GISdevelopment.net ---> AARS ---> ACRS 1990 ---> Poster Session

On the genetic model of The Fenhe-Weihe Rift

Yan Xinmin and Bai Xinfeng
Remote Sensing Center of Shaanxi Province, Xian, China


Abstract
The Fenhe-weihe Rift, one of the typical epigenotypes of continental rifts in China, is located the middle east region of the conjunction part between the two great-circle tectonic belts in the Northern Shaanxi and in Shangyun-Bashan area. The satellite picture shows that the north margin and the south margin of the rift can be matched. In the rift, the curie isothermal plane reflects a very peculiar regional distribution of the Weihe rift which are reflected by gravity-magnetic field, water system, crustal strees, geothermals, neotectonic movement and undulatory Moho etc. according to the properties mentioned above, a new recognition of the Fenhe-Weihe Rift was proposed in the paper. Thus a new model of formation and evolution of the Fenhe-weihe rift was established. The rift was formed by the crustal extension caused by the mantle upwaring mainly in the areas of Weinan, Linfen, Jingning and Hanzhong, and by counter clockwise rotation of the rock mass in the North Shaanxi great-circle tectonic and clockwise rotation of the rock mass in the Shangyun Bashan great-circle tectonic caused by the mantle upwarping.

The feature of The Fenhe-Weihe Rift
  1. Geological and Geomorphologic Feature
    The Fenhe-Weihe Rift is situated at the southern part between Shanxi up warping region and the Jishaan fault valleys in font of the estern Qinlin mountain . It has the largest area and the deepest depth in the valleys. From Baoji in Shaanxi province to Huoxian in Shanxi province, the land form looks like a crescent graben basin, extended southeast six hundred kilometres in length, and ninety kilometers in width, and measures four thousand square kilometers [1] . The average height is about two thousand metres about the Qinling and Huoshan mountain in southeast, and nine hundred to thirteen hundred metres about the Beishan and Luliangshan mountains in northwest. The vast and flat Fenwei plain between them is about four hundred metres high and is a down-warped basin filled with thicker and looser sediments. The Weihe Rive and the Fenhe River flow into the centre of this basin from west and north separately. Some sub-basin and sub-swells are distributed alternately in this basin.

  2. satellite Images Feature
    We interpreted the main faults in the Fenhe-weihe Rift and its neighbourhood from the satellite images and other aerial remote sensing data in this area (see appendix 1), some direct and eye-catching information was discovered as follows:

    1. There are two circular structures that correspond to the northern Shaanxi roke mass(I) and the Shangyunbashan roke mass (II) [2] in addition, the Yikezhaomeng roke mass (III) also showsup obviously. The northern shanxi roke mass is on the north of the Qinling mountain and south of the Great Wall, the Qingyang area and the Pingliang area in Gansu province are also included, they form a big circular image. The Shanctunbashan roke mass is located in shangxian area and the Bashan are between the Qinling mountain and the Yangtze River. The Yikezhaomeng roke mass is on the north of the Great Wall.

    2. It has very clear boundary between the northern edge and the southern edge in the Fengweihe Rift. Both edges can be joined with each other on the images, for example, the Fuping -Yanling - Sanyuan triangular area near the northern edge of the valley corresponds with the source area of the Chane River near the northern edge of the valley corresponds with the source area of the Chane River near the northern edge, and the Hancheng corresponds with the northern foot of the Zhongtiaoshan mountain and so on.

    3. The image feature between the Lishan land mass and the Shougtaoshan land mass corresponds with that near Qinling mountains, and they are same in lithogy particularly.

    4. There are two groups of X shear faults at the tangent parts among the shandei roke mass and the Bashan roke mass and the Yikezhaomeng roke mass. they are F3 and F4, F1 and F2

    5. The Feng -Weihe Rift isjust located between the Shanbei roke mass and the Shangyunbashan roke mass, and the graben is inclined towards the Shanbei roke mass and the Yikezhaomeng roke mass.

    6. The points of intersection at this two groups of the X shear faults are separately located at the narrow ends of Fenhe-weihe Rift and Dingbian Rift.


    Appendix I

  3. Buckling and Defoming Feature about Drainage System
    The fractures associated with Fenhe-weihe Rift make the drainage system present a strange buckling and deforming. The rivers and rivulets flowing north from the southern part of this Rift generally buckle towards the west, than flow east into the Weihe River. The rivers and rivulets in the northern part of this rift also have this feature. The appearance are all shown in the shape of the letter S.

  4. Magnetic and Gravity Feature
    The Bouguer gravity abnormality inside Fenhe-Weihe Rift are mainly negative and the values can be about 200 microgal. Those around the edge of this rift are mainly positive. That aeromagnetic abnormality shows a North-Northeast axial negative abnormal belt, and corresponds basically with the Bouguer gravity abnormality. They are all related to the deeper structure feature of Fenhe-Weihe Rift.

  5. Geothermal Field Feature
    The warm springs and hot wells in the account for eighty percent of the total in Shaanxi province. They correspond to the underground abnormality and are related with the rising of the upper mantle[1]

  6. Stirain Field Feature
    The vertical axis of the rift is basically vertical with the tectonic strain axis determined by the tectonic joints in the loess, and the trend of the strain fields axis follows that of the rift and then deflects with them.

