Is There Exhumed Continental Mantle in the Northeast South China Sea?

Abstract:

Seismic reflection profiles and tomographic models developed from wide-angle seismic traveltime data document a 200-300-km-wide zone of thinned continental crust between the shelf and the continent-ocean transition (COT) across the NE South China Sea (SCS) rifted margin. Despite this wide rift zone and an apparent lack of significant synrift magmatism, a zone of exhumed continental mantle has not been identified near the COT. At other well-studied magma-poor rifted margins such as Iberia and Newfoundland, Tethyan margins now exposed in the Alps, and the southern rift margin between Australia and Antarctica, there is clear evidence that extreme crustal thinning can result in exhumation of the sub-continental mantle across the continent-ocean-transition. In the northeast SCS, there is a zone of extremely thin crust, perhaps thinner than 2-5 km, but this margin-parallel zone is only ~50-75 km seaward of the shelf edge. No data clearly document mantle exhumation here, but shallow levels of mantle may well have been serpentinized during the rifting process; we interpret this zone as a failed rift. More than 150 km to the south, ridges and seamounts become more prominent approaching the continent-ocean transition. In the COT zone, Moho reflections are present on some seismic profiles below 8-12 km-thick crust on the continental side and below 6-8 km-thick crust on the oceanic side, apparently excluding mantle exhumation as a possibility. The nature of the COT is not completely clear from the seismic reflection data, and neither these data nor magnetic anomalies indicate unambiguously how heavily this zone is influenced by magmatism. We suggest that this apparently abrupt transition from stretched continental crust to oceanic crust without a zone of exhumed mantle may represent an important variant in the spectrum of magma-poor rifted margin structures. However, additional imaging and extensive sampling must be conducted to constrain the nature and primary characteristics of this margin and continent-ocean transition.