Revealing the upper crustal structure in the Fujian region using deep seismic profiling

The Fujian region, located along the southeastern margin of the Chinese continent, constitutes an active tectonic belt along the coastal area in southeast China, with complex formation and evolution histories. To understand the characteristics of upper crustal structures and basement structures in the Fujian, this study collected the Pg travel time data from four NE-trending deep seismic sounding (DSS) survey lines in the region. Employing finite-difference tomography, this study obtained the fine-scale velocity structure of the upper crust along the four profiles, revealing the lateral variations in the upper crustal velocity structure, basement structures, and shallow fault characteristics in the region. Key findings are as follows:(1) The velocities along the four profiles range between 4 700 and 6 100 m/s, gradually increasing from west to east. They exhibit noticeable lateral variations in basement velocities on either side of tectonic unit boundaries; (2) The burial depth of crystalline basement interface along the four profiles varies between 1.0 and 6.0 km, deepening from north to south. The undulations in the basement surface correspond to the structural uplifts and depressions; (3) The upper structures in the uplift zone in northwest Fujian exhibit overall high velocities and shallow burial depths, which are related to the metamorphic basement; (4) Regarding the upper crust in the depression zone in southwest Fujian, its structures exhibit significant variations velocities, and its interface morphology displays alternating patterns of deep and shallow parts. These correspond to the geological structures characterized by fault basins and depressions interlacing with uplifts in the region; (5) The upper crust of the Mesozoic magmatic zone along the coastal area in southeast China exhibits high and greatly varying velocities, indicating that stronger tectonic activity in the eastern part of South China Continent compared to its western part; (6) The NW-trending fault zone where the DSS profiles pass through cut through the basement, showing dominant strikes consistent with those of the geological structures and tectonic deformations in Fujian. This finding reflects the NW-directed subduction-collision compression from the Paleo-Pacific Plate. These findings provide shallow seismic evidence for obtaining significant depth relationships of NW-trending structures along the coastal area in southeast China.