PETROGRAPHY OF THE ORDOVICIAN QOMOLANGMA FORMATION AND YELLOW BAND, MOUNT EVEREST, NEPAL

The extreme challenges of conducting geologic field work above 8,000 meters (in the “death zone”) on the Mount Everest massif have limited the number of rock samples available for research over the decades since Everest was first climbed. However, an expedition to Mount Everest in the spring of 2012, sponsored by National Geographic, The North Face and Montana State University, provided a unique opportunity to collect a suite of rock samples from the upper elevations of Everest on the Nepal side, specifically along the Southeast Ridge from the South Col to the highest outcrops immediately below the summit snow cornice. This part of our Everest research focuses on two formations that comprise the upper 400 meters of Mount Everest: the Ordovician Qomolangma Formation which forms the summit pyramid and the subjacent calc-silicate Yellow Band (North Col Formation). These units are separated by the NNE-dipping Qomolangma detachment, a low angle normal fault which has played a significant role in the magmatic and tectonic evolution of the Greater Himalaya. Thirteen samples were collected from seven locations across the Qomolangma Formation and Yellow Band. Petrographic analysis of these samples has revealed that the degree of strain and low-grade metamorphism within the Qomolangma Formation is not homogenous. Rather, both seem to systematically increase from sheared dolomitic limestone on the summit (8850m) to significantly recrystallized limestone or marble on the South Summit (8750m), approximately 30 meters above the Qomolangma detachment. This gradient has been quantified using grain size and subgrain size of dynamically recrystallised quartz and calcite, calcite E-twin morphology, and correlation of microstructures to specific dislocation creep regimes. Numerous studies posit that the Qomolangma detachment marks a sharp discontinuity in metamorphic grade between ‘unmetamorphosed’ Tethyan limestone of the Qomolangma Formation in the hanging wall and greenschist facies rocks of the North Col Formation in the footwall below. However, our data suggest that the difference in metamorphic grade across the Qomolangma detachment on Everest may be significantly less than previously reported which, in turn, is due to the gradational nature of strain and metamorphism partitioned within the Qomolangma Formation.