LARGE-SCALE, DISTRIBUTED STRUCTURAL THINNING IN THE HIMALAYAN OROGEN: A CASE STUDY FROM CENTRAL BHUTAN

One of the key debates in the Himalayan orogen is over the structural significance of the normal-sense South Tibetan detachment (STD) system. In Bhutan, multiple segments of the STD have been mapped. Along the Bhutan-Tibet border, a discrete shear zone has been mapped that correlates with the STD mapped across the Himalaya. To the south, in central Bhutan, multiple studies have also correlated several isolated exposures of north-vergent shear zones with the STD. However, each study maps these shear zones at a different structural level, and other studies have interpreted the rocks in this region to reflect a thick zone of distributed shearing. Here, we attempt to resolve this debate by presenting T, P, finite strain, and shear-sense data from an 11 km-thick structural transect along the Dang Chu. RSCM and garnet-biotite thermometry define a gradual, structurally-upward decrease from 600-700°C to 400-500°C, and compiled P data from central Bhutan define a structurally-upward decrease from 9.5-11.5 kbar to 4.0-6.5 kbar between 0 and 11 km above the MCT. The P data define a field gradient that is 1.2-2.4 times lithostatic, which indicates that 9±7 km of post-peak metamorphic structural thinning was accommodated above the MCT. These P and T gradients do not support the existence of a discrete, normal-sense shear zone. Strain analyses illustrate that pure shear-dominant (Wm ≤0.4), layer-normal flattening was broadly distributed above the MCT. Transport-parallel stretching ranges between 20-110% at 2-5 km above the MCT and between 5-55% at 5-11 km above the MCT. This imparted an overall top-to-north shear strain, which is corroborated by north-vergent shear-sense indicators. The strain data allow for up to 85 km of north-vergent shearing distributed through these rocks, which may be related to a vertical gradient in southward displacement magnitude that developed during MCT motion. Alternatively, shearing may have been translated northward into the discrete STD system mapped along the Bhutan-Tibet border. Timing estimates of ~23-15 Ma for shearing on the MCT and STD allow for both possibilities. These results represent a case study for large-scale, distributed structural thinning, and highlight the diverse range of processes that accommodate tectonic denudation during orogenesis.