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

Abstract

S U M M A R YThe depths to the magnetic layer bottom (Z b ) 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 Z b , larger windows do not necessarily incur presumable increases in Z b in the SCS area. Although the centroid method is taken as our primary technique for estimating Z b , 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 Z b estimated from different techniques, but for all windows showing spectral peaks, the estimated Z b 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 Z b than the surrounding continental blocks. In the surrounding continental regions Z b are averaged at about 34 km, a depth close to the Moho depth. The average Z b 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 Z b near the northern continental margin. The estimated Z b 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 Z b from different techniques as well as the good correlation between surface heat flow and Z b support the reasoning that our estimated Z b are within an acceptable range of accuracy.

Estimating depth to the bottom of magnetic sources at Wikki Warm Spring region, northeastern Nigeria, using fractal distribution of sources approach

“…One of the methods for examining the thermal structure of the crust is the estimation of the CPD using aeromagnetic data (Smith et al, 1977;Okubo et al, 1985Okubo et al, , 1989Okubo et al, , 1994Okubo et al, , 2003Tsokas et al, 1998;Tanaka et al, 1999;Stampolidis and Tsokas, 2002;Dolmaz et al, 2005;Ates et al, 2005;Aydin et al, 2005;Qingqing et al, 2008;Li et al, 2010;Li, 2011;Bilim, 2011;Manea and Manea, 2011;Abd El Nabi, 2012;Hisarli et al, 2012;Saleh et al, 2013;Sayed et al, 2013;Abraham et al, 2014;Obande et al, 2014;Starostenko et al, 2014). Various studies have shown correlations between Curie temperature depths and average crustal temperatures, leading to viable conclusions regarding lithospheric thermal conditions in a number of regions around the world (Ross et al, 2006;Rajaram, 2007;Bouligand et al, 2009).…”

“…A multilayer situation is seen in the analysis of windows 1, 6, 7, 8, 12, 13, 14, and 15. by enlarging the window, long wavenumber components and/or spectra of uniformly magnetized layer could also gain more portions of power, enough to disguise the peak on the amplitude spectrum (Li et al, 2010). The shape of the radial power spectrum as calculated in previous studies ideally should display a peak at low wavenumbers.…”

Tracing the life history of a marginal sea—On “The South China Sea Deep” Research Program

“…As shown by a recent study, the uppermost mantle can be magnetized by in-situ serpentinization caused by infiltration of seawater through large transform faults [109]. In the SCS, Curie isotherm is recently found to lie underneath the Mohorovicic discontinuity [110], hence raising the question whether the uppermost mantle here is also magnetized because of serpentinization. Before the origin of magnetism in the uppermost mantle is found, we need to investigate first the sub-surface liquid activities in the SCS and their impacts by testing against the above hypothesis [111].…”

Curie Point Depth from Spectral Analysis of Magnetic Data in the Southeast Tibet

“…It indicates that the heat flows are likely more influenced by thermal conductivity. Previous studies on the eastern and southeastern Asia, the north Atlantic and the South China Sea show that the low thermal conductivity of crustal rocks may induce a delay between the changed thermal field and the effect of heat flow [47,[73][74]. It is supposed that the low thermal conductivity might probably cause the low CPD with low heat flow in the Eastern Yunnan Block.…”