Serpentinized Peridotite as Source of Aeromagnetic Anomalies

The source of long-wavelength aeromagnetic anomalies appears to originate from the earth's deep crust. Constrained by previous studies on geochemical, petrologic analysis, the eclogite and serpentinized peridotite samples from drill hole ZK703 at Donghai in the western Sulu ultrahigh-pressure (UHP) terrane, East China, were unambiguously exhumed from the lower crust and the upper mantle, providing significant information about the magnetic properties of rocks at a deeper part of the crust. Results show that the serpentinization process favors the neoformation of nearly stoichiometric magnetite, resulting in the enhancement of its magnetization up to 8.6 A/m, which is sufficient enough to contribute to some magnetic anomalies. In contrast, eclogite samples have only weaker magnetization (generally less than 0.05 A/m) compared to serpentinized peridotite. Nevertheless, experiments under the lower crustal conditions are necessary to further support these conclusions.

Tectonic framework of Qilian orogen: reveal from an aeromagnetic anomaly feature

The Qilian Orogenic belt is one of the typical orogenic belts globally and a natural laboratory for studying plate tectonics.Many researchers have studied the ophiolite and high pressure and ultra-high pressure metamorphic rocks in the Qilian orogen and obtained valuable achievements.However,a hot debate exists on the basement property,the distribution of ophiolite,and the boundaries of tectonic units.Large-scale high-precision aeromagnetic surveys have recently been conducted in the Qilian Orogenic belt and adjacent areas.In this study,we are trying to analysis the tectonic framework of the Qilian Orogen using 1:500,000 aeromagnetic data.The results provide geophysical perspectives for studying the structural framework and deformation of this area.According to the aeromagnetic∆T anomaly map,the central and Southern Qilian have the same magnetic anomaly feature that noticeably differs from the North Qilian Orogenic belt and the Qaidam Block.This result indicates that the central and Southern Qilian have a unified magnetic basement and differ from the North Qilian orogenic belt and Qaidam Block.The map shows the distribution of ophiolite in the North Qilian orogenic belt.Linear magnetic anomalies represent the ophiolites because the mafic–ultramafic rocks usually have high magnetic susceptibility.The ophiolite belts are continuously distributed in the western part of North Qilian orogenic belt and have a large scale.However,the scale of the ophiolite belt and the outcropping of mafic–ultramafic rocks reduces when they pass through Qilian County to the east.The results indicate differences in the evolution process between the eastern and western parts of North Qilian,with Qilian County as the transition zone.This study also systematically defines the geophysical boundaries of the Qaidam Block,Qilian Block,North Qilian Orogenic belt,and Alxa block.It is proposed that the sinistral displacement of the Altun Fault is adjusted and absorbed by the series of NE-trending faults in the Qilian orogen and merge into the Longshoushan–Gushi Fault.The extension of the North Qilian Orogenic belt is strengthened by the neotectonics movement along the shearing direction,which separated the North Qilian Orogenic belt into several segments and formed a series of northeast-trending faults.

Aeromagnetic data and geological structure of continental China:A review

We review the latest aeromagnetic geological data of continental China. We discuss the latest achievements in geological mapping and the newly detected features based on aeromagnetic data. Using aeromagnetic data collected for more than 50 years, a series of 1：5000000 and 1：1000000 aeromagnetic maps of continental China were compiled using state-of-the-art digital technology, and data processing and transformation. Guided by plate tectonics and continental dynamics, rock physical properties, and magnetic anomalies, we compiled maps of the depth of the magnetic basement of continental China and the major geotectonic units, and presented newly detected geological structures based on the aeromagnefic data.

Regional metallogenic structure based on aeromagnetic data in northern Chile

Chile is a very important country that forms part of the Andean metallogenic belts. The Atacama and Domeyko fault systems in northern Chile control the tectonic- magmatic activities that migrate eastward and the types of mineral resources. In this paper, we processed and interpreted aeromagnetic data from northern Chile using reduction to pole, upward field continuation, the second derivative calculation in the vertical direction, inclination angle calculation, and analytical signal amplitude analysis. We revealed the locations and planar distribution characteristics of the regional deep faults along the NNE and NS directions. Furthermore, we observed that the major reasons for the formation of the tectonic-magmatic rocks belts were the nearly parallel deep faults distributed from west to east and multiple magmatic activities along these faults. We ascertained the locations of volcanic mechanisms and the relationships between them using these regional deep faults. We deduced the spatial distributions of the basic-intermediate, basic, and acidic igneous rocks, intrusive rocks, and sedimentary sequences. We showed the linear positive magnetic anomalies and magnetic anomaly gradient zones by slowly varying the background, negative magnetic anomaly field, which indicated the presence of strong magmatic activities in these regional deep faults; it also revealed the favorable areas of copper and polymetallic mineralization. This study provides some basic information for further research on the geology, structural characteristics, and mineral resource prospecting in northern Chile.

