The Carbonate Veins Documenting the Tectonic Evolution of the South
China Sea Continental Margin from Early Cretaceous to Early Cenozoic
Abstract
The transition from active to passive continental margin of the South
China Sea (SCS) is usually inferred to occur in the Late Mesozoic to
Early Cenozoic. However, it is less known about the tectonic
characteristics of active continental margins before the Late Mesozoic,
which hampers the recognition of integral evolution of the SCS. The
International Ocean Discovery Program (IODP) site U1504 has sampled
greenschist facies mylonite from the basement in the Outer Margin High
of the northern SCS continental margin, which potentially record the
Mesozoic and Cenozoic tectonic evolution of the SCS region. The
microstructure has identified two episodes of deformation in the
mylonite, namely early ductile and late brittle deformation, but without
age constraints. Here, we further identify three episodes of carbonate
veins (pre-mylonite, syn-mylonite and post-mylonite) in the greenschist
facies mylonite according to the intersecting relationship between the
veins and the mylonite foliation. Then we select 10 carbonate samples
for in situ U-Pb dating, and obtain three accurate ages. The
pre-mylonite carbonate veins are dated to 210 ± 20 Ma and 195 ± 32 Ma,
respectively, which might denote the age of the protolith clast. The age
of the syn-mylonite carbonate vein is 135 ± 12 Ma. But for the
post-mylonite carbonate veins, no effective age was obtained using U-Pb
dating method. Post-mylonite carbonate veins and late brittle fractures
were formed at the same time, and the formation environment is similar
to the overlying Late Eocene bioclastic limestone. Therefore, combining
the microstructure, geochemistry and seismic profile, we speculate that
the post-mylonite carbonate veins and brittle fractures may be formed
during the Early Cenozoic rifting. These dating ages of the three
episodes of carbonate veins suggest that the mylonite records at least
two main periods of continental extension in the SCS region since the
Early Cretaceous. In reference to the Mesozoic tectonic settings, we
infer that, due to the slab rollback of the subducting paleo-Pacific,
the SCS continental margin started significant extension during the
Early Cretaceous as shown by the ductile deformation of the mylonite. In
the Early Cenozoic, the mylonite was exhumated to the seafloor along
with further continental extension, and weak brittle deformation
occurred in the mylonite. Therefore, the Early Cretaceous extension of
the SCS active continental margin may have a certain promotion effect on
the rupture of the passive continental margin in the Cenozoic. Keywords:
Greenschist facies mylonite; Carbonate U-Pb dating; Continental margin
of the SCS; Early Cretaceous; IODP 368