FEATURES OF ANCIENT RIVER-BEDS AT THE CONFLUENCE OF RED, ĐÀ AND LÔ RIVERS AND THEIR RELATIONS WITH NEOTECTONIC ACTIVITIES

PHẠM ĐÌNH THỌ¹, HẠ VĂN HẢI² , HẠ QUANG HƯNG²

¹Department of Geology and Mineral Resources of Việt Nam, 6 Phạm Ngũ Lão, Hà Nội
²Hà Nội University of Mining and Geology, Đông Ngạc, Từ Liêm, Hà Nội

Abstract: The confluence area of the Red, Đà and Lô rivers is the place where three Red River, Chảy River and Lô River great faults meet one another. Geologists have been proving that there are indications of contemporaneous activities of these fault, that made these rivers to leave the northwest-southeast direction. New research results have been showing that these fault systems were cross-cut by a younger sub-meridional fault zone. Just neotectonic activities were the main causes of current change of these rivers as well as their reunion in the Việt Trì Town, and made the Đà River to change suddenly its flowing direction from northwest-southeast to the north, then to empty into the Red River in Ba Vì District (Hà Tây Province). The reasearch results displayed in this article present our new judgements about the relationship between neotectonic movements and the river network in the studied area.

I. INTRODUCTION

During recent years, the Red River fault zone has been studied by home and foreign geologists and geophysicists. Almost all research works have been recognizing the largest scale of the Red River fault zone in Việt Nam and its clear contemporaneous activities. According to P. Tapponier et al [8], during Pliocene-Quaternary, the Red River fault zone is a dextral strike-slip with maximal horizontal compression stress field of submeridional direction.

The newest results of geodetic and geophysical measurements (2001-2005) of the Institute of Geosciences (VAST) have been showing that the Red River fault is suffering the strike-slipe with the rate of < 1 mm/year.

So, why the Red River which flows in the northwest-southeast direction from the Vietnam-Chinese border through Lào Cai, Yên Bái provinces suddenly winds and changes its direction at Phú Thọ and Việt Trì? Why this area became the confluence of the four Red, Lô, Đà and Đáy rivers ? Is there any relation between the change of flowing direction in this area and the geological structure and neotectonic activities. We will touch upon these problems in this article.

II. FEATURES OF ANCIENT RIVER-BEDS DURING CENOZOIC

The confluence area of the Red, Lô and Đà rivers is of hilly and plain relief with a thick cover and few outcrops. Therefore, we chose the way to apply methods of remote sensing, geomorphology and geophysics. These methods allow to restore the ancient river networks through different geological periods and to draw judgements about neotectonic activities. In the isobed map of Cenozoic sediments [1] (established on the basis of deep seismic data of PetroVietnam Corporation), we can note that, at the beginning of Cenozoic, the relief of Bắc Bộ Plain from Việt Trì Town to East Sea was a low depression of northwest-southeast trend. Basing on the topographic form and facies of sediments in boreholes of PetroVietnam, we drew the position of ancient beds of the Red and Lô rivers. In this map, at the beginning of Cenozoic, these rivers were clearly oriented in northwest-southeast trend (Fig. 1).

By using the morphometric method, the map of summit surface of the hilly and mountainous areas in the East Bắc Bộ has been established. Based on deep-seismic data, the isobed map of Pliocene sediments in plain area has been also compiled. The content of these two maps is displayed in association in the Fig. 2. On the isobed map of Pliocene sediments, the position of ancient bed of the Lô River in Pliocene has been assumed and localized, but one can see that the ancient bed of the Red River does not pass through the present Bắc Bộ Plain.

The geological map of Việt Nam [7] displays a band of N₂-Q₁ alluvial sediments which extends up to 100 km from Sơn Tây through Lương Sơn (Hòa Bình Province) to Ninh Bình. By analyzing the topographical form and sedimentary facies, we can easily recognize that the band of above alluvial sediments is just the ancient bed of the Red River in Pliocene - early Pleistocene (Fig. 2) [1].

This event conforms to the tectonic regime of the Hà Nội Depression. During Oligocene-Miocene (28-32 Ma.), this depression suffered the spreading and acted as a negative flower structure [1]. Under the submeridional compression effect (at 5 Ma.) from the south, it was compressed, uplifted and became a positive flower structure. These movements made the Red River to change its bed to the west and to flow along the margin of the Bắc Bộ Plain. And as a result, it formed the above said band of N₂-Q₁ alluvial sediments.

In the geological map of Hà Nội City, the Lower Pleistocene alluvial sediments (Q₁¹) are absent [6], that is in accordance with above judgement – i.e. in early Pleistocene, the Red River did not pass through the Hà Nội area. To middle Pleistocene, the bed of the Red river moved eastwards and passed through this area.

