The role of protothrusts in frontal accretion and accommodation of plate convergence, Hikurangi subduction margin, New Zealand

摘要

Protothrusts mark the onset of deformation at the toe of large subduction accretionary wedges. They are recognized in seismic reflection sections as small-displacement (tens of meters) faults seaward of the primary frontal thrust fault. Although assumed to reflect incipient accretionary deformation and to mark the location of future thrusts, few studies discuss their displacement properties, evolution, and kinematic role during frontal accretion and propagation of the subduction décollement. We analyze high-quality geophysical and bathymetric images of the spectacular 25-km-wide Hikurangi margin protothrust zone (PTZ), the structure of which varies along strike north and south of the colliding Bennett Knoll seamount. We provide a quantitative data set on protothrust scaling relationships and fractal fault population characteristics. Our analyses lead us to speculate on the importance of stratigraphic heterogeneity in structural development, and highlight the role of protothrust arrays in the formation of the frontal thrust. We document a migrating wave of protothrust activity in association with forward advancement of the décollement and deformation front. Shortening east of the present frontal thrust, calculated from displacements on seismically imaged faults and from subseismic faulting derived from power law relationships, reveal the significant role of the PTZ in accommodating shortening. There is possibly as much as ～7.4 km and ～4.0 km of shortening accommodated by the PTZ south and north, respectively, of Bennett Knoll seamount. As much as ～90% of the total shortening may be accommodated at subseismic scale. These data indicate that the active PTZ, together with older accreted PTZs, may accommodate ～10%–50% of the total margin-normal convergence rate at the Hikurangi margin.