ALBERT

Description: New marine geophysical data acquired across the partly ice‐covered northern East Greenland continental margin highlight a complex interaction between tectonic and magmatic events. Breakup‐related lava flows are imaged in reflection seismic data as seaward dipping reflectors (SDRs), which are found to decrease in size both northwards and southwards from a central point at 75° N. We provide evidence that the magnetic anomaly pattern in the shelf area is related to volcanic phases and not to the presence of oceanic crust. The remnant magnetization of the individual lava flows is used to deduce a relative timing of the emplacement of the volcanic wedges. We find that the SDRs have been emplaced over a period of 2‐4 Ma progressively from north to south and from landward to seaward. The new data indicate a major post‐middle Eocene magmatic phase around the landward termination of the West Jan Mayen Fracture Zone. This post‐40 Ma volcanism likely was associated with the progressive separation of the Jan Mayen microcontinent from East Greenland. The break‐up of the Greenland Sea started at several isolated seafloor spreading cells whose location was controlled by rift structures and led to the present‐day segmentation of the margin. The original rift basins were subsequently connected by steady‐state seafloor spreading that propagated southwards, from the Greenland Fracture Zone to the Jan Mayen Fracture Zone. Key Points Polyphase Cenozoic volcanic rifting and consecutive emplacement of breakup‐related lava flows units along the northern East Greenland margin Breakup along restricted margin segments is followed by north to south directed progressive opening of the Greenland Sea Widespread post‐middle Eocene (〈 40 Ma) offshore magmatism, associated with the breakup of the Jan Mayen microcontinent from East Greenland

Description: Key points:  First insights into the crustal structure of the northeastern Lau Basin, along a 290 km transect at 17°20’S.  Crust in southern Fonualei Rift and Spreading Center was created by extension of arc crust and variable amount of magmatism.  Magmatic underplating is present in some parts of the southern Niuafo’ou Microplate The northeastern Lau Basin is one of the fastest opening and magmatically most active back‐arc regions on Earth. Although the current pattern of plate boundaries and motions in this complex mosaic of microplates is reasonably understood, the internal structure and evolution of the back‐arc crust are not. We present new geophysical data from a 290 km long east‐west oriented transect crossing the Niuafo’ou Microplate (back‐arc), the Fonualei Rift and Spreading Centre (FRSC) and the Tofua Volcanic Arc at 17°20’S. Our P‐wave tomography model and density modelling suggests that past crustal accretion inside the southern FRSC was accommodated by a combination of arc crustal extension and magmatic activity. The absence of magnetic reversals inside the FRSC supports this and suggests that focused seafloor spreading has until now not contributed to crustal accretion. The back‐arc crust constituting the southern Niuafo’ou Microplate reveals a heterogeneous structure comprising several crustal blocks. Some regions of the back‐arc show a crustal structure similar to typical oceanic crust, suggesting they originate from seafloor spreading. Other crustal blocks resemble a structure that is similar to volcanic arc crust or a ‘hydrous’ type of oceanic crust that has been created at a spreading center influenced by slab‐derived water at distances 〈 50 km to the arc. Throughout the back‐arc region we observe a high‐velocity (Vp 7.2‐7.5 km s‐1) lower crust, which is an indication for magmatic underplating, which is likely sustained by elevated upper mantle temperatures in this region.