THE MAGELLAN SEAMOUNT TRAIL - IMPLICATIONS FOR CRETACEOUS HOTSPOT VOLCANISM AND ABSOLUTE PACIFIC PLATE MOTION

The Magellan Seamount Trail (MST) delineates a northwest trending chai n of four Cretaceous guyots in the West Pacific Seamount Province (WPS P). Seamount morphology, Ar-40/Ar-39 geochronology and Sr-Nd-Pb geoche mistry of the MST provides evidence for a hotspot origin between the S amoa, Rarotonga and Society hotspots of the South Pacific Isotopic and Thermal Anomaly (SOPITA). The MST yields an excellent linear age prog ression of 47.6 +/- 1.6 mm/yr (r(2) = 1.000; MSWD = 0.23; 1 sigma SE) including Vlinder guyot (95.1 +/- 0.5 Ma, n = 5; 2 sigma SD), Pako guy ot (91.3 +/- 0.3 Ma, it = 3) and Ioah guyot (87.1 +/- 0.3 Ma, it = 2). The MST also exhibits a small range in Sr-Nd-Pb isotopic compositions indicating enriched mantle sources with an affinity of EMI. Neverthel ess, three volcanic events are found out of sequence with linear MST h otspot volcanism: (1) an independent volcanic pedestal was formed 4-7 Myr before shield-volcanism started at Vlinder guyot, (2) a post-erosi onal volcanic cone was formed at least 20-30 Myr after drowning of Vli nder guyot, and (3) Ita Mai Tai guyot (118.1 +/- 0.5 Ma, it = 3) was f ormed 34-36 Myr before the MST hotspot arrived at the predicted locati on of this guyot. By identifying and ruling out discordant volcanic ev ents, we can use the age progression in MST to test the fixity of its hotspot. When presuming the fixed hotspot hypothesis, the local age pr ogressions of the MST (47.6 +/- 1.6 mm/yr) and the copolar Musicians s eamount trail (55.8 +/- 6.4 mm/yr) are not compatible with their 100-8 0 Ma Euler pole. We investigate two options: (1) acceptance of a 'forc ed' Euler pole obeying the hotspot hypothesis by using both the age pr ogressions and the azimuths of the studied seamount trails, or (2) acc eptance of a 'best-fit' Euler pole by using the azimuths of the studie d seamount trail exclusively. In the first option, the angular speed o f the Pacific plate during the 100-80 Ma stage pole is calculated at 0 .502 +/- 0.017 degrees/Myr. In the second option, the 'best-fit' Euler pole is found approximately 35 degrees different from the 'forced' Eu ler pole. We argue that the observed age progressions can only be reco nciled with the 'best-fit' pole when allowing for the relative movemen t of the MST and Musicians mantle plumes with respect to one another. The calculated maximum velocity component parallel to the line of age progression could then be as much as 23 mm/yr for the mantle plumes - when assuming one tired hotspot in this alternate model. (C) 1998 Else vier Science B.V. All rights reserved.