Obduction: Why, how and where. Clues from analog models
Abstract
Obduction is an odd geodynamic process characterized by the emplacement of dense oceanic “ophiolites”
atop light continental plates in convergent settings. We herein present analog models specifically
designed to explore the conditions (i.e., sharp increase of plate velocities — herein coined as ‘acceleration’,
slab interaction with the 660 km discontinuity, ridge subduction) under which obduction may develop as
a result of subduction initiation.
The experimental setup comprises an upper mantle modeled as a low-viscosity transparent Newtonian
glucose
syrup filling a rigid Plexiglas tank and high-viscosity silicone plates. Convergence is simulated by
pushing a piston with plate tectonics like velocities (1–10 cm/yr) onto a model comprising a continental
margin, a weakness zone with variable resistance and dip (
W
), an oceanic plate (with or without a
spreading ridge), a preexisting subduction zone (
S
) dipping away from the piston and an upper active
continental margin, below which the oceanic plate is being subducted at the start of the model (as for
the Neotethyan natural example).
Several configurations were tested over thirty-five parametric models, with special emphasis on
comparing
different types of weakness zone and the degree of mechanical coupling across them.
Measurements of displacements and internal deformation allow for a precise and reproducible tracking
of deformation. Models consistently demonstrate that once conditions to initiate subduction are reached,
obduction may develop further depending on the effective strength of
W
.Results(1)constrainthe
range of physical conditions required for obduction to develop/nucleate and (2) underline the key
role of such perturbations for triggering obduction, particularly plate ‘acceleration’. They provide an
explanation to the short-lived Peri-Arabic obduction, which took place along thousands of km almost
synchronously (within
∼
50–10 Myr), from Turkey to Oman, while the subduction zone beneath Eurasia
became temporarily jammed. They also demonstrate that the emplacement of dense, oceanic material
on continental lithosphere is not a mysterious process requiring extraordinary boundary conditions but
results from large-scale, normal (oceanic then continental) subduction processes.