A MULTIFACETED APPROACH TO DATE TECTONICALLY-DRIVEN EXHUMATION OF THE HIMALAYAN MIDDLE CRUST

Precisely bracketing the age of ductile deformation along crustal-scale shear zones is crucial to assess the time scale at which they operate and calculate accurate creep rates. The onset of deformation, in particular, is challenging to pinpoint with traditional methods (e.g. deformed intrusions, thermochronology). We present a multi-faceted approach combining in-situ laser-ablation split-stream inductively-coupled plasma mass spectrometry monazite petrochronology with monazite trace element analysis, microstructural analysis, petrography, and metamorphic phase equilibria modelling to precisely date the onset of deformation along the South Tibetan detachment (STD) in the Karnali klippe, west Nepal Himalaya. This approach is complemented by ⁴⁰Ar/³⁹Ar geochronology to constrain the termination of shearing.

The STD is an orogen-wide shear zone that exhumed high metamorphic-grade mid-crustal rocks in the footwall and juxtaposed them with low metamorphic-grade supracrustal rocks in the hanging wall. A sheared garnet-muscovite-biotite schist at the base of this ~1-km-thick shear zone contains pre-kinematic garnet that grew at ~600 °C and 900-1000 MPa between ca. 36 and 30 Ma. Garnet breakdown during decompression is inferred to have started at ca. 30 Ma from the increasing Y content of monazite. Syn-kinematic asymmetric Y-rich rims of monazite indicate the base of the STD was active from ca. 29 to <24 Ma at 600 °C based on quartz crystallographic <c>-axis preferred orientations fabrics. The schist yields an 18.8 ± 0.3 Ma white mica ⁴⁰Ar/³⁹Ar date, interpreted as a cooling age, which constrains the minimum age of ductile deformation at the base of the STD. A second white mica ⁴⁰Ar/³⁹Ar date from the upper part of the STD, which was deformed at lower temperature (<500 °C), is interpreted to indicate syn-kinematic recrystallization at 16 ± 2 Ma, suggesting that deformation propagated up-section within the shear zone.

These data constrain the onset of mid-crustal tectonically-driven exhumation along the STD to the early Oligocene, several m.y. before commonly reported values. The well-constrained duration of activity of the STD in western Nepal (ca. 30 to <16 Ma) is combined to published estimates of dip-slip displacement (150-200 km) to calculate long-term average creep rates of 1.1 to 1.4 cm/year.