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.

Abstract The northern part of the western continental margin of India formed due to the separation of the Seychelles from India at c. 63 Ma. This produced offshore tectonic elements such as the Gop Rift, the Saurashtra Volcanic Platform (SVP) and the Laxmi Ridge, as well as numerous seamounts, e.g. the Raman and Panikkar seamounts. The Laxmi Ridge and the Laxmi Basin have been studied using high-resolution 2D reflection seismic data and well data. Patch and pinnacle carbonate reefs, indicating shallow waters, are common in the north, whereas large, isolated platforms are usually noted in the south. Palaeo-depth estimates are made from well biostratigraphy. Subsidence studies of the SVP suggest that the burial history is consistent with the anomalously hot Réunion plume. We have performed a subsidence analysis south of the SVP on the Laxmi Ridge and Laxmi Basin. The sediment-unloaded basement depths, estimated using using flexural isostasy with effective elastic thicknesses of 10–40 km have been found to be 2000–4000 m in areas where carbonates exist. These carbonates indicate <200 m bathymetry at c. 65 Ma, and the subsidence discrepancy is thus due to thermal cooling or anomalous heating due to the Deccan plume. Patch and pinnacle reefs in the north suggests that either the rise in sea-level or the rate of subsidence of the basement were fast. The presence of large platforms in the south indicates otherwise. This is possibly due to a greater influence from the Indus Fan sediments towards the north. In addition, the Laxmi Ridge is a spreading centre that remained emergent near or above sea-level due to plume support, which was also greater in the south due to proximity to the plume. When the plume support discontinued, the ridge subsided quickly to present-day depths, which matches the subsidence expected for 60–70 Myr old oceanic crust. Supplementary material: A table is available at https://doi.org/10.6084/m9.figshare.c.3470751