Most models of melt generation beneath mid-ocean ridges predict significant reduction of melt production at ultraslow spreading rates (full spreading rates &<20 mm x yr(-1)) and consequently they predict thinned oceanic crust. The 1,800-km-long Arctic Gakkel mid-ocean ridge is an ideal location to test such models, as it is by far the slowest portion of the global mid-ocean-ridge spreading system, with a full spreading rate ranging from 6 to 13 mm x yr(-1) (refs 4, 5). Furthermore, in contrast to some other ridge systems, the spreading direction on the Gakkel ridge is not oblique and the rift valley is not offset by major transform faults. Here we present seismic evidence for the presence of exceptionally thin crust along the Gakkel ridge rift valley with crustal thicknesses varying between 1.9 and 3.3 km (compared to the more usual value of 7 km found on medium- to fast-spreading mid-ocean ridges). Almost 8,300 km of closely spaced aeromagnetic profiles across the rift valley show the presence of discrete volcanic centres along the ridge, which we interpret as evidence for strongly focused, three-dimensional magma supply. The traces of these eruptive centres can be followed to crustal ages of approximately 25 Myr off-axis, implying that these magma production and transport systems have been stable over this timescale.

S U M M A R YSeismic velocities and the associated thicknesses of rifted and igneous crust provide key constraints on the rifting history, the differentiation between non-volcanic and volcanic rifted margins, the driving force of magmatism at volcanic margins, that is, active or passive upwelling and the temperature anomaly in the lithosphere. This paper presents two new wideangle seismic transects of the East Greenland margin and combines the velocity models with a compilation of 30-wide-angle seismic velocity models from several publications along the entire East Greenland margin. Compiled maps show the depth to basement, depth to Moho, crustal thickness and thickness of high velocity lower crust (HVLC; with velocities above 7.0 km s −1 ). First, we present two new wide-angle seismic transects, which contribute to the compilation at the northeast Greenland margin and over the oceanic crust between Shannon Island and the Greenland Fracture Zone. Velocity models, produced by ray tracing result in total traveltime rms-misfits of 100-120 milliseconds and χ 2 values of 3.7 and 2.3 for the northern and southern profiles with respect to the data quality and structural complexity. 2-D gravity modelling is used to verify the structural and lithologic constraints. The northernmost profile, AWI-20030200, reveals a magma starved break-up and a rapidly thinning oceanic crust until magnetic anomaly C21 (47.1 Ma). The southern seismic transect, AWI-20030300, exhibits a positive velocity anomaly associated with the Shannon High, and a basin of up to 15 km depth beneath flood basalts between Shannon Island and the continent-ocean boundary. Breakup is associated with minor crustal thickening and a rapidly decreasing thickness of oceanic crust out to anomaly C21. The continental region is proposed to be only sparsely penetrated by volcanism and not underplated by magmatic material at all compared to the vast amount of magmatism further south. Break-up is proposed to have occurred at the seaward boundaries of the continent-ocean transition zones at between ∼50 and ∼54 Ma, propagating from north to south based on a joint analysis incorporating transects from the Kejser Franz Joseph Fjord and Godthåb Gulf. Secondly, the variation of the HVLC along the East Greenland margin from 60 • to 77 • N and from transects of its conjugate margin shows inverted emplacement of prominent landward and seaward HVLC thickness portions from north to south in a distribution chart. The differences in the HVLC distribution are attributed to one or more of the following three models. In the first model it is inferred that a transfer zone/detachment acts as a barrier to northward magma flow. In the second model, underplating results in thicker and highly intruded lower crust with several small-scale feeder dykes that locally increase the lower crustal velocities. In the third model, a second magmatic event associated with the separation of the Jan Mayen microcontinent is considered. Lithosphericscale inhomogeneities might be responsible for the heterogene...

