Deep Seismic and geochemical constraints on the nature of rift-induced magmatism during breakup of the North Atlantic

Abstract The deep crustal structure of the Norwegian and East Greenland conjugate passive margins has been investigated with two-ship multichannel reflection and refraction techniques in the region from the Iceland-Faeroe Ridge to the Greenland Fracture Zone. The data demonstrate the presence of thick (15–20 km) igneous crust that was emplaced along the conjugate margins during the initiation of seafloor spreading. Seaward dipping units comprise the uppermost 3–6 km of the thick crust. The thickness of igneous crust diminishes seaward, with oceanic layer 2 approaching normal thickness nearer to the margin than does oceanic layer 3. Wide-angle reflection techniques, used in conjunction with expanded spread profiling, reveal deep Moho reflections beneath 20 km thick oceanic crust that continue seaward into the ocean basin. Margins that have experienced unusually prolific magmatic activity during breakup have been termed “ volcanic” to distinguish them from “ non-volcanic” margins that have evolved into accreting plate boundaries without excessive volcanism. The origin of volcanic margins cannot be determined from Seismically derived crustal thicknesses alone. However, thermal and chemical properties of the mantle source may be revealed in the compositions of magmatic products emplaced within volcanic passive margins. A combined investigation of crustal structure and associated basalt composition could better resolve processes that result in volcanic passive margins. On the Voring Plateau of the Norwegian margin, Seismic lines coincide with DSDP Leg 38 drill sites. Whole rock major element abundances in the freshest basalts are compatible with moderate extents of partial melting of mantle much like that believed to underlie modern mid-ocean ridge systems and would be expected to result in the emplacement of oceanic crust about 9 (± 2) km thick. This is about the thickness of oceanic crust located seaward of the dipping basaltic units off the Hatton, More and Voring Plateau margins and 10 km south of Iceland on the Reykjanes Ridge. These results, together with regional heat flow, depth, gravity and geoid anomalies, suggest that the mantle beneath this region has experienced a moderate, long-lived temperature increase compared to mantle beneath normal ocean basins. The much greater thicknesses of igneous crust adjacent to the margins do not appear to reflect an unusually hot flux associated with their construction, since this would lead to much greater extents of partial melting than are inferred from the basalt geochemistry. Evidently, the difference between volcanic and non-volcanic margins is not simply proximity to a hot spot. Instead, we suggest that the magmatism is closely related to the continental rifting, phase. Tectonism resulting in an abrupt transition from continental to oceanic lithosphere may induce small-scale convection that results in the circulation of a greater amount of mantle material into the pressure-temperature field where partial melting begins than would occur through passive upwelling alone. Initial crustal thicknesses are enhanced as a result of the convective partial melting. Crustal thickness is expected to diminish as this secondary convection slows, but the degree of partial melting of the source should not change dramatically.