The HimachalHimalaya is known to have a complextectonic setting that has evolved during the Cenozoic era (Webbet al. 2011).
The Kullu valley, occurring between 31° 21′ N and 32°59’N latitude and 76° 49′ E and 78° 59′ E longitude, with an area of 4782 km2 (falling in Survey of India Quadrangle sheet No 52H and 53E), is a prominent tectonic window referred in literature as Larji-Kullu-Tectonic Window (LKTW, Fig.1) (Heim and Gansser 1939; LeFort 1996; Hodges 2000; Yin 2006; Upreti 1999; Thakur 1998; Webb et al. 2007). The LKTW has structurally evolved in four phases during EarlyMiocene to Pleistocene, where the Greater Himalayan Crystalline Sequence (GHCS) of rocks are brought over the units of Lesser HimalayanCrystalline Sequence(LHCS) rocks along the Main Central Thrust (MCT), and the window has been geomorphologically carved out and exhumedduring late phasesof movement spanningacross Pliocene- Pleistocene and Recent times (Vannay et al.
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2004; Yin 2006; Remington M. Leger et al. 2013). The present studyarea lies in the north-western part of LKTW, where the MCT tectonically juxtaposes rocks of the Higher HimalayanSequence with the Lesser Himalayan Sequence.LKTW, the par-autocthnous structure (Dubey and Bhat 1991), is characterized by an antiformal fold structure in LHCS circumscribed by two prominentthrusts MCT-I (Vaikrita Thrust) and MCT-II (Kullu thrust) with the axial zone of the fold trending NNW-SSE. The most conspicuous feature of the area is the presence of “Window in Window” structure, where a20 km long and 6 km wide, lozenge-shaped “Larji window”is bound by Banjar thrust and this Larji window lies within a larger window structure bounded by the Kullu Thrust (Fig- 2). In addition,the Munsiari Thrust, the thrust/fault line of regionalimportance in the Himalayasalso traverses the area along the axial zone of Kullu Window (KW) and it has been described as an out-of-sequence thrust (OOST: Thiede 2004). The presentday topography was developed by the lastphase of thrusting activity in this region.
Recent seismic events clustering data indicatesthat SW-directed thrustingis still going on along the MunsiariThrust in the Garhwalregion (Kayal 1996), and at the same time east-directed extension is taking place in the MCT hanging wall in the Sutlej sectionof the Larji-Kullu Tectonic window(Molnar and Lyon-Caen 1989; Vannay2004). The morpho-tectonic featuresand a number of seismic events of moderatemagnitude in the region indicate active tectonicmovement in the Larji-Kulu TectonicWindow (Dubey et al. 2003).
The Kullu Valley is characterized by highly variable lithologyconsisting of a dominantassemblage of low to high grade meta-sedimentary rocks belonging to different litho-tectonic groups. The upper part of the valley consists of gneisses,granites and migmatites of the Vaikrita Group, locallydesignated as the RohtangGneissic Complex.Micaceous Slates, garnetiferous quartzite and schist, green massive quartziteand limestone of the Jutogh Group (in pockets limestones of Shali Formation as well) lie in the middle part. The Lesser Himalayan Sequenceconsists of at least two tectono-metamorphic units: (i) The very low- to medium-grade Proterozoic meta-quartzites, shales, dolomitesand meta-igneous rocks (Frank et al. 1973, 1977; Miller et al. 2000) of the Lesser Himalayan Sedimentary Succession and (ii) the amphibolite-facies ortho- and paragneisses of the Lesser Himalayan Crystalline Unit (LHC),i.e. the Jeori–Wangtu graniticgneisses (Thiedeet al.
2004; Thoni et al. 2012). Geomorphologically, the area is characterized by high relief and variable degreesof dissection with deep narrow gorges, steep slopes,escarpments and cliff faces. The relief of the area varies in altitudebetween 900 m to above 5000 m msl at Rorung Dhar. The upper reaches with high relief zone is mainly characterized by funnel-shaped valley with steep and variable slopeelements and also it is under the influenceof seasonal snow and mass movementactivity. The steepness of the topography can be judged from the fact that the altituderises from about 2000m near Manikaranto about 5000m on Rorung Dhar within a distance of about 8 km and also experiences heavy snowfall.
The middle reachesof the Beas valley constitutes and characterized by wide open valleys, large alluvial fans, fluvial terracesand flood plains belonging to the Quaternary period. This area is drainedby Beas River and its tributaries viz Parvati,Hurla, Sainj and Tierthan rivers joining Beas from the east. The Beas River, flowing southerly, transectsthe area, with streambase level ranging from 2800min the north to 900m in the south. The river is characterized by fluvio-glacial Quaternary alluvial deposits with three to four levels of fill and cut-fillterraces (Sah and Mazari 2007).
Bajada fans of large dimensionare seen on the eastern bank of the river offeringsuitable space for urban development. Near Bhuntar, the channel changes its course from north- south to south-east direction where the processofchannel avulsionof about 45m is noticed.However, the three sub-parallel tributaries (viz Parvati, Sainjand Hurla) on the easternflank are primarilybedrock rivers with unpaired strathterraces. The ParvatiRiver, the main tributary of Beas is flowingin E-W direction with the elevation drop from 4500 to 1090 m above msl. Climatically, the upper Beas basin experiences a subtropical to temperate type of climate.The uppermost part of the Beas basinexperiences cold temperateconditions, whereas temperate and subtropical conditions prevailin the middle and lower zone. The rainfall patternclearly demonstrates that the influenceon precipitation is altitudedepended within the basin.
The upper reaches of the Beas basin experience more rainfall as compared to the lower as indicated by the average rainfalldistribution curve for the yearsof 1983, 2004 and 2015.