Major lineaments and gravity-magnetic trends in Saurashtra, India

The lineament map of Saurashtra prepared from false colour, thematic maps indicates four major structural trends. The NE-SW trend reflects the Precambrian Aravalli trend which is dominant in the SE part of Saurashtra and largely represents the basic dykes and plugs exposed in this sector. The ENE-WSW to E-W trend represents the Precambrian trend of Narmada–Son lineament in southern Saurashtra, volcanic pipes of late Cretaceous in central Saurashtra and Gulf of Kutch and Kutch rift basin of Jurassic times, north of Saurashtra. The NW-SE trend parallel to the west coast of Saurashtra is possibly related to coastal tectonics which evolved during late Jurassic due to the break-up of Africa from India. The N-S to NNE-SSW trends prevalent in the eastern and the central parts of Saurashtra are parallel to the Cambay rift basin which evolved during late Cretaceous, due to interaction of the Reunion plume with the Indian lithosphère. It is significant to note that N-S trends occur in pairs, indicating fracture zones. The Bouguer anomaly map also reflects similar structural trends in different parts of Saurashtra, where individual trends are predominant compared to the others. Some of the N-S structural trends coincide with gravity gradients or linear gravity anomalies, indicating fracture zones/faults which may be important for groundwater exploration. Besides, the Bouguer anomaly map has also delineated six circular gravity 'highs' of 40–60 mGal over the volcanic plugs/stocks. The large wavelength gravity 'low' over the Jasdon plateau is partly caused by thickening of the crust due to isostatic compensation and a part of this anomaly may also be caused by some deeper sources. The total intensity magnetic map depicts well-defined pairs of magnetic lows and highs of approximately 500–1000 nT over the volcanic plugs, which belong to the Deccan eruption. The same order of gravity and magnetic anomalies observed over the volcanic plugs suggest almost similar bulk composition for them, although large variations in compositions are reported from the exposed rock types. It also suggests that there must be mafic/ultramafic components under all the pipes, though they are reported mainly from the Junagadh pipe.