The studied area is restricted between the latitudes 12^o12’ and 15^o22’ N. The main geological structures of this region consist of the Precambrian Kon Tum Massif in the north and a part of the Mesozoic Đŕ Lạt Depression in the south. This region underwent dramatically a complex tectonic development history. During the Cenozoic era a large-scale volcanic activity phase occured, forming number of basalt plateaux. In many localities, the thickness of the basalt cover reaches hundred metres, in some places up to 500 m. Therefore, this cover generally hides geological features of the basement, among them the distribution characteristics of fault systems (Fig. 1). Because of that, geophysic investigations can play a significant role in the geological study in this region.

Up to this time, the whole studied region was covered by the Bouguer gravity map at 1:500,000 scale and aero-magnetic map at 1:200,000 scale. Besides, in some restricted areas the electrical, magnetic, gravimetric and gamma spectral surveys at larger scale have been carried out with mineral investigation purposes. In fact, the results of geophysic survey during last years have been showing to be very useful for the interpretation of problems related to the history of tectono-geological evolution of the region. Among these results, those of the analysis of magnetic data have been contributing actively to the discovery of main faults hidden under the basaltic cover. However, the effect caused by the magnetic field of low latitude zone in Việt Nam is still a noteworthy problem in the link of data interpretation. Many traditional methods having high effectiveness in the interpretation of magnetic data in high latitude zone have been showing that they are not so effective in low latitude one, because in the last zone the vector of magnetization of both geomagnetism and causative bodies of magnetic anomalies has not the vertical trend, leading to the situation that the distribution of magnetic field in low latitude zone does not obey the distributive rules of magnetic field in high latitude. It is worthy to pay attention to that, the projected position of causative bodies of magnetic anomalies on the observed plane does not coincide with the position of these anomalies as in high latitude zone. In this case, if we base on the correlation between the distribution of magnetic field and that of causative bodies of anomalies in the low latitude zone, the results of interpretation may be distorted [1, 6]. In this sense, the technique used for reduction to the pole of magnetic anomalies is needed before applying traditional algorithms, which suits the interpretation of magnetic data in high latitude zone. In fact, after reduction to the pole the relationship between the spatial distribution of magnetic anomalies and magnetic bodies can be regarded as that in high latitude zone.

As above mentioned, the interpretation of magnetic data from the studied region may be begun by the reduction to the pole of the survey magnetic field. The results of calculation in this first step can be considered as initial data for applying traditional techniques in further steps. In this paper, the algorithm proposed by J. Arkani-Hamed [1] has been used for reducing magnetic anomalies to the pole. In difference from previously used algorithms, the variations in direction of the magnetization vector of the geomagnetic field and the causative bodies of magnetic anomalies are taken into account. This approach may allow us to diminish the errors of calculation in comparing with formerly used algorithms. Commonly, the used magnetic data consist of observed data from a large area, therefore if the magnetization vectors of the geomagnetic field and causative bodies of magnetic anomalies are considered as constant that does not reflect faithfully the real situation.

According to this algorithm, the calculations are realized in the frequency domain with the use of two-dimensional fast Fourier transformation. The pole magnetic field in the Cartesian coordinates can be calculated thanks the inverse Fourier transformation from the spectral frequency domain [1]. The results of calculation allow to create initial data for using in other methods commonly used in the interpretation of data from high latitude zone. If they are combined with existing geologo-geophysic materials, they let see that, the magnetic field after reducing to the pole reflects better the factors of geological structure of the studied area than the observed magnetic field (Fig. 1, 2). Since this moment, the further interpretation methods use only the newly reduced to the pole magnetic field. It is to note that, the variations of magnetic anomalies in spatial distribution are closely related to the distributive characteristics of geological structures and the composition of rocks. The separation of the field into regional and local components allows us to study on characteristics either related to regional structure, or to local factors. This operation in certain degree can be done by applying the upward continuation algorithm.

According to the theory on the transformation of potential fields, the upward-continued magnetic field reflects structural elements lying deeper than the observed magnetic field. By carrying out the upward continuation to different levels we are able to observe the quantitative change of the structure following the depth, among this the anomalies upward-continued to bigger height reflect structural elements bearing more regional character than those upward-continued to smaller height. These calculations are realized by using the two-dimensional fast Fourier transformation [1]. By the above said calculations, the reduced to the pole magnetic field of the studied area is upward-continued to the levels: Z = 2, 3, ..., 10 km. Obviously, the picture of the upward-continued magnetic field promises a better correlation with geological structure elements in the area (Fig. 3, 4). All these anomaly fields are used in the calculation for compiling the sketch map of magnetic discontinuities, which are closely related to the fault distribution in the area. For realizing this, the algorithm calculating the maximum of horizontal gradient of anomaly fields has been used as the third technique for data interpretation. In fact, the vertical boundary of bodies causing anomalies has the vertical angle that often produces the extremes of the horizontal gradient field. Usually,

