Deep Lithospheric Density Structure and Tectonic Significance of the Yidun Island Arc in the Tethyan Tectonic Domain: Evidence from 3D Gravity Inversion

The Yidun Island Arc, located along the eastern margin of the Tibetan Plateau, represents a Late Triassic active continental margin arc closely associated with the subduction–collision evolution of the Paleo-Tethys Ocean (Fig. 1). Although previous geochemical and geophysical studies have revealed the presence of deep-seated thermal and material anomalies beneath this region, significant controversies remain regarding the geometry of the subducting slab, the pathways of mantle upwelling, and their coupling with mineralization processes. Gravity anomalies are highly sensitive to subsurface density variations and therefore provide direct constraints on deep structures. In this study, we construct a lithospheric density model of the Yidun Island Arc based on three-dimensional gravity inversion, with a focus on resolving the deep structural characteristics of the suture zone and the associated mantle flow patterns.

Fig. 1: Tectonic–geomorphological features and regional setting of the Yidun Island Arc. (a) Topography and distribution of major tectonic units in the Yidun Island Arc. LCJF, Lancangjiang Fault; NJF, Nujiang Fault; JSJFZ, Jinshajiang Fault Zone; JQF, Jinhe–Qinghe Fault; MYF, Mopan Mountain–Yuanmou Fault; ANHF, Anninghe Fault; LTF, Litang Fault; DLSF, Daliangshan Fault. (b) Geographic location of the Yidun Island Arc within the Tibetan Plateau and surrounding tectonic framework.

This study utilizes high-precision gravity data to construct a lithospheric density model for depths of 0–150 km beneath the study area by removing the Moho effect and performing three-dimensional gravity inversion. The results indicate that: (1) pronounced high-density anomalies (Δρ ≈ +0.02–0.03 g/cm³) occur beneath the Jinsha River Suture (~99°E) and the Ganzi–Litang Suture (100–100.5°E), extending to depths of ~120 km. These anomalies are interpreted as remnants of eclogitized slabs formed during westward subduction of the Paleo-Tethys Ocean; (2) a near-vertical low-density channel (Δρ ≈ −0.08–0.12 g/cm³; width ~50–100 km) is developed between the two sutures, extending continuously from the asthenosphere to the lower crust. This channel spatially coincides with low-velocity zones revealed by seismic tomography and high-conductivity anomalies identified by magnetotelluric data (Fig. 2), suggesting a mantle channel flow induced by blockage from the rigid Yangtze Block; (3) the low-density channel shows strong spatial overlap with the porphyry–skarn Cu–Mo polymetallic mineralization belt in the southern Yidun Island Arc, indicating that deep mantle upwelling provided essential thermal input and fluid sources for shallow ore-forming systems.

This study provides the first direct geophysical evidence, from the perspective of three-dimensional density structure, for westward subduction polarity and a mantle channel flow-controlled metallogenic model in the Yidun Island Arc, thereby advancing our understanding of the coupling between deep geodynamic processes and shallow mineralization in the Tethyan collisional belt.

Fig. 2: Vertical slices of three-dimensional density structure derived from gravity inversion. (a) Locations of three vertical profiles in different orientations; (a.1)–(a.3) Density anomaly sections along profiles AA′, BB′, and CC′ obtained from gravity inversion; (b.1)–(b.3) Density anomaly sections along profiles AA′, BB′, and CC′ converted from seismic velocity models.

Finally, we would like to express our special gratitude to the National Natural Science Foundation of China (Grant No. 4223031) for the financial support of this paper.