Deformation of the Shillong Group rocks, Shillong Basin, Meghalaya, north-east India: Implication on the Proterozoic supercontinent build-up

The effects of deformation and concomitant high-grade metamorphism during the Cenozoic period in the active Himalayan orogen hinder the reconstruction of the original geological framework and thus the evolution history. The Shillong Group in the Shillong Plateau, northeastern India, rests unconformably above the Himalayan crystalline basement gneiss, and is divided in two formations, the Lower Metapelite Formation (LMF) and the Upper Quartzite Formation (UQF). There are three horizons of conglomerate: (a) Basal, between the basement and the Shillong Group, (b) an inter-formational conglomerate, between the LMF and UQF, and (c) an intra-formational conglomerate (within the UQF). The clasts of the latter two conglomerates are suitable strain markers. Principal compressional structures in the Shillong Group were developed in two successive deformation episodes during the Meso- to Neo-Proterozoic period. The earlier episode was of progressive general shear deformation, while the later one possibly occurred in a transpressive mode similar to that of a fold-thrust belt. Remarkably, both episodes underwent NW-SE regional compression. The inter-formational conglomerate represents a flattening type strain of relatively lesser magnitude (R-f-phi X:Y:Z = 1.35:1:0.76), in contrast to the constriction type strain of relatively higher magnitude (R-f-phi X:Y:Z = 2.42:1:0.44) of the intra-formational conglomerate. Field relations of the Shillong Group with the deformed and undeformed granite plutons suggest that the earlier deformation episode possibly took place during Rodinia assembly, due to India-Antarctica collision at 1,100 Ma, while the later one happened in a time frame close to 500 Ma, during the amalgamation of Eastern Gondwana.