Introduction – Basic Considerations Today

Precambrian geotectonics in the Himalayan terrain in totality is a very challenging subject of study; Cenozoic hangover overlaps almost every aspect!. The present publication is the outcome of hard task of a very old and experienced worker to unravel very pertinent or revolutionary yet fact-based startling observations which may revolutionalise our long-held ideas on orogenesis in the Himalayan terrain. It also covers the Precambrians of the frontal peninsula which existed nearby or connected. Regional Precambrian geological events in the greater Indian domain are therefore discussed and the prominent geotectonic aspects are addressed. The phenomenon of a big gap in the Precambrian geological history in both the peninsula and the Himalaya is reviewed. A brief introductory lithotectonic framework of the major domains has been described. As recent work specially on geochronology and geochemistry have induced us to revise our various ideas on geology of the major tectonic domains like the HHC – its protolith and geotectonics, the geotectonic divisions are facing rethinking! The MCT is commonly held as a movement zone and the HHC as a tectonically Phanerozoic neighbour of the LHC and LHS. I may like to qualify MCT not so much as a later movement zone but sort of a old decollement(unconformity) feature as taught during our student days. Some very relevant observations of the author need serious attention! Not Cenozoic in age, the nappes, klippes etc. of the LHC are the products of regional Paleoproterozoic deformation(F1) and metamorphism(M1) and the extent of the MCT into the iLHS to the south has now to be reconsidered afresh. The Chail-Jutogh and the like sequences display a continuous Paleoproterozoic structural and metamorphic history. Like Martin et al.(2010),Chakrabarti(1989) had already highlighted on failure of a multi-thrust theory to explain a continuous reversed metamorphic picture. The regionally conspicuous Rampur Quartzite(+volcanics) framing the oldest Paleoproterozoics exposed in the Rampur-Wangtu-Karcham domain is correlated with Gahr Gneiss-Berinag Quartzite-Ulleri-Lingtse gneiss thereby supporting a lot to Precambrian tectonostratigraphy. The protolith boundary is the most likely locale of MCT which very commonly represents a Cenozoic slip zone with associated folding across such zone; slip zones like the HHT are quite likely developments associated with deformation partitioning. The NNE-SSW trending mineral lineation(L1) parallel to Paleoproterozoic F1 fold axial trend is a b-lineation so conspicuous in the entire Precambrian Himalaya with superposed similar trending a-lineation of later generation(Cambrian/Tertiary) defined by lower grade mineral commonly mimetically recrystallized. The so-common consideration of two generations of garnet across a prograde sequence with structurally and chemically zoned and unzoned grains matters review. Contribution of Mitra et al.(2010) on duplex formation in LHS and movement of MCT would be more rewarded if we try to identify positive sites of the Neoproterozoic HHC south of the MCT or south of the protolith boundary and workers on systematic geological mapping in the LHS terrains report inability to continue the marker horizons areally. I recommend to discontinue the decades of practice to show Himalayan tectonostratigraphy only along NE-SW sections; regional Precambrian(Paleoproterozoic) folding fabric should not be so ignored amidst so much in-depth(? neo-) tectonic studies.