Dynamic implications of drowned reefs and raised coastlines along the Bismarck volcanic arc

Dynamic implications of drowned reefs and raised coastlines along the Bismarck volcanic arc Raised and drowned reefs reveal the distribution of uplift and subsidence along the north coast of New Guinea and the island volcanoes of the Bismarck volcanic arc. Extinct volcanic islands in the Bismarck volcanic arc are fringed by well developed coral reefs. Drowned reefs offshore from these islands provide evidence for subsidence in the central section of the arc, north of the Finisterre Range, which has been colliding with the Australian continent since the Pliocene. Bathymetric and backscatter data collected from R/V Kilo Moana in 2004 reveal regularly spaced drowned reef terraces and drowned atolls at up to 1100 meters below sea level. The interval between terraces varies from about 200 meters around Bagabag Island to the west, to about 100 meters around Umboi Island to the east. Across-arc variations are poorly constrained because of the narrow width of the arc. However, the adjacent mainland coast has well documented raised reefs indicating long-term uplift. Farther from the Finisterre Range, islands show evidence for either constant sea level or slight uplift. Supposing that each terrace represents drowning of a lowstand reef during postglacial sea level rise, then roughly uniform subsidence rates of 1 to 2 mm per year can be inferred, decreasing from Bagabag to Umboi. Subsidence may be due to cessation of magmatic activity and cooling, flexural loading by the uplifting Finisterre Range, and/or sediment loading on the seafloor north of the Finisterre Range.