GEOCHEMISTRY OF TRAVERTINE-DEPOSITING SPRINGS NEAR AND ALONG THE SOUTH TIBETAN DETACHMENT SYSTEM, MOUNT EVEREST REGION, TIBET

Travertine deposits and active travertine-depositing springs in the Mt. Everest region provide new insights into the nature and scale of fluid circulation beneath the southern margin of the Tibetan plateau. We present new data from three travertine systems that spatially coincide with the generally east-west trending South Tibetan detachment system (STDS) and north-south trending, graben forming normal faults; north of Nylam, northwest of Kharta, and near Old Tingri.

North of Nylam travertine deposits exposed along the STDS contain at least two previously undocumented active travertine spring mounds. Spring temperatures range from 10-20 ºC, with pH between 6.3-6.6, conductivities from 506-1955 µS, and alkalinities from 207-1397 mg/l HCO3-. Carbonate cemented river terraces and massive carbonate cemented breccias form a travertine complex along the STDS northwest of Kharta (~120 km east of Nylam). Here, travertine deposits include flowstone and vent structures located ~200 m above the present river level and carbonate cemented fluvial terraces located at present river level. Carbonate cemented breccias are enigmatic and may represent fault breccias, cemented landslide deposits, or cemented colluvium. Although active travertine deposition was absent here, we sampled gas and water from two springs with distinctly lower pH and higher conductivities than adjacent surface water. We compare our results from travertine occurrences associated with the STDS to previously investigated travertine systems along north-south grabens (Old Tingri). The graben-associated Tsamda and Gondasampa (previously undocumented) thermal springs had temperatures of 43-25 ºC, were bubbling, ranged in pH between 6.3-6.8, had conductivities from 1914-3440 µS, and alkalinities from 1500-2840 mg/l HCO3-.

Depressed pH and high alkalinities of these springs strongly suggest a signature from deeply circulated, CO2-charged fluids. Their spatial association with the STDS suggests a link between this crustal-scale shear zone, fluid circulation, and mechanisms for generating CO2. Gas composition and He isotopic analysis will strengthen our preliminary results by constraining the origin of the spring gases and explore their implications for fluid circulation and crustal structure beneath the southern Tibetan plateau.