Vector magnetic anomaly maps derived at sea-level from Magsat data show a large positive anomaly above the Middle America Trench Relative highs and lows correlate with variations in the seismicity of the subducted Cocos plate. These anomalies can only be modelled assuming complete demagnetization of the slab near seismic discontinuities. We propose that demagnetization is necessarily associated with tearing of the plate. Magnetic anomalies indicate tearing of of the Cocos plate beneath Mexico and near Panama but further east than previously proposed on the basis of seismic data. Although the demagnetization process is likely to be related to the temperature increase, it is shown that thermal diffusion is not sufficient to produce the observed anomaly and that the slab must be heavily fractured in the vicinity of teared zones. ¿ American Geophysical Union 1987