Sustainable County-Seat Urbanization in China’s Inter-Provincial Border Areas: Spatiotemporal Patterns, Cross-Provincial Spatial Dependence, and Spatial Explanatory Factors

China’s inter-provincial border areas (IPBAs) are largely located in mountainous, riverine, and other ecologically sensitive regions, where human activities interact intensively with natural ecosystems. Their urbanization is often constrained by geographical peripherality, administrative fragmentation, population and industrial outflow, inadequate public services, and weak cross-regional linkages, posing persistent challenges to sustainable urbanization, resource efficiency, and balanced urban–rural development. Promoting sustainable county-seat urbanization (CSU), with county seats serving as key nodes linking urban and rural areas, is therefore critical to improving resource efficiency, narrowing urban–rural disparities, and advancing sustainable human–environment relations. To operationalize this sustainability goal, this study develops a three-dimensional framework of concentration, functional capacity, and spatial connectivity from an “elements–functions–structure” systems perspective. Using 889 counties in China’s IPBAs, we examine the spatiotemporal evolution of CSU from 2010 to 2023. A cross-provincial contiguity matrix is further constructed and compared with conventional spatial contiguity to identify changes in spatial dependence across provincial administrative boundaries. An optimal-parameters geographical detector (OPGD) is then applied to assess spatial explanatory power and interaction patterns. The results show that CSU in IPBAs improved substantially over the study period, although pronounced regional disparities and internal imbalances persisted. Spatial clustering was considerably weaker across provincial boundaries than under conventional contiguity, with functional capacity, concentration, and connectivity displaying distinct levels of cross-provincial continuity. Industrial and economic foundations, public-service provision, and transport conditions showed the strongest spatial explanatory power. This study contributes a process-oriented Concentration–Capacity–Connectivity framework and cross-provincial contiguity approach for understanding sustainable urbanization in peripheral regions.