Paper No. 139-8
Presentation Time: 4:10 PM
GEOCHRONOLOGY AND THERMAL HISTORY OF THE MOUNT EVEREST MASSIF
LARSON, Kyle P.1, COTTLE, John2, SHRESTHA, Sudip3, BUTTON, Mark3, CAMACHO, Alfredo4, STEELE-MACINNIS, Matthew5, LAGESON, David6 and CREIGHTON, Steven7, (1)Earth, Environmental and Geographic Sciences, University of British Columbia, Okanagan, 1177 Research Road, Kelowna, BC V1V 1V7, Canada, (2)Department of Earth Science, University of California Santa Barbara, Santa Barbara, CA 93106, (3)Fipke Laboratory for Trace Element Research, University of British Columbia, Okanagan, 3247 University Way, Kelowna, BC V1V 1V7, Canada, (4)Department of Earth Sciences, University of Manitoba, Winnipeg, MB R3T 2N2, Canada, (5)Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada, (6)Department of Earth Sciences, Montana State University, 226 Traphagen Hall, P.O. Box 173480, Bozeman, MT 59717, (7)Saskatchewan Research Council, Advanced Microanalysis Centre, Saskatoon, SK S7K 0X8, Canada
From the earliest expeditions on Mount Everest, the geology of the massif has been the subject of scientific observation. While the existing geologic information from the massif, which includes the occurrence of the large-scale Qomolangma and Lhotse detachment faults, facilitates a basic assessment of the geological history of the rocks therein, there remain significant gaps in spatial and thermal representation of the available data. No data, for example, have been published from the massif between the Yellow Band (~ 8500 m), in the immediate footwall of the Qomolangma detachment, and the structural level of the Nuptse granite below the Lhotse detachment (~ 7000 m), limiting interpretations about the kinematics within the massif.
The present study re-examines specimens collected from across the Mount Everest massif as part of the 2012 National Geographic Legacy Expedition using in situ geochronometers including apatite U-Pb, white mica and biotite Rb-Sr, single crystal white mica and biotite 40Ar/39Ar and thermometry based on Raman spectroscopy of carbonaceous material (RCSM). New specimens from the South Col and the Lhotse face were also examined to extend spatial coverage farther across the massif. The apatite U-Pb data show mixing or partial resetting of early Paleozoic (detrital?) material during Himalayan orogenesis. Rb-Sr and 40Ar/39Ar data is entirely Himalayan (<55 Ma) and records a significant break in dates from early Eocene (ca. 40-48 Ma) at an elevation of ~ 8500 m in the Yellow Band to early Miocene (ca. 18 Ma) at lower (~7900 to 7500 m) elevations. At ~7500 m, apparent prograde biotite yield overlapping ca. 18 Ma Rb-Sr and 40Ar/39Ar dates, whereas later, alteration-related white mica yield distinctly younger ca. 16 Ma dates. A similar break in RCSM temperature is noted over the same interval where temperatures increase from ~ 430 ˚C (8840 m) to ~510 ˚C (8520 m) before dropping to ~440˚C (7520 m). The apparent temperature break is further consistent with a ~ 400 ˚C Ti-in-biotite temperature at an elevation of 7906 m.
These new data demonstrate the geological complexity within Mount Everest massif. The new dataset can be reconciled by early Miocene movement across a previously unrecognized thrust fault, here termed the Adrishya thrust.
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