Abstract
Based on the new and higher degree geomagnetic model NGDC-720-V3, we have investigated the spatial distribution, the altitude decay characteristics of the crustal magnetic anomaly, the contributions from different wavelength bands to the anomaly, and the relationship among the anomaly, the geological structure, and the geophysical field in Sichuan-Yunnan region of China. It is noted that the most outstanding feature in this area is the strong positive magnetic anomaly in Sichuan Basin, a geologically stable block. Contrasting with this feature, a strong negative anomaly can be seen nearby in Longmen Mountain block, an active block. This contradiction implies a possible relationship between the magnetic field and the geological activity. Completely different feature in magnetic field distribution is seen in the central Yunnan block, another active region, where positive and negative anomalies distribute alternatively, showing a complex magnetic anomaly map. Some fault belts, such as the Longmen Mountain fault, Lijiang-Xiaojinhe fault, and the Red River fault, are the transitional zones of strong and weak or negative and positive anomalies. The corresponding relationship between the magnetic anomaly and the geophysical fields was confirmed.
Highlights
The crustal magnetic field generated by the remanent magnetization and the induced magnetization in the crust and upper mantle is an important part of the geomagnetic field
Complete separation of the main field and the crustal field has not been achieved in theory. They overlapped each other in measurements. It can be inferred from the geomagnetic power spectrum with the spherical harmonic degree n that n ≤ 13 is for the main field, n = 14∼15 is the transition of the main field and the lithospheric field, and n ≥ 16 is for the crustal field [4,5,6], which provides quantitative basis for the study of the crustal field
Based on the NGDC-720-V3 model derived from satellite, airborne, ground, and marine magnetic survey data and constructed by the American National Geophysical Data Center (NGDC) as discussed by Maus [15], we investigate spatial distribution features of the crustal field in the Sichuan-Yunnan region and seek for the connection between the crustal anomaly and the geological structure, including the corresponding relationship between the transitional zone of strong and weak magnetic anomalies and the fault zone, and characteristics of crustal anomalies in basins, orogens, and fracture zones
Summary
The crustal magnetic field (or lithospheric field) generated by the remanent magnetization and the induced magnetization in the crust and upper mantle is an important part of the geomagnetic field. Following the advances of satellite magnetic survey and data processing technology, a high degree geomagnetic spherical harmonic model compiled from satellite, marine, aeromagnetic, and ground magnetic surveys can be evaluated at any desired location to provide the magnetic field components, or the anomaly of the field, and to study their spatial and temporal distribution regularities. Map (WDMAM) was developed through a collaborative effort led by staff at the International Association of Geomagnetism and Aeronomy (IAGA) [11,12,13,14] These models are important and fundamental data for studying the crustal magnetic field. The connection between the anomaly and the crust structure is revealed by comparing with geophysical fields such as the seismic wave velocity anomaly, the gravity anomaly, and the lithospheric thermal structure
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