| Across rifted margins, the prerift continental crust thins from ~ 30 km, reaching zero at the
continent-ocean transition (COT) beyond which either oceanic crust or unroofed mantle
forms top basement. As a result of the crustal thinning, considerable subsidence is both
expected and observed. However at several margins, subsidence appears to have occurred
largely after rather than during rifting. Examples of such behaviour described in
the literature include the West Iberia margin, the salt basins of the South Atlantic,
and the Exmouth Plateau margin. This synrift subsidence deficit can be explained
by crustal depth-dependent stretching, in which much of the crust is withdrawn
after the end of rifting, but considerable problems arise with this model. They can
however also be explained at magma-rich margins by thermal uplift during rifting,
the addition of igneous intrusions to the lithosphere during rifting, and the partial
depletion of the mantle. At magma-poor margins, mantle serpentinization has a similar
effect, although as serpentinization can only occur once the entire curst has become
brittle, this is likely to be important only at high degrees of stretching. An alternative
explanation may be the influx of asthenosphere warmer than the relatively cool
sublithospheric mantle observed beneath several continents and which is one explanation
for the lack of melt at many rifted margins. These different models would thus
imply some modification to the McKenzie model for lithospheric stretching, arising
because of the geodynamic processes accompanying continental breakup. But it
is also possible that synrift subsidence has been systematically underestimated if
local water level was substantially below global sealevel. The presence of thick
evaporites at many rifted margins indicates that this was true at the end of rifting.
As rifting leading to continental breakup by definition occurs within a continent,
it may be expected that the rift initially develops isolated from the global ocean,
with consequently unusual salinities and water levels very different from global
sealevel. In summary, there are thus several explanations for the synrift subsidence
discrepancy which are to be expected at rifted margins formed during continental breakup. |