Constraining the age and origin of the seamount province in the Northeast Indian Ocean using geophysical techniques

The breakup of western margin of Australia from Greater India started around 155 Ma and progressed southwards. After the separation, the interceding intraplate region experienced large volumes of submarine volcanism, extending over 100 Myrs. The Christmas Island Seamount Province (CHRISP, as it has been dubbed) lies south of the Java-Sunda Trench, and contains numerous submerged volcanic seamounts, and two sub-aerially exposed island groups—Cocos (Keeling) Islands, and Christmas Island. While recent geochronological investigations have shed light on the diverse eruption ages of the volcanics of this region, some islands/seamounts have demonstrated protracted volcanic histories, and it is not clear how the volcanic loading, tectonic subsidence, and subsequent emergence history of the islands relates to these discrete volcanic episodes. This study utilises a number of geophysical techniques to determine the crustal structure, loading and subsidence history, and last sub-aerial exposure age for the CHRISP. The study shows that flexural and subsidence modelling are reliable techniques in constraining the age of the seamounts when geochronological techniques are not possible. Utilising regional gravity signatures, we model the crustal structure underneath the Cocos (Keeling) Island, and constrain the thickness of the limestone cover between 900 and 2,100 m. Using age-depth subsidence curves for oceanic lithosphere the time since these seamounts were exposed above sea-level was determined, and a trend in exposure ages that youngs towards the west is observed. Two episodes of volcanism have been recorded at Christmas Island and they are of different origin. The younger phase in the Pliocene is a manifestation of flexure induced cracks produced in the lithosphere as it rides the subduction fore-bulge, whereas a low velocity seismic zone rising from the lower mantle, and tectonic reorganization, may be associated with the older Eocene volcanic phase, as well as much of the rest of the province. Our modelling also supports the existence of an older, undated volcanic core to Christmas Island, based on the loading ages from flexural modelling.