Structure, metamorphism and tectonics of the Central Nepal Himalayas

Sapkota, Jyotindra (2011) Structure, metamorphism and tectonics of the Central Nepal Himalayas. PhD thesis, James Cook University.

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Abstract

Analysis of mylonitic garnetiferous rocks within the Kathmandu Thrust Sheet, which overlies the Lesser Himalayas along the southern part of the Main Central Thrust (MCT) and constitutes a leading edge of the Higher Himalayas, reveals a complex tectonometamorphic history. This portion of the MCT in the Central Nepal Himalayas, which is folded by the Gorkha-Kathmandu fold couplet, contains tectonometamorphic signatures that were difficult to recognize because of formation of the MCT and subsequent deformations. These have been accessed quantitatively via inclusion trails preserved in garnet porphyroblasts, in spite of their truncation by all matrix foliations.

They preserve structural and metamorphic history obliterated in the matrix accessed by methodologies including:

• identification and measurement of various microfabrics preserved in the matrix and shear senses revealed by them, • measurement of Foliation Intersection/Inflection Axes (FIAs) preserved in garnet porphyroblasts and shear sense interpretation from asymmetrically curved inclusion trails, • measurement of pitches of internal foliations within garnet foliations in vertical thin sections cut at a high angle to the related FIA trend, • modelling of PT pseudosections using bulk rock compositions analysed at XRF facilities, • calculation of PT conditions of garnet core nucleation using Fe, Mn and Ca isopleths calculated from garnet core compositions analysed on electron microprobe and • calculation of average PT conditions with various rim geothermobarometric methods from the compositions of garnet rims and surrounding biotite, muscovite and plagioclase grains.

Different generations of crenulated matrix foliations from various parts of the Gorkha-Kathmandu fold couplet constitute a comprehensive archive of structural history related to movement along the MCT, its folding by the couplet and subsequent deformation events. The oldest foliation preserved in the matrix formed sub-vertically as can be observed in strain shadows of some garnet porphyroblasts. Curvature of this foliation into the sub-horizontal foliation that formed subsequently indicates top to the south sense of movement in spite of different geometries preserved in parts of the central limb of the fold couplet. Antithetic reactivation of the central limb during folding has locally switched this shear sense to top to the north. A sub-vertical foliation that formed axial planar to the Gorkha-Kathmandu fold couplet exhibits a switch in shear sense across each of the hinges with a south side up shear sense on the external limbs and north side up on the central limbs. Moderately to steeply north dipping foliation observed in the Kalphu river section overprints the axial plane foliation with a top to the south sense of shear. The youngest foliation in the region is a sub-horizontal foliation that shows a top to the north sense of shear. This feature, observed close to the MCT in the Kathmandu Thrust Sheet, suggests that the shear sense reversed from top to the south to top to the north when these rocks were uplifted into a zone that was affected by gravitational collapse as India-Eurasia collision continued.

Inclusion trails preserved within garnet porphyroblasts contain a succession of five FIA sets trending SSE-NNW, ENE-WSW, NW-SE, E-W and NNE-SSW. These FIA sets formed as a result of change in the direction of bulk horizontal shortening associated with India-Eurasia collision. The intersections of Fe, Ca and Mn isopleths for garnet cores in PT pseudosections modelled from bulk rock compositions reveal that FIAs 1-5 formed respectively at 6.2 kbar and 515°C, 6 to 7 kbar and 545 to 550°C, 6.6 kbar and 530°C, 5.6 to 6.2 kbar and 525 to 550°C and 6.8 to 6.9 kbar and 520 to 560°C. This data suggest that the PT conditions varied little during regional Barrovian metamorphism that commenced after India-Eurasia collision and stopped prior to development of the MCT. Pressures of about 11 kbar yielded by various rim geothermobarometric methods do not accord with those in the succession of garnet cores indicating that their rims are not in equilibrium with the surrounding silicates in the matrix. Pervasive matrix foliations in the rocks that overlie the MCT in the Kathmandu Thrust Sheet are mylonitic. The matrix grains deformed plastically during mylonitisation and did not equilibrate with garnet rims that probably underwent some dissolution as garnet porphyroblasts are competent and tend not to deform internally. Furthermore, the significantly higher pressures calculated using the rims do not accord with the development of all matrix foliations as the rocks were carried towards the surface along the developing MCT.

Multiple generations of internal foliations in garnet porphyroblasts developed sub-vertically and sub-horizontally. The preservation of these orientations in the porphyroblasts would not have been possible if they had rotated during their growth or subsequent deformations. A very small number of foliations that deviate from these orientations predated garnet growth and are present only in garnet cores. Some rocks belonging to each FIA set in the succession contain at least three and up to four foliations in garnet porphyroblasts. This suggests that Kathmandu Thrust Sheet rocks were affected by at least 15 events of horizontal crustal shortening and gravitational collapse, which occurred before mylonitic foliations related to the MCT truncated the internal foliations in garnet porphyroblasts.

The succession of five FIA sets exhibits a remarkable correlation with the motion of India between 50 and 29 Ma relative to Eurasia that was arbitrarily kept stationary in its present position. The direction of bulk horizontal shortening perpendicular to FIA 2 aligns with the appropriate segment of India's motion. If Eurasia moved along the trend perpendicular to FIA 2 but at different speeds to form the five FIA sets, then it should have moved NNW during FIAs 1, 3 and 4 and SSE during FIA 5 in order to produce the required resultant motion with the overall northward moving Indian plate.

Looking overall west, curvatures of steeply dipping foliations into gently dipping ones in garnet porphyroblasts are dominantly CW in FIAs 1, 3 and 4; coaxial in FIA 2; and dominantly anticlockwise in FIA 5. Therefore, the shear sense switched from being top towards north or coaxial during the development of the first four FIAs to being top towards south during the fifth. These developments occurred at a depth of at least 23 km as revealed by 6-7 kbar of pressures associated with all garnet nucleation. The presence of both asymmetries in each FIA with a dominance of clockwise asymmetry or coaxiality in FIAs 1 through 4 and an anticlockwise asymmetry in FIA 5 suggest that the central Nepal rocks migrated from north side of the orogen core to south side when FIA 5 developed. This occurred as India, lying some 7kms deeper, was displaced northwards below Eurasia during the 50 to 20 Ma period while FIAs 1 through 5 developed. It was accompanied by many phases of horizontal shortening and crustal thickening leading to periods of gravitational collapse and vertical shortening. This eventually resulted in lateral southwards extrusion of the rocks towards the earth's surface as the MCT developed. The top to the south sense of shear in FIA 5 is similar to that recorded by mylonitic foliations, which truncate inclusion trails in garnet porphyroblasts and transported the Higher Himalayan rocks southwards by about 100 km on the MCT.

Item ID: 26380
Item Type: Thesis (PhD)
Keywords: FIAs, foliation, garnet core isopleths, geothermobarometry, garnet porphyroblasts, Higher Himalayan crystallines, inclusion trail asymmetries, inclusion trails pitches, Kathmandu Thrust Sheet, Main Central Thrust, MCT, MCT FIAs, MCT Higher Himalayan crystallines, foliation, orogeny, petrology, plate tectonics
Date Deposited: 27 May 2013 01:32
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040312 Structural Geology @ 34%
04 EARTH SCIENCES > 0403 Geology > 040313 Tectonics @ 33%
04 EARTH SCIENCES > 0403 Geology > 040304 Igneous and Metamorphic Petrology @ 33%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 100%
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