Sp. Huestis et Gd. Acton, ON THE CONSTRUCTION OF GEOMAGNETIC TIMESCALES FROM NON-PREJUDICIAL TREATMENT OF MAGNETIC ANOMALY DATA FROM MULTIPLE RIDGES, Geophysical journal international, 129(1), 1997, pp. 176-182
When marine magnetic-anomaly data are used to construct geomagnetic po
larity timescales, the usual assumption of a smooth spreading-rate fun
ction at one seafloor spreading ridge forces much more erratic rate fu
nctions at other ridges. To eliminate this problem, we propose a forma
lism for the timescale problem that penalizes non-smooth spreading beh
aviour equally for all ridges. Specifically, we establish a non-linear
Lagrange multiplier optimization problem for finding the timescale th
at (1) agrees with known chron ages and with anomaly-interval distance
data from multiple ridges and (2) allows the rate functions for each
ridge to be as nearly constant as possible, according to a cumulative
penalty function. The method is applied to a synthetic data set recons
tructed from the timescale and rate functions for seven ridges, derive
d by Cande & Kent (1992) under the assumption of smooth spreading in t
he South Atlantic. We find that only modest changes in the timescale (
less than 5 per cent for each reversal) are needed if no one ridge is
singled out for the preferential assumption of smoothness. Future impl
ementation of this non-prejudicial treatment of spreading-rate data fr
om multiple ridges to large anomaly-distance data sets should lead to
the next incremental improvement to the pre-quaternary geomagnetic pol
arity timescale, as well as allow a more accurate assessment of global
and local changes in seafloor spreading rates over time.