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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.

The Knipovich Ridge situated in the North Atlantic Ocean represents an ultraslowspreading member of the mid-ocean ridge system. Its spreading direction is highly oblique (35° - 50° between the spreading axis and the normal to the ridge trend) with a spreading rate of ~ 15 mm/yr. During the AWI expedition ARK-XXIV-3 a 125 km long N-S orientated seismic refraction profile between 75° 30’ N and 77° 15’ N was shot along the Knipovich Ridge valley. The obtained velocity distribution was used to locate local earthquakes which were acquired using a local OBS array. The crustal thickness along the Knipovich Ridge varies from ~ 5.5 km below strong magmatic centres and ~ 4 km between these centres in the amagmatic part. Just before the southern strong magmatic centre the crust thins temporarily to 3.4 km. Seismic velocities of the crust do mostly not exceed 6.6 km/s . The velocity distribution of the crust suggests the absence of an oceanic Layer 3. The obtained upper mantle velocity is 7.6 km/s . During two days of processed local earthquake data 85 earthquakes were located with a maximum magnitude of Ml= 2.7. The detection threshold of the local OBS array was Ml= 1.3. The earthquake distribution is scattered along the ridge valley with slightly clustering. Two third of the earthquakes were located within the upper mantle which indicates that the upper mantle is not strongly serpentinised.