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ON THE NATURE OF THE RAMGARH THRUST IN KUMAON HIMALAYAS S. S. MERH, N. M. VASHl AND J. P. PATEL Department of Geology, M. S. University, Baroda The Almora Thrust (Heim & Gansser 1939) in Kumaon Himalaya separates the underlying younger rocks of Krol nappe from those of Almora nappe above it. South of Someswar-Dwarahat, this thrust is seen synformally folded, and its northern flank has been termed as North Almora thrust; its southern flank is somewhat controversial. The town of Bhowali is situated on an anticline which perhaps is complementary to the above mentioned synform to the north. Between Bhowali and Ranikhet-Almora, most workers have shown two thrusts dipping due NE, one is ideally seen near Ramgarh and the other is observed on the Ranikhet road near Upradi (Vashi & Merh 1965). Heim and Gansser have joined up the Ramgarh Thrust with North Almora thrust, though they have shown the thrust at Upradi as South Almora thrust. Gansser (1964) too has considered the Ramgarh thrust as the southern limb of the synformally folded Almora thrust. He has not given any name to the thrust at Upradi. Vashi & Merh (1965) have however suggested that the Upradi thrust is in fact the southern limb of the folded Almora thrust. Merh (1968) does not believe that the thrust at Ramgarh is the southern extension of Almora thrust, forming, a nappe of Chandpur rocks (Ramgarh nappe). According to Heim & Gansser (1939, p. 28) the region between Bhowali and Upradi Thrust ( = South Almora thrust) comprises a recumbent syncline overturned to the sw. There are however few field data to support the existence of such folded structure as visualised by Heim & Gansser, in the Ramgarh area. The rocks hardly show any evidence to suggest the possibility of a folded thrust, open or tight to form the so called Ramgarh nappe. The present authors' investigations have made it clear that the entire succes­ sion from Bhowali northward'up to the South Almora thrust (Upradi thrust) consti­ tutes an uninverted sequence, cut by a thrust a little to the south of Ramgarh (Fig. 1). This thrust has brought the older sheared migmatites (' quartz porphyries' of Heim & Gansser) over the Nagthat quartzites of Bhowali. Merh (1968) has estab­ lished a sequence of three major deformational events in Central Kumaon Himalaya. The first deformation comprises large scale isoclinal folding (F,) which synchronised with the progressive phase of the regional metamorphism of the Almora nappe, and the resulting metamorphic foliation (S,) characterises the axial plane of the isoclinal folds. The axes of the minor folds related to this deformation so commonly recorded in the Almora Ranikhet area, mark the dominant lineation (L,) and generally plunge at various angles due N to NE. The stresses which overfolded the rocks, ultimately culminated into the Almora thrust. The dislocation metamorphism along the thrust has given rise to an inverted sequence of metamorphic zones in the Almora nappe with increasing grade upward. At a later date the area was subjected to another folding (F2) which resulted in major structures like Almora synform and Bhowali anticline. It is to this folding that the various anticlines and synclines recorded in the Khairana-Sinwalbari area and discussed below are related. A chevron type microfolding of the schistosity and the development of a strain-slip cleavage (S,) at a number of places both in the Almora and Krol nappes, is relaled to the folding F2. The axes of the minor folds and puckers (L?) are seen plunging due WNW to NW. The third and possibly the last major folding (F8) has been responsible for the deve­ lopment of N-s to NE-SW folds on all scales. It is due to this folding that the SHORTER COMMUNICATIONS 381 regional foliation trends fluctuate from as much as N-S to E-W. Minor folds and the related linear structures (L„) are recorded all over in a scattered manner. The present investigation of the Khairna-Sinwalbari region between the Ramgarh thrust and the South Almora thrust has futher elucidated the structure of the area, and thrown more light on the exact nature and evolution of the Ramgarh thrust. Figure 1. Geological sketch map of the region between Ramgarh and Ranikhet in Kumaon Himalaya. Structure of the region: The rock succession below the South Almora thrust and above the Ramgarh thrust, represents an uninverted sedimentary sequence which has been correlated as under: 382 SHORTER COMMUNICATIONS Crystalline schists, gneisses and Chandpur flaggy quartzites South Almora thrust Quartzites with phyllites (?) Nagthat (Lower) Limestones with slates ] Slates with interbedded I quartzites y (?) Deobans Sheared sericitic and chloritic | migmatites J Ramgarh thrust Pebbly quartzites (?) Nagthat (Upper) The above formations are seen to have been folded into a series of asymmetrical synclines and anticlines (F2). The various