Progressive development of E-W extension across the Tibetan plateau: A case study of the Thakkhola graben, west-central Nepal

The Thakkhola graben is a large-scale N-S striking, E-W extensional structure located in west-central Nepal that was actively extending ca. 17 Ma. New multi-system geochronological data from the immediate footwall of the Dangardzong fault, the main graben-forming structure in the Thakkhola graben, outline decelerating cooling paths. The cooling rate of the footwall progressively decreases from 55 ± 10°C/Myr in the early Miocene (~22–13 Ma, monazite U-Th/Pb, mica ⁴⁰Ar/³⁹Ar and zircon U-Th/He), to 23 ± 8°C/Myr in the middle to late Miocene (~13-8 Ma, zircon and apatite U-Th/He), and finally to 10 ± 2°C/Myr from 8 Ma to present day (cooling post apatite U-Th/He closure). The deceleration in cooling rate is interpreted to reflect the widespread development of N-S striking graben structures in the Tibetan plateau in the middle Miocene and the progressive partitioning of strain away from the Thakkhola graben into other, younger, extensional features. The shift in the locus of strain may reflect the progressive under-thrusting of India beneath the Tibetan plateau and/or the eastward flow of mid-to-lower crustal material away from the plateau.