ALBERT

Description: Knipovich Ridge passive seismic experiment (KNIPAS) is a state-of-the-art seismological project that studies on segment scale the active spreading processes of an ultraslow mid-ocean ridge. The generation of new ocean floor is accompanied by characteristic seismicity that reflects ongoing spreading events and the physical state of the young lithosphere, and differs widely depending on spreading rate. While fast spreading ridges hardly show earthquakes that are large enough to be recorded on land, magmatic spreading events at the slowest spreading centres seem to be regularly preceded by earthquakes larger than M 5. The depth limit of earthquakes and their presence and absence reveal along-axis variations in the thermal and mechanical regime of the lithosphere. Therefore, it is necessary to record earthquakes locally with ocean bottom seismometers (OBS). Such surveys, however, typically have limited spatial extent and cannot reveal segment-scale spreading processes like along-axis melt flow, while spatially more extended data sets of hydro-acoustically recorded earthquakes yield no information on focal depth and can therefore not constrain lithospheric thickness or temperature. The project KNIPAS instrumented for the first time an entire ridge segment with OBS. During Polarstern cruise PS100 in July-September 2016 we deployed 23 OBS of the German Instrument Pool for Amphibian Seismology (DEPAS) along a 160 km long ridge section that covers Logachev Seamount and a neighbouring volcanic centre. An additional 3 OBS of the Institute of Geophysics, Polish Academy of Sciences, were deployed around Logachev Seamount. The instruments recorded seismicity until July-October 2017 depending on capacity. Cruise MSM67 of Maria S. Merian acquired wide-angle seismic profiles across Logachev Seamount and the subsequent cruise MSM68 successfully recovered all OBS. We now have a comprehensive seismological dataset at hand that will contain despite partly high noise levels in the vicinity of Logachev volcano an expected 9000 earthquakes M〉1 and several dozens of well-recorded teleseismic events to study spatial variations of seismicity, thermal structure and lithospheric thickness of an ultraslow spreading ridge. In a joint project we will combine the expertise of our work groups to study seismicity pattern, analyse the large-scale lithospheric structure with modern passive seismic methods to be adapted for the special conditions of marine seismic surveys and to image at high resolution the structure of a volcanic centre.

Description: Knipovich Ridge passive seismic experiment (KNIPAS) is a state-of-the-art seismological project that studies on segment scale the active spreading processes of an ultraslow mid-ocean ridge. The generation of new ocean floor is accompanied by characteristic seismicity that reflects ongoing spreading events and the physical state of the young lithosphere, and differs widely depending on spreading rate. While fast spreading ridges hardly show earthquakes that are large enough to be recorded on land, magmatic spreading events at the slowest spreading centres seem to be regularly preceded by earthquakes larger than M 5. The depth limit of earthquakes and their presence and absence reveal along-axis variations in the thermal and mechanical regime of the lithosphere. Therefore, it is necessary to record earthquakes locally with ocean bottom seismometers (OBS). Such surveys, however, typically have limited spatial extent and cannot reveal segment-scale spreading processes like along-axis melt flow, while spatially more extended data sets of hydro-acoustically recorded earthquakes yield no information on focal depth and can therefore not constrain lithospheric thickness or temperature. The project KNIPAS instruments for the first time an entire ridge segment with OBS. During Polarstern cruise PS100 in July-September 2016 we deployed 23 OBS of the German Instrument Pool for Amphibian Seismology (DEPAS) along a 160 km long ridge section that covers Logachev Seamount and a neighbouring volcanic centre. An additional 5 OBS of the Institute of Geophysics, Polish Academy of Sciences, were deployed around Logachev Seamount. The instruments will record seismicity until July-October 2017 depending on capacity. Cruise MSM67 of Maria S. Merian will acquire wide-angle seismic profiles across Logachev Seamount and the subsequent cruise MSM68 will recover all OBS. By the end of 2017 we will have a comprehensive seismological dataset at hand consisting of an expected 9000 earthquakes M〉1 and several dozens of well-recorded teleseismic events to study spatial variations of seismicity, thermal structure and lithospheric thickness of an ultraslow spreading ridge. In a joint project we will combine the expertise of our work groups to study seismicity pattern, analyse the large-scale lithospheric structure with modern passive seismic methods to be adapted for the special conditions of marine seismic surveys and to image at high resolution the structure of a volcanic centre.

