DEFORMATION AND STRAIN PATTERNS OF THE CENTRAL HIMALAYAN METAMORPHICS FROM NORTHWESTERN GARHWAL

Abstract

The Himalayan Arc has been considered as a typical example of large-scale continental convergence where the Indian Plate with the Precambrian shield has collided along its northern boundary with the Eurasian Plate. It is interred that the rnetamorphic pile of the Central Himalaya constitutes a morphotectonic unit of maximum crustal shortening and uplift as a result of collision of the two plates. Therefore, deformational and strain patterns of this metamorphic belt may provide significant evidences for the mechanism of intracontinental crustal shortening. The area under investigation along the Tons-Supin-Rupin Valleys forms a part of Lesser and Central Himalaya in the NW-Garhwal in the districts of Uttarkashi and Rohru, Uttar Pradesh and Himachal Pradesh. Low grade metamorphosed quartzite of the Lesser Himalayan Garhwal Group is thrust over by predominantly gneissose and schistose metamorphic pile of the Central Himalaya along the Main Central Ihrust (MCT). This important intracont-nental boundary, though traceable in the Garhwal-Kumaon-Nepal Himalaya for considerable distance, is largely concealed beneath the Jutogh Thrust sheet of Himachal Himalaya in this area. The Jutogh Thrust imbricates the Lesser Himalayan sequence with the metamorphics of the Jutogh and Naitwar Groups. The lower metamorphic thrust sheet of the Naitwar Group may be considered as an extension of the Himalayan basement of 1900-2300 my from the Garhv.al-Kurnaon Himalaya. The polymetamorphosed and deformed Central Crystallines and Garhwal Group are affected oy four important phases of

deformation. The earliest Dj deformational phase has produced rarely noticeable tight to isoclinal Fj folds with long-drawn limbs and tightly appressea narrow hinges of little-known orient ation. These folds possess an axial plane foliation Sj which cuts across aplite veins. Sj parallels a lithological layering of metamorphic or sedimentary origin except in the hinge zones. As a result of D2 deformational phase, a later foliation S2 transposes the Sj into the most prominent planar structure. Litho logical layering, gneissosity or foliation Sj is involved in (probably coaxial) reclined to recumbent F2 folds. These folas persistently trend NNE to NE and parallel the stretching lineation L2 on S2 planes - an axial plane foliation to the F2 folds. The L stretching lineation is contemporaneously developed during the D ductile shear deformation, irrespective of the orientation of the S foliation. Peak of metaraorphisw during the D2 deformation is inaicated by syntectonic porphyroblastic kyanite, staurolite, garnet ar.a actinolite growths with simultaneous rotational fabric during the ductile shearing. Asymmetric pressure fringes around porphyroblasts and S and C relationships between the foliations support predominantly southward verging ductile shear during this deformation. During the b compressive deformational phase, the F3a folds on the earlier foliation but mainly on S2 are isoclinal to close with low to moderately NW-plunging fold axes. Axial surfaces S3a of these folas cut across the folded gneissosity and foliation S2 in the hinge zones of the Y^ folds and moderately dip towards NE/SW.

Subsequently, F3b folds of open to close and inclined type are developed on &v S3 and S3a foliations. The F3b folds have characteristic crenulation foliation S3b that is fairly common in pelitic-rich sequence. Kink, brittle-ductile discrete shear zones ano tension gashes are developed during the D^ deformational phase. Superpositions of F2 folds on F± and of F3b on F3a folds have resulted in the Type 3 interference pattern. The overprinting of subhorizontal F3b folds with NW-striking axial planes on the northeasterly trending reclined ?2 folds has led to the Type 2 interference pattern. Type 1 interference pattern between F2 and F folds indicate their approximate orthogonal orientation at a 3b few localities. Morphological and geometric aspects of t^, F3a and F3b folds reveal that the shapes of ?2 folds depend upon the competency contrast between the folded layers and surrounding medium and also upon the distance from the main tectonic boundaries. Dip isogon pattern and $*/ * ratios of most of F2 are characteristic of IC class. A/W ratio increases with an increase in amount of flattening strain. All these characters confirm that the F2 folds are initiated by buckling and modified by homogeneous flattening. F folds also are developed by buckling, while flexural-slip was 3 a an important mechanism or folding at some places. These folds show comparative less amount of flattening ana low A/W ration. The F3h folds have large interlimb angles and low a/w ratio. Dip isogon characters inaicate a smaller amount of flattening in these folas.

