Remote Sensing of active structure and geothermal in Tibet

Remote Sensing of active structure and geothermal in Tibet

He Yunzhong Liu Shangzhng Yuan Peixin
The Remote Sensing Station of the Sichuan Bureau of Geology and
Mineral Resources Chengdu, China

Tian Youhua Qinchang loug
The Geothermal-geological Team of the Tibet bureau of Geology and
Mineral Resources Lasa, China

Abstract
On the basis of the interpretation for the false color landsat MSS mosaic (1/500,000) of Mid-south Tibet with an area of more than one million Km2 we have found out different orientation and orders of active structural zones, and circular structures. Using the known hydrothermal active spots, we have discovered that hydrothermal activity is mainly located to the south of the latitudinal Banggong-Nujian structural zone, and the most located to the south of the latitudinal Banggong-Nujiang strcutrual zone, and the most intense is near the Yarlung Zangbo structural zone. The high-temperature hydrothermal spots are mainly distributed over these active structural zones of different orientations, such as the longitudinal fault depression zones, the NWW-NW trending fault depression zone, the NE trending fault depression zone and the latitudinal fault depression zone, the are-shaped margin of the Bomi massif and the arc-shaped fault depression zone in the margin of the circular structures.

We have finished the false color composite of 47 scenses MSS images with the I² S 101 image system and made the processing mosaic by hand, so that we can investigate rich geothermal resources at a high-speed and coordinate regionalism study of geothermal resource in Tibet. The mosaic is at the scale of 1/500,000 which is 4.5 m long and 1.2m wide, and covers an area of more than one million Km², which is characterized by its clearness and is good for geological interpretation (Fig.1)

For the sake of enhancing the useful information, we have performed various image processings for some important subareas. In addition, for some important hydrothermal area, we have used black-and-white aerial photographs for interpreting.

Indexes of geological interpretation of hydrothermal activity
We have got some interpretation key of hydrothermal spots by the interpretation of known spots and investigation in the field they are as follows:

Sediment Index

Underground thermal water contains large amount of carbonate and a small amount of halides[1]. Reaching the surface, the thermal water discharges above-mentioned salts around hydrothermal sports and their neighborhood. These salt sediments are white.
Vegetational Index

The hydrothermal areas are Long-term frostless, and even perennially ice-free, because of hydrothermal activities. Thanks to the condition above mentioned with hydrologic factors, the hydrothermal areas process a microclimate suitable to the growth of plants, thus the hydrothermal areas are covered perennially by green vegetation.
Hydrothermal Alteration Index

Underground thermal water alternates the wallrocks, especially, feldspare-rich rocks such as granitoids. The resulted hydrothermal alteration zone especially when it contains a vast of kaolinite and sericite, may be used as an element of image interpretation too.
In addition, perennially ice-free river and perennially snow-free sector can be used as an element of image interpretation

Fig.1 It is a sketch of the masaic map of Landsat MSS images in Middle South Tibet, the scale of 1:500,000, 4.5m long, 1.2m wide, and composed of 47 standard images. The semi-circular white image is Bomi mass if in the eastern masaic map.

An analysis of the active structures
Development of hydrothermal activity depends on three factors: 1}, thermal source, 2}, heat discharger conduit and heat-bearing reservoir, 3), water with heat energy. All of these are obviously in close relationship with geological structures.

Fig. 2 Active Structure Schematic Map in Tibet

According to this concept, we have interpreted some active structures, circular structures, principal graben valleys and basins and the uplift in whole Tibet, and draw up an interpretation map of linear and circular structures in Tibet and structural schematic map showing the prin-cipal structural zones. (Fig.2)

From the mosaic and the interpretation, the active structures can be divided into latitudinal, longitudinal and oblique zones. Each of the active structural zones is composed of linear structures, fracture structures, graben valleys, fault block uplift and magmation zones and some circular structures which are relate with the S-N compressive stress due to the subductin of the Indian plate down the Eurasian plate.

Latitudinal Active Structural zone Latitudinal active structures are predominant ones in the area, and run parallel to the geosuture in space. They include the high Himalayan fault block uplifting zone, the Yarlung Zangbo structural zone, the Bangong-Nujiang structural zone and kunlun Kekexili structural zone. In addition, the secondary structures among them are pengqu fault depression zone, The Lower Watershed Uplifting Zone in South Tibet. Gangdise fault block uplifting region and subsidence region with many large lakes in North Tibet, from south to north.

Most of the structure zones near the Xiwalic thruat fault zone of the India plate are of new-born characteristics and are formed in the Himalayan Movement. For example, the high Himalayan fault block upkifting zone, the pengqu graben fault depression zone, etc. Northwardly, the structural zones are characterized by inheritance in the space. For example, the Yarlung Zongbo structural zone inherited the geosuture zone, which have been formed in late yanshan-Early Himalayan Period. Bengong-Nujiang structural zone inherited the geostructure zone, in Indo-China period. They occure as fault depression and uplift. There is a graben valley with a length of more than 500 km in the Bangong-Nujiang structural zone.
Longitudinal Active Structural ZoneThe S-N compressive stress combined with the mantle upwarping is responsible for 6 large longitudinal fracture zones, which are several hundreds km wide between each neighbouring zones[2].