The traces of this ancient bed of the Red River are fluvial sediments of the Middle-Upper Pleistocene Hà Nội Formation (Q₁^2-3 hn), seen in the Sóc Sơn and Đông Anh areas of the geological map, as well as in boreholes within the Hà Nội City area. And they proved that the Red River started to pass through the Bắc Bộ Plain since the Middle Pleistocene up to present time.

In the Geological map of Việt Nam [9], we can recognize the existence of a band of Miocene sediments along the riverside of Red River extending from Lào Cai, Yên Bái to Ba Vì. The distributive area of these sediments lets see that, during Miocene there existed a long lake extending in northwest-southeast direction up to nearly hundreds of kilometres, that coincides with the position of the Red River Fault (Fig. 2). The Miocene sediments with the thickness of hundreds metres contain coal seams formed in lacustrine environment. Along the riverside of Lô River, a band of similar coal-bearing sediments occurs in Tuyên Quang.

The existence of these two bands of lacustrine sediments allows to draw the following conclusion: during Miocene, the Red River was blocked in Việt Trì, and the Lô River – in Tuyên Quang. This led to the formation of lakes with the length of nearly tens to hundreds kilometres in the above areas.

Ancient river bed

Ancient river bed

Figure 1. Ancient bed of the Red and Lô rivers [1].

Figure 2. Ancient bed of the Red and Lô rivers displayed on the summit surface
and isobasal map of Pliocene sediments in the Bắc Bộ Plain [1].

In our recent articles, we have presented some new discoveries about the submeridional fault extending through the Hòa Bình Town and Ba Vì and northwards to the Vietnam-Chinese border. There were clear signs of this fault in geological, geomorphological, geophysical, remote sensing and geochemical data. Just this fault zone blocked the Red River in Việt Trì and the Lô River in Tuyên Quang to form the above said long lakes. The change of flowing direction of the Đà River is also related to the activities of this fault zone.

As above analyzed, during Miocene, a long lake was located in the Ba Vì area with a length of hundreds kilometres. That’s why it could not be reasonable for the existence of the Đà River in this area. At that time, this river flowed in the northwest-southeast direction passing through the Mai Châu Townlet and, probably, emptying into the Mã River (?).

At present, the Đà river is flowing in northwest-southeast direction to the Hòa Bình Town, then winds its current northwards and empties into the Red River at Ba Vì. So, in the Quaternary the Đà River flowed across a band of Miocene sediments. Studying on the Hòa Bình - Ba Vì section of the Đà River, we could not find Lower Pleistocene fluvial sediments. This leads to the opinion that, this section was formed only in Middle-Late Pleistocene. Since Middle Pleistocene, the Đà River changed its current, flowed northwards and emptied into the Red River as seen at present.

Thus, we can come to the judgement that, the activities of the Hòa Bình - Ba Vì - Sơn Dương - Mèo Vạc fault zone changed the river network, leading to the confluence of the Red, Lô and Đà River at Việt Trì.

III. MANIFESTATIONS OF CONTEMPORANEOUS ACTIVITIES OF TECTONIC FAULTS

As presented above, the river network in the study area has close relationship with neotectonic activities, especially, with the activities of the Hòa Bình - Ba Vì - Sơn Dương - Mèo Vạc submeridional fault zone. Evidences about its existence and recent activities were shown in articles [2, 3, 4] in 2003 - 2007 years.

The activities of this fault zone are related to the phenomena of strong land splitting and landslide in the area of Ông Tượng Hill in 1998, and water loss of the Phương Lâm swamp in the Hòa Bình Town [2]. The phenomena of current change and strong scouring of the Đà River at the Tân Đức Commune (Việt Trì Town) are also related to the activities of this submeridional fault zone [4].

During the process of study, searching and prospect evaluation of thermal mineral water at La Phù (Phú Thọ Province) of the Association of Sciences and Production of Mineral Water in 2001 [5], geologists drilled some wells and determined that, this is thermal mineral water with the temperature of 37-44°C and the total dissolved solids of 2500-3500 mg/l. The radon content of the mineral water in the borehole LK.12 of this area was analyzed, reaching 4.28 nCi/l. The results of geothermal measurement of 83 points in the La Phù area by geophysical instruments of the Geophysic Division (VAST) associated with results of measuring temperature of mineral water from boreholes have been allowing to establish the geothermal map of the area. However, due to the limited expenditure the studied depth reaches only about 40 m.

This map shows that the geothermal anomalies extend in the submeridional direction along the left dyke of Đà River [5] in La Phù and Bảo Yên communes, Thanh Thủy District, among them, there are 2 occurrences having the maximal temperature of 44°C (Fig. 4).

