<p>The traditional gravity and magnetic correspondence analysis tends to have high correlation outside the field source area. In order to overcome the disadvantage, we propose a new method for identify the source position and attribute, which is based on similarity and vertical derivative of potential field. In this method, we put forward a new gravity and magnetic correlation parameter (GMCP), which can effectively reduce the range of potential field source and indicate the field intensity information. The distribution of the non-zero areas of GMCP reflects the size of the source. GMCP discriminant parameter values of positive and negative reflect the source attribute. When GMCP is greater than zero, it is a positive correlation indicating that there are high-density and high-magnetization or low-density and low-magnetization homologous bodies in this region; When GMCP is less than zero, it is negative correlation indicating that there are high-density and low-magnetic or low-density and high-magnetic density homologous bodies in this region. GMCP goes to zero, which means no gravity-magnetic homologous geological body. Complex models test results with different noise level and actual data processing of South China Sea Basin show the correctness and validity of identification of the proposed methods.</p>
The Qinghai-Tibet Plateau (also referred to as the Plateau) has long received much attention from the community of geoscience due to its unique geographical location and rich mineral resources. This paper reviews the aeromagnetic surveys in the Plateau in the past 60 years and summarizes relevant research achievements, which mainly include the followings. (1) The boundaries between the Plateau and its surrounding regions have been clarified. In detail, its western boundary is restricted by West Kunlun-Altyn Tagh arc-shaped magnetic anomaly zone forming due to the arc-shaped connection of the Altyn Tagh and Kangxiwa faults and its eastern boundary consists of the boundaries among different magnetic fields along the Longnan (Wudu)-Kangding Fault. Meanwhile, the fault on the northern margin of the Northern Qilian Mountains serves as its northern boundary. (2) The Plateau is mainly composed of four orogens that were stitched together, namely East Kunlun-Qilian, Hoh-Xil-Songpan, Chamdo-Southwestern Sanjiang (Nujiang, Lancang, and Jinsha rivers in southeastern China), and Gangdese-Himalaya orogens. (3) The basement of the Plateau is dominated by weakly magnetic Proterozoic metamorphic rocks and lacks strongly magnetic Archean crystalline basement of stable continents such as the Tarim and Sichuan blocks. Therefore, it exhibits the characteristics of unstable orogenic basement. (4) The Yarlung-Zangbo suture zone forming due to continent-continent collisions since the Cenozoic shows double aeromagnetic anomaly zones. Therefore, it can be inferred that the Yarlung-Zangbo suture zone formed from the Indian Plate subducting towards and colliding with the Eurasian Plate twice. (5) A huge negative aeromagnetic anomaly in nearly SN trending has been discovered in the middle part of the Plateau, indicating a giant deep thermal-tectonic zone. (6) A dual-layer magnetic structure has been revealed in the Plateau. It consists of shallow magnetic anomaly zones in nearly EW and NW trending and deep magnetic anomaly zones in nearly SN trending. They overlap vertically and cross horizontally, showing the flyover-type geological structure of the Plateau. (7) A group of NW-trending faults occur in eastern Tibet, which is intersected rather than connected by the nearly EW trending that develop in middle-west Tibet. (8) As for the central uplift zone that occurs through the Qiangtang Basin, its metamorphic basement tends to gradually descend from west to east, showing the form of steps. The Qiangtang Basin is divided into the northern and southern part by the central uplift zone in it. The basement in the Qiangtang Basin is deep in the north and west and shallow in the south and west. The basement in the northern Qiangtang Basin is deep and relatively stable and thus is more favorable for the generation and preservation of oil and gas. Up to now, 19 favorable tectonic regions of oil and gas have been determined in the Qiangtang Basin. (9) A total of 21 prospecting areas of mineral resources have been delineated and thousands of ore-bearing (or mineralization) anomalies have been discovered. Additionally, the formation and uplift mechanism of the Plateau are briefly discussed in this paper.