Description: To better understand the lithospheric structure beneath the ultraslow-spreading ridges the active seismic survey within the Knipovich Ridge Passive Seismic Experiment (KNIPAS) was carried out. The aim of this work was to provide a segment-scale image of lithosphere structure, velocity field and its boundaries beneath the Logachev Seamount on the Knipovich Ridge. Active seismic profiles were acquired during cruise no. MSM67 in September 2017. On the ocean floor at depths from 2.3 to 3.3 km seismic energy was recorded by 8 ocean bottom seismometers (OBS). In total 320 km of seismic data was collected along 6 profiles with lengths varying from 30 to approximately 60 km covering the area of around 2200 km2. The profiles are crossing each other over the center of the Logachev Seamount. High resolution bathymetric data acquired during the cruise combined with previous bathymetry data sets were utilized as an ocean bottom layer within the seismic model. Our intention underlying this work is to provide evidence of crustal thickness variation beneath the Logachev Seamount and therefore substantially contribute to an understanding of this type of ridges. For the 2D modeling process only data from OBSs near the profiles were used. Seismic model was prepared for each seismic line by iterative trial-and-error ray tracing. After preparation and initial processing of the acquired data, picking of visible first breaks on all seismic sections had been done. Layers of the model were added to assume the best fit between calculated travel times and picks. Five lithospheric layers for the longest profiles were separated with substantial velocity contrasts at the boundaries. Besides first arrivals, later phases and multiples were used. Water wave and its multiples allowed estimation of the velocity in the sea water. Available non-linear information from all profiles will be used for further 3D tomography modeling. By combining the available observables from all seismic profiles we draw the following conclusions. The resulting 2D lithosphere models show relatively high velocity gradients especially for the middle oceanic crust. High velocities 5.3 – 5.8 km/s are observed just below the surface over the seamount center. We found ca. 1.5 km uplift of the lower oceanic crust layer to the East of the Logachev Seamount. For the longest profile layer with velocity above 8 km/s was distinguished at depth of approximately 10 km which can suggest presence of the Moho discontinuity.

Description: The Knipovich Ridge is part of the Arctic Ridge System comprising very slow spreading ridges. In the class of ultraslow spreading ridges, the Knipovich Ridge with its full spreading velocity of 14 – 17 mm/yr is one of the slowest and most obliquely spreading ridges. Magmatic centres along the Knipovich Ridge are mostly defined by seamounts. Amagmatic segments, where tectonism dominates the spreading, act as transfer regions between magmatic centres, since transform faults are absent. The detailed spreading processes at ultraslow spreading ridges still remain unclear. We want to study tectonics and magmatism and their interplay along the Knipovich Ridge by the distribution of local seismicity at segment-scale. We further are interested in how ridge segmentation works in the absence of transform faults. Knipovich Ridge was equipped with a maximum of 30 ocean bottom seismometers along a length of 160 km. The seismometers are positioned between 75.7 and 77.2°N to both sides of the rift valley. They recorded seismicity continuously for on average 11.5 months between summer 2016 and 2017. We used the detection algorithm Lassie and a Kurtosis-based picking algorithm followed by review of the picks by an analyst. The velocity model used for location is defined by well constrained events. We present here first results of this project. We found that earthquakes are not equally distributed along the ridge axis. We observe regions of enhanced seismicity and regions with no or very little seismic activity. Focal depths undulate along the ridge axis up to depths of 20-25 km. We also found clusters of events, one in the north, close to volcanic features, and one close to station 19, south of the Logachev Seamount, a prominent volcanic edifice. The depth distribution of earthquakes reflects the boundary between brittle and ductile deformation, depending on temperature and composition of rocks. This thermal boundary has a varying depth along the rift axis and allows the focussing of melts, e.g. towards Logachev Seamount, where deep seismicity is entirely absent. Seismically less active regions above the band of seismicity may be due to specific composition of rocks, e.g. serpentinised peridotite that leads to ductile reaction on applied stresses. Seismicity clusters may be related to magmatic activity or tectonism of transfer regions.

Description: The Polish Geophysical Expedition to West Antarctica in 1979–1980 was carried out by the Institute of Geophysics, Polish Academy of Sciences. Beside deep seismic soundings, 12 multi-channel seismic profiles, with a total length of ca 1000 km have been recorded north and east of the South Shetland Islands and in the Bransfield Strait, but they have never before been completely interpreted and published. All profiles have been processed with modern processing flow including time migration. Profiles crossing the South Shetland Trench revealed distinct reflector inside continental slope, which has been interpreted as border between buried accretionary prism and overlying slope sediments of glacial-marine origin. Profiles in the Bransfield Strait show traces of the Last Glacial Maximum (LGM) in the form of glacial foreground valleys, with some of them used as weak spots for young age volcanic intrusions. This paper is the first comprehensive geological interpretation of collected dataset and differences between results from other expeditions are discussed.