Geodynamic models of the opening of the northwest Indian Ocean

Recent seismic experiments showed that the northwest Indian Ocean opened in two phases of rifting. The first brief phase of rifting formed the failed Gop Rift at some time between 71 and 64 Ma. This was then followed by rifting and sea-floor spreading between the Laxmi Ridge and the Seychelles, which is well-dated by magnetic anomalies at 63.4 Ma. Both of these rift events occurred around the time of the formation of the Deccan flood basalts (65± 1 Ma), so it would be expected that both pairs of rifted margins would show signs of excessive magmatism. However, this is not the case. Whilst the Gop Rift margins show modest excess magmatism, with underplating and thickened oceanic crust, the Seychelles-Laxmi Ridge margins do not, rather they show thin oceanic crust. To explore the reasons for the unexpected crustal structure, we employ a geodynamic model of rift evolution where melt volumes, seismic velocity of the lower crust and rare-earth element (REE) chemistry of the melt are estimated. The observed excess magmatism at the Gop Rift is consistent with a model where the margin opened at 71 Ma, ~ 6 Myrs prior to the Deccan, with half spreading rates of 80 mm yr-1. The opening occurred above a warm mantle layer (Δ T=200°C, thickness of 50 km) that we interpret as incubated Deccan plume material, which had spread laterally. This scenario also recreates the observed lower seismic velocities of 7.4 km s-1 and 12 km igneous crustal thickness. The model indicates that when the rifting migrated to the Seychelles Laxmi Ridge at 63 Ma, the thermal anomaly was greatly reduced. To match the observations of 5.2 km thin oceanic crust, low 6.9 km s-1 seismic velocities and flat REE profile, melting likely occurred above a mantle that became depleted by the prior Gop Rift event. As well as its significance for the evolution of the northwest Indian Ocean, our work shows that small changes in the composition and temperature of the mantle can cause dramatic changes in melt production during continental breakup. The Seychelles Laxmi Ridge margins have previously been assumed to be magmatic. We have however found that if the mantle was depleted, then despite the apparent proximity of this margin to a thermal plume, thin oceanic crust can be generated.