【作者】 汪兴旺；

【导师】 李瑞； 熊盛青；

【作者基本信息】 成都理工大学 ， 地球探测与信息技术， 2008， 博士

【摘要】 1998年中国国土资源航空物探遥感中心在青藏高原中西部地区完成了1∶100万航磁概查,并首次发现雅鲁藏布江双磁异常带、青藏高原中部北北东向负磁异常区等重要的航磁异常;2000年至2005年,又在西藏一江两河地区和西藏申扎一那曲地区开展了1∶20万固体矿产航空磁测勘查,这为进一步探讨特提斯洋的演化、雅鲁藏布江缝合带的形成以及青藏高原的隆升机制提供了新的资料、新线索、新的角度。青藏高原航磁双磁异常带(雅鲁藏布江双磁异常带)的南磁异常带与已发现的雅鲁藏布江缝合带的蛇绿岩带相吻合,并且两磁异常带具有较为相似的磁场特征,但磁异常北带位置地表出露的是冈底斯花岗岩,所以此磁异常北带的性质(是冈底斯花岗岩引起的?还是基性或超基性岩引起的?)是本论文研究的内容之一。青藏高原区域航磁异常总体走向为北西西—东西走向,航磁反映无论是磁异常分区或规模较大磁异常带均为近东西走向,磁异常的这种特征显然是客观地反映了高原各块体的构造走向。根据航磁不同高度上延结果,随着上延高度的增加,在测区最东部自柴达木向西南延伸的广大地区出现一条北北东向的负磁异常区,这一负磁异常区东西宽可达300～400km,南北长则纵贯整个高原,可达1200km以上。而在原航磁△T平面图(图1-1)上,这一北北东向的区域负磁异常背景上一些浅层局部磁异常又显示为北西西—东西走向,与高原的区域构造走向一致,在东昆仑中段、可可西里、羌塘东等地均有清晰反映,形成类似于“立交桥式”的磁异常结构。这是在其它地区很少见到的可能仅出现在青藏高原中部的一种独特的地球物理现象,本论文对这一现象的形成机理进行研究。岩矿石的磁性不仅是产生航磁异常的物性基础,也是进行航磁成果解释的重要依据。本论文在研究区内收集磁化率数据25655个,基本掌握了区内地层岩石和岩浆岩的磁性特征,为航磁资料的解释提供了重要依据;并从航遥中心航磁资料中,优选出青藏高原中西部地区航磁全部资料,塔里木盆地西部、塔里木盆地东部、西藏中部地区、川西藏东地区、青海中南及西南、柴达木盆地、罗布泊—阿拉善、西藏一江两河地区1∶20万的部分资料,保持青藏高原中西部航磁概查的测区西部、南部边界不变,北部边界扩展到北纬40°,东部边界扩展到东经96°,通过对这一范围的航磁数据进行位场转换处理,编制了系列航磁图件,结合其它地质及地球物理研究成果,采用磁性场源深度成像、物性参数正反演计算、小波变换滤波、磁异常正反演拟合计算等前沿技术方法,对青藏高原航磁双磁异常带及北北东向负磁异常区的异常性质及地质意义进行了研究,取得了以下成果:1、编制了高质量的航磁位场转换处理系列图件。通过对青藏高原航磁△T资料进行化极、向上延拓、垂向二次导数和不同高度层场差处理,编制了△T频率域化极上延1km、5km、10km、20km、30km、50km、70km,△T不同高度层场差1-5km、5—10km、10—20km、20-30km、30-50km,△T频率域化极上延垂向二次导数1km、5km、10km、20km、30km、50km、70km等系列等值线平面图。可以看出青藏高原航磁异常分布与该区域构造方向基本一致,均呈近东西向。在高原边缘以及各地体边界磁异常强度较大,多呈近东西向的条带状、线性磁异常,其大多是由出露地表且有较大延深的基性岩、超基性岩以及蛇绿岩套引起的,具有磁异常形状狭窄尖锐等特征,表明其与断裂有密切关系,反映了各个地体之间的结合状况。各个地体内部磁异常相对平静。加深了青藏高原原航磁特征的认识,为地质构造和矿藏资源的研究提供了更为丰富的磁场信息和航磁系列图。2、采用磁源参数成像法和可视化重磁异常正反演软件,研究雅鲁藏布江缝合带的空间分布特征。