Basin-Mountain Tectonic Framework And Deep Structure Of The Northeastern Xinjiang: Constraint Of Geophysical Data

The Northeastern Xinjiang, located in the center of Asia continent, displays the style of Basin-Range tectonics, i.e., two basins of the Junggar-Santanghu basin and Turpan-Hami surrounded by three mountain ranges of the Altai Mountain, Bogda-Harlik Mountain, and Jueluotag-Beishan Mountain. The scientific questions such as whether the two basins own the Precambrian crystalline basement and how these orogenic belts controlled the two basins are the focus for consideration. The basement properties of the two basins and the relationship between these basins and orogenic belts are also one important part of tectonic studies in Xinjiang. The geophysical techniques have a unique advantage to reveal the Basin-Range tectonic framework and deep structure. Base on the integrated geophysical studies in the Northeastern Xinjiang, this paper tries to study the basement properties of the three basins, the relationship between the three basins and their neighboring orogenic belts, and the locations of plate boundaries. By discussing these issues, this paper expires to better understand the tectonic evolution of the Northeastern Xinjiang.Three typical faults of EW-trending, NWW-trending and NEE-trending are developed in the study area, the trend of which is consistent with that of the regional tectonics. The three faults, characterized by large scale and long-distance extension, are the basement faults constituting the boundaries between the Basin-Range tectonic units in the Northeastern Xinjiang. The study area can be divided into 12 tectonics units, i.e., the north margin of the Tarim structural belt, the Beishan tectonic area, the East Tianshan structural belt, the Jueluotag fold zone, the Turpan-Hami basin, the Bogda-Harlik fold zone, the Junggar basin, the East Junggar-South Mongolia fold zone, the Erqisi-South Gobi fold zone, the Altai orogenic belt, the Mongolia Lake fold zone and the Tuwa-Mongolia block.The fault contact is presented as the relationship between the Junggar basin, the Turpan-Hami basin, the Santanghu basin and their neighboring orogenic belts. The development of the Junggar basin was controlled by both of the Kelameili fault inclining from southwest to northeast and the northern margin fault of Bogda inclining from north to south. Wherein, the latter fault possibly extended vertically to upper mantle. The development of the Turpan-Hami basin was controlled by both of the southern margin fault of Bogda inclining from south to north and the Kanggurtag fault inclining from north to south. While the development of the Santanghu basin was controlled by the thrust faults on both sides. The deep structure of the Santanghu basin should be controlled by the Kelameili fault.The comprehensive analysis indicates that the southern margin of the Altai fault zone should be the boundary between the Siberia plate and the Kazakhstan plate, while the Yamansu fault zone should be the boundary between the Kazakhstan plate and the Tarim plate. The most notable feature on the Yamansu fault zone is that magnetic gradient zone is presented according to the magnetic anomalies by reduction to the pole and upward continuation of 50 km. The reduced-to-the-pole anomalies and their residual anomaly demonstrate that there is an EW-trending beaded magnetic anomaly zone with 35 km in width and 600 km in length along the Alatage-Yamansu-South Jingquan limit. The southern margin of the Altai fault zone locates in the gravity gradient zone according to the Bouguer gravity anomalies and their upward continuation. In addition, the gradient zone does not disappear with the increasing height of upward continuation.Based on the isostatic gravity anomalies of the Airy model, the feature of the Moho surface and the analysis of geoscience transect profiles, this paper suggests that there exists an apparent "root" below the Bogda Mountain, but the real "root" is much thinner than the theoretical one. It means that the Bogda Mountain is still unbalanced. The "root" becomes thinner from west to east. The root which is about 8 km in thick at the west margin of the Bogda Mountain disappears at about 50 km to the east of Qijiaojing.The comprehensive discussion indicates that both basements of the Turpan-Hami basin and the Santanghu basin belong to the Precambrian metamorphic crystalline basement, while the basement of the Junggar basin is constituted of the Precambrian metamorphic crystalline basement and the basic and ultrabasic substances. The regional high magnetic anomalies in the Turpan-Hami basin and the Santanghu basin are mainly caused by the Precambrian metamorphic crystalline basement which shows stronger magnetism, while the regional low gravity anomalies respond to Mesozoic-Cenozoic strata with relatively lower density and thicker deposition. The regional high magnetic anomalies in the Junggar basin are caused by the Precambrian metamorphic crystalline basement together with the basic and ultrabasic substances formed as mantle minerals entered the crust, which also caused the regional high gravity anomalies in this basin.