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The mountain system in Papua New Guinea (PNG) is growing by accretion of Australian margin strata to the front of the Papuan fold‐and‐thrust belt in the south and by accretion of exotic terranes along its NE margin. Presently, the Finisterre Terrane is being accreted along the NE margin of PNG, and the collision point is migrating eastward within the Solomon Sea. East of the collision point the New Britain Trench marks the tectonic front between the Solomon Sea and South Bismarck Sea plates, whereas to the west the tectonic front lies in the southern part of the Papuan fold‐and‐thrust belt. Present‐day sedimentary environments within the western Solomon Sea are inferred from interpretation of seismic reflection data. Similar facies are observed accreted to the front of the Finisterre Terrane in the Markham valley. The frontal Leron Formation is largely fluvial and shallow marine, and the Erap Complex contains elements of turbidites and pelagic deposits. There appears to be a smaller volume of coarse, proximal marine strata accreted to the front of the Finisterre Terrane than is expected from interpretation of reflection profiles in the western Solomon Sea. This difference may be attributed to a combination of submarine erosion in the Markham canyon, partial incorporation of these deposits into the Erap Complex, and a difference in depositional rates and products carried to the Solomon Sea in the past relative to what is observed today. Based on plate motions, alternative collision geometries, and the youngest dates available for the Erap Complex, we determine a rate of progression of the collision point of about 212 km/m.y. during the past 1 m.y.

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 up‐lift 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 back‐stop 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 coll...

By studying seafloor morphology we can make associations between near surface deformation, fluid flow and the overall structural framework of accretionary prisms. In February, 1994 a DS/RV ALVIN program to the Costa Rica accretionary prism investigated the relationship of fluid seepage and sediment deformation by using the distribution of chemosynthetic communities and heat flow anomalies as indicators of fluid flow. The active normal faults that cut the hemipelagic section on the Cocos plate may provide conduits for fluids that cause the regional heat flow to be extremely low. These normal faults intersect the toe of the prism at an oblique angle, creating localized regions of increased deformation. Positive heat flow anomalies observed at the deformation front indicate diffuse fluid flow, however, we discovered no seep communities indicative of focused flow. The seaward‐most seep communities discovered are in a region of active out‐of‐sequence thrusts that cut a sediment apron which covers the complex to within 3 km of the prism toe. Vents occur consistently at the base of the fault scarps. Dives on a mud diapir show extensive seep communities, pock marks, and authigenic carbonates. Evidence of fluid release is on the crest which implies a low viscosity fluid migrating upward in the center of the structure. Normal faults on the upper slope can be seen in cross‐section in the walls of a submarine canyon. The faults cut the slope apron and displace the seafloor, actively maintaining the critical taper of the prism.