Does unsteady overriding plate kinematics explain variability of slab dip and upper plate deformation?
Résumé
Plates reorganization associated with mantle convection leads
to changes in absolute plate velocities which may in turn impact
the geometry of slabs and the upper plate regime of deformation.
Analogue experiments suggest a very good correlation between
overriding plate absolute motion and, slab dip and upper
plate deformation: steep slab and extension are associated with
overriding plate moving away from the trench while shallow slab
and shortening occur when the upper plate goes the other way.
However, these correlations are established on models in which
the motion of the overriding plate is constant and the subduction
steady-state which may not correspond to the general case:
How does the subduction adapt to the changes of overriding plate
absolute velocity? What is the impact on slab dip and upper
plate deformation? Could it explain why correlations in Nature
between overriding plate absolute motion and slab dip are poorer
than expected? We herein present a study coupling 3D analogue
models of subduction and observations of present-day subduction
parameters. The models show that instantaneous changes in the
overriding plate absolute velocity produce a transient stage during
which both upper plate deformation and slab dip adjust, followed
by a period of stabilization. Specifically, deformation velocity adjusts
rapidly while slab dip requires a larger time. For deep slab
dip this time delay is on average 17 ± 4 Myr and is independent of
both the absolute velocity and changes in absolute velocity. There
is a systematic trend toward lower than expected slab dip when
upper plate slowdowns and higher when it accelerates. In Nature,
there is the same general trends but correlations are poorer than
in laboratory models. The distribution of slab dip as a function
of overriding plate accelerations or decelerations above specific
periods of time supports the idea of a global adjustment time of
10 Ma, close to what found in analogue models. Over this time
period, deep slab dips tend to be lower than expected when the
overriding plate constantly decelerated and higher when it accelerated.
The same analysis for upper plate deformation gives a
best result for an adjustment time of 5 Ma. Thus, there is a delay
in the response of the system to the changes of overriding plate
absolute velocity. The poor correlation in Nature between presentday
subduction kinematics, slab dip and upper plate deformation
may explain because the present-day velocities correspond to a
snapshot in the evolving history of subduction