Seismic tomography
shows
that subducting slabs can either sink
straight
into the
lower
mantle,
or
lie down in the mantle transition zone.
Moreover, some
slabs
seem to
have
change
d
mode from stagnation to penetration
or
vice
-
versa.
We investigate the
dynamic controls on these modes and particularly the transition between the
m
using
2D self
-
consistent
thermo
-
mechanical
subduction models
.
Our models confirm that
the ability of the trench to move is key for slab flattening in the transition zone
. O
ver
a wide range of plausible Clapeyron slopes and viscosity jumps at the base of th
e
15
transition zone,
hot
young slabs (
25
Myr
in our models
)
are
most likely
to
penetrate
,
while
cold
old slabs (
150
Myr
) drive more trench motion and
tend to
stagnate.
Several
mechanisms are able
to
induce
penetrating slabs to stagnate
:
ageing of the
subduct
ing
plate,
decreasing upper plate forcing
,
and
increasing
Clapeyron slope
(
e.g.
due to
the
arrival of a more hydrated slab
).
Gett
ing
stagnating slabs
to
penetrate is more
difficult. It can be accomplished
by
an instantaneous change in
the forcing of the
up
per plate
from free to
motionless
,
or
a sudden
decrease
in
the Clapeyron slope.
A
rapid change
in plate
age at the trench from old to
young
cannot easily
induce
penetration
.
On Earth
,
ageing of the
subducting plate
age
(with accompanying upper
plate rifting)
may be the
most
common mechanism for
causing slab stagnation
,
while
strong changes in
upper plate forcing
appear required for triggering slab penetration
.