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The Geologic Interpretation Research Of Double-magnetism Anomaly And Negative Magnetism Anomaly Zone In Qing-Tibet Plateau

Posted on:2009-04-26Degree:DoctorType:Dissertation
Country:ChinaCandidate:X W WangFull Text:PDF
GTID:1100360242493053Subject:Earth Exploration and Information Technology
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In 1998, China Aero Geophysical Survey & Remote Sensing Center for Land and Resources has completed the aeromagnetic survey of Midwest zone of Qinghai-Tibet Plateau (1:1000000) and found the double- magnetism anomaly zone in Q-T Plateau and the negative magnetism anomaly zone with the direction of NNE in the middle of Q-T Plateau, etc for the first time. Form 1998 to 2005, we also complete the aeromagnetic survey of solid mineral (1:200000) in Yijianglianghe area and Shenzha-Naqu area of Tibet. This data offered the new information and new key and new point for discussing the evolvement of Thetis Ocean and the formation of the suture zone of Brahmaputra and the uplift mechanism of Q-T Plateau.The south of the double- magnetism anomaly zone in Q-T Plateau (Brahmaputra double- magnetism anomaly zone) matches the ophiolite zone of Brahmaputra suture zone which has been discovered. Both the north and the south magnetism anomaly zones have the similar magnetic characters. But the outcrop of the north magnetism anomaly zone is Gangdise granite. So the research on the character of the north magnetism anomaly zone (is it caused by Gangdise granite, basic rocks, or ultrabasic rock?) is one part of this paper.The main trend of the magnetism anomaly zone in the area of Q-T Plateau is NWW-EW. The aeromagnetic survey shows that either the magnetism anomaly division or the major magnetism anomaly zone is mostly E-W trend. Obviously, the character of the magnetism anomaly shows the structural trend of the blocks under the Q-T Plateau. According to the upward continuation, as the height increases, there is a negative magnetism anomaly zone with NNE direction in the very east part of the surveyed area (from the Qaidam to the wide southwestern area).The negative magnetic anomaly zone is as wide as 300-400km from east to west and more than 1200km from north to south which is across the whole Plateau. However, on the aeromagnetic map ofΔT (figure 1-1), some shallow part of the negative magnetic anomaly zone shows the NWW-EW direction on the background of magnetic anomaly with NNE direction. This direction matches the structural trend of Q-T Plateau and it is showed clearly in the middle of eastern Kunlun, Kekexili, eastern Qiangtang and so on. All of these form the magnetism anomaly structure similar with "overpass". This is a kind of typical geophysical phenomenon which maybe only appears in the middle of Q-T Plateau and is rare in other areas. We'll discuss the cause of the phenomenon in this paper.The magnetism of the rocks is not only the physical basis of the aeromagnetic anomaly, but also important foundation for interpretation of aeromagnetic data. In this paper, we collect 25655 samples of susceptibility in the researched area and we almost get the magnetic characters of the rocks and the igneous rock of the layer in the area which provide the important basis for aeromagnetic interpretation. From the aeromagnetic data provided by Aero Geophysical Survey & Remote Sensing Center, we choose all the data of the middle west areas of Q-T Plateau, and the data of the middle west area of Tarim Basin, the middle area of Tibet, the west of Sichuan and east of Tibet, the middle south and southwest of Qinghai, Qaidam Basin, Lop Nur- Alxa, Yijianglianghe area of Tibet (1:200000). We keep the west and south boundary of the aeromagnetic survey for the middle west areas of Q-T Plateau unchanged, and extend the north boundary to 40°north latitude and the east boundary to 96°east longitude. By converting and processing the potential field of this area, we establish a series of aeromagnetic maps. With other geology and geophysics outcomes, such as vertical imagery of aeromagnetic source, forward modeling and inversion of physical parameters, wavelet transform filtering and adaptation calculation of magnetism anomaly, etc, we researched the characters and geologic interpretation of the double- magnetism anomaly zone in Q-T Plateau and the negative magnetism anomaly zone with the direction of NNE, getting the following results.1. We established a series of aeromagnetic maps with potential field converted and processed. After reduction-to-the-pole, upward continuation, and the processing of vertical second derivative and difference in field for different heights, we established a series of contour plans includingΔT reduced-to-the-pole and upward continued in frequency domain for 1km,5km,10km,20km,30km,50km,70km, andΔT with difference in field for different heights 1-5km,5—10km,10—20km,20-30km, 30—50km, andΔT with vertical second derivative reduced-to-the-pole and upward continued in frequency domain for 1km,5km,10km,20km,30km,50km,70km, etc.It appears that the magnetism anomaly distribution in Q-T Plateau matches the structural trend of this area (EW direction). The edge of the Plateau and the boundary of terrains are the high-field magnetic anomaly zones, which are almost linearity and like a belt with EW direction. Most of them are caused by the basic rock, ultrabasic rock and ophiolite which are outcrop and extensible. So the shape of the magnetism anomaly is confined and keen-edged, which means that anomaly is related closely to the fault of this area and it reflects the formation picture of the terrains. The internal anomaly of those terrains is relatively calm. We deepen the cognition of the aeromagnetic characteristic in Q-T Plateau, and provide more abundant magnetic information and aeromagnetic map for the study of geological structure and mineral resources.2. Introducing vertical imagery of aeromagnetic source method and software of visual forward modeling and inversion, we study the characters of the distribution of Brahmaputra suture zone.The double- magnetism anomaly zone in Qinghai-Tibet Plateau (the Brahmaputra zone) generally directs to NWW, and is made up of two anomaly zone in north and south. It appears linear distribution along with the Brahmaputra suture zone, which reflects the magnetic characters of Brahmaputra suture zone.