Earth and Planetary Science Letters, 95 (1989) 307-320 307 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands [5] Constraints of Sea Beam data on crustal fabrics and seafloor spreading in the South China Sea Anne Briais 1, Paul Tapponnier 1 and Guy Pautot 2 1 lnstitut de Physique du Globe de Paris, 4, Place Jussieu, 75252 Paris C~dex 05 (France) 2 IFREMER, Centre de Brest, B.P. 70, 29263 Plouzan~ (France) Received April 19, 1988; revised version accepted August 3, 1989 The South China Sea is a mid-late Tertiary marginal basin. The magnetic anomaly lineations in the eastern part of the basin trend approximately east-west [1,2], suggesting a north-south direction of spreading. In the spring of 1985, two cruises on the French research vessel "Jean Charcot" provided Sea Beam coverage, seismic reflection, magnetic and gravity profiles. The Sea Beam data exhibit two major structural trends: scarps striking N50 o E + 15 o, interpreted as normal faults, and scarps striking N140°E_+15 °, interpreted as fracture zones. This fabric implies a northwest-southeast direction of spreading, up to about 100 km north and south of the inferred spreading axis [3]. Dense Sea Beam coverage of a roughly 1 ° square area northwest of the Scarborough Seamounts chain shows that the emplacement of these seamounts was, at least in its initial stage, controlled by faulting in two orthogonal directions, N50 o E and N140 o E. Magnetic and gravimetric maps of the same area also reveal anomalies trending roughly N50 o E, which are disrupted by transform zones striking N140 o E. This detailed study indicates that the fracture zones may be closely spaced (less than 20-30 km) east of Macclesfield Bank. Furthermore, magnetic anomalies identified as 6 and 6a (20 Myr) along two north-south profiles located at both edges to the north of this detailed study area may be correlated with the N80 °E trend characteristic of such anomalies in the eastern part of the South China Sea. The east-west trend of magnetic anomalies 6 and 6a, south of Scarborough seamount chain, recognised by Taylor and Hayes (1983) [1] is incompatible With the trend of the fault scarps observed on Sea Beam data. We infer that progressive, right-lateral offsets of the ridge across closely spaced discontinuities may account for the nearly east-west average trend of some of the magnetic anomalies, and of the Scarborough seamount chain, which represents the location of the relict spreading axis, in spite of a N50 °E spreading direction. The whole central part of the basin east of 115 °E may have formed in this way, since N130-140 °E striking fracture zones are observed on the Sea Beam swaths. This model may correspond to the second of two successive spreading phases, with N-S and NW-SE directions of extension respectively, as was presented by the authors in a previous paper (Pautot et al., 1986 [3]). Alternatively, we may assume that the direction of extension did not change radically during the opening history [3], implying that oblique spreading occurred along NS0 o E trending ridge segments, creating the N80 o E trending scarps and magnetic anomalies observed to the north and to the south of the study area. Such a direction of extension is compatible with mid-late Tertiary left-lateral movements along large N130-150 °E strike-slip faults, such as the Red River and Wang Chao faults in South China and Sundaland. 1. Introduction The South China Sea basin opened between Eocene and late Miocene times [1,4,5] in a com- plex tectonic setting. Like several other marginal basins of the western Pacific (Japan Sea, Okhotsk Sea), this basin may not be the result of back-arc extension only (e.g. [6,7]). The Philippine arc- trench system, bounding the basin to its east, has an independent history, with poor constraints on the age of the onset of the present-day active, east-dipping subduction [8-11]. The southern margin, including the deep sedimentary Palawan 0012-821X/89/$03.50 © 1989 Elsevier Science Publishers B.V. trough just northwest of Borneo and Palawan islands, had been considered to be a relict conver- gent margin in various kinematic models [1,12]. New multi-channel seismic and drilling surveys suggest it may instead be a passive continental margin affected by convergence from the Sulu Sea in the south [4,13]. To the west and to the north, the history of the continental margins must be placed within the framework of that of the Asian mainland. Since no deep-sea drilling has been performed in the South China Sea, attempts to date the oceanic crust have utilized seafloor mag- netic anomalies. Taylor and Hayes [1,2] identified