Investigating the cooling history of the eastern Himalaya Greater Himalayan rocks: A U-Pb and 40Ar/39Ar thermochronology study from central and eastern Bhutan
AuthorZamora, Carolina Zamora
AdvisorGordon, Stacia M
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Within the Himalayan orogen, the Greater Himalaya (GH) is a tectonostratigraphic unit that consists of mid- to lower-crustal rocks that were exhumed during convergence between the Indian and Asian plates. In the eastern Himalaya within the kingdom of Bhutan, the GH has been divided into two structural units, the upper- and lower-GH, by an intra-GH shear zone, the Kakthang thrust (KT). Many studies across the Himalaya have investigated the importance of intra-GH structures and their role in exhuming the GH to the near surface. Inversions within an overall upright pressure gradient across the GH, combined with older ages recording the initiation of melt crystallization with increasing structural distance, suggest initial exhumation at ca. 27 Ma in the upper-GH and ca. 15 Ma in the lower-GH. This initial exhumation was likely driven by progressive ductile underplating, which constructed the GH as a composite tectonic unit by ca. 15 Ma; however, the GH rocks remained at >550 ºC until ca. 13 Ma, indicating that they were not exhumed to shallow crustal levels during initial underplating. To investigate the final exhumation and cooling history of the GH within Bhutan, rutile and titanite were analyzed via split-stream laser-ablation inductively coupled plasma mass spectrometry to obtain U-Pb ages and trace-element analyses from samples collected along two transects within central and eastern Bhutan. In addition, 40Ar/39Ar muscovite cooling ages were obtained. These two transects cross the lower-GH, the KT zone, and the upper-GH. Trace-element thermometry indicates that the accessory phases crystallized at high-temperature conditions: Zr-in-titanite and Zr-in-rutile temperatures ranged from ~700–825 ºC and 645–735 ºC, respectively, in central Bhutan and Zr-in-rutile temperatures ranged from 625–725 ºC in eastern Bhutan. The U-Pb rutile dates, however, likely record cooling. The titanite in the upper-GH from central Bhutan (re)crystallized at ca. 19.3 Ma, whereas U-Pb rutile cooling ages ranged from 11.4 to 10.4 Ma in central Bhutan and 9.5 to 8.6 Ma in eastern Bhutan. The central Bhutan transect shows older rutile cooling ages with increasing structural distance above the KT, from 10.4 Ma in the lower-GH to 11.4 Ma in the upper-GH. In comparison, the rutile in the eastern transect do not show a systematic trend with structural distance but are ~2 Myr younger than the ages observed in central Bhutan. Furthermore, 40Ar/39Ar muscovite cooling ages indicate that the upper-GH cooled through ~425 ºC from ca. 10.6–9.5 Ma in central Bhutan and from ca. 9.1–7.7 Ma in eastern Bhutan. In comparison, the lower-GH in central Bhutan cooled through ~425 ºC from ca. 9.9–9.5 Ma, while in eastern Bhutan cooling is recorded from ca. 9.2–8.1 Ma. Overall, the U-Pb rutile cooling ages are slightly younger in eastern versus central Bhutan, while the 40Ar/39Ar muscovite cooling ages overlap for both transects. Lesser Himalayan (LH) rocks lie structurally-below the GH rocks. Previous studies in eastern Bhutan document that LH rocks cooled through ~425 ºC by ca. 13.0 to 8.4 Ma, coeval with the cooling in the GH of central and eastern Bhutan. Furthermore, duplexing in the LH occurred from ca. 15–8 Ma and the overlying GH rocks were passively translated southward and structurally-elevated during this duplexing. This uplift of GH rocks likely coincided with enhanced erosion and resulted in the final exhumation of the GH to the near surface.