The 3-D Density Structure of the Large Shield Volcanic Structure in the Gardner Region Revealed by a New Gravity Inversion

The Gardner region is a well-known shield volcanic complex on the Moon. Its magma origin and formation mechanism are of significant interest but still enigmatic. To reveal the subsurface structure of this volcanic complex, we propose a new 3-D inversion method of the gravity field based on the regularizations of the L1 norm of the model and its gradients. The model test indicates that our proposed method can recover the density structures with high resolution. Subsequently, we apply it to the Bouguer gravity data in the Gardner region. Our result shows a large, dense body with a volume of about 45 x 45 x 13 km3 centered under the topographic bulge of the Gardner Plateau. We infer that this structure is most likely dense basalts trapped at the crust-mantle boundary as a sill and acted as the magma reservoir that has fed the volcanic complex in the Gardner area. Volcanism is common on the Moon, which provides an important window into its topography and thermal evolution. The surface topography of the Gardner region displays a series of small shield-like domes, sinuous rilles, and irregular depressions. It has been identified as one of the eight large volcanic complexes on the Moon. However, little is known about its internal structure, and many speculations exist about its formation. Gravity observation is one of the important means to probe the inner structure. In the Gardner area, a distinct quasi-circular positive Bouguer gravity anomaly is observed, indicating the presence of subsurface mass surplus. Exploring the cause of the gravity anomaly can provide a basis for revealing the origin of volcanism in this area. However, due to the non-uniqueness of potential field interpretation, available inversion methods of large-scale gravity data face the problem of low-depth resolution. In order to reveal the deep structure of the Gardner area, a new gravity inversion method is developed and applied. The results reveal a large, dense, sill-like structure under the Gardner plateau. It is very likely the magmatic reservoir of the large shield volcanic complex, which provides profound evidence for confirming the volcanic structure of the Gardner region. A new inversion method of the gravity field based on the regularizations of the L1 norm of the model and its gradients is proposed The practical inversion reveals a large dense sill at the 9-22 km depth centered under the lunar Gardner volcanic complex The estimated dense sill is most likely the magma chamber of the large shield volcanic structure in the Gardner region