(1) We developed and used the imagery software of aeromagnetic source parameters into local wave-number imagery for the aeromagnetic data of Yijianglianghe area of Tibet. Local wave-number image shows that the Brahmaputra double-magnetism anomaly zone is caused by buried rock, which provides more evidence to further research of this zone.From the local wave-number image we can see the local wave-number extreme of aeromagnetic source which is corresponded to the south of Brahmaputra double- magnetism anomaly zone is 0.2-0.5 and its buried depth is 0km-3km. However, the wave-number extreme of aeromagnetic sources which are corresponded to east and middle of the north of this double- magnetism anomaly zone are about 0.25. So their buried depths of east and middle are similar and are all bout 3km. in the local wave-number image, the blanket outcrop granite of north of Brahmaputra appears lumpy, and its local wave-number extreme is about 0.8-0.9, which is corresponded to the physiognomy. This shows the granite mass can cause the local anomaly, but hardly cause the extensive linear anomaly zone like Brahmaputra double- magnetism anomaly zone because of its nonuniform magnetism.From the local wave-number image we can also see the magnetic outcrop granite lying on the north of double- magnetism anomaly north zone is as far as 12km to the south boundary. The aeromagnetic survey (1:200000) of Yijianglianghe area of Tibet shows the granite is controlled by the faults with NE direction and NW direction but not along the Brahmaputra suture zone emplacement.Above all, whether on the north or south of Brahmaputra double- magnetism anomaly zone, the local wave-number shows the most of magnetic body of Brahmaputra suture zone is buried, barely outcropping. The depths of magnetic bodies of east, middle and west of double- magnetism anomaly north zone are the similar, but it's wider for the middle than the east, and they're maybe broken by the fault with NW direction at Renbu and the east moves some distance to south.(2) We use the visual software of forward modeling and inversion into completing forward modeling and inversion adaption of the double- magnetism anomaly zone in Brahmaputra suture zone and establish the 3D magnetic distribution of this suture zone, with gravity and seismic data as condtraints and considering magnetic characteristics of rock. It shows that the northern double- magnetism anomaly zone has similar character of approach -to-south attitude as the south, which appears continuity.(3) Combining gravity, seismic and other geologic data, we study the characters of the double- magnetism anomaly zone in Brahmaputra suture zone.a) The north of the double- magnetism anomaly zone in Brahmaputra suture zone shows continuous linear distribution on magnetism anomaly map, magnetic field and magnetic capacity of ground truth indicate that this distribution has noting to do with outcrop granite;b) Brahmaputra suture zone locates on Bouguer anomaly gradient area where variance is slow, that means the suture zone is correspondent to the relatively ascended gravimetric Bouguer anomaly.c) Seismic velocity reveals that the velocity of the shell on the suture zone is 6.0km/s-6.2km/s. According to the relation of velocity and density, the rock density of this zone is 2.70g/cm~3-2.77g/cm~3' which is higher the average of the shell (2.67g/cm~3).In conclusion, the rock in Brahmaputra suture zone is characterized by high magnetism and density. A few of ophiolite outcrop in the north of aeromagnetic anomaly zone, such as Luobusha. We can infer that the northern aeromagnetic anomaly zone in Brahmaputra is caused by buried basic and uitrabasic ophiolite.3. Adopting the series of aeromagnetic maps, we research the character and geologic meaning of the NNE negative magnetic anomaly in the middle of Qinghai-Tibet Plateau.(1) Negative magnetic anomaly zone in Qinghai-Tibet Plateau heads to NNE, and it ranges 300-400km from east to west, and 1200km across the whole Plateau from south to north. This carries a lot of information of deep earth.(2) The cause of Negative magnetic anomaly in Qinghai-Tibet Plateau is local in some depth, and it depends on local structural evolution, and is affected by the variance of thickness and formation lithology. The deep seated structure and magmatic activity influence magnetic characteristics of rock. Hydrothermal activity and the uplift of high-temperature and low-velocity substance work as demagnetizer in some degree.(3) Negative magnetism anomaly in Q-T Plateau indicates that the south-to-north fracture belt found in southern Xizang has extended to north, which means the same structure exists in deep lithosphere of Q-T Plateau hinterland. This fracture develops on the basis of the whole east-to-west structure of the Plateau, and it is the assorted fracture of the frontal arc fracture in south of the Plateau.
Keywords/Search Tags:Qinghai-Tibet Plateau, forward modeling and inversion of magnetism anomaly, double-magnetism anomaly zone, negative magnetic anomaly zone, Brahmaputra suture zone
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