Precambrian Geotectonics in the Himalaya: Sans Cenoxoic Hangover

By B.K. Chakrabarti

Precambrian Geotectonics in the Himalaya provides an overview on general geology and tectonics of all the Precambrian domains of the Himalayan terrain. Authored by an expert with over five decades of laboratory, field and publication experience, the book studies the “Window zones to provide a scope for understanding Precambrian deformation effects. The book fills a gap in literature, specifically covering the Precambrian geotectonic picture of the terrain. Considering Precambrian regional events are not clearly recognized or visualized in many sectors due to overlapping crystallines, this book details a Precambrian geotectonic framework of the terrain on which the Himalayan event evolved.

This book is a necessary reference guide for Earth scientists, exploration and hazard management scientists, professors, students and anyone who carries out research that requires a comprehensive picture of the Precambrian Himalaya and the adjacent peninsula.

• Features comprehensive data gathered from decades of research on the Himalaya
• Includes numerous detailed case studies that allow readers to comprehensively consider the data presented
• Describes the Precambrian tectonostratigraphic history of the Himalayan terrain

• Language: English
• Publisher: Elsevier Science
• Release date: Jun 22, 2023
• ISBN: 9780323983440

B.K. Chakrabarti

B.K. Chakrabarti Ph.D. is an Emeritus Scientist with the Geological Survey of India and a Sir J. Coggin Brown Gold Medalist for his work on Precambrian geology. Dr. Chakrabarti served with the Geological Survey of India for 35 years and has published more than 30 articles on the subject. He was also a Fellow at the W.B. Academy of Science & Technology in Kolkata, India. Now retired, Dr. Chakrabarti is also editing a book on the Precambrian of the Indian Peninsula to be published by the Indian Journal of Geosciences.

Book preview

Precambrian Geotectonics in the Himalaya

Sans Cenozoic Hangover

B.K. Chakrabarti

Director(retd.), Geological Survey of India

Table of Contents

Cover image

Title page

Copyright

Dedication

Foreword

Acknowledgment

About the book

Chapter 1. Introduction – Basic Considerations Today

1.1. Lithotectonic subdivisions of the Himalaya

1.2. Precambrian Himalaya—geological background

Chapter 2. Precambrian Himalayan Terrain—Pakistan to Arunachal

2.1. Pakistan Himalaya

2.2. Jammu-Kashmir Himalaya

2.3. Himachal Himalaya

2.4. Garhwal–Kumaon Himalaya

2.5. Nepal Himalaya

2.6. Darjeeling–Sikkim Himalaya

2.7. Bhutan Himalaya

2.8. Arunachal Himalaya

Chapter 3. Precambrian Himalayan terrains–geotectonic perspective

3.1. Basic premises of Precambrian Himalaya

3.2. Structure and metamorphism

3.3. Thrust zones

3.4. Ulleri gneiss and associates

3.5. Late Neoproterozoic–Cambrian evolution

3.6. Isotopic and geochemical Feedback

Chapter 4. Precambrians of the frontal peninsula

4.1. Rajasthan sector

4.2. Great Boundary Fault

4.3. Vindhyan sector

4.4. Indo-Gangetic basin

4.5. Central Indian Tectonic Zone

4.6. Eastern Peninsula and Himalaya

4.7. Peninsula—Himalaya and beyond

Chapter 5. Precambrian Himalayan Terrains – A Review

5.1. Magmatic and associated events

5.2. Lesser Himalayan and higher Himalayan sequences—overview

Chapter 6. Precambrian Geotectonics in the Himalaya—Sans Cenozoic Hangover

6.1. Paleoproterozoic Himalaya—a field perspective

6.2. Shimla Himalaya—an illustration

6.3. Paleoproterozoic Himalaya—no Cenozoic Hangover

References

Index

Copyright

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Dedication

Dr. O.N. Bhargava, FNA

Director (retd.), Geological Survey of India

Honorary Professor, Punjab University, Chandigarh

Foreword

Traditionally, the Himalaya is considered a product of the Cenozoic Himalayan orogeny. The idea got formulated in the Lesser Himalaya as the pre-Cenozoic rocks are thrust over the Cenozoic sediments and the Eocene Subathu Series is exposed in four tectonic windows. Initially, the Blaini Boulder Bed, correlated with the Late Palaeozoic Gondwana glaciation, was used as a datum line; the sequences below it were assigned Palaeozoic age and those above a Mesozoic age. Based on superficial and flimsy parallelism with the Tethyan sequences, all clastics were correlated with Cambro-Silurian sequences of the Spiti Valley, quartzites were equated with the Muth, and the carbonates with the Triassic Lilang System. Only the crystalline thrust sheets were regarded as Precambrian.

