Along-strike breakup variations of the continent-ocean transition zone in the northern South China Sea

To understand breakup styles and syn-rifting magmatism in the northern South China Sea (SCS), we analyze how the continent-ocean transition zone (COT) varies along strike in its potential field and deep structures. High free-air gravity anomaly and accompanied basement structures evidence significant mantle upwelling and serpentinization in the northeastern COT. 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 reduced gravity anomaly reflects subdued mantle upwelling but perhaps stronger syn-rifting magmatism. In the northwestern margin, low gravity anomaly suggests fairly limited mantle upwelling, but more syn-rifting magmatic intrusions in the crust, and the top basement of the COT is smooth and slightly deepened. The COT widths are mostly less than 30 km, and within the COT the oldest legible magnetic anomaly C11r is related to the final continental breakup. 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 Northwest Subbasin. The coexistence of mantle upwelling/serpentinization, magmatic underplating, and volcanisms atop the COT during continental breakup characterizes a typical intermediate rifted margin that shows, nonetheless, 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