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  • 1
    Call number: 21/STR 05/16
    In: Scientific Technical Report STR
    Type of Medium: GFZ publications
    Pages: 126 S.
    Series Statement: Scientific Technical Report STR 05/06
    Note: Zugl.: Berlin, FU, Diss., 2004
    Location: Reading room
    Branch Library: GFZ Library
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  • 2
    Publication Date: 2019-02-01
    Description: The Gulf of Cadiz seismicity is characterized by persistent low to intermediate magnitude earthquakes, occasionally punctuated by high magnitude events such as the M ~ 8.7 1755 Great Lisbon earthquake and the M = 7.9 event of February 28th, 1969. Micro-seismicity was recorded during 11 months by a temporary network of 25 ocean bottom seismometers (OBSs) in an area of high seismic activity, encompassing the potential source areas of the mentioned large magnitude earthquakes. We combined micro-seismicity analysis with processing and interpretation of deep crustal seismic reflection profiles and available refraction data to investigate the possible tectonic control of the seismicity in the Gulf of Cadiz area. Three controlling mechanisms are explored: i) active tectonic structures, ii) transitions between different lithospheric domains and inherited Mesozoic structures, and iii) fault weakening mechanisms. Our results show that micro-seismicity is mostly located in the upper mantle and is associated with tectonic inversion of extensional rift structures and to the transition between different lithospheric/rheological domains. Even though the crustal structure is well imaged in the seismic profiles and in the bathymetry, crustal faults show low to negligible seismic activity. A possible explanation for this is that the crustal thrusts are thin-skinned structures rooting in relatively shallow sub-horizontal décollements associated with (aseismic) serpentinization levels at the top of the lithospheric mantle. Therefore, co-seismic slip along crustal thrusts may only occur during large magnitude events, while for most of the inter-seismic cycle these thrusts remain locked, or slip aseismically. We further speculate that high magnitude earthquake's ruptures may only nucleate in the lithospheric mantle and then propagate into the crust across the serpentinized layers.
    Type: Article , PeerReviewed
    Format: text
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  • 3
    Publication Date: 2019-02-01
    Description: 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 ~ 100 km down to a depth of 250 km. We relate this structure to the origin of Tristan da Cunha and name it the Tristan conduit. Below 250 km the low velocity structure ramifies into narrow veins, each with a radius of ~ 50 km. Furthermore, we imaged a linkage between young seamounts southeast of Tristan da Cunha and the Tristan conduit.
    Type: Article , PeerReviewed
    Format: text
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  • 4
    Publication Date: 2014-05-12
    Description: During RV Polarstern cruise ARK-XXIV/3 (2009), a geophysical study along the ultraslow spreading Knipovich Ridge was conducted. The survey, located in the rift valley south of the Logachev Seamount (7636′N), provides a crustal thickness of 4.5 km in the amagmatic parts and 5.7 km underneath the seamount itself. The velocity-depth function indicates the presence of a thick oceanic layer 2 (4.5 km), but no indication for a thick oceanic layer 3. The only exception is the area underneath the Logachev Seamount, where velocities higher than 6 km/s are detected. This indicates a stronger and focussed melt supply underneath the seamount. Local seismicity was analysed for two days. In total, 191 quakes were identified in the vicinity of the rift valley with magnitudes up to ML = 2.6. At least 48 of them are located in the upper mantle (up to 18 km below sea level), supporting models predicting a cold mantle or conductive cooling underneath ultraslow ridges to explain the reduced melt supply.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 5
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    In:  [Talk] In: 2016 DGF SPP1375 SAMPLE Colloquium, 06.-08.06.2016, München, Germany .
    Publication Date: 2017-01-17
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 6
    Publication Date: 2019-02-01
    Description: 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.
    Type: Article , PeerReviewed
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  • 7
    Publication Date: 2016-09-13
    Description: 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.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 8
    Publication Date: 2017-04-28
    Type: Conference or Workshop Item , NonPeerReviewed
    Format: text
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  • 9
    Publication Date: 2017-01-13
    Description: The Yermak Plateau is a prominent bathymetric feature of the Arctic Ocean. To the west it is bordered by the Fram Strait, which forms the only deep-water connection between the Arctic and the other global oceans. Origin, crustal nature and age of the Yermak Plateau are largely unknown. For this study, we investigated dredged rocks of two sites from the Yermak Plateau. Based on petrography, geochemistry, and geochronology, we distinguished between ice-transported and in-situ rocks. Ice-transported material was most likely derived from outcrops of the High Arctic Large Igneous Province (HALIP) on Franz Josef Land, the Siberian trap province, and presumably from northern Svalbard. Our data from the in-situ rocks, in conjunction with previously published geophysical data, show that the investigated parts of the Yermak Plateau are composed of stretched continental crust strongly affected by alkaline magmatism. The continental rocks represent a direct continuation of the exposures on northern Svalbard. Alkaline magmatism took place at ~ 51 Ma and was related to continental rifting in an extensional setting. The melts were formed by low degrees of partial melting of the sub-continental lithospheric mantle and are probably associated with the high-amplitude magnetic anomalies described for the northeastern Yermak Plateau. Extension of the Yermak Plateau was contemporaneous with spreading of the adjacent young Eurasian Basin, and occurred during the peak of compressional deformation affecting North Greenland, Svalbard, and Ellesmere Island. These contrasting regimes were probably compensated by transpression and strike-slip movements along the DeGeer and Wegener Faults. The date of ~ 51 Ma for extension-related magmatism also provides age constraints for the extension-related formation of the Sophia Basin (and thus for water exchange between the Eurasian Basin, the area of the DeGeer Fault and the young Norwegian-Greenland Sea), and for the sediments covering the horst-and-graben structures of the Yermak Plateau.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 10
    Publication Date: 2018-06-12
    Description: Abstract
    Description: The Teisseyre-Tornquist Zone (TTZ) as part of the Trans-European Suture Zone (TESZ) is one of the most prominent suture zones in Europe separating the young Palaeozoic platform from the much older Precambrian East European craton. The knowledge of deep structure of the TESZ is very important for the understanding of various tectonic processes in Europe. The PASSEQ 2006-2008 seismic experiment was performed thanks to a big international effort of 17 institutions from 10 countries. A total of 139 three-component temporary short-period and 49 temporary broadband seismic stations provided continuous recordings between May 2006 and June 2008 with the main period of recordings during 2007, in an array about 1200 km long and 400 km wide running from Germany through the Czech Republic and Poland to Lithuania. The average spacing between all stations was about 60 km, attaining about 20 km in the central part. The configuration of the seismic network was a compromise among needs of different seismic methods. The dense central profile allows the use of modern passive 2-D imaging techniques, while the distribution of broadband sensors was designed for surface wave and receiver function studies of the upper mantle down to the transition zone in a wide frequency range. Waveform data is fully open, with network code 7E.
    Keywords: Seismic waveforms, PASSEQ ; Monitoring system ; Seismological stations
    Type: Other , Seismic Network
    Format: Approx 1684 GB
    Format: SEED data
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