The utility of stromatolites as chronometers relegated many carbonate sequences to the Riphean (Mesoproterozoic) age. Here, we had a sequence ranging from the Mesoproterozoic to the Cenozoic with several unconformities (e.g. the Blaini Formation over different units of the Simla/Jaunsar sequences; Subathu Formation over sequences ranging from Palaeoproterozoic to Cretaceous), yet pre-Cenozoic tectonic disturbances were ruled out—a unique example of remarkable tectonic quiescence stretching over 1500 million years, whereas elsewhere in the world quite a few older orogenies are well recognized.

Though Dr. J.B. Auden did refer to the NE trend in the Himalaya as an extension of the peninsular structural grain, in course of time this observation got lost, and the NE trend was ascribed to cross-folds. Any talk of pre-Cenozoic tectonics in the Himalaya was a geological blasphemy. Field observations like disoriented xenoliths in older granite, Palaeozoic granites cutting foliation of the country rock, and observation under an ordinary microscope of idioblastic minerals abutting the pervasive foliation were pooh-poohed and looked at with derision. Initially, even the significance of Late Cambrian orogeny and extensive occurrence of Lower Palaeozoic granitoids were dwarfed. The differential regional deformation in the Himalaya too is conspicuous; intense deformation is confined to the vicinity of the Indus Suture and the Crystalline thrust sheets; and the Tethyan sequence between these two and the Lesser Himalayan formations south of the thrust sheets, by and large, lacked such deformation. To me, the deformation in ISZ is owed to the collision of plates, and in the thrust sheets mainly due to Precambrian and/or Cambrian tectonics. We did not have isotopic numbers and field and simple microscopic observations were treated with contempt, even by close friends. It was mighty difficult to publish any paper propounding pre-Cenozoic tectonics/metamorphism. I had several occasions to discuss my new-fangled ideas with Dr. Benoy K. Chakrabarti, who had independently arrived at more or less similar conclusions.

Then came some Palaeozoic isotopic dates of metamorphic minerals and evidence of Late Cambrian (Kurgiakh) orogeny. To borrow a Churchillian quote, it proved a ‘Gathering Storm’; earth scientists, though minuscule, started getting reconciled to the pre-Cenozoic tectonic events—mainly limited to the Late Cambrian/Ordovician interval; Precambrian events were still big ‘No–No.’

Presumably, at least some of the events in the Himalaya would have been registered in the peninsula or vice versa, which deserved critical appraisal. It required a person with unbiased vision and intellectual conviction than mere textual knowledge; he should have a masterly grasp on tectonics and metamorphism to collate and reinterpret the scattered data in various sectors of the Himalaya suggestive of pre-Cenozoic tectonic episodes and present a panoramic history of the Himalaya, linking it with the events in the peninsula, paucity of which is poignantly felt.

Thus, it is a matter of great satisfaction that Dr. Chakrabarti has picked the gauntlet and decided to write this book. The assertions in the book have moorings in the field, thus standing in a class apart. Whether the reader agrees or opposes, he would not be left in suspense or doubt as to what the book conveys. The narration is categorical, positive, direct, and unambiguous. May be not all answers are provided in this book, for that matter no geological write-up can be a full stop. What is important is that the right questions have been pondered and framed with rational elucidations, which shall stimulate fresh thinking, and hopefully, in the near future, we will hear more about the Precambrian and Cenozoic tectonics in the Himalaya.

Om N. Bhargava

Panchkula

Acknowledgment

Whatever I knew, felt, perceived, and kept inside low, often abnegate, is expressed here in clear terms at the end of my days for my dear juniors who may afford to pass on a little passage money to this octogenarian for his last journey to the wonderland! I guess my learned readers bear with my long talks!

Nevertheless, my teachers in Presidency College, University of Calcutta and their legendary guides like Profs. Hess (Poldervaart and Hess), Ramsay, MRW Johnson, Moorhouse, and others inspired me a great deal. Our professors with Prof. S. Ray as the Head tried to provide a good upbringing to make us qualified to work in any type of terrain. Field work under them in the different parts of India made me quite comfortable to carry out the assigned mapping work 6 months a year in the Geological Survey of India during the initial years in the remote hilly terrains. Besides them, Profs. FJ Turner, LS Hollister, PH Reitan, D Bhattacharyya, Bapi Goswami, Ashok Patwardhan, Aaron Martin, Hafiz Ur Rehman, Mallickarjun Joshi, DM Banerjee, Dilip Saha, Anupam Chattopadhyay, and Tapos Goswami, to name a few, have kept me indebted with their scholarship and human values. I am enriched in association with my GSI colleagues – young and old! My nearby friends and colleagues far and near, especially during the corona bereavement and poor health period, were a constant source of inspiration—CP Vohra, SV Srikantia, KK Ray, US Mukhopadhyay, SK Ray, Gautam Mukherji, Debi P Das, Bhaskar Chakraborti, A Chaudhuri, Amaresh Chatterjee, Sabyasachi Shome, Subrata Chakraborti, RC Dey, RK Gaur, KC Prashar, Satyabrata Guha, Vikram Rai, Surendra Pandhare, VK Joshi, Niteesh Dutta, Sudip Bhattacharyya, Sambhunath Ghosh, Ranjit K. Datta, Gautam Ghosh, Pronab Naskar, Samir Sengupta, Bashab N Mahanta, Mriganka Ghatak, Avisekh Ghosh, Gouri Bandyopadhyay, and others including my son Sumon and wife Sabita. My teachers and specially Prof. AK Saha and my four deceased family members would have been so happy that I pursued the task so long with illness and all odds to completion at this age! I am thankful to Elsevier and specially to Peter Llewellyn, Helena Beauchamp and Sruthi Satheesh for their sincere involvement.

