Study On The Gravity And Magnetic Fields And Their Relationships With The Tectonic Frame In Back Mountain Area Of Karamay

Back Mountain area of Karamay located at the southeast of Western Junggar, is an important constitute part of the Central Asian Orogenic Belt. During the Paleozoic era, the complicated ocean-continent conversion and crustal accretion processes occurred in the Back Mountain area of Karamay, which has been focused by academic community. In this thesis, aiming to the crustal and mantle structures and combing with the new knowledge of the petrology and tectonics, the gravity and magnetic anomaly data with different scales are utilized to reveal the tectonic frame and geological evolution in the Back Mountain area of Karamay and thus this research has important scientific significances.Based on the latterly released satellite gravity and magnetic data, regional terrestrial gravity and aeromagnetic data and four profiles of terrestrial gravity and magnetic data, the distribution features of the gravity and magnetic anomaly in the study area are analyzed from the different spatial scales. Moreover, the gravity and magnetic anomaly are interpreted qualitatively and quantitatively by new techniques. Furthermore, adopting the three-dimensional (3D) inversion, the 3D density and susceptibility are imaged, respectively.Because of the high altitude, nearly global coverage, data processing uniformly and the consistencies of the accuracy and resolution globally, the satellite gravity and magnetic data can fill the gaps in the terrestrial and aerial gravity and magnetic surveys and importantly improve the reliability of the long wavelengths. Therefore, the newly released satellite gravity and magnetic anomaly data are used in the Western Junggar and its surroundings. First, the distribution features of the anomaly data are analyzed and then the large-scale 3D density and susceptibility structures in the crust and uppermost mantle are constructed by the 3D inversion. At last, the distribution characteristics of the density and susceptibility and their implications on the regional tectonics are revealed.In the current status of geophysical work in the Back Mountain area of Karamay, the terrestrial gravity and aeromagnetic data are compiled by the surveys in different epochs and with different resolutions, thus to separate the residual and regional anomalies, the wavelet decomposition is applied to the gravity and magnetic anomaly data. According to the results by trial-and-error procedure, the 5-order approximation and 6-order approximation of the wavelet decomposition using db4 (Daubechies-4) are treated as the regional gravity and magnetic anomaly, respectively. Multiple techniques such as the wavelet modulus maxima method, the normalized derivative deviation method, tilt-angle of the total gradient and the Canny edge detection, are applied to the linear signal extraction of the terrestrial gravity and aeromagnetic anomaly data. Combining the local gravity anomaly and its first order vertical derivative, the geological units and faults are recognized. Using the separated local anomalies, the source nature is discussed and then by the inversion, the 3D density and susceptibility structures in the middle and upper crust are obtained in the Back Mountain area of Karamay.The 2.5D density and susceptibility are imaged, respectively, by the four profiles of terrestrial gravity and magnetic data with high accuracy and resolution. Then the 2.5D density and susceptibility structures are compared with the results by the inversion of the magnetotelluric data along the same profiles.The main achievements in this thesis are as following:(1) Characteristics of the satellite gravity and magnetic anomaly and their implications on the regional tectonic frameAccording to the characteristics of the gravity and magnetic anomalies, the feature of gravity and magnetic anomaly in the Altai, Kazakhstan-Junggar, Tianshan and the northern margin of Tarim tectonic zone is obvious. Regional gravity field well reflects the deep lithosphere gravity field background of the major tectonic units in this region (i.e.40°N～50°N, 80°E～90°E). The local gravity anomalies depict the lithospheric structure and lithology in the contact zone between the different tectonic units. The piedmont high gravity anomalies in the south-east of Altai fold belt in the north of region refulects the characteristics of high density of lithosphere, it was obviously controlled by the structures of NW trending, especially the Fuyun-Fuhai area with the high gravity anomalies corresponds to the late Paleozoic active continental margin in the Junggar basin, which is considered to be the largest secondary tectonic units of Siberian plate in China. Tianshan fold belt, in south of the region, is the most severe changes area in gravity anomalies. Positive and negative anomalies stripe shows the distribution of lithosphere deep faults in this zone and their relationships, high gravity values in the south of the Tianshan orogenic zone shows the presence of high density materials in the tarim plate and lithosphere on its interface zone; the interface zone of Tianshan fold belt and the Junggar basin in the zone is characterized by low gravity value band of the north Tianshan belt. NW and NE trending high gravity anomalies band presented in the interchange of Kazakhstan plots and Junggar Basin, is located in Junggar and the south edge of Tal Bach taishan in its north, it’s considered to be the subduction hyperplasia belt within ocean, has a variety of different types of tectonic setting such as fossil basin, island arc, posted