Reflection profiles taken by the Hawaii Institute of Geophysics in 1970 and 1971 are the basis for a new bathymetric chart of the East Caroline and Lyra Basins. Bordered on the east and west by areas of high elevation, the Ontong Java Plateau and the Eauripik Ridge, respectively, the two basins are separated from each other by the Mussau Trough and Mussau Ridge and bounded by the Manus Trough on the south. An analysis of the reflection profiles shows that the rectilinear pattern evident throughout the region results from a set of northeast and northwest cross-trends that appear to be structurally controlled. Using the concept of an extinct sea-floor spreading center to explain the formation of most of the topographic features found in this region, these cross-trends become northeast traces of transform faults that intersect northwest-trending structural features originally developed along the axis of spreading. We interpret data obtained at Deep Sea Drilling Project (DSDP) sites 62 and 63 as evidence for a late Oligocene cessation of spreading. Subsequent motions of the Pacific and Indian plates resulted in strike-slip motion along the previously active transform faults, thus increasing offsets observed in the southern Eauripik Ridge and forming the grabens associated with the Kiilsgaard and other fracture zones.

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