I dedicate the volume to Dr. ON Bhargava who inspired me throughout; he was always there, although in poor health, to answer my odd queries on regional geology especially of the Himachal terrain. Dr. Bhargava had scanned the NW Himalaya (Bhutan for some time) on foot for long 6 decades and it is a matter of immense satisfaction that I dedicate my last major review work on tectonics of the Precambrian footing of the Himalaya to him and through him to all field geoscientists who mostly remain unsung!

Dr. B.K. Chakrabarti

Kolkata

About the book

My study of the recent contributions on the Precambrians of the Himalaya and the adjacent peninsula induced me to look back into my old observations and ideas of regional implication as the tradition has perhaps not sung all in proper perspective!

The so-established Cenozoic nappe and klippe structures in the Lesser Himalayan Crystalline (LHC) are no more considered by this author as Cenozoic in age; these are the product of Paleoproterozoic deformation (F 1 ) and metamorphism; and the extent of the MCT to the iLHS has now to be considered afresh. The so much reported Jutogh Thrust does not exist and the Chail-Jutogh sequence displays a continuous structural and metamorphic history. The regional F 2  event (Cambrian ± Cenozoic) had obvious superimposed effects; also, the classic inverted metamorphic (Paleoproterozoic) picture is restricted to the Paleoproterozoic sequence lying below the HHC/MCT with an unconformity and decollement in between. Also, a multi-thrust theory to explain the conspicuous reversed Barrovian metamorphic picture needs on-field review and a microscope to help!

As in the Shimla Himalaya (Jutoghs and Rampur-Wangtu granite-gneiss-migmatite body tied up as one with oldest Paleoproterozoic age), the iLHC must have a root zone to the north not exposed due to the overriding HHC or shared partially with equivalent/same sequence.

The Rampur Quartzite (+volcanics) framing the oldest Paleoproterozoics exposed in the Rampur-Wangtu-Karcham domain becomes a mylonite gneiss or blastomylonite in different structural levels between Rampur and the upper levels in Shimla-Chor area (with F 2  refolding, slip, and associated recrystallization), has a sedimentary plus volcanic origin, carries high-grade mineral, and is correlated with Garh Gneiss-Berinag Quartzite-Ulleri-Lingtse gneiss.

The protolith boundary is the most likely locale of MCT which very commonly represents a slip zone with associated folding across such zone; slip zones like the HHT of Goscombe et al. (see inside) are quite likely developments associated with deformation partitioning.

The conspicuous NNE-SSW trending mineral lineation (L 1 ) parallel to Paleoproterozoic F 1  fold axial trend is a b-lineation so common in the Paleoproterozoics of the Himalaya and superposed similar trending a-lineation of later generation (Cambrian/Tertiary) is defined by lower grade mineral commonly mimetically recrystallized.

The common consideration of two generations of garnet across a prograde sequence with structurally and chemically zoned and unzoned grains has been reviewed (thanks Prof. FJ Turner for advice on growth kinetics).

Contribution of  Mitra et al. (2010)  on duplex formation in LHS and movement of MCT is quite interesting, yet systematic mapping in the LHS must identify positive sites of the Neoproterozoic HHC south of the MCT or south of the protolith boundary and let workers on systematic geological mapping in the LHS terrains report inability to continue specially the marker horizons areally due to interplayed Cenozoic tectonics—that is always not!

I decry the common practice over decades to exhibit a NE-SW cross-section of the Himalaya; such section shows a single folded (F 1 : Paleoproterozoic) unit as disconnected units; make sections across the NNE-SSW trending old F 1  mega-folds, or search out the F 1  folds in such sections, not the NE-SW section which exhibits Cambrian/Cenozoic effects.

And about the so-commonly observed Mesoproterozoic gap in sedimentation in both the frontal peninsula and the LHS, the Rajasthan Vindhyans may lead us aboard: the provenance in the western sector was stable not to cause any gap in sedimentation between the Lower and the Upper Vindhyans; change in the provenance during the Kaimur to Rewa group in the western sector caused a break in the deposition.

And lastly, the Paleoproterozoic MCTZ rocks commonly look markedly more tectonized than the HHC with an ingrained old and solid structural fabric and much later superposed refolding and tectonic interplay with a contrast! My case studies especially in the Shimla Himalaya have spelt out a little in this direction! Further studies on Tertiary tectonics and domain of activation in the lofty terrain are perhaps inviting!

