NEW DATA ON BASALTS IN THE BOTTOM
OF THE SOUTH CENTRAL VIỆT NAM CONTINENTAL SHELF  TAKEN BY CRUISES VG-05

1NGUYỄN TIẾN HẢI, 2K. STATTEGGER

1Institute of Marine Geology and Geophysics (IMGG), Việt Nam;
 2Institute of Geosciences of Kiel University (CAU), Germany

Abstract: Within the framework of a Scientific Research Joint Project between Institute of Marine Geology and Geophysics (IMGG, Việt Nam) and Institute of Geosciences of Kiel University (CAU, Germany), an expedition has been carried out by VG-05 cruises during the year 2004 in the East Việt Nam Sea. The interpretation of new data taken by this cruise along high resolution shallow seismic profiles has defined some volcanos/diapirs (basalts, mouths) in the bottom of the South Central Việt Nam continental shelf, namely:

     - A volcano on profile 10050403 (about 108o.7891 E and 10o.5960 N, having diameter of about 800 and 2.400 m) in the bottom of the Vũng Tàu - Bình Thuận continental shelf (in the northeast vicinity of Hòn Tro volcano island).

     - A volcano on profile 09050403 (about 108o.8699 E and 10o.2528 N, having diameter of about 700 m) in the bottom of the Vũng Tàu - Bình Thuận continental shelf.

     - A volcano on profile 14050407-14050420 (diameter of about 150 m, 400 m and 10 km) in the bottom of the Vũng Tàu - Bình Thuận continental shelf.

     - A volcano on profile 15050401-15050411 (diameter of about 5 km) in the bottom of the Vũng Tàu - Bình Thuận continental shelf.

     - A volcano on profile 28050415 (about 109o.3566 E and 12o.3568 N, having diameter of about 320 m) in the bottom of the of Nha Trang continental shelf.


I. INTRODUCTION

The continental shelf off the southeast Vietnamese coast has extension of varied dimensions: in the vicinity of the Mekong River and where the Vietnamese continental shelf merges with the Sunda continental shelf it expands to more than 500 km. The central part of the Vietnamese continental shelf is very narrow with lateral extension as low as 20 km between 13° and 15° N.

The results of this study aim to provide with the principal knowledge on the geologic history and evolution of the SE-Vietnamese coastal zone over the last 10,000 years. In addition, these results should support planning strategies for integrated coastal zone management with respect to sustainable development. Reference should be given to competing interests between potential users of the coastal zone, e.g. tourism, fisheries, exploitation of mineral resources.

The studied region is located at the borders of the Sundaland continental plate and is assumed to be tectonically stable during the Quaternary (Tjia & Liew, 1996). An important dislocation, termed as the 110º-fault runs parallel to the Vietnamese coast reaching the study area in the east. Two fault zones in the region of Nha Trang trending SW-NE join the 110º-fault (Fontaine & Workman, 1997). They were formed by the SE movement of the Indochina block during the Himalayan orogeny. Granite dominates in the vicinity of the Nha Trang Bay.

The coastal zone in SE Việt Nam shows a variety of morpho-structural types and landforms which are genetically related to different types of coastline comprising sea terraces, sandy coasts, salt water swamps, erosional and denudation-accumulation coastal plains, basaltic plateaux, inselbergs. The volcanic activities in the region began and developed along regional rifts in the activated period of late Cenozoic magma in the East Sea [3].

Widespread volcanic activities of basaltic and basanitic type persisted during Tertiary and Quaternary times. The last large eruption was noted in 1923 on the continental shelf in the area of Catwick Islands, some 200 km south of Nha Trang Bay (Simkin et al., 1981).

II. MATERIAL AND METHODS

The project “Holocene coastal evolution, sea-level fluctuations, terrigenous sedimentation and sediment dynamics on the continental shelf between the Mekong Delta and Nha Trang, SE Việt Nam” had been carried out by Institute of Marine Geology and Geophysics (IMGG, Việt Nam) and Institute of Geosciences of Kiel University (CAU, Germany) from 2003 to 2005. The project was implemented in close cooperation with the other participating groups of the Project “Land-ocean interaction in the coastal zone of South Việt Nam”.

