Crustal-scale structural and compositional distinction beneath NE Tibet: Evidence from reflection-seismology and Vp/Vs imaging

Abstract

Knowledge of the crustal structures and rock compositions of the Tibetan Plateau is key to understanding its growth and deformation. We utilized a ∼ 400 km multi-data seismic profile in NE Tibet that was constructed using deep seismic reflection data and a dense broadband array. The seismic reflection and Vp/Vs characteristics point to the existence of a crustal-scale structural and compositional boundary that corresponds to the Hezuo–Tanchang Fault in the West Qinling Orogen. South of the fault there is a decoupling of the upper and middle–lower crust. The upper crust is characterized by thrust structures and relatively low Vp/Vs ratios (<1.70), typical of felsic quartz-rich rocks, while the middle crust exhibits anomalously high Vp/Vs ratios (1.8–2.0) and multiple north-dipping thrust–nappe structures that indicate regional mylonitization. Relatively transparent reflections with Vp/Vs ratios of ∼1.82 in the lower crust refer to the mafic rocks, likely composed of anorthosite/metagabbro resulting from the underplating from mantle-derived magmas. North of the fault, the entire crust exhibits weak reflections associated with the crust-cutting West Qinling Fault, with low Vp/Vs ratios (<1.70) related to felsic rocks. The structural distinction is probably due to the differentiation in rock compositions that resulted from the Triassic amalgamation of the Songpan–Ganze Block and the Qilian Orogen. Cenozoic reactivation due to the northward movement of the Indian Plate produced different structures on either side of the West Qinling Orogen. The structural and compositional boundary would have constrained the northward growth of the plateau from central Tibet and resulted in a differential uplift of NE Tibet.