Chapter 1: Introduction – Basic Considerations Today

Abstract

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.

Keywords

Deformation and metamorphism; Future trend in Himalayan tectonics; Garnet and mylonite study; Lithotectonic zones; Main Central Thrust; New tales of isotope and geochemical studies; Precambrian peninsula; Protolith boundary

Devaprayag : confluence of Alaknanda and Bhagirathi R. (Courtesy Debi P. Das, GSI.)

The present publication deals with Precambrian geotectonics in the Himalayan terrain. It also covers the Precambrians of the frontal peninsula. Regional Precambrian geological events in the greater Indian domain are 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. The very major issues of concern specially to the younger generation have been addressed perhaps with a new outlook, based on analysis of available data and personal observations. Some of the important disputable issues related to Precambrians of the Himalaya discussed with logical solutions in the following chapters are mentioned below.

My studies of the recent contributions on the Precambrians of the Himalaya and the adjacent peninsula inspired me at this age to furnish my old work specially on the Shimla Himalayan terrain and venture to utter some new facts and ideas of regional implication, not all sung so long in proper perspective (also see Chakrabarti, 2016).

1. The so-long described Cenozoic nappe and "klippe" structures in the Lesser Himalayan Crystallines (LHC) are no more considered by this author as Cenozoic in age, these are the product of Paleoproterozoic deformation (F1) and metamorphism (as shown for the Jutoghs and its root zone); the extent of the Main Central Thrust (MCT) to the inner Lesser Himalayan sequence (iLHS) has now to be reconsidered afresh. The so much reported Jutogh Thrust does not exist, and the Chail-Jutogh sequence displays a continuous structural and metamorphic history. The regional F2event (Cambrian ± Cenozoic) had obvious superimposed effects; also, the classic inverted metamorphic (Paleoproterozoic:M1) picture is restricted to the Paleoproterozoic Jutogh-Chail, Daling-Darjeeling (now Paro Gr.), etc., lying below the Higher Himalayan Crystallines (HHC)/new MCT. As reported by Martin et al. (2010), the prograde metamorphic picture in the Modi Khola section has break zones of discontinuity; Chakrabarti (1989, 2009 and 2016) had already highlighted on failure of a multithrust theory to explain a continuous reversed metamorphic picture (thrust units reportedly maintaining normal stratigraphy).

2. The LHC bodies in the Himachal-Uttarakhand sector in particular are considered Paleoproterozoic, carry both Cambrian and Cenozoic orogenic signatures (minus Tertiary granite) and, as in the Shimla Himalaya (Jutoghs and Rampur-Wangtu granite-gneiss-migmatite body tied up as one with oldest Paleoproterozoic age), have a root zone not commonly exposed or shared partially (with the Ramgarh Gr.) with the Munsiari Group. It is now commonly reported that the Cenozoic and Cambrian regional structural trends are mutually parallel (about related products discussed in section 6.2); the alignment of the western closures of the Lansdowne, Almora, and Baijnath LHC bodies has a striking similarity with the mega-F1axial trend.

3. The Rampur Quartzite (+volcanics) framing the oldest Paleoproterozoics exposed in the Rampur-Wangtu-Karcham domain becomes a mylonite gneiss or blastomylonite in different structural levels between Rampur and the upper levels in Shimla-Chor area (affected by later F2 refolding, slip and associated recrystallization) (sec. 6.2), has a sedimentary plus volcanic origin, carries high-grade mineral, and is correlated with Garh Gneiss-Berinag Quartzite-Ulleri-Lingtse Gneiss. It is also held that the Ulleri Gneiss alike the Lingtse Gneiss has no connection with the MCT (compare its setting with the Rampur Quartzite-mylonite gneiss in sec. 6.2).

4. The Salkhalas/Chails/Ramgarh/Dalings had the same Paleoproterozoic history as the older, and it has (normally) a tectonic contact (VT) with the Neoproterozoic HHC across a decollement-unconformity zone. The protolith boundary is the most likely locale of MCT which very commonly represents a slip zone with associated folding across such zone; slip zones like the HHT ofGoscombe et al. (2006) are quite likely developments associated with deformation partitioning.

5. The north-northeast–south-southeast (NNE-SSW) trending (general) mineral lineation (L1) parallel to Paleoproterozoic F1fold axial trend is a b-lineation so conspicuous in the entire Precambrian Himalaya and superposed similar trending a-lineation of later generation (Cambrian/Tertiary) is defined by lower-grade mineral (discussed in sec. 6.2) commonly mimetically recrystallized.

6. The common consideration of two generations of garnet (so common in lower amphibolite facies)across a prograde sequence with structurally and chemically zoned and unzoned grains matters review (see Chakrabarti, 1983; thanks Prof. FJ Turner for advice to study on growth kinetics). The topic has been illustrated in detail under section 6.2.3.3.2 (Deformational and Chemical Growth Pattern in Syntectonic Garnet of Metamorphites).

