A P–T–t–D discontinuity in east-central Nepal: Implications for the evolution of the Himalayan mid-crust

Abstract Rock specimens collected from lower, middle, and upper portions of the exhumed Himalayan mid-crust in the upper Tama Kosi region of east-central Nepal yield pressure–temperature–time–deformation (P–T–t–D) paths that demonstrate the presence of a structural, metamorphic, and geochronologic discontinuity. A P–T–t–D path from below the discontinuity defines a loading path with deformation at staurolite-grade metamorphic conditions occurring at ~ 10–8 Ma. P–T–t–D paths from above the discontinuity, in contrast, record an earlier and protracted metamorphic history that includes decompressional heating. Monazite ages from specimens collected above the discontinuity constrain prograde garnet growth to be > 21 Ma with decompression-related garnet breakdown initiating at c.19 Ma and likely continuing until c.15 Ma. These contrasting P–T–t–D histories separated by a tectonometamorphic discontinuity are consistent with an evolutionary model for the Himalaya in which rocks above the discontinuity were metamorphosed in the deep hinterland of the orogen and deformed during lateral extrusion from beneath Tibet while rocks below the discontinuity were deformed and metamorphosed later within the shallower foreland of the orogen. These results demonstrate the spatial and temporal relationships and compatibility between lateral midcrustal extrusive flow and wedge taper processes within an evolving orogen.