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Dykes are abundant in the Deccan Large Igneous Province, and those to the west are referred to as the ‘coastal swarm’. Most of the coastal swarm dykes appear in the Western Deccan Strike-slip Zone (WDSZ). Faults with N–S, NE–SW and NW–SE trends (brittle shears) have been reported in the WDSZ around Mumbai. However, details of their relationships with Deccan dykes, which can easily be studied at sub-horizontal outcrops, have remained unknown. Previous authors have classified dykes in the WDSZ according to their isotopic ages as group I (c. 65.6 Ma), group II (c. 65 Ma) and group III (64–63 Ma). Dykes have also been categorized on the basis of field observations; group I dykes were found to pre-date deformation related to the separation of Seychelles and India, whereas group II and III dykes post-date this event. Our field studies reveal group I dykes to be faulted/sheared and lacking a uniform trend, whereas group II and III dykes have approximately N–S, NW–SE and NE–SW trends and intrude brittle shears/fault planes. We have also found evidence of syn-deformation intrusion in the group II and III dykes: e.g. P-planes along the dyke margins and grooves in the baked zone of dykes. These two groups of dykes match the trends of dominantly sinistral brittle shears. Of the 43 dykes studied, only ten belong to group I, and we conclude that a large proportion of the dykes in the WDSZ belong to groups II and III. It is erroneous to interpret the Seychelles–India rifting as simple near-E–W extension at c. 63–62 Ma from the general approximately N–S trend of the dykes; the direction of brittle extension must instead be deduced from brittle shears/fault planes.

Supplementary material: Stereo plots and reduced stress tensors for all faults and brittle shears are available at

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