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Abstract

This paper studies the magma-rich Gop Rift–Laxmi Basin, West India, which underwent the mantle first–crust second break-up mode. It draws from reflection seismic and gravity data from this abandoned system. Seismic images document that the crustal necking was associated with the development of seawards-dipping reflector wedges deformed by landwards-dipping detachment faults. A wide crustal necking zone indicates that the ductile lower crust was still present during necking. Observed uneven detachment fault spacing indicates the effect of upper-crustal anisotropy. Comparison of the seismic images through progressively more mature stages of the rift–drift transition documents that the final stages of thinning represented the time period when the upper-crustal wide and symmetrical rift architecture changed to the asymmetrical one, and the decoupled system to the coupled one. It further indicates that the last crustal layer was broken with a convex-up fault that was associated with an excess magmatic event. The fault propagation represented the first spontaneous deformation unaffected by the pre-existing anisotropy. Subsequent drift of the two plates was associated with melt-assisted spreading and spontaneous faulting. The faulting geometry and sequence controlled which of the conjugate margins ended up with a volcanic outer high, representing the record of the break-up-locating excess magmatism.

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