collision and so on,it may be one of the causes of high gravity anomalies.There being a low regional magnetic anomaly zone with the strike of NW-SE in the north of Altay fold belt, which reflects that the region controlled by NW trending structures significantly; The region of the low magnetic anomaly with the strike of NW-SE in the south of the region is Tianshan fold belt, which shows the lithosphere deep fault zone distribution and its relationships, and the low magnetic anomaly in the south Tianshan orogenic belts, which indicate that there exits the substances of low magnetic susceptibility in the tarim plata and its cohesion; There is the body of high magnetic susceptibility at lower crust and upper mantle in the Kazakhstan-Junggar block, Yinning, Turpan and Tarim basin, this kind of high magnetic susceptibility abnormal body can be the reason of the high satellite magnetic anomalies in this area. There exists an arc low magnetic susceptibility of the banded anomalies in the west, north of Junggar basin, which is located on the western extension of Kalamaili tectonic belt, and it is considered to the suture zone of the Siberia plate and Kazakhstan plate.In order to understand the lithospheric structure better, the density and susceptibility anomalies in the different depth are imaged by the 3D inversion of satellite gravity and magnetic anomaly data. In the upper crust, the density variation corresponds well with the surface structure. Especially in the Tianshan fold belt, there are sharply density changes at the sides of the ultra deep faults, which shows a complex tectonic frame. The sheet density anomalies with trend of NW-NWW are dominated in the Altay fold belt. In the Western Junggar, there are high density anomalies in the southern part of the Tarbagatai Mountains, and the high density anomalies are weak in the Back Mountain area of Karamay while the density anomaly changes very smoothly in the Junggar basin. In the middle crust, the density changes intensely in the Tianshan fold belt and the huge low density anomalies are dominated in the North Tianshan fault zone and northern part of the South Tianshan fault zone. The density varies slightly in the uppermost mantle. The Kazakhstan-Junggar basin is dominated by the high density anomalies and the variation tends to be simple while the crustal tectonic trace still is kept. The ultra lithospheric faults in the Tianshan fold belt may extend more deeply. Seeing from the inversion results of magnetic susceptibility in different depths, Yili basin, Tuha basin, Junggar basin and Tarim basin appear high magnetic distribution. It is very obvious that Junggar basin inserts into the northwest Kazakhstan plate with high magnetic "mushroom" shape. In collision contact area, namely the back mountain area of Karamay, shows low magnetic characteristics with north east to south west direction, which is different with the high magnetic distribution in Kazakhstan plate and Junggar basin. The Altay fold belt in the north, as the high-value belt in the NW-SE direction, reflects that this region is clearly controlled by the construction in the NW direction, and corresponds to the late Paleozoic active continental margin board with density anomalies. The ophiolite and acidic volcanic rock in early devonian, the representative products of spreading ridge, are the reason of the deep high magnetic. The high magnetic and low magnetic belt (in the NE-SW direction) in the northwest of the junggar basin is likely to the result of collision and extrusion. In the west and north rim of the junggar basin, there is a arc banded anomaly with low susceptibility. It is located in the western extension part of Carla Mic tectonic belt. This large fault belt plays an important and controlled role for the mantle-derived magma and the differentiation of the residual melt rising to the upper crust emplacement. The tectonic regime from initial collision extrusion to late collision tension, became the favorable conditions for the rising of mantle-derived magma and its differentiation to the crust emplacement.We cut out three typical profiles from the imaging results of three-dimensional density or magnetic susceptibility, then analyze the structure characteristics and tectonic implications of density and magnetic of the lithosphere around the west Junggar. The high density and high magnetic susceptibility of the middle and lower crust is the main feature in the northern section of Junggar Basin. They have the trend that insert under the northward, so we can suggest that the ancient oceanic plate may be dive to underneath of the Altai land lithosphere. On AA’profile, low density and low magnetic susceptibility of southern section of Junggar basin locate on west kalamaili tectonic extension belt, and Kalamaili tectonic zone is considered to be the Siberian Plate and Kazakhstan plate’s suture zone. The density and magnetic susceptibility on this segment of the crust-mantle shows that Junggar block lithosphere is relatively hard, remained relatively intact structure during the Indian plate and Eurasian plate collision process. On the Tianshan fold belt-Tarim Basin edge segment, Tianshan fold belt density structure is complex, showing that with the pushing of north and south sides, the crust and lithosphere folds, fracture, overlap and oblique cut. But the magnetic susceptibility of the structure is relatively simple, the low magnetic susceptibility reflecting that the crust and lithosphere has a similar material composition. The two profiles, crossing the west Junggar tectonic belt, show the presence of high density and high magnetic susceptibility in the Junggar Basin crust-mantle, and insert from NW to SE. Near the fault zone in Darfur bout, there is high-density crust and mantle body aggregation. There is low density and low magnetic susceptibility anomalies segment on north Darfur bout fault, similar to the magnetic susceptibility structure of profiles A A’.