Diachronous deformation along the base of the Himalayan metamorphic core, west-central Nepal

Abstract

Geologic mapping combined with microstructural and geochronologic analyses from the lower Himalayan metamorphic core in west-central Nepal record along-strike similarity in flow style despite variability in the timing of metamorphism and deformation. The Main Central thrust zone at the base of the Himalayan metamorphic core varies in thickness, tectonostratigraphy, and metamorphic gradient along the 250 km of strike length studied. In situ U-Th/Pb geochronology of monazite sampled from an along-strike transect at the top of the high-strain zone records Eocene−Oligocene prograde metamorphism followed by Miocene retrograde metamorphism. The timing of prograde and retrograde metamorphism and the muscovite 40 Ar/ 39 Ar dates gradually decrease along strike from northwest to southeast. This age trend is punctuated by an abrupt ∼3−8 m.y. decrease in the age of prograde and retrograde metamorphism and muscovite 40 Ar/ 39 Ar dates near the Marsyandi River in central Nepal. Quartz crystallographic preferred orientation fabrics from a parallel transect along the base of the high-strain zone document similar flow style at ∼440 °C in central Nepal. Muscovite 40 Ar/ 39 Ar ages, interpreted to approximate the age of deformation at this structural level, decrease from ca. 7 to 4 Ma along strike from northwest to southeast. Diachronous deformation and metamorphism along strike in west-central Nepal demonstrate the necessity of incorporating more than a single transect into tectonic models. Along-strike tectonometamorphic variability in west-central Nepal spatially corresponds to faults in the Indian basement bounding the subsurface Faizabad ridge, highlighting the possible influence of inherited basement faults on the geometry of the basal Himalayan detachment, the Main Himalayan thrust, as well as the tectonic evolution of the structurally overlying Himalayan metamorphic core. This study highlights the potential influence of inherited structures on the overlying orogenic wedge and the probability of along-strike diachroneity of deformation in the Himalaya.