Multigeneration mafic dyke-swarm related to Deccan magmatism South of Bombay: Implications on the evolution of the Western Indian continental margin

Abstract

The mafic dyke-swarm intrusive into Deccan basalts, south of Bombay straddles the "Panvel Flexure" and comprises dykes belonging to four generations: (i) An E-W system of thick dykes (2.5 to 40m wide) having gradational contacts with the lavas (ii) an intense N-S pre-"flexure" system of dykes dipping 60 to 80 degrees east (iii) an alkaline zig-zag en echelon, NNW to NNE trending, vertical syn-to post-"flexure" dykes and (iv) felsic dykes and plugs which show evidences of magma mixing. The "Panvel Flexure" is constituted of blocks of lava flows tilted towards west along easterly dipping normal faults which have produced variation in the dip of the lava. The "flexuring" is the result of foundering of stretched and thinned continental lithosphere following a period of massive magmatic discharge preceding rifting of India from Seychelles Bank. The lava pile attains maximum thickness in the vicinity of the "flexure" region diminishing both towards the east and the west. The present elevation of the Deccan plateau is associated with subsidence in the axial region of the plume attendant on thermal relaxation of the stretched lithosphere.