(2) The understanding of the regional tectonic of the Back Mountain area of Karamay from the surface gravity anomaly and the aeromagnetic anomalyFour techniques were chosen to process the surface gravity and magnetic data, including the wavelet modulus maxima method, the normalized derivative deviation method, the total horizontal derivatives method and the canny edge detection. These methods were applied to the study area to recognize and infer the edges of the geological body and fault structure. The results, in conjunction with the local gravity anomaly and its first-order vertical derivative, recognized the Darbut Fault, the Hatu Fault, the Kewu Fault buried in the covered area of the basin and the regional fault F4 trending near NS direction in the south, and it extended to the west part of Xiaoguai over 80km. The characteristic of the local gravity anomaly between the two sides of the fault is apparently different. The application of the edges detection of the magnetic geological body and the analytic signal amplitude inferred the distribution of high magnetic susceptibility, and the ophiolite melange (including basic rocks) belts with high magnetic susceptibility were all forecasted, and this class of magnetic rock may exist in the basin probably.The difference between the original gravity anomalies and the 5th order approximation of wavelet decomposition were treated as the local (residual) gravity anomalies for inversion. The local anomalies in the north of Kewu Fault and in the south of the Darbut Fault represented the gravity belts with the highest amplitude and the widest scope. There are two low gravity anomaly zone in Tiechang Groove and the east, located in the Cenozoic cover area of the south piedmont of Ur Kashgar Mountain, and they may be a reflection of the piedmont fault basin. The 500m downward aeromagnetic anomalies were as light source anomalies, and mainly related to the distribution of the magmatic rocks and ophiolite melange belts in the area. Using the ground gravity and aeromagnetic anomaly, the 3D inversion of the mid-upper crust density and magnetic susceptibility in mesoscale depicted the density and magnetic susceptibility structure of the Back Mountain area of Karamay.The mesoscale 3-D density and magnetic susceptibility characteristics reveal the structure and rock mass distribution in the Back Mountain area of Karamay. The boundaries of the density and magnetic susceptibility anomaly stripes fit well with fracture tectonic belt on the surface and the NE-SW Hatu Fault and Darbut Fault, and reflect the buried NE-SW Kewu Fault and the S-N F4 fault obviously. Density and magnetic susceptibility anomalous bodies existwidely surrounding The Hatu Rock, Akbarstau Rock, Hongshan Rock and Karamay Rock, and the development depth is deeper. This kind of anomalous bodies are closely associated with linear structure, and can be considered in the control of faults. The anomalous belt with the highest density and magnetic susceptibility is located in the northwest of the Kewu Fault, and likely to produced by the opiolite belt and associated with Karamay ophiolite belt. The low density anomalous belt in the south of Tiechang Groove and the nearly EW anomalous belt with low magnetic susceptibility in the shallow part and deep high magnetic susceptibility in the deep control the tectonic framework of the north region of the Back Mountain area of Karamay. The collision extrusion and tension in the NW-SE direction to this region formed a series of NE-SW faults including Darbut Fault, Hatu Fault and Kewu Fault. They may be the regional faults in the subduction accretionary wedge environment,with a close relationship with the formation of the massive granite nearby. The fracture system takes control of the formation and tectonic framework in this region including the junggar Devonian and early carboniferous island arc(the junggar mesozoic-cenozoic superpose basin), the ordovician and late devonian ophiolite melange belt in the Back Mountain area of Karamay, the early carboniferous residual sea basin in the Back Mountain area of Karamay, the middle adakite and middle devonian ophiolite melange belt in Darbut, the devonian island arc in Tiechang Groove and Baiyang Town.(3) The understanding of the tectonic of the Back Mountain area of Karamay in terms of high precision gravity and magnetic profilesThe density imaging profiles of Line I and Line II both reflect the outcropped faults including the Hatu and Darbut and the buried Kewu Fault. The high density sections of the Line I correspond well to the Karamay and Darbut ophiolite belts, and the low ones to the Hongshan Rock. The high density sections of the Line II correspond well to the Karamay and Darbut ophiolite belts, and the low ones to the Karamy Rock. The density imaging profile of the line III also reflect the outcropped Hatu Fault and Darbut Fault. The magnetic susceptibility imaging results of the Line Ⅰ,Ⅱ,Ⅲ and Ⅳ cannot reveal the structure fracture clearly, but can show the high-magnetic Karamay and Darbut ophiolite belts obviously. It relate to the lithologic structure similarity between the two sides of the regional tectonic. Through the physical analysis, the rock with high and middle magnetic susceptibility and high density is consistent with the related results of the ophiolite belts.