We present a model for microcontinent formation that is based on the structure of the Tamayo trough in the Gulf of California, Mexico. Potential field modeling of a transect through the Tamayo trough and Tamayo Bank suggests that the crust of the Tamayo trough is oceanic, and that the Tamayo Bank is a detached continental fragment. The oceanic crust that separates the Tamayo Bank from the mainland of Mexico is thin (5 km), so oceanic spreading was probably magma starved before it ceased. Such a thin crust has also been described on the Aegir Ridge in the North Atlantic, which became extinct after the Jan Mayen microcontinent separated from Greenland. In our model for the origin of microcontinents, the locus of plate spreading jumps to the weakened continental margin when the spreading ridge becomes amagmatic and the force required for continued extension at the ridge increases. A microcontinent is formed when the ridge jumps into a continental margin, and an asymmetric ocean basin or microplate is formed when the ridge jumps within oceanic crust.