Lithosphere formation at the ultraslow-spreading Gakkel Ridge: From reconnaissance seismicity studies towards passive seismology with under-ice ocean bottom seismometers

Vera.Schlindwein [ at ]


At the 1800 km long Gakkel Ridge, the lithosphere underlying the Eurasia Basin is formed at rates of less than 15 mm/y full rate. Contrary to any faster spreading ridges, the melt production and hence the composition and geophysical properties of the lithosphere vary dramatically along axis, reflected in provinces of pronounced geological differences. Magma-rich segments with a thin basaltic crust alternate with stretches exposing mantle rocks at the seafloor of a deep rift valley. Earthquakes accompany the formation of the lithosphere at mid-ocean ridges and shed light on the active spreading processes. We present here a comprehensive seismicity analysis that compares the teleseismic earthquake record of 35 years drawn from the catalogue of the International Seismological Centre with reconnaissance-style local earthquake records at five locations along the ridge that were instrumented with seismometers on drifting ice floes. The teleseismic earthquake activity varies along the ridge and reflects ultraslow spreading processes with more and larger earthquakes produced in magma-rich regions than in magma-starved areas. Large magnitude earthquakes M > 5.5 are common along this ultraslow spreading ridge especially in the presence of a basalt-containing lithosphere. A sharp contrast in size, frequency and clustering behaviour of the seismicity along with a change in magnetic anomaly pattern and geochemical signature is seen across the 3°E boundary. This boundary may have persisted for a long time and influenced the formation of the Eurasia Basin. Locally recorded earthquakes are of small magnitude (M< 2) and probably reflect the formation of the pronounced topographic relief of Gakkel Ridge. Their hypocentres extend into the upper mantle. Focal depths do not depend on spreading rate but rather reflect the thermal state of the lithosphere with very deep earthquakes indicating an exceptionally cold lithosphere. The duration and resolution of the ice floe based surveys is limited, but the first longterm ocean bottom seismometer (OBS) surveys of the geologically similar Southwest Indian Ridge indicate that seismicity can image the lithosphere in detail, constraining its mechanical thickness and temperature gradient, and giving further hints on its composition and the extent of serpentinization. To prepare for comparable studies at Gakkel Ridge, we undertook a pilot test of an OBS deployment in dense sea ice of westernmost Gakkel Ridge during Polarstern cruise PS86 in summer 2014 (Fig. 1). We successfully recovered a prototype ice-OBS that can be tracked during ascent from the seafloor. Its 10-day continuous data show an almost complete absence of microseismic noise due to the ice cover yielding ideal conditions for future passive seismic studies that employ wave trains of regional and teleseismic earthquakes to study the Arctic Basin lithosphere.

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3P Arctic, 29 Sep 2015 - 02 Oct 2015, Stavanger, Norwat.
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Schlindwein, V. and Schmid, F. (2015): Lithosphere formation at the ultraslow-spreading Gakkel Ridge: From reconnaissance seismicity studies towards passive seismology with under-ice ocean bottom seismometers , 3P Arctic, Stavanger, Norwat, 29 September 2015 - 2 October 2015 .

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