Geoscience Reference
In-Depth Information
with a chemically buoyant region. Plate creation
may superpose a new TBL on top of the old one,
particularly if ridges migrate and jump. If one
can slow down the process of heat conduction for
old plates one can slow down the flattening pro-
cess. Candidates for this include sedimentation,
or a decrease in conductivity at depth due to tem-
perature, mineralogy or crystal orientation.
If the mantle at depth is partially molten then
dikes may be injected to shallower depths when
the lithospheric stress conditions become appro-
priate. This serves to reheat the plate and par-
tially reset the thermo-magmatic age. This would
not uniformly affect all plates or affect them at
the same age. The reheating/diking events would
be milder versions of the plate creation process
itself, at the ridge. The plate does not have to
be impacted by a hot plume or over-ride hotter-
than-average mantle for this mechanism to be
effective.
One explanation for the 'flattening' of the
seafloor at old age, and the motivation for the
plate model, was that it might be caused by
the lower oceanic lithosphere becoming convec-
tively unstable once a critical age is reached.
This is a form of delamination. Small-scale con-
vection beneath the plate is assumed to main-
tain an isothermal boundary at a specific depth,
and cause the thermal structure of the cooling
lithosphere to resemble that of a finite-thickness
plate. The stirring action of small-scale convec-
tion alone may act to cool the upper mantle,
leading to increased subsidence, not flattening of
the depth--age curve. What is needed is a method
to reheat the plate or to slow down the cooling.
Dike or sill injection, or magma underplating,
does this without the involvement of particularly
hot mantle.
Hotspots are often held responsible for adding
heat to the system and retarding lithospheric
subsidence. A low-viscosity plume is very ineffi-
cient at thinning the overlying plate, and there is
insufficient time for conductive thinning except
for slowly moving plates. The excess buoyancy of
dike and sill-injected plates could contribute to
the anomalous subsidence of the seafloor, with-
out the need for plumes or extra basal heating.
The shallow depths of the seafloor in the
western Pacific and the superswell in French
Polynesia have been attributed to the residual
effect of a hot Cretaceous superplume but evi-
dence used in support of this is largely spurious.
The
superswell is well explained by a
warm or buoyant low-viscosity astheno-
sphere
, not by lithospheric reheating or thin-
ning (McNutt and Judge, 1990); a deep source of
heat is not required.
Hypsometric curves predicted by plate cooling
models are not a good match to the bathymetry
observations. Reheating or intrusion models may
be required to explain the heatflow, the hyp-
sometry, the abrupt flattening of median depth
with age, and the increased variability of depth
at older ages. The cooling half-space model pre-
dicts that bathymetry and heat flow will follow
a square-root age relation for all time. The plate
model assumes that a fixed temperature is main-
tained at a fixed depth everywhere and so pre-
dicts that subsidence curves should flatten at the
same age everywhere; they do not. From time to
time some areas will probably be extended and
intruded; uplift and rejuvenated subsidence may
occur anywhere at any time, depending on the
stress-state of the plate.
Attempts to 'correct' heat flow and ocean
bathymetry by avoiding hotspots or correcting
for their effects are misguided if dikes and sills
and underplating are ubiquitous in oceanic litho-
sphere. This mechanism for explaining heat flow
and bathymetry is basically a stress mechanism
since it does not depend on high temperatures,
only stress conditions in the lithosphere. The
plate can be treated as semi-permeable to magma
and a partially open system, rather than a rigid
impermeable LID over the asthenosphere. The
stochastic nature of heat sheet penetration is
a combination of stress conditions in the plate
and the variable fertility and melting point of
the asthenosphere. Underplating and intrusion
of magmas in the ordinary range of temperatures
may be responsible for the background heat flux
rather than numerous high temperature deep-
mantle plumes.
Flexural rigidity, a measure of elastic plate
thickness, appears to decrease and then to
increase rapidly after the imposition of a large
volcanic load such as at a 'hotspot.' This is
usually taken as an indication of
thermal rejuve-
nation
or heating and weakening of the litho-
sphere by a plume. However, the load itself causes
