What built Shatsky Rise, a mantle plume or ridge tectonics?

Abstract

Shatsky Rise is an oceanic plateau that formed at the Pacific-Farallon-Izanagi triple junction during the Late Jurassic to Early Cretaceous. Its origin is unclear, but volcanism from a mantle plume or plume head is accepted as an explanation because many observations from the plateau are consistent with the plume head model. Initial eruptions were massive and rapid, with emplacement rates estimated at 1.2-4.6 km 3 /yr, similar to continental flood basalts. The plateau exhibits an age progression, with igneous output waning over time, possibly representing the transition from plume head to plume tail. Shallow water fossils imply that the rise top was subaerial and that thermal and dynamic uplift was significant. Furthermore, the age progression and trends of Shatsky and Hess rises are mimicked by the Mid-Pacific Mountains, to be expected if these features formed by the drift of the plate over mantle plumes. In contrast, several observations do not fit the plume head model. The initial eruption was coincident with a reorientation of the Pacific-Izanagi ridge and an 800-km jump of the triple junction, a low probability occurrence if plume heads behave independently of plate motions. Moreover, this same type of event may have occurred repeatedly, as other western Pacific plateaus occur near the trace of this triple junction (Hess Rise) and the Pacific-Farallon-Phoenix triple junction (Magellan Rise, Manihiki Plateau, MidPacific Mountains). If these other plateaus formed from plumes, either there were many plumes or the plumes defining the triple junction paths exhibited large relative motion. Moreover, rocks recovered from the main Shatsky Rise edifices have midocean ridge basalt (MORB) geochemistry and isotopic signatures, whereas most plume head models imply that lower mantle material, with a different signature, will be carried to the surface. A simpler hypothesis is that Shatsky Rise and other neartriple-junction plateaus were formed as a result of ridge tectonics. One possibility is that decompression melting occurred near the triple junctions during the Mesozoic because the northwest Pacific was located over a region of anomalous asthenosphere that was susceptible to melting given only small perturbations in lithospheric stress. A pitfall for this argument is that changes in the thin, fast-spreading Pacific lithosphere must result in massive volcanism. Although both plume and ridge tectonics hypotheses explain some observations from Shatsky Rise, uncertainties make it premature to conclude which, if either, is correct. Resolution awaits future investigations, which must reveal missing pieces to this puzzle.

IODP Expedition 324: Ocean Drilling at Shatsky Rise Gives Clues about Oceanic Plateau Formation

“…Apropos of plate-edge genesis, Shatsky Rise displays a connection with tions come from decompression melting of unusually fusible upper mantle at plate edges (Foulger, 2007). This explanation is bolstered by the observation that many plateaus formed near ocean ridges, especially at triple junctions, a situation that is unlikely if the volcanism results from deep mantle convection independent of plate tectonics (Sager, 2005).…”

“…2; Koppers et al, 2010). triple junction tectonics that is repeated elsewhere in the Pacific (Sager, 2005). Moreover, basalts from ODP Site 1213 (ODP Leg 198), on Tamu Massif (Fig.…”

“…Although some aspects of Shatsky Rise appear consistent with the plume head hypothesis, others are not. Emplacement apparently began with the construction of a huge volcanic edifice, Tamu Massif (~2.5 x 10 6 km 3 ), which possibly erupted in <1 Myr, coincident with an ~800-km jump of the triple junction Sager et al, 1999;Sager, 2005). Tamu Massif appears to be a large, central "supervolcano" with an area similar to that of Olympus Mons on Mars (Fig.…”

Formation and evolution of Shatsky Rise oceanic plateau: Insights from IODP Expedition 324 and recent geophysical cruises

“…1). Shatsky Rise is unique among large Mesozoic oceanic plateaus because geologic evidence suggests two competing hypotheses for plateau formation: the plume head and fertile mantle models (Sager, 2005) and it also contains…”

Composition and timing of carbonate vein precipitation within the igneous basement of the Early Cretaceous Shatsky Rise, NW Pacific

“…Sager, 2005). The proposed NW-directed age progression across Shatsky Rise is solely based on the interpretation of magnetic reversals since only a single radiometric age determination of basement rock from the southern flank of Tamu Massif was published so far (144.6±0.8 Ma, Mahoney et al, 2005).…”