Geoscience Reference
In-Depth Information
'stress-rejuvenation' and an apparent thinning
of the plate and does so without a heat flow
anomaly. Thus, not only can volcanic chains
themselves be caused by stress rather than tem-
perature, but other features such as subsidence
and heatflow can also be explained by ather-
mal stress mechanisms. The
subsidence of
oceanic plateaus
andupliftofCFBsaresome
of the paradoxes of thermal mechanisms. All
of these considerations show that, while sur-
face hotspots may represent local rise of normal
asthenopspheric mantle to shallow depths, they
do not require abnormally hot mantle. In global
compilations of heatflow, one should use all the
data instead of attempting to mask out hotspots,
and one should not use simple theoretical mod-
els to correct measured heatflow to the value that
it should have for the appropriate age crust.
spreading? If the heat flux to the surface has
changed significantly in the last 200 Myr it
should show up in variations of seafloor spread-
ing rates and sealevel.
The creation of new
oceanic plates appears to have been
constant during the past 180 Myr
. Detailed
plate reconstructions and sea-level variations pre-
clude large global thermal pulses. There is no
evidence for a major increase in plate rates dur-
ing the Cretaceous. Likewise, there is no evi-
dence for the superplumes, mantle overturns and
avalanches that were spawned by the specula-
tion that there are large and rapid variations in
plate-creation rates. Long-term (hundreds of mil-
lions of years) variations of 5 or 10% in surface
heat flow, however, are expected simply from the
nature of high-Rayleigh-number convection, even
if the boundary conditions are constant. Plate
reorganizations may release pent up heat at new
plate boundaries since mantle temperatures tend
to increase beneath large long-lived plates. Heat-
flow also depends on the aspect ratio and style
of mantle convection and this may change in
the order of 50% over a supercontinent cycle of
500 Myr.
Temporal changes in heat flux
The
question
arises:
are
present
estimates
of
global
heatflow
representative
of
the
present
cycle
of
continental
break-up
and
seafloor