Subduction of oceanic plates beneath continental lithosphere is critical for understanding tectonic evolution and evaluation of prospecting and exploration. Within the Yidun Terrane (YDT) of southwestern China, a number of Mesozoic to Cenozoic granitoid intrusions are exposed and they are useful for investigating the tectonic evolution of the Paleo-Tethys system. However, Mesozoic magmatism of the northern portion of the YDT, remains ambiguous regarding to their magmatic spatial–temporal evolution and their mineralization potential. As the largest pluton in the northern YDT, the Cuojiaoma batholith mainly consists of monzogranite and granodiorite. In this study, we present new zircon U–Pb and molybdenite Re–Os ages, whole-rock geochemical, and zircon Hf–O isotopic data for the Cuojiaoma batholith. LA-ICP-MS zircon U-Pb dating of granodiorite and monzogranite exhibit ages of 221.8 ± 1.4 Ma (n = 22, MSWD = 2.4) and 216.7 ± 2.2 Ma (n = 14, MSWD = 0.03), respectively. A total of 9 molybdenite samples differing Re-Os model ages of 205.4 ± 3.3 and 220.6 ± 5.5 Ma, yield a robust weighted mean model age of 209.9 ± 1.8 Ma (MSWD = 2.4, n = 9) representing the depositional age of molybdenite. The monzogranite and granodiorite's mineralogical and geochemical characteristics indicate they are classified as (medium-) high-K calc-alkaline and metaluminous to weakly peraluminous I-type granite. Geochemically, they are enriched in large-ion lithophile elements (LILEs, e.g., Rb, U, K) and light rare earth elements (LREEs), and depleted in high-field-strength elements (HFSEs, e.g., Nb, Ta, Ti and P) and heavy rare earth elements (HREEs), and contain distinctly or slightly negative Eu anomalies and no significant Ce anomalies, indicating an affinity to classical island arc magma. Combined with their negative zircon εHf(t) values (−16.24 to −2.49 and −16.41 to −1.43) and two-stage Hf model ages (2019–1255 Ma and 2027–1200 Ma), plus their zircon δ18O values, which range from 5.96 to 8.01 and from 5.05 to 7.61 for monzogranite and granodiorite, respectively, these geochemical indexes indicate that the Cuojiaoma batholith shares similar petrogenesis to other intrusions within the YDT. The formation of the early granodiorite may be genetically related to the slab subduction and is most likely formed by mixture of lower crustal melts and mafic magma derived from partial melting of mantle wedge induced by the influx of slab derived melt (fluid). Subsequent slab break-off and the upwelling asthenosphere at ∼ 216 Ma to 210 Ma led to high heat flow and extensive melting of the overlying mantle wedge, followed by the highly crystallization differentiation, which finally contributed to the monzogranite and the subsequential disseminated molybdenite in a post-subduction extension setting. The northern YDT possesses high Mo metallogenic prospectivity, especially for the magmatic activity that dominantly by lower crustal melt and genetically related to the slab break-off occurred at ∼ 216 Ma and represented by the highly fractionated granite derived from the large Cuojiaoma granitic batholith.
Abstract. Although many geophysical models have been proposed in the Longmenshan fault zone (LFZ) and its surrounding areas, the deep structure of the seismic gap and its constraint of the Wenchuan and Lushan earthquakes remain uncertain. Based on the compiled aeromagnetic data and Bouguer gravity data, we have tried to create a more detailed and visible magnetic and density model beneath the LFZ using 2D forward modeling and 3D inversion. The research shows that structure heterogeneities are widely distributed beneath the LFZ. The earthquake epicenters show high magnetic anomalies and the edge of high Bouguer gravity anomalies that consist of rigid blocks where apt to accumulate stress. However, the seismic gap shows low magnetic anomalies and transition of Bouguer gravity anomalies related to a weak zone. The Sichuan Basin has two NE-trending banded high magnetic blocks extending beneath the LFZ that firmly support the crust of the Sichuan Basin was downward subduction toward the LFZ. More importantly, the basement subducts to approximately 33 km west of the Wenchuan-Maoxian fault with a low dip angle beneath the middle segment of the LFZ, whereas the distance decreases to approximately 17 and 19 km under the southern segment. Thus, the crust of the Sichuan Basin beneath the middle segment extends farther than that beneath the southern segment with the seismic gap as the transition zone. Therefore, we propose that the structural heterogeneity of the basement on the western margin of the Sichuan Basin may be the main reason for the different focal mechanisms and geodynamics of the Wenchuan and Lushan earthquakes.
