The origin of seamount volcanism in the northeast Indian Ocean

The Northeast Indian Ocean formed during the breakup of Australia and India at 155 Ma, and spreading continued along the Wharton Ridge until 43 Ma. The region encompasses the Christmas Island Seamount Province, which comprises of at least ~50 seamounts distributed over a region of 2000 km east-west by 500 km north-south. In addition to the submerged seamount, the seamount province also contains two sub-aerially exposed island groups, the Cocos (Keeling) Archipelago, and Christmas Island. The evolutionary history of the intraplate region and the oceanic-continental margins surrounding the seamount province has been constrained by interpretation of magnetic and gravity anomalies, ocean drilling, and dredged samples. Recent work in the region has suggested the Christmas Island Seamount Province contains a recycled continental lithosphere component. However the ultimate formation mechanism for this disseminated seamount province is not well understood. This study undertakes a geophysical investigation of the area, including gravity modelling, oceanic subsidence modelling, and flexure modelling, to constrain the lithospheric characteristics, and loading and subsidence ages of these submerged seamounts. We coupled our observations with paleomagnetic analysis, and integrate our results with seismic tomography models, and plate reconstructions. The study also undertakes a geochemical and ⁴⁰Ar/³⁹Ar geochronological analyses on the volcanic rocks from Christmas Island to constrain source characteristics, and the duration and age of volcanism at Christmas Island. The crustal structure of the Cocos (Keeling) Islands and Christmas Island, constrained by gravity modelling, suggests a ~2100 m thick limestone cover overlays the submerged basaltic surface at Cocos (Keeling) Island. Modelling has revealed a deeper basaltic core underneath the centre of the islands, followed by a 5 - 7 km thick gabbro layer, extending down to depth of 15 - 18 km. The last exposure ages calculated from age-depth subsidence modelling highlights that the older seamounts formed in the east, and seamount ages young towards the west, and this agrees with the seafloor ages. The last exposure ages of these seamounts are comparable with radiometrically determined ages by Hoernle et al. (2011), while the loading ages suggests they formed on very young oceanic crust. ⁴⁰Ar/³⁹Ar geochronology has yielded an Eocene (43 - 37 Ma) and a Pliocene (5 - 4 Ma) age for Christmas Island. Although no geochronological dating exists for the oldest volcanic core of the island, this study has been able to propose a loading age of the island between 88 – 75 Myrs using lithospheric flexural modelling. Paleomagnetism of Christmas Island basalts have suggested a paleolatitude between −43.5°−+9.0°/11.2° , suggesting a position further south than Christmas Island's reconstructed position in the Eocene (~30°S), and a paleolatitude of 13°S ± 11° for the volcanic activity in the Pliocene. The Pliocene position of the island is consistent with plate reconstructions models and backtracking of the island using plate motions rates. Seismic tomography models support a low velocity zone underneath the reconstructed position of the seamount province, which may have been an important component in the formation of the seamount province. These tomographic models also conform with upwelling mantle flow models calculated in the East Indian Ocean. Geochemical analyses of volcanic rocks from Christmas Island have a trace element pattern that is similar to OIB’s. Intermediate Pb (⁽²⁰⁶Pb/²⁰⁴Pb, 18.7874-18.9725 and ²⁰⁷Pb/²⁰⁴Pb,15.5585-15.6417) and Nd (¹⁴³Nd/¹⁴⁴Nd, 0.512684-0.512798) isotopic ratios for the Eocene phase of volcanism point towards an enriched (EM-2) type composition that is inherited from continental (or recycled sediment) contamination. In addition, the Eocene phase has a distinct Indian Ocean DUPAL anomaly signature. This suggests a mixing of sources contributed to the volcanics, including a lower-mantle DUPAL contribution, and an upper mantle continental lithosphere signature - a possible remanent of Gondwana dispersal. The Pliocene phase of volcanism on Christmas Island is a product of melt rising through flexure-induced cracks, produced in response to changing lithospheric stresses at the flexural fore-bulge as the lithosphere subducts. The Pliocene phase has sampled both EM-1 (from Hoernle et al.,2011, ²⁰⁶Pb/²⁰⁴PbPb, 17.8498-17.8692 and ²⁰⁷Pb/²⁰⁴Pb, 15.4550-15.5481) and EM-2 (this study) type composition suggesting that older material is still present underneath the lithosphere.