The structural evolution of the Qomolangma Formation, Mount Everest, Nepal

Re-examination of specimens collected across the Qomolangma Formation exposed in the summit pyramid on Mount Everest confirms a complex, multi-deformational history. Two pre-Oligocene deformational events are recorded within the calcite-dominated rocks from the main summit (8848 m). The first event is characterized by an R_XZ of ~1.3–1.4 in detrital quartz grains, which increases to ~1.6 after the second event. The finite strain recorded in calcite in the same rocks is higher with an R_XZ of ~2.7–2.8. The deformation in the main summit appears to have been accommodated dominantly through grain boundary sliding in calcite in a slightly constrictive environment with a vorticity of >0.81. Deformation in the South Summit (8750 m) contrasts with that of the top and is instead interpreted to reflect Miocene movement along the Qomolangma detachment with no earlier deformation preserved. U–Pb geochronology on synkinematic rutile indicates that movement on the structure occurred at 16.3 ± 5.0 Ma. Rocks within the Qomolangma Formation in the summit pyramid of Mount Everest appear to record the basic history of the Himalaya with early deformation likely related to crustal thickening and later, more spatially restricted deformation reflecting the Miocene extrusion of the metamorphic core.