Abstract:

Since the Early-Cenozoic onset of the Indian plate subduction beneath the Eurasian plate along the Yarlung-Zangbo suture zone (YZSZ), the subduction process has gained wide interests among geologists. However, crustal-scale vertical interactions between the two plates beneath the dominant domain in the collision zone remains unclear owing partly to the lack of high-resolution datasets, which has severely limited the understanding of the crustal thickening mechanism of the dominant collision domain and its deep geodynamic processes. In this study, fine structural interpretation of two deep seismic reflection profiles—180 km and 100 km long cutting through the middle and eastern part of the YZSZ, respectively—revealed the crustal-scale lateral and vertical contact relationships between the subducting Indian plate and the overriding Lhasa terrane. (1) Laterally, the Indian lower crust subducts northward, with limited subduction front beneath the southern margin of the Lhasa terrane (SLT) which is shown as non-reflective crust, while the Central Lhasa terrane (CLT) is north-dipping. (2) Vertically, the Indian lower crust undergoes subduction, while crustal duplexing occurs in the middle-upper crust. Nearly three quarters of the SLT crust are non-reflective crust, while the rest, the SLT upper crust, is south-dipping. The CLT crust can be divided into two domains: north-dipping lower crust and concave-downward upper crust. Differential vertical zonation is observed in all three tectonic units. (3) The upper crust of the dominant collision domain has a consistent deformation pattern, where a sequence of break-backward imbricate structures is present. This break-backward imbricate system can be traced from the Luobadui-Mila fault of the northern edge of SLT, beyond YZSZ, to the northern edge of the North Himalaya dome belt. Combing with the previous findings based on coincident magnetotelluric data on the southward migration of high conductive barrier of SLT thrusting along the main Himalayan into the northern Himalayas, we believe the episodic magmatism in the Tethyan Domain beneath SLT generated juvenile crust that is prone to anomalous thickening. Meanwhile, during India-Eurasia plate interaction, mantle-sourced magmatism in SLT—generated from northward subduction of the Neo-Tethyan oceanic slab and subsequent collision between the India and Eurasia plates—caused southward thermal migration, which induced anatexis in the northern Himalayas and weakened the crustal strength of the region. The ongoing crustal-scale duplexing therefore leads to antiformal stacking and causes crustal thickening. Rapid exhumation of the North Himalayan dome by the increasing antiformal stacking, meanwhile, exerts sudden northward compression to the overlying Tethyan Himalayan sequence, which eventually creates fault-propagation folds following a break-backward sequence in the upper crust through the whole dominant collision domain. Overall, vertical and lateral tectonic interactions within the dominant collision domain in the India-Eurasia collision zone played an important role in producing such anomalous thick crust, but the break-backward imbricates system in the upper crust lowered topographic relief in the dominant collision domain as well.

Key words: collision between Indian and Eurasian plates, dominant domain in collision zone, crustal-scale architecture, tectonic interactions