Computer Science
Scientific paper
Nov 1989
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989e%26psl..95..307b&link_type=abstract
Earth and Planetary Science Letters, Volume 95, Issue 3-4, p. 307-320.
Computer Science
5
Scientific paper
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°E +/- 15°, 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°E and N140°E. Magnetic and gravimetric maps of the same area also reveal anomalies trending roughly N50°E, which are disrupted by transform zones striking N140°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 N80°E trending ridge segments, creating the N80°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.
Briais Anne
Pautot Guy
Tapponnier Paul
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