7. Paleoproterozoic F1 mega-folding is not uncommon in the Himalaya (see section .6.2; mega-folding of the Ulleri Gneiss reported by Upreti, 1999), and the common practice of showing a northeast–southwest (NE–SW) cross-section shows a single-folded (F1) unit as disconnected units; make sections across the NNE–SSW or NE–SW trending mega-folds or search out the F1folds in such sections, not the NE–SW section which exhibits Cambrian/Cenozoic effects. Further, the Paleoproterozoic–Neoproterozoic break between the Lesser Himalayan sequence (LHS) and the HHC represents a decollement and unconformity zone, a most likely site of the MCT.

8. Contribution of Mitra et al. (2010) on duplex formation in LHS and movement of MCT is quite significant. According to them, MCT movement intensity and its areal variations depended on LHD geometry related to kinematic impact experienced by the LHS domain below the thrust. Nevertheless, let us try to identify positive sites of the Neoproterozoic HHC south of the MCT or south of the protolith boundary and let workers on systematic geological mapping in the LHS terrains report inability to continue specially the marker horizons areally due to interplayed Cenozoic tectonics as above!

9. And about the so-commonly observed Mesoproterozoic gap in sedimentation in both the frontal peninsula and the LHS, the observation of Shukla et al. (2020) on the Rajasthan Vindhyans may lead us aboard: the provenance in the western sector was stable not to cause any gap in sedimentation between the Lower and the Upper Vindhyans; change in the provenance during the Kaimur to Rewa group in the western sector caused a break in the deposition.

And lastly, the Paleoproterozoic MCT Zone (MCTZ) rocks look markedly more tectonized (than the HHC; may vary in sectors) with an ingrained old and solid structural fabric and much later superposed refolding and tectonic interplay with a contrast! Perhaps my case studies in the Shimla Himalaya has spelt out a little in this direction!

1.1. Lithotectonic subdivisions of the Himalaya

The Himalayan belt has been traditionally subdivided into a few observed longitudinal lithotectonic zones bounded by thrusts or faults (Fig. 1.1). From south to north, the zones and associated thrusts or faults are—the Main Frontal Thrust (MFT), the sub-Himalayan Zone bounded between the MFT and the Main Boundary Thrust (MBT), the Lesser Himalayan Zone between the MBT and the MCT, the HHCs above the MCT, the Tethyan Zone separated by the South Tibetan Detachment System (STDS) from the HHC, and the Indus-Tsangpo Suture Zone where the Indian plate is subducted below the Asian plate. There are diverse views on such a lithotectonic subdivision.

Figure 1.1  Lithotectonic zones in the Himalaya and the adjacent peninsula. B, Bundelkhand granite; GBF, Great Boundary Fault; GD and SD, Gandak and Sarda Depressions; HHC, Higher Himalayan Crystallines; black, granitoid bodies; ITSZ, Indus-Tsangpo Suture Zone; LHS, Lesser Himalayan Sedimentaries; MBT, Main Boundary Thrust; MCT, Main Central Thrust; MFT, Main Frontal Thrust; STDS, South Tibetan Detachment System; SR, FR, MS, Sargodha, Faizabad, and Monghyr-Saharsa ridges; THBV, Trans-Himalayan Batholiths and Volcanics. After Chakrabarti, B.K., 2016. Geology of the Himalayan Belt–Deformation, Metamorphism, Stratigraphy, Elsevier, 248.

The sub-Himalayan zone is occupied by the Siwaliks with rich content of vertebrate fossils, the lesser Himalayan Zone displays the classic reversal of metamorphic grades, and the HHC sequence shows both normal and reversed ordering of metamorphic grades. The position of the MCT, two MCT—MCT¹ and MCT², and the Munsiari Thrust in place of the original MCT are subject of debate. Also, contributions of recent workers on demarcation of different zones of the Himalayan belt, geotectonic evolution of the HHC, demarcating the MCT, etc., are worth consideration for review of the above conventional subdivisions of the Himalaya.

Before the controversies on the age and original site of deposition of the HHCs (and the extent of areal space it occupies) were being published during the last two decades, a dawn to dusk field worker of Geological Survey of India in the Himalayan terrain for over two decades (and mostly jointly with O.N. Bhargava) S.V. Srikantia (we have lost him recently) had very clearly differentiated three decades back (Srikantia, 1987) two distinct geological domains separated by the MCT: the Lesser Himalaya and the Tethys Himalaya are two contrasting and distinct geotectonic zones within the realm of the Himalaya separated from each other by the MCT. Also, the Lesser Himalaya was considered like most others continuous with the Northern Peninsular India and the Tethys Himalaya demarcated to the north by zones of ophiolitic melange of the Indus Tectonic zone. According to Srikantia (1987), the Tethys Himalayan zone has all the elements of a microcontinent independent of the Tibetan block (Karakorum-Lhasa) in the north and Gondwanic India-Lesser Himalaya to the south. The upper Carboniferous-Permian period probably witnessed a drift of the Proto-Tethys plate and its suturing with the Proto-Karakorum-Tibet plate as evidenced by a Permian-Triassic marine sedimentation all along the northern boundary of the Tethys zone and in Karakorum, volcanic activity and emplacement of ophiolite.

