Structural segmentation and tectonic transition of a coastal South China fault driven by asymmetric pull-apart basins during the Late Pleistocene

Abstract

This study investigates the Late Quaternary activity and dynamics of the Qinzhou section of the Maoling-Jiulong Fault (MJF), a key part of the coastal active fault system along the northern Beibu Gulf. Structural geometric segmentation delineates three distinct segments: the Pingji Basin (PJB), Jiulong (JL), and Qinzhou Basin (QZB). Radiometric dating constrains fault-related deposits in the PJB and QZB to 21-22 ka, while undeformed terraces in JL younger than 10.8 ka indicate cessation of motion after the Late Pleistocene. Integrated trench logging and structural profiling reveal a negative flower structure governing both shallow faulting and basin-controlling deformation. Dynamic analysis suggests localized transtensional tectonics since the Late Pleistocene, with asymmetric pull-apart basins (PJB and QZB) modulating strain partitioning. The QZB remains in an active extensional pull-apart phase, strictly controlling modern geomorphology from the Qinjiang River to Qinzhou Bay. Situated at the intersection of faults between NE-trending West Pacific-South China Sea and NW-trending Himalayan-Indochina tectonic domains, and under NNE-SSW oriented regional stress, the Qinzhou section exhibits weaker seismogenic potential than adjacent sections, reflecting its role as a structural pivot in intraplate tectonic transition.