Geoscience Reference
In-Depth Information
characteristic of the mantle but not of laboratory
fluids.
labeled as 'hotspots.' Once a sufficiently thick
eclogite layer forms, it will detach and founder
because of its high density. This results in uplift,
extension and magmatism. Delamination of a
10-km-thick eclogite layer can lead to 2 km of
uplift and massive melt production within about
10--20 Myr. Lower-crust density exceeds the man-
tle density by 3--10% when it converts to eclogite.
Crustal delamination is a very effective and non-
thermal way of thinning the lithosphere, extend-
ing the melting column and creating massive
melting and uplift. In contrast to thermal mod-
els, uplift occurs during and after the volcanism
and the crustal thinning is rapid. Delamination
may also be involved in the formation of oceanic
plateaus such as Ontong Java plateau.
There are several ways to generate massive
melting; one is to bring hot material adiabat-
ically up from depth until it melts; the other
is to insert low-melting-point fertile material --
delaminated lower arc crust, for example -- into
the mantle from above and allow the mantle to
heat it up. Both mechanisms may be involved in
the formation of large igneous provinces -- LIPs.
The time-scale for heating and recycling of lower-
crust material is much less than for subducted
oceanic crust because the former starts out much
hotter and does not sink as deep.
The removal of dense mafic roots from
over-thickened continental crust may not be a
Rayleigh--Taylor instability; roots may be sheared,
faulted or peeled off, or entrained, but the
results regarding the fertilization of the mantle
and local lowering of the melting point are the
same.
Anti-plumes: when the crust gets too thick
Plumes are Rayleigh--Taylor instabilities caused
by temperature. A chemical layer can also get
denser than the underlying layers or less dense
than the overlying layers. A salt dome is an
example of the latter. Many of the phenom-
ena that have been attributed to thermal insta-
bilities from a thermal boundary layer at the
core--mantle boundary can also be explained by
lithologic instabilities of the upper boundary
layer. Plumes, as used in the geophysical litera-
ture, are thermal instabilities of the lower ther-
mal boundary layer, heated from below by the
core. Anti-plumes are chemical instabilities trig-
gered by phase changes in the upper boundary
layer, which is cooled from above. They are 'anti-'
in all these respects. These are generally due to
delamination of the lower continental
crust
.
If continental crust gets too thick, the dense
eclogitic (or garnet pyroxenitic) bottom detaches,
causing uplift, asthenospheric upwelling and
pressure-release melting. Delamination of lower
crust explains a number of geologic observations,
such as uplift, uncompensated high elevations,
high heat flow and magmatism. These are the
predicted consequences of passive upwelling of
hot asthenospheric mantle in response to the
removalofthelowercrust.Delaminationmay
be important in creating compositional hetero-
geneity in the mantle. It may also be impor-
tant in creating magmatic regions that have been
