Some of the most important unanswered questions about Cenozoic plate motions concern the
magnitude and timing of deformation within Antarctica and other plates in the southern ocean
basins, and their effects on plate circuits used to reconstruct
Cenozoic plate positions. Acton and Gordon [1994] examine whether published plate
reconstructions for the period 
70-20 Myr can bring paleomagnetic poles of similar ages
but from different plates into coincidence after they are rotated into a common reference frame.
They find that paleomagnetic poles from sites located on plates other than the Pacific, when rotated
into a fixed Pacific reference frame via a circuit that incorporates plate boundaries surrounding
Antarctica, differ significantly from coeval paleomagnetic poles determined from sites on the
Pacific plate. They suggest that a systematic error within the global plate circuit,
possibly as a result of deformation within Antarctica or the southern Pacific, is the most likely
of several alternative explanations for the angular discrepancy between fixed Pacific and
rotated non-Pacific paleomagnetic poles. New marine geophysical data from the southern Pacific
and Antarctic margins however suggest that any internal deformation of the Antarctic plate
probably occurred earlier than Chron 27 (
64 Myr), although a full description of this work
has not yet been published [ Cande et al., 1992; Raymond et al., 1993].
Two studies of Indian, Australian, and Eurasian plate motions have provided useful information
related to the Himalayan mountain belt and initiation of distributed deformation within the seafloor
south of India. Using paleomagnetic observations derived from Ocean Drilling Project
(ODP) Sites 756-758 along the Ninetyeast Ridge,
Klootwijk et al. [1991] conclude that India's rapid northward motion of 180-195 mm yr
prior to 55 Myr slowed dramatically to 45 mm yr
after 
55 Myr, and possibly again
at 17 Myr. They interpret the decrease at 55 Myr as evidence for completion of the suture between
India and Asia. The possible decrease at 17 Myr coincides roughly with rapid denudation and possibly,
enhanced uplift of the Himalayas after 20 Myr
[ Harrison et al., 1992]. Results reported in
Royer and Chang [1991] raise the intriguing possibility that the initiation of deformation
across a wide equatorial zone south of India may coincide with the latter event.
Royer and Chang demonstrate that magnetic anomaly and fracture zone crossings
from the Indian, Africa, Australian and Antarctic plates require that motion between India and
Australia across their oceanic boundary south of India pre-dates Chron 5 (
11 Myr), implying
that a prominent 
7-8 Ma unconformity present in drill cores from ODP Leg 116 in the
central Indian basin [ Stow et al., 1989] does not mark the onset of regional-scale deformation
related to India-Australia motion.