Abstract:
Mafic dyke swarms, when controlled by a regional stress field, are emplaced perpendicular to the minimum principal compressive stress direction. However, mafic dyke swarms may also exploit pre-existing structural fabric in their host rocks. Distinguishing between the two scenarios is important for correct geodynamic understanding of magmatic events. Continental flood basalt provinces such as the Deccan Traps of India represent huge fissure eruptions produced by swarms of mafic dykes. The Coastal dyke swarm of the Deccan, along the western Indian rifted margin, is one of its three major dyke swarms. We present a field study of the Goa dykes, part of the Coastal swarm. The dykes are typical dolerites and basalts exposed in Proterozoic-age basement; lava flows which these dykes may have once fed have been stripped away by erosion, though exposed in the Western Ghats escarpment approx. 100 km to the east. The Goa dyke swarm contains 60 dykes, some of which form larger multiple-injection dykes. The dykes are short compared to typical (kilometers or tens of kilometers long) dykes in flood basalt provinces such as the Deccan or Iceland, and range in length from approx. 15 to approx. 220 m and in thickness from 3 cm to 20 m. Most dykes are vertical and the others dip steeply in various directions. Their structural attributes and internal features (such as vesicular zones and columnar jointing) provide clues to the processes of magma flow and solidification in the dykes. Field measurements yield an average crustal dilation of 8 percent due to dyke emplacement. Calculations of magmatic overpressures and magma chamber depths generally yield unrealistic values owing to the low aspect ratios of the dykes. Based on comparisons between the dyke trends and the host rock joint trends, we consider that a few of the Goa dykes were emplaced along pre-existing fractures, but the orientations of the great majority of the dykes reflect contemporaneous stress fields. Based on crosscutting relationships, we infer that the regional minimum horizontal compressive stress (sigma 3) changed over time from a approx. E-W orientation to a approx. NE-SW orientation and finally to a approx. N-S direction.
