New paleomagnetic results from 101-89 Ma Laytonville Limestone of Franciscan Central Belt m¿lange of northern California confirm a southern hemisphere origin as first proposed by Alvarez et al. (1980). Discovery and study of new Laytonville limiestone outcrops extend the paleomagnetic record and provide a definitive determination of age, stratigraphic polarity, and hemisphere of deposition. Paleontological determinations how that five sections are presently overturned and four are presently right side up, as expected from a random distribution of nine blocks in a m¿lange. The exact accretion time is unknown, but geologically reasonable estimates range between 70 and 50 Ma, leading to ''averaged'' estimates of poleward velocity between 30 and 14 cm/yr. The ''averaged'' values are obtained by dividing the total paleolatitude shift by the difference between magnetization and estimated accretion ages; they reflect net poleward transport over a long time interval and are minimum estimates because they record only the poleward component of motion. An ''instantaneous'' poleward velocity is recorded by the systematic inclination decrease of 160 paleomagnetic samples with respect to decreasing age over the 12 m.y. section. Conservative estimates place this minimum velocity at 15 cm/yr. The consistency between values predicted by the average velocity calculations and those measured within the outcrops serves as an important check on the primary nature of the magnetization. This minimum velocity is much higher than any present-day plate velocity. The subduction of old oceanic crust at the boundary of a relatively small plate is hypothesised as a driving mechanism to explain the measured velocities. Coupled with the minimum instantaneous poleward velocities, this hypothesis suggests that the velocities of subducting plates may vary up to a factor of 5. Similar plate geometries and resultant velocities have probably existed at other times in the past, but their discovery may have been hindered by low preservation potential. ¿ American Geophysical Union 1990