The Finisterre Mountains and western Solomon Sea of northern Papua New Guinea are the site of an active, oblique arc-continent collision. Comparison of structures along the length of the collision zone provides a history of its structural evolution. Here we compare the results of structural mapping in the Finisterre Mountains, where collision is in an advanced state, with side scan sonar images and potential field data from the western Solomon Sea, which illuminate the structure of the juvenile collision zone. An accretionary wedge complex, composed of a southwestward younging stack of imbricate thrust sheets, developed along a westward extension of the modern New Britain Trench prior to collision. This accretionary wedge crops out in the Finisterre Mountains as the Erap Structural Complex. Consequences of collision were doubling of the crustal thickness to 50--52 km, rapid uplift of the Finisterre terrane, and initiation of a major out-of-sequence thrust that displaced the Finisterre Volcanics over younger sediments of the accretionary wedge. This out-of-sequence thrust is mapped in the Finisterre Mountains as the Wongat Thrust, and it is recognized in the western Solomon Sea on side scan sonar images. A second generation out-of-sequence thrust formed after substantial uplift had subaerially exposed the collision zone. This thrust crosscuts the Wongat Thrust, forming a window of accretionary wedge sediments bounded to both the NE and SW by Finisterre Volcanics. This younger out-of-sequence thrust is the Gain Thrust of the Finisterre Mountains.

At least 15 km of crustal shortening has occurred across the Wongat Thrust at 146¿30'E longitude, and the Gain Thrust has accommodated up to 7.4 km of shortening in the same area. It is probable that the well-lithified Finisterre Volcanics serve as the backstop for the accretionary wedge/collision complex that is forming in the collision zone of the western Solomon Sea. Potential field models indicate that this backstop is arcward dipping. Thrusting of the backstop material over the accretionary wedge, as we observe along the Wongat Thrust, is expected for wedges with arcward dipping backstops. A series of NE-SW trending cross faults, perpendicular to the dominant structural trend, exist in the collision zone. These faults have components of left-lateral strike-slip and SE down dip-slip motion. It is likely that the Finisterre terrane is being segmented along these faults, with the segmentation caused by differential resistance to convergence between the downgoing continental crust of the collision zone and the oceanic crust of the subduction zone to the east. The origin of the faults is uncertain. Previous authors believed that they formed during Paleogene volcanic activity in the Finisterre terrane. The colinearity of several of these cross faults with major mapped faults on the downgoing plate raises the possibility that the Finisterre Range cross faults are upper plate manifestations of lower plate structures and, as such, reveal strong interplate coupling in the collision zone. ¿ American Geophysical Union 1994