Surface friction of subducting seamounts influences deformation of the accretionary wedge

Abstract

Undulating topography on a subducting oceanic plate causes substantial deformation of the accretionary wedge. In particular, the subduction of a seamount is considered to induce 1) the development of landward concave topography such as reentrant and indented features, 2) compression within the accretionary wedge, and 3) disturbance of the primary thrust-sheet structures by uplift associated with the formation of multiple faults. However, according to previous seismic profiles, some subducting seamounts have provided suggestive evidence for weak wedge deformation rather than strong compression and uplift that were indicated in the previous model experiments. As the cross-sectional shape of an accretionary wedge is known to be strongly constrained by the internal and basal friction of the wedge, this study evaluates the effect of the frictional conditions of a subducting seamount on the deformation of the upper plate using analog experiments of two seamount models with low and high friction, respectively. Results show that subduction of a high-friction seamount causes continuous compression and uplift of the accretionary wedge, with the plate boundary fault zone composed of discontinuous multiple faults that offset vertically and partially overlapped to form a thick deformation zone. In contrast, a low-friction seamount is associated with a relatively thin wedge and a single plate boundary fault, which results in less compression compared with a high-friction seamount. During the subduction of a low-friction seamount, the accretionary wedge grows locally as the thickness of sediment on the seamount decreases, which has not been reported in previous studies. The wedge deformation pattern around the subducting Bennett Knoll Seamount in the Hikurangi Margin could be explained by the mechanism observed in the low-friction seamount subduction. From these results, we conclude that the frictional conditions on the seamount surface are a dominant control on the process of wedge deformation during the early stages of seamount subduction.