青藏高原航磁双磁异常带(即雅鲁藏布江双磁异常带),总体呈北西西走向,由南北两条磁异常带组成,沿雅鲁藏布江缝合带呈线性展布,是雅鲁藏布江缝合带磁性特征反映。(1)开发并使用磁源参数成像软件对西藏一江两河地区的航磁数据进行局部波数成像;局部波数图像明显地显示出雅鲁藏布江双磁异常带是由隐伏岩体引起的。为进一步研究雅鲁藏布江缝合带的双磁异常带的性质提供了证据。从局部波数图像上可知:与雅鲁藏布江航磁双磁异常南带相对应的磁源体(蛇绿岩带)局部波数极值在0.2～0.5之间,埋深约0km～3km。航磁双磁异常带北带无论是东段异常,还是中段异常,其相对应的磁源体,波数极大值均在0.25左右,因此东段与中段磁性体埋藏深度相差不大,约3km。在局部波数图上,雅鲁藏布江北部大面积出露地表的花岗岩,表现为一些团块状,局部波数值在0.8～0.9左右,与地貌有较好的对应关系,这说明花岗岩体虽能引起较强的局部异常,但花岗岩体磁性不均匀,很难产生像雅鲁藏布江航磁双磁异常带这样规模宏大的线性异常带。由局部波数图还可看出,位于航磁双磁异常带北带的北部地表出露的具有磁性的花岗岩,距南部边界有12km的距离。另西藏一江两河地区东段1:20万航磁测量结果,反映出花岗岩受北东向和北西向两组断裂控制的,并不是沿着雅鲁藏布江缝合带侵位的。总之,无论是雅鲁藏布江航磁双磁异常带的北带还是南带,局部波数反映出雅鲁藏布江缝合带的磁性体大部分是隐伏的,极少出露地表,航磁双磁异常北带东、中、西各段的磁性体埋深相近,只是中段磁性体比东段更宽;并且两段可能在仁布被一北西向断裂错断,东段向南位移了一段距离。(2)使用可视化重磁异常正反演软件和已知的重力、地震资料作为约束条件,依据岩石的磁性参数,对雅鲁藏布江缝合带的双磁异常带进行正反演拟合计算,给出了雅鲁藏布江缝合带上磁性体的空间分布特征;说明了雅鲁藏布江双磁异常北带磁性体具有相似的南倾产状,在空间分布上具有连续性。(3)结合重力、地震资料以及其它地质资料对雅鲁藏布江缝合带的双磁异常带的性质进行了研究a雅鲁藏布江缝合带的双磁异常带北带在航磁异常图上呈连续的线性展布,地面实测磁场、磁化率等综合剖面说明它与地表出露的花岗岩没有对应关系;b雅鲁藏布江缝合带位于缓变化区域布格异常梯度带上,这说明了雅鲁藏布江缝合带与相对升高的重力布格异常相对应;c地震波速揭示在雅鲁藏布江缝合带处上地壳的波速在6.0km/s～6.2km/s,按密度与速度的关系式计算出缝合带处岩体的密度在2.70～2.77g/cm~3之间,高于上地壳平均密度2.67 g/cm~3;综上所述,雅鲁藏布江缝合带上的岩体应具有强磁性、高密度特性,在航磁异常北带上多处有蛇绿岩出露,如罗布莎等,从而推断雅鲁藏布江航磁异常北带是蛇绿岩型隐伏的基性和超基性岩引起的。3、利用航磁系列图件,研究青藏高原中部北北东向负磁异常区的性质及地质意义(1)青藏高原负磁异常区,呈北北东向,东西宽可达300～400km,南北长则纵贯整个高原,可达1200km以上,包含着更为丰富的地壳深部信息。(2)青藏高原负磁异常的成因具有一定深度的区域性质特征,它受区域构造格局演化的控制;受地层岩性与厚度变化的影响;深部构造及其岩浆活动,影响了地壳中岩石磁性及磁场的变化,热液作用、高温低速体的上升,在一定程度上起着退磁作用。(3)青藏高原负磁异常区出现预示在西藏南部地表观察到的南北向断裂已向北延伸,也就是说青藏高原腹地的深部和岩石圈存在近南北向的构造。这种近南北向的断裂是在青藏高原总体东西构造基础上形成的,是整个青藏高原南部边缘前弧断裂的配套断裂。更多还原

【Abstract】 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.更多还原