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We report a reanalysis of active‐source wide‐angle seismic data collected along the Greenland‐Iceland Ridge during the 1996 Seismic Investigation of the Greenland Margin (SIGMA) experiment. Interpreting the crustal structure of volcanic rifted margins has suffered from nonuniqueness because thick crust at the continent‐ocean transition may not be totally of igneous origin. In this regard, the Greenland‐Iceland Ridge presents a unique opportunity because, together with the Faroe‐Iceland Ridge, it is generally considered to constitute the Iceland hotspot track, and as such, the bulk of its crust may be considered to be of igneous origin. From 15 ocean‐bottom instruments deployed along a 290‐km‐long refraction transect, we collect 5,383 Pg and 1,118 PmP travel times, and we use joint refraction and reflection seismic tomography with adaptive importance sampling to invert them to construct a two‐dimensional compressional wave speed (Vp) model across the Greenland‐Iceland Ridge. Based on the covariation of crustal thickness and Vp, the western part of the transect may be almost entirely continental. Even the eastern part could include a significant fraction of preexisting continental crust. When considered together with the seismic structure of the Icelandic crust and its geochemical characteristics, our results suggest that the putative Iceland mantle plume could be considerably weaker than commonly assumed. Key Points: A compressional‐wave speed model was built for the Greenland‐Iceland Ridge crustThe ridge may contain a substantial amount of preexisting continental crustThe putative Iceland mantle plume can be considerably weaker than commonly thought [ABSTRACT FROM AUTHOR]

Thin oceanic crust is formed by decompression melting of the upper mantle at mid-ocean ridges, but the origin of the thick and buoyant continental crust is enigmatic. Juvenile continental crust may form from magmas erupted above intra-oceanic subduction zones, where oceanic lithosphere subducts beneath other oceanic lithosphere. However, it is unclear why the subduction of dominantly basaltic oceanic crust would result in the formation of andesitic continental crust at the surface. Here we use geochemical and geophysical data to reconstruct the evolution of the Central American land bridge, which formed above an intra-oceanic subduction system over the past 70 Myr. We find that the geochemical signature of erupted lavas evolved from basaltic to andesitic about 10 Myr ago-coincident with the onset of subduction of more oceanic crust that originally formed above the Galápagos mantle plume. We also find that seismic P-waves travel through the crust at velocities intermediate between those typically observed for oceanic and continental crust. We develop a continentality index to quantitatively correlate geochemical composition with the average P-wave velocity of arc crust globally. We conclude that although the formation and evolution of continents may involve many processes, melting enriched oceanic crust within a subduction zone-a process probably more common in the Archaean-can produce juvenile continental crust. [ABSTRACT FROM AUTHOR]

Deposits within the floor of the Norwegian Basin were sampled to characterize the deposition from the Storegga Slide, the largest known Holocene-aged continental margin slope failure complex. A 29 to 67 cm thick veneer of variable-coloured, finely layered Holocene sediment caps a homogeneous, extremely well-sorted, poorly consolidated, very fine-grained, grey-coloured sediment section that is >20 m thick on the basin floor. This homogeneous unit is interpreted to represent the uppermost deposits generated by a gravity flow associated with the last major Storegga Slide event. Sediments analogous to the inferred source material of the slide deposits were collected from upslope on the Norwegian Margin. Sediments sampled within the basin are distinguishable from the purported source sediments, suggesting that size sorting has significantly altered this material along its flow path. Moreover, the very fine grain size (3·1 ± 0·3 μm) suggests that the >20 m thick homogeneous unit which was sampled settled from suspension after the turbulent flow was over. Although the turbulent phase of the gravity flow that moved material out into the basin may have been brief (days), significantly more time (years) is required for turbid sediments to settle and dewater and for the new sea floor to be colonized with a normal benthonic fauna. Pore water sulphate concentrations within the uppermost 20 m of the event deposit are higher than those normally found in sea water. Apparently the impact of microbial sulphate reduction over the last ca 8·1 cal kabp since the re-deposition of these sediments has not been adequate to regenerate a typical sulphate gradient of decreasing concentration with sub-bottom depth. This observation suggests low rates of microbial reactions, which may be attributed to the refractory carbon composition in these re-deposited sediments. [ABSTRACT FROM AUTHOR]