The Malaita accretionary prism (MAP) along the northern flank of the Solomon Islands consists of an accreted wedge of pelagic sediment and oceanic plateau basalt offscraped from the actively subducting Ontong Java Plateau (OJP). Over 2000 km of 120-channel marine seismic data from 18 reflection profiles reveal five structural provinces defined on the basis of tectonic strike and structural style. Ulawa and Malaita structural zones consist of accreted basalt and early Cretaceous-Cenozoic sedimentary rocks from the OJP that are modified by seaward dipping normal faults. Ndai and Ramos transition zones are framed by NS- to NE-striking high-angle strike-slip faults and are characterized by a smooth transition from the OJP to the MAP. Santa Isabel structural zone is composed of several NE-verging to upright anticline-syncline pairs consisting of accreted basalt and pelagic sediment correlatable with the incoming OJP section. Anticlinal ridges plunge northwest to form the western edge of the MAP.

Differences in style and orientation of the provinces are correlated with the dip of the incoming OJP and depth of the basal décollement. The Ulawa structural zone is adjacent to the deepest part of the North Solomon Trench (NST) (~5700 m) and the steepest dip of the OJP (7.0 degrees). The steepness of the trench wall, the small active accretionary wedge, and the presence of normal faults may be attributed to the steeper dip of the downgoing OJP. The shallow sediment deformation at the toe-of-slope in the Malaita structural zone indicates a relatively shallow depth for the décollement. The Santa Isabel structural zone is adjacent to the shallowest part of the NST (~3300 m) and the shallowest-dipping area of the OJP (<2.0 degrees). The low-relief trench-wall, thick accretionary wedge, and long-wavelength folds may be attributed to shallow subduction and a thicker offscraped section of the OJP. The Ndai and Ramos transition zones exhibit elements from the other structural provinces and represent a tectonic transition between the areas of steeper-dipping OJP to the southeast and shallow dipping OJP to the northwest.

The MAP is expanding to the northwest as new blocks of upper crust and sediment are offscraped from the incoming OJP. Due to east-to-west convergence of the OJP, a NW to SE transect across the MAP is analogous to a temporal view of progressive oceanic plateau accretion. Upon arrival at the NST, the OJP resists subduction due to increased buoyancy related to its anomalous 35 km thickness. The interplate slip is transferred to a plane of weakness 9 km below the ocean floor in the OJP crust allowing the upper crust and sediments of the OJP to be accreted to the Solomon Island arc as well as the seaward transfer of the deformation front and NST. Continued shortening increases the amount of deformation within individual thrust sheets and causes arcward rotation of thrust faults. As the prism builds to a critical taper, continued convergence results in shoaling of the décollement for each new thrust sheet until only the uppermost crust and sediments are accreted.

Learn more about tectonic history of the Malaita Accretionary Prism

View free-air gravity map and hypocenter profiles of region

See seismic data examples from the Malaita Accretionary Prism

*Project funded by National Science Foundation Grant #OCE:9301608