HR: 09:40h
AN: V21B-07    [PDF]
TI: Lithospheric and Melt Anomaly Control of Foundation Chain Volcanism
AU: * O'Connor, J M
EM: joconnor@gpi.uni-kiel.de
AF: Department of Isotope Geochemistry, Vrije Universiteit, Amsterdam, 1081 HV Netherlands
AU: * O'Connor, J M
EM: joconnor@gpi.uni-kiel.de
AF: Institute for Geosciences, Christian-Albrechts Universitaet, Kiel, D-24118 Germany
AU: Stoffers, P
EM: pst@gpi.uni-kiel.de
AF: Institute for Geosciences, Christian-Albrechts Universitaet, Kiel, D-24118 Germany
AU: Wijbrans, J R
EM: Jan.Wijbrans@falw.vu.nl
AF: Department of Isotope Geochemistry, Vrije Universiteit, Amsterdam, 1081 HV Netherlands
AB: The Foundation Chain is a small chain of seamounts and volcanic ridges extending northwestward from the Pacific-Antarctic spreading ridge. $^{40}$Ar/$^{39}$Ar age data show linear migration of volcanism along-chain at a rate of 91$\pm2$mm/yr for the past 22 Myr (O'Connor et al., 1998). The case history of the Foundation Chain is notable because it is a rare example of a hotspot melting anomaly that has been traversed by a fossil microplate and is now being encroached by the active Pacific-Antarctic spreading ridge. Prior to the Selkirk Microplate encountering the melt anomaly the Foundation Chain formed as broad elongate zones of scattered, synchronous volcanism cross-cutting the overall NW-SE trend of the chain (O'Connor et al., 2002). But once the significantly older microplate began capping the melt anomaly about 14 Myr ago, the chain narrowed abruptly into a single line of discrete seamounts, only broadening again about 5 Myr ago when sufficiently young lithosphere again started drifting over the melting anomaly. Measured ages show a dominant trend of coeval, yet structurally disconnected, segments of Foundation Chain VERs developing in a series of en echelon, elongate 'zones' of coeval volcanism cross-cutting the overall NW-SE seamount trend (O'Connor et al., 2001). These elongate zones developed at intervals of approximately 1 Myr while maintaining a basically steady-state orientation and size as the Pacific-Antarctic spreading ridge migrated closer to the melt anomaly. Although VER development was controlled in part by local factors (e.g. location of nearest spreading ridge segment, lithospheric thickness and stress), long-lived attributes of the Foundation melt anomaly (e.g. size, orientation, periodicity) must have played a pivotal role. Foundation volcanism can be suppressed across elongate melt 'zones' if the capping tectonic plate is too thick for melts to penetrate to the surface (O'Connor et al., 2001, 2002). The lack of a seamount chain connecting the Foundation and the Austral volcanoes can be similarly explained, thus extending the age of the Foundation melting anomaly back to at least 34 Myr ago (McNutt et al., 1997). While lithospheric architecture controls if and where Foundation volcanism occurs (e.g., chain broadening and narrowing), it cannot explain the origin of the underlying long-lived melting anomaly. The timing and distribution of Foundation Chain volcanism requires a long-lived process that creates broad melting anomalies of fundamentally constant size and orientation under a moving Pacific lithosphere with an apparent periodicity of about once per Myr (O'Connor et al., 2001, 2002). Thus, the Foundation Chain is a product of lithospheric architecture and a first-order mantle process controlling the existence and behavior of an underlying long-lived melt anomaly.
DE: 1035 Geochronology
DE: 3035 Midocean ridge processes
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 5480 Volcanism (8450)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting