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374 GEOLOGY, April 1995geophysical survey. The gravity field is
rougher southwest of the fracture-zone
trace than it is to the northeast; I interpret
this as being due to rougher buried topog-
raphy to the southwest, consistent with slow-
ing of spreading as a section of the Pacific-
Phoenix ridge approached the subduction
zone. A prominent gravity low follows the
north edge of the Chatham Rise, a location
where there is no sea-floor trough; this may
locate the east Gondwana paleosubduction
zone (Wood and Davy, 1994). Another grav-
ity low extends northeast from the east end
of the Chatham Rise. This may mark the rift
valley of a stalled ridge. Similar gravity sig-
natures are associated with stalled ridges
mapped west of the Antarctic Peninsula and
in the western Scotia Sea (Sandwell and
Smith, 1992). Sea-floor spreading anomaly
chron 34 (84Ma; Kent and Gradstein, 1985)
can be found adjacent to the southeast mar-
gin of the Chatham Rise (Fig. 3; Cande
et al., 1989). Thus, the gravity data may lo-
cate a spreading system older than chron 34
north of Zealandia and trending at a low
angle into the paleosubduction zone.
The marine magnetic anomaly map of
Cande et al. (1989) locates the trace of the
Pacific-Farallon-Aluk (the Phoenix plate is
renamed Aluk for chron 34 and later time)
triple junction; they have marked its loca-
tion for times prior to chron 34. The north-
ernmost (oldest) point on the triple junction
trace is now at lat 25.58S, long 152.58W. Ex-
trapolating the spreading rate for the chron
34–33 period back in time, the age of the
starting point on this triple junction trace is
about 100 Ma. I have extended the Pacific-
Phoenix (Aluk) spreading center from this
triple junction location to the Chatham Rise
perpendicular to the fracture zone trend in
the ocean reentrant (Fig. 4). These obser-
vations imply that the geometry of the
spreading system in mid-Cretaceous time
was similar to that of the spreading systemin
effect at chron 34 time after sea-floor
spreading between Zealandia and Antarc-
tica started.
Marine geophysical data from the equa-
torial and southwestern Pacific also permit
the Pacific-Phoenix ridge to be located off-
shore east Gondwana in mid-Cretaceous
time (Fig. 4). Joseph et al. (1993) proposed
(following Larson, 1976; Winterer et al.,
1974) that at or soon afterM0 time (ca. 119–
118 Ma; Kent and Gradstein, 1985), the Pa-
cific-Phoenix ridge jumped 108 south from a
location at the Nova-Canton Trough. Lar-
son et al. (1992) placed the ridge between
258 and 358S prior to the jump, so afterward
the ridge was as far south as 458S. Using the
East Antarctica Apparent Polar Wander
Path of DiVenere et al. (1994), the Chatham
Figure 3. Interpretation
of Figure 2. Pre–chron
34 fracture zone ex-
tends northwest from
east end of Chatham
Rise. Gravity lineations
perpendicular to frac-
ture-zone trend are
seen east of North Is-
land of New Zealand.
Gravity low along north
edge of Chatham Rise
marks paleosubduction
zone; low trending
northeast from east end
of Chatham Rise may
mark rift valley of stalled
spreading center. Con-
tours mark gravity
highs; lows are shaded
areas or dark lines;
thicker black lines are
positive lineations
shown in Figure 4. Mag-
netic anomalies
(striped) are fromCande
et al. (1989).