Foreland Flexure and Lower Crustal Flow as Controls on Cenozoic Subsidence Anomalies in the Zengmu (Sarawak) Basin, South China Sea

Abstract While the evolution of individual passive marginal and foreland basins is well understood, the subsidence pattern of passive margins near convergence zones—where thrust loading overprints earlier extension and the typical wedge and foredeep are obscured—remains poorly understood. Herein, we present an integrated geological interpretation and quantitative backstripping analysis of 24 seismic sections to calculate the Cenozoic tectonic subsidence across the entire Zengmu (Sarawak) basin in the southwestern South China Sea. Additionally, we apply flexural–dynamic subsidence modeling to quantify the contribution of orogenic loading to the basin's development. Our results reveal an initial rifting stage (Paleocene to Eocene) unaffected by foreland flexure, featuring significant subsidence localized along faults. The subsequent Oligocene and Early Miocene periods were punctuated by relatively rapid (413.6 ± 179.2 m) and slow syn‐rift (230.8 ± 120.6 m) subsidence, interpreted as the respective signals of the flexural backbulge and forebulge. This basin exhibits anomalous post‐rift subsidence, characterized temporally by Middle Miocene acceleration and subsequent deceleration, and spatially by contrasting patterns of greater subsidence in the north versus uplift in the south. This pattern can be primarily attributed to lower crustal flow deriving from thick sedimentary accumulation and lithospheric weaknesses. We propose that the observed spatio‐temporal subsidence anomaly is not unique to this basin, but a manifestation of a globally common phenomenon affecting passive margins near convergent zones.