ALBERT

Description: 〈span〉〈div〉SUMMARY〈/div〉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 (∼9 km) magmatic crust transitioning into thin oceanic crust (∼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.〈/span〉

Description: 〈p〉Rifted margins are commonly defined as magma-poor or magma-rich archetypes based on their morphology. We re-examine the prevailing model inferred from this classification that magma-rich margins have excess decompression melting at lithospheric breakup compared with steady-state seafloor spreading, while magma-poor margins have inhibited melting. We investigate the magmatic budget related to lithospheric breakup along two high-resolution long-offset deep reflection seismic profiles across the SE Indian (magma-poor) and Uruguayan (magma-rich) rifted margins.〈/p〉 〈p〉Resolving the magmatic budget is difficult and several interpretations can explain our seismic observations, implying different mechanisms to achieve lithospheric breakup and melt production for each archetype. We show that the Uruguayan and other magma-rich margins may indeed involve excess decompression melting compared with steady-state seafloor spreading but could also be explained by a gradual increase with an early onset relative to crustal breakup. A late onset of decompression melting relative to crustal breakup enables mantle exhumation characteristic of magma-poor margin archetypes (e.g. SE India).〈/p〉 〈p〉Despite different volumes of magmatism, the mechanisms suggested at lithospheric breakup are comparable between both archetypes. Considerations on the timing of decompression melting onset relative to crustal thinning may be more important than the magmatic budget to understand the evolution and variability of rifted margins.〈/p〉

Description: 〈span〉〈div〉Summary〈/div〉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 (RDA) 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 (∼9 km) magmatic crust transitioning into thin oceanic crust (∼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.〈/span〉

Description: Rifted margins are commonly defined as magma-poor or magma-rich archetypes based on their morphology. We re-examine the prevailing model inferred from this classification that magma-rich margins have excess decompression melting at lithospheric breakup compared with steady-state seafloor spreading, while magma-poor margins have inhibited melting. We investigate the magmatic budget related to lithospheric breakup along two high-resolution long-offset deep reflection seismic profiles across the SE Indian (magma-poor) and Uruguayan (magma-rich) rifted margins. Resolving the magmatic budget is difficult and several interpretations can explain our seismic observations, implying different mechanisms to achieve lithospheric breakup and melt production for each archetype. We show that the Uruguayan and other magma-rich margins may indeed involve excess decompression melting compared with steady-state seafloor spreading but could also be explained by a gradual increase with an early onset relative to crustal breakup. A late onset of decompression melting relative to crustal breakup enables mantle exhumation characteristic of magma-poor margin archetypes (e.g. SE India). Despite different volumes of magmatism, the mechanisms suggested at lithospheric breakup are comparable between both archetypes. Considerations on the timing of decompression melting onset relative to crustal thinning may be more important than the magmatic budget to understand the evolution and variability of rifted margins.