Characteristics of the crystalline basement beneath the Ordos Basin:Constraint from aeromagnetic data

Aeromagnetic anomaly zonation of the Ordos Basin and adjacent areas was obtained by processing high-precision and large-scale aeromagnetic anomalies with an approach of reduction to the pole upward continuation. Comparative study on aeromagnetic and seismic tomography suggests that aeromagnetic anomalies in this area are influenced by both the magnetic property of the rock and the burial depth of the Precambrian crystalline basement. Basement depth might be the fundamental control factor for aeromagnetic anomalies because the positive and negative anomalies on the reduction to the pole- upward-continuation anomaly maps roughly coincide with the uplifts and depressions of the crystalline basement in the basin. The results, together with the latest understanding of basement faults, SHRIMP U-Pb zircon dating of metamorphic rock and granite, drilling data, detrital zircon ages, and gravity data interpretation, suggest that the Ordos block is not an entirety of Archean.

Characteristics of Zhangsanying-Tongshanzi aeromagnetic anomaly zone and prospecting potential of iron deposits in northern Hebei,China

Based on the latest high-precision aeromagnetic data,an aeromagnetic anomaly zone is identified at Zhangsanying--Tongshanzi in northern Hebei Province.By the potential field conversion processing,including the reduction to the pole,vertical derivative,upward continuation and residual anomaly,the authors analyzed the characteristics of three typical aeromagnetic anomalies in Zhangsanying--Tongshanzi aeromagnetic anomaly zone and their geological origin.The methods include the forward and inversion methods,such as 2.5D optimization fitting and Euler deconvolution.Moreover,combined with the geological outcrop,known iron deposits,ground magnetic survey and verification,the authors studied the relationship between the aeromagnetic anomalies and iron deposits.The result shows that the Zhangsanying--Tongshanzi aeromagnetic anomaly zone is composed of 10 large magnetic anomalies with high amplitude and clear boundary.The aeromagnetic anomalies are comparable and intrinsically related to the ground magnetic anomalies and IP anomalies,indicating that the anomalies are caused by magnetite deposits.It has good magnetite prospecting potential in the Zhangsanying--Tongshanzi aeromagnetic anomaly zone.

Geophysical prospecting of copper-nickel deposits in Beishan rift zone, Xinjiang

The Beishan rift zone in Xinjiang Uygur Autonomous Region was formed due to strong activities of faults on the basement of the Tarim continental crust.Despite the fact that many geological research results of the rift zone have been achieved,only a few studies have been conducted on its regional geophysical characteristics.In this paper,the gravity and magnetic anomalies of the rift zone were highlighted through specific data processing of 1∶50000 high-precision aeromagnetic data and gravity data with a grid spacing of 2 km×2 km.Based on this,the geophysical evidence for the scope and internal structures of the Beishan rift zone was obtained for the first time.The distinct characteristics of magnetic and gravity fields in the areas to the north and south of the Beishan rift zone reveal that deep faults exist between the Beishan rift zone and the geological units on the southern and northern sides.Furthermore,the faults on the two areas contain the bidirectional thrusts and have flower-shaped structures according to the characteristics of the magnetic and gravity fields.The Beishan rift zone can be divided into two tectonomagmatic zones,namely the Zhongposhan-Bijiashan-Cihai-Baishanliang zone(the northern zone)and the Bayiquan-Qixin-Baishan zone(the southern zone).The northern zone can be further subdivided into three comet-shaped anomaly groups(tectonomagmatic areas),while the southern zone can be further subdivided into two tectonomagmatic areas.According to the characteristics of aeromagnetic anomalies and gravity field,19 mafic-ultramafic complexes were delineated.The known Pobei,Hongshishan,and Qixin complexes are all located within the inferred complexes,with estimates of total explored resources of Ni,Cu,and Au of 3×10^(6) t,10×10^(3) t and 10 t,respectively.The prospecting of high-grade copper-nickel deposits should focus on the periphery and deep parts of the known and inferred mafic-ultramafic complexes.Among them,the peripheral strata of the complexes specifically have great prospecting potential of large-scale high-grade copper-nickel deposits of magma injection type.Finally,this paper analyzed the application effects of the rapid airborne-ground-drilling synergetic exploration method in the prospecting of copper-nickel deposits in Qixin,Beishan,Xinjiang,which will provide references for further exploration of copper-nickel deposits in Beishan area,Xinjiang.

