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

Ground magnetic observations were used to investigate the crustal magnetic structure beneath the Ulaanbaatar region of central Mongolia. The dataset combines 100 northern sites acquired in 2012 with 283 southern sites acquired in 2022. After correcting each campaign for temporal variations and the main geomagnetic field, we integrated residual total-field anomalies, reduction to the pole, magnetic derivatives, spectral source-depth estimates, and three-dimensional susceptibility inversion. Short-wavelength anomalies are concentrated in the south and southeast, whereas broader magnetic domains dominate the north. Spectral analysis gives characteristic source-top depths of 3–5 km, centroid depths of 7–10 km, and derived basal depths of 10–15 km. These values describe statistical source populations and are not interpreted as Curie-point depths. The inversion fits the observations with an RMS residual of 2.59 nT and resolves numerous localized upper-crustal features that merge into fewer, broader susceptibility domains at depth. The mapped Bogd Uul granitoid has a comparatively weak magnetic interior, while stronger responses along its southern and southeastern margins point to a composite contact-zone signal. The Avdar and Sharkhai faults show contrasting spatial and directional associations with magnetic boundaries, consistent with present-day deformation exploiting inherited crustal heterogeneity in different ways. Together, the results provide a three-dimensional framework for interpreting the faulted and repeatedly intruded crust beneath Ulaanbaatar.