Integrated Geophysical Evidence for the Middle-Lower Crust Melting of the Songpan-Aba Terrain, NE Tibetan Plateau

The Songpan−Aba region is located on the northeastern edge of the Tibetan Plateau. Tectonically, the area is surrounded by the West Qinling orogenic belt in the north, the Longmenshan orogenic belt in the southeast, and the East Kunlun and Sanjiang orogenic belts in the west and southwest, forming a triangle that provides an ideal location to study the crust-mantle structure and deep tectonics of the eastward extrusion of the Tibetan Plateau. In this study, the magnetic and electrical structures of the Songpan−Aba area were investigated by inversion using high-precision magnetic anomaly and magnetotelluric data to obtain the subsurface magnetization inversion intensity and resistivity of Songpan–Aba and adjacent areas. The results revealed a continuous magnetic layer up to 20 km below Songpan–Aba and its surrounding areas in the south, possibly originating from a magma root southwest of the Longmenshan massif. In the West Qinling, Songpan–Aba, and Longmenshan areas, pervasive low-resistance, weakly magnetic, or magnetic layers were identified below 20 km that might be formed from the molten mantle material extruded from the eastern edge of the Tibetan Plateau.

Strain Transformation Adjacent to the West Qinling Orogen: Implications for the Growth of the Northeastern Tibetan Plateau

The West Qinling orogen has played an important role in accommodating the deformation in the northeastern Tibetan Plateau induced by the India-Eurasia convergence. Here we construct a vertical land motion (VLM) model based on the latest leveling observations adjacent to the West Qinling orogen. Combined with the horizontal deformation field, the crustal deformation pattern in this area is investigated. Additionally, slip rate and coupling coefficients of the West Qinling fault, the longest fault separating the West Qinling orogen from the Lanzhou (Longxi) block, are inverted and constrained with GPS and VLM observations. Results show that the West Qinling fault slips slowly at a rate of 1–2 mm/yr and is strongly coupled with a moment magnitude deficit of Mw7.4. The crustal uplift rates adjacent to the West Qinling orogen are 0–3 mm/yr; which combined with 0–12.5 × 10−9/yr contraction rates, suggests that strain transformation plays a key role in controlling the tectonic uplift in the West Qinling orogen, and furthers our understanding of the contemporary geomorphic and topographic features. We identify a significant deformation transition belt at longitudes of 105°–106°E, which indicates that crustal deformation, induced from the northeastern expansion of the Tibetan Plateau, is mainly constrained to the plateau, rather than accommodated by crustal materials escaping eastward along the Qinling Mountains.