S U M M A R YThe northernmost spreading centre of the world, the Gakkel Ridge, is also an end-member in terms of global spreading velocities. Models show that full spreading rates vary between 1.3 and 0.63 mm yr −1 along the almost 1800 km long ridge system in the Central Arctic Ocean. The western part of the ridge was investigated in great detail by a two-ship expedition in summer 2001. The complete data sets and the modelling of the seismic refraction and aeromagnetic experiments gathered during this expedition are shown in this study. The magnetic signals along the dense (2 km spacing) aeromagnetic flight lines acquired at the same time show a good correlation between high amplitudes and a shallowing of the rift valley and the presence of large volcanic constructions at the rift shoulders. The magnetic anomalies rapidly fade out east and west of these centres of focused magmatism. This might indicate that the basaltic layer producing the magnetic anomaly thins away from the volcanic centres. A continuous magnetic anomaly is observed along the rift valley west of 3 • 30 E, consistent with increasing and more robust magmatism.The crustal thickness along the Gakkel Ridge varies greatly. Beneath some of the centres of focused magmatism, the oceanic crust thickens up to 3.5 km. In the amagmatic segments in between the crust thins to 1.4-2.9 km. This observation is also valid for the Western Volcanic Zone west of 3 • 30 E, where despite the stronger magnetic anomaly the crust does not significantly thicken. The strength of the magnetic anomaly along the rift valley is thus not a reliable indicator of crustal thickness beneath the Gakkel Ridge. The data show that the crustal thickness does not change dramatically across 3 • 30 E. Only the occurrence of a large elongate volcanic ridge significantly influences this parameter. More frequent volcanic eruptions along such ridges are most likely responsible for the basalts found in the westernmost part of the Gakkel Ridge. In the non-transform segments some seismic stations indicate that mantle rocks are exposed at the seafloor, with no indication of the presence of a basaltic cover or normal oceanic crust. Both the seismic and magnetic data support models in which the uppermost basaltic cover is responsible for the magnetic anomaly in the rift valley.

Abstract. RHUM-RUM is a German-French seismological experiment based on the sea floor surrounding the island of La Réunion, western Indian Ocean (Barruol and Sigloch, 2013). Its primary objective is to clarify the presence or absence of a mantle plume beneath the Reunion volcanic hotspot. RHUM-RUM's central component is a 13-month deployment (October 2012 to November 2013) of 57 broadband ocean bottom seismometers (OBS) and hydrophones over an area of 2000 × 2000 km 2 surrounding the hotspot. The array contained 48 wideband OBS from the German DE-PAS pool and 9 broadband OBS from the French INSU pool. It is the largest deployment of DEPAS and INSU OBS so far, and the first joint experiment.This article reviews network performance and data quality: of the 57 stations, 46 and 53 yielded good seismometer and hydrophone recordings, respectively. The 19 751 total deployment days yielded 18 735 days of hydrophone recordings and 15 941 days of seismometer recordings, which are 94 and 80 % of the theoretically possible yields.The INSU seismic sensors stand away from their OBS frames, whereas the DEPAS sensors are integrated into their frames. At long periods (> 10 s), the DEPAS seismometers are affected by significantly stronger noise than the INSU seismometers. On the horizontal components, this can be explained by tilting of the frame and buoy assemblage, e.g. through the action of ocean-bottom currents, but in addition the DEPAS intruments are affected by significant selfnoise at long periods, including on the vertical channels. By comparison, the INSU instruments are much quieter at periods > 30 s and hence better suited for long-period signals studies.The trade-off of the instrument design is that the integrated DEPAS setup is easier to deploy and recover, especially when large numbers of stations are involved. Additionally, the wideband sensor has only half the power consumption of the broadband INSU seismometers.

S U M M A R YWe present a regional crustal model of the East Greenland Fjord Region between 69 • N and 74 • N which spans the Caledonian fold belt and the adjoining Devonian and Mesozoic basins. The model is a compilation of existing seismic models that were partly reinterpreted and newly derived results from different modelling approaches. Remodelling of 33 stations on three deep seismic lines in the southern area yielded consistent P-wave velocity models for the entire Fjord Region. Seismic velocities, between 5.5 km s −1 near the surface and 6.9 km s −1 in the lower crust are typical for regions of Palaeozoic age. Moho depths up to 48 km in the seismic models suggest the existence of a crustal root beneath the Caledonian orogen. Shear wave modelling of 51 stations on six refraction seismic profiles resulted in S-wave velocities between 3.2 km s −1 in the upper crust, 4.0 km s −1 at the crust-mantle boundary and 4.2 km s −1 in the partial magmatic underplating of the lower crust in the northern area. Calculation of Poisson's ratio portrays a fairly homogeneous crust with only slight variations in Poisson's ratio of 0.26-0.30 for the uppermost crystalline crust and 0.22-0.24 for the middle crust. These values cannot be linked to lithological variations because they are either small-scale or span several tectonic provinces. Finite-difference modelling and amplitude analysis confirm models without magmatic underplating in the lower crust of the southern Hall Bredning, Scoresby Sund area.