Evaluating magmatic additions at a magma-poor rifted margin: an East Indian case study

Rifted margins are often classified as magma-poor or magma-rich based on a magmatic budget interpretation from seismic reflection data. The southern segment of the East Indian rifted margin is often regarded as a type-example of a magma-poor margin displaying exhumed mantle. However, in its southern segment, 9 km thick transitional crust, previously interpreted as magmatic crust, separates the exhumed mantle from thin oceanic crust. Such thick transitional crust is atypical for a magma-poor margin, so we investigate its likely formation and potential implications for the evolution of magma-poor margins. Using an integrated set of geophysical techniques alongside seismic reflection data, we test the existence of exhumed mantle and the composition of the transitional crust. These geophysical techniques consist of gravity inversion, residual depth anomaly analysis, flexural subsidence analysis and joint inversion of gravity and seismic data. We apply these methods to high-quality seismic reflection data (ION line INE1-1000) on the southern segment of the East Indian rifted margin and test a series of geological scenarios for the margin structure using our integrated quantitative analysis. Of these, our quantitative analysis, seismic observations and the regional plate kinematic history support a structure consisting of thinned continental crust inboard of exhumed, serpentinized mantle followed by thick (similar to 9 km) magmatic crust transitioning into thin oceanic crust (similar to 5 km). The juxtaposition of exhumed mantle and thick magmatic crust is explained by the occurrence of a jump in seafloor spreading during the Early Cretaceous formation of the south-east Indian Ocean. The final rifted margin structure contains characteristics of both magma-poor and magma-rich rifted margins resulting from two distinct rift events with different magmatic budgets. The investigation of the East Indian rifted margin structure and evolution shows the importance of incorporating the plate kinematic history and quantitative validation of seismic interpretation into the analysis. Classifying the East Indian margin as a typical magma-poor rifted margin is misleading causing us to question the use of end-member terminology to describe rifted margins.