Tuesday, September 9, 2014

Remotely India # 7: Mesozoic Domes Of The Kachchh Basin

Remotely India # 7

A recent paper in Current Science (open access) on the geomorphology and tectonics of domal structures in Mesozoic strata prompted me to resurrect this old series titled Remotely India. In these posts I put up a satellite image(s) of an interesting geological feature somewhere in India followed by a brief explanation.

Today's post is on the Kachchh basin in western India. In Mesozoic times this region was a long lasting marine basin in which hundred's of feet of sediments accumulated. Occasionally the sea withdrew and fluvial and lacustrine deposits formed. But most the sedimentary sequence represents marine conditions. Over much of the region the sedimentary strata are horizontal to very gently dipping in disposition indicating very little tectonic disturbance of the basin since the Mesozoic.

At places though these Mesozoic sedimentary rock has been upwarped into domes. These appear as a linear series of blisters in the landscape. The observation that they occur in a linear arrangement is significant and geologists have understood that these blisters and domes occur along mostly east-west trending faults. Map below show the geomorphological and tectonic elements of the region. Red cross is the town of Bhuj.

Source: Kachchh Mesozoic Domes, Current Science 

And this image below is of the Habo dome near the town of Bhuj.

Observations reveal that these domes show an antiformal structure with diverging dips i.e. beds (strata) bending or dipping outwards from the core area.

Here is a series of  domes (Dudhai domes) east of the town of Bhuj.

How did these structures form? Geologists reasoning is as follows: a) The domes don't have corresponding basin like structures, so they did not form through compressional forces buckling the crust into swells and depressions b) the domes occur in Mesozoic rocks on the uplifted blocks of faults but they are not found on the other side of the fault blocks. This suggests that the domes are not "drape folds" formed by vertical movement and adjustment of the sedimentary blanket during faulting. Also supporting the view that faulting did not form the domes is the observation that at places the domes are truncated by faults indicating that this type of structural disturbance occurred much later than the formation of the domes.

Image below shows a truncated dome north of the town of Bhuj

c) there is a close association of magmatic bodies of mafic and ultra-mafic composition with the domes. In some domes plug like magmatic bodies occur in the core of the domes. At places small dykes and stringers of magma are seen to dart from the main magmatic body into the sedimentary beds. This indicates that the magmatic bodies intruded the sedimentary rocks. Based on this the best explanation geologists feel is that these domes formed when magma rose through the crust and impinged the base of the Mesozoic sediments causing bending and doming of the rocks.

When did this occur?  Since Mesozoic times as Gondwanaland split apart into fragments forming the western margin of India, the Kachchh region has been subjected to extensional forces which resulted in several rift type basins forming there. East West trending faults and fracture systems are typical structural elements of this region since the mid Mesozoic.  Radiometric dating says the magmatic intrusions into these sedimentary rocks took place about 68 mya to 64 mya (million years ago).  This was just before and overlapping the Deccan volcanism in the Late Cretaceous which suggests that during magmatic episodes molten material was channelized along older fracture and fault systems of the Kuchchh region resulting in a localization of the domes along faults.

Topographic features, the undulations and swells and mountains that wrinkle the earth's crust form in the variety of ways. Classic orogenic mountains like the Himalayas form when tectonic plates collide and deform and thicken and lift up the earth's crust along thrust faults. Prolonged explosive outpouring of lava can form the beautiful yet deadly mountains like Vesuvius, Mount St. Helens and Mount Fuji. Yet, quieter outpouring of lava like the one experienced during Deccan volcanism can form enormous piles of lava. This pile can later be affected by gentler vertical movements of the crust coupled with differential erosion, forming a landscape of cliffs and gorges as seen in the Western Ghats... and sometimes magma ascends and pushes the crust upwards to form blisters like the domes of Kachchh. Coincidently, David Bressen has a blog post on the history of the idea of plate tectonics. Before this theory explained the origin of large mountain chains, the prevailing view was that mountains are blisters and bumps formed by magma pushing the crust from underneath..!!

Interactive Map:

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