Fig. 3 The image shows Shenza-Xietongmen longlitudial fault depression zone. It is obvious that graben valley always accompany fault black uplifts.

Langtutidinal active structural zone is characterized by its tensional forming graben valleys which extend to 10.20 km wide. This structural zone cut almost off all other oriented structural lines. In addition, this active structural zone is characterized by not only the flourishing longitudinal fault graben valleys but also the associated fault uplifts in the graben valleys as well as yarlung zangbo River Canyon in Nimu structural zone.
Oblique Active Structural Zone. They are mainly composed of NWW and NEE structural zones and secondarily, of NW and NE trending structural zones. They are characterized by sinistral or dextral equidistant trick-slip with an interval of about 150-200 km. The NWW trending structures are of great importance and may be devided into six structural zones from east to west, showing as fault depression, uplift, sinistral strike slip fault zone, long term active and trancriustal zones extending into the mantle[3]. For example, Anglaren Yangzhouyong structural zone, which lies in Central Tibet is up to 900 km long, in the central sector, it consists of a series of fracture zones, The eastern part of the zone is formed after the Yarlung Zangbo geosuture, cutting off the Yarlung Zangbo structural zone.

This structural zone is transcrustal structural zone extending into the lower crust and the mantle, it is demonstrated by the existence of Mohe mose upwarping on the mantle slope.
Bomi MssifIt is the most outstanding structure in East Tibet. It is about 800km long from Lancang River in the east to Nimu-Yang bajing in the west. We suggest that in the Early Yan shan Period the Gangdise structural zone was pregnant with the Bomi massif, which formed eventually in the Himalayan period.

The core of the massif is located at the turning point of the Yarlung zangbo River, in which the south Jiabawa Peak with elevation of 7756m, is the highest part in East Tibet. All the peaks have an elevation of more than 5500m, except for the Himalayan mountains which are located in the Bomi massif. During the Miocene Epoch, the Bomi massif obducted and napped the Chengdu structural basin.
Circular Structures Numerous circular structures in the area, for example, Naqu circular structures, Pengcuo circular structures etc. are active structures with the mantle upwarping and magmation.

Characteristics of the distribution of the hydrothermal spots.
Except for East Tibet, most of the hydrothermal sports are located in the region to the south of Bangong-Nujiang zone, increasing southwardly. The spots to the west of Nimu are distributed mainly along Yarlung Zangbo structural zone and NW trading India fault depression. The spots to the east of Nimu are distributed mainly over the arc-shaped structural zone in the margin of the Bomi massif, forming as S-shaped hydrothermal belt with a width of over 100km and a length of about 2500km.

There is a high-temperature hydrothermal zone in the hydrothermal belt. The zone lies to the. South of Gangdise and danxung Jiuzila.

Controlling Factors of Hydrothermal Activity

The Hydrothermal Areas Controled by the Following Structural Zones:
- Bongong-Nujiang structural zone is the north boundary of Gangwana, there aren't almist hydrothermal activities to north or the zone.
- In Yarlun Zangbo structural zone and Pengqu fault depression zone and uplifts and circular structures in the Loser Watershed upli8fting zone in South Tiet, hydrothermal activity is more strong.
Bomi Massif Gets Hydrothermal Activity Under Control.
- The are-shaped margin of the massif, the abduction zone in the east and tentional fracture depression zone in the west, cause high-temperature hydrothermal activity under control.
- The core and intersection of different structural lines.
The Hydrothermal Area are Controlled by the Graben Valleys in Different Structural Zones.
- It is evident that longitudinal graben valleys bring the hydrothermal activities under control, such as the high-temperature activity at well known Yanbajan, Yangyixian in Nimi graben valley; Chabu and Dagjia high temperatural geyseres at the intersection of longtudinal graben valley and different structural lines , etc.
- It is important that NWW and NW trending graben valleys bring hydrothermal activity under control, for example, the strong hydrothermal eruption at Qupu in the Mopangyong graben valley; the high-tempretural hydrothermal activity at the intersevtion of the NWW trending graben valley and NW trending strike-slip tensional valley in Mensi; the hydrothermal activity in Yangzhuong, etc.
- In addition, NEE and NE trending graben valley get the hydrothermal activity under control.
The Hydrothermal Activity is Control by the Circular Structures.
- The margin of the circular structures and intersetion of different linear structures, such as Naqu geothermal field [Fig.4]
- shaped fault depression zone in the margin of the circular structures gets hydrothermal activity structures gets hydrothermal activity under control for exampole, the arc-shaped fault depression zone in the sestern margin of Gamba circular structure.
Fig. 4 it is image inlaid with computer in Naqu area. The cricular structure is obvious in central part. The white images are of manifestation hydrothermal activity.

The above mentioned characteristics indicate that the hydrothermal activity has close relation. Ship with geotectonics activity structures, in addition , with geohydrologic climatic, formational conditions which go beyond the scope of this study.

References

Toughwei et al: Geothermal in Tibet, Science Press, 1981.
Hantonglin: Activity Structure in Tibet, Geological Publishing house, 1987.
E.S.T.O' Driscoll: The Application of Lineament Tectonics in the Discovery of the Olympic Dam CU. Cu-Au-V Deposit at Roxby Downs south Australia. "Global Tectonics and Meta Uogeng" 1985, VOl. 3 pp43-57.