Sean Long

Geometry and crustal shortening of the Himalayan fold-thrust belt, eastern and central Bhutan

We present a new geologic map of eastern and central Bhutan and four balanced cross sections through the Himalayan fold-thrust belt. Major structural features, from south to north, include: (1) a single thrust sheet of Subhimalayan rocks above the Main Frontal thrust; (2) the upper Lesser Himalayan duplex system, which repeats horses of the Neoproterozoic–Cambrian(?) Baxa Group below a roof thrust (Shumar thrust) carrying the Paleoproterozoic Daling-Shumar Group; (3) the lower Lesser Himalayan duplex system, which repeats horses of the Daling-Shumar Group and Neoproterozoic–Ordovician(?) Jaishidanda Formation, with the Main Central thrust (MCT) acting as the roof thrust; (4) the structurally lower Greater Himalayan section above the MCT with overlying Tethyan Himalayan rock in stratigraphic contact in central Bhutan and structural contact above the South Tibetan detachment in eastern Bhutan; and (5) the structurally higher Greater Himalayan section above the Kakhtang thrust. Cross sections show 164–267 km shortening in Subhimalayan and Lesser Himalayan rocks, 97–156 km structural overlap across the MCT, and 31–53 km structural overlap across the Kakhtang thrust, indicating a total of 344–405 km of minimum crustal shortening (70%–75%). Our data show an eastward continuation of Lesser Himalayan duplexing identified in northwest India, Nepal, and Sikkim, which passively folded the overlying Greater Himalayan and Tethyan Himalayan sections. Shortening and percent shortening estimates across the orogen, although minima, do not show an overall eastward increase, which may suggest that shortening variations are controlled more by the original width and geometry of the margin than by external parameters such as erosion and convergence rates.

Tectonostratigraphy of the Lesser Himalaya of Bhutan: Implications for the along-strike stratigraphic continuity of the northern Indian margin

New mapping in eastern Bhutan, in conjunction with U-Pb detrital zircon and δ 13 C data, defi nes Lesser Himalayan tectonostratigraphy. The Daling-Shumar Group, 2–6 km of quartzite (Shumar Formation) overlain by 3 km of schist (Daling Formation), contains ~1.8–1.9 Ga intrusive orthogneiss bodies and youngest detrital zircon peaks, indicating a Paleoproterozoic deposition age. The Jaishidanda Formation, 0.5– 1.7 km of garnet-biotite schist and quartzite, stratigraphically overlies the Daling Formation beneath the Main Central thrust, and yields youngest detrital zircon peaks ranging from ~0.8–1.0 Ga to ca. 475 Ma, indicating a Neoproterozoic–Ordovician(?) deposition age range. The Baxa Group, 2–3 km of quartzite, phyllite, and dolomite, overlies the DalingShumar Group in the foreland, and yields ca. 0.9 Ga to ca. 520 Ma youngest detrital zircon peaks, indicating a Neoproterozoic– Cambrian(?) deposition age range. Baxa dolo mite overlying quartzite containing ca. 525 Ma detrital zircons yielded δ 13 C values between +3‰ and +6‰, suggesting deposition during an Early Cambrian positive δ 13 C excursion. Above the Baxa Group, the 2–3 km thick Diuri Formation diamictite yielded a ca. 390 Ma youngest detrital zircon peak, suggesting correlation with the late Paleo zoic Gondwana supercontinent glaciation. Finally, the Permian Gondwana succession consists of sandstone, siltstone, shale, and coal. Our deposition age data from Bhutan: (1) reinforce suggestions that Paleoproterozoic (~1.8–1.9 Ga) Lesser Himalayan deposition was continuous along the entire northern Indian margin; (2) show a likely east ward continuation of a Permian over Cambrian unconformity in the Lesser Himalayan section identifi ed in Nepal and northwest India; and (3) indicate temporal overlap between Neoproterozoic–Paleozoic Lesser Himalayan (proximal) and Greater Himalayan–Tethyan Himalayan (distal) deposition.

Geologic Map of Bhutan

Abstract Please click here to download the map associated with this article. We present a new, 1:500,000-scale geologic map of the kingdom of Bhutan, and surrounding areas of India and southern Tibet. The map is a compilation of the most complete and most recent mapping datasets available, and presents an unprecedented amount of structural data when compared to previous published geologic maps of Bhutan. The map is a combination of: 1) new data presented in this study; 2) compilation of small-scale, published geologic maps of specific areas of Bhutan, Tibet, and parts of India; and 3) compilation of specific areas of published, country-scale geologic maps of Bhutan. Mapping detail is focused primarily on Subhimalayan, Lesser Himalayan, and Greater Himalayan rocks, with a lower level of detail on Tethyan Himalayan rocks. We present new 3-part stratigraphic divisions for the Siwalik Group and the structurally-lower Greater Himalayan section, and compile detailed stratigraphic divisions and structural geometries for the Lesser Himalayan section and Paro Formation. We also compile detailed mapping of the Yadong cross-structure and other structurally-complex areas of southern Tibet, and present new locations for the South Tibetan detachment. Our map compilation also highlights specific areas in Bhutan that would benefit from future geologic mapping. It is our hope that this map will be a valuable tool to be utilized by Bhutanese geologists, future researchers visiting Bhutan, and travelers and trekkers as well. We also hope that this map will serve as a new starting point for future mapping in Bhutan and throughout the Himalayan orogen.