Infiltration of meteoric water in the South Tibetan Detachment (Mount Everest, Himalaya): When and why?

Abstract

The South Tibetan Detachment (STD) in the Himalayan orogen juxtaposes low‐grade Tethyan Himalayan sequence sedimentary rocks over high‐grade metamorphic rocks of the Himalayan crystalline core. We document infiltration of meteoric fluids into the STD footwall at ~17–15 Ma, when recrystallized hydrous minerals equilibrated with low‐δD (meteoric) water. Synkinematic biotite collected over 200 m of structural section in the STD mylonitic footwall (Rongbuk Valley, near Mount Everest) record high‐temperature isotopic exchange with D‐depleted water (δDwater = −150 ± 5‰) that infiltrated the ductile segment of the detachment most likely during mylonitic deformation, although later isotopic exchange cannot be definitively excluded. These minerals also reveal a uniform pattern of middle Miocene (15 Ma) 40Ar/39Ar plateau ages. The presence of low‐δD meteoric water in the STD mylonitic footwall is further supported by hornblende and chlorite with very low δD values of −183‰ and −162‰, respectively. The δD values in the STD footwall suggest that surface‐derived fluids were channeled down to the brittle‐ductile transition. Migration of fluids from the Earth's surface to the active mylonitic detachment footwall may have been achieved by fluid flow along steep normal faults that developed during synconvergent extension of the upper Tethyan Himalayan plate. High heat flow helped sustain buoyancy‐driven fluid convection over the timescale of detachment tectonics. Low δD values in synkinematic fluids are indicative of precipitation‐derived fluids sourced at high elevation and document that the ground surface above this section of the STD had already attained similar‐to‐modern topographic elevations in the middle Miocene.

Meteoric fluid‐rock interaction in Variscan shear zones

“…These results are in good agreement with previous studies conducted on ductile shear zones in the Pyrenees, the New Zealand Alps, the North American Cordillera and the Himalaya that highlight the infiltration of meteoric fluids at similar depths (e.g. Gébelin et al., , ; McCaig, ; Menzies et al., ; Mulch et al., , ; Person, Mulch, Teyssier, & Gao, ; Upton et al., ). As proposed for detachment zones in Western USA and the South Tibetan Detachment (Gébelin et al., , , , ; Mulch et al., ; Person et al., ), three main conditions are essential to explain the downward infiltration of meteoric fluids at depth and imply a combined effect of brittle normal faults in the upper crust, a high geothermal gradient and the presence of a hydraulic head.…”

“…Gébelin et al., , ; McCaig, ; Menzies et al., ; Mulch et al., , ; Person, Mulch, Teyssier, & Gao, ; Upton et al., ). As proposed for detachment zones in Western USA and the South Tibetan Detachment (Gébelin et al., , , , ; Mulch et al., ; Person et al., ), three main conditions are essential to explain the downward infiltration of meteoric fluids at depth and imply a combined effect of brittle normal faults in the upper crust, a high geothermal gradient and the presence of a hydraulic head. These criteria were very likely met in the southern Armorican domain where brittle normal faults have been recognized and linked to the exhumation of high‐grade metamorphic rocks at ~300 Ma, but also where the thickened crust would have provided the necessary hydraulic head (Figure ; e.g.…”

“…They represent critical interfaces where metamorphic, magmatic and surface‐derived fluids meet (e.g. Gébelin, Teyssier, Heizler, & Mulch, ; Gébelin et al., ; Mulch et al., ; Nesbitt & Muehlenbachs, ; Upton, Koons, & Chamberlain, ; Gardien et al, ). Characterization of a meteoric component of crustal fluids is crucial to better understand ore deposition at the orogen scale (e.g.…”

“…This method has proven to be one of the cornerstones when recovering the isotopic composition of ancient rainfall that infiltrated the upper crust through a brittle deformation network down to 15 km depth in the footwall of detachment zones in the North American Cordillera (e.g. Fricke et al., ; Gébelin et al., , , ; Losh, ; Mulch et al., , ), in the European Central Alps (Campani, Mulch, Kempf, Schlunegger, & Mancktelow, ) but also in the Himalaya (Gébelin et al., , ).…”

The Himalayan cryosphere: past and present variability of the ‘third pole’

“…A major intensification of the SAM took place c. 13-15 myr ago (Gupta et al 2015;Betzler et al 2016), by which time the Himalaya had attained a height of c. >5000 m a.s.l. (Gébelin et al 2013(Gébelin et al , 2017. This period of monsoon intensification also coincided with a period of warmth known as the Middle Miocene Climate Optimum, when global temperatures were 3°C higher than at present.…”