Dense Array Imaging of the Upper Crust at Bayan Obo Region: Implications for Deep‐Seated Faults and Mineralization

Abstract The Bayan Obo area, which hosts the world's largest deposit of rare earth elements (REE), is located near the Northern Marginal Fault Zone of the North China Craton (NMFZ‐NCC). This fault system may play a critical role in controlling mineralization processes, but its detailed surface position and subsurface extension remain speculative. In this study, we deployed 158 short‐period seismic stations throughout the Bayan Obo region and applied ambient noise tomography to constrain upper crustal structures. The resulting 3D shear‐wave velocity model resolves fine‐scale heterogeneities that corresponds well to known rock types in the shallow crust (<2 km). At depths greater than 3.5 km, the velocity structure exhibits a pronounced north‐south dichotomy, delineating a sharp contrast that traverses north of the Bayan Obo mining area. Intriguingly, this boundary zone deflects abruptly southward by about 20 km, circumventing the high‐velocity structure of the Bayan Obo and closely following a band of Bouguer gravity low. We suggest this seismic and potential field lineament delineates the subsurface location of the NMFZ‐NCC. Its curved geometry may represent an inherited structure during the Mesoproterozoic rifting, established by the subduction and closure of the intervening (Paleo‐Asian) ocean. This bend zone may represent the foci of stress concentration and release, ideal for developing complex fault zones. Taken together with the widespread occurrence of emplaced dikes, we suggest that this fault system may serve as pathways for fluid migration, likely providing conditions for the formation of the Bayan Obo giant REE deposit.