Strain analysis and stratigraphic status of Nongkhya, Sumer and Mawmaram conglomerates of Shillong basin, Meghalaya, India

Abstract

Shillong basin, one of the Purana basins of the Indian peninsula is situated in the central and eastern parts of the Shillong plateau of NE India. Metasedimentary and metavolcanic rocks of the basin are of Mesoproterozoic age and lithostratigraphically belong to Shillong Group (erstwhile Shillong series) comprising Lower Metapelitic Formation (LMF) and Upper Quartzitic Formation (UQF). A long, persistent, faulted and tectonically attenuated conglomerate known as Nongkhya-Sumer-Mawmaram-conglomerate separates these two formations. In the present work, quantitative strain analyses of the pebbles of Sumer, Nongkhya and Mawmaram conglomerates of Ri-Bhoi and West Khasi Hills districts of Meghalaya are carried out using manual and computerized programmes. Eight different techniques for intrapebble, interpebble and bulk rock strain estimation are applied and results are compared systematically. Sumer and Mawmaram conglomerates bear the testimony of broadly flattening type of deformation (0 < k < 1) while Nongkhya conglomerate shows constriction type (1 < k < α). The change in strain ellipsoid shape from Nongkhya to Mawmaram area is accompanied by a change of fabric from S < L to L < S tectonites. Affinity of rotational strain is more in Nongkhya conglomerate as compared to Sumer and Mawmaram conglomerates. The compactness of pebbles is high in case of Nongkhya conglomerate and low to moderate in Sumer and Mawmaram conglomerates indicating high strain in Nongkhya conglomerate (northeastern part of Shillong basin) relative to Sumer and Mawmaram conglomerates (southwestern part of Shillong basin). Thus strain magnitude increases from SW to NE direction of the Shillong basin.The tectonostratigraphic status of these conglomerates suggest that the Sumer and Mawmaram conglomerates were initially a part of one conglomerate horizon of interformation type between LMF and UQF of the Shillong Group. With progressive deformation, the northeastern part of the Sumer conglomerate suffered tectonic attenuation and separation and eventually thrusted over the Basement Gneissic Group (BGG) as a tectonic mélange. This sector of the conglomerate is known as Nongkhya conglomerate. The tectonic configuration of the Nongkhya conglomerate is the effect of right and left lateral strike slip movement of Sumer conglomerate at Sumer and Adabasti points, respectively. This is a positive signature of post D 3 deformation on the Sumer conglomerate. The regional sigmoidal pattern of the interformational conglomerate broadly correlate with the Tyrsad-Barapani Shear Zone (TBSZ) of sinistral nature.