PALEOMAGNETISM OF THE LATE ARCHEAN NYANZIAN SYSTEM, WESTERN KENYA

The Nyanzian System lavas of western Kenya are believed to be the oldest rocks of the Tanzanian Craton. Intrusive age relationships suggest an age greater than or equal to 2850 Ma although direct attempts at dating the Nyanzian have produced disparate results. Our study involves a suite of samples collected from the Nyanzian basalts, pillow basalts, andesites and rhyolites from sixteen sites in western Kenya. These rocks yield a tilt-corrected paleomagnetic pole at 14 degrees N, 150 degrees E (K=59, dp=5 degrees, dm=7 degrees). This pole is constrained to be older than the first (D1) deformation (>2472+/-30 Ma) by positive fold, conglomerate and reversal tests. Analysis of the paleomagnetic data base for three African cratonic nuclei (Tanzanian, Kaapvaal/Zimbabwe and West Africa) for the time period from 2.0 Ga to 3.0 Ga demonstrates a paucity of well-dated poles, although there are several poles from the Kaapvaal/Zimbabwe and Tanzanian Cratons which allow ''spot-readings'' of their relative positions. We demonstrate, based on these data, that the Kaapvaal/Zimbabwe and Tanzanian Cratons were drifting independently at similar to 2875 Ma, similar to 2700 Ma and similar to 2450 Ma. This independent motion of the Tanzanian and Kaapvaal/Zimbabwe Cratons indicates that previously proposed models involving African cratonic coherence can no longer be considered valid for the time period from 2850 to 2500 Ma.