ON THE CONSTRUCTION OF GEOMAGNETIC TIMESCALES FROM NON-PREJUDICIAL TREATMENT OF MAGNETIC ANOMALY DATA FROM MULTIPLE RIDGES

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.