ORIGINAL RESEARCH PAPER Seismological constraints on the thermal structure along the Lucky Strike segment (Mid-Atlantic Ridge) and interaction of tectonic and magmatic processes around the magma chamber D. Dusunur Æ J. Escartı ´n Æ V. Combier Æ T. Seher Æ W. Crawford Æ M. Cannat Æ S. C. Singh Æ L. M. Matias Æ J. M. Miranda Received: 28 February 2009 / Accepted: 4 August 2009 / Published online: 25 August 2009 Ó Springer Science+Business Media B.V. 2009 Abstract The crust at mid-ocean ridges is formed through a combination of magmatic and tectonic processes. Along slow-spreading ridges, magmatism is inferred to be discontinuous and episodic, and lithospheric faulting may strongly interact with the melt supply system. These interactions can be studied for the first time at the Lucky Strike segment along the Mid-Atlantic Ridge (MAR), where a 3.4 km deep magma chamber (AMC) extending *6 km along-axis is found at its centre (Singh et al. in Earth Planet Sci Lett 246:353–366, 2006). With an array of ocean bottom seismometers we have detected along this ridge segment approximately 400 microseismic events during a total of 6 days, and located 71 of them, whose local magnitudes ranged from 0.2 to 1.8. While most of the events were concentrated at non-transform offset and inside corners, three events with well-constrained locations were detected beneath the central volcano and at the edges of the AMC. Two of the microearthquakes, which occur in a brittle lithosphere and therefore at temperatures lower than 750°C, are deeper than the AMC and therefore very steep thermal gradients both along- and across-axis. Regionally seismicity deepens from *6 km at the segment center to [ 10 km towards the ends. Keywords Axial-magma chamber Á Microearthquakes Á Lucky Strike segment Á Mid-Atlantic Ridge Introduction The SISMOMAR cruise (Crawford et al. 2005), which took place between 1st June and 3rd of July 2005 onboard N/O l’Atalante, was primarily designed for active seismic refraction and reflection studies (Singh et al. 2006). How- ever, microseismicity was recorded over a total of 6 days distributed in 4 time windows, during which no active seismic source was used. Despite the limited length of the study (6 days), we are able to carry out a preliminary study of the levels of microseismicity, its distribution throughout the segment, and of the magnitude of events. Seismic reflection data revealed the presence of an AMC (Fig. 1) at a depth of about 3.4 km below the sea floor, and a system of faults penetrating the crust above and next to the AMC (Singh et al. 2006; Combier et al. 2007; Combier 2007). This system is underlain by a seismic low velocity zone (LVZ) extending to the Moho (Seher et al. 2007; Seher 2008). This AMC is the likely heat source for the Lucky Strike hydrothermal system (Fig. 1) found at the summit of the volcano at the segment center (Langmuir et al. 1997). This central volcano was emplaced within the well-developed rift valley, bounded by faults with scarps more than 500 m high. Smaller fault scarps, with vertical throws ranging from 10 to 100 m, are present throughout the rift valley floor, dissecting the volcanic edifice. Here we report the distribution of microseismic events along the Lucky Strike segment (Fig. 2), which result from D. Dusunur (&) Á J. Escartı ´n Á V. Combier Á T. Seher Á W. Crawford Á M. Cannat Á S. C. Singh CNRS/Institut de Physique du Globe de Paris, Geosciences Marines, Case 89 IPGP, 4 P. Jussieu, 75252 Paris, Cedex 05, France e-mail: dogadusunur@yahoo.com Present Address: D. Dusunur Istanbul Technical University, Istanbul, Turkey L. M. Matias Á J. M. Miranda Centro de Geofı ´sica da Universidade de Lisboa/IDL, Lisbon, Portugal 123 Mar Geophys Res (2009) 30:105–120 DOI 10.1007/s11001-009-9071-3