Mountain Uplift and Related Processes in Antarctica

Tim Stern
School of Earth Sciences and Institute of Geophysics, Victoria 
University of Wellington, Wellington, New Zealand.

Uplift of the Transantarctic Mountains (TAM) has been the focus of a 
number of workers over the past 20 years. Fission track analysis show 
the TAM have a modest rate of exhumation of ~ 100-200 m / my,  which 
is two orders of magnitude less than say the Southern Alps of New 
Zealand , or the eastern Himalaya.. But interest in the TAM uplift 
prevails because of questions related to climate, erosion and a 
possible link between the rise of the TAM and the development of the 
East Antarctic ice cap. Moreover, there are some more prosaic 
questions that arise like how such a long mountain chain remains high 
for so long, and why is this rift shoulder so much higher than 
others. Smith and Drewry's (1984) initial idea of a mantle plume as 
the cause of uplift was a purely thermal concept. A low-angle 
detachment model from Fitzgerald et al was next, then Stern & ten 
Brink in 1989 followed with "Flexural Uplift of the Transantractic 
Mountains". Central to this model was the 3 way interaction of 
thermal expansion, isostatic forces in the vicinity of a normal 
fault, and isostatic rebound as a response to erosion. Erosion, both 
incision and mountain-top, was considered the dominant factor. Other 
more detailed models have followed, but most contained some mix of 
the three basic elements of thermal expansion, erosion and isostatic 
forces linked to extension.  For example, lithospheric necking as 
proposed by Busetti et al required that the starting point was a cold 
and fairly rigid West Antarctic lithosphere. More recent ideas 
revolve around a hot lithosphere with thick crust in West Antarctica 
that has been stretched, leaving behind the edge of a pre-existing 
plateau ( i.e. the TAM). It would be difficult to distinguish between 
some of these variations using seismic observations. But models that 
require uplift from crustal underplating could probably be tested by 
active source seismic methods to measure crustal thickness.  Surface 
wave tomography and receiver function methods are, however, providing 
some of the best constraints on processes beneath the TAM.  The 
contrast in S-wave speed seen in the mantle beneath the TAM is one of 
the strongest seen in any continental area and implies high lateral 
thermal gradients.  
Other important global problems can be addressed by studies linked to 
the rise of the TAM. For example differential erosion in the TAM is 
spectacular, and only matched by that of the great Himalayan gorges. 
A numerical estimate of the isostatic response to the incision is 
about 1500 m of rock uplift. Although this result depends on what 
rigidity profile one wishes to adopt for the edge of the TAM, the 
predicted rebound here is surely one of the highest on earth. Behind 
the TAM lies the Wilkes subglacial basin. One interpretation is that 
the basin represents a broad flexural downwarp as a compensation for 
the uplift of the TAM. If so, the wavelength of this basin, and its 
associated negative, free-air gravity anomaly, suggest an elastic 
thickness ~  80 km. A line of thinking that is currently gaining 
popularity is that the elastic thickness in continental regions 
should not exceed the crustal thickness. Why then does the East 
Antarctic lithosphere appear to be so strong and defy this principle 
?  Is the apparent strength related to the polar position, the ice 
cap or the apparent aseismicity ?  If we can find some link here, we 
may learn something fundamental about the creep strength of the 
mantle.