Crustal manifestations of a hot transient pulse at 60°N beneath the Mid-Atlantic Ridge

Abstract

Since its inception at 62Ma, mantle convective upwelling beneath Iceland has had a significant influence on Cenozoic vertical motions, magmatism and paleoceanography in the North Atlantic Ocean. Crucially, intersection of the Reykjanes Ridge with the Icelandic Plume provides us with a useful window into the transient activity of this plume. Here, the spreading ridge acts as a linear sampler of plume activity, which is recorded as a series of time-transgressive V-shaped ridges and troughs. We present the results of a detailed study of the spreading ridge close to 60°N, where the youngest V-shaped ridge of thickened oceanic crust is forming today. A combination of multibeam bathymetry and seismic reflection profiles, acquired along and across the ridge axis, is used to map the detailed pattern of volcanism and normal faulting. Along the ridge axis, the density of volcanic seamounts varies markedly, increasing by a factor of two between 59°N and 62°N. Within this zone, seismic imaging shows that there is enhanced acoustic scattering at the seabed. These observations are accompanied by a decrease in mean fault length from ∼12km to ∼6km. A 1960–2009 catalog of relocated teleseismic earthquake hypocenters indicates that there is a pronounced gap in seismicity between 59°N and 62°N where the cumulative moment release is two orders of magnitude smaller than that along adjacent ridge segments. A steady-state thermal model is used to show that a combination of increased melt generation and decreased hydrothermal circulation accounts for this suite of observations. The predicted decrease in the thickness of the brittle seismogenic layer is consistent with geochemical modeling of dredged basaltic samples, which require hotter asthenospheric material beneath the spreading axis. Thus, along-axis variation in melt supply caused by passage of a pulse of hot material modulates crustal accretion processes and rheological properties.