Intra-basement structures and their implications for rifting of the northeastern South China Sea margin

•Intra-basement structures in the northeastern South China Sea margin are imaged by reflection seismic data.•A group of intra-basement reflectors are shear zones inherited from the former Andean-type orogeny.•Offshore shear zones are similar to onshore counterparts in the footwall of extensional detachments.•Late Mesozoic orogenic collapse reactivated basement structures and changed basement rheology. The present-day passive northern South China Sea (SCS) margin has evolved from an Andean-type margin since the Late Cretaceous and subsequently undergone multiple rifting events. However, few studies focused on the role of pre-existing basement structures and their influence on the Early Cenozoic SCS rifting. This paper addresses the nature and tectonic significance of intra-basement structures in outer northeastern SCS margin, based on 2D seismic reflection, well data and onshore geological studies. We identified three subunits (SF1, SF2 and SF3) of intra-basement reflections by their seismic amplitude and geometrical characteristics. By correlating to drilling and waveform modelling results, we interpret that these subunits represent magmatic and/or metamorphic rocks, residual Mesozoic strata, and layered deformation zone in the basement, respectively. The SF3 shows various spatial contacts with the rift-related faults and has influences on rifting fillings. For faults merging downwards with SF3 shear zones, obvious syn-rift depocenters are located in their hanging walls, while faults with higher dip angles above SF3 almost kept inactive during the rifting. The SF3 extend and rise upward to the shallow level, and form a preferential zone for fault activity. In analogy to the onshore extensional structures, structures in SF3 are most likely mylonitized shear zones in the footwall of detachment faults. The interaction of SF3 with the faults produces antiformal structures in the footwall of extensional detachments, recording ductile to brittle transition of the basement. The intra-basement structures inherited from the Early Mesozoic subduction were probably reactivated with significant rheological change following the Late Cretaceous orogenic collapse.