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

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

Sep 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002geoji.150..706d&link_type=abstract

Geophysical Journal International, Volume 150, Issue 3, pp. 706-714.

Astronomy and Astrophysics

Astronomy

8

Analogue Modelling, Lithospheric Strength, Transform Zones

Scientific paper

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.

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