Dyke–brittle shear relationships in the Western Deccan Strike-slip Zone around Mumbai (Maharashtra, India)
Achyuta Ayan Misra, Soumyajit Mukherjee, 2017. "Dyke–brittle shear relationships in the Western Deccan Strike-slip Zone around Mumbai (Maharashtra, India)", Tectonics of the Deccan Large Igneous Province, S. Mukherjee, A. A. Misra, G. Calvès, M. Nemčok
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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 https://doi.org/10.6084/m9.figshare.c.3259627
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Understanding the Deccan Trap Large Igneous Province in western India is important for deciphering the India–Seychelles rifting mechanism. This book presents 13 studies that address the development of this province from diverse perspectives including field structural geology, geochemistry, analytical modelling, geomorphology and geophysics (e.g., palaeomagnetism, gravity and magnetic anomalies, and seismic imaging). Together, these papers indicate that the tectonics of Deccan is much more complicated than previously thought. Key findings include: the Deccan province can be divided into several blocks; the existence of a rift-induced palaeo-slope; constraints on the eruption period; rift–drift transition mechanisms determined for magma-rich systems; the tectonic role of the Deccan or Réunion plumes; sub-surface structures reported from boreholes; the delineation of the crust–mantle structure; the documentation of sub-surface tectonic boundaries; post-Deccan-Trap basin inversion; deformed dykes around Mumbai, and also from the eastern part of the Deccan Traps, documented in the field.