The asymmetry file contains three columns, beginning of period (Myr), end of period (Myr) and asymmetry (%). To use the asymmetry file filename when running Magellan the -a option can be used followed by the asymmetry file (mageallan -a filename). Equivalently the asymmetry file can be specified in the configuration file, with the key asymmetry=filename. A negative asymmetry represents faster spreading to the westerly part of the profile and a positive asymmetry represents faster spreading to the easterly part of the profile (for a north-south striking profiles the north is negative and south positive). For instance if the asymmetry is 0.05 the spreading rate to the left of the ridge is (1 - 0.05) * 1/2 spreading rate and to the right of the ridge the spreading rate is (1 + 0.05) * 1/2 spreading rate. Thus, when the left and right spreading rates are added, the outcome is always the full spreading rate.
Here is an example of an asymmetry file
% Everything written after the percentage sign is a comment
% This is a plain text file with the format below
% start(Myr) end(Myr) asymmetry(%)
0 50 0.04
50 89 0.01
89 118 -0.02
2010-08-03