The Tethys Himalayan Tectogen has a thick basement composed of gneiss-migmatite-metasedimentaries with a 15–20 km thick fossiliferous marine Phanerozoic sequence. This succession, according to Srikantia, is exclusive and indigenous to the Tethys Himalaya and is in contrast with the geological picture of the Lesser Himalaya, Indian Peninsula, or the Karakorum-Tibet block. Also, the sedimentation was mostly of shallow marine type except during the early Paleozoic when it was geosynclinal (Srikantia, 1977). In the history of the Tethys Himalaya, the Devonian represents a period of remarkable tectonic stability followed during the Carboniferous to Asselian by stable to unstable shelf condition. However, not noted by many workers, the post-Sakmarian witnessed regression of sea all along the Tethys Himalaya. This phase also coincides with volcanic activity which lasted till the Kungurian particularly in the Kashmir and Zanskar. Srikantia pointed out that there is no evidence of Permian volcanism anywhere in the Indian Shield or in the Lesser Himalaya. Tethys Himalaya is believed to be part of Gondwanaland particularly because of the presence of Gangamopteris, Eurydesma, Deltopecten, and a few productids. However, the pre- and post-Permian fauna of these areas are compared with northern forms. The Permian and Carboniferous flora like Lepidostrobus and Racopteris (unknown to Gondwana) are also found in the Tethys Himalaya of Kashmir and Spiti-Zanskar. Further, Srikantia pointed out that forms such as Schizoneura gondwanensis which are extensively known in the Talchir, Damuda, and Panchet formations of India and also reported from New South Wales are present along with Gangamopteris cf. buriacica, Glossopteris indica, G. augustifolia in the Kusnezk Series of Siberia. Remnants of Glossopteris flora also occur in Tonking (China). We may view Srikantia and Bhargava (1983) in this context. Srikantia leaves for us to consider that Spiti, Kashmir, Siberia, and China formed parts of Gondwanaland during the Permian and parts of Laurasia during the pre- and post-Permian periods. He mentioned another possibility: could there be a certain amount of parallel evolution in widely separated areas! He like many believed that palaeontological evidence bereft of related geotectonic information is not infallible. Therefore, the Himalaya comprising the Tethyan belts of Kashmir, Spiti-Zanskar, and higher Garhwal-Kumaun-Nepal-Sikkim-Bhutan-Arunachal-South Yalutsangpo north of the MCT may be considered as a separate geotectonic entity that was not part of Gondwanaland.

The geological history of the Lesser Himalaya is perhaps much more complex than the Tethys domain. Also the Lesser Himalayan zone was long being regarded as part of the Indian Peninsula (Pascoe, 1950; T. Holand in Wadia, 1957). The seismic studies in the Ganga Plain and deep drilling close to the foothills have indicated continuation of the Vindhyan and the pre-Vindhyan rocks including perhaps the Bundelkhand granite massif of the Peninsula into the Ganga basin and beyond into the Lesser Himalayan domain. Srikantia, therefore, related the peninsular setting to the Proterozoic carbonate belt all along the Lesser Himalaya with a profuse development of columnar stromatolites comparable with the Vindhyan belt of the Indian Peninsula. Further, he mentioned about the thick LHS of argillo-arenaceous sediments (Srikantia, 1978) which are possibly time equivalents of the Upper Vindhyan (Srikantia and Sharma, 1976).

Three decades have gone by, and this octogenarian survives to read a lot of published work on the Himalaya specially during the last two decades. Richards et al., 2005, Martin et al., 2005 (and few others) gave our old outlook an altogether new dimension. They report the first geochemical traverses to integrate U–Pb ages and Hf data on single detrital zircons with bulk-rock Sm–Nd–Rb–Sr isotopic measurements across Himalayan sectors. Their data confirm geochemical distinction between the older Lesser Himalayan Formations and the younger units including the HHC. Paleoproterozoic Jutogh Group and Rampur Formation yielded eNd(500) values below −17, while Neoproterozoic and younger metasediments of the Vaikrita-Haimanta and outer LHS groups have eNd(500) above −13 (distinguish them from the Paleoproterozoic Inner LHS). Zircon populations from the Inner Lesser Himalaya are characterized by Paleoproterozoic–Late Archaean ages (2.6–1.8 Ga), whereas the depositionally younger units contain populations both of this age and a younger period (Meso- to Neo-Proterozoic; 1.1–0.8 Ga). Detrital zircon ages are younger than their respective Hf-isotope derived crustal formation ages by 0.7–2.1 Ga, indicating that the source domains of the detrital zircons consisted of older terrains with considerable amounts of reworking and renewed magmatism.

