Seismic Imaging of Thickened Lithosphere Caused by Plume Pulsing beneath Iceland

Abstract:

Mid-ocean ridges and hotspots are ideal environments to constrain the definition of the lithosphere-asthenosphere boundary (LAB). LAB depth is expected to be significantly different in these regions depending on the definition of the plate. A purely thermal definition should deepen with age and distance from the ridge and shallow in the presence of a thermal plume anomaly. Whereas, compositional boundaries could exist at constant depth, possibly deepened by a thermal plume anomaly. We use S-to-P receiver functions to image upper mantle discontinuities beneath Iceland. We image a strong velocity decrease with depth at 45 – 80 km depth that is likely associated with the LAB. The phase undulates in depth, shallowest beneath the ridge, deeper in adjacent regions near ~9 My old seafloor, and shallow again at great distances from the ridge. The depth variations do not show simple thickening with seafloor age, even when effects of thermal plume anomalies are taken into account. The phase is also deeper and its magnitude greater than predictions for a thermal plate based on experimental constraints. The observation is better explained by a compositionally defined lithosphere with thickness dictated by temporal variation in thermal plume anomaly magnitude. Indeed, periodic plume pulsing has been hypothesized to explain variations in crustal thickness and gravity at the nearby Reykjanes Ridge. Our model suggests that thicker lithosphere is created at the ridge during times of plume pulsing, e.g., ~9 My. The thickened plate is subsequently rifted apart and thinner lithosphere is created until another pulse occurs. Comparison to experimental predictions for half-space cooling adjusted for regional crustal thickness suggests melt is likely required beneath the discontinuity to explain large velocity drops (~10 – 15%). Indeed, a velocity increase with depth is imaged at 160 km depth just east of Iceland that is likely the base of the melt rich layer. This suggests the plume approaches the island from the east, consistent recent eastward ridge jumps. Comparisons with other hotspot environments including Galapagos, Hawaii, and Afar suggest compositional components to the definition of the lithosphere, plume ascents that are non-vertical, and melt residence in the mantle at time and length scales that are seismically imageable.