--> Crustal structure of the non-volcanic continental margin of the South China Sea: many rifts in one

AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Crustal structure of the non-volcanic continental margin of the South China Sea: many rifts in one

Abstract

Rifted continental margins along the Atlantic resulted from extension of continental lithosphere leading to breakup and the onset of seafloor spreading, with the modes of extension and transition to spreading typically described based on end-member conceptual models. An important obstacle in discussing such models and advancing the key aspects of formation of rifted margins has been the limited number of conjugate margin pairs on which high-quality data are available, the poor knowledge of opening rates, and the uncertainty in the kinematic reconstruction of the locations of conjugate pairs. Today, the models strongly rely on the extensively surveyed Iberia–Newfoundland, East Greenland–mid-Norwegian and fewer examples in the South Atlantic conjugate margins, adapted to explain other rift systems. However, as observational techniques have improved in recent years, an increasing complexity has been observed in the range of rifted margins. New advances have boosted the understanding of formation of rifted continental margins, suggesting a wider range of structural evolution that challenges the classical extensional models. Here, we examine the rifting evolution of the South China Sea (SCS) from geophysical and geological observations, traditionally interpreted as a non-volcanic (magma-poor) margin. We analyse the tectonic structure of a ~1100 km-long transect to understand the mechanisms for crustal extension and final breakup. The complementary images of conjugated rift segments permit us to study the symmetry/asymmetry of conjugate margins and the evolution of the processes controlling their contrasting geometry and crustal architecture. The high quality of the resulting images allows us interpreting a series of structural features that challenge the typical end-member non-volcanic model. The seismic images show a tectonic structure characterized by a ~850 km-wide area of stretched continental crust abutting ~200 km of oceanic crust. Our observations indicate that the tectonic structure is the result of several ~100 km wide segments underwent largely simultaneous extensional deformation at both conjugate margins of the South China Sea from early Eocene (~45 Ma) to late Oligocene – early Miocene (~23 Ma). The seismic profiles show several areas where continental crust is thinned down to <10 km (ß ~3.3 – 6.6), alternated with areas where continental crust is comparatively little or very little thinned (ß ~1.4 – 2). The absence of sills in the syn-tectonic strata or magmatic constructs in the upper crust support the idea that lithospheric thinning was not accompanied by any significant melt production even in highly stretched areas. Though, the continent ocean transition is abrupt lacking evidence for mantle exhumation. The final morphology of ~850 km-wide area of stretched continental and rift asymmetric straddling the axis of final breakup may be reminiscent of the structure observed in segments of the South Atlantic.