Rifting processes at ultraslow mid-ocean ridges

Mid-ocean rift systems opening at full spreading rates < 20 mm/y differ in their structure from all faster spreading ridges. One of the key differences is that the small amount of available melt is not distributed evenly along the ridge axis. Instead, rift sections of robust magmatism form with a crustal thickness of the order of 3-4 km. At more magma-starved locations, a magmatic crust may be entirely absent apart from pronounced volcanic centres where up to 9 km of crustal thickness can be achieved. Gakkel ridge and Knipovich Ridge in the Arctic show several of these volcanic centres, each of them can be traced off-axis by a bathymetric ridge in spreading direction that indicates increased crustal production. \nWe collected and compiled for several years local, regional and global seismicity data of the ultraslow spreading ridges, the Arctic Ridge System and the Southwest Indian Ridge, to examine the structure and the still poorly known spreading processes of this class of mid-ocean ridge. \nAt Knipovich Ridge, the maximum depth of faulting of local earthquakes marks the 600°C isotherm, which rises underneath Logachev Seamount. An undulating lithosphere-asthenosphere boundary has been postulated by many authors to explain the uneven melt distribution. They assume that melts travel horizontally along this sloping boundary towards the volcanic centres. Our observation of an undulating 600°C isotherm may thus provide the first geophysical evidence for this hypothesis. \nExtraordinarily deep local earthquakes (16 km - 20 km below sea floor) at several sites of ultraslow spreading ridges indicate that the lithosphere is colder than expected by thermal models. Confusingly, we observed such deep earthquakes also at the 85°E volcanic complex at Gakkel Ridge, 6 years after a major eruption when the lithosphere should still be warm. \nMagma-starved ridge portions of ultraslow spreading ridges show only few and weak teleseismically recorded earthquakes, while the volcanic centres may emit large numbers of strong earthquakes often in swarms. This contradicts the stripe-and-gap pattern of seismicity observed for example along the entire Mid-Atlantic ridge, where volcanic ridges are connected with reduced seismicity. In order to better understand the different spreading processes in magmatic and amagmatic rift sections of ultraslow spreading ridges, we performed the first comparative local seismicity study of an ultraslow spreading ridge. From Nov 2012- Nov 2013, two similarly designed networks of ocean bottom seismometers were recording local earthquakes at two geologically contrasting sites of the Southwest Indian Ridge. We will present a short impression of the recent recovery of the instruments during Polarstern cruise ANT-XXIX/8 and Meteor Cruise M101. \n