Lithosphere strength controls oceanic transform zone structure: insights from analogue models

Research areas:
Oceanic transform zones have often been regarded as plate boundaries.
The origin of their structural variability is poorly constrained. A
simple observation indicates that the transform zone is narrow and
linear when the offset is large; while it is wide with a complex
faulting pattern in the case of a small offset. On the other hand, for a
given offset, large structural differences exist between transform zones
located on the fast-spreading South-East Pacific Rise and on the
slow-spreading Mid-Atlantic ridge. In general, the transform zones in
slow-spreading environments are linear with a simple fault pattern,
while in fast-spreading systems they are wide with a complex pattern of
deformation. We perform small-scale analogue modelling to constrain the
influence of lithospheric strength on the development of deformation
above a transform boundary. The models are made up of sand and silicone
putty as analogues of the brittle layer and the viscous layer of the
lithosphere, respectively. Two plastic sheets coming from shifted gashes
form a set-up of two diverging discontinuities connected by a transform
boundary. The rheological layering and strength of the model were
modified using different shapes of the viscous layer placed on the
transform boundary. Above the divergent discontinuities, the faulting
pattern is always formed by parallel normal faults. When no viscous
layer is placed on the transform boundary (strong discontinuity), the
deformed zone is narrow and has few linear faults. By adding a narrow
and thin viscous layer, the deformed zone becomes wider with a complex
faulting pattern formed by oblique-slip faults on the limits and by pure
strike-slip faults in the inner part. These latter strike-slip faults
trend oblique to the transform boundary. When a viscous layer with a
wide lateral extent overlays the transform discontinuity (weak
strength), the faulting is dominated by obliquely normal faults
extending over a wide zone, and the strike-slip is restricted to the
inner part of the deformed zone. Therefore, the mechanical strength of
the small scale-model controls the shape of the deformed zone and the
deformation partitioning. These results were applied to 24 oceanic
transforms zones: we point out that the spreading rate and the transform
offset are the two dominant parameters controlling the deformation
pattern. These two factors directly control the lithospheric strength at
the transform boundary. However, the distance to the nearest hotspot,
which may generate warmer thermal conditions even in slow-spreading
environments, should modify this result.