Crustal Magnetic Structure of the Ulaanbaatar Region, Mongolia, Revealed by 3-D Magnetic Inversion and Spectral Analysis

Ground magnetic data were used to investigate crustal magnetic structure beneath the Ulaanbaatar region of central Mongolia. Residual total-field anomalies were analyzed using reduction to the pole, magnetic derivatives, spectral source-depth estimation, and three-dimensional susceptibility inversion. The magnetic field is characterized by short-wavelength anomalies in the south and southeast and broader magnetic domains toward the north. Spectral analysis produces characteristic source-top depths of approximately 3–5 km, centroid depths of 7–10 km, and derived basal depths of about 10–15 km; these values are interpreted as statistical source-depth parameters rather than Curie-point depths. The three-dimensional inversion reproduces the observations with an RMS residual of 2.59 nT and resolves numerous localized upper-crustal susceptibility bodies that merge into fewer, broader domains with depth. Several magnetic boundaries extend beyond mapped lithological contacts and coincide spatially with major structures. In particular, the Avdar and Sharkhai faults are preferentially associated with susceptibility gradients and magnetic-domain boundaries, consistent with reactivation of pre-existing crustal heterogeneity. These results define a three-dimensional magnetic framework for the faulted and repeatedly intruded crust beneath Ulaanbaatar.