Velocity structure of the western North China basement from high-resolution wide-angle reflection/refraction profiles

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Two 1420 km-long, high-resolution wide-angle reflection/refraction profiles were produced for the western part of the North China Craton. Twenty-one shots were fired along the two profiles and seismic data was collected by 700 portable three-component digital seismographers. The crust-mantle structure was investigated in different regions, including the Ordos Block, Haiyuan arcuate tectonic region, Alashan Block, and Yinchuan Basin. First-arrival Pg waves were analyzed using the finite-difference method of first-arrival traveltime tomography to construct the 2D interface morphology and velocity structure of the crystalline basement. The study identified the location and shape of major faults, and notable variations in basement depth and velocity structure in different regions. The crystalline basement in the Ordos Block is typically stable with a depth of 4.5–5.5 km and exhibits only minor fluctuations in the lateral velocity gradient. This is evidenced by Pg wave features, including stable travel time, long tracing distance and high velocity. Between the Helan Mountains and the Yinchuan Basin, notable variations in topography, basement depth, and velocity structure were identified, indicating strong divergence in vertical movement and deformation. In the Yinchuan Basin, the Pg wave traveltime curve revealed the complex structure of the crystalline basement and variations in the velocity gradient. Since the Cenozoic Era, the western North China Craton has experienced strong and frequent tectonic activity causing strong deformation on the periphery of the Alashan Block. The presence of small, low-velocity, near-surface basins, and variations in the crystalline basement interface and velocity isolines provides evidence of strong tectonic deformation of the basement near Bayan Ura Hill, on the eastern margin of the Alashan Block. The Haiyuan arcuate tectonic region is complex and interacts with the Qilian Mountains, Alashan Block, and Ordos Block. Basement depth extends to 6.0 km and varies considerably from the adjacent uplift zone, which has a depth of 2.0 km. The highly deformed and unstable basement structure at the northeastern margin of the Tibetan Plateau is probably related to the collision and squeezing of the Indian Plate and the Eurasian Plate. Seismology data indicates differences between the crystalline basement interface and the velocity structure, as well as strong transverse inhomogeneity. Differences in basement depth in the Ordos Block, together with significant lateral heterogeneity, compressional fold deformation, and the presence of fault zones in the marginal basement, block the continuous compressive movement of the Tibetan Plateau in a northeast direction.