Elm Bay Fault Zone In The Northeast Of Ordos Basin Tectonic Characteristics And Its Significance

Ordos Basin has been well known of stability, with the weakly tectonic movement, gentle stratum. There is few folds and faults developed interior the basin.However, there are orogenic belts surrounded the basin, resulting in strongly post-transformation on the basin boundary accompanying with faults and folds developed.Located in the northeast of the Ordos Basin,Yushuwan fault zone is situated on the borderland of Yimeng uplift, Jinxi flexual fold belt and Shanbei slope. Previous researchers was inclined to general description and geological survey of faults and associated structures, often with inadequate attention. On the geological map of Ordos basin, the fault is exactly the boundary of the Paleozoic and Mesozoic, with obvious linear relationship on remote sensing image. Aeromagnetic anomaly and gravity and other geophysical information showed a significant northwest banded anomalies and linear gradient zone, indicating the presence of NW-trending basement deep faults. Deep structural information obtained by the seismic array also show that the crustal thickness occur abrupt change of3.7km in Zhungeerqi, suggest the existence of rupture in Moho surface, and the location is coincidently agree with Yushuwan fault zone on the surface. By field geological survey, the stratum attitude is suddenly changed near the Yushuwan fault, which is inharmonious between different stratum.Combined with geological survey and geophysical data, our research show that Yushuwan fault zone,northeast of the Ordos Basin,is a cutting deeper NW-trending strike-slip basement deep fault.It extends about150km, and about29km in outcrop. Vertical fault distance is about900m,the possible slip distance is about10km.On southwest side of the deep fault exists a tectonic deformation, about21km wide.The application of fission track dating methods, to limited the northeastern Ordos Basin uplift period.According to a total of17apatite and zircon fission track data in the northeast of the Ordos Basin, the results showed that the northeast of the basin has experienced190Ma,150-170Ma,110-130Ma,60-80Ma,20-40Ma phased uplift.May be begin activity prior to the Triassic,the Yushuwan fault controlled the northeastern boundary of Yanchang Formation in Ordos basin.Late Triassic-Early Jurassic.the fault acted strongly, and controlled the tectonic deformation strength on both sides, and its driving force stem from the collision of the Yangtze and North China plate.Early-Middle Jurassic to Early Cretaceous,the fault activity was not obvious.Since Late Cretaceous,the fault activity enhanced, and produced near900m vertical displacement,and then resulted in strong erosion up to2000m in northeast of the Ordos basin. Previously, it was thought that the Northeast of Ordos Basin was a single monoclinic structure, northeast tilting and southwest plunging. With the discovery of the Yushuwan fault, northeast of the basin is not a simple slop, but a fault terrace with a large drop.Fault terrace blocking and buried relatively deep in southwest of the fault zone, the natural gas from basin interior accumulate and enrichment in the northeast basin. Therefore, in the wider region of west Yushuwan fault, there should be abundant natural gas resource potential and good exploration prospects. Today, widely distributed in shallow Jurassic Yan’an Formation, the higher maturity of the coal gas dissipation caused "bleaching" phenomenon. It is to prove that the scale of Upper Paleozoic coal-type gas from basin abdomen has migrated the area. In the area of northeast of Yushuwan fault, due to the blocking effect of the fault, less natural gas has transported to the east of the fault. Coupled with stronger erosion of higher uplift, most of the Mesozoic strata have been eroded completely, resulting in the lack of good preservation conditions. There is no basic conditions for gas preserve and possible exploration potential.The discovery of the Yushuwan fault zone has important geological significance:The fault may be the eastern boundary of the Yimeng Uplift in the Ordos Basin. Yimeng Uplift was E-W extending, high in the north and east, low in the south and west, the eastern boundary has been blurred for a long time, not conclusive. According to aeromagnetic anomaly, the southwest side of the fault the magnetic anomaly contour value is higher and denser, the northeastern side is lower and flat. Yushuwan deep fault on both sides of aeromagnetic characteristics is very different, obvious partition. The fault restricts on the high magnetic anomaly of Yimeng Uplift eastward extension, to determine the eastern boundary of Yimeng uplift.