Along-strike break-up variations of the continent–ocean transition zone in the northern South China Sea

Abstract

We analyse how the potential field and deep structures of the continent–ocean transition (COT) zone in the northern South China Sea vary along-strike with the aim of understanding the break-up styles and synrift magmatism in this region. The high free-air gravity anomaly and the accompanying basement structures are evidence of significant mantle upwelling and serpentinization in the northeastern COT zone. The top-basement of the COT is uplifted and rough, but gradually retrogrades into a relatively flat and low relief towards the mid-northern margin, where the reduced gravity anomaly reflects subdued mantle upwelling, but perhaps stronger synrift magmatism. On the northwestern margin, the low gravity anomaly suggests fairly limited mantle upwelling, but more synrift magmatic intrusions in the crust; the top-basement of the COT zone is smooth and slightly deepened. The width of the COT zone is mostly <30 km and the oldest legible magnetic anomaly (C11r) within the zone is related to the final continental break-up. The seaward limit of the COT should be relocated further south beyond anomaly C11r, pointing to a very narrow zone of true oceanic lithosphere in the NW Sub-basin. The coexistence of mantle upwelling/serpentinization, magmatic underplating and volcanism on top of the COT during continental break-up characterizes a typical intermediate rifted margin, although there are significant along-strike variations. Thematic collection: This article is part of the Emerging knowledge on the tectonics of the South China Sea collection available at: https://www.lyellcollection.org/topic/collections/south-china-sea