Crustal structure and seismicity of the ultraslow spreading Knipovich Ridge

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Kollofrath, J. (2011): Crustal structure and seismicity of the ultraslow spreading Knipovich Ridge , Master thesis, University Kiel.
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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.

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