SEAMOUNT Subduction

Posted onAuthorStéphane Dominguez

ACTIVE MARGIN DEFORMATION RELATED TO SEAMOUNT SUBDUCTION

-> Based on Dominguez et al., 1998a, 1998b, 2000 and Dominguez’s PhD Thesis

As revealed by marine geophysics, large domains of oceanic plates are characterized by a very rough seafloor covered by numerous seamounts, aseismic ridges, and volcanic plateaus. In many active margins, severe deformations, affecting the morphology and internal structure, are observed where these volcanic highs subduct. When they reach the seismogenic zone, they also modify the seismic coupling and might favor the nucleation of strong earthquakes. It is even suggested that the subduction of very large seamounts (e.g. the Louisville ridge in the Tonga-Kermadec Trench) could fracture the surrounding oceanic crust.

Geophysical data recorded during oceanographic cruises allow detailed studies of the morphology and structure of convergent margins. Nevertheless, while seismic profiles succeed in resolving the geologic structure of the upper plate in relatively undeformed regions of a margin, interpretation becomes difficult in regions where seamount subduction occurs. In such areas, the accretionary wedge and the frontal margin are so strongly deformed that the internal structure is poorly resolved and very difficult to interpret.

Analog Modeling

To study the different stages of seamount subduction, we performed classical sandbox experiments to investigate in detail the evolution of deformation both in space and time. Objectives are to better understand the deformation mechanisms and to improve our interpretations of seismic profiles and margin morphologies. 

Swath-bathymetric data from the Costa Rica and Ryukyu active margins reveal detailed surface deformation of the margin above subducting seamounts and ridge. Shaded perspective views highlight the detailed structure of the seafloor and compare well with surface deformation observed in the sandbox experiments.

The indentation of the margin by the seamount inhibits frontal accretion and produces a large V-shaped re-entrant. The margin uplifts along seaward dipping backthrusts initiated from the base of the landward seamount flank. A transtensive slip-line network propagates landward in response to the indentation and uplift of the margin. At depth, a thin shielded zone develops on the landward flank of the asperity. It gradually disappears as the seamount subducts.

In the wake of the seamount, out-of-sequence forethrusts, deflected upward by the landward flank of the subducting relief, create a stress shadow zone. Consequently, a section of the frontal margin detaches and follows behind the subducting seamount. At the final stage, normal faults, geometrically controlled by the shape of the seamount, develop in the subsiding wake of the asperity, triggering submarine landslides.

Learn more:

-> Dominguez S., Malavieille J. & Lallemand S.E., 2000. Deformation of margins in response to seamount subduction – insights from sandbox experiments; Tectonics, 19, n°1, 182-196, https://doi.org/10.1029/1999TC900055.

-> Dominguez S., Lallemand S.E., Malavieille J. & Schnürle P., 1998b. Oblique subduction of the Gagua Ridge beneath the Ryukyu accretionary wedge system: Insights from marine observations and sandbox experiments, Marine Geophysical Researches, 20, 5, 383-402, https://doi.org/10.1023/A:1004614506345.

-> Dominguez S., Lallemand S.E., Malavieille J. and Von Huene R., 1998a. Upper plate deformation associated with seamount subduction, Tectonophysics, 293, 207-224, https://doi.org/10.1016/S0040-1951(98)00086-9.

See also:

Dominguez S., Lallemand S.E., Malavieille J. and Pouclet A., 1995. Nouvelle interprétation structurale du mont sous-marin Daiichi-Kashima et essai de reconstitution géodynamique, C. R. Acad. Sci. Paris, 320, IIa, 403-409, PDF.

Lallemand S.E., P. Schnurle, and J. Malavieille – Coulomb theory applied to accretionary and non-accretionary wedges – Possible causes for tectonic erosion and/or frontal accretion, J. Geophys. Res., 99, B6, 12033-12055, 1994, PDF.