Abstract: The airborne gravimetry was an important leap and innovation in the world's history of geophysical exploration. China's first test of the airborne gravity geological survey in the onshore‐offshore transitional area of the western and southern part of the Bohai Sea was successful and effective in geology. Based on the airborne gravity data, and combining previous ground gravity, seismic and drilling data etc., we carried out the geological interpretation by forward and inverse methods. The result shows that the airborne Bouguer gravity anomaly was clear, the fracture interpretation was reliable, and the inversion depth of the main geological interfaces was relatively accurate. This airborne gravity geological survey not only filled the exploring gaps in the onshore‐offshore transitional area, and realized the geological and tectonic junction between the sea and the land, but also discovered four local gravity anomalies, 11 fractures and three sags or subsags, and so on. The good geological effect of airborne gravimetry not restricted by terrain condition shows that it can be served as a new geophysical method in the exploration of complex terrain physiognomy area such as mountain, jungle, desert, marsh, onshore‐offshore transitional area and so on, and has an extensive application prospect in China in the future.
Abstract. Although many geophysical models have been proposed in the Longmenshan fault zone (LFZ) and its surrounding areas, the deep structure of the seismic gap and its constraint of the Wenchuan and Lushan earthquakes remain uncertain. Based on the compiled aeromagnetic data and Bouguer gravity data, we have tried to create a more detailed and visible magnetic and density model beneath the LFZ using 2D forward modeling and 3D inversion. The research shows that structure heterogeneities are widely distributed beneath the LFZ. The earthquake epicenters show high magnetic anomalies and the edge of high Bouguer gravity anomalies that consist of rigid blocks where apt to accumulate stress. However, the seismic gap shows low magnetic anomalies and transition of Bouguer gravity anomalies related to a weak zone. The Sichuan Basin has two NE-trending banded high magnetic blocks extending beneath the LFZ that firmly support the crust of the Sichuan Basin was downward subduction toward the LFZ. More importantly, the basement subducts to approximately 33 km west of the Wenchuan-Maoxian fault with a low dip angle beneath the middle segment of the LFZ, whereas the distance decreases to approximately 17 and 19 km under the southern segment. Thus, the crust of the Sichuan Basin beneath the middle segment extends farther than that beneath the southern segment with the seismic gap as the transition zone. Therefore, we propose that the structural heterogeneity of the basement on the western margin of the Sichuan Basin may be the main reason for the different focal mechanisms and geodynamics of the Wenchuan and Lushan earthquakes.
Abstract In order to quantitatively evaluate the performance index of airborne gravity system, repeated flight lines are usually used to test the coherence of dynamic survey of airborne gravimeter. However, there is no corresponding criterion in the current code to evaluate the quality of repeat line test data of airborne gravity survey. We research and summarize a method of RMS (root mean square) accuracy calculation for the test data of repeat lines. Based on the calculation results of internal and external accord accuracy, we can objectively evaluate the dynamic accuracy index and working status of the instrument.
The West Qinling Orogenic Belt (WQOB) in central China records the tectonic evolution and deep geodynamics process associated with plate collision on the northeastern margin of the Qinghai–Tibet Plateau. The study of the deep–seated West Qinling structure is beneficial for revealing the evolution of the West Qinling crust during the continental collision orogeny, and also has significance for mineral exploration. In this study, the crustal structure and the distribution of major faults in the West Qinling Orogen are calculated by processing geophysical aeromagnetic and ground gravity data. The density and magnetic susceptibility differences between the West Qinling crust and the upper mantle are calculated by fitting, and the depth and trend of the faults related are inferred from the gravity and magnetic inversion results. Due to the overall subduction of the lower crust of the Ruoergai to the West Qinling Orogenic Belt, the structural deformation within the West Qinling is strong, and the geophysical magnetic field and gravity field are suddenly changed. It is indicated that the formation of the Diebu–Sanhe fault may have a strong correlation with the Mianlue suture zone, and their intersection may penetrate into the lower crust at a depth of about 50 km or more. The West Qinling structure represents important conduits for migrating magmatic–derived hydrothermal fluids. During the continental collision, the West Qinling major faults provided hydrothermal migration channels for ore–forming materials in different tectonic periods. Meanwhile, multiple the geological activities led to the formation of mineral deposits related to magmatic hydrothermal fluids.
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