关键词: 敦化-密山断裂带, 航磁特征, 深部构造, 小波多尺度分析, 居里面反演

Abstract:

This paper focuses on the in-depth analysis of the aeromagnetic characteristics of the Dunhua-Mishan fault zone and its surrounding areas using wavelet multi-scale analysis. In order to analyze the anomalies of the crustal structure at different depths, wavelet multi-scale decomposition is used to separate the deep field from the shallow field sources, superimpose the aeromagnetic anomalies on different anomalies of different geological bodies, extract the required information, analyze the local field anomalies caused by the field sources, and invert and interpret the geological bodies. In this paper, wavelet multi-scale analysis is used to decompose the aeromagnetic data, separate the deep and shallow field sources of aeromagnetism in the study area, and obtain wavelet detail maps of order 1 to 4. The wavelet transform detail maps are a response to high frequency anomaly information, and also a reflection of local field aeromagnetic anomaly information, which can be used to infer information such as fault depth and basement depth of basin. The experimental results are used to analyze the anomaly characteristics at different depths, invert and analyze the characteristics of the aeromagnetic anomalies and crustal structure at different depths, explore the deep basement and fault tectonic features and the intersection relationship between the Dunhua-Mishan Fault and the surrounding faults, calculate the approximate field source depth by wavelet detail map and power spectrum method, and infer the fault cut-through depth. The results of the analysis can provide geophysical research information for the study of geotectonics and the evaluation and exploration of hydrocarbon resources.
Based on the original aeromagnetic anomaly map, aeromagnetic anomalies ranging from -494~2022nT can be obtained, with the highest anomaly located at about 50km from Baoqing County. The anomalies in the central part of the study area are high, while those in the eastern and western parts are low. The deposition of basal and ultramafic magmatic rocks in the Dunhua-Mishan area has caused massive high anomalies, while deep and large faults caused basement uplift or decline, shown as high and low anomaly zones. In the aeromagnetic shallow source field, the shallow surface and upper crustal media are more complex, and the Dunhua-Mishan fault zone shows multi-pearl-like small-scale anomalies, resulting mainly from the intrusion of basal or ultramafic magmatic rocks in the shallow part of the fault. In the deep source field, the magnetic anomalies in middle and lower crust are mainly caused by different magnetic properties of basin bedrock. The large fault zone presents as the dividing line of different trajectory feature zones, and the deep large fault cuts deeper and presents as the dividing line of different trajectory feature zones. The cut-through depth of the deep major faults is larger and affects the aeromagnetic characteristics of the deep tectonic zone. The paper further discusses the cut-through depth of the major faults of this region by analyzing the characteristics of the aeromagnetic anomalies at different depths and finds that there are the three deep major faults in the region, namely, the Dunhua-Mishan Fault, the Dahezhen Fault and the Yilan-Yitong Fault, while the Hulin River Fault, the Muling River Fault, the Fujin-Xiaojia River Fault and the Nanbeihe-Boli Fault only cut through the shallow crust; the Muling River Fault, the Dunhua-Mishan Fault, the Dahezhen Fault and the Fujin-Xiaojia River Fault only intersect in the shallow crust. The Parker method was used to invert the depth of the Curie points in the area, and the results show that the depth of the Curie points in the area ranges from 22.3~29.9km, with the deepest area in the south of Hulin County, which is a depressional basin formed by plate subduction and extrusion, and the Dunhua-Mishan fault zone has a controlling effect on the morphology of the Curie points. Seismic activity is low in the region as a whole, and earthquakes are densely distributed in the northwest of the study area along the Yilan-Yitong fault zone, and less distributed along the Dunhua-Mishan fault zone and the Dahezhen fault zone. In the vicinity of the Dunhua-Mishan fault zone, small earthquakes are mainly concentrated in the area south of the Mishan sub-uplift, and the northern section of the Dunhua-Mishan fault zone is generally more stable. The gravity field in this area has been studied in depth by previous authors. The area belongs to the Moho surface uplift zone in Heilongjiang Province, with the Moho depth of about 30~32.5km. The Yilan-Yitong rift zone is deep to the Moho surface, and the Moho surface often shows uplift in the seismically active area. The local deformation and uplift of the crust-mantle provides the possibility of stress concentration, while the existence of deep major faults provides a channel for material transport. The overall level of seismic activity in the region is low, and the areas with intense activity are mainly concentrated in the Yilan-Yitong fault zone, with small earthquakes also gathering near the Jixi area. Seismicity of Qitaihe-Jixi area is mainly influenced by the Mudanjiang Fault and the Nanbei River Fault. The Dunhua-Mishan Fault has a strong influence on the distribution of Curie points and also influences the formation of several major tectonic units. So, more attention should be paid to the crustal activity of areas around the faults and at the intersections of faults in the future.

Key words: Dunhua-Mishan fault zone, aeromagnetic characteristics, deep structure, wavelet multiscale analysis, Curie surface inversion