  7. Rising and Falling feature about the Moho
    The Trend of the Moho is distributed east-west from Baoji to Huaxian, then turned northeast from Huaxian and extended to Jizhong plain . Parts of Weihe and Fenhe drainage basin belong to this area. The mantle gradient is about 0.36 from Zhouzhi to Xian city [3]

  8. The depth feature of Curie Isothermal Surface
    The Curie isothermal surface in Fenghe-Weihe Rift forms. Fenwei rise[3], which consist of Zhouzhi-Yuncheng and Linfen rise. Thermal flux in the centre of Fenwei rise is about 1.99 HFU which is part of high thermed flow zone. This rise overlaps the Fenhe rift and Weihe rift . The two rift meet in Weinan area. The Curie isothermal surface is Eastern Qinling is a declivity zone of intermediate temperature, which shows that there is a comparative high geothermal in the deep of this area. The Curie thermal surface is a steep zone in Tongchuan area, besides, there are two rises in Jingning and Hanzhong area. There is a flat-slope in Northwestern of Jingling rise and a steep-slope in Southwestern. Hanzhong rise is a flat -slope in southwestern and steep slope in Northestern, only a flat-zone in Ordos area.

  9. Feature of Seismic Activity
    Seismic activity is very frequent in Fenhe-weihe Rift, the depth of seismic focus is about 10-30 Kilometers. The greatest magnitude of earthquake is 8 occurring in Huaxian area on the 23th of January, 1556. But there in no record of mega seism in the rock-block especially in Ordos rock block. The earthquake mainly occurred in Fenwei basin and nearly.

  10. The Distributive feature of Ore Deposits
    Beside the geothermal resource, there are many kinds of endogenic deposits in Fenwei Rift and areas nearby following the strongly intrusive and activity of magma, for instance, jinduicheng molybdenum are deposit, Huanglongpu molybdenum-reverberate ore deposit and Tongchuan gold are deposit. Accompanying the tectonism , coal , salt, oil and natural gas were deposited. They are east-Shanxi, Tongchuan and Huncheng coal field, Xiehu salt deposit in Yuncheng , Yanchang oil and natural gas field and others . All the deposits distribute mainly in the eastern and northern parts of Fenwei Rift, rarely in Baoji area and nearby.

    In the same way, oil and natural gas, salt deposits are in Dingbian Rift in northern part of Shaanxi province. For instance Hongliugou and Pengtan areas have industrial oil. Also salt deposits widely distribute in Yanchi county.
The genetic model of The Fenhe-Weihe Rift
  1. The intensively active surface belt (Weihe fault) appears at first on Fennhe-Weihe area, and shows an are along north margin of Weihe and Zhong Tiao Shan Mountain. In early Mesozoic era, the southern area of Weihe folds and rise to become QinLing mountain.

  2. In the end of Jurassic period , during Yanshanin movement period, because earth mantle matter issued up (from below to up and incline toward northwest), Rift area (weihe inclinedly uplift) followed along, and the earth crust thickness reduced and broke, At the same time, the Weihe river rose inclinedly and had a strong impact to shanbei and Shang-Yan_Bashan rock masses. On the coordinated action of anormal mantle in LinFen, Jing Ning and HanZhong , Shanbei rock mass shears and shifts on counter clockwise direction (rock block in mass centre move toward nortest) . In this process, Shanbei rock mass down throws and Shang- Yun-bashan rock mass up throws relatively, as yet this movement still continues. On the east side the tangential point of the two large rock mass, there is greater tensioned stress that that generated by crust thinning. In other words the distance between two blocks can enlarge and widen, and E-W stuructures and N-E structures were more developed . On the west side of the tagential point of the two rock masses, compressed structure exists in beels, this is about east side of Huaxian.

  3. After Tertiary Period, Qutermantle on the north side of Huanxian surged up, which made northern part of Northern Shaxi mass uplift and Fenhe belt fall down. Until Quaternary Period, uplifted regions of each block mantle move and issure up, and accelerate the process to inherit the former active characters. It follows that Fenghe-Weihe Rift had various landform and geological structures, and the mineral characters come into being.

  4. In effect, the uprises in Linfen, Lishan, Zhongtiaoshan etc. fault valleys are residual fault blocks that Fenhe-Weihe Rift developed and evolved. Just as the residual is lands in ocean were due to continental drift.

  5. The rotational stress and tension of rock mass form smaller rift in the direction vertical to its long axis. So that there are more NW structures on northern side of Weihe river and NE structure of southern side of Weihe river in the rift valley, that is also the reason that Xian surface crack is mostly in NE direction.
The theoretical and practical significance of establiching the model of Fenhe-Hwihe Rift
  1. The theoretical significance

    • It is possible and effective to apply remote sensing imagery regional geological structure.
    • The rift as a special geological appearance has not been understood sompletely, but it is a new train of thought to understand the rift through establishing the model of Fenhe-Weihe Rift.
    • The established model of Fenhe-Weihe Rift confirmed the correctness and paracticalibility of S theory.

  2. The practical siginificance

    • New information of regional deep structure was provided for the research of regional environment geology in the area, and the existence of Fenhe-Weihe Rift and the correctness of seeking mineral deposits near the rift was affirmed.
    • The consideration of seeking different mineral deposits in different part of the rift is further explicated.
    • The genuine ground rift beet direction in Xian being Northeast structure was testified.
    • The statement that Lishan and Zhongtiaoshan mountain are partial swell was denied.
References
  • Wang Jingming, A discussion of Fenhe-weihe Rift. Xian College of Gedogy journal, 1986, vol.8, No3, pp 36-40
  • Ding bing et al;, ShanBei great circular structure and a serise of large scal X shear fractures. Shaanxi Geology, 1983, voll.1, No.1, pp 65-70
  • Pan zuoshu and Ding Fengyi et al, A research in crustal structure and mineral prediction by using physical exploration data of Quin region 1989, pp 41-43.