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.
In particular, the rifting event initiated the opening of the Tethys Ocean during which the Cimmerian Plate travelled north and moved away from Gondwana.
Immediately, primary evidence from detrital zircon ages, paleocurrent recorded in the units, and animal similarities supports these predictions.
Furthermore, when samarium–neodymium dating of the entire North Indian sequence was made, significant overlap of Nd isotopic signatures between the different rocks units indicates a sharing of similar sources.
[16] Finally, both sequences contain the same Early Cambrian equatorial trilobite species, reinforcing the likelihood of the passive continental margin model.
[24] In spite of the metamorphic grade of Greater Himalaya, the protolith lithology is nevertheless similar to the other zones and possibly shares the same depositional setting.
[27] Results of the zircon ages and possible protolith lithologies and their corresponding first-order similarities between the rock units from Myrow et al. have generally discredited this model.
[2] Other models would require greater slip along the main central thrust during the Cenozoic orogeny to achieve the present stratigraphical arrangement.
Deformation produced by Paleozoic tectonics may have been overprinted by the Cenozoic reactivation of the main central thrust, which result in the lack of old suture zone rocks.
[25] Further investigation is required to support more predictions of this model, since metamorphism of the Greater Himalayan strata in the early Paleozoic, and Cambrian to Ordovician syn-tectonic sediments in Tethyan Himalaya, are lacking.
[17] This model fails to reconcile with detrital zircon ages, and paleontological evidence with respect to the similarities between Tethyan and Lesser Himalaya.
Trilobite fauna, paleocurrent and lithological similarities would be unlikely if the rock units were formed in separate terranes as explained by Myrow et al.[16] Earlier predictions[17] that suggest a thrust/ shortening event in Tethyan Himalaya in the early Paleozoic, and the simultaneous formation of granitic intrusions under the sequence, were also undermined by the apparent rifting isotopic signatures in the granites.
[8] This model illustrates that in Carboniferous, due to possible rifting,[4] the Lesser and Greater Himalaya were separated by a north-dipping normal fault.
This model explains the apparent missing lower Paleozoic strata in Lesser Himalaya due to footwall uplift and erosion, and provides a possible solution to the age relationship across the main central thrust by the reactivation of this ancient normal fault.
