Structural Evolution of Thakkhola Graben: Implications for the Architecture of the Central Himalaya, Nepal

The Thakkhola Graben of central Nepal is one of several north-south-trending rifts and grabens throughout the Himalayan hinterland and southern Tibet. The faults bounding the graben run from the crest of the Himalayas at the Annapurna and Dhaulagiri Ranges to the suture between India and Asia. This suture is reported to separate two disparate stress regimes, arc-perpendicular compression in the Nepal Himalayas to the south and arc-parallel extension in the Tibetan Plateau to the north. However, two lines of evidence suggest the India-Asia Suture no longer functions as a structural boundary between these stress fields: (1) the Lopukangri Rift cuts and offsets the suture, and (2) the Thakkhola Graben is located south of the suture. Despite decades of research in the Thakkhola Graben, an accurate account of fault geometry and kinematics has yet to be presented. Therefore its relationship to extensional structures in Tibet and its role in Himalayan tectonics is undetermined. Field mapping, combined with kinematic modeling and reconstruction of offset piercing points, has confined the geometry and kinematics of the faults bounding the Thakkhola Graben. The western boundary of the graben is the Dangardzang Fault and the eastern boundary is the Muktinath Fault, both of which are steeply dipping and cut down into the middle crust. The Dangardzang Fault consists of two parallel fault strands, with the easternmost accommodating the majority of strain. This fault has accommodated 4.5 kilometers of dip-slip displacement, 5.3 kilometers of dextral strike-slip displacement, and 1.4 kilometers of horizontal extension. The Muktinath Fault consists of one dominant fault strand with several subsidiary faults of lesser magnitude. This fault has accommodated 4.2 kilometers of dip-slip displacement, 1.9 kilometers of sinistral strike-slip displacement, and 0.8 kilometers of horizontal extension. Comparison of the Thakkhola Graben with the Lopukangri Rift to the north shows both of these structures share similar fault geometries, magnitudes of slip, and kinematics. This relationship implies the same stress field is active on both sides of the India-Asia Suture and therefore, in this region it no longer operates as a structural boundary between the Himalayas and Tibet.