## 2 Observation Data and Data Processing Methods
### Decomposition of the Lithospheric Magnetic Field  
The upward continuation method is a conventional gravity and magnetic data processing method that mainly highlights deep anomalies. It has an important role in the study of deep structures, the tendency of structures, and the depth of faults. In this paper, the upward continuation method was employed to decompose the lithospheric magnetic field; the specific formula is defined as follows [3]:  
\[\Delta T^{\prime}(X_{i},-h)=\frac{h}{\pi}\sum_{k=1}^{k=n-1}\frac{\left(X_{k}- X_{i}\right)}{\left(X_{k}-X_{i}\right)^{2}+h^{2}}\times\Delta T(X_{k},0) \tag{2}\]  
Here, \(\Delta T\) and \(\Delta T^{\prime}\) are the lithospheric magnetic field values before extension and after extension, respectively, in nT; h is the upward continuation height; \(X_{i}\) and \(X_{k}\) are the projection distances of each measuring point, where \(X_{i}\) is given, \(X_{k}\) is variable, and \(k\) changes from 1 to 159; and \(n\) is the total number of measuring points, that is, \(n=160\).  
The obtained \"residual\" anomalies can be considered an approximation of local anomalies (caused by shallow materials) by subtracting the anomalies at a certain height from the measured anomalies [11]. The majority of the surface at depths of 0-5 km in most areas is sedimentary rock. Most of the surface at depths of 5-30 km is intrusive rock, and the temperature of most rocks is below the Curie temperature [3]. The rock temperature below 30 km is evaluated to be higher than the Curie temperature of the ferromagnetic material, so it is concluded that the lithospheric magnetic field generated by the rock below 30 km is very weak. Based on these above characteristics, the process of lithospheric magnetic field decomposition by the upward continuation method is described as follows: the lithospheric magnetic field is extended at a height of 30 km, and the obtained magnetic field components reflect the distribution characteristics of the interior, which is referred to as the basement part f\({}_{\rm b}\). After subtracting the basement part f\({}_{\rm b}\) from the lithospheric magnetic field F\({}_{\rm I}\), i.e., F\({}_{\rm I}\)-f\({}_{\rm b}\), and then extending upwards to a height of 10 km, the obtained lithospheric magnetic field component reflects the characteristics of the magnetic susceptibility and remanent magnetization structure of the medium at a depth of approximately 5-25 km, which is referred to as the middle layer f\({}_{\rm m}\). The lithospheric magnetic field F\({}_{\rm I}\) is subtracted from the basement part f\({}_{\rm b}\) and the middle layer f\({}_{\rm m}\). The obtained lithospheric magnetic field component reflects the characteristics of the magnetic susceptibility and remanent magnetization structure of the medium from the surface to a depth of approximately 5 km, referred to as the superficial part f\({}_{\rm s}\). The lithospheric magnetic field F\({}_{\rm I}\) is decomposed into the basement part f\({}_{\rm b}\), middle layer part f\({}_{\rm m}\), and superficial part f\({}_{\rm s}\) by the upward continuation method, namely  
\[{\rm F_{I}=f_{b}+f_{m}+f_{s}} \tag{3}\]