folds are fairly open with one limb steep and almost vertical while the other gently dipping due NE. These folds extend NW- SE and are overturned due south such that their axial planes show moderate dips due NE. Their fold axes plunge gently due NW. Numerous minor folds related to these structures are recorded which show a geometry identical to the major struc­ tures. Ideal development of these folds is seen in the Khairna-Sinwalbari region on the two sides of the river Kosi. These structures gradually tend to smoothen out south-eastward and in Ramgarh-Nathuakhan area, the folds practically disappear. Behaviour of these folds in NW is not yet clearly established, but from preliminary studies it appears that these structures continue to extend in the area to the NW-of Khairna bridge across the Kuchgad stream. Here, the authors suspect some struc­ tural complexity due to a superimposition of N-S trending F3 folds on the Fa anti­ cline that flanks the Ramgarh Thrust. Folding in relation to Ramgarh thrust: The Ramgarh thrust is perhaps more of the nature of a big reverse fault. It could also be considered a high angled thrust. It extends roughly NW-SE with occasional fluctuations in its strike to as much as E-W to NNW-SSE. Its dip also varies from sub-vertical to as much as 60° due NE. This variation in the dip and strike is due to the late folding (F8). It appears that the Ramgarh thrust originates from the Krol thrust in the SE, and extends across Ramgarh northwestward up to Khairna and further west following the course of Kosi river before it dies out. A major anticline flanks this thrust to its NE. The thrust extends parallel to the fold and cuts its steeper limb almost near the crest. Thus, on going from SE to NW, this trfrust is seen progressively cutting the migmatites, quart­ zites and then the overlying limestones. The manner in which it is cutting the successively younger rocks on going NW suggests that the folding preceded the thrusting. A careful scrutiny of the rocks in the immediate neighbourhood of the thrust, especially the sheared migmatites, clearly establishes the following sequence of defor- mational events: (1) Development of NW plunging synclines and anticlines, at the time of folding of the Almora thrust. It was during this folding that a group of migmatised slates and basic rocks (epidiorites), forming the lowermost formation of the area, got intensely sheared giving rise to the chlorite and sericitic rocks SHORTER COMMUNICATIONS 383 erroneously called as 'porphyries' by Heim & Gansser (1939). Small puckers and microfolds.in slate related to this folding are quite common and show orientation of fold axes and axial planes identical to the major folds. (2) Rupturing of the vertical limb of an anticline, giving rise to a high angled thrust (Ramgarh thrust). Along this thrust, due to drag effect, the sheared migmatites, developed monoclinal or open angular folds whose axial planes are seen dipping towards the thrust at low angles. What is most interesting about these drag fold? is that their fold axes are parallel to those of regional flexures and associated minor folds (Fj). This is a clear indication of the fact that the folding and thrusting were genetically related and the thrusting followed the folding, both forming a connected sequence of a single defor- mational episode. The authors are of the opinion that the Ramgarh thrust is an offshoot of Krol thrust and is perhaps connected with the movement along the Krol thrust. Acknowledgement: The authors wish to express their grateful thanks to C. P. Shah who very kindly made available valuable information about Garampani area. REFERENCES GANSSER, A., (1964) The geology of the Himalayas—Inlerscience Publishers Wiley, N. Y. HEIM, A. and GANSSER, A., (1939) Central Himalayas—Geological observations of the Swiss Expedition of 1936. MERH, S. S., (1968) A preliminary note-on the structural history of the Central Kumaon Himalayas—5«//. Geol. Soc. Ind., v. 5, No. 1. VASHI, N. M. and MERH, S. S., (1965) Structural elements of the rocks in the vicinity of the S. Almora Thrust near Upradi, Almora District, U. ?.~Jour. M. S. Univ., Baroda, v. 14, No. 3, pp. 27-32. GEOLOGY AND MINERALOGY OF THE CHROMITE DEPOSITS OCCURRING NEAR KONDAPALLE, KISTNA DISTRICT, A.P. S. CHAKRAVARTI AND S. MUKHERJEE Department of Geology, University of Calcutta, Calcutta The geological importance of the Kondapalle area is well known due to the economic concentration of chromite within the ultramafic rocks; and the area has been studied by several workers like Srirama Rao (1946, 1947), Krishna Rao (1964), and Leelanandam (1967a, 1967b). Srirama Rao (1946, 1947) studied the geology and mineralogy of the Bezwada gneiss and the associated rocks, charnockite suite of rocks and the chromites. He considered the chromite as late magmatic and hydrothermal in origin.

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