Title: Tertiary seafloor spreading between East and West Antarctica and implications for 
Antarctic lithospheric structure

Authors: Joann Stock and Steve Cande

Motion between East and West Antarctica, which is proposed to have occurred in 
Cretaceous and/or Tertiary time, has long been a concern for Antarctic tectonics. The 
amount and direction of any such motion needs to be known in order to address global 
problems such as hotspot fixity and reference frames of plate motion, as well as for any 
plate circuits that go through the Antarctic plate. It also has important implications for 
the lithospheric structure of the Antarctic continent. Marine geophysical studies of the 
seafloor of the Ross Sea region (cruise NBP9702), along with studies of the conjugate 
region on the Australia plate, showed that Antarctica acted as two independently moving 
plates separated by a slow oceanic spreading center along the Adare Trough during part 
of the Tertiary, from at least chron 20 time to chron 9 time (40 to 28 Ma; Cande et al., 
Science, 2000). This spreading produced a 150 km width of new seafloor that is 
identified in the Adare Basin (surrounding the Adare Trough) with conjugate magnetic 
anomalies on both sides. Recently, on cruise NBP0209, additional geophysical 
measurements were collected at the southern end of the Adare Trough. These results 
show that some of the linear magnetic anomalies associated with the Adare Trough, as 
well as the gravity low that is one of the defining characteristics of the Trough, continue 
southward into shallower regions of the Ross Sea without any apparent offset or 
termination. This result thus rules out some previous models that invoked NW-striking 
strike-slip faults here as a means for the extension to have been transferred laterally away 
from the southern Ross Sea. The 150 km of Tertiary ocean floor in the Adare Basin 
therefore projects along strike south-southeastward into the region between Cape Adare 
and the Iselin Bank. We hypothesize that the material now present between Cape Adare 
and the Iselin bank represents new crustal area formed during Tertiary extension between 
the East Antarctic plate and the West Antarctica plate. The nature of this crust is 
unknown but it may be oceanic (or transitional) crust, or highly extended continental 
crust. There was also some displacement between E and W Antarctica prior to chron 20 
time, east of Iselin Bank; the net amount of this displacement is not yet well constrained. 
The continuation of the fossil E-W Antarctic plate boundary into the modern Antarctica 
plate, and where and how it would connect back out to the margins (i.e. west of the 
Antarctic peninsula?) is not known. From Aus-Ant plate reconstructions, we know that 
since about chron 9 time (28 Ma) there is no resolvable relative motion between East and 
West Antarctica in the Adare Basin region. A better knowledge of the seismic wave 
speed structure in the southern Ross Sea and West Antarctica, such as could be 
determined from a more dense seismic array, is critical for understanding the nature of 
extended lithosphere (oceanic? Transitional crust? Basin and Range style extended 
continental crust?) that originally formed between East and West Antarctica, and for 
tracing the fossil East-West Antarctica plate boundary along strike away from the Ross 
Sea region.