3D Seismic observations into the Development of Extensional Low-Angle Normal Faults and Metamorphic Core Complexes: South China Sea, Malaysia

Over the past decade, high-quality industrial 3D seismic surveys acquired along both the northern (Lei et al., 2018; Zhang et al., 2020) and southern (Legeay et al., 2024) rifted margins of the South China Sea (SCS) have imaged and documented the development of low-angle normal faults (<30°) locally accommodating the formation of metamorphic core complexes. Such structures had previously been recognized only in field analogues, such as the Aegean region or the Basin and Range Province. In this contribution, we present new high-resolution 3D seismic data that enable us to map, observe, and characterize the tectono-sedimentary evolution of hyper-extended rift basins associated with crustal thinning and deformation.

The SCS exhibits a wide rift domain characterized by large-scale crustal boudinage, expressed as a series of hyper-extended basins separated by basement highs. Rifting in the SCS initiated during the early Cenozoic and evolved with a NE–SW-oriented oceanic propagation between 32 and 16 Ma. This extensional event overprinted a pre-existing Andean-type orogenic system, defined by a Mesozoic magmatic arc extending from Borneo to Korea above the west-dipping Pacific subduction zone. A major tectonic reorganization occurred during the Late Cretaceous following the collision of the Luconia block with the active Eurasian margin. This pre-rift configuration resulted in a highly heterogeneous basement composition, including remnants of the magmatic arc, the Luconia block, and intervening thrust wedges.

Our observations across several hyper-extended rift basins from both the northern and southern SCS margins highlight the three-dimensional geometry of low-angle fault systems characterized by corrugated and domal morphologies. The hanging walls are composed of extensional allochthonous blocks consisting of pre-rift sediments and basement rocks, while the syn-rift infill is organized into alternating wedges bounded by antithetic faults relative to the main low-angle detachment systems.

At depth, the footwalls display shallow-dipping seismic reflections interpreted as reactivated thrust wedges that likely facilitated the development of low-angle extensional structures.

The exhumation of deeper crustal levels is spatially correlated with zones of maximum displacement along the normal faults, which locally exhibit domal geometries and evidence for sheath folding.