Nubra Valley: Geology of Suture Zone, NW India

 

Rocks from Charasa village to the confluence between the Nubra and Shyok rivers to the south, comprise a volcanic and plutonic sequence comprising the Charasa porphyritic andesite lava flow, an aphanitic red volcanic (rhyolite), and a fine-grained light grey volcanic rock, interlayered with a red or red and green volcanic breccia. This sequence is in contact in the south with a plutonic sequence comprising a grey Bt-Hbl granite, a green Plagioclase porphyritic facies, and a pink granite facies. In the contact region there are intrusive rocks with sub-volcanic porphyritic texture (phenocrysts in a fine matrix) with flow banding locally. All plutonic facies have volcanic xenoliths, angular to round. The contact region is strongly hydrothermally altered and brecciated, with large amounts of Py and Cpy, which produce a ferruginous weathering. Bedding defines an open fold (Fig. 1(a)) but none of these rocks have a pervasive solid state deformation, except in one locality. There is pervasive greenschist facies (epidote) metamorphism of all rocks.

Fig. 1a) Lower hemisphere stereonet projection of bedding from Charasa to the south. No tectonic foliation was documented in these rocks

 

The volcanic sequence continues to the north of Charasa, lacking plutonic rocks. In between Kuri and Murgi villages, there is a band of approximately 100 m of conglomerate (Figure 2a-g). This is the famous Saltoro molasse. It contains rounded pebbles up to 30 cm in diameter, including pebbles of the Charasa porphyritic andesite, subvolcanic granite, pink granite, and red, aphanitic volcanics (Fig. 2d). From which we conclude a local origin for the pebbles. This layer is underlain by a coarse, red-brown pebbly sandstone with pebble rich layers, and overlain by fine-grained, grey massive volcanic rocks, variably sheared. The conglomerate itself is sheared by thrusting to the SW with a sinistral component (Fig. 1(b)). This is the first outcrop of sheared rocks, and deformation intensifies northwards.

Fig. 1b) Hensa Gompa Shear Zone (DSK56-57): thrusting to the S-SE and characterized by ramps and flats, strking ESE.

 

Immediately north of Charasa village, we note a change in the orientation bedding planes in volcanic rocks, lacking tectonic foliations and no longer fitting with the large scale fold defined to the south. This new orientation is parallel to the tectonic foliation associated with the thrusting first found to the north in DSK50 (Fig. 1(c)). In other words, beds north of Charasa rotate into parallelism with the thrust plane but do not record a foliation.

Fig. 1c) Red circles are bedding planes measured between Charasa and the confluence to the south. These rocks define a fold, marked by the best fit red great circle defined by bedding. These beds have no tectonic foliation. North of Charasa, starting at DSK47, bedding dips moderately north, parallel to the tectonic foliation (and bedding) that becomes dominant further to the north (starting in DSK50, the conglomerate beds). This is indicated by the black arrow. Black dots combine bedding in DSK47-49 and the foliations measured in DSK50-51.

 

The cliffs behind Murgi, at the waterfall and southwards, expose a normal shear zone (Murgi Waterfall Normal Shear Zone) a few hundred meters thick trending 100/70N with a down dip lineation, and recording a normal shear sense. This overprints earlier thrusting to the SW. Overprinting relatioships are clear in outcrop (S-C fabrics and ramps and flats). North of this normal shear zone, behind Murgi there is complex zone of mylonites and folding, which marks a preserved early deformation phase.

 

North of Murgi fan, around Hensa Gompa (Fig. 3), there is a transitional contact between the volcano-sedimentary sequence and the Kubed granite that outcrops to the north. The volcano-sedimentary sequence is essentially a fine-grained, greenish-grey, chlorite-biotite schist and a clean, white quartzite layers up to 3 m thick (Fig. 3a), interlayered with porphyritic granite and fine-grained granite bands and gabbros (Fig. 3e). The quartzite layers are distributed over a band of 100-150m (Fig. 3b), and there is evidence for very tight folding indicative of transposition (Fig. 3g and h). The sequence lacks volcanic breccias and the red, aphanitic rholite. Deformation returns to thrusting to SW (Fig. 1(b)) with lineations varying in trend and plunge from 360/50 to 050/50, this thrusting characterized by ramps (dipping 50-60N) and flats (15-30N) (Fig. 3c-e). This is the Hensa Gompa shear zone.

