Himalayan Geology, Vol. 42 (2), 2021, pp. 263-289, Printed in India
Evolution of the Lesser Himalaya in space and time
OM N. BHARGAVA1*, BIRENDRA P. SINGH1, WOLFGANG FRANK2, SUNIL K. TANGRI3
1Centre of Advanced Studies in Geology, Panjab University, Chandigarh-160014, India
2Koehldorfergasse 26, A-3040 Neulengbach, Austria, 31268 Sector 15, Chandigarh-160015, India
*Email (corresponding author): onbhargava@gmail.com
Abstract: The gneisses and associated metasediments and intrusive ca. 1860 Ma Wangtu Granite, designated as the Jeori-Wangtu-Bandal Gneissic Complex (JWBC), formed the basement for the oldest Lesser Himalayan rocks, when India was part of the Columbia supercontinent. A mega rift extending over the entire length of the present Himalaya developed around ca. 1800 Ma. It was accompanied with outpouring of the tholeiitic volcanics, the event was diachronous that younged towards the east. In this shallow marine basin, the Rampur/ Berinag/ Kuncha/Kushma/Ranimata Daling/ Shumar groups accumulated. The lowest thrust sheet, labelled as the Kulu/ Munsiari/Ramgarh Porphery/ Daling/ Jashidanda thrust sheets, were nucleated a bit later or more or less coinciding with the rifting. The thrusting involved the basement rocks, which occur as tectonic slices in all the sectors of the Himalaya.
Sedimentation of the Rampur Group and equivalent sequences ceased and after a pronounced hiatus, a carbonate shelf developed during the Mesoproterozoic-Early Neoproterozoic, in which sequences, variously designated as the Shali/Larji/Deoban/Calc “Series”/Dhadang/Baxa/Dedza groups were deposited over the Rampur and equivalent sequences. At that time India formed part of the Rodina.
Rifting ca. 830 Ma, demolished the Shali-Baxa carbonate shelf. The succeeding rifted basin, located over the disrupted Mesoproterozoic-Early Neoproterozoic sequences, was also extensive and covered the nascent Kulu and equivalent thrust sheets. More or less simultaneously or with some time lag, the Chaur Granite and the embryonic Jutogh Thrust Sheet were emplaced. In this rifted basin, the Simla/ Jaunsar/ Nathuakhan and allied groups were deposited. The sedimentation in this basin terminated ~730 Ma, its sediments were raised to form an upland, which became abode of the Blaini valley glaciers. The glaciers descended in a marine basin with the Simla (which constituted the Outer Krol Belt) and the Jaunsar (Inner Krol Belt) forming the floors. A post-Blaini marine regression restricted the sea in which the Ediacaran Infra Krol and Krol sequences conformably succeeded the Blaini Formation. At the termination of the Krol sedimentation, the Outer Krol Belt was uplifted, the event coincided with the rifting reported in the Tethyan Bhutan. The sedimentation of the Tal Group continued in the Inner Krol Basin sited over the Jaunsar Group. Another basin (Chilar) was carved towards the SE of the Outer Krol basin, which possibly lasted up to the end of the Ediacaran. The late Cambrian-Early Ordovician Kurgiakh Orogeny terminated the Tal basin. As a component of the Kurgiakh orogeny in its early phase, the Cambrian Tethyan basin together with the underlying Vaikrita rocks was accredited to the Lesser Himalaya Tectogen; the event was associated with Early Palaeozoic granite emplacement and metamorphism. The Early Ordovician witnessed a rather extensive marine transgression, which possibly lasted till the Middle Silurian.
Post Middle Silurian, in short intervals during the Permian and Cretaceous, no sedimentation took place in the Lesser Himalayan terrain which remained a positive area; its erosion contributed sediments to the Tethyan basin. The rifting in the Gondwanaland/ Pangea, opened old lineaments, mainly close to the present foothill region to provide pathways to the Permian and Cretaceous seas. The subduction of the Indian Plate commenced during the Thanetian, consequently, the foreland basin was formed at the present foothill region; its embayments extended in the Lesser Himalaya over the folded and deeply eroded pre-Paleocene sequences.
The continued subduction of the Indian Plate caused the earlier nucleated thrust sheets to advance southward and as a consequence of which the foreland basin shifted farther south. The Lesser Himalaya formed the provenance of this basin in which the Siwalik succession was deposited.
Due to impact of advancing crystalline thrust sheets, the Shali and Simla Group rocks moved partly over the Paleogene of the foreland basin. The Outer and Inner Krol Belts slid on their floors as superficial thrust sheets. The Late Cambrian folds, which are co-axial with the Himalayan folds, got accentuated and also selectively overturned. The Lesser Himalaya part was elevated. Weathering led to extensive erosion and release of the stresses, which periodically activated various faults causing earthquakes.
Keywords: Lesser Himalaya, Paleoproterozoic, Mesoproterozoic, Tonian, Ediacaran, Cambrian, Paleogene, Neogene
