in narrow veins below that depth. A recent link from the conduit to a seamount chain shows, that melt is channelled towards seamounts and islands in the study area. High seismicity within an oceanic plate segment north of Tristan da Cunha can be related to the internal stresses of the fragment. Differently directed forces act at the northern boundary of this plate. An earthquake free zone coincides spatially with the location of the Tristan mantle plume. This indicates a ductile regime in the lithosphere above the plume. Furthermore, hints for an incipient ridge jump towards a parallel line to the actual location of the Tristan plume were found. Several earthquakes were localised close to the archipelago of Tristan da Cunha. The locations of these earthquakes are related to young surface eruptions like small volcanic cones or seamounts

Hunting for the Tristan plume - An upper mantle tomography around the volcanic island Tristan da Cunha

Tristan da Cunha is a volcanic island in the South Atlantic close to the Mid-Atlantic Ridge. It is part of an area consisting of widely scattered seamounts and small islands at the western and youngest end of the aseismic Walvis Ridge. Tristan da Cunha together with the Walvis Ridge represents the classical example of a mantle plume track, because of the connection to the Cretaceous Etendeka flood basalt province in NW Namibia. The genesis of the island has so far remained enigmatic. It is hotly debated, if Tristan da Cunha sits actually above a deep mantle plume or if it is only originated by upwelling material from weak (leaky) fracture zones. It also has to be clarified if there are any indications for a plume-ridge interaction. Geochemical investigations have shown complex compositions of magmatic samples from Tristan da Cunha, which could be interpreted as a mixing of plume-derived melts and depleted upper mantle sources. To improve our understanding about the origin of Tristan and to test the mantle plume hypothesis, we deployed 24 broadband ocean-bottom seismometers and 2 seismological land stations around and on the island during an expedition in January 2012 with the German research vessel Maria S. Merian. After acquiring continuous seismological data for almost one year, the seismometers were recovered in early January 2013. We cross-correlated the arrival times of teleseismic P and PKP phases to perform a finite-frequency tomography of the upper mantle beneath the study area. Here we show the 3D mantle structure in terms of velocity variations: We do not image a “classical” plume-like structure directly beneath Tristan da Cunha, but we observe regions of low velocities at the edges of our array that we relate to local mantle upwelling from potentially deeper sources. Additionally we discuss local seismicity within the Tristan da Cunha region, which show processes along the nearby mid-ocean ridge and transform faults. Furthermore, the local seismicity indicates spots of recent magmatic activity in close vicinity to the islands

Hunting for the Tristan mantle plume – An upper mantle tomography around the volcanic island of Tristan da Cunha

The active volcanic island Tristan da Cunha, located at the southwestern and youngest end of the Walvis Ridge – Tristan/Gough hotspot track, is believed to be the surface expression of a huge thermal mantle anomaly. While several criteria for the diagnosis of a classical hotspot track are met, the Tristan region also shows some peculiarities. Consequently it is vigorously debated if the active volcanism in this region is the expression of a deep mantle plume, or if it is caused by shallow plate tectonics and the interaction with the nearby Mid-Atlantic Ridge. Because of a lack of geophysical data in the study area, no model or assumption has been completely confirmed. We present the first amphibian P-wave finite-frequency travel time tomography of the Tristan da Cunha region, based on cross-correlated travel time residuals of teleseismic earthquakes recorded by 24 ocean-bottom seismometers. The data can be used to image a low velocity structure southwest of the island. The feature is cylindrical with a radius of ∼100km down to a depth of 250km. We relate this structure to the origin of Tristan da Cunha and name it the Tristan conduit. Below 250km the low velocity structure ramifies into narrow veins, each with a radius of ∼50km. Furthermore, we imaged a linkage between young seamounts southeast of Tristan da Cunha and the Tristan conduit