Hawaiian volcanic propagation and Hawaiian swell asymmetry: evidence of northwestward flow of the deep upper mantle

Abstract

Bathymetry and the geoid anomaly of the northern flank of the Hawaiian swell is broader and higher than the southern flank, and it is characterized by higher heat flow than the axis or southern flank. It is here proposed that the northern flank of the Hawaiian swell has been augmented by heat conducted from the hotspot conduit into the upper mantle then transported northward of the volcanic axis by flow in the upper mantle (∼325°) that is more northerly than Pacific plate motion (292°). By assuming that the deep upper mantle is decoupled from the Pacific plate and is flowing at 325° to the northwest, changes in direction and rate of volcanic propagation and in geochemistry along individual volcanic segments of the Hawaiian volcanic chain can be interpreted in terms of tank experiment results showing that a volcanic hotspot conduit breaks into diapirs when tilted by mantle flow. Hawaiian volcanoes are aligned in en-echelon segments, and the Hawaiian Islands are the two most recent segments. For an individual segment, older northwestern volcanoes are aligned nearly parallel to the 292° plate motion direction, and they propagated to the southeast at approximately the same rate as the 92 km/m.y. speed of northwestward plate motion. In contrast, the alignment of the younger southeastern volcanoes is close to 325°, and they show a conspicuous acceleration in propagation of volcanism marked by out-of-sequence eruptions. Within the model proposed here, diapirs rise from instability nodes that develop along the tilted conduit of a mantle hotspot plume as it is sheared in the direction of deep upper-mantle flow and each diapir gives rise to a single volcanic center. As tilting progresses, diapirs form at lower levels along the conduit in more upstream positions of the mantle flow zone, rise sequentially into the decoupled lithosphere, erupt sequentially, and are translated in the direction of plate motion (older, northwestern Hawaiian Islands). Eventually, flow in the highly tilted conduit is impeded to the degree that the remaining upstream conduit breaks into a number of diapirs that rise together into the lithosphere. These late diapirs, translated as a group aligned in the direction of horizontal mantle flow, erupt over a relatively short time span and show out-of-sequence volcanism (younger, southeastern Hawaiian Islands). At this stage, a new cycle of rising and tilting will initiate the next en-echelon segment.