 

Further north from Hensa Gompa, thrusting becomes overprinted by dextral shearing , trending 330/70-350/Sv which we interprete to mark the Karakoram SZ itself, close to parallel to the trend of the Nubra valley. In terms of strain intensity, the highest was found around Hensa Gompa in the quartzite-layered sequence and in the earliest of deformations (not detailed here) behind Murgi. The KSZ as documented here does not record a particularly high strain.

 

From DSK61 to the north, but particularly in DSK62, the Hensa Gompa thrust is overprinted by dextral shearing related to the Karakoram shear zone (Fig. 1(d)). Overprinting is expressed in two ways: rotation of thrust planes and new foliation formation. The main lineation in most granitic rocks is a combination of inherited and intersection lineation. Dextral shear sense is clearly defined by asymmetries on horizontal plane, and lineations associated with dextral shearing is only developed in a particular kind of fine-grained muscovite planes.

Fig. 1d) Lower hemisphere stereonet projection of foliations in outcrop DSK62 showing rotation of ESE trending planes associated with the Hensa Gompa thrust towards steep NNW-trending dextral shear zone (Karakoram Shear Zone).

 

So, from south of Charasa to DSK62 the pattern is: large scale open folds with no foliation overprint. The N dipping limb is rotated towards parallelism with the Hensa Gompa SZ way before any foliation is developed. At DSK50 conglomerates the foliation starts and intensifies towards Hensa Gompa which is the contact between the volcano-sedimentary sequence and the Kubed Granite. This shear zone dominates until at DSK62 it is overprinted by the dextral Karakoram Shear Zone, which causes a rotation and overprints foliation related to the Hensa Gompa Shear Zone. Significantly, all rock types on this side of the Nubra valley are significantly retrogressed to greenschist facies. This is not true of the granites exposed on the NE side of the valley at Panamik. In fact, a key feature of this thrusting package from DSK50 (conglomerate) where it starts, all the way to Kubed is that it is all retrogressed to chlorite-epidote greenschist facies with no indication of changes in metamorphic facies.

 

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Saltoro Molasse

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Figure 2a) Conglomerate layer known as the Saltoro Molasse, exposed south of Murgi. The car is close to the base of the layer and the top contact can be seen. The total thickness is approximately 100m.	2b) Lower contact of the conglomerate with a coarse red-brown coarse sandstone.
2c) Internal layering of the conglomerate.	2d) Rounded pebbles in conglomerate including a Charasa plagioclase porphyritic andesite in the lower left, plus a pebble of a fine subvolcanic granite, red aphanitic volcanic and on the right a pink granite. The conglomerate also included pebbles of the red, aphanitic volcanic rock that outcrops in the underlying volcanic sequences.
2e) Sheared conglomerate, with top to the left (thrust) indicated by the foliation and pebble asymmetries. The lineation in this outcrop is close to down dip.	2f) Detail of (e). C=090/45N l plunges towards 050, making this a thrust with a left-lateral component.
2g) Larger scale fish-shaped blocks in the grey volcanic rocks immediately overlying the conglomerates.

 

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Hensa Gompa Shear Zone

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Figure 3a) Mylonitic, white, clean, quartzite, interlayered with a grey, fine-grained felsic volcanic rocks. C//S0 = 107/52N stretching lineation = 047/48	3b) Cliff behind Hensa Gompa, showing an ~100m thick region of interlayered white quartzite and volcanics. To the right (N) lies the main contact zone between the volcano-sedimentary sequence shown (that includes both granite and gabbro layers) and plutonic rocks (mostly porphyritic granite).
3c) Thrusting to the left (SW) in volcano-sedimentary sequence, below Hensa Gompa, indicated by the drag fold on the low angle plane. The thrust plane is 110/locally horizontal, lineation = 355/36.	3d) Thrusting to the left (SW)in volcano-sedimentary sequence, below Hensa Gompa. Note asymmetric sheared quartz vein. Thrust plane = 110/15N.
3e) Thrusted gabbroic layer, below Hensa Gompa .
3g) Interleaving of grey volcanics with quartzite as a result of folding and shearing.	3h) Rootless, isoclinal fold of thin quartzite layers.

 

 

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