『Abstract
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 55.7 Ma, 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 29.1 Ma. 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
ages, 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 o 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.』
Why is extrapolation important?
Three modes of extrapolation
Unconstrained extrapolation: Two case studies from the Pacific
Case 1: Pacific-Farallon extrapolated age distribution
Case 2: Pacific-Phoenix extrapolated age distribution
Case 3: Tasman Sea extrapolated age distribution
Discussion
Conclusions
Acknowledgments
References cited