Abstract:

As the oldest seamount and the biggest guyot found in the South China sea (SCS) central basin, Guanshi guyot is located on the 17°N fossil spreading center near the Manila trench. The dredged plagiogranite sample on Guanshi guyot shows trace element and isotopic composition similar to those of mid-oceanic ridge basalts (MORB), and its pyroxene and whole-rock 40Ar/39Ar dating yields ages of (32.3±0.5) Ma and (28.9±1.9) Ma, which is inconsistent with popular research acquired from marine magnetic anomalies identifying (~23.6 Ma). So further study is required to inspect the opening history of SCS central basin. The density of the dredged plagiogranite sample is measured for many times, which ranges from 2.757 3 to 2.941 6 g/cm3, and the median density is 2.846 5 g/cm3. The stratum velocity field originated from multi-channel seismic (MCS) data is used to transform the time-domain MCS interpreting data into the depth-domain. By creating many crust density models across Guanshi guyot with constraint of the depth-domain sediment basement and gravity anomalies, we infer that the most possible internal density of Guanshi guyot  mainly ranges from 2.70 to 2.75 g/cm3, higher than the average density of younger seamounts with SCS central basin (2.60-2.70 g/cm3), and lower than the average density of gabbro from lower oceanic crust (2.80-2.90 g/cm3). To be precise, higher density (>2.80 g/cm3) rock which can be drilled in the local area in the southern part of Guanshi guyot can’t be excluded in this study. Comprehensive considering the crust density models across Guanshi guyot, MCS interpreting data, and life cycle of seamount, we infer that Guanshi guyot is an intraplate seamount (or oceanic island) during the seafloor spreading period of SCS central basin. Further studies have to be done to understand the relationship between dredged plagiogranite sample and Guanshi guyot. For better understanding the origin of the plagiogranite during the evolution of intraplate seamount, a more reasonable tectonic model should be explored.

Key words: Guanshi guyot, the South China sea central basin, joint modeling of gravity and reflection seismic data, oceanic plagiogranite, intraplate seamount