The sea floor of the Việt Nam continental shelf was surveyed using the shipboard parasound system to characterize sea-floor morphology and geometry of subsurface seismic reflectors, as well as to ensure coring at suitable locations. Using the parametric effect, the system emits two different frequencies of 18 kHz and 20.5-23.5 kHz, generating a working frequency of 2.5-5.5 kHz. These results in narrow beam characteristics return a well resolved signal of the sea-floor and subsurface reflectors with a penetration of up to 150 m, depending on bottom characteristics. Correlation of sedimentary strata with similar physical characteristics is thus simplified and the lateral extent of sediment bodies is traceable. In combination with ground truthing by sedimentological and geochemical investigations, the abundance and distribution of sedimentary beds on the continental shelf can be estimated.

The data interpretation from high resolution shallow seismic profiles was taken in cruises VG-05 within the framework of the Project in 2004 [2].

III. RESULTS

On the basis of the data interpretation from high resolution shallow seismic profiles, some volcanos/diapirs (basalts, mouths) have been found in the bottom of the South Central Việt Nam continental shelf:

In the bottom of the Vũng Tàu - Bình Thuận continental shelf:

- Seismic line 3: Volcano on profile 10050403 (about 108o.7891 E and 10o.5960 N), having size of about 800 and 2.400 m (in NE vicinity of Hòn Tro volcano island, Fig. 1).

- Seismic line 1: Volcano on profile 09050403 (about 108o.8699 E and 10o.2528 N) having size of about 700 m (Fig. 2).

- Seismic line 6+7a: Volcano on profile 14050407-14050420 (Fig. 2) having size of about 150 m, 400 m and 10,000 m.

- Seismic line 7b: Volcano on profile 15050401-15050411 (Fig. 2) having size of about 50,000 m).


Figure 1. Seismic line 1, volcano on profile 09050403 in the bottom
of the Vũng Tàu - Bình Thuận continental shelf.

Figure 2. a) Seismic line 3, volcano on profile 10050403;
b) Seismic line 6+7a, volcano on profile 14050407-14050420;
c) Seismic line 7b, volcano on profile 15050401-15050411
in the bottom of the Vũng Tàu - Bình Thuận continental shelf.

Fig.3. Seismic line 14, volcano on profile 28050415 in the bottom
of the Nha Trang continental shelf.


In the bottom of the Nha Trang continental shelf:

- Seismic line 14: Volcano on profile 28050415 (about 109o.3566 E and 12o.3568 N) with the size of about 320 m (Fig. 3). 

IV. DISCUSSION

Presented characteristics of the volcanoes show that the volcanic activities in the studied region began and developed along regional rifts in the activated period of late Cenozoic magma in the East Sea [3] and maybe the determine ages into four volcanic period: Late Miocene, Pliocene, Middle Pleistocene and actual, in themselves - into two groups: hawaian and tholeiitic [1] .

REFERENCES

1. Đỗ Minh Tiệp, 1996. Time and space differentiation of Cenozoic submarine basalts on the bottom of the East Sea. Contr. of Marine geol. and geoph., II : 179-194, Sci. & Techn. Publ. House, Hà Nội.

2. Nguyễn Tiến Hải et al, 2006. Holocene coastal evolution, sea-level fluctuations, terrigenous sedimentation and sediment dynamics on the continental shelf between the Mekong Delta and Nha Trang, SE Việt Nam. Coop. Proj. between IMGG (Việt Nam) and CAU (Germany) in 2003-2005.

3. Nguyễn Xuân Hãn et al, 1996. Features of late Cenozoic volcanic activities in the East Sea of Việt Nam. Contr. of Marine geol. and geoph., II : 88-95. Sci. & Techn. Publ. House, Hà Nội.

4. Schimanski A., K. Stattegger, 2004. Deglacial and Holocene evolution of the Vietnamese continental shelf: Stratigraphy, sediments and sea-level change. Marine Geology, pp. 1-24.

5. Schimanski A., N.T. Hai et al, 2005. Seismic stratigraphy and Late Pleistocene - Early Holocene evolution of the Paleo-Mekong Delta: Insights from high resolution reflection seismics. Intern. Conf. on Deltas (Mekong Venue): Geol. Modelling and Manag., HồChíMinh City.