Preliminary stratigraphic and structural architecture of Bhutan: Implications for the along strike architecture of the Himalayan system

Preliminary mapping and stratigraphic correlation of Lesser Himalayan rock in eastern Bhutan using field characteristics, U–Pb detrital zircon dating, and ɛNd geochemistry define the first-order stratigraphic architecture of the Indian passive margin sequence in the eastern Himalaya. We use this new image of the lateral and vertical relationships of the original stratigraphy to determine the structural framework of the eastern Himalayan fold-thrust belt in Bhutan. We propose that Lesser Himalayan rock in Bhutan can be divided into lower Lesser Himalayan rocks with a Paleoproterozoic detrital zircon signal, and upper Lesser Himalayan rocks with detrital zircon signals of ∼ 1000–500 Ma. The ∼ 500 Ma detrital grains are from rocks in the frontal portions of the fold-thrust belt north of the Main Boundary thrust as well as directly in the footwall of the Main Central thrust in the hinterland. Our preliminary stratigraphic study coupled with mapping allows us to construct a composite balanced cross-section through the Kuru Chu valley in eastern Bhutan which provides the first image of the geometry and amount of shortening through Bhutan. The Main Frontal thrust tilts a 6 km of thick section of Neogene foreland basin deposits. These deposits are separated from folded and faulted upper Lesser Himalayan rocks by the Main Boundary thrust. North of the Main Boundary thrust, we propose two duplex systems. The southernmost duplex contains 9 repeated sections of upper Lesser Himalayan units, including the Permian Gondwana Sequence and the Cambrian (or younger) Baxa Group. The northernmost duplex is located in the footwall of the Main Central thrust, and is comprised of two repeated sections of the Proterozoic Shumar and Daling Formations. The southern boundary of this northern duplex is the Shumar thrust which acts as a roof thrust for the southern duplex system and may correlate to the Ramgarh thrust in Nepal and India. We propose that the development of the duplex systems passively folded overlying Tethyan and Greater Himalayan rocks. Minimum shortening for this part of the Himalayan fold-thrust belt is 359 km, all of which has occurred from 22 Ma to present defining a long-term shortening rate of 16 mm/yr.