In 2005, at Phong Vân Commune, Ba Vì District, situated on the right side of the Đà River, the thermal mineral water similar to that in La Phù has been discovered in boreholes. The above places have manifestations of thermal mineral water almost coinciding with the position of submeridional faults, which were discovered on the basis of remote sensing data and of the isobed map of Cenozoic sediments in Ba Vì area. This allows to judge about the close relationship between thermal mineral water and the submeridional fault. The thermal mineral water is the manifestation of neotectonic activities, and as a clue, the submeridional faults in this area are contemporaneous active faults.

In the other side, the radon anomaly is also the evidence of neotectonic activities. The existence of radon in mineral water has been adding more evidences to make the above judgements more reliable.

By analyzing the neotectonic stress field we recognize that, under the horizontal compression of submeridional direction, the northwest-southeast trending faults, such as Red River, Chảy River and Lô River ones, became dextral strike-slip faults; as for the submeridional one acted like a spreading fault.

Besides, many veins of hydrothermal quartz cross-cutting all formations in the La Phù thermal mineral water have been observed. They could be related to above said submeridional faults.

IV. CONCLUSIONS

The above research results have been proving the close relationship between the river network and neotectonic activities. Some conclusions can be drawn from this as follows:

1. During Oligocene - early Miocene, the Red and Lô rivers flowed in the northwest-southeast direction through the Việt Trì Town to the East Sea. To Miocene, the Red River was blocked in Việt Trì to form a long lake with the length of hundreds kilometres. It led to the formation of Miocene coal-bearing beds. At the same time, the Lô River was blocked in Tuyên Quang Town to form similar type of coal-bearing beds.

2. During Pliocene - early Pleistocene, the Red River flowed along the western margin of the Bắc Bộ Plain, passing through Sơn Tây and Lương Sơn townlets (Hòa Bình Province) to Ninh Bình Town.

In Middle Pleistocene, the Red River changed the flowing direction eastwards to flow across the Bắc Bộ Plain, forming the alluvial Hà Nội Formation.

3. The Hòa Bình - Ba Vì section of the Đà River flowed in the submeridional direction cros-cutting Miocene sediments, therefore, it is younger than Miocene. Its age probably is Middle Pleistocene - Holocene.

4. The submeridional fault zone (Hòa Bình - Sơn Dương - Mèo Vạc) cross-cutting the Ba Vì area is a great fault zone with the destructive zone of nearly tens kilometres. Its clear manifestations can be found in materials of geomorphology, remote sensing and aeromagnetic survey. It has also relation with thermal radon mineral water in this area, therefore is neotectonic active faults. It has serious influences in the structural plan of this area and is the main reason of the change of flowing direction of the Red, Lô and Đà rivers in this area.

This article is completed under the support of the Project 7.204.06 of the Basic Research Program, Ministry of Sciences and Technology.

REFERENCES

1. Hạ Văn Hải, 1993. Geomorpho-logical features and the history of recent tectonic development of the Northeast part of Vietnam. Geography-Geology Doctor thesis. Library of the Hà Nội University of Mining and Geology (in Vietnamese).

2. Hạ Văn Hải, 2003. Some new discoveries about recent activities of the Hòa Bình - Sơn Dương - Mèo Vạc sub-meridional fault zone in the North Việt Nam. Scientific-technological J. of Mining and Geology, 3/2003 (in Vietnamese).

3. Hạ Văn Hải, 2005. About the causes of dyke splitting and riverside erosion in Ba Vì, Hà Tây. Scientific-technological J. of Mining and Geology, 1/2005 (in Vietnamese).

4. Hạ Văn Hải, 2005. About the sub-meridional faults active during Pliocene-Quaternary in the East Bắc Bộ. J. of Earth sciences, 2/2005. Hà Nội (in Vietnamese).

5. Lê Tứ Hải (Editor), 2001. Report Research of mineral water in Thanh Thủy District, Phú Thọ Province. Phú Thọ (in Vietnamese)

6. Nguyễn Đức Đại, 1996. Report of Urban geology of the Hà Nội City. Division of Hydrogeology and Engineering geology of the North, Hà Nội (in Vietnamese).

7. P. Tapponnier et al., 1982. Propogating extrusion tectonic in Asia: New insights from simple experiments with plasticine. J. Geology, 10 : 611-616.

8. Trần Đức Lương, Nguyễn Xuân Bao (Editors-in-chief), 1988. Geological map of Việt Nam at 1:500,000 scale. Gen. Dept of Mines and Geology, Hà Nội.

9. Vũ Nhật Thắng (Editor), 2003. Geology and mineral resources of Hà Nội City. Division of Geological Mapping of the North. Hà Nội (in Vietnamese).

10. Phạm Đình Thọ et al., 2006. Evolution of Cenozoic sedimentation in the Tuyên Quang Depression. J. of Earth sciences (in Vietnamese).