Regional strain variations within three lithotectonic units have been analysed using qaartz as strain marker from quartzite and gneiss. A comparison of various two-dimensional finite strain analysis methoas reveal, an overestimetion of strain values in aifferent techniques assuming the Rf/# technique as iaeal. R values calculated from Rf/0 method show close approximation only to the R values obtained by centre-to-centre method of Fry. Two aimensional finite strain Rf/0 data, combined to determine three dimensional strain ellipsoid reveals the following strain patterns during the late D„ deformational phase. (a) Strain magnitude ( cT ) generally is seen to B increase near the proximity of the major tectonic boundaries where it attains a maximum value. (b) The finite strain ellipsoias are of flattening type. (c) In the monotonous quartzite of the Garhwal Group, high values of % occur near the thrusts followed by low strain magnituae to the south. A zone of high strain magnituae is delineated away from the MCT in the lowermost section probably representing another teatonic boundary within the monotonous quartzite. Post-collision neotectonic uplift and cooling rates of the metamorphic pile, aecipherable from the fission track data (F/T) of apatite grains from the Central Crystalline rocks, indicate average cooling rates of 11.1 /my and uplift rates of 0.37 mm/yr in the Tons Valley section. Satellite imagery and aerial photo geological coverage in these valleys and adjoining areas reveal

IOUr important set, of lineaments and fracture traces, all of then, revealing strike-slip sense of movements. L, lineaments trend N-S and show dextral sense of displacement, while its connate sinistral set L, trends H«°. The other striKe-sliP fracture traces and lineaments trend «20° with dextral sense CL3> and „90° (l.) with sinistral sense of displacement respectively. Ongoing detorm-tion in the area is inaicated by soft-sediment aeformational structures in the quaternary sediments as aresult of episodic earthquake recurrence. These small-scale subtle structures are mostly strata-hound and exposed in three deformed sequences. ». structures vary in style from normU faults in the lower zone to intensely compressions! structures in the middle zone like folds having drscrete dlspiacements along their axiai surfaces and thrusts with duplex style, besioes load slumping. «. upper zone is marked by iow-angie thrusts with left-h.nded displacement with associated shears and small-scaie sporadic kinks. Earthquake-induced liguificatron Of the sott-sedrments has primarily controlled slumping in the middle zone, while their deformation is basically controlled by iocalised tectonic stressed due to earthquake whose present-day recurrence exhibit similar characteristics. The trenas or regional foliatron ana orientation of rtrain ellipsoras in the lesser ana Central Himalayan metamorphics reveal that the central Himalayan .one prooably constituted abroad ductUe shear zone curing the b, deformation with bulK strain field of flattening type throughout the progressive deformation. This

helps in working out the sh.--ar zone rnoael within the metamorphic pile in relation to the intracontinental crustal shortening involving heterogeneous simple shear with either volume loss or layer-parallel shortening. The whole metamorphic pile seems to have thrust southward during the late D2 deformational phase along major dislocation surfaces evolving from the zone of maximum strain. The thrust sheets continued their progressive southward translation during the early D~ deformation ana were involved in frontal folding during the late D deformation in Late Cenozoic. Continued intracontinental deformation in parts of the Himalaya is evidenced by brittle-ductile kinks and shear banos during the D. deformational phase followed by differential uplift of the m<. tamorphic pile. The ongoing Holocene movements presently are reflectea in its seismically active character and sporadic soft-sediment deformation.