Aude Gébelin

The Miocene elevation of Mount Everest

The Neogene elevation history of the Mount Everest region is key for understanding the tectonic history of the worlds highest mountain range, the evolution of the Tibetan Plateau, and climate patterns in East and Central Asia. In the absence of fossil surface deposits such as paleosols, volcanic ashes, or lake sediments, we conducted stable isotope paleoaltimetry based on the hydrogen isotope ratios (dD) of hydrous minerals that were deformed in the South Tibetan detachment shear zone during the late Early Miocene. These minerals exchanged isotopically at high temperature with meteoric water (dDwater = m156p p 5p) that originated as high-elevation precipitation and infiltrated the crustal hydrologic system at the time of detachment activity. When compared to age-equivalent near-sea-level foreland oxygen isotope (d18O) paleosol records (d18Owater = m5.8p p 1.0p), the difference in d18Owater is consistent with mean elevations of g5000 m for the Mount Everest area. Mean elevations similar to modern suggest that an early Himalayan rain shadow may have influenced the late Early Miocene climatic and rainfall history to the north of the Himalayan chain.

Meteoric water circulation in a rolling-hinge detachment system (northern Snake Range core complex, Nevada)

Combined petrofabric, microstructural, stable isotopic, and 40 Ar/ 39 Ar geochronologic data provide a new perspective on the Cenozoic evolution of the northern Snake Range metamorphic core complex in east-central Nevada. This core complex is bounded by the northern Snake Range detachment, interpreted as a rolling-hinge detachment, and by an underlying shear zone that is dominated by muscovite-bearing quartzite mylonite and interlayered micaschist. In addition to petrofabric, microstructural analysis, and 40 Ar/ 39 Ar geochronology, we use hydrogen isotope ratios (δD) in synkinematic white mica to characterize fluid-rock interaction across the rolling-hinge detachment. Results indicate that the western flank of the range preserves mostly Eocene deformation (49–45 Ma), characterized by coaxial quartz fabrics and the dominant presence of metamorphic fluids, although the imprint of meteoric fluids increases structurally downward and culminates in a shear zone with a white mica 40 Ar/ 39 Ar plateau age of ca. 27 Ma. In contrast, the eastern flank of the range displays pervasive noncoaxial (top-to-the-east) fabrics defined by white mica that formed in the presence of meteoric fluids and yield Oligocene–Miocene 40 Ar/ 39 Ar ages (27–21 Ma). Evolution of the Oligocene–Miocene rolling-hinge detachment controlled where and when faulting was active or became inactive owing to rotation, and therefore where fluids were able to circulate from the surface to the brittle-ductile transition. On the western flank (rotated detachment), faulting became inactive early, while continued active faulting on the eastern flank of the detachment allowed surface fluids to reach the mylonitic quartzite. The combined effects of synkinematic recrystallization and fluid interaction reset argon and hydrogen isotope ratios in white mica until the early Miocene (ca. 21 Ma), when the brittle-ductile transition was exhumed beneath the detachment.

Eocene and Miocene extension, meteoric fluid infiltration, and core complex formation in the Great Basin (Raft River Mountains, Utah)

Metamorphic core complexes (MCCs) in the North American Cordillera reflect the effects of lithospheric extension and contribute to crustal adjustments both during and after a protracted subduction history along the Pacific plate margin. While the Miocene-to-recent history of most MCCs in the Great Basin, including the Raft River-Albion-Grouse Creek MCC, is well documented, early Cenozoic tectonic fabrics are commonly severely overprinted. We present stable isotope, geochronological (40Ar/39Ar), and microstructural data from the Raft River detachment shear zone. Hydrogen isotope ratios of syntectonic white mica (δ2Hms) from mylonitic quartzite within the shear zone are very low (−90‰ to −154‰, Vienna SMOW) and result from multiphase synkinematic interaction with surface-derived fluids. 40Ar/39Ar geochronology reveals Eocene (re)crystallization of white mica with δ2Hms ≥ −154‰ in quartzite mylonite of the western segment of the detachment system. These δ2Hms values are distinctively lower than in localities farther east (δ2Hms ≥ −125‰), where 40Ar/39Ar geochronological data indicate Miocene (18–15 Ma) extensional shearing and mylonitic fabric formation. These data indicate that very low δ2H surface-derived fluids penetrated the brittle-ductile transition as early as the mid-Eocene during a first phase of exhumation along a detachment rooted to the east. In the eastern part of the core complex, prominent top-to-the-east ductile shearing, mid-Miocene 40Ar/39Ar ages, and higher δ2H values of recrystallized white mica, indicate Miocene structural and isotopic overprinting of Eocene fabrics.

Seismic anisotropy of the Archean crust in the Minnesota River Valley, Superior Province

The Minnesota River Valley (MRV) subprovince is a well-exposed example of late Archean lithosphere. Its high-grade gneisses display a subhorizontal layering, most likely extending down to the crust-mantle boundary. The strong linear fabric of the gneisses results from high-temperature plastic flow during collage-related contraction. Seismic anisotropies measured up to 1 GPa in the laboratory, and seismic anisotropies calculated through forward-modeling indicate ΔVP ~5–6% and ΔVS ~3%. The MRV crust exhibits a strong macroscopic layering and foliation, and relatively strong seismic anisotropies at the hand specimen scale. Yet the horizontal attitude of these structures precludes any substantial contribution of the MRV crust to shear wave splitting for vertically propagating shear waves such as SKS. The origin of the regionally low seismic anisotropy must lie in the upper mantle. A horizontally layered mantle underneath the United States interior could provide an explanation for the observed low SWS.

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

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