Extrapolating Oceanic Age Distributions: Lessons from the Pacific Region

Extrapolation of the age distribution of oceanic lithosphere has played a significant role in assessments of variations in global mean spreading rate, global mean ocean basin depth, and implications for global mean sea level. Subduction has already removed 50% of oceanic lithosphere younger than 55.7Ma, making some level of extrapolation a necessary part of global plate reconstructions. An area equal in size to the Pacific Basin oceanic lithosphere must be extrapolated for ages older than 29.1Ma. Three modes of extrapolation are identified. Mode 1 extrapolation uses the preserved history as recorded on one plate to infer the history of the previously adjacent plate. This mode of extrapolation is exemplified by the inferred history of the Farallon, Vancouver, Nazca, and Cocos plates relative to the Pacific Plate, on which this record is preserved. Mode 2 involves extrapolation beyond the preserved age extent of a given ridge system. No observable data exist that directly constrain the motions beyond the youngest magnetic reversal–dated oceanic lithosphere along such a boundary. This mode has, for example, been employed to extrapolate the age distribution resulting from spreading along the Izanagi-Pacific ridge system for as much as 60 m.yr. beyond the last directly determined record preserved on the Pacific Plate. Mode 3 is extrapolation of age distributions of entirely subducted ocean basins where no information explicitly constrains the relative-motion history of such basins. The age distributions in various neo-Tethyan basins require mode 3 extrapolation. This article examines extrapolations specifically using modes 2 and 3, employing the known spreading histories of the Pacific-Farallon/Vancouver and Pacific-Phoenix plate systems and the Tasman Sea as case studies. These tests demonstrate that extrapolated distributions of ages do not match preserved ages. Important events recorded in the preserved oceanic lithosphere, including both initiation and extinction of spreading ridges, cannot be inferred from the extrapolations and yet constitute important events that control aspects of the preserved oceanic lithosphere age distribution. Hence, reconstructed age distributions that require significant mode 2 and 3 extrapolations cannot provide a rigorous basis for testing hypotheses related to global histories of ridge production, mean age, mean depth, or other potentially correlated phenomena. This may appear to be an obvious result, and hence not worth publishing, but the persistent use of extrapolated age distributions in the published literature suggests that problems with extrapolation have not been appreciated by all.