Further, the distinct feature of the HHC, also shared with the Outer Lesser Himalaya of the Sutlej traverse, is the presence of Meso- to Neo-Proterozoic detritus derived from a complex source area. They have also observed that unlike the 500 Ma granitoids apparently confined to Neoproterozoic units or HHC and Haimanta Group, 1.8–1.9 Ga granitoid bodies (e.g., the Wangtu gneiss in the Sutlej valley) are exclusively associated with the Inner Lesser Himalayan units. According to them, while the Inner Lesser Himalaya sediments derived their detritus from partially reworked Archaean crust similar to the Aravalli craton, depositionally younger units such as the HHC represent a mixture of detritus from this ancient crust and a more juvenile source region. The present contribution would try to review the above observation based on his and available tectonostratigraphic studies (also see Chakrabarti, 2016). Some workers, however, considered the above Rampur-Wangtu body as Central Crystallines; these were mapped by me (Chakrabarti, 1972) as representing core region of regional synformal anticline plunging toward NNE and framed by Rampur Quartzite, and these are now the oldest exposed rocks in Himachal Himalaya (details in Chapter 2 and section on Himachal Himalaya).

Martin et al. (2005) reported on geology of the southern Annapurna Range of central Nepal, specially on structural geometries near the MCT together with whole-rock Nd isotopes and U–Pb ages of detrital zircon. The study distinguished the HHC from the underlying Lesser Himalayan metasedimentary rocks. εNd(0) values for the Lesser Himalayan rocks typically range from −20 to −26, while the HHCs have εNd(0) values of −19 to −12. Further, the Lesser Himalayan rocks yielded detrital zircons with an age peak at ca.1880 Ma and no detrital zircons younger than ca. 1550 Ma. The HHC rocks gave detrital zircons with a prominent broad peak of ages at ca. 1050 Ma and no detrital zircons younger than ca. 600 Ma.

Yin (2006) made notable contribution on along-strike variation of some major features related to Cenozoic evolution of the Himalayan belt. To my interest to the present context, his observation like—in the western Himalaya, the MCT exhibits a major lateral ramp (the Mandi ramp) west of which the MCT places the low-grade Tethyan Himalayan Sequence (THS) over the low-grade LHS, whereas east of the ramp, the MCT places the high-grade HHC over the low-grade LHS (MCT merges with the STDS in Zanskar). This along-strike change in stratigraphic juxtaposition and metamorphic grade across the MCT indicates a westward decrease in slip magnitude along MCT, possibly a result of a westward decrease in total crustal shortening along the Himalayan orogen.

Martin (2016) made a review on the MCT, and we may value his opinion while revising the lithotectonic zones in the Himalaya. The MCT is most commonly identified as a high-strain zone in the Himalaya as the common models demand at least 90 km of shortening. He maintains–Geologists currently employ three unrelated definitions of the MCT: metamorphic-rheological, age of motion-structural, or protolith boundary-structural. These disparate definitions generate map and cross-section MCT positions that vary by up to 5 km of structural distance. The lack of consensus and consequent shifting locations impede advances in our understanding of the tectonic development of the orogen. According to Martin, and judiciously, identification of the MCT should depend on all the above three considerations, the last one most dependable. Searle (2008) had a point that MCT does not always follow one particular stratigraphic horizon, and Martin counters with a reactivation history of the MCT. Martin maintains that It is important to state explicitly that …. If there is a thrust at the Searle et al. (2008) location, the thrust should be labeled with a name other than the MCT unless that position also corresponds to the contact between Himalayan Assemblage A and Assemblage B. Further, Martin makes us remember —… mapping one thrust within a high-strain zone and also a second thrust at the edge of the same high-strain zone is misleading because such a practice gives the appearance that there are two high-strain zones when in fact there is only one.

1.2. Precambrian Himalaya—geological background

The Precambrian regime in the Himalayan domain is now confined between the MBT and the STDS (where identified). A lot of work in field and laboratory has been carried out during the recent years to change much of our understanding of geology of this vast terrain including the nearby areas. It is not possible to reconstruct the Precambrian picture of the Himalayan terrain along with the adjacent peninsula as the Cambrian–Cenozoic events have modified and concealed parts of the terrain, comparable sectors across remain.

1.2.1. The Lesser Himalaya

The Lesser Himalayan zone between the MBT and the Proterozoic HHC consists of a sequence of sedimentaries and overlying crystallines (LHC) occurring either as isolated nappe/klippe or as bodies continuous for long distance. The rock sequence of LHC zone is commonly seen to display reversal of regional metamorphic grades, the higher-grade rocks occurring in progressively higher tectonic levels (may say topographic, as syndeformational picture is not that much dealt with).

