The Askot Crystallines

The metamorphics and associated granitic rocks of the Askote Crystallines (= Didihat Crystallines, form a very asymmetrical syncline with a flat thrust to the South and a steep to vertical northern border (see map).  They are surrounded and underlain by the younger Berinag Formation of the Inner Sedimentary Belt.

The Berinag Formation, consisting of quartzites, sericite-­quartz schists interbedded with chlorite schists and amphibolites, has been compared with the Nagthat and Jaunsar Series of the Garhwal and Punjab Himalaya, and with the Kaimur Formation of Upper Vindhyans in the peninsular India.

The contact of the Crystallines with the Berinag Quartzites has been considered a thrust contact.  Some workers put forward the view that the contact is normal and the Crystallines constitute an autochthonous unit and have not been transported from the Central Axial Crystalline Zone.  Retrogressive metamorphism in the form of cataclasis, mylonitization, chloritization of garnet and biotite near the contact zone, and the restriction of metamorphosed basic sills (amphibolite and chlorite schists) within the Berinag Quartzites, however, speak volumes in favour of a tectonic plane separating the two groups, which is hereby referred to as the Askote Thrust.

On a regional basis the Askote Crystallines are folded into a large synform together with the surrounding sedimentaries.  The Crystallines occupy the core of the syncline, which is very asymmetrical and isoclinal in nature with a flat thrust to the south and a steep to vertical (locally overturned) northern flank.  The synclinal axis roughly trends NW-SE with minor variations up to WNW-ESE.  This axis is itself folded imparting the syncline a double plunge.  The exposures of the Crystallines die out towards east and west because of the double plunge of the synclinal axis towards its core.

A large mass of tourmaline-bearing muscovite-alkali feldspar gneiss forms the bulk of the Crystallines.  Dating by the Rb-Sr method gives an age of 1960±100 m.y. for these granitic gneisses.  This age agrees, within limits of experimental error, with the age of the Central Crystallines, and hence supports the view that the Askote Crystallines constitute a nappe of the Central Crystallines.  Along the contact with the Crystallines the Berinag Quartzites are highly sericitic.  Dioritic sills, more or less altered to amphibolite, are generally interbedded with the Quartzites near the Crystalline-Quartzite contact.  Riddled with secondary silica veins, the occurrence of amphibolites is of much significance and may possibly be related to the thrusting event of the Crystalline mass.   The occurrence of huge lenticles of quartzite within the lowermost horizon of the Crystallines and also in the amphibolite, is noteworthy.  Megascopically and microscopically these quartzites bear close similarity with the Berinag Quartzites, and are possibly torn off pieces of the underlying unit picked up during overthrusting of the Crystalline mass.

The southward movement from the Central Crystalline Zone in the form of a nappe as suggested by Heim and Gansser, (1939), Gansser (1964), and Valdiya (1978) may be responsible for the suprasedimentary disposition of the Askote Crystallines, which may be described as a recumbent nappe, only the inverted limb of which is now exposed.

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S. Farooq

Department of Geology

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email: farooqs@sancharnet.in