Flattening of the Greater Himalayan Zone within the Eastern Himalaya: Insights from Pressure–Temperature–Structural Distance Trends from Central and Eastern Bhutan

Abstract:

Quantifying pressure–temperature (P–T) trends across structural thickness provides insight into the tectonic mechanisms for emplacement and exhumation of orogen-scale ductile thrust sheets in active continental collision zones. Furthermore, these trends can reveal, via metamorphic discontinuities, the presence or absence of intra-thrust sheet structures. The Greater Himalayan zone (GHZ) contains mid-crustal rocks exhumed during Neogene convergence between India and Asia; however, the mechanisms that drove their emplacement and exhumation are debated. Within the Bhutan Himalaya, an intra-GHZ structure, the Kakhtang thrust (KT) is proposed to separate the GHZ into structurally upper and lower levels, and is interpreted to extend across much of the eastern Himalaya. In this investigation, peak P–T conditions were determined for rocks from two transects through both levels of the GHZ in north-central and northeastern Bhutan, in order to evaluate the significance of the KT, and to better understand the emplacement mechanism of GHZ rocks. The composition of the phases Grt + Pl + Bt ± Ms ± St ± Amp ± Kfs from twelve metapelite and orthogneiss samples were analyzed using the electron microprobe at UC-Santa Barbara. Peak P–T estimates, determined via THERMOCALC and supplemented by Ti-in-biotite thermometry, show similar T- but differing P versus structural distance trends across each transect. A gradual decrease in temperatures structurally upsection (~750 ºC to 600 ºC) was observed on both transects, and in northeast Bhutan, a steep decreasing pressure trend (0.40 ± 0.08 kbar/km) is documented. In comparison, in north-central Bhutan, a super-lithostatic pressure gradient (0.74 ± 0.16 kbar/km) is observed across >5.5 km of structural thickness, and is interpreted to represent >60 % layer-normal flattening strain, similar in magnitude to flattening documented in GHZ rocks in south-central Bhutan. The documented gradients show no significant change in P–T conditions crossing the KT, and therefore argue that the KT is likely not a first-order intra-GHZ shear zone. Furthermore, the super-lithostatic pressure trends favor large-scale, post-peak metamorphic flattening of the GHZ during emplacement.