Potash–Forming Regularity of the Ordovician Marine Northern Shaanxi Salt Basin: Insight from Review and Prospect of the Deep Geology of the Ordos Basin

The Ordovician Northern Shaanxi Salt Basin （ONSSB）, located in the east-central Ordos Basin, western North China Craton （NCC）, is one of the largest marine salt basins yet discovered in China. A huge amount of halite deposited in the Mid-Ordovician Majiagou Formation, and potash- containing indication and local thin layer of potash seam were discovered in O2m^65 （6th submember, 5th member of the Majiagou Formation）. This makes ONSSB a rare Ordovician potash-containing basin in the world, and brings new hope for prospecting marine solid potash in this basin. However, several primary scientific problems, such as the coupling relationship between ONSSB and the continent nucleus, how the high-precision basement fold controls the ONSSB, and how the basement faults and relief control ONSSB, are still unclear due to the limitations of the knowledge about the basement of the Ordos Basin. This has become a barrier for understanding the potash-forming regularity in the continental nucleus （CN） area in marine salt basin in China. Up to now, the material accumulation has provided ripe conditions for the answers to these questions. Latest zircon U-Pb ages for the basement samples beneath the Ordos Basin reveal that there exists a continental nucleus （Yi-Meng CN） beneath the northern Ordos Basin. And this brings light into the fact that the ONSSB lies not overlying on the Yi- Meng CN but to south Yi-Meng CN. Both do not have superimposed relationship in space. And borehole penetrating into the basement reached Palaeoproterozoic meta-sedimentary rocks, which suggests the ONSSB is situated in the accretion belt of Yi-Meng CN during geological history. Basement relief beneath the ONSSB area revealed by seismic tomography and aeromagnetic anomaly confirms the existence of basement uplift and faults, which provides tectonic setting for sedimentary center migration of the ONSSB. Comparative research with various data sources indicates that the expanding strata in the ONSSB adopted the shape of the basement folds. We found that the orientations of the potash sags showed high correlation with those of several basement and sedimentary cover faults in the ONSSB. The secondary depressions are also controlled by the faults. Comparative research between all the global salt basins and continental nuclei distribution suggests that distribution of the former is controlled by the latter, and almost all the salt basins developed in or at the margin of the continental nucleus area. The nature of the tectonie basement exerts a key controlling effect on potash basin formation. And on this basis we analyzed in detail the geological conditions of salt-forming and potash-forming in the ONSSB.

Multi-scale wavelet separation of aeromag-netic anomaly and study of faults in Beijing area

In this paper, through a multi-scale separation of the aeromagnetic anomaly by wavelet transform technique, we reprocessed the aeromagnetic data collected 20 years ago in Beijing area and analyzed the aeromagnetic anomaly qualitatively, integrating geological structure features in the area. In particular, we studied the spatial distributions of the two main faults called Shunyi-Liangxiang fault and Banqiao-Babaoshan-Tongxian fault, which have cut and gone through the central Beijing area striking in NE and EW directions, respectively. The influences of these two faults on the earthquakes have also been discussed briefly.

Investigation of Curie Point Depth in Sulu Ultrahigh-Pressure Metamorphic Belt,Eastern China

The Curie point depth of continental crust can reflect the regional tectonic pattern and geothermal structures. Analysis of magnetism is an efficient way to obtain the Curie point depth on a regional scale. This study systematically investigated the Curie point depth of Sulu （苏鲁） ultrahigh pressure （UHP） metamorphic belt （33°40＇N to 36°20＇N and 118°E to 120°E, ca. 60 000 km^2）, eastern China using aeromagnetic data. The results show that the Curie point depth of the Sulu region varies from 18.5 to 27 km. The shallowest Curie point depth （ca. 18.5 km） is located in Subei （苏北） subsidence, where the estimated temperature gradient value is about 31.35℃/km, which is comparable with the measured value of 30 ℃/km. In addition, a two-dimensional numerical solution of the heat conduction was used to calculate the temperature field to a depth of 30 km along the profile from Tancheng （郯城） to Lianshui （涟水） with a length of 139 km. The steady state model solved using the finite element method shows that the temperature around the Curie point depth is about 585.36 ℃, which is close to the Curie temperature （580℃） of magnetite at atmospheric pressure. These results provide new insights into the tectonic and continuous thermal structures of the Sulu UHP metamorphic belt.