Magnetotelluric study of the mechanism of the Abaga and Dalinor volcanic groups in Central Inner Mongolia, China

Abstract

Central Inner Mongolia is one of the areas with intense volcanic activity in the Cenozoic. In order to better understand the origin of these volcanoes, a study of magnetotelluric sounding (MT) was carried out. This study aims to learn about the mechanism of deep volcanic areas from the electrical point of view. Broadband and long-period MT data were collected on a 500-kilometer section at central Inner Mongolia. Two-dimensional inversion was applied since the data show no obvious three-dimensional effect. Impedance tensor decomposition and phase tensor analysis show that the electric strike angle of the study area is N45°E. A 2-D resistivity model was obtained with a depth range from surface to the uppermost mantle (0–150 km). The resistivity model imaged the ascending channels and the sources of the magma beneath the Abaga and the Dalinor volcanic groups. There are low resistivity anomalies (C1 and C2) related to volcanic activities at depths of 60–100 km below the two volcanic groups. The percentage of melt in the low-resistivity areas below the Abaga volcanic group and the Dalinor volcanic group are as high as 5–10% and 3–5% respectively. This difference in terms of melt fraction explains why the volcanic activity of the Abaga volcanic group is larger than that of the Dalinor volcanic group. Two magma chambers were found in the lower crust of the Dalinor volcanic group, and no magma chamber was found beneath the Abaga group. Based on the resistivity model, it is further speculated that the formation mechanism of volcanic activity in central Inner Mongolia is that the partial molten asthenosphere in an extensive environment flowed up to the surface through structural fissures and formed a large area of basalt platform. The magmatic body imaged at the depths of 60–100 km may be the source of volcanic eruptions of the study area.