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Pre-collisional Himalaya
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Pre-collisional Himalaya
Pre-collisional Himalaya is the arrangement of the Himalayan rock units before mountain-building processes resulted in the collision of Asia and India. The collision began in the Cenozoic and it is a type locality of a continental-continental collision. The reconstruction of the initial configuration of the rock units and the relationship between them is highly controversial, and major concerns relate to the arrangements of the different rock units in three dimensions. Several models have been advanced to explain the possible arrangements and petrogenesis of the rock units.
In the Himalaya, the rock units are conventionally divided into four major sections. From North to South, they are:
The Tethyan Himalayan sequence, Greater Himalayan crystalline complex, and Lesser Himalayan sequence are grouped together as the North Indian sequence due to the overlapping age from Proterozoic to Phanerozoic. For Pre-collisional Himalaya, only the North Indian Sequence is of concern as the Sub-Himalayan sequence is a rock unit that was deposited at the same time as the India and Asia collision and the resulting mountain-building process.
The Tethyan Himalayan sequence is composed of mainly siliciclastic and carbonate sedimentary rocks deposited from 1840 Ma to 40 Ma. These are inter-bedded with volcanic rocks of Paleozoic and Mesozoic age. This sequence is divided into several sub-units due to the different lithofacies present in the sequence. The lithofacies of the rocks are a result of the shift in the depositional environment. Specific to this sequence, the Carboniferous to Jurassic rifting event was the cause of the changing depositional environment. In particular, the rifting event initiated the opening of the Tethys Ocean during which the Cimmerian Plate travelled north and moved away from Gondwana. The boundary and age between the several sub-units are poorly constrained, yet the whole sequence is generally considered to have first developed in Neoproterozoic. The 1840 Ma of the rocks was determined by rubidium–strontium dating of the Baragoan gneiss, however some have allocated the gneiss into the Lesser Himalayan Sequence instead.
Generally, the Greater Himalayan crystalline complex is a belt of high-grade metamorphic rocks that extends along the east-trending length of the Himalayan range. It contains leucogranites interspersed throughout the entire complex, and they are early to middle Miocene of age. The complex is sandwiched by two major faults, with the Main Central Thrust in the south and the South Tibetan Detachment to the North. Additionally, the Tethyan Himalayan sequence overlies the complex. The estimated age of the complex ranges from 1800 Ma to 480 Ma, however these ages are poorly constrained. From bottom to top, the metamorphic grade of the complex first increases up section, it is then reversed, with the metamorphic grade decreasing up section. The transition occurs between the middle and top portions of the complex. In addition, inverted metamorphism appears at Central Nepal.
The Lesser Himalayan sequence is characterized by low-grade meta-sedimentary rocks, metavolcanic rocks and augen gneiss. The majority of the sequence is marine deposits, however, most of the sequence does not contain any fossils; only in rare instances will a formation contain fossils. Major formations of the sequence include the Tal Formation, Gondwana Strata, Singtali Formation, and Subathu Formation. The entire sequence has an age range of 1870 Ma to 520 Ma. In North West India, the sequence is overlain by the Cambrian strata, while in Pakistan, Cambrian or Carboniferous strata from the Tibetan Himalaya sequence overlays the Mesoproterozic strata of the Lower Himalayan sequence.
The initial configuration of pre-collisional Himalaya can be expressed in the four following models:
This model is a single margin model. Here, the North Indian sequence was deposited on a continental margin of northern India that was facing north. The units in the North Indian sequence represents the same set of sediments, but became distinct, since nearshore and offshore conditions changes the characteristics of the rock units during deposition.
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Pre-collisional Himalaya
Pre-collisional Himalaya is the arrangement of the Himalayan rock units before mountain-building processes resulted in the collision of Asia and India. The collision began in the Cenozoic and it is a type locality of a continental-continental collision. The reconstruction of the initial configuration of the rock units and the relationship between them is highly controversial, and major concerns relate to the arrangements of the different rock units in three dimensions. Several models have been advanced to explain the possible arrangements and petrogenesis of the rock units.
In the Himalaya, the rock units are conventionally divided into four major sections. From North to South, they are:
The Tethyan Himalayan sequence, Greater Himalayan crystalline complex, and Lesser Himalayan sequence are grouped together as the North Indian sequence due to the overlapping age from Proterozoic to Phanerozoic. For Pre-collisional Himalaya, only the North Indian Sequence is of concern as the Sub-Himalayan sequence is a rock unit that was deposited at the same time as the India and Asia collision and the resulting mountain-building process.
The Tethyan Himalayan sequence is composed of mainly siliciclastic and carbonate sedimentary rocks deposited from 1840 Ma to 40 Ma. These are inter-bedded with volcanic rocks of Paleozoic and Mesozoic age. This sequence is divided into several sub-units due to the different lithofacies present in the sequence. The lithofacies of the rocks are a result of the shift in the depositional environment. Specific to this sequence, the Carboniferous to Jurassic rifting event was the cause of the changing depositional environment. In particular, the rifting event initiated the opening of the Tethys Ocean during which the Cimmerian Plate travelled north and moved away from Gondwana. The boundary and age between the several sub-units are poorly constrained, yet the whole sequence is generally considered to have first developed in Neoproterozoic. The 1840 Ma of the rocks was determined by rubidium–strontium dating of the Baragoan gneiss, however some have allocated the gneiss into the Lesser Himalayan Sequence instead.
Generally, the Greater Himalayan crystalline complex is a belt of high-grade metamorphic rocks that extends along the east-trending length of the Himalayan range. It contains leucogranites interspersed throughout the entire complex, and they are early to middle Miocene of age. The complex is sandwiched by two major faults, with the Main Central Thrust in the south and the South Tibetan Detachment to the North. Additionally, the Tethyan Himalayan sequence overlies the complex. The estimated age of the complex ranges from 1800 Ma to 480 Ma, however these ages are poorly constrained. From bottom to top, the metamorphic grade of the complex first increases up section, it is then reversed, with the metamorphic grade decreasing up section. The transition occurs between the middle and top portions of the complex. In addition, inverted metamorphism appears at Central Nepal.
The Lesser Himalayan sequence is characterized by low-grade meta-sedimentary rocks, metavolcanic rocks and augen gneiss. The majority of the sequence is marine deposits, however, most of the sequence does not contain any fossils; only in rare instances will a formation contain fossils. Major formations of the sequence include the Tal Formation, Gondwana Strata, Singtali Formation, and Subathu Formation. The entire sequence has an age range of 1870 Ma to 520 Ma. In North West India, the sequence is overlain by the Cambrian strata, while in Pakistan, Cambrian or Carboniferous strata from the Tibetan Himalaya sequence overlays the Mesoproterozic strata of the Lower Himalayan sequence.
The initial configuration of pre-collisional Himalaya can be expressed in the four following models:
This model is a single margin model. Here, the North Indian sequence was deposited on a continental margin of northern India that was facing north. The units in the North Indian sequence represents the same set of sediments, but became distinct, since nearshore and offshore conditions changes the characteristics of the rock units during deposition.
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