Tectonic Evolution of the Pacific Margin of Antarctica Since 100 Ma

Robert D. Larter (1), Alex P. Cunningham (1,3), Karsten Gohl (2), 
Graeme Eagles (2), Peter F. Barker (1) & Frank O. Nitsche (2,4)

(1) British Antarctic Survey, High Cross, Madingley Road, Cambridge, 
CB3 0ET, UK (e-mail: r.larter@bas.ac.uk)
(2) Alfred Wegener Institute for Polar and Marine Research,
P.O. Box 120161, D-27515 Bremerhaven, Germany
(3) Now at ARK CLS Limited, Milton Keynes, UK
(4) Now at Lamont-Doherty Earth Observatory of Columbia University,
Palisades, New York, USA

West Antarctica is generally thought to have behaved as a single terrane
since mid-Cretaceous time. Subsequently some limited internal deformation,
below the resolution of paleomagnetic methods, may have taken place, and
there has been some extension between East and West Antarctica. However,
the most dynamic part of the Antarctic plate during Late Cretaceous and
Tertiary times was the Pacific margin of West Antarctica and adjacent
oceanic areas. Unfortunately this is one of the least studied sectors of
the Antarctic margin, due to its remoteness and inaccessibility.

We present new reconstructions of the Pacific margin of Antarctica based on
constraints from marine magnetic data and regional free-air gravity fields.
Recent results from interpretation of seismic reflection and gravity
profiles collected in the Bellingshausen Sea are also incorporated. The
reconstructions show regional constraints on the complex tectonic evolution
of the Amundsen and Bellingshausen Seas during and after the breakup
between New Zealand and West Antarctica. The Late Cretaceous to early
Tertiary development of the region involved interactions between the 
Pacific, West Antarctic, Phoenix (Aluk), Charcot and Bellingshausen plates.

The West Antarctic margin consists of two main sectors. The western sector
comprises the parts of the margin that Chatham Rise and Campbell Plateau
separated from at about 90 Ma and 80 Ma, respectively. Part of this sector,
probably including much of the Amundsen Sea continental shelf, was
subsequently affected by independent motion of the Bellingshausen plate. In
the eastern sector, Phoenix plate subduction continued through the Late
Cretaceous and early Tertiary, then terminated progressively from southwest
to northeast as segments of the Antarctic-Phoenix ridge migrated to the
Antarctic Peninsula margin. Between these two sectors is an area with
complex gravity anomalies near Peter I Island. We suggest that a fragment
of the former Charcot plate, containing the oldest ocean floor adjacent to
West Antarctica, is preserved in this area. Some implications of our new
reconstructions can be tested by onshore and offshore deployments of
seismometers along the Pacific margin. Ideally such deployments should be
co-ordinated with controlled-source seismic investigations.