this boundary is also the boundary between units of geological structures [2, 4, 5]. Just this property allows us to use the scheme of maximal horizontal gradient (discontinuities) of different upward-continued fields for determining approximately the position of faults on the plan. For the studied region, the anomaly fields upward-continued to the levels Z = 2, 3, 5, 6, 8 km have been used (examples in Fig. 3). The results of calculation have been showing that the integration of the scheme of magnetic discontinuities allows not only to affirm many informations on the distribution of existing fault systems, but also to provide with new informations about them. Except that, the dipping direction of a limited number of faults can be determined by comparing different schemes of magnetic discontinuities. In fact, these discontinuities of different levels have the tendency to displace toward the dipping direction of faults [4]. Thus, the data interpretation has been realized by using a combination of techniques including: reduction to the pole of magnetic field, upward continuation and calculation of maximal horizontal gradient in four directions. The effect of using this method combination has been received good evaluation after the test study on models [6] and the interpretation of magnetic data from the territory of North Việt Nam [7].

The results of calculation have been allowing us to receive the magnetic anomalies reduced to the pole, some maps of upward-coninued magnetic anomalies, and schemes of distribution of magnetic discontinuities. Analyzing the characteristics of maps of magnetic anomaly field, we can carry out the structural zoning of magnetic fields. The more detailed distribution of fault systems can be obtained after superimposing the maps of magnetic discontinuities, the maps of magnetic anomaly field and other available geologo-geophysic materials.

By analyzing the characteristics of structural trends and the distribution of the intensity of anomaly field, including the observed anomalies, the anomaly field reduced to the pole and upward-continued fields, the magnetic field of the studied area can be divided into 3 main structural zones:

1) North Structural zone: this zone occupies the northern part of the studied area. Its southern boundary is the line linking the north of the Ngọc Hồi District Centre and the north of Kbang District Centre toward the sea-shore to a point near the Phù Mỹ District Centre. The small size and small amplitude with negative value are predominant characteristics of the anomalies in this zone (Fig. 2-4).

2) Central Structural zone: the southern boundary of this zone is the line stretching from the point near Ea Súp District Centre southeastward to north Krông Buk District Centre, north Ma Đrăk District Centre and to the sea-shore near Ninh Hoà District Centre. In general, the structure of regional field of this zone is of NW-SE trend. On the common background of regional anomalies, local anomalies of medium size with large amplitude and high value of horizontal gradient are largely developed. The strong differentiation in the structure of anomaly field of this zone, possibly, conforms to the complex character of the geological structure in this area.

3) South Structural zone: consisting of the remaining area. In this zone, almost all magnetic anomalies have the sublatitudinal trending axis, that differs it from the two above zones. They are distributed rather thinly on the area, but their size is usually bigger than those of the Central Structural zone.

In general, the magnetic anomalies upward-continued to different levels reflect rather well the distribution of fault systems in the region.

The use of new results of magnetic data interpretation in the study on the distribution of fault systems in the South of Central Việt Nam has been carried out by combining the maps of upward-continued magnetic anomaly field, the schemes of magnetic discontinuities and geologo-geophysic available materials.

According to the signs, the boundary of causative bodies of magnetic anomalies is expressed not only by maximal horizontal gradient bands, but also by boundaries separating field areas having structure which strongly differs from one another by the trend and intensity of anomaly field. These boundaries can be determined by the direct analysis of structural features of upward-continued anomaly field; a great number of boundaries have been determined on the schemes of magnetic discontinuities. It is worthy to note that the schemes calculated for anomaly field upward-continued to high levels reflect mainly structural elements of regional character. Synthesizing all above calculated results in combining with the analysis of characteristics of geological structures from the studied area, a sketch map of distribution of magnetic boundaries has been compiled (Fig. 7). By correlating this map with existing results of previous studies we can make more accurate the position of faults on the plan, classify them and discover new informations on the distribution of the fault network.

These results have been showing that almost all main faults found in previous studies have been reconfirmed by magnetic data interpretation in this work. They have been allowing also to find some fault sections in many localities of the studied area, especially in those hidden beneath a thick basaltic cover (Fig. 8).

According to our study, in the South of Central Việt Nam fault systems of different trends are widespread, among them the sublatitudinal system is predominant in the area of the North Structural zone, while the NW-SE and sublatitudinal trending fault systems are predominant in the Central Structural zone, and the NE-SW trending fault system – in the South Structural zone. The study results have been allowing also to estimate the dipping direction of main faults in the region (Fig. 8).

1) By applying the technique of reduction to the pole of magnetic field, the low latitude effect seems to be significantly reduced. It is obvious that, after reduction to the pole the magnetic field reflects better geological structure elements of the studied region.

2) The results of magnetic data interpretation presented in this paper can be used for improving the sketch map of distribution of fault systems in the South of Central Việt Nam. In fact, many results of former studies have been affirmed through the combination with present materials. This is also the basis proving that new data on fault systems of this study have a given reliability. They can be useful for further studies on regional geology of this territory.

It seems that, new informations on the distribution of faults suggested in this paper are only the results from magnetic data interpretation. Therefore, their usage must be combined with other materials for enhancing their reliability.

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