Information on the host rocks

 

Petrography

Petrochemistry
Rock Alteration

 

Sulfide Mineralization

Structure of The Askot Crystallines

Systematic mapping and field observations around the Askote area revealed that various planer and linear structural elements are superimposed upon each other.  Structural studies are best made in schistose rocks, for the granitic ones have mostly reacted, particularly to later tectonic movements, by brittle fracture, and hence the imprint of the tectonic activity is but vague.  Migmatites and associated high-grade metamorphic rocks are considered to be products of deep-seated conditions and must have been quite hot and plastic, flowing beneath the overlying low-grade rocks during regional metamorphism.  Upon cooling they become more rigid than the low-grade rocks and thus deform in a different manner.  The structural studies are based on field observations of megascopic and mesoscopic structures, and on microscopic structures studied in oriented thin sections.  It is observed that microscopic structures are genetically related with the megascopic and mesoscopic structural elements of the Crystallines and are their microscopic versions.

The earliest deformation of the Askote Crystallines was the development of the bedding cleavage (S0 due to the weight of the overlying sediments.  This cleavage is largely obliterated by later structural elements and its sole relicts are found in the carbonaceous shaly bands that occur as boudins in the low-grade schists.  Preserved in the Crystallines are structural elements of at least three generations, which are related to the major tectonic episodes to which the rocks have been subjected.  The earliest compressive movements folded the rocks into numerous folds (referred to as F1 ) trending NW-SE.  This folding episode is most remarkable and widespread, being represented by numerous large scale and reclined folds in quartzitic bands within the schistose rocks.  The main schistosity (S1 marks the axial plane cleavage of the F1 folds and constitutes the most pervasive and well developed of all planer structures present.  Continued compressions culminated in dislocation and large scale southward thrusting of the Crystallines over the younger Sedimentaries.  The shearing accompanied with the thrusting of the Crystallines over the Sedimentaries locally produced a strong shear cleavage (S2) in the schistose rocks near the Askote Thrust.  S1 has largely been obliterated where S2 is developed.  Drag folds F2 trending NW-SE were formed during thrusting and are restricted to the low-grade schists (fig. 1).

Fig. 1:  The S1 schistosity forming the axial plane cleavage of F1 folds thrown into numerous drag folds F2 trending NW-SE.  Note that the shear cleavage S2 makes low angles with S1.

The Crystallines, occurring in the form of an overturned sequence over the Sedimentaries, were later folded into a broad, asymmetric, syncline on a WNW-ESE axis.  During this folding the rocks developed extensive open folds (F3) of mesoscopic scale trending WNW-ESE with amplitudes ranging from 2-10 m and open microfolds also trending WNW-ESE.  The F3 folds are superimposed upon the F1 folds.  The S3 schistosity, forming axial plane cleavage of the F3 folds is pervasive but not as well developed as the S1 planes.

Locally S3 makes low angles with the S1 cleavage (fig. 2).  The synclinal axis was subsequently folded gently on a NNE-SSW axis giving the WNW-ESE trending synform a double plunge.  

Fig 2:  The S3 schistosity forming the axial plane cleavage of F3 folds locally makes low angles with S1 schistosity which has been folded on a WNW-ESE axis.

This folding was accompanied by slight rotational movements that resulted in a shearing of the schistose rocks along planes trending NNE-SSW and dipping steeply towards east.  A strong shear cleavage S4, restricted to the shear zones, is imprinted on all other structural features and is hence the youngest structural element.  The shearing also gave rise to tiny chevron folds, homoclinal kinks, and microscopic drag folds F4 (fig. 3) trending in a NNE-SSW direction and plunging towards NNE.

Fig 3:  Microscopic drag folds F4 associated with the S4 shear cleavage in the schistose rocks near Barigaon.  The direction of movement can be easily deciphered.




On the basis of small-scale structures Schwan (1980) has postulated five phases of deformation in  the Himalayan region.  The F
1, F2 and F3 deformations described herein correspond well in respect of direction and chronological sequence with the D3, D4 and D5 deformations of Neogene age in the Himalaya (Schwan, 1980).  The D1 and D2 deformations of Precambrian or Paleozoic age have, however, not been identified.  The inferences as regards the chronology of structural elements are in fair agreement with those of Vashi and Laghate (1972) for the rocks of the Almora Crystallines.

 

Notes & Handouts

The Himalayas

Kumaon Himalayas

Askot Basemetals

University

   


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