1.2.1.1. Lesser Himalayan Sedimentaries

Neoproterozoic rocks are known from many sectors of the Himalaya; however, the older Paleoproterozoic to Mesoproterozoic sequences are almost exclusively restricted to the Lesser Himalaya. The Tons Thrust separates Lesser Himalaya into two parts: the southern outer Lesser Himalaya and the more northern inner Lesser Himalaya. In the inner Lesser Himalaya, a thick lower Paleoproterozoic succession (the youngest rocks of which were deposited at about 1.6 Ga: Kumar et al., 2020) and an unconformably overlying thick succession of uppermost Mesoproterozoic and Neoproterozoic rocks, the oldest of which are ∼1.1 Ga; the most significant break in depositional age thus occurs within the rocks of inner Lesser Himalaya where early Neoproterozoic rocks of the Mandhali Formation unconformably overlie Paleoproterozoic rocks of the Damtha Group and Gangolihat Dolomite (following Kumar et al., 2020). Upper Mesoproterozoic and Neoproterozoic rocks of the outer Lesser Himalaya are mostly siliciclastic which continue upwards to Cambrian strata. According to Kumar et al. (2000: specially co-authors Hughes and Myrow), these sequences resemble the age-equivalent passive margin successions worldwide following the breakup of Rodinia. Recent detrital zircon age data on the Blaini Formation suggests correlation of this diamictite with the Manjir Formation (also Tanakki Conglomerate) and indicates a likely Marinoan (i.e., ∼635 Ma) age (Myrow and Hughes in Kumar et al., 2020).

According to Celerier et al. (2009), the ca.1800-Ma Berinag quartzite is the bottom sequence of the Inner Kumaon-Garhwal Lesser Himalaya overlain across a regional unconformity by the Chakrata and Rautgara Formation slate and turbidite which are succeeded by Neoproterozoic Deoban dolomites and Mandhali carbonaceous slates and carbonates. The Outer LHS comprises ca.850-Ma Chandpur Formation (mostly turbidites), Nagthat quartzite, and Blaini conglomerate. The Outer Lesser Himalayan Sequence units are capped by the late Neoproterozoic Krol limestone and Tal clastic sediments. Trilobites from the Tal Formation indicate an early Cambrian age.

As I have quite long association specially with the Shimla Himalaya, work of specially Bhargava and Srikantia who had field geological contribution in a big way need mention to facilitate my later discussion on various issues. My earlier book publication (Chakrabarti, 2016) covered other areas also, and I would update information on Precambrian geology of those terrains. I summarize Bhargava (2000) to help me add tectonic considerations I nourished so long and apply my old published and unpublished contributions (GSI reports during 1968–1986 and publications in Indian journals during all these years and my two books—Chakrabarti, 2009 and 2016) and have some confidence to complete my so-challenging project at this age!

Bhargava (2000) considered the Bandal-Jeori-Wangtu group of gneisses as the oldest sequence in the western Himalaya (see Chakrabarti, 1972 and with R.C. Dey in 1973, 2009, 2016) over which the Rampur (Sundernagar–Berinag–Rautgara) quartzite with basics was deposited in a rifted basin over a period of 100–200 Ma at around 1.8–1.9 Ga (= Kishtwar window sequence). With a gap of nearly 300 Ma at around 1500 Ma and in a shallow warm sea was deposited the carbonate sequence. At around 1000 Ma, Tattapani-Peontra flows erupted and mineralization was marked in this rifting phase. From Basantpur-Mandhali through Chandpur-Chaosa to Nagthat was a phase of shallowing of the basin. Shimla-Jaunsar deposition was followed with a gap by a transgressive phase and development of the basin at around 750 Ma for the Blainis. The Krol Group in the inner belt is followed up by a full column of the Tals, while the nearby outer Krol belt is devoid of the Tals. According to Bhargava (2000), a fault-triggered uplift was experienced in the outer belt at around 600–550 Ma, also realized in the Batal Formation (possibly onset of Tethyan basin).

1.2.1.2. Lesser Himalayan Crystallines

As described by me (Chakrabarti, 2016), the crystallines from south and southeast of Nanga Parbat separate out as a lower belt of discontinuous crystalline bodies and klippe into Jammu and Kashmir, Himachal Pradesh, and further east. However, in a few places, as in Himachal Pradesh, Nepal, and Darjeeling-Sikkim Himalaya, the lower belt of crystallines (LHC) was reported continuous with the upper belt of crystallines (HHC). The lesser Himalayan sedimentaries (LHS) are often exposed as window due to erosion of the overlying LHC. The LHC is exposed as so-described thrust sheets/nappe and klippes like the Shimla klippe and Jutogh nappe in Himachal Pradesh, the nappe/klippe of Almora, Lansdowne, Askot, etc., of Kumaon Himalaya, the Jajarkot and Kathmandu nappe/klippe of Nepal Himalaya, etc. Valdiya reported metamorphic intensity in the Almora klippe/nappe reaching only lower amphibolite facies; however, later workers could find assemblage