Prytulak, Julie; Widdowson, Mike and Littler, Kate
Expedition 324: the shatsky rise – testing the plume head hypothesis (4th September–4th November 2009).
UK-IODP, UK newsletter number 35, pages 38-45.
Knowledge about large igneous provinces (LIPs) has played a fundamental role in shaping the prevailing view of mantle geodynamics, that of largely plate–driven flow in the upper mantle punctuated by rising, thermally-driven plumes from the lower mantle. The largest LIPs, including oceanic plateaus, reach volumes ranging from one million cubic km to greater than 10 million cubic km, and are apparently the product of relatively short-lived massive magmatic episodes that represent the largest non–ridge volcanic process on Earth (e.g., Coffin and Eldhom, 1994). In terms of magma flux, volume, and extent, such LIPs dwarf even the most prodigious present day hotspots, such as Iceland and Hawaii. Magma production rates for the largest LIPs rivalled, or even surpassed, that of the global mid–ocean ridge system for short periods of time. Moreover, because many of the largest continental LIPs formed during the Mesozoic, they may represent a mantle convection regime different from that of the ridge volcanism– dominated Cenozoic.
A widely accepted explanation for oceanic plateaus and continental flood basalts is the plume head hypothesis, which posits that large, bulbous, primarily thermal diapirs are created at depth in the mantle, and then rise toward the surface, causing cataclysmic volcanism when they impact the lithosphere. Similar to the related plume hypothesis for hotspot volcanism, the plume head hypothesis has been accepted by many scientists because it provides a simple framework tying together many observations. Moreover, the plume head phenomenon occurs naturally in many numerical and laboratory experiments. The trouble is that there is currently no unequivocal geological evidence proving the plume head mechanism has operated within the Earth. Many existing data are indirect indicators of eruption rate and magmatic volume and could be explained by alternative hypotheses.
Ongoing debate about the number, characteristics, and even existence of mantle plumes makes it desirable to consider alternative explanations for oceanic plateaus. Because these plateaus are arguably the most direct expression of mantle plume heads (in contrast to continental LIPs, whose magma must have first passed through continental lithosphere), understanding oceanic plateau formation is considered critical to understanding mantle geodynamics. In order to address the plume head versus alternative hypothesis, it is necessary to study a plateau for which the relation to contemporaneous mid–ocean ridges is known. Unfortunately, this condition is no met for plateaus formed during the Cretaceous Normal Superchon (a.k.a., the Cretaceous Quiet Period)–like Ontong Java, Manihiki, and Kerguelen plateaus–because of the lack of magnetic reversals and thus linear seafloor magnetic anomalies to mark the locations of spreading ridges.
Shatsky Rise is the only large intraoceanic plateau formed at a time of magnetic reversals. Contemporaneous magnetic lineations exist around and within the Shatsky Rise plateau, providing a framework that allows the development of a tectonic model. However, this model is currently based on geophysical inferences with little geological evidence from sampling.
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