The 600-km-long Magallanes fault zone and the newly discovered 100-km-long Deseado fault zone represent large but little understood segments of the South American-Scotia (SAM-SCO) transform on the South American continent (~50--56 ¿S latitude). Kinematic analyses of fault populations near Mount Hope on Tierra del Fuego indicate that these fault zones have accommodated sinistral strike-slip motion with a probable component of normal slip on vertical-subvertical faults during Cenozoic time. Regional geological data and overprinting relationships between the strike-slip/oblique-slip faults and mid-Cretaceous to Oligocene contractional structures suggest that sinistral motion in the Mount Hope segments of the two fault zones occurred significantly since ~60 Ma and dominantly since ~30 Ma with the most recent episode of activity occurring during the Quaternary. Results from mapping show 20--25 km of cumulative sinistral separation of a thrust contact across a well-exposed segment of the Magallanes fault zone north and east of Mount Hope. Although the exact amount of displacement accommodated by this segment of the Magallanes fault zone remains uncertain, the 20--25 km of cumulative sinistral separation combined with an estimate of 3 km of vertical separation is consistent with small magnitudes of displacement of the order of tens of kilometers. This result apparently contrasts with predictions based on plate reconstructions that large amounts of strike-slip displacement (>200 km) have been accommodated internally within southernmost South America since early Cenozoic time. A low magnitude of displacement along the Mount Hope segment of the Magallanes fault zone suggests that (1) strike-slip motion has been partitioned across the continent since about 30 Ma and/or (2) the Mount Hope segment of the Magallanes fault zone represents a young part of the modern plate boundary that migrated to its present location during the last few million years. ¿ American Geophysical Union 1994