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two rifting stages, a Neo-Proterozoic stage associated with
ʲ
¼
The melt depletion within the lithosphere is modelled using
a parameter,
X
, that represents a completely compatible
tracer (see Appendix 2). This tracer has an initial condition
of a linear reduction from
X
1.24 and a late Paleozoic stage with
ʲ
¼
1.15
(Fig.
12.5
).
However, several short period anomalies (20-40 Ma)
superposed to the long term subsidence of the CB
(Fig.
12.5
) cannot be explained by successions of tectonic
episodes. Alternatively, in-plane stresses could change sub-
sidence patterns of actively stretching or folding regions
(Karner
1986
), allowing for distal continental collisions to
affect subsidence, but the continual change is suggestive of
an internal mechanism rather than far-field events. Dynamic
subsidence related to sub-lithospheric mantle flow can
explain such anomalies, as for instance the mechanism pro-
posed by Crosby et al. (
2010
) to explain the increase of
subsidence during Cenozoic. Here we suggest that the
20-40 Ma subsidence perturbations are related to the gravi-
tational instabilities caused by the changes in lithospheric
thickness (and hence contrasts of density, temperature and
viscosity) at the edges of the CB. The present-day thickness
of lithosphere (Fishwick
2010
) shows large variations
around the CB from roughly 100 to 200 km that will cause
movement of the lithosphere and asthenosphere. The form of
mantle flow most often considered in relation to a change in
lithosphere thickness is a
¼
2to
X
¼
1 at the base of the
thermally defined lithosphere (Fig.
12.6
). The 2-D model is
set up with a 100 km wide transition zone in melt depletion
and temperature such that the base of the compositional and
thermally defined lithosphere increases from 100 to 200 km
(Fig.
12.7a
). This initial condition is then allowed to evolve
through time within a Cartesian domain of fixed boundaries
of dimensions 2,800 km long by 700 km deep (257
257
nodes) with boundary conditions of free-slip. Temperature is
held fixed at 0
C at the top and 1,315
C at the base. The
sides have a zero temperature gradient. The equations of
viscous mantle flow are solved using Citcom (Moresi et al.
1996
; Nielsen and Hopper
2004
), and the topography is
calculated from the normal stress acting on the fixed upper
surface of the model.
The upper mantle evolves through time, and for the first
50 Ma there is a single large
instability that pulls
the surface down within the transition zone of the thicker
region of lithosphere (Fig.
12.7b
and
12.8
red line). The
return flow causes a topographic high above the edge of
the thinner region of lithosphere (Fig.
12.8
). This return
flow however does not generate melt due to decompression
as it is too week and deep (as also predicted in Nielsen and
Hopper
2004
). As the system continues to evolve, Rayleigh
Taylor instabilities form with a characteristic wavelength
that is 3-4 times the thickness of the lithosphere (Jaupart
et al.
2007
, Fig.
12.7c, d
).
Crucially, the lateral change in density set up by the
change in lithosphere thickness drives migration of the tran-
sition in lithosphere domains, changing the shape of the
transition in lithosphere thickness and hence altering topog-
raphy. The result is a dynamically changing topography
through time (Fig.
12.8
), with an episodic rise and fall as
convection cells form and move. By plotting the change in
“
drip-like
”
(King and Anderson
1998
; Sleep
2007
; van Wijk et al.
2010
; Hardebol et al.
2012
), where the change in thickness drives a downward
drip to form at the point of change in lithosphere thickness.
What has received little attention is the lateral movement of
the lithosphere and asthenosphere where there are lateral
changes in density (e.g. Huismans and Beaumont
2011
;
Armitage et al.
2013
).
To understand how such a change of lithospheric thick-
ness can drive the mantle flow and affect the subsidence with
the proper periods, we used an idealised viscous flow model
described in Appendix 2. The initial thermal structure is of a
linear geotherm from 0
C to a temperature of 1,315
C at the
base of the lithosphere at either 100 or 200 km (Fig.
12.6
).
“
corner flow
”
Fig. 12.6
Initial condition for
the upper mantle. (
a
)
Temperature of the upper mantle
for the 100 km thick lithosphere,
dashed line
, and the 200 km thick
lithosphere,
solid line
.(
b
) Melt
depletion, the concentration of a
hypothetical completely
compatible trace element that
remains in the solid as melt is
generated. Dashed line for
100 km thick lithosphere and
solid line
for 200 km thick
lithosphere. (
c
) The resulting
density structure for the 100 km
thick (
dashed line
) and 200 km
thick lithopshere (
solid line
)
a
b
c
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