Evolution of the Paleogene succession of the western Himalayan foreland basin

The Paleogene succession of the Himalayan foreland basin is immensely important as it preserves evidence of India-Asia collision and related records of the Himalayan orogenesis.In this paper,the depositional regime of the Paleogene succession of the Himalayan foreland basin and variations in composition of the hinterland at different stages of the basin developments are presented.The Paleogene succession of the western Himalayan foreland basin developed in two stages,i.e.syncollisional stage and post-collisional stage.At the onset,chert breccia containing fragments derived from the hanging walls of faults and reworked bauxite developed as a result of erosion of the forebulge. The overlying early Eocene succession possibly deposited in a coastal system,where carbonates represent barriers and shales represent lagoons.Up-section,the middle Eocene marl beds likely deposited on a tidal flat.The late Eocene/Oligocene basal Murree beds,containing tidal bundles,indicate that a mixed or semi-diurnal tidal system deposited the sediments and the sedimentation took place in a tidedominated estuary.In the higher-up,the succession likely deposited in a river-dominated estuary or in meandering rivers.In the beginning of the basin evolution,the sediments were derived from the Precambrian basement or from the metasediments/volcanic rocks possessing terrains of the south.The early and middle Eocene（54.7-41.3 Ma） succession of the embryonic foreland possibly developed from the sediments derived from the Trans-Himalayan schists and phyllites and Indus ophiolite of the north during syn-collisional stage.The detrital minerals especially the lithic fragments and the heavy minerals suggest the provenance for the late Eocene/Oligocene sequences to be from the recycled orogenic belt of the Higher Himalaya,Tethyan Himalaya and the Indus-suture zone from the north during post-collisional stage.This is also supported by the paleocurrent measurements those suggest main flows directed towards southeast,south and east with minor variations.This implies that the river system stabilized later than 41 Ma and the Higher Himalaya attained sufficient height around this time.The chemical composition of the sandstones and mudstones occurring in the early foreland basin sequences are intermediate between the active and passive continental margins and/or same as the passive continental margins.The sedimentary succession of this basin has sustained a temperature of about 200 C and undergone a burial depth of about 6 km.

Magnetic fabric and geomorphic characteristic of Neotectonic activity along strike direction of North Almora Thrust,Kumaun Lesser Himalaya,India

Structurally,the North Almora Thrust(NAT),is placed in the northern proximity of the Almora Nappe(AN)in the Kumaun Lesser Himalaya.The NAT is characterized by the presence of a zone of mylonitic rocks of basal Saryu Formation(1800±100 Ma)in the hanging wall and the metasedimentaries of Inner Lesser Himalayan rocks as the footwall block.The NAT has been dextrally offset by two estabilished maj or faults i.e.Saryu River Fault(SRF)in Saryu Valley and Dwarahat-Chaukhutia Fault(DCF)in Ramganga valley.In present study,we identified four new faults,based on field study,geomorphic landform and magnetic fabric analysis.These faults are N-S trending Pancheshwar Fault,NE-SW Rameshwer Fault,NNE-SSW trending Kosi Fault,and NNE-SSW trending Gagas Fault(GF).Our results show that the zone is bound by cross cutting relation with NAT and these fault zones are comparatively more active than other regions.Furthermore,we suggest that the steep and NW-SE orientation of magnetic foliation within the NAT zone is a result of NE-SW oriented progressive regional compression.The magnetic foliations represent the unseen internal foliations in the rocks developed due to preferred alignment of magnetic minerals and can be found through the Anisotropy of magnetic susceptibility(AMS)study.Variation in the alignment of the field and magnetic foliations are developed due to superimposed brittle deformation along the faults over the pre-existing field foliations.Magnetic foliations represent the impact of last stage deformation and significant to find brittle deformation and finite strain in the rocks of the study area.The lowering of anisotropy(Pj)away from the fault zone represents distribution of strain across the NAT zone.AMS fabric confirms the presence of faults developed across the NAT zone and also explains the deformation pattern along these faults.The geomorphic anomalies and steepness changes across the NAT zone are correlated with active deformation along the NAT and associated transverse faults.