eLearning

Geology; April 1995; v. 23; no. 4; p. 373–376, 4 figures. 373SUBDUCTED SLAB CAPTURE
In the subducted slab capture process, a
spreading ridge approaches subparallel to a
subduction zone following the trailing edge
of a downgoing plate (Fig. 1). The age of the
subducting plate is decreasing with time. As
the young subducting plate approaches the
subduction zone it may fragment into mi-
croplates outlined by transform faults that
intersect the trench. Eventually it may be-
come too small to subduct, and spreading
slows and 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. If the outboard plate
is converging with the overriding plate, sub-
duction continues, and the welded ridge and
captured plate are consumed. However, if
the outboard plate is diverging, then rifting
and/or strike-slip faulting may occur in the
overriding plate. The ridge is not subducted,
and there is no ‘‘ridge collision’’ (Nicholson
et al., 1993, 1994). Any back-arc spreading
could produce a weak zone in the overlying
plate and provide a locus for rifting within it.
The slab-capture hypothesis requires sup-
porting observations in order to be fully ap-
plied, including an extinct spreading center
outboard of the paleosubduction zone and
an abrupt change in tectonic regime in the
overlying plate. The sudden change ob-
served in east Gondwana (Bradshaw, 1989;
Weaver et al., 1994) is the primary clue and
is the observation most demanding of an ex-
planation; subduction is quickly replaced by
extension. Locating an extinct system of
ridges, as was successfully done offshore
southwestern North America using marine
magnetic anomalies (Atwater and Severing-
haus, 1989; Lonsdale, 1991), is more chal-
lenging in the southwest Pacific sector.
Spreading ended during the Late Creta-
ceous period of no magnetic reversals, so a
symmetric magnetic anomaly pattern can-
not be found. Furthermore, except for a re-
entrant of prerift ocean crust north of the
Chatham Rise, all older crust bordering the
outboard edge of Zealandia has been con-
sumed by Tertiary subduction and disrupted
by back-arc spreading in the Tonga-Kerma-
dec subduction zone.
PACIFIC, PHOENIX, AND ANTARCTIC
PLATES IN CRETACEOUS TIME
Recently declassified Geosat altimetry
and gravity data reveal important sea-floor
features near New Zealand and within the
oceanic crust reentrant north of the
Chatham Rise (Sandwell and Smith, 1992;
Morgan and Sandwell, 1994). A trough
aligned with and parallel to the Udintsev
Fracture Zone can be clearly traced beyond
chron 34, northwest and into the Tonga
trench (Figs. 2 and 3); this trough probably
marks a fracture zone of a past spreading
system. The northwest end of this trace cor-
responds to the Rapuhia Scarp on the north-
east margin of the Hikurangi Plateau
(Wood and Davy, 1994). Subtle gravity lin-
eations (buried topography?) that strike
northeast normal to the fracture-zone trend
can be seen southwest of the inferred frac-
ture zone. Wood and Davy (1994, Fig. 3)
mapped northeast-trending buried base-
ment features here from a reconnaissance
Figure 2. Satellite gravity map of
southwest Pacific sector (Sand-
well and Smith, 1992). Gray areas
are North Island and South Is-
land of New Zealand. Gravity
high extending east from South
Island is Chatham Rise. Simu-
lated illumination is from north.