Depths to the magnetic layer bottom in the South China Sea area and their tectonic implications

Abstract

The depths to the magnetic layer bottom (Zb) in the South China Sea (SCS) area are estimated by computing radially averaged amplitude spectra of total field magnetic anomalies. We test different sizes of moving windows in which the spectra are calculated to better understand how window sizes affect the depth estimations. Apart from lowering the resolutions of estimated Zb, larger windows do not necessarily incur presumable increases in Zb in the SCS area. Although the centroid method is taken as our primary technique for estimating Zb, for cross check, the spectral peak and the non-linear inversion methods are also applied to those windows where spectral peaks do appear. In a single window we may find discrepancy in Zb estimated from different techniques, but for all windows showing spectral peaks, the estimated Zb from one technique are grossly correlated with those from another. Our results show that most parts of the central SCS ocean basin and the northern continent–ocean transition (COT) zone have significantly smaller Zb than the surrounding continental blocks. In the surrounding continental regions Zb are averaged at about 34 km, a depth close to the Moho depth. The average Zb is about 22 km in the central basin, but this value is much larger than the Moho depth, signifying that the uppermost 10 km or so of the mantle beneath the central basin is also magnetized. The strong faulting and recent magmatism within the COT zone can account for the small Zb near the northern continental margin. The estimated Zb are also found very correlative to surface heat flow. This observation verifies dominant contributions to surface heat flow from incoming mantle heat flow due to thermal conduction. The positive correlations observed among Zb from different techniques as well as the good correlation between surface heat flow and Zb support the reasoning that our estimated Zb are within an acceptable range of accuracy.