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Bruce P. Luyendyk Institute for Crustal Studies and Department of Geological Sciences, University of California,
Santa Barbara, California 93106-1100
ABSTRACT
In the process of subducted slab capture, a spreading ridge approaches subparallel to
a subduction zone following the trailing edge of a downgoing plate. Eventually the down-
going plate is too young and small to subduct, and spreading stops. The spreading ridge
stalls many tens of kilometres outboard of the subduction zone. The subducted plate welds
to the outboard plate across the dormant spreading center and is captured by it. The
captured plate then acquires the motion of the plate it welded to. In the southwest Pacific
the Pacific-Phoenix ridge approached the east Gondwana margin as the Phoenix plate
subducted beneath New Zealand, the Chatham Rise and Campbell Plateau, the Lord Howe
Rise (collectively, Zealandia), and Marie Byrd Land in Cretaceous time. Spreading and
subduction shut down here between 110 and 105Ma, and some sections of the Phoenix plate
became welded to (captured by) the Pacific plate. Pacific plate northward motion began in
Aptian time, pulling the captured subducted microplates with it. This movement exerted a
basal traction on the overlying east Gondwana margin and resulted in extension of Zea-
landia and Marie Byrd Land. Continued Pacific northward motion rifted Zealandia from
Marie Byrd Land at about 85 Ma.

MESOZOIC EVENTS IN EAST
GONDWANA
A sudden change in tectonic regime from
subduction to extension occurred in New
Zealand in mid-Cretaceous time (105 6 5
Ma; Bradshaw, 1989). Bradshaw ascribed
this change to collision of the Pacific-Phoe-
nix spreading center with the paleosubduc-
tion zone along the margin of east Gond-
wana. Ridge collision was followed by the
rifting away during sea-floor spreading of
New Zealand, the ChathamRise and Camp-
bell Plateau, and the Lord Howe Rise (col-
lectively, Zealandia) from east Gondwana
shortly before 85 Ma (Lawver et al., 1992;
Mayes et al., 1990). A similar sudden change
occurred in southwestern North America
during late Cenozoic time (Nicholson et al.,
1993, 1994; R. Bohannon and T. Parsons,
unpublished) and also in the Antarctic Pe-
ninsula in Early-Middle Jurassic time (Sto-
rey and Alabaster, 1991). I propose that the
scenario described by Bradshaw (1989) is
correct, but that ridge collision did not oc-
cur. Instead, Phoenix microplate slabs un-
derlying Zealandia in the subduction zone
were captured by the northward-moving Pa-
cific plate and rifted away overlying Zea-
landia from east Gondwana.
Extension in New Zealand starting with
mid-Cretaceous rifting has been substantial.
For the Campbell Plateau it has been esti-
mated at 200 km (Bradshaw, 1991). In the
western province of New Zealand, mylo-
nites of the Paparoametamorphic core com-
plex were uplifted to the surface by 105–100
Ma (Tulloch and Kimbrough, 1989). Exten-
sion there closely followed crustal thickening
and compression, ending in Early Cretaceous
time. Western New Zealand has extensional
basins like the Taranaki with 4 km or more of
nonmarine sediments of Albian-Cenomanian
age and younger (see Bradshaw, 1991). Off-
shore, the Great South Basin off the southeast
South Island of New Zealand is a complex
graben structure with more than 7 km of
mid-Cretaceous and Tertiary sedimentary
rock (Carter, 1988; Cook and Beggs,
1990). Well data there show that subsid-
ence began at 103 Ma or just before (Car-
ter, 1988). Extended structures of Creta-
ceous age are found on the Lord Howe
Rise (Lister et al., 1991; Bentz, 1974), and
on the Queensland margin (Symonds et
al., 1987). The Bounty Trough between
the Campbell Plateau and the Chatham
Rise is a Late Cretaceous failed rift that
preceded Zealandia–West Antarctica
spreading (Carter et al., 1994).
In westernMarie Byrd Land ofWest Ant-
arctica, the migmatites of the Fosdick
Mountains are interpreted to have been up-
lifted during north-south extension from
depths of 15 km or more in the period
100–94 Ma (Richard et al., 1994). Paleo-
magnetic studies suggest that tilting of fault
blocks here occurred in the interval 105 to
103 Ma (Luyendyk et al., unpublished), im-
plying that rifting between the Campbell
Plateau and western Marie Byrd Land be-
gan near this time. Igneous rocks in central
Marie Byrd Land have been interpreted to
record a sudden change from subduction-
related to rift-related magmatism in mid-
Cretaceous time (Weaver et al., 1994).
Figure 1. Slab capture model ap-
plied to east Gondwana (modi-
fied from illustration for Califor-
nia by Craig Nicholson). A:
Pacific-Phoenix ridge ap-
proaches trench and subduction
zone of east Gondwana. Vectors
atop plates are relative to east
Gondwana or West Antarctica;
half-spreading-rate vectors are
shown on side. Spreading center
approaches subduction zone at
rate r. B: Spreading and subduc-
tion rates decrease as slab pull
force decreases. Ridge stops
approaching trench when half-
rate equals vector of Pacific
plate, and retreats slowly from
trench as spreading stops. C:
Spreading stops and Phoenix
plate is captured by Pacific.
Basal traction of Phoenix on
overlying Zealandia margin ex-
tends and rifts it away from West
Antarctica, opening new ocean
basin.