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

Bathymetry and the geoid anomaly of the northern flank of the Hawaiian swel l 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 conduc ted from the hotspot conduit into the upper mantle then transported northwa rd of the volcanic axis by flow in the upper mantle (similar to 325 degrees ) that is more northerly than Pacific plate motion (292 degrees). By assumi ng that the deep upper mantle is decoupled from the Pacific plate and is fl owing at 325 degrees to the northwest, changes in direction and rate of vol canic 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 t ilted by mantle flow. Hawaiian volcanoes are aligned in en-echelon segments , and the Hawaiian Islands are the two most recent segments. For an individ ual segment, older northwestern volcanoes are aligned nearly parallel to th e 292 degrees plate motion direction, and they propagated to the southeast at approximately the same rate as the 92 km/m.y. speed of northwestward pla te motion. In contrast, the alignment of the younger southeastern volcanoes is close to 325 degrees, and they show a conspicuous acceleration in propa gation of volcanism marked by out-of-sequence eruptions. Within the model p roposed here, diapirs rise from instability nodes that develop along the ti lted 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 cen ter. 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 di rection of plate motion (older, northwestern Hawaiian Islands). Eventually, flow in the highly tilted conduit is impeded to the degree that the remain ing upstream conduit breaks into a number of diapirs that rise together int o the lithosphere. These late diapirs, translated as a group aligned in the direction of horizontal mantle flow, erupt over a relatively short time sp an and show out-of-sequence volcanism (younger, southeastern Hawaiian Islan ds). At this stage, a new cycle of rising and tilting will initiate the nex t en-echelon segment. (C) 1999 Elsevier Science B.V. All rights reserved.