Tectonics, Structure, and Metamorphic Evolution of the Himalayan Fold-Thrust Belt, Western Bhutan

Abstract

Field mapping in western Bhutan in combination with U-Pb ages, geochemical data, stratigraphic columns, mineral assemblages and reaction textures, micro- and macro-scale structural observations, and balanced cross sections have allowed us to: 1) evaluate the use of detrital zircon and geochemical signatures for tectonic interpretation, 2) define tectonostratigraphy of litho-units in western Bhutan, particularly the Paro Formation, 3) produce pressure-temperature paths of deformed rocks, and 4) evaluate the magnitudes and rates of shortening through this portion of the Himalayan orogen.

We divide the Lesser Himalayan (LH) section into four map units that range from Paleoproterozoic to Ordovician in age. The Paro Formation is interpreted as the distal equivalent of the Jaishidanda Formation based on a similar structural position immediately below the Main Central thrust (MCT) as well as similarity in detrital zircon signatures. Th-Pb ages of metamorphic monazite from Greater Himalayan (GH) rocks and a single age from the upper LH rocks bracket the minimum age of the MCT displacement between 20.4 \(\pm\) 1.0 and 15.1 \(\pm\) 0.4 Ma. Young monazite ages indicate that GH rocks continued to cool even until ~10 Ma. A total displacement of ~230 km achieved over 5 Myr yields a long-term horizontal shortening rate of 4.3 \(\pm\) 1.2 cm/yr.

In western Bhutan, patterns of metamorphic isograds show an inversion of metamorphic field gradient extending from the upper LH section to the higher structural levels of GH right below the lower-South Tibetan Detachment. In the GH section, deformation postdates peak metamorphic conditions that prevailed at ~20 Ma. In the Paro Formation, the presence of deformed kyanite at the base of the section and presence of undeformed sillimanite at the upper part of the section suggests burial to the kyanite stability field and syn- to post-deformational growth of sillimanite.

A balanced cross-section across western Bhutan illustrates three endmember solutions for the geometry of deformation and highlights implications for shortening magnitude, kinematics and rates. Retro-deformation of cross sections indicates a minimum of ~466-566 km crustal shortening (72-77%). A comparison of structural geometry, thrust kinematics, shortening, and the rate of shortening highlights significant